Biomes of western North America at 18,000, 6000, and 0 14C ...

Biomes of western North America at 18,000, 6000, and 0 14C yr B.P. reconstructed from pollen andpackrat midden data

Robert S. Thompson1 and Katherine H. Anderson2

1 U.S. Geological Survey, Earth Surface Processes Team, Box 25046, MS980, Denver, CO 80225, USA.

2 Katherine H. Anderson, Institute of Arctic and Alpine Research (INSTAAR), University of Colorado,Boulder, CO 80303, USA.

Address for correspondence: Dr. R.S. Thompson, U.S. Geological Survey, Earth Surface Processes Team,Box 25046, MS980, Denver, CO 80225, USA (fax +1 303 2365349, e-mail:[email protected])

Ms. for Journal of Biogeography, BIOME 6000 special issue.

1 March, 2000

Biome reconstructions for western North America 2

(A) ABSTRACT

1 A new compilation of pollen and packrat midden data from western North America provides a refined

reconstruction of the composition and distribution of biomes in western North America for today and for

6000 and 18,000 radiocarbon years before present (14C yr B.P.).

2 Modern biomes in western North America are adequately portrayed by pollen assemblages from lakes and

bogs. Forest biomes in western North America share many taxa in their pollen spectra and it can be difficult

to discriminate among these biomes. Plant macrofossils from packrat middens provide reliable identification

of modern biomes from arid and semiarid regions, and this may also be true in similar environments in other

parts of the world. However, a weighting factor for trees and shrubs must be used to reliably reconstruct

modern biomes from plant macrofossils.

3 A new biome, open conifer woodland, which includes eurythermic conifers and steppe plants, was defined

to categorize much of the current and past vegetation of the semiarid interior of western North America.

4 At 6000 14C yr B.P., the forest biomes of the coastal Pacific Northwest and the desert biomes of the

Southwest were in near-modern positions. Biomes in the interior Pacific Northwest differed from those of

today in that taiga prevailed in modern cool/cold mixed forests. Steppe was present in areas occupied today

by open conifer woodland in the northern Great Basin, while in the central and southern Rocky Mountains

forests grew where steppe grows today. During the mid-Holocene, cool conifer forests were expanded in the

Rocky Mountains (relative to today) but contracted in the Sierra Nevada. These differences from the forests

of today imply different climatic histories in these two regions between 6000 14C yr B.P. and today.

5 At 18,000 14C yr B.P., deserts were absent from the Southwest and the coverage of open conifer woodland

was greatly expanded relative to today. Steppe and tundra were present in much of the region now covered

by forests in the Pacific Northwest.

Key words: packrat middens, pollen data, plant macrofossil data, plant functional types, biomes, vegetation

changes, western North America, last glacial maximum, mid-Holocene


(A) INTRODUCTION

Biomes represent broad physiognomic vegetation types that are based on the co-occurrence of plant species

that respond individualistically to climatic gradients and climatic change. Biomes depict vegetation

formations on a global basis based on the structure, physiognomy and climatic adaptations of the plant

functional types of which they are composed (Prentice et al., 1992). Plant functional types (PFTs) are

collections of plant taxa grouped by stature, leaf form, phenology and climatic adaptations. The

characterization of vegetation in terms of biomes provides the basis for modelling past and future vegetation

changes (e.g. Harrison et al., 1995; Kutzbach et al., 1998; Neilson et al., 1998) and using the same terms

for vegetation from different regions facilitates comparisons between the results of numerical climate

modelling and evidence of past vegetation (e.g. Jolly et al., 1998a; Joussaume et al.,1998; Williams et al.,

1998).

Prentice et al. (1992) provided the original definitions of PFTs and biomes used in this paper. Subsequent

regional syntheses for Europe (Prentice et al., 1996), the former Soviet Union and Mongolia (Tarasov et al.,

1998), China (Yu et al.,1998; Yu et al., this issue), Africa and Arabia (Jolly et al., 1998b; Elenga et al., this

issue), Beringia (Edwards et al., this issue), and Canada and the eastern United States (Williams et al.,1998,

this issue) refined these definitions and methodology, and extended them into a wide range of climatic and

vegetational circumstances. In this paper, we use pollen and plant macrofossil data to portray biomes in

western North America at 18,000 radiocarbon years before present (18,000 14C yr B.P.: the last glacial

maximum or LGM), 6000 14C yr B.P. (the mid-Holocene), and 0 14C yr B.P. (today). We discuss the

changes in biomes between these time periods in terms of broad-scale changes in climate.

Western North America is treated separately from Canada and the eastern United States for the following

reasons: (1) it has an endemic flora that differs from those of Canada and the eastern United States; (2) it

has a unique data source (packrat middens) and palynological settings in desert and mountain environments

that are different from the other two regions; (3) its climate has sharp gradients, is generally more arid than

that of the eastern United States, and its winter temperatures are generally milder than those of Canada or

the northeastern and midwestern United States; and (4) its mountainous terrain requires special data analysis

and can limit interpolation among sites.

(B) Modern climate and vegetation

The modern climate of western North America has strong geographic and elevational gradients in both

temperature and precipitation (Fig. 1). Winter precipitation from the North Pacific supports wet

environments in the Pacific Northwest, along the coast of California and in the northern Rocky Mountains.

Rain shadows associated with the Sierra Nevada, Transverse and Cascade Ranges cause aridity in the


interior western United States. This aridity is offset in some areas near the Mexico-United States border and

in the southern Rocky Mountains by summer monsoonal rainfall from subtropical sources. Seasonal and

diurnal temperature ranges are low in coastal regions, but are high in many parts of the arid and semiarid

interior. These geographic gradients of climate interact with the mountainous physiography of western

North America to create a mosaic of forest, woodland, steppe/grassland, and desert vegetation. These

patterns are depicted in Fig. 2, where the categories representing potential natural vegetation from Küchler

(1964) have been grouped (Table 1) to approximate the biomes defined by Prentice et al. (1992). These

categories are based on observations of the current and historic vegetation and are purposefully simplified

to facilitate comparison between the fossil and modern data (for example, grassland and steppe are merged

here as it is difficult to distinguish these in the fossil record).

(A) DATA AND METHODS

(B) Pollen data

Pollen records provide quasi-continuous evidence of vegetation change through time. These data are

quantified as percentages of each taxon for each spectrum in the record; however, modern pollen

percentages frequently do not have linear relationships with the abundances of species within the terrestrial

vegetation. Pollen data are generally of low taxonomic resolution, and can usually be identified only at the

family or genus level. Fossil pollen data have been the primary source of information on biomes in other

regions, and their inclusion here allows some degree of comparability with previous studies. The pollen data

included here are from original counts wherever possible, or were digitized from published diagrams when

counts were unavailable. Raw pollen counts appear to provide a better discrimination between non-arboreal

biomes (Jolly et al., 1998b; Yu et al., this issue), but digitized data have been used to reconstruct biomes

successfully in other regions (e.g. Prentice et al., 1996; Tarasov et al., 1998; other papers in this issue).

(B) Packrat midden data

Packrats (Neotoma spp.) are small rodents that collect leaves, sticks, fruits and other materials from the area

within tens-of-meters of their nests and bring these items into their homes for food, nesting material, etc.

Through time their nests can become cemented by their desiccated urine, and in dry caves and rockshelters

in western North America these urine-cemented "packrat middens" can be preserved for tens-of-thousands

of years (see papers in Betancourt et al., 1990 for further discussion). Packrat middens are unique to

western North America, although similar deposits left by other animals have been found in arid regions in

the Middle East, South America, South Africa and Australia (Betancourt et al., 1990). Plant remains from

packrat middens: (1) can generally be identified to the species level, and a given midden plant assemblage

appears to provide a detailed inventory of the species growing within 50m (or less) of the packrat’s nesting


site; (2) are extremely well-preserved and provide excellent material for radiocarbon dating; and (3) occur

in assemblages that appear to represent a short interval of time, perhaps as short as the few years of an

individual packrat’s lifespan. In sum, packrat middens provide a detailed inventory of plant species for short

intervals of time in the past.

Although vegetation assemblages preserved in packrat middens have been studied since the 1960s, no

uniform method of quantification has been developed, and indeed there have been more quantification

schemes than investigators in the field. In any case, there is little evidence that relative abundance within the

assemblages has much meaning, except to indicate the past presence or absence of a given plant species at a

given midden site. In this paper we used data from the original publications (USGS/NOAA NGDC Packrat

Midden database) and converted the various quantification schemes into a simple four-digit scale where: 0 =

absent, 1 = rare (and possibly a contaminant); 2 = present; and -9 = cannot determine presence or absence

(this usually pertains when the original investigator published an incomplete list of plant species and it is

thus not possible to determine if something is really absent). Packrat middens with very few reported taxa

were omitted from the analysis.

(B) Data sets

Modern pollen assemblages were obtained for 66 sites (Table 2). Raw pollen counts were obtained for 16

sites, and digitized data were used for the rest. Modern (and near-modern) packrat midden assemblages (35

samples) were obtained from 18 sites (Table 3). To facilitate comparisons with the older time slices, we

used only the core tops of pollen sediment cores for the 0 14C yr B.P. time horizon. We did not use the

diverse array of surface samples available for western North America. For packrat middens, we used all

samples with radiocarbon ages within the last 1000 radiocarbon years.

The data set for 6000 14C yr B.P. consists of 76 pollen sites (Table 2), of which 14 were obtained as raw

pollen counts and 62 were digitized, and 34 packrat midden assemblages (23 sites) (Table 3). The data set

for 18,000 14C yr B.P. consists of 21 pollen sites (Table 2), of which 7 were obtained as raw pollen counts

and 14 were digitized, and 17 packrat midden assemblages (13 sites) (Table 3). With original pollen data

for the 6000 14C yr B.P. time slice we used the pollen spectrum closest to 6000 14C yr B.P., whereas for the

digitized pollen data we interpolated to determine the approximate 6000 14C yr B.P. level. With packrat

midden data for 6000 14C yr B.P., we included all samples dated within 1000 radiocarbon years of 6000 14C

yr B.P. (in other words, those samples that dated between 7000 and 5000 14C yr B.P.). The same protocols

were followed for the 18,000 14C yr B.P. time slice.


(B) Method of assigning biomes to pollen and packrat midden assemblages (biomization)

The biomization procedure has been fully described by Prentice et al. (1996) and Prentice & Webb (1998).

There are four steps in the biomization procedure: (1) assignment of individual plant taxa to plant functional

types (PFTs); (2) specification of the set of PFTs that can occur in each biome; (3) calculation of the affinity

score of a given vegetation assemblage to every biome; and (4) assignment of the vegetation assemblage to

the biome for which it has the largest affinity score. In cases where the affinity score for two or more biomes

is equal, a tie-breaking rule is applied to determine the biome attributed to the sample, following Prentice et

al. (1996).

We prepared separate plant taxon-PFT matrices for pollen (Table 4) and for packrat midden assemblages

(Table 5) based on our knowledge of the ecology and biology of the individual plants, and on the

descriptions of the flora and vegetation given in Benson (1982), Benson & Darrow (1981), Kearney &

Peebles (1960), Little (1971, 1976, 1977), Thompson et al. (1999a, 1999b) and Turner et al. (1995). The

pollen matrix includes 74 individual pollen taxa and the packrat midden assemblage matrix includes 418

individual species.

We began with the PFT classification based on pollen data used for Europe (Prentice et al., 1996) and

subsequently tested and modified for other northern hemisphere regions (e.g. Jolly et al., 1998b; Tarasov et

al., 1998; Yu et al., 1998; Williams et al., 1998), and modified the pollen to PFT assignments to fit the

North American situation. The PFT definitions, the assignment of individual pollen taxa to PFTs, and the

assignment of PFTs to biomes are broadly consistent with the definitions used in adjacent regions of north

America by Williams et al. (this issue) and Edwards et al. (this issue).

There are difficulties with the pollen-to-PFT relation in western North America for conifers, as many pollen

taxa include both boreal and temperate species (most notably Picea, Abies and Larix). In Eurasia, Larix and

Picea are confined to the boreal zone (Prentice et al., 1996; Tarasov et al., 1998). However, both taxa also

occur in more temperate settings in North America (Little, 1971; Thompson et al., 1999a, 1999b). For

example, Larix lyallii and L. occidentalis both live in temperature environments in the interior regions of

the Pacific Northwest, while Picea sitchensis occurs in temperate coastal forests from southern Alaska to

northern California. We therefore allowed Larix to occur as both a boreal summergreen conifer (as in

Eurasia) and a cool-temperate conifer, while Picea was classified as both a boreal evergreen conifer (as in

Eurasia) and a cool-temperate conifer.

The species-level identifications of the midden plant macrofossils led us to define five new PFTs: woodland

conifer (wc), woodland shrub (ws), frost-sensitive desert shrub or succulent (ds2), desert shrub or succulent

(ds) and steppe shrub (ss). Woodland conifers include pinyon pine and woodland juniper, whereas the


woodland shrub PFT includes species in the genera Amelanchier, Berberis, Cercocarpus, Cowania,

Fraxinus, Garrya, Prunus and Ribes, among others. Frost-sensitive desert shrubs or succulents include

plants characteristic of the present-day Sonoran Desert, including species in the genera Agave, Bursera,

Cereus, Cercidium and Ferocactus, among others. The desert shrub PFT includes less-frost-sensitive

species including species in the genera Acacia, Celtis, Coleogyne, Dalea and Ephedra. Steppe shrubs

include species in the genera Artemisia, Atriplex, Chrysothamnus, Ephedra, Gutierrezia and Grayia, among

others.

We created a PFT to biome matrix (Table 6) to assign the PFTs in Tables 4 and 5 to the biomes defined in

Prentice et al. (1992) and Prentice et al. (1996). In the case of tie-breaks, biomes are assigned in the order

they appear in Table 6.

We identified a new biome (open conifer woodland: OC) to represent the pinyon-juniper woodlands and

subalpine conifer woodlands of the American Southwest, Great Basin and Colorado Plateau. This biome is

characterized by the co-occurrence of eurythermic conifer, woodland conifer, woodland shrub, steppe shrub

and steppe forb PFTs. The biome occurs in dry conditions near the lower moisture requirements of conifers,

and is associated with colder conditions than the xerophytic woods/scrub biome in Europe (Prentice et al.,

1996), Africa (Jolly et al., 1998b) and southern California (see discussion above). The open conifer

woodland biome can be identified from both pollen and midden data.

The biomization procedure required some modifications when applied to the packrat midden data. In

particular, the highly localized sampling area represented by packrat midden assemblages, coupled with the

depauperate arboreal flora of much of the region, means that they characteristically contain relatively few

fossil remains from trees and shrubs but an abundance of material from forbs, many of which may have

been represented by only a few individual plants. If the standard method of calculating affinity scores was

applied in this situation, it would be extremely difficult to generate assignments to arboreal biomes. We

therefore applied a weighting scheme to the packrat midden assemblages, whereby tree macrofossils were

given a weighting of 3, shrub macrofossils were given a weighting of 2 and forbs were given a weighting of

1. These weighting factors were determined through experimentation: we tried several schemes and selected

the weightings that provided the best biomization of present-day midden assemblages when compared with

the mapped modern extents of biomes interpreted from the Küchler (1964) map of potential natural

vegetation in North America. Except for these weighting factors, the calculation of affinity scores and the

assignment of packrat midden samples to biomes was done in the standard way.


(A) RESULTS

(B) Predicted vs observed modern biomes

Pollen data provide the evidence for biome reconstructions in the more northerly and higher elevation

biomes (Fig. 3), whereas packrat middens are largely restricted to the Southwestern deserts, Great Basin and

Colorado Plateau. The pollen- and macrofossil-based reconstruction of modern biome distributions (Fig. 3)

corresponds reasonably well with the observed patterns in vegetation distribution (Küchler, 1964; Fig. 2).

The desert, steppe, conifer forest and taiga vegetation are correctly placed geographically. The predicted

and observed geographic patterns for selected biomes are as follows:

Tundra. Only one pollen site (Little Lake, OR; Worona & Whitlock, 1995) is reconstructed as representing

this biome and it occurs within a present-day region of cool conifer forest. This misassignment is due to the

high percentages of Alnus, Cyperaceae and Poaceae that occur at this site, presumably due to historic land

use.

Taiga. It is difficult to use pollen data to discriminate among the taiga, cool conifer forest, cold mixed forest

and cool mixed forest biomes in western North America. However, biomization of the modern pollen data

do result in the correct placement of taiga along the Idaho/Montana border and in south-central Utah.

Cold deciduous forest. The core-top pollen spectrum from Waits Lake, WA (Mack et al., 1978), in a region

of present-day cool conifer forest, is incorrectly assigned to the cold deciduous forest biome. This biome

has an identical taxon list with the taiga, cool conifer, cold mixed and cool mixed forest biomes, except that

the cold deciduous forest biome lacks Picea and Abies. The Waits Lake surface spectrum is dominated by

Pinus pollen with significant representations of Alnus and Larix. The allocation to cold deciduous forest

occurs because Abies is present in a very small amount and Picea is absent.

Cool conifer forest. Cool conifer forest is correctly reconstructed from pollen spectra in the Pacific

Northwest and Sierra Nevada, as well as in the northern panhandle of Idaho and in adjacent Montana.

However, in the Yellowstone (Wyoming) region, Colorado and Utah, modern core-top pollen spectra from

this biome (as mapped in Fig. 2) are often misassigned to the open conifer woodland biome. Pinus, the most

prolific pollen producer in western North America, is present in both cool conifer forest and open conifer

woodland as is Cupressaceae, which in these regions is represented by the genus Juniperus (another

abundant pollen producer). Cool conifer forest includes many taxa not present in open conifer woodland,

including: (1) coniferous forest trees (Abies, Pseudotsuga, Picea); (2) riparian hardwoods with restricted

geographic coverages and low representations in pollen spectra in these regions (Alnus, Betula, Cornus,

Corylus, Salix); and (3) taxa that are poorly recorded in pollen spectra (Ericaceae, Populus, Shepherdia


canadensis). Our data set indicates that most sites in these cool conifer forests have less than 1%

Pseudotsuga (although this tree is common in these forest), and at most only a few percent of Abies and

only rare Picea. The under-representation of major taxa in the pollen assemblage makes it difficult to obtain

a correct biome assignment. For example, Edwards et al. (this issue) have shown that it is difficult to

correctly predict the extent of the Larix-dominated cold deciduous forests in eastern Siberia because Larix

is chronically under-represented in the pollen record. The cool conifer forest samples in the Rocky

Mountains have an additional problem in that they contain significant amounts of Artemisia, Poaceae and

other steppe taxa. These taxa occur in open conifer woodland but not in cool conifer forests. Their

occurence at high elevation sites in the Rocky Mountains is presumably due to pollen blown upslope from

lower elevation habitats. Collectively, the under-representation of key species of the cool conifer forest and

the presence of steppe taxa in the pollen spectra results in the misassignment of samples to the open conifer

woodland biome.

Open conifer woodland. The distribution of open conifer woodland (Fig. 2) is reasonably well captured in

the biome reconstruction (Fig. 3), although pollen-based reconstructions overestimate the extent of the

biome in the central and northern Rocky Mountains. Pollen spectra in modern steppe/grassland

environments in eastern Washington, western Montana, central Wyoming, northeastern Colorado and the

Great Basin are also mistakenly categorized as open conifer woodland. This is the reverse of the situation

described in the preceding paragraph, as here the long-distance blow-in of Pinus pollen (e.g. Mack &

Bryant, 1974) combined with the local steppe/grassland taxa in the pollen spectra, results in a wrong

assignment to open conifer woodland. Packrat midden assemblages, with their species-level inventory of

plant remains, do not suffer the same problems as pollen data in representing this biome.

Xerophytic woods/scrub. The single midden-based reconstruction of xerophytic woods/scrub in California is

correctly placed. There are no pollen or midden records yet from the larger area of present-day xerophytic

woods/scrub in southern California (Fig. 2).

Steppe/grassland. There are cases where the biomization of pollen data results in an incorrect categorization

of steppe or grassland as open conifer woodland due to long-distance transport of conifer pollen. In other

cases, such as the Ruby Marshes in northeastern Nevada (Thompson, 1992), the pollen site itself is located

in steppe, but woodland vegetation occurs within a few kilometers. Here the steppe/grassland and open

conifer woodland biomes form a mosaic, and we do not consider this to be a failed biomization from a

regional perspective.

Desert. The modern hot deserts of the American Southwest and northwest Mexico are recorded only in

packrat midden assemblages, as this environment is typically too arid for lakes. The present-day desert

biome is correctly placed by the midden data, even in a situation such as the Grand Canyon where pockets


of modern desert occur (and are correctly reconstructed from the midden data; Table 3, Fig. 3) but are too

small to appear on the present-day vegetation map (Fig. 2).

0 14C yr B.P. Summary. Although there are minor problems with the assignment of individual sites to

biomes, it is clear that the biomization technique adequately captures the complex patterns in the vegetation

of western North America and thus can be used to reconstruct past changes in vegetation distribution with

some confidence. However, there are problems, including: (1) the differing taxonomic resolution and spatial

coverage of pollen spectra and packrat midden plant assemblages affect the results in that middens provide

an extremely local but taxonomically precise view of biomes, whereas pollen data provide a more

taxonomically generalized and regional perspective (especially in mountainous regions where significant

amounts of pollen from one vegetation association can blow into another); (2) present-day land use can

skew pollen assemblages and lead to misassignments of surface pollen samples to biomes; (3) the

depauperate tree flora can make it difficult to segregate forest biomes, particularly because trees such as

Picea and Abies occur in both boreal and temperate plant assemblages; and (4) certain pollen types that are

key markers for specific biomes (e.g. Pseudotsuga, Larix, and Abies) are under-represented in pollen

assemblages.

The taiga, cool conifer, cold mixed and cool mixed forest biomes in western North America are difficult to

differentiate on the basis of pollen assemblages. All four share the following taxa: Abies, Alnus, Betula,

Cornus, Cupressaceae, Ericaceae, Larix, Larix/Pseudotsuga, Myricaceae, Picea, Pinus, Populus, Salix,

Shepherdia canadensis and the combined taxon Taxodiaceae/Cupressaceae/Taxaceae. In their original

definitions (Prentice et al., 1992, 1996), the cold mixed forest biome was characterized by the presence of

the ctc1 PFT (see Table 5 for key to abbreviations), whereas cool conifer forests were characterized by the

bec PFT. In Europe, obligate ctc1 taxa (e.g. Cedrus and Taxus) are not common in the pollen records and

the distinction between cold mixed and cool conifer forests largely rests on the presence of obligate bec taxa

such as Picea and Pinus (Hyploxylon). Undifferentiated Picea is not confined to ctc1 in western North

America because it is also considered a cool-temperate conifer, and Pinus (Hyploxylon) in this region

covers a range of environments from subalpine to open conifer woodland. Consequently, for western North

American pollen records, the cool conifer and cold mixed forest biomes have identical taxon lists and are

assigned to the cool conifer forest biome.

The pollen-based definition of the taiga biome lacks Corylus, Pseudotsuga, Taxaceae, Taxodiaceae and

Tsuga, taxa that are present in the cool conifer, cold mixed and cool mixed forest biomes. Most

palynologists have not distinguished Pseudotsuga from Larix, nor Taxaceae and Taxodiaceae from each

other or from Cupressaceae. Corylus is rarely recorded, and is never abundant, in western pollen records. In

practice, then, it is the presence, absence, or abundance of Tsuga pollen that allows us to distinguish taiga

from the other three biomes. Tsuga is present only in the northern Rocky Mountains, Cascade Range and


Sierra Nevada, so it is only in those regions that one could potentially differentiate taiga from the other

forest biomes. The cool mixed forest biome includes several taxa that are not present in the other three

forest biomes: Acer, Ceanothus, Fraxinus, Quercus, Rhamnus. These taxa are usually found in minor

amounts in pollen records in western North America, and it is probable that in most cases the cool mixed

forest biome cannot be reliably separated from the other three forest biomes.

The mountainous terrain of western North America may further confound attempts to differentiate taiga,

cool conifer, cold mixed and cool mixed forest biomes in pollen spectra. Wind-blown pollen can be

translocated from one vegetation association to another in a short geographic (but perhaps large elevational)

distance. In addition, many of the potentially distinctive taxa for these biomes are poor pollen producers

(e.g. Acer, Ceanothus, Corylus, Fraxinus and Rhamnus) and may not be recorded in pollen spectra from

their native vegetation.

The taxonomic resolution of packrat midden macrofossil assemblages makes it possible to segregate the

species characteristic of the forest biomes. Unfortunately, packrat middens are not preserved in wet

environments, and although cool conifer and cold mixed forest biomes are recognized in fossil middens, the

vast majority of the midden assemblages reflect open conifer woodland, steppe and desert biomes.

(B) Western North American biomes at 6000 14C yr B.P.

The predicted distribution of biomes across western North America at 6000 14C yr B.P. (Fig. 3) is similar to

the modern distribution, in that forests were established in the Pacific northwest, open conifer woodland

occurred in the central Great Basin and the southwestern deserts had reached their modern configuration.

However, there are subtle regional differences between the reconstructed biomes at 6000 14C yr B.P. and the

modern vegetation patterns. Three biomes were reconstructed from pollen data at one site each for 0 14C yr

B.P. (cold deciduous forest, xerophytic woods/scrub, and, incorrectly, tundra), and none of these was

reconstructed from the 6000 14C yr B.P. data. The other biomes reconstructed for 0 14C yr B.P. were present

at 6000 14C yr B.P. in western North America and are discussed below.

Taiga. Taiga is rare in the 0 14C yr B.P. reconstruction (Fig. 3) but is present at several sites in northern

Idaho and adjacent eastern Washington and western Montana at 6000 14C yr B.P. (Fig. 3). Relatively small

changes in pollen percentages occurred at the sites where taiga was replaced by cool conifer forest between

6000 and 0 14C yr B.P. The key factor appears to be the arrival of, or increase in, Tsuga pollen in these

records. Although this genus is present at relatively low levels (1 to 5%) in the modern spectra, the presence

of this key taxon is decisive in discriminating cool conifer forest from taiga.


At Waits Lake (Mack et al., 1978b), taiga at 6000 14C yr B.P. was replaced by cold deciduous forest at 014C yr B.P. These two biomes share all PFTs except bec (Tables 4 and 6), which is represented only by

Abies and Picea in western North America. At Waits Lake, Abies was present at approximately 1% in the

mid-Holocene and was absent in the modern spectrum, and this minor difference caused the shift from one

biome to another. At the Lost Trail Pass Bog site in western Montana (Mehringer et al., 1977) slightly

higher levels of Larix/Pseudotsuga pollen at 6000 14C yr B.P. resulted in identification of cool conifer

forest, and its decline to modern levels led the biomization procedure to identify taiga (which lacks ctc)

from the present day spectrum. Taiga was identified in southern Utah at both 0 and 6000 14C yr B.P. (Fig.

3).

Cool conifer forest. Fossil pollen data indicate that this biome attained its modern extent in the Pacific

Northwest prior to 6000 14C yr B.P. (Fig. 3), and was also present in western Montana. It apparently

covered more area than today in parts of Colorado and northeast Arizona because it was reconstructed for

sites that were identified as having open conifer woodland at 0 14C yr B.P. (although, as previously

discussed, the reconstructions for 0 14C yr B.P. are probably in error; the sites today host a depauperate

form of cool conifer forest). For several of the Colorado pollen records (e.g. Cottonwood Lake and

Keystone Iron Bog: Fall, 1985, 1988), Picea and Abies pollen grains were more abundant at 6000 than at 014C yr B.P., and declined as Pinus and Artemisia increased through the late Holocene. This taxonomic shift

from greater ctc pollen taxa to increased ec and ss pollen resulted in the change in biomes reconstructed

between the two time periods. The opposite trend occurred in the Sierra Nevada of California, where open

conifer woodland was replaced by cool conifer forest after 6000 14C yr B.P. at several sites. Here ctc pollen

taxa (Abies, Tsuga) arrived or increased between the mid- and late Holocene, while Pinus pollen declined.

Open conifer woodland. In the Great Basin and Colorado Plateau regions, open conifer woodland occupied

essentially the same area at 6000 14C yr B.P. that it occupies today. In southeastern Idaho and adjacent

Wyoming, steppe was present at a few sites occupied today by woodland. In the 0 14C yr B.P. reconstruction

(Fig. 3), depauperate cool conifer forests in the Rocky Mountains were incorrectly reconstructed as open

conifer woodland. This is probably also the case in the same region for 6000 14C yr B.P.

Steppe/grassland. Pollen data indicate that this biome had the same geographic range at 6000 14C yr B.P. as

it has today in the Great Basin, eastern Washington, Wyoming and on the Great Plains.

Desert. Biomization of packrat midden plant assemblages indicate that the Southwestern deserts had

attained their modern northern limits by 6000 14C yr B.P.

6000 14C yr B.P. Summary. Pollen and packrat midden data indicate that the mid-Holocene biomes of

western North America were similar to those of the present day. Cool-temperate conifer (ctc) trees were


more abundant than today in pollen records in the Colorado Rockies, but less abundant in records in the

interior of the Pacific Northwest and in the Sierra Nevada. Small shifts in the percentages of these trees

between the two time periods caused apparent changes in biomes through time, although the overall region

covered by conifer and mixed forests remained essentially the same between the two time periods.

Steppe/grassland and desert biomes at 6000 14C yr B.P. occupied largely the same areas as they do today.

(B) Western North American biomes at 18,000 14C yr B.P.

While the environments of western North America appear to have been quite similar at 0 and 6000 14C yr

B.P., they were radically different at the last glacial maximum (LGM, 18,000 14C yr B.P.). During the LGM,

the Laurentide Ice Sheet covered most of what is now Canada and montane glaciers were present in many of

the western mountains. Large lakes were present across the Great Basin and in parts of the Southwest (e.g.

Thompson et al., 1993). The reconstructed vegetation patterns were also very different (Fig. 3). The taiga

and desert biomes were apparently absent. The remaining biomes are discussed below:

Cool conifer, cold mixed, and cold deciduous forests. In the Pacific Northwest, biome reconstructions from

pollen data indicate that the areas of modern conifer and mixed forests were largely occupied by steppe

(although bec elements were present) and perhaps even tundra at the LGM (Barnosky et al., 1987).

However, cool conifer forests were present in western Oregon. This biome, along with cold mixed and cold

deciduous forest, was identified from plant macrofossils in packrat middens from various elevations in the

eastern Grand Canyon (Fig. 3).

Open conifer woodland. Pollen and packrat midden data both indicate that this biome had a greatly

expanded range (relative to today) at the LGM, suggesting that the vegetation at that time was very open.

The open conifer woodland biome occurred within its current range in the Great Basin and Colorado

Plateau, and was also present in the modern desert regions of the Southwest. Pollen records indicate its

presence in Colorado and Wyoming, and there is little evidence that these are misassigned cool conifer

forests as were the Holocene sites in these states. Cool-temperate conifers (ctc) are largely absent from

macrofossil records in the interior north of the Grand Canyon, so essential elements of the cool conifer

forests appear to have been missing. In the Great Basin and surrounding areas, the LGM open conifer

woodlands were characterized by subalpine pines and by prostrate juniper with steppe plants. These

woodlands were comprised of woodland conifers (pinyon pines and woodland junipers) in association with

steppe and cold-tolerant desert elements in those areas that are desert today.

Xerophytic woods/scrub. Warm-temperate sclerophyll scrubs (wte2) were recovered from late Pleistocene

packrat middens on the western side of the southern Sierra Nevada in California, near sites where these


plants grow today. Warm-temperate sclerophyll scrubs have not been recovered from LGM sites elsewhere

in western North America.

Tundra. This biome was reconstructed for one site in western Washington, in a region where other sites

were reconstructed as steppe (Fig. 3). The presence of lemming and reindeer fossils in southern Idaho and

geomorphic evidence of frost wedges in Wyoming (Thompson et al., 1993 and references therein) suggests

that this biome may have been more widespread in the northern interior of western North America than

indicated by the currently available botanical data.

Steppe/grassland. Steppe elements were present in LGM vegetation across western North America during

LGM time. However, eurythermic conifers were present (at least regionally) in the present domain of steppe

in the Great Basin and surrounding region, so pollen and macrofossil data from this region are classified as

open conifer woodland (Fig. 3). Steppe is reconstructed for the LGM for the presently tree-covered western

Washington.

(B) Climatic implications of changes in biomes

The correspondence of biomes interpreted from the Küchler (1964) map of potential natural vegetation map

(Fig. 2, Table 1) with climatic parameters (MTCO: mean temperature of the coldest month; GDD5: growing

degree days on a 5°C base; and α: a moisture index ranging from 0=extremely dry to 1=extremely wet

based on Thornthwaite and Mather 1955, 1957) are shown in Figs 4 and 5 and in Table 7. These climatic

data were obtained by using a 25-km equal-area grid of present-day climate values (Bartlein et al., 1994;

Thompson et al., 1999a, 1999b) and assigning each grid point in western North America to a biome based

on the Küchler (1964) map.

Tundra is restricted to mountain tops and occurs where MTCO is less than -2.5° C, GDD5 is less than 1000

and α is generally above 0.75. The GDD5 upper limit for tundra seems high when compared to other regions

(cf Prentice et al., 1992); this could reflect the difficulty in estimating present-day climate at high elevations

from the nearest low-elevation sites. However, preliminary analysis of tundra in Alaska suggests that tundra

in that region commonly occurs with GDD5 values as high as 500 to 750. The cool conifer forest biome is

the most widespread forest vegetation in the western United States and occurs under a variety of climatic

conditions (Table 7). It can occur under somewhat colder winter temperatures, and generally requires more

moisture, than open conifer woodland. Open conifer woodland is comprised of eurythermic conifers and

steppe plants and can survive under colder conditions than xerophytic woods/scrub and drier and warmer

climates than cool conifer forest. Xerophytic woods/scrub vegetation cannot survive freezing temperatures

and, as its name implies, grows under warm and dry conditions.


Grassland and steppe are not distinguished as separate biomes in our reconstructions based on pollen and

macrofossil data. However, they are segregated on the Küchler (1964) map (Fig. 2), and Figs 4 and 5

illustrate their climatic differences. Grassland occurs on the Great Plains, in parts of the Southwest, in the

Central Valley of California and in the interior Pacific Northwest. Steppe is widespread across the semiarid

interior of the western United States from the Great Plains to the lee of the Sierra Nevada and Cascade

Ranges. Grassland survives under a wide variety of winter temperatures (MTCO -15 to 10° C), and is

associated with GDD5 2000 to 4500 and α of 0.4 to 0.7. Steppe overlaps with grassland in its climatic

tolerances, but can live in drier environments (α as low as 0.15), has lower GDD5 requirements (GDD5 as

low as 1000) and does not occur under warm winter conditions (MTCO is below 0°C). Desert occurs in the

Southwest and in southeastern-most California. It has lower moisture requirements than steppe and

grassland (α between 0.05 and 0.4), higher GDD5 requirements (3000 to 6000) and MTCO above freezing.

The climates associated with our 0 14C yr B.P. reconstructed biomes are illustrated in Fig. 6 (see also Table

7b). Where we do not have precise elevational data for a site, we associated the climate of the closest

gridpoint on our 25-km grid with the pollen or packrat midden site. Climatic “envelopes” for the Küchler-

based biomes were visually interpreted from Fig. 4 and are included on Fig. 6 to allow comparisons of the

climates of the Küchler-based biomes and the reconstructed biomes. The left-hand panels of Fig. 6 illustrate

the reconstructed climates for the cool conifer forest localities. Nearly all of the sites identified as belonging

to this biome fall within the climate envelope from the Küchler-based biome, and thus it appears that the

biomization assigns these surface pollen samples correctly.

The central panels of Fig. 6 plot the climates for the 0 14C yr B.P. sites that were reconstructed as having

open conifer woodland vegetation. In these panels the climatic envelopes (from Fig. 4) for this biome are

shown, along with those for cool conifer forest. Many sites in modern cool conifer forest in the northern

Rocky Mountains were incorrectly placed in open conifer woodland by the biomization procedure. The data

in Fig. 6 show these sites occurring within the moisture requirements of the cool conifer forest biome

(which exceed those of open conifer woodland) but at the cold end of the cool conifer forests. Inspection of

these data indicate that the cool conifer forest sites that are misidentified as open conifer woodland occur

near the lower limits of the forest in relatively dry regions of northeastern Nevada, central Utah, and western

Colorado. At the cold end of its tolerance many of the ctc trees are absent from this biome, and pollen

spectra converge on those of the open conifer woodland.

The right-hand panels of Fig. 6 illustrate the apparent modern climates of sites reconstructed as

steppe/grassland or desert biomes at 0 14C yr B.P. In this plot the steppe/grassland and desert samples

adhere closely to the freezing-line divide apparent in the Küchler-based data. However, sites within both


biomes appear to exceed the permissible values for the moisture index. Here the assignment of nearest grid-

point climates may be causing difficulties, as in this region of high relief we may be incorrectly using

precipitation values from higher elevation sites.

(B) Vegetation and climate of western North America at 6000 14C yr B.P.

The presence of cool conifer forest in the maritime region of the Pacific Northwest at 6000 14C yr B.P.

suggests that relatively little climatic change occurred there between the mid-Holocene and the present. In

the interior part of this region, the shift from taiga at 6000 14C yr B.P. to cool conifer and cold deciduous

forests at 0 14C yr B.P. may reflect a shift to milder climates in the late Holocene, with modern warmer

winters perhaps being the key factor. The reconstruction of cool conifer forests at several sites in Colorado

and northeastern Arizona that are characterized as open conifer woodland or steppe in the modern

reconstruction indicates that conditions were wetter than present at 6000 14C yr B.P. In contrast, steppe was

more extensive in the northern Great Basin at 6000 14C yr B.P. compared to present, and sites in the Sierra

Nevada that are in cool conifer forest today were in open conifer woodland during the mid-Holocene. These

shifts in vegetation imply drier than modern conditions in the northern Great Basin and Sierra Nevada at

6000 14C yr B.P. As noted by Williams et al. (this issue), warmer and drier climates at 6000 14C yr B.P. are

also reflected in the contraction of forests and expansion of grassland in the upper Midwest.

The contrast between drier conditions in these regions and wetter conditions in the southern Rocky

Mountains has been noted in previous reconstructions of the mid-Holocene climates of western North

America (e.g. Thompson et al., 1993), and, as discussed in that paper, appears to reflect an enhanced

summer monsoon circulation in the Southwest coupled with warmer than modern conditions across most of

western North America (as illustrated in Fig. 1, the area of relatively moist mid-Holocene climates lies

within the modern track of the summer monsoon). Despite this proposed augmentation of the summer

monsoon, the biome reconstructions imply little discernible difference between 6000 and 0 14C yr B.P. in

the deserts of the Southwest.

(B) Vegetation and climate of western North America at 18,000 14C yr B.P.

The occurrence of steppe and tundra at the LGM in now-maritime western Washington indicates conditions

both colder and drier than today (Barnosky et al., 1987). Cold climates associated with steppe are also

inferred for the northern Great Plains near the Laurentide Ice Sheet (Williams et al., this issue). The

presence of cool conifer forest in western Oregon at 18,000 14C yr B.P. indicates that there was a strong

temperature and moisture gradient along the coast of the Pacific Northwest.


Open conifer woodland was present at most of the LGM sites from Wyoming south to the Mexican border,

indicating open vegetation with MTCO remaining above -10°C (Fig. 4). In the Great Basin and adjacent

regions, this woodland included xerophytic subalpine conifers instead of the current dominance of

woodland conifers, suggesting that the climate was at the cold and dry end of the range of open conifer

woodland. In contrast, this biome in the present-day Southwestern deserts included pinyon pine and

woodland juniper, suggesting that the climate was cool (not cold) and wet relative to today. Open conifer

woodland also occurred in the southeastern United States during the LGM (Williams et al., this issue), and

it is possible that it is favored by the lower levels of atmospheric carbon dioxide that occurred in the late

Pleistocene (as may have occurred with dry-tropical biomes: Jolly & Haxeltine, 1997).

Overall, the distributions of biomes in western North America at 18,000 14C yr B.P. are consistent with

earlier palaeoclimatic reconstructions (Thompson et al., 1993) and modelling (Kutzbach et al., 1993, 1998;

Thompson et al., 1993). Key factors of the LGM climate that differed significantly from today include: (1) a

southward displacement of the westerlies, and their enhanced persistence through the year; (2) virtual

elimination of the present summer monsoon circulation; and (3) a strong temperature gradient through the

interior region, with very cold temperatures to the north and mild conditions along the Mexican border.

(A) DISCUSSION AND CONCLUSIONS

(B) The biomization method

Pollen and packrat midden data appear to provide realistic reconstructions of present-day biomes in western

North America. However, to achieve these results the weighting had to be increased for trees (3x) and

shrubs (2x) to be able to reconstruct the woodland, forest and desert biomes. Our ability to differentiate

among taiga, cool conifer forest, cold deciduous forest and cold mixed forest from pollen data is limited due

to a depauperate flora and taxonomic overlap among the PFTs. In addition, our ability to discriminate

between cool conifer forest (at the cold end of its range) and open conifer woodland appears to be limited

by the very low representations of key ctc trees (especially Pseudotsuga) and by blow-in of pollen from

steppe plants in regions of high relief.

The new biome, open conifer woodland, provides a good description of both the modern and late

Pleistocene vegetation of the semiarid interior of western North America. It likely occurs in other regions of

the world. For example, the vegetation of the colder, drier parts of the circum-Mediterranean region is also

characterised by an intimate mix of conifers and steppic vegetation (e.g. the juniper woodlands of interior

Spain and Greece: Polunin & Walters, 1985). However, vegetation types that are apparently analogous to

our open conifer woodland biome occupy rather limited areas in Europe and the Middle East today. This

may explain why they were not classified as a separate biome in the original biomization of Europe


(Prentice et al., 1996) and have not been recognised in subsequent biomizations (e.g. Jolly et al., 1998b).

Given that juniper woodlands may have been more widely distributed in Eurasia during other periods of the

late Quaternary, it would be useful to re-examine the vegetation records from these other regions in the light

of our recognition of open conifer woodlands as a distinct biome.

Single examples of plant macrofossils from a few sites have been used to supplement pollen data in

biomizations of other regions (e.g. northern Russia: Texier et al., 1997; Africa: Jolly et al., 1998b).

However, the biomization procedure has never been applied to assemblages of plant macrofossils or to an

extensive plant macrofossil data set encompassing a range of different biomes. Our reconstructions show

that identical biome reconstructions are obtained using plant macrofossils and pollen spectra, when both

types of data are available from the same region. Specifically, the changing extent of open conifer woodland

since the LGM is clearly documented from both pollen and plant macrofossil sites. The comparability of

biome reconstructions from the two data sources opens up new possibilities for the systematic mapping of

vegetation changes during the late Quaternary in more arid regions where pollen data are sparse. Although

packrat middens are unique to western North America, similar deposits have been found in e.g. the Middle

East (Fall et al., 1990), South Africa (Scott, 1990) and Australia (Nelson et al., 1990), as well as in South

America. The systematic exploitation of the macrofossil records from these deposits could substantially

enhance our understanding of the Late Quaternary dynamics of arid regions.

Pollen and plant macrofossils record vegetation at rather different spatial scales: pollen data provide

regional reconstructions of vegetation changes at a coarse level of taxonomic precision, while macrofossil

data provide records of more local changes based on species-level identifications. Potentially, these

differences in spatial resolution could be exploited in several ways. For example, macrofossil data could be

used to determine whether specific taxa represented in a pollen record were locally present or were derived

by long-distance transport. Takahara et al. (this issue) have shown that the upslope transport of pollen from

extra-local vegetation leads to incorrect biome reconstructions for nearly half of the pollen records from

Japan. The use of plant macrofossil data to establish local presence, in conjunction with pollen data, could

therefore substantially improve biome reconstructions in mountainous regions. In cases where different

species within a genus are characteristic of different biomes, such as is the case with different species of

Pinus, Abies and Picea in western North America, it may also be possible to exploit the higher taxonomic

resolution of plant macrofossil data to guide biomizations at pollen sites (Jolly et al., 1998b; Takahara et

al., this issue).

(B) Changes in the distribution of PFTs in western North America since the last glacial maximum

Plant species have responded to changes in climate and atmospheric chemistry in individualistic fashions

over the time since the LGM (Thompson, 1988). We have explored aspects of this history in regard to


changes in biomes. Aspects of the vegetation history of western North America can be also seen in the

changing affinity scores of PFTs. For example, boreal evergreen conifers (bec) were present across most of

the region at 18,000 14C yr B.P. (Fig. 7), except for the present-day low elevation deserts of the Southwest.

By 6000 14C yr B.P. this PFT attained its modern range in the Rocky Mountains and Pacific Northwest, and

lost much of its late Pleistocene habitat in the Southwest. Woodland conifers (wc) also changed their

distributions through time: they lived in the modern Southwestern deserts at 18,000 14C yr B.P. and

dispersed northward and upward in elevation to near their modern limits by 6000 14C yr B.P. (as they lost

their late Pleistocene habitats in the Southwestern deserts).

The steppe shrub PFT (ss) was important across its modern range at 18,000 14C yr B.P., and was also

present in the Pacific Northwest and the Southwestern deserts (Fig. 8). This implies that this complex of

species has been a major constituent of western vegetation under both late Pleistocene and Holocene

climatic regimes. In contrast, taxa from the desert shrub or succulent PFT (ds) were minor components of

western vegetation at the LGM, and have expanded into their modern range during the Holocene.

Collectively the histories of the four PFTs in Figs 7 and 8 illustrate differing patterns of PFT response to

climatic change. Some, such as the steppe shrub PFT, have maintained near-modern geographic (although

perhaps not elevational) ranges since the LGM, whereas others have modified their geographic ranges to

greater or lesser extents.

(B) Vegetation and climate changes in western North America in the Late Quaternary

The vegetation reconstructed through the objective biomization method in western North America is in

good agreement with more informal reconstructions for 18,000, 6000 and 0 14C yr B.P. (e.g. Thompson et

al., 1993). The biomization procedure provides a uniform method of describing vegetation changes through

time, as well as providing a potential means to identify underlying climatic causes. Geologic and biologic

data indicate that the present-day climatic patterns (Fig. 1) of western North America were greatly changed

at the LGM. For example, the present-day climate of western Washington State in the Pacific Northwest is

characterized by high levels of precipitation and mild temperatures. This was apparently not true at 18,00014C yr B.P.; instead glacial, faunal and vegetation data suggest that this region experienced dry climates with

cold winters (Barnosky et al., 1987; Thompson et al., 1993). In contrast, the desert regions of the

Southwestern United States today have hot and dry climates, whereas palaeolacustrine, faunal and

vegetational data indicate that the climate was relatively cool and moist at 18,000 14C yr B.P. (Street-Perrott

et al., 1989; Thompson et al., 1993), apparently due to a southerly displacement (relative to today) of the

westerlies. The biomes reconstructed for both of the regions at the LGM fit well within this framework, with

the cold dry climates of the Northwest being associated with steppe and tundra, and the cool moist

Southwestern climates with open conifer woodlands (Fig. 3). The reconstructed biomes for the mid-

Holocene are also well aligned with previous palaeoclimatic syntheses. Particularly noteworthy are the


contrasting changes in biomes in the southern Rocky Mountains and Sierra Nevada. The former area

apparently fell within a region of greater-than-modern summer monsoonal precipitation at 6000 14C yr B.P.

(Fall, 1988; Thompson et al., 1993), while the latter area experienced greater-than-present drought at this

time (Anderson, 1987; Thompson et al., 1993). The reconstructed biomes reflect this pattern, with the

Rocky Mountains having more sites covered with cool conifer forest at 6000 14C yr B.P. than today, while

several present-day cool conifer forest sites in the Sierra Nevada were open conifer woodland in the mid-

Holocene (Fig. 3).

(A) ACKNOWLEDGEMENTS

This paper is a contribution to BIOME 6000. Sandy Harrison graciously provided editorial advice and

motivation. Our assignment of pollen taxa to PFTs was made in consultation with Pat Anderson, Pat

Bartlein, Mary Edwards, Eric Grimm, Steve Jackson, Pierre Richard, Tom Webb, Cathy Whitlock and Jack

Williams. We thank Jack Williams for the software used in the biomization procedure, and Tom Webb, Dan

Muhs and Jack Williams for detailed and constructive reviews. We thank Laura Strickland for her help in

data preparation. Silvana Schott provided a detailed technical edit of the manuscript. We are grateful to

Colin Prentice for advice on the application of the biomization procedures to Western North America and

for comments on an earlier draft of the manuscript.

(A) BIOPIC

Robert S. Thompson is a geologist with the Global Change and Climate History Team of the U.S.

Geological Survey who studies late Cenozoic vegetation and climate change, primarily in western North

America. Katherine H. Anderson is a research associate with the Institute for Arctic and Alpine Research

(INSTAAR) at the University of Colorado. She applies quantitative techniques to reconstruct past climates

from pollen and plant macrofossil data in North America.


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TABLE AND FIGURE CAPTIONS:

Table 1. Present-day biomes in western North America interpreted from Küchler (1964; see Fig. 2).

Table 2. Characteristics of the 0, 6000 and 18,000 14C yr B.P. pollen sites from western North America.

Site names with asterisks indicate digitized data, those without an asterisk were taken from the North

American Pollen Database from the sample level closest to the target age. Negative radiocarbon ages

indicate dates that are younger than A.D. 1950. Dating control (DC) codes are based on the COHMAP

dating control scheme (Webb, 1985; Yu & Harrison, 1995). For site with continuous sedimentation

(indicated by a C after the numeric code), the dating control is based on bracketing dates where 1 indicates

that both dates are within 2000 years of the selected interval, 2 indicates one date within 2000 years and the

other within 4000 years, 3 indicates both within 4000 years, 4 indicates one date within 4000 years and the

other within 6000 years, 5 indicates both dates within 6000 years, 6 indicates one date within 6000 years

and the other within 8000 years, and 7 indicates bracketing dates more than 8000 years from the selected

interval. For sites with discontinuous sedimentation (indicated by D after the numeric code), 1 indicates a

date within 250 years of the selected interval, 2 a date within 500 years, 3 a date within 750 years, 4 a date

within 1000 years, 5 a date within 1500 years, 6 a date within 2000 years, and 7 a date more than 2000

years from the selected interval. Biome codes (Biome) are given in Table 6. For mapping purposes some

sites (indicated by ‡) which are too close to one another have been displaced slightly.

Table 3. Characteristics of the 0, 6000 and 18,000 14C yr B.P. packrat midden sites from western North

America. Dating control codes (DC) follow the scheme described in Table 2. Biome codes (Biome) are

given in Table 6. For mapping purposes some sites (indicated by ‡) which are too close to one another have

been displaced slightly.

Table 4. Assignments of pollen taxa from western North America to the PFTs used in the biomization

procedure.

Table 5. Assignments of plant macrofossil (midden) taxa from western North America to the PFTs used in

the biomization procedure.

Table 6. Assignment of PFTs to biomes used in the biomization for western North America.


Table 7. Climatic tolerances of biomes in western North America, based on (a) present-day potential natural

vegetation, and (b) present-day pollen and plant macrofossil assemblages. MTCO temperatures are

approximated to the nearest 2.5° C; GDD5 values to the nearest 500 growing-degree days; and α values to

the nearest 0.05. The values in 7 (a) were obtained through visual inspection and interpretation of Figs 5

and 6; the values in 7 (b) through visual inspection and interpretation of Fig. 5. Cold deciduous forests and

xerophytic woods/scrub are omitted from 7 (b) because they only occur once.

Figure 1. Present-day climate of western North America. These four panels illustrate the biseasonal

precipitation regime and the range of temperature and precipitation conditions experienced over this

mountainous region. Modern mean temperature (right) and mean precipitation (left) in January (top) and

July (bottom) over western North America.

Figure 2. Vegetation of the western United States based on the Küchler (1964) map of potential natural

vegetation. The categories used by Küchler have been grouped to approximate the biomes used by Prentice

et al. (1996) and in this paper (with the exception of the steppe and grassland categories, which can be

differentiated here but are more difficult to discriminate in pollen and macrofossil data).

Figure 3. Biomes for 0, 6000, and 18,000 14C yr B.P. in western North America. The 0 14C yr B.P. biomes

are on: (1) pollen assemblages from modern core tops from fossil pollen sites and lacustrine surface

samples, and (2) packrat middens from the past 1000 radiocarbon years. Pollen sites are represented by a

circle, midden sites by a triangle.

Figure 4. Bivariate plots of the estimated modern climatic ranges of the vegetation categories shown in Fig.

2.

Figure 5. Histograms illustrating the univariate estimated modern climatic ranges of the vegetation

categories shown in Fig. 2.

Figure 6. Bivariate plots of the estimated modern climatic ranges of the western North American biomes

mapped for 0 14C yr B.P. in Fig. 3. Pollen sites are represented by a circle, midden sites by a triangle.

Figure 7. Affinity scores for selected plant functional types in western North America for 18,000, 6000, and

0 14C yr B.P. The upper series of panels illustrates the occurrence of boreal evergreen conifers including

species of Picea, Abies, and Larix. The lower series illustrates the past occurrence of species of pinyon


pines and woodland junipers (woodland conifers). Pollen sites are represented by a circle, midden sites by a

triangle.

Figure 8. Affinity scores for selected plant functional types in western North America for 18,000, 6000, and

0 14C yr B.P. The upper series of panels illustrates the occurrence of steppe plants, such as Artemisia,

whereas the lower series illustrates shrub and succulent species that are now largely confined in hot deserts.

Pollen sites are represented by a circle, midden sites by a triangle.


Table 1 Present-day biomes in western North America interpreted from Küchler (1964; see Fig. 2).

Biome Küchler categories Key generatundra alpine meadows and barren Agrostis, Carex, Festuca, Poacool conifer forest spruce-cedar-hemlock forest, cedar-hemlock-

Douglas fir forest, silver fir-Douglas fir forest,fir-hemlock forest, mixed conifer forest,redwood forest, red fir forest, lodgepole pine-subalpine forest, pine-cypress forest, westernponderosa forest, Douglas fir forest, cedar-hemlock-pine forest, grand fir-Douglas fir forest,western spruce-fir forest, eastern ponderosaforest, Black Hills pine forest, pine-Douglas firforest, Arizona pine forest, spruce-fir-Douglas firforest, Southwestern spruce-fir forest, westernponderosa forest, Oregon oak/California mixedevergreen forest

Abies, Cupressus, Picea, Pinus,Pseudotsuga, Sequoia, Thuja, Tsuga

temperatedeciduous/cool mixedforest

cedar-hemlock-Douglas fir forest, Oregon oakwoodlands, California mixed evergreen forest,California oaklands, oak-juniper woodlands,mountain mahogany-oak (in part), northernfloodplain forest

Arbutus, Cercocarpus, Juniperus,Populus, Pseudotsuga, Quercus, Salix,Thuja, Tsuga, Ulmus

open coniferwoodland

juniper-pinyon woodland, juniper woodland Artemisa, Juniperus, Pinus

xerophyticwoods/scrub

chapparal, coastal sagebrush, California oaklands(in part)

Adenostoma, Arctostaphylos,Ceanothus, Eriogonum, Quercus,Salvia

grassland fescue-oatgrass, California steppe, Californiatule marshes, fescue-wheatgrass, wheatgrass-bluegrass, grama-galleta steppe, grama-tobosaprairie, wheatgrass-needlegrass shrubsteppe,galleta-three awn shrubsteppe, mesquite-buffalograss, mesquite-acacia-savanna, foothills prairie,grama-needlegrass-wheatgrass, grama-buffalograss, wheatgrass-needlegrass, wheatgrass-bluestem-needlegrass, wheatgrass-grama-buffalograss, bluestem-grama prairie, sandsage-bluestem prairie, shinnery, northern cordgrassprairie, oak savanna, juniper-oak savanna,fescue-mountain muhly prairie

Acacia, Agropyron, Andropogon,Aristida, Artemisia, Bouteloua,Buchloe, Danthonia, Distichlis,Festuca, Hilaria, Juniperus,Muhlenbergia, Poa, Prosopis,Quercus, Setaria, Spartina, Stipa

steppe sagebrush steppe, Great Basin sagebrush steppe,saltbush-greasewood, mountain mahogany-oak(in part)

Agropyron, Artemisia, Atriplex,Cercocarpus, Quercus, Sarcobatus

desert mesquite bosques; blackbrush, creosote bush,creosote bush-bur sage, palo verde-cactus shrub,grama-tobosa shrubsteppe, ceniza shrub, grama-tobosa shrubsteppe, Trans-Peco shrub savanna

Cercidium, Colegyne, Flourensia,Franseria, Larrea, Leucophyllum,Opuntia, Prosopis


Table 2 Characteristics of the 0, 6000 and 18,000 14C yr B.P. pollen sites from western North America. Sitenames with asterisks indicate digitized data, those without an asterisk were taken from the North AmericanPollen Database from the sample level closest to the target age. Negative radiocarbon ages indicate dates thatare younger than A.D. 1950. Dating control (DC) codes are based on the COHMAP dating control scheme(Webb, 1985; Yu & Harrison, 1995). For site with continuous sedimentation (indicated by a C after thenumeric code), the dating control is based on bracketing dates where 1 indicates that both dates are within2000 years of the selected interval, 2 indicates one date within 2000 years and the other within 4000 years, 3indicates both within 4000 years, 4 indicates one date within 4000 years and the other within 6000 years, 5indicates both dates within 6000 years, 6 indicates one date within 6000 years and the other within 8000years, and 7 indicates bracketing dates more than 8000 years from the selected interval. For sites withdiscontinuous sedimentation (indicated by D after the numeric code), 1 indicates a date within 250 years ofthe selected interval, 2 a date within 500 years, 3 a date within 750 years, 4 a date within 1000 years, 5 a datewithin 1500 years, 6 a date within 2000 years, and 7 a date more than 2000 years from the selected interval.Biome codes (Biome) are given in Table 6. For mapping purposes some sites (indicated by ‡) which are tooclose to one another have been displaced slightly.

Siteno.

Site name Lat.(°N)

Long.(°W)

Elev.(m)

No. of14C

dates

DC Age ofchosendepth

Targetage

Biome Reference

MODERN SAMPLES1 Alkali Creek* 38.75 106.83 2800 3 n/a n/a 0 OC Markgraf & Scott, 19813 Antelope Playa* 43.50 105.45 1450 3 n/a n/a 0 STEP Markgraf & Lennon, 19864 Balsam Meadows* 37.17 119.50 2005 6 n/a n/a 0 COCO Davis et al., 19855 Barrett Lake* 37.60 119.02 2816 6 n/a n/a 0 OC Anderson, 19906 Battle Ground Lake* 45.67 122.48 <300 11 n/a n/a 0 COCO Barnosky, 1985a

18 Como Lake* 37.55 105.50 3523 2 n/a n/a 0 OC Shafer, 198920 Cottonwood Pass Pond* 38.83 106.41 3700 3 n/a n/a 0 OC Fall, 198821 Creston Fen* 47.58 118.75 n/a 3 n/a n/a 0 OC Mack et al., 197622 Cub Creek Pond* 45.17 110.17 2500 3 n/a n/a 0 COCO Waddington & Wright, 197424 Cygnet Lake Fen 44.65 110.60 2530 6 n/a 0 0 OC Whitlock, 199326 Dead Man Lake* 36.24 108.95 2780 5 n/a n/a 0 OC Wright et al., 197328 Diamond Pond* 43.25 118.33 1265 11 n/a n/a 0 STEP Wigand, 198729 Divide Lake* 43.95 110.23 2628 3 n/a n/a 0 TAIG Whitlock & Bartlein, 199330 Dome Creek Meadow* 40.02 107.03 3165 5 n/a n/a 0 OC Feiler et al., 199731 Emerald Lake 44.07 110.30 2634 3 n/a 458 0 OC Whitlock, 199332 Exchequer Meadow* 37.00 119.08 2219 6 n/a n/a 0 COCO Davis & Moratto, 198837 Fryingpan Lake* 38.62 111.67 2720 5 n/a n/a 0 TAIG Shafer, 198940 Gold Lake Bog 43.65 122.05 1465 5 n/a 29 0 COCO Sea & Whitlock, 199542 Gray's Lake* 43.00 111.58 1946 15 n/a n/a 0 OC Beiswenger, 199143 Great Salt Lake* 41.00 112.50 1280 1 n/a n/a 0 OC Mehringer, 198544 Guardipee Lake 48.55 112.72 1233 3 n/a -34 0 OC Barnowsky, 198945 Hager Pond* 48.67 116.92 860 12 n/a n/a 0 COCO Mack et al., 1978b,d46 Hall Lake* 47.82 122.30 104 6 n/a n/a 0 COCO Tsukada et al., 198147 Hay Lake, Arizona 34.00 109.43 2780 6 n/a 106 0 STEP Jacobs, 198548 Head Lake* 37.70 105.50 2300 6 n/a n/a 0 OC Shafer, 198949 Hedrick Pond 43.75 110.60 2073 5 n/a 0 0 OC Whitlock, 199350 Hidden Cave* 39.33 118.75 1251 16 n/a n/a 0 OC Wigand & Mehringer, 198551 Hoh Bog* 47.75 124.25 n/a 8 n/a n/a 0 COCO Heusser, 197853 Hurricane Basin* 37.97 107.55 3650 6 n/a n/a 0 STEP Andrews et al., 197554 Ice Slough* 42.48 107.90 1950 4 n/a n/a 0 OC Beiswenger, 198755 Indian Prairie Fen 44.63 122.58 988 5 n/a 350 0 COCO Sea & Whitlock, 199557 Jacob Lake 34.33 110.83 2285 3 n/a 25 0 STEP Jacobs, 198359 Keystone Iron Bog* 38.87 107.03 2920 7 n/a n/a 0 COCO Fall, 1985, 198860 Kirk Lake* 48.12 121.50 190 8 n/a n/a 0 COCO Cwynar, 198761 La Poudre Pass Bog* 40.48 105.78 3103 3 n/a n/a 0 STEP Short, 198562 Lake Cleveland* 42.32 113.63 2519 3 n/a n/a 0 OC Davis, 198156 Lake Isabel Bog* 40.07 105.62 3310 3 n/a n/a 0 OC Short, 198565 Lily Lake ‡ 43.77 110.32 2469 2 n/a 0 0 OC Whitlock, 199368 Little Lake, Oregon 44.17 123.58 217 13 n/a 243 0 TUND Worona & Whitlock, 199569 Long Lake* 40.07 105.60 3210 5 n/a n/a 0 OC Short, 198570 Lost Lake, Montana 47.63 110.48 1019 5 n/a -35 0 STEP Barnosky, 198971 Lost Trail Pass Bog* 45.75 113.97 2152 16 n/a n/a 0 TAIG Mehringer et al., 197773 Marion Lake* 49.33 123.00 305 7 n/a n/a 0 COCO Mathewes, 197374 Mariposa Lake, WY 44.15 110.23 2730 3 n/a 0 0 OC Whitlock, 199375 Mayberry Well 33.70 108.30 2080 1 n/a 211 0 OC Markgraf, unpub76 Mckillop Creek Pond* 48.33 115.45 920 5 n/a n/a 0 COCO Mack et al., 198377 Mineral Lake* 46.73 122.20 436 7 n/a n/a 0 COCO Tsukada et al., 198178 Mission Cross Bog* 41.78 115.48 2424 7 n/a n/a 0 STEP Thompson, 198479 Molas Lake 37.75 107.68 3200 2 n/a 81 0 OC Maher, 196183 Nichols Meadow* 37.43 119.57 1509 2 n/a n/a 0 COCO Koehler & Anderson, 199485 Pangborn Bog* 48.83 122.58 n/a 3 n/a n/a 0 COCO Hansen & Easterbrook, 197488 Posy Lake* 37.95 111.70 2653 4 n/a n/a 0 COCO Shafer, 198989 Potato Lake* 34.08 111.50 2222 4 n/a n/a 0 OC Anderson, 199392 Rattlesnake Cave* 43.52 112.62 1996 1 n/a n/a 0 OC Davis, 198194 Ruby Marshes* 41.13 115.48 1818 25 n/a n/a 0 OC Thompson, 1992


100 Slough Creek Pond 44.93 110.35 1884 4 n/a 0 0 OC Whitlock & Bartlein, 1993102 Soleduck Bog* 47.92 124.47 73 1 n/a n/a 0 COCO Heusser, 1973103 Splains Gulch* 38.83 107.08 3160 4 n/a n/a 0 TAIG Fall, 1988105 Swamp Lake* 37.95 119.82 1554 5 n/a n/a 0 COCO Smith & Anderson, 1992106 Swan Lake* 42.33 112.42 1452 3 n/a n/a 0 OC Bright, 1966107 Teepee Lake* 48.33 115.50 1270 8 n/a n/a 0 COCO Mack et al., 1983109 Tioga Pass Pond* 37.92 119.27 3018 4 n/a n/a 0 STEP Anderson, 1990113 Waits Lake* 48.17 117.67 n/a 8 n/a n/a 0 CLDE Mack et al., 1978c,d116 Wessler Bog* 48.17 124.50 25 1 n/a n/a 0 COCO Heusser, 1973118 Williams Fen* 47.33 117.58 n/a 6 n/a n/a 0 STEP Nickmann, 1979119 Woski Pond 37.73 119.63 1212 3 n/a -36 0 COCO Anderson & Carpenter, 1991

6000 14C yr B.P. SAMPLES1 Alkali Creek* 38.75 106.83 2800 3 2C n/a 6000 OC Markgraf & Scott, 19813 Antelope Playa* 43.50 105.45 1450 3 1C n/a 6000 STEP Markgraf & Lennon, 19864 Balsam Meadows* 37.17 119.50 2005 6 2C n/a 6000 OC Davis et al., 19855 Barrett Lake* 37.60 119.02 2816 6 3C n/a 6000 OC Anderson, 19906 Battle Ground Lake* 45.67 122.48 <300 11 1C n/a 6000 COCO Barnosky, 1985a9 Big Meadow* 48.92 117.42 1040 8 2C n/a 6000 TAIG Mack et al., 1978a

10 Blacktail Pond* 44.97 110.60 2018 3 5C n/a 6000 OC Gennett, 197711 Bogachiel River Site* 47.88 124.33 533 2 7C n/a 6000 COCO Heusser, 197812 Bonaparte Meadows* 48.75 119.08 1021 14 1C n/a 6000 TAIG Mack et al., 197913 Buckbean Fen* 44.30 110.25 2363 4 2C n/a 6000 OC Baker, 197614 Carp Lake* 45.92 120.88 714 13 2C n/a 6000 COCO Barnosky, 1985b18 Como Lake* 37.55 105.50 3523 2 1C n/a 6000 OC Shafer, 198919 Copley Lake* 38.87 105.08 3250 7 1C n/a 6000 COCO Fall, 198820 Cottonwood Pass Pond* 38.83 106.41 3700 3 2C n/a 6000 COCO Fall, 198821 Creston Fen* 47.58 118.75 n/a 3 4C n/a 6000 OC Mack et al., 197622 Cub Creek Pond* 45.17 110.17 2500 3 7C n/a 6000 OC Waddington & Wright, 197424 Cygnet Lake Fen 44.65 110.60 2530 6 1C 6124 6000 OC Whitlock, 199325 Davis Lake* 46.58 122.25 282 16 1C n/a 6000 COCO Barnosky, 198128 Diamond Pond* 43.25 118.33 1265 11 n/a n/a 6000 OC Wigand, 198729 Divide Lake* 43.95 110.23 2628 3 3C n/a 6000 TAIG Whitlock & Bartlein, 199330 Dome Creek Meadow* 40.02 107.03 3165 5 2C n/a 6000 STEP Feiler et al., 199731 Emerald Lake 44.07 110.30 2634 3 4C 6018 6000 TAIG Whitlock, 199332 Exchequer Meadow* 37.00 119.08 2219 6 1C n/a 6000 COCO Davis & Moratto, 198837 Fryingpan Lake* 38.62 111.67 2720 5 1C n/a 6000 TAIG Shafer, 198938 Gardiners Hole* 44.92 110.73 n/a 2 4D n/a 6000 OC Baker, 198340 Gold Lake Bog 43.65 122.05 1465 5 2C 6147 6000 COCO Sea & Whitlock, 199542 Gray's Lake* 43.00 111.58 1946 15 2C n/a 6000 STEP Beiswenger, 199143 Great Salt Lake* 41.00 112.50 1280 1 3D n/a 6000 OC Mehringer, 198544 Guardipee Lake 48.55 112.72 1233 3 5C 6166 6000 STEP Barnowsky, 198945 Hager Pond* 48.67 116.92 860 12 1C n/a 6000 TAIG Mack et al., 1978b46 Hall Lake* 47.82 122.30 104 6 2C n/a 6000 COCO Tsukada et al., 198147 Hay Lake, Arizona 34.00 109.43 2780 6 6C 5639 6000 OC Jacobs, 198548 Head Lake* 37.70 105.50 2300 6 2C n/a 6000 OC Shafer, 198949 Hedrick Pond 43.75 110.60 2073 5 4C 6077 6000 OC Whitlock, 199350 Hidden Cave* 39.33 118.75 1251 16 2C n/a 6000 OC Wigand & Mehringer, 198553 Hurricane Basin* 37.97 107.55 3650 6 2C n/a 6000 TAIG Andrews et al., 197554 Ice Slough* 42.48 107.90 1950 4 3C n/a 6000 STEP Beiswenger, 198755 Indian Prairie Fen 44.63 122.58 988 5 2C 5919 6000 COCO Sea & Whitlock, 199559 Keystone Iron Bog* 38.87 107.03 2920 7 2C n/a 6000 COCO Fall, 1985, 198860 Kirk Lake* 48.12 121.50 190 8 3D n/a 6000 COCO Cwynar, 198761 La Poudre Pass Bog* 40.48 105.78 3103 3 2C n/a 6000 OC Short, 198562 Lake Cleveland* 42.32 113.63 2519 3 2C n/a 6000 OC Davis, 198163 Lake Emma* 37.90 107.63 3730 5 3D n/a 6000 OC Carrara et al., 198456 Lake Isabel Bog* 40.07 105.62 3310 3 2C n/a 6000 OC Short, 198564 Lake Washington* 47.67 122.22 6 5 4C n/a 6000 COCO Leopold et al., 198265 Lily Lake ‡ 43.77 110.32 2469 2 2C 5892 6000 TAIG Whitlock, 199366 Lily Lake Fen ‡ 43.77 110.32 2469 6 4D 6285 6000 OC Whitlock, 199368 Little Lake, Oregon 44.17 123.58 217 13 2C 5990 6000 COCO Worona & Whitlock, 199569 Long Lake* 40.07 105.60 3210 5 2C n/a 6000 OC Short, 198570 Lost Lake, Montana 47.63 110.48 1019 5 1C 5978 6000 STEP Whitlock, 198971 Lost Trail Pass Bog* 45.75 113.97 2152 16 1C n/a 6000 COCO Mehringer et al., 197773 Marion Lake* 49.33 123.00 305 7 2C n/a 6000 COCO Mathewes, 197374 Mariposa Lake, WY 44.15 110.23 2730 4 4C 6505 6000 OC Whitlock, 199376 Mckillop Creek Pond* 48.33 115.45 920 5 1D n/a 6000 TAIG Mack et al., 198377 Mineral Lake* 46.73 122.20 436 7 2C n/a 6000 COCO Tsukada et al., 198178 Mission Cross Bog* 41.78 115.48 2424 7 1C n/a 6000 STEP Thompson, 198479 Molas Lake 37.75 107.68 3200 2 7C 5818 6000 OC Maher, 196181 Mud Lake* 48.50 119.75 655 5 7D n/a 6000 OC Mack et al., 197983 Nichols Meadow* 37.43 119.57 1509 2 1D n/a 6000 OC Koehler & Anderson, 199485 Pangborn Bog* 48.83 122.58 n/a 3 5D n/a 6000 COCO Hansen & Easterbrook, 197486 Pinecrest Lake* 50.50 121.50 320 1 7D n/a 6000 COCO Mathewes & Rouse, 197588 Posy Lake* 37.95 111.70 2653 4 1C n/a 6000 OC Shafer, 198991 Rapid Lake* 42.62 109.20 3135 4 2C n/a 6000 OC Fall, 198892 Rattlesnake Cave* 43.52 112.62 1996 1 4D n/a 6000 OC Davis, 198193 Redrock Lake* 40.67 105.50 3095 7 1C n/a 6000 OC Maher, 197294 Ruby Marshes* 41.13 115.48 1818 25 1C 6000 OC Thompson, 1992100 Slough Creek Pond 44.93 110.35 1884 4 2C 6128 6000 OC Whitlock & Bartlein, 1993102 Soleduck Bog* 47.92 124.47 73 1 4C 6000 COCO Heusser, 1973103 Splains Gulch* 38.83 107.08 3160 4 1C 6000 COCO Fall, 1988105 Swamp Lake* 37.95 119.82 1554 5 4C 6000 COCO Smith & Anderson, 1992106 Swan Lake* 42.33 112.42 1452 3 4C 6000 STEP Bright, 1966


107 Teepee Lake* 48.33 115.50 1270 8 1C 6000 TAIG Mack et al., 1983109 Tioga Pass Pond* 37.92 119.27 3018 4 1C 6000 OC Anderson, 1990113 Waits Lake* 48.17 117.67 n/a 8 2C 6000 TAIG Mack et al., 1978c116 Wessler Bog* 48.17 124.50 25 1 7C 6000 COCO Heusser, 1973118 Williams Fen* 47.33 117.58 n/a 6 1C 6000 STEP Nickmann, 1979

18,000 14C yr B.P. SAMPLES6 Battle Ground Lake* 45.67 122.48 <300 11 2C 18000 STEP Barnosky, 1985a8 Bechan Cave 38.00 111.00 1370 1 7D 18236 18000 OC Davis et al., 1984, Davis, 1990

11 Bogachiel River Site* 47.88 124.33 533 2 7C 18000 STEP Heusser, 197814 Carp Lake* 45.92 120.88 714 13 1C 18000 STEP Barnosky, 1985b25 Davis Lake* 46.58 122.25 282 16 3C 18000 STEP Barnosky, 198126 Dead Man Lake* 36.24 108.95 2780 5 7C 18000 OC Wright et al., 197327 Devlins Park* 40.02 105.55 2953 6 1C 18000 OC Legg & Baker, 198033 Fargher Lake* 45.88 122.52 n/a 3 2C 18000 COCO Heusser, 198342 Gray's Lake* 43.00 111.58 1946 15 6C 18000 OC Beiswenger, 199147 Hay Lake, Arizona 34.00 109.43 2780 6 6C 17770 18000 OC Jacobs, 198549 Hedrick Pond 43.75 110.60 2073 5 4D 17408 18000 OC Whitlock, 199351 Hoh Bog* 47.75 124.25 n/a 8 7D 18000 TUND Heusser, 197857 Jacob Lake 34.33 110.83 2285 3 7C 18144 18000 OC Jacobs, 198358 Kalaloch* 47.55 124.33 35 3 6D 18000 STEP Heusser, 197268 Little Lake, Oregon 44.17 123.58 217 13 2C 17618 18000 COCO Worona & Whitlock, 199575 Mayberry Well 33.70 108.30 2080 1 7C 18332 18000 OC Markgraf, unpub NAPD77 Mineral Lake* 46.73 122.20 436 7 2D 18000 STEP Tsukada et al., 198179 Molas Lake 37.75 107.68 3200 2 7D 18062 18000 OC Maher, 196183 Nichols Meadow* 37.43 119.57 1509 2 7C 18000 OC Koehler & Anderson, 199489 Potato Lake* 34.08 111.50 2222 4 4C 18000 OC Anderson, 199394 Ruby Marshes* 41.13 115.48 1818 25 1C 18000 OC Thompson, 1992


Table 3 Characteristics of the 0, 6000 and 18,000 14C yr B.P. packrat midden sites from western North

America. Dating control codes (DC) follow the scheme described in Table 2. Biome codes (Biome) are

given in Table 6. For mapping purposes some sites (indicated by ‡) which are too close to one another have

been displaced slightly.

Site name Sample Lat. Long. Elev. Sample C14 Target DC Biome References

Code (°N) (°W) (m) type age age

MODERN SAMPLES

Ajo Loop 4 31.97 112.78 550 midden 980 0 4D DESE Van Devender, 1987

Ajo Loop 3A 31.97 112.78 550 midden 990 0 4D DESE Van Devender, 1987

Ajo Loop 2C 31.97 112.78 550 midden 130 0 1D DESE Van Devender, 1987

Ajo Loop 2B 31.97 112.78 550 midden 30 0 1D DESE Van Devender, 1987

Bison Alcove MOD 38.73 109.50 1317 midden 405 0 2D OC Sharpe, 1991

Bison Alcove MOD 38.73 109.50 1317 midden 355 0 2D OC Sharpe, 1991

Dog Canyon 1 32.83 105.92 1615 midden 360 0 2D DESE Van Devender et al., 1984

Dolores 4(1) 37.52 108.55 2100 midden 900 0 4D OC Van Devender, 1985

Dolores 3 37.52 108.55 2100 midden 580 0 3D OC Van Devender, 1985

Dolores 4(2) 37.52 108.55 2100 midden 990 0 4D OC Van Devender, 1985

Eureka View ‡ #3 37.33 117.78 1450 midden 535 0 3D DESE Spaulding, 1980

Gatecliff #5 39.00 116.78 2319 midden 0 0 1D OC Thompson & Hattori, 1983

Greenwater Valley ‡ G11C 36.17 116.60 1410 midden 740 0 3D DESE Cole & Webb, 1985

Greenwater Valley ‡ G11A 36.17 116.60 1410 midden 0 0 1D DESE Cole & Webb, 1985

Greenwater Valley ‡ G9D 36.17 116.60 1350 midden 290 0 2D DESE Cole & Webb, 1985

Greenwater Valley ‡ G15 36.17 116.60 1380 midden 0 0 1D DESE Cole & Webb, 1985



Greenwater Valley ‡ G6B 36.17 116.60 1350 midden 270 0 2D DESE Cole & Webb, 1985

Greenwater Valley ‡ G11B 36.17 116.60 1410 midden 400 0 2D DESE Cole & Webb, 1985

Greenwater Valley ‡ G4A 36.17 116.60 1350 midden 200 0 1D OC Cole & Webb, 1985

Hance Canyon ‡ HC2B 36.03 111.97 1200 midden 0 0 1D DESE Cole, 1981

Hidden Forest #3B 36.57 115.10 2380 midden 820 0 4D OC Spaulding, 1981

Horse Thief Hills ‡ #3(1) 37.35 117.80 1575 midden 200 0 1D DESE Spaulding, 1980



Horse Thief Hills ‡ #2 37.35 117.80 1635 midden 200 0 1D OC Spaulding, 1980

Horseshoe Mesa ‡ HM1B 36.03 111.98 1100 midden 0 0 1D DESE Cole, 1981

Kings Canyon KC7 36.80 118.80 1270 midden 90 0 1D XERO Cole, 1983

Kings Canyon MODINDUR 36.80 118.80 1270 midden 0 0 1D XERO Cole, 1983

McCullough Range 1(1)1 35.75 115.17 1045 midden 960 0 4D DESE Spaulding, 1991

Navar Ranch 18C 31.90 106.15 midden 789 0 4D DESE Van Devender & Toolin, undat.

San Andres 1 32.83 105.92 1555 midden 430 0 2D DESE Van Devender et al., 1984

Sierra Bacha 3 29.83 112.47 100 midden 320 0 2D DESE Van Devender et al., 1994

Wide Rock Butte LEVEL1 36.12 109.33 2100 midden 0 0 1D OC Schmutz et al., 1976

6000 14C yr B.P. SAMPLES

Ajo Loop 1A 31.97 112.78 550 midden 5240 6000 4D DESE Van Devender, 1987

Chuar Valley CH1 36.17 111.92 1430 midden 6830 6000 4D OC Cole, 1981

Council Hall Cave #1A 39.33 114.10 2040 midden 6120 6000 1D OC Thompson, 1984

Eureka View #4(3) 37.33 117.78 1435 midden 5435 6000 3D DESE Spaulding, 1980




Fishmouth Cave 4 37.42 109.65 1585 midden 6100 6000 1D OC Betancourt, 1984

Grandview Point ‡ GP4 36.00 111.98 2200 midden 5510 6000 2D OC Cole, 1981

Hornaday Mountains 1C 31.98 113.60 240 midden 6065 6000 1D DESE Van Devender et al., 1990

Lava Tube 99999 43.55 122.57 1640 midden 5690 6000 2D STEP Bright & Davis , 1982

Lubkin Canyon ‡ 1 36.53 118.05 1264 midden 5090 6000 4D STEP Koehler & Anderson, 1995

Lubkin Canyon ‡ 2 36.53 118.05 1264 midden 5610 6000 2D OC Koehler & Anderson, 1995

Lucerne Peak ‡ #1 34.50 117.00 1097 midden 5800 6000 1D DESE King, 1976

Marble Canyon 1A 32.83 105.92 1580 midden 5430 6000 3D DESE Van Devender et al., 1984

McCullough Range 3(5) 35.75 115.17 1045 midden 5060 6000 4D DESE Spaulding, 1991




Navar Ranch 1C1 31.90 106.15 midden 6360 6000 2D DESE Van Devender & Toolin, nd

Rhodes Canyon 4 33.18 106.60 1700 midden 6330 6000 2D DESE Van Devender & Toolin, 1983

Rhodes Canyon 6B 33.18 106.60 1700 midden 6950 6000 4D DESE Van Devender & Toolin, 1983

Sierra Bacha 1J 29.83 112.47 200 midden 5340 6000 3D DESE Van Devender et al., 1994

Sunset Cove ‡ #1 34.50 117.00 972 midden 5880 6000 1D OC King, 1976

Tse an Bida Cave BI6B 36.00 112.00 1450 midden 6800 6000 4D OC Cole, 1981

Valleyview #1 39.50 114.72 2350 midden 6250 6000 1D OC Thompson, 1984

Valleyview #1 39.50 114.72 2350 midden 6670 6000 3D OC Thompson, 1984

Waterman Mountains 9D 32.35 111.45 760 midden 5540 6000 2D DESE Andersen & Van Devender, 1991

Waterman Mountains 9D 32.35 111.45 760 midden 4845 6000 5D DESE Andersen & Van Devender, 1991

Waterman Mountains 12A 32.35 111.45 760 midden 6195 6000 1D DESE Andersen & Van Devender, 1991

Waterman Mountains 12A 32.35 111.45 760 midden 5920 6000 1D DESE Andersen & Van Devender, 1991

Wellton Hills #5A 32.60 114.12 175 midden 6600 6000 3D DESE Van Devender, 1973

Wellton Hills #5A 32.60 114.12 175 midden 8150 6000 7 DESE Van Devender, 1973

Wide Rock Butte LEVEL4 36.12 109.33 2100 midden 6210 6000 1D COMX Schmutz et al., 1976

Wolcott Peak 4 32.45 111.47 862 midden 5350 6000 3D DESE Van Devender, 1973

18,000 14C yr B.P. SAMPLES

Big Boy 2 32.83 105.92 1555 midden 18300 18000 2D OC Van Devender et al., 1984

Chuar Valley ‡ CH18C2 36.17 111.92 1770 midden 18490 18000 2D COMX Cole, 1981

Chuar Valley ‡ CH18B 36.17 111.92 1770 midden 18800 18000 4D OC Cole, 1981

Eyrie #3(2) 36.63 115.28 1855 midden 18890 18000 4D OC Spaulding, 1981

Flaherty Mesa #2 36.48 115.25 1770 midden 18790 18000 4D OC Spaulding, 1981

Hance Canyon ‡ HC4 36.03 111.97 1100 midden 17400 18000 3D OC Cole, 1981

Horseshoe Mesa ‡ HM6 36.03 111.98 1450 midden 18630 18000 3D XERO Cole, 1981

Kings Canyon #5A2 36.80 118.80 1275 midden 19130 18000 5D XERO Cole, 1983

Kings Canyon #5A2 36.80 118.80 1275 midden 17520 18000 2D XERO Cole, 1983

Nankoweap ‡ NA9C 36.25 111.95 2020 midden 18130 18000 1D COCO Cole, 1981

Nankoweap ‡ NA9B 36.25 111.95 2020 midden 17950 18000 1D COCO Cole, 1981

Pontatoc Ridge #4B 32.35 110.88 1463 midden 17950 18000 1D OC VanDevender & Thompson, unpub.

Rampart Cave RatLayer 36.10 113.93 535 midden 18890 18000 4D OC Phillips, 1977

Streamview #2 39.33 114.10 1860 midden 17350 18000 3D OC Thompson, 1984; Thompson & Mead, 1982

Vulture Canyon #6 36.10 113.93 645 midden 17610 18000 2D OC Phillips, 1977

Vulture Canyon #17 36.10 113.93 645 midden 17100 18000 4D OC Mead & Phillips, 1981

Willow Wash #4D 36.47 115.25 1585 midden 17070 18000 4D OC Spaulding, 1981


Table 4 Assignments of pollen taxa from western North America to the PFTs used in the biomizationprocedure.

Abbr. Plant functional type Pollen taxaaa arctic/alpine dwarf shrub Alnus undiff., Betula undiff., Brassicaceae, Caryophyllaceae,

Dryas-type, Fabaceae, Oxyria, Polygonum bistortoides-type,Potentilla-type, Ranunculaceae, Salix, Saxifragaceae, Thalictrum,Umbelliferae/Apiaceae

bec boreal evergreen conifer Abies, Piceabs boreal summergreen Alnus undiff., Betula undiff., Cornus, Larix, Larix/Pseudotsuga,

Myricaceae, Populus, Salix, Shepherdia canadensisctc cool-temperate conifer Abies, Larix/Pseudotsuga, Picea, Pseudotsuga, Taxaceae,

Taxodiaceae (Other, in California),Taxodiaceae/Cupressaceae/Taxaceae, Tsuga

df desert forb Ambrosia-type, Brassicaceae, Caryophyllaceae,Chenopodiaceae/Amaranthus, Ephedra, Eriogonum,Euphorbiaceae, Fabaceae, Nyctaginaceae, Ranunculaceae,Sphaeralcea-type, Tubuliflorae/Other Asteraceae,Umbelliferae/Apiaceae

ds desert shrub or succulent not presentds2 frost-sensitive desert shrub or

succulentnot present

ec eurythermic conifer Cupressaceae/Taxaceae, Pinus,Taxodiaceae/Cupressaceae/Taxaceae

g grass Poaceaeh heath Ericaceaes sedge Cyperaceaesf steppe forb Ambrosia-type, Artemisia, Brassicaceae, Caryophyllaceae,

Cercocarpus-type, Chenopodiaceae/Amaranthus, Ephedra,Eriogonum, Fabaceae, Potentilla-type, Ranunculaceae, Sarcobatus,Saxifragaceae, Sphaeralcea-type, Tubuliflorae/Other Asteraceae,Umbelliferae/Apiaceae

ss steppe shrub not presentts temperate summergreen Acer, Alnus undiff., Anacardiaceae, Aquifoliaceae, Carya,

Ceanothus, Clethra, Cornus, Fraxinus, Myricaceae,Ostrya/Carpinus, Populus, Quercus, Rhamnus, Salix, Ulmus

ts1 cool-temperate summergreen Betula undiff., Corylus, Fagus, Larix, Larix/Pseudotsuga, Tiliats2 intermediate-temperate summergreen Castanea, Celtis, Cephalanthus, Fabaceae, Juglans, Magnoliaceae,

Morus, Nyssa, Platanusts3 warm-temperate summergreen Celtis, Cephalanthus, Liquidambar, Nyssawc woodland conifer not presentws woodland shrub not presentwtc warm-temperate conifer Taxodiaceae (Other, in California),

Taxodiaceae/Cupressaceae/Taxaceae, Taxodiumwte warm-temperate broadleaved

evergreenAquifoliaceae, Chrysolepis/Lithocarpus, Magnoliaceae, Quercus

wte1 cool-temperate broadleavedevergreen

Aquifoliaceae

wte2 warm-temperate sclerophyll shrub Ceanothus, Chrysolepis/Lithocarpus, Quercus, Rhamnus


Table 5 Assignments of plant macrofossil (midden) taxa from western North America to the PFTs used in the biomization procedure.

Abbr. Plant functional type Plant macrofossil taxaaa arctic/alpine dwarf shrub Cirsium sp., Phlox sp.bec boreal evergreen conifer Juniperus communis, Picea engelmannii, Picea pungens, Picea engelmanii, Pinus flexilisbs boreal summergreen Ribes cf. montigenum, Shepherdia canadensis, Shepherdia sp.ctc cool-temperate conifer Abies concolor, Abies magnifica, Calocedrus decurens, Juniperus communis, Juniperus occidentalis, Juniperus

scopulorum, Juniperus sp., Pinus flexilis, Pinus lambertiana, Pinus longaeva, Pinus ponderosa, Pseudotsuga menziesiidf desert forb Abutilon sp., Allionia incarnata, Amaranthus cf. albus, Amaranthus fimbriatus, Amaranthus sp., Amaranthus/chenopodium,

Ambrosia confertifolia, Ambrosia sp., Amsinckia intermedia, Amsinckia sp., Amsinckia tesselata, Amsinkia/Cryptantha,Amsonia sp., Argenome sp., Argythamnia lanceolata, Artemisia ludoviciana, Astragalus sp., Bahia absinthifolia,Boerhaavia sp., Boerhaavia wrightii, Brickellia arguta, Chaenactis sp., Chenopodium (cf.), Chenopodium sp., Chorizanthebrevicornu, Cirsium sp., Coldenia canescens, Cryptantha barbigera cf., Cryptantha maritima, Cryptantha racemosa,Cryptantha sp., Cryptantha virginensis, Cucurbita sp., Daucus pusillus, Descurainia sp., Dithyrea californica, Erigeronsp., Eriogonum fasciculatum, Eriogonum sp., Erodium sp., Eucnide urens, Euphorbia cf. polycarpa, Euphorbiamicromera/polycarpa, Euphorbia sp., Gilia cf. latifolia, Gilia sp., Gutierrezia lucida, Gutierrezia sarothrae, Gutierreziasp., Haplopappus brickelliodes, Hedeoma nana, Hedeoma sp., Helianthus sp., Heterotheca sp., Ipomoea or convolvulus,Iva cf. ambrosiaefolia, Kallstroemia sp., Lappula redowskii, Lappula sp., Lepidium fremontii, Lepidium sp., Leptodactylonpungens, Lesquerella sp., Lotus sp., Lupinus sp., Machaeranthera sp., Mentzelia multiflora-type, Mentzelia sp., Mirabilisbigelovii, Mirabilis multiflora, Mirabilis sp., Oenothera (cf.), Oenothera pallida, Pectocarya cf. recurvata, Pectocaryaheterocarpa, Pectocarya sp., Penstemon sp., Perityle emoryi, Physalis crassifolia, Physalis sp., Plantago insularis,Plantago sp., Polygonum sp., Portulaca oleracea, Psoralea sp., Salazaria mexicana, Salvia sp., Senecio sp., Sphaeralcea-type, Sphaeralcea ambigua, Sphaeralcea sp., Stanleya pinnata, Stephanomeria exiqua, Streptanthus sp., Strepthanthuscordatus, Thysanocarpus sp., Tidestromia oblongiflolia, Tidestromia lanuginosa, Verbena sp., Viguiera reticulata,Viguiera sp.

ds desert shrub or succulent Acacia greggii, Agave lecheguilla, Agave neomexicana, Agave sp., Agave utahensis, Aloysia wrightii, Ambrosia dumosa,Atriplex canescens, Atriplex polycarpa, Berberis haematocarpa, Brickellia desertorum, Brickellia longiflora, Brickellia oreupatorium, Brickellia sp., Buddleja utahensis, Cactaceae undet., Ceanothus greggii, Celtis pallida, Choisya dumosa,Chrysothamnus nauseosus ssp. bigelovii, Chrysothamnus teretifolius, Coleogyne ramosissima, Coryphantha strobiliformus,Cowania mexicana (cf.), Croton sonorae, Dalea fremontii, Dalea sp., Dasylirion sp., Dasylirion wheeleri, Ditaxislanceolata, Echinocactus horizonthalonius, Echinocereus sp., Encelia farinosa, Encelia frutescens, Encelia virginensis,Ephedra aspera, Ephedra nevadensis, Ephedra sp., Ephedra torreyana, Ephedra trifurca-type, Ericameria larcifolia,Eriogonum heermannii, Eriogonum sp., Fallugia paradoxa, Ferocactus-type, Ferocactus covellei, Forsellesia nevadensis,Forsellesia spinescens, Fouquieria columnaris, Fouquieria splendens, Grayia spinosa, Gutierrezia microcephala,Haplopappus cooperi, Haplopappus laricifolius, Horsfordia, Hymenoclea salsola, Hyptis emori, Janusia gracilis,Koeberlinia spinosa, Krameria grayi, Krameria parvifolia, Larrea divaricata, Lycium andersonii, Lycium cf. berlandieri,Lycium pallidum, Lycium sp., Machaeranthera tortifolia, Mammillaria tetrancistra, Menodora spinescens, Mortoniascabrella, Neolloydia johnsonii, Nicotiana trigonophylla, Nolina micrantha, Opuntia, Opuntia acanthocarpa, Opuntiaarbuscula, Opuntia basilaris, Opuntia bigelovii, Opuntia chlorotica, Opuntia echinocarpa, Opuntia erincea, Opuntia


fulgida, Opuntia imbricata, Opuntia kunzei, Opuntia leptocactus, Opuntia phaeacantha-type, Opuntia ramosissima,Opuntia sp., Opuntia versicolor, Peucephyllum schottii, Phoradendron californicum, Prosopis glandulosa, Prosopisjuliflora, Prunus fasciculata, Prunus sp., Psoralea sp., Psorothamnus fremontii, Rhus diversiloba, Rhus microphylla, Salviamohavensis, Salvia sp., Simmondsia chinensis, Solanum hindsianum, Tetradymia axillaris, Tetradymia spinosa,Thamnosma montana, Thamnosma sp., Trixis californica, Viquiera stenoloba, Yucca angustifolia, Yucca angustissima,Yucca baccata (cf.), Yucca baccata/torreyi, Yucca brevifolia, Yucca schidigera, Yucca sp., Yucca torreyi, Yucca whipplei,Yucca whipplei ssp.caepitosa

ds2 frost-sensitive desert shrub orsucculent

Agave subsimplex, Bursera micophylla, Cactaceae undet., Carnegiea gigantea, Cercidium, Cercidium floridum, Cercidiummicrophyllum, Cereus giganteus, Ferocactus-type, Ferocactus acanthodes, Ferocactus cylindraceus, Jatropha cuneata,Lophocereus schottii, Olneya tesota, Pachycereus pringlei, Prosopis juliflora var. velutina, Prosopsis velutina, Sapiumbiloculare, Stenocereus thurberi, Tetradymia sp.

ec eurythermic conifer Cupressaceae, Pinus sp.g grass Agropyron cf. spicatum, Agropyron sp., Andropogon barbinodis/saccharoides, Aristida adscencionis, Aristida sp.,

Avena/festuca type, Bouteloua aristidoides, Bouteloua barbata, Bouteloua curtipendula, Bouteloua eriopoda, Boutelouagracilis, Brickellia arguta, Bromus anomalus, Bromus rubens, Bromus sp., Bromus tectorum, Digitaria californica,Echinochloa crusgallii, Elymus sp., Enneapogon desvauxii, Eragrostis sp., Erioneuron grandiflorum, Erioneuronpulchellum, Festuca sp., Filago sp., Gramineae undet., Hilaria jamesii, Leptochloa dubia, Muhlenbergia microsperma,Muhlenbergia monticola, Muhlenbergia pauciflora, Muhlenbergia sp., Oryzopsis hymenoides, Oryzopsis sp., Panicumcapillare, Panicum cf. hallii, Poa sandbergii, Poaceae undet, Setaria cf. leucophila, Setaria macrostachya, Sitanionhystrix, Stipa arida, Stipa cf. lobata, Stipa comata, Stipa neomexicana, Stipa occidentalis, Stipa sp., Stipa speciosa, Stipiaarida cf., Tridens muticus

h heath Arctostaphylos pungens, Arctostaphylos sp.s sedge not presentsf steppe forb Allionia incarnata, Amaranthus cf. albus, Amaranthus sp., Amaranthus/chenopodium, Ambrosia confertifolia, Ambrosia

sp., Amsinckia sp., Amsinkia/Cryptantha, Angelica sp. cf., Argenome sp., Artemisia ludoviciana, Artemisia sp., Astragalussp., Chaenactis sp., Chenopodium (cf.), Chenopodium sp., Cirsium sp., Cryptantha sp., Cryptantha virginensis, Cucurbitasp., Daucus pusillus, Descurainia sp., Ericameria cuneata, Erigeron sp., Eriogonum fasciculatum, Eriogonum sp., Erodiumsp., Euphorbia sp., Gilia sp., Gutierrezia lucida, Gutierrezia sarothrae, Gutierrezia sp., Hedeoma nana, Hedeoma sp.,Helianthus sp., Ipomoea or convolvulus, Lappula redowskii, Lappula sp., Lepidium fremontii, Lepidium sp., Leptodactylonpungens, Lesquerella kingi, Lesquerella sp., Linum lewisii, Linum sp., Lithospermum sp., Lotus sp., Lupinus argenteus,Lupinus sp., Machaeranthera sp., Mentzelia multiflora-type, Mentzelia sp., Mirabilis bigelovii, Mirabilis multiflora,Mirabilis oxybaphoides, Mirabilis sp., Oenothera (cf.), Oenothera pallida, Pectocarya heterocarpa, Pectocarya sp.,Penstemon breviflorus, Penstemon sp., Phacelia sp., Phlox sp., Physalis sp., Plantago insularis, Plantago sp., Polygonumsp., Portulaca oleracea, Psoralea sp., Senecio sp., Sphaeralcea-type, Sphaeralcea sp., Stanleya pinnata, Stephanomeriaexiqua, Streptanthus sp., Strepthanthus cordatus, Verbena sp., Viguiera sp.

ss steppe shrub Amelanchier sp., Amelanchier utahensis, Artemisia frigida, Artemisia sp., Artemisia spinescens, Artemisia tridentata,Atriplex canescens, Atriplex confertifolia, Atriplex polycarpa, Berberis fremontii, Berberis haematocarpa, Brickellia sp.,Buddleja utahensis, Cactaceae undet., Ceanothus greggii, Ceratoides lanata, Cercocarpus ledifolius, Chamaebatiariamillefolium, Chrysothamnus greenei, Chrysothamnus nauseosus, Chrysothamnus nauseosus ssp. bigelovii, Chrysothamnussp., Chrysothamnus teretifolius, Chrysothamnus viscidiflorus, Coryphantha strobiliformus, Cowania mexicana (cf.),


Dasylirion sp., Dasylirion wheeleri, Echinocactus polycephalus, Echinocereus sp., Ephedra nevadensis, Ephedra sp.,Ephedra torreyana, Ephedra trifurca-type, Ephedra viridis, Ericameria larcifolia, Eriogonum sp., Fallugia paradoxa,Forsellesia nevadensis, Grayia spinosa, Gutierrezia microcephala, Lycium pallidum, Mammillaria-type, Mammillariamicrocarpa, Mammillaria sp., Menodora spinescens, Nolina micrantha, Opuntia, Opuntia arbuscula, Opuntia basilaris,Opuntia bigelovii, Opuntia chlorotica, Opuntia erincea, Opuntia fulgida, Opuntia imbricata, Opuntia leptocactus, Opuntiaphaeacantha-type, Opuntia polyacantha, Opuntia sp., Opuntia spinosior, Opuntia whipplei, Petrophytum caespitosum,Psoralea sp., Purshia tridentata, Rhus diversiloba, Ribes cereum, Ribes sp., Ribes velutinum, Salvia carnosa, Salvia sp.,Shepherdia argentea, Symphoricarpo ssp., Symphoricarpos longiflorus, Symphoricarpos sp., Tetradymia axillaris,Tetradymia sp., Tetradymia spinosa, Tidestromia oblongiflolia, Yucca angustifolia, Yucca angustissima, Yuccaharrimaniae, Yucca sp.

ts temperate summergreen Amelanchier sp., Amelanchier utahensis, Ceanothus cuneatus, Ceanothus integerrimus, Celtis reticulata, Cercisoccidentalis, Cercocarpus sp., Fraxinus anomala, Berberis repens, Ostrya knowltoni, Physocarpus alternans, Prunus sp.,Ptelea trifoliata var. pallida, Quercus arizonica x Q. grisea, Quercus gambellii, Quercus grisea, Quercus sp., Rhusaromatica, Rhus aromatica or virens, Rhus diversiloba, Rhus sp., Rhus trilobata, Ribes cf. montigenum, Ribes sp., Rosa sp.,Rubus sp., Sambucus neomexicana, Shepherdia sp.

ts1 cool-temperate summergreen not presentts2 intermediate-temperate

summergreenRobinia neomexicana

ts3 warm-temperate summergreen Prosopis glandulosawc woodland conifer Juniperus deppeana, Juniperus erythrocarpa, Juniperus monosperma (cf.), Juniperus occidentalis, Juniperus osteosperma,

Juniperus pinchotii, Juniperus scopulorum, Juniperus sp., Parthenium incanum, Parthenium sp., Pinus cembroides, Pinusdiscolor, Pinus edulis, Pinus edulis/remota, Pinus flexilis, Pinus monophylla, Pinus monophylla x P. edulis

ws woodland shrub Aloysia wrightii, Amelanchier sp., Amelanchier utahensis, Aplopappus cuneatus, Berberis fremontii, Berberishaematocarpa, Berberis repens, Berberis sp., Brickellia watsonii, Cactaceae undet., Ceanothus cuneatus, Ceanothusgreggii, Cercocarpus betuloides, Cercocarpus intricatus, Cercocarpus ledifolius, Cercocarpus montanus, Cercocarpus sp.,Chamaebatiaria millefolium, Cowania mexicana (cf.), Ericameria cuneata, Eriogonum heermannii, Eriogonum sp.,Fendlerella utahensis, Forsellesia nevadensis, Forsellesia pungens, Fraxinus anomala, Garrya flavescens, Haplopappuslaricifolius, Haplopappus nanus, Holodiscus dumosus, Holodiscus microphyllus, Jamesia americana, Mammillaria-type,Mammillaria grahamii, Mammillaria sp., Menodora spinescens, Opuntia polyacantha, Opuntia sp., Pediocactus mesae-verdae, Philadelphus microphyllus, Physocarpus alternans, Prunus fasciculata, Prunus sp., Psoralea sp., Ribes cereum,Ribes cf. montigenum, Ribes sp., Ribes velutinum, Rosa cf. stellata, Salvia carnosa, Shepherdia argentea, Shepherdia sp.,Symphoricarpo ssp., Symphoricarpos longiflorus, Symphoricarpos sp., Tetradymia axillaris, Tetradymia spinosa,Thamnosma montana

wtc warm-temperate conifer Cupressus arizonicus, Pinus cembroides, Pinus discolor, Torreya californicawte warm-temperate broadleaved

evergreenArctostaphylos pungens, Arctostaphylos sp., Cercocarpus betuloides, Cercocarpus sp., Quercus chrysolepis, Quercuspungens, Rhamnus crocea, Rhamnus crocea var. Ilicifolia, Umbellularia californica

wte1 cool-temperate broadleavedevergreen

not present

wte2 warm-temperate sclerophyll shrub Berberis sp., Ceanothus greggii, Quercus sp., Quercus turbinella, Quercus undulata, Vauquelinia californica


Table 6 Assignment of PFTs to biomes used in the biomization for western North America.

Biome Code Plant functional types

cold deciduous forest CLDE bs, ec, h

taiga TAIG bec, bs, ec, h

cold mixed forest CLMX bs, ctc, ec, h, ts1

cool conifer forest COCO bec, bs, ctc, ec , h, ts1

temperate deciduous forest TEDE bs, ec, h, ts, ts1, ts2, wte1

cool mixed forest COMX bec, bs, ctc, ec, h, ts, ts1

broadleaved evergreen/warm mixed forest WAMX ec, wtc, h, ts, ts3, wte, wte1

xerophytic woods/scrub XERO ec, wc, ws, wte, wte2

steppe STEP g, s, sf, ss

desert DESE df, ds, ds2, g

tundra TUND aa, g, h, s

open conifer woodland OC ec, sf, ss, wc, ws


Table 7 Climatic tolerances of biomes in western North America, based on (a) present-day potential natural

vegetation, and (b) present-day pollen and plant macrofossil assemblages. MTCO temperatures are

approximated to the nearest 2.5° C; GDD5 values to the nearest 500 growing-degree days; and α values to

the nearest 0.05. The values in 7 (a) were obtained through visual inspection and interpretation of Figs 5

and 6; the values in 7 (b) through visual inspection and interpretation of Fig. 5. Cold deciduous forests and

xerophytic woods/scrub are omitted from 7 (b) because they only occur once.

a)

Biome MTCO MTCO GDD5 GDD5 α αmin max min max min max

cool conifer forest -12.5° 5.0° 500 2500 0.45 0.95

xerophytic woods/scrub 2.5° 12.5° 2000 5000 0.30 0.70

grassland -15.0° 7.5° 1500 5000 0.35 0.70

steppe -10.0° 0.0° 1000 3000 0.15 0.55

desert 0.0° 12.5° 3000 6500 0.05 0.40

tundra -12.5° -2.5° <500 1000 0.75 1.00

open conifer woodland -10.0° 5.0° 1000 3500 0.20 0.65

b)

Biome MTCO MTCO GDD5 GDD5 α αMin Max Min Max Min Max

taiga -10.0° -7.5° 500 500 0.75 0.85

cool conifer forest -7.5° 5.0° 500 2500 0.60 1.00

grassland/steppe -10.0° 0.0° 500 2000 0.40 1.00

desert 0.0° 7.5° 2000 6500 0.05 0.45

open conifer woodland -10.0° 5.0° 500 3500 0.20 0.70

Biomes of western North America at 18,000, 6000, and 0 14C ...

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