Introduction

In boreal forests with discontinuous permafrost,"drowning" of forests can occur as the surface subsidesdue to thawing of ice-rich soils. Such thawing can beinitiated by climate warming, forest fire, thermal heattransfer from adjacent water bodies, tree fall andchanges in groundwater flow and other factors.Recently, Racine and Walters (1994) and Jorgenson et al.(1996) identified an area of widespread and rapid per-mafrost degradation and forest drowning in the TananaFlats area of interior Alaska. Here the most intensethawing occurs in lowland birch forests (Figure 1)rather than in black spruce woodland where most stu-dies of taiga thermokarst development have occurred(Drury, 1956; Luken and Billings, 1983; Zoltai, 1993).Possible reasons for more rapid thawing in birch foreststhan in black spruce woodlands are described else-where in these proceedings (Walters et al. 1998). Theobjective of this study is to describe the thermokarstfeatures and associated vegetation in birch forests onthe Tanana Flats.

Study area

The Tanana Flats occupies a 3600 km2 portion of theTanana lowland basin bordered on the north by the

Tanana River, Fairbanks and the Yukon-Tanana uplandsand on the south by the Alaska Range. The Tanana Flatsis therefore situated on the toe slope of a large alluvialfan complex built out from the north side of the AlaskaRange. Both surface and subsurface water move acrossthe Tanana Flats on a very low gradient from southeastto northwest, flowing from Alaska Range glaciers to the

Abstract

The thawing of ice-rich permafrost beneath birch forests in the Tanana Flats area of Interior Alaska has pro-duced thermokarst features colonized by a range of species and wetland vegetation types. As the forest drownsalong its border with fens, an open-water moat is colonized by minerotrophic species and a floating mat deve-lops. At the same time, thawing in the birch forest interior produces water-filled thaw pits and collapse scarbogs in which ombrotrophic vegetation develops through several stages to Sphagnum bogs. As the thawingfront moves into the birch forest from the fen, these latter features are incorporated into the floating mat, acce-lerating the expansion of fens.

Charles H. Racine, et al. 927

THERMOKARST VEGETATION IN LOWLAND BIRCH FORESTS ON THETANANA FLATS, INTERIOR ALASKA, U.S.A.

Charles H. Racine1, M. Torre Jorgenson2, James C. Walters3

1. U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755 U.S.A.e-mail: cracine@crrel.usace.army,mil

2. Alaska Biological Research Inc. , P.O. Box 80410, Fairbanks, Alaska 99708 U.S.A.e-mail: tjorgenson@abrinc.com

3. Department of Earth Science, University of Northern Iowa, Cedar Falls, Iowa 50614 U.S.A.e-mail: walters@uni.edu

Figure 1. Low level oblique aerial photo across the Tanana Flats showing aforested island bordered on both sides by floating mat fen. The forest is pre-dominately Betula papyrifera with darker patches of Picea mariana forest.Vegetation, soils, permafrost and water were sampled along a 600 m transectpositioned across this island and adjacent fen in the direction of viewing.Visible thermokarst features include the fen and moat along the edge of thebirch forest and a collapse scar bog visible in the middle of the island. A smallembayment area along the foreground edge is visible and represents a col-lapse scar bog being incorporated into the fen.

Tanana River. The subsurface groundwater portionappears to discharge to the surface in springs in thenorthwest corner and probably accounts for the exten-sive development there of fens (Racine and Walters,1994). Here the taiga vegetation is a complex mosaic ofthese fen meadows, paper birch, mixed birch-spruceand black spruce forests, alder or ericaceous shrub-scrub and scattered bogs (Figure 1). This study concernsthermokarst development in the birch forests whichborder fens.

The climate of Fairbanks is continental and subarctic,with a mean annual temperature of -3.3ûC with a pro-nounced warming trend from 1976 to the present(Osterkamp,1994). The climate is subarid, with little orno water surplus; yearly precipitation averages 28.4 cm,with a minimum in late winter and maximum in latesummer (Viereck et al. 1993).

Methods

TRANSECTS

Vegetation, topography, soil, permafrost and watersampling were conducted in detail along two 600-mlong transects over four km apart, located in birchforests and adjacent floating mat fens (Figure 1). Inaddition, less detailed information was obtained at anumber of remote sites on floating mat fens and in

birch forests and associated thermokarst features. Ontransects, relative elevations of the ground and watersurface were measured at 2 m intervals with an autolevel. Permafrost presence and active layer thicknesswere determined in late August at the same intervalswith a 4 m-long tile probe. Soil pits were dug and peatcores were obtained to describe profiles.

VEGETATION SAMPLING

Birch forest vegetation was sampled at four sitesalong the two transects and in eight other birch foreststands (n=12) at widely scattered sites in the northwestTanana Flats. A 20 x 20m quadrat was established ineach birch stand and the height and diameter of alltrees and shrubs with stem diameters over 5 cm weremeasured. Four to six of the largest-diameter birch treeswere cored at 15 cm above the ground surface to deter-mine their age. Percent cover estimates of all species inboth the overstorey and understorey were also recor-ded. The vegetation of 42 thermokarst sites associatedwith the sampled birch forest stands was also sampledby estimating percent cover of all species in 5 x 5 mquadrats. A number of additional fen sites were alsosampled. Surface water pH and conductivity were mea-sured at most of the sample sites with a YSI 3500 waterquality meter. Vegetation data analysis was carried outusing summary statistics and multivariate methodsincluding cluster analysis and ordinations using

The 7th International Permafrost Conference928

Figure 2. Cross-sectional profile along an actual transect in the Tanana Flats from floating mat fen and moat (left) through a birch forest with thaw pits and collapse scar bogs showing relationships to topography, soils and permafrost.

Detrended Correspondence Analysis (DCA) (Hill andGauch, 1980) in PC-Ord, a commercial analysis package.

Results

TRANSECT PROFILE

The profile from a fen into and through a birch forest(Figure 2) shows the distribution of surface and subsur-

face features related to thermokarst development. Thebirch forest here is raised 1 to 2 m above the fen watertable. At the forest-fen border there is a 0.5 to 1.5-mdeep open water moat with dead or dying birch trees(Figure 3a) extending out into the fen for a distance of10 to 50 m. Within the forest, there are both small ope-nings or thaw pits, 2 to 15-m-wide (mean = 13 m, SD = ±9, n = 15) (Figure 3b) and larger 75 to 100-m-diameter collapse scar bogs (Figure 3c). Up to one halfof the transect distance through the birch forest may beoccupied by thermokarst pits and bogs.

Permafrost is sporadic in the moat area, and absentbeneath the fen, collapse scar bogs and the larger thawpits in Figure 2. In the birch forest, the active layer pro-file in late August is deep (1 m) and uneven in relationto developing thaw pits. The organic soil horizon in thesampled birch forests was relatively thick (0.5-1 m).Peat horizons in the collapse scar bogs are on the orderof 1 m. In the fen, the floating mat is 0.5 to 0.7 m thickconsisting of small rootlets, rhizomes and some peat,overlying a 0.2 to 0.4 m thick sapric organic layer overthe silt. Part of this lower organic layer may representthe decomposed birch forest organic horizon shown inFigure 2 as continuous beneath the thaw pits, bogs andfen. Silts underlie the organics and range in depth from3 to 5 m where gravels and sands are present. Thesesands and gravels probably serve as a conduit for themovement of groundwater.

VEGETATION

BIRCH FORESTS

The 10 to 15-m-tall canopy of all 12 sampled standswas clearly dominated by Betula papyrifera with occa-sional understorey trees of Salix bebbiana, Picea glauca orP. mariana. Tree cover was 60 to 80% and diameters(dbh) were mostly in the 10-25 cm range with standbasal area from 15 to 37 m2 ha1 (mean = 21, SD = (7.4, n = 9) and densities from 1000 to 3000 treesha1. (mean = 1450, SD = (695, n=9). The age of thelargest trees was 50 to 60 years in all stands. There waslittle or no evidence of tree regeneration (either spruceor birch) in any of the sampled stands. Understoreyvegetation varied from a continuous 1-2 m-tall shrublayer of Rosa acicularis or a grass layer of Calamagrostiscanadensis to almost no understorey vegetation or withonly scattered shrubs of Rosa acicularis, Ribes triste,Salixbebbiana, Ledum palustre and Rubus idaeus. Other groundcover species included Vaccinium vitis-ideae, Moehringialatifolia, Pyrola acerifolia and Epilobium angustifolium.Moss and lichen cover was absent or very sparse (<5%).

THAW PITS

Thaw pits occurred at variable frequency, diameterand vegetation composition (Table 1) on the shadedbirch forest floor in all sampled birch stands (Figure

Charles H. Racine, et al. 929

Figure 3. Three thermokarst features in lowland birch forests on the TananaFlats include: a) open water moat along birch forest-fen edge with standingdead birch trees, b) a thaw pit in the birch forest floor with open water andfloating Lemna minor and c) a collapse scar bog.

3b). Within a single birch stand, the cover in differentpits may range from open water with floating aquaticsto continuous vegetation. Ordination of the vegetationin 21 sampled pits (Figure 4a) shows three stand clus-ters which may represent three stages of vegetationdevelopment following thaw subsidence.

Group I samples represent an early stage in pit forma-tion with deeper (20-50 cm) standing water and stand-ing dead or dying birch trees (Figure 3b). The dominantspecies is the floating aquatic, Lemna minor (Table 1).Pits in group II contain less water, with only a few shal-low pools, exposed wet organic soils, woody birch twigdebris and small clumps of vegetation. The annualBidens cernua is frequently dominant (>50% cover) bylate August and there are clumps of Carex canescens andCarex aquatilis, Calamagrostis canadensis and forbsincluding Ranunculus gmelini and Cicuta virosa. GroupIII stands have a continuous vegetation mat of variablecomposition but characterized mainly by the presenceof Spagnum squarrosum and Potentilla palustris.

The 7th International Permafrost Conference930

Table 1. Summary table of major plant species that composethe vegetation of four thermokarst features in birch forests

on the Tanana Flats, interior Alaska. H = high frequency(over 50% of sampled stands); L = low frequency (10 to 50%

of sampled stands). I, II and III under thaw pits refers tovegetation type developmental stage in Figure 4a.

Figure 4. Ordinations of vegetation samples using detrended correspondentanalysis (DCA) from (A) 21 thaw pit samples and (B) 34 fen samples show-ing outlines of different vegetation groups or types.

Associated species include Calla palustris and the mos-ses, Sphagnum riparium and Calliergon sp.

Permafrost is generally present in group I, occasionalin II and absent in III (Figure 2). The lowered water levels in group II stands may be related to thawing ofthe permafrost and drainage of pore space beneaththese pits. The surface water in Group I had the highestpH and conductivities of the three groups while thosein Group III had the lowest values (Table 2).

COLLAPSE SCAR BOGS

Collapse scar bogs occur as large openings (30 to 150 m in diameter; mean = 75m) in the birch forest(Figures 1, 2 and 3c). A moat is common around theperiphery of these bogs but was not sampled. The vege-tation composition of the 13 sampled bogs was relative-ly uniform and simple with Sphagnum riparium clearlydominant with no or few additional species of moss.Scattered sedges (Carex limosa, C. aquatilis andEriophorum scheuchzeri) covered about 5 to 10% and lowshrubs of Oxycoccus microcarpus, Ledum groenlandicum,Chamaedaphne calyculata and Spirea beauverdiana accountfor an additional 5 to 25% cover (Table 1). In some bogsthere are also scattered and stunted trees of Larix larici-na, Picea mariana and Betula papyrifera. The pH and con-ductivities were lowest of any thermokarst featuredescribed here (Table 2).

MOATS

Along the birch forest/fen border of rapid thawingand collapse (Figures 2, 3a), there is a broad or narrowtension zone of open water, remnant birch forest, stan-

ding dead and dying birch trees and developing vege-tation. Water depths are variable but may be up to 1 mdeep near the birch forest bank; the water is frequentlyseen to be flowing in these moats unlike water in theother features. A diverse assemblage of aquatic speciescan occur here including Utricularia vulgaris, Sparganiumsp., Lemna minor and Riccia fluitans. The emergentHippuris vulgaris and Glyceria maxima were found onlyin moats and not in the other thermokarst features(Table 1). On the moat edge where there is wet organicsoil, the vegetation is similar to that of thaw pits withBidens cernua, Epilobium palustre, Calamagrostis sp. andoccasional sedges. Floating mat development begins inthese moats with loose mats of Calla palustris. As thefloating mat develops, Menyanthes trifoliata eventuallyinvades and completely replaces these Calla palustrismats. Water pH in these moats averaged 6.3 (SD = ±0.5;n = 7) and conductivity averaged 300 µs cm-1 (SD = ±105, n = 7).

FLOATING MAT FENS

Floating mat fens consist of a floating rhizome, rootand peat mat composed of various species of forbs,graminoids and in some places low shrubs and moss(Table 1). The vegetation composition of these floatingmats varies locally across a single fen and regionallyover their wide extent. The most basic and extensivefloating mat community consists of almost pure standsof tall (0.5 m) buckbean (Menyanthes trifoliata) forming agroup of stands (I) clearly visible on the left in Figure4B. From this basic buckbean type, increasing amountsof Potentilla palustris, sedges and low shrubs (Salix can-dida) form a second group (II in Figure 4B) followed bya third distinct group (III) with Equisetum fluviatile do-

Charles H. Racine, et al. 931

Table 2. Surface water chemistry for thermokarst features and associated vegetation types

minant. A fourth type of floating mat fen is representedby a group of two outlying stands dominated by sedgemeadow (Carex aquatilis or C. lasiocarpa) meadow withlittle or no buckbean. Minerotrophic moss species suchas Calliergon sp. are fairly common in group II and IIIstands. Other species that are usually present but notabundant at most floating mat sites include Cicutavirosa, Typha latifolia and Rumex arctica (Table 1). Waterchemistry of surface water in fens is quite variable butis generally circumneutral with a pH over 6.5 and highconductivities of about 275 µs cm-1 (Table 2).

Discussion

The thawing of permafrost underlying birch forests inthe Tanana Flats has produced several distinctthermokarst features and associated wetland vegetationtypes. Two different pathways of wetland vegetationsuccession are represented: (1) a minerotrophicsequence involving forest drowning adjacent togroundwater fens and development of a highly produc-tive forb-dominated floating vegetation mat; (2) anombrotrophic sequence within the birch forest interiorinvolving the development of small water-filled thawpits with dead birch trees and possibly progressingthrough several stages to simple Sphagnum bogs.

Convergence of these two pathways occurs when theprogressive thawing front (subsidence of the birch for-est along its boundary with floating mat fens) movesinto the birch forest and incorporates the thaw pits andcollapse scar bogs within the birch forest into the floa-ting mat fen. We observed several sites on aerial photosand in the field where this is occurring (Figure 1). Thisprocess accelerates the expansion of the fens and mayaccount for variation in the floating mat vegetation.Under this scenario, the final birch forest thermokarststage is the fen. We have seen little evidence for reestab-lishment of forest or permafrost on these floating matfens although this is a common stage in peatland deve-lopment (Zoltai, 1993).

Most studies of thermokarst development in borealforests take place in black spruce woodlands ratherthan birch forests (Drury, 1956; Thie, 1974; Luken andBillings, 1983; Zoltai, 1993; Vitt et al. 1994; Halsey et al.,1995; Laberge and Payette, 1995). These studies all listSphagnum riparium and Carex limosa and sedge lawns asthe major features of early vegetation developmentwith Sphagnum fuscum, S. angustifolium, low ericaceousshrubs and black spruce appearing later in succession.In the Tanana Flats birch forests, only the Sphagnumriparium-Carex limosa vegetation of collapse scar bogsresemble this black spruce thermokarst vegetation type.Bogs in the latter stages of thermokarst succession (withS. fuscum, S. angustifolium and abundant shrubs) are rare

in birch forests. This observation and one radiocarbondate of 490 ±70 BP (Beta-97563) at the bottom of the col-laspse scar bog in Figure 2 suggest that the bogs herehave formed within the past 500 years (cf. Zoltai, 1993).

The thawing of ice-rich permafrost beneath lowlandbirch forests in the Tanana Flats represents a majorecosystem change from terrestrial forest to wetlandwith associated changes in biological productivity, bio-mass, gas exchange, nutrient cycling, vegetation pat-terns and biodiversity. With respect to the effects ofthermokarst on biodiversity, over 60 species ofhydrophytic plants were sampled in the four types ofthermokarst features associated with these birch forests(fens, moats, pits and bogs). This represents at least 20%of the flora of the Tanana Flats (Racine et al., 1997). Inaddition, the majority (76%) of the species associatedwith the thermokarst complex (Table 1) occur in onlyone (44%) or two (32%) of the four features.

In interior Alaska, birch forests are usually associatedwith upland areas on well-drained and permafrost-freesouth-facing slopes. Here they usually represent a 30 to60 year-old fire-succession with white spruce replacingthe birch after 100 years (Van Cleve et al., 1996). It istherefore difficult to understand the origin of the low-land birch forests described here and why they areassociated with ice-rich permafrost areas of rapid ther-mal degradation. It is clear however, that they representone of the most sensitive ecosystem in interior Alaskato the pronounced climate warming which is occurringthere (Osterkamp, 1994),

Acknowledgments

This work was funded by US Army Alaska IntegratedTraining Area Management program under the direc-tion of William Gossweiler, Gary Larsen, and PamBruce. Field assistance was provided by PeggyRobinson (CRREL), Marilyn Racine, and Robert Lichvar(CRREL).

The 7th International Permafrost Conference932

Charles H. Racine, et al. 933

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Halsey, L.A., Vitt, D.H and Zoltai, S.C.(1995). Disequilibriumresponse of permafrost in boreal continental westernCanada to climate change. Climatic Change, 30, 57-73.

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Racine, C., Lichvar, R., Murray, B., Tande, G., Lipkin, R. andDuffy, M. (1997). A floristic inventory and spatial databasefor Fort Wainwright, Interior Alaska. CRREL SpecialReport 97-23.

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