Past, present, and future in the forests of the Sierra Nevada: variability in forest response to environmental change, and management strategies to promote ecosystem resilience

Hugh Safford USDA Forest Service, Pacific Southwest Region

University of California-Davis, Dept. of Environmental Science and Policy

hughsafford@fs.fed.us 707-562-8934

Outline • Sierra Nevada: a tale of two forests

– Lower elevation, yellow pine and mixed conifer forests

– High elevation, red fir and subalpine forests

• Differences in environments and trends between low and high elevation forests

• Interactive impacts of environmental stressors – Climate change, wildfire, invasive species/pest outbreaks

• Management response to global change – Some general strategies for resilience

Study area: Sierra Nevada Forest Plan Amendment area

120,000 km2; 11 National Forests, 3 National Parks

Köppen climate zones

Yellow pine & mixed conifer Oak

Subalpine Red fir

http://www.sierranevadaphotos.com/geography/sierra_climate.asp

Climate and vegetation zones

Yellow pine Mixed conifer Red fir Subalpine

Pinus ponderosa (ponderosa pine)

A. concolor Abies magnifica (red fir)

Tsuga mertensiana (mountain hemlock)

P. jeffreyi (Jeffrey pine)

P. ponderosa P. contorta (lodgepole pine)

Pinus albicaulis (whitebark pine)

P. lambertiana (sugar pine)

P. lambertiana P. monticola (western white pine)

P. monticola

Quercus kelloggii (black oak)

Calocedrus decurrens (incense cedar)

P. contorta

Abies concolor (white fir)

Pseudotsuga menziesii (Douglas-fir)

A. magnifica

Quercus spp. (oaks) Other 5-needle pines

Other hardwoods

Dominant tree species Lower montane

Upper montane

Stations from 37.5° to 39.5° latitude (in this graph only from west slope)

In the Sierra Nevada, the transition from mixed conifer to red fir is a major ecotone

Sierra Nevada: transition is at 1700-

2000 m

Ponderosa pine

White fir Red fir

Subalpine

West side stations only

Water balance

Low elevation forests are more moisture limited

than high elevation forests

Mediterranean (C)

Boreal (D)

Hot Mediterranean (Csa)

Cool Mediterranean (Csb)

Based on Köppen climate classification

High elevation forests are more energy limited than low

elevation forests

Tree species above this ecotone are less drought and fire adapted and better adapted to cold

High

Low

Safford and Stevens 2013

But even within forest types, there is high

variation in tolerances

Van de Water and Safford 2011

Presettlement fire return intervals

>2x longer in high elevation forests

Differences in fire tolerance lead to different relationships with fire: fire return interval

Percent high severity fire

Mallek et al. 2013

Before settlement: more high severity fire

in high elevation forests (but nothing like Rocky

Mtn lodgepole!)

Summary I: A tale of two forests

Yellow pine and mixed conifer forests – Lower elevation

– More arid in summer, moisture limited

– <50% of ppt falls as snow

– Highly frequent, mostly low severity fire before settlement

– Original dominant species highly fire and drought tolerant • Ponderosa, Jeffrey, and sugar pine; black oak

• White fir: less fire tolerant, but possible most fire-tolerant Abies sp in NA

Red fir and subalpine forests

– Higher elevation

– Lower PET and periodic thunderstorms = more mesic summer environment

– Most ppt falls as snow

– Lower frequency, somewhat more severe fire before settlement

– Dominant species more susceptible to fire and drought mortality

• Red fir, mountain hemlock, lodgepole pine, high elevation 5-needle pines

Trends in temp and ppt over the last century have been +/- similar in low and high elevation forests…

B

Graphic courtesy of S. Dobrowski

early 20th century vs. early 21st century, from PRISM

Temp has increased 0-1.5°

C

Precip is steady to increasing, except

on east side

But low and high elevation forests show some notably different trends over the last century

Changes in tree spp frequencies,

1930s to 2000s

Dolanc et al. 2014

High Low

Tree composition

Most notable changes are at lower elevations, where ponderosa and sugar pine are being replaced by shade-tolerant competitors, and

evergreen hardwood densities are increasing

Trends in tree density: 1930s-2000s

High (>2000 m) Low

Dolanc et al. 2014

Over last eight decades, tree densities are up at all elevations, but greatest increases are in yellow pine and mixed conifer forests

500 m elevation bands

Trends in fire severity: 1980s-2010s

On Forest Service lands: Fire severity is increasing in low elevation forests (mixed conifer, white fir, black oak)

but not in high elevation forests (e.g., red fir)

Percent of annual area burned where

tree mortality is >95%

Miller et al. 2009, Miller & Safford 2012, Mallek et al. 2013

But overall burned area is (proportionally) increasing more rapidly in high elevation forests

Overall increase in median annual area burned, 1984-2009

Mallek et al. 2013

What is driving recent ecological trends in low and high elevation forests?

1. Fire suppression

Min yrs since Federal fire suppression

80+ years of fire exclusion has differentially affected Sierra Nevada forests:

Time since last fire is beyond HRV

in most low elevation forests

Fire frequency

Van de Water and Safford 2011

Measures of fire frequency departure: National Park Service FRID Index

Low

Moderate

Extreme

High

Safford and Van de Water 2014

Low elevation forests – high to extreme departures; high elevation forests – low to moderate

80+ years of fire exclusion has differentially affected Sierra Nevada forests:

Mallek et al. 2013

On USFS lands: Current fire severities at low elevation are

much higher than before settlement; high elevation

forests are not notably different

Fire severity

Very reduced

(CC3)

Reduced

(CC2)

“HRV”

(CC 1 to -1)

Enhanced

(CC -2)

Very enhanced

(CC -3)

Fuel

quantity

(Fire Regime I)

Inter-

mediate

(Fire Reg. III)

Fuel quality

or ignition

(Fire Regimes

IV & V)

Bigcone Douglas-fir

Dry mixed conifer

Yellow pine

Current frequency of fire vs. presettlement frequency

Pre

sett

lem

ent

fuel

load

s

Pre

sett

lem

ent

fire

ret

urn

inte

rval

Moist mixed conifer

Limiting factor

Presettlement Conditions

Redwood (NI)1

Chaparral

Moist subalpine

Redwood (HI)

Red fir

Moist coastal forests

Dry subalpine1

Mixed evergreen

Oak woodland

Overall, high elevation forests are closer to the historical range of variation than low elevation forests

Steel et al. 2015

What is driving recent ecological trends in low and high elevation forests?

1. Fire suppression 2. Changing climate

Mt. Tallac, Lake Tahoe Basin:

December 2013

In Sierra Nevada forests, precipitation is turning more to rain, and the freezing line is moving uphill

Tahoe City (1900 m): strong reduction in % of ppt falling as

snow over last century

Number of days with mean temp <0° is dropping rapidly

Trends in snow water equivalent on April 1,

1950-1997.

Moser et al. 2009

Major losses in snowpack over last 50 years…

…except in highest mtns in southern SN

For the time being, high elevation warming under steady to increasing precip is actually improving growing conditions in many high elevation forest

stands…

Mtn. Hemlock expanding into former perennial snowfield

Lodgepole pine expanding into drying meadow

Increased variability in precipitation*: wetter wet years and drier dry years

0

10

20

30

40

50

60

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

5-y

r c

oe

ffic

ien

t o

f v

ari

ati

on

in

an

nu

al p

rec

ipit

ati

on 5-yr running coefficients of

variation in mean annual precipitation

WRCC 2009

Meyer and Safford 2012

Tahoe City Huntington Lake

* a common pattern, but not happening everywhere

Current California drought is “worst in 1200 years”

Griffin and Anchukaitis 2014

What is driving recent ecological trends in low and high elevation forests?

1. Fire suppression 2. Changing climate 3. Invasive species and pest outbreaks

Bark beetle mortality driven by drought cycles

Mortality is typically higher in lower

elevation forests, which are more

moisture limited, and where densities are

enhanced by fire suppression

Low

High

Data from Zack Heath, USFS-FHP

…but high elevation forests are experiencing growing outbreaks of pine beetle mortality

Whitebark pine, Inyo National Forest, 2013

Warner Mtns, Modoc NF, 2013 Photo: Danny Cluck

Thus far, beetle outbreaks have been minor compared to Rocky

Mtns. Current drought may change this: 300% increase in

mortality between 2013 and 2014

Invasive annual grasses and fire cycle feedbacks

Cheatgrass is actively invading yellow pine forests on both slopes of the Sierra Nevada. It and red brome have greatly increased fire

frequencies along the Great Basin edge of the Sierra Nevada.

Currently only a problem in lower elevation forests, but for how long?

White pine blister rust

Having major impacts on sugar pine and

western white pine populations, expanding

into whitebark populations in Sierra

Nevada. Other 5-needle pines?

Center of the problem is in lower elevation forests, but whitebark pine infection is

increasing

Jeffrey pine killed by fire in S. California, 6 yrs post-fire with no

regeneration: fire X temperature X drought X pine beetles

Loss of piñon pine near Topaz Lake, western Great Basin: invasive species

X fire X pine beetles

Interactions among stressors are creating threshold conditions on the fringes of the Sierra Nevada

Frequent anthropogenic fire reducing shrubland to grassland: fire X exotic

species X drought

Massive tree mortality in San Bernardino Mtns, early 2000s:

drought X pine beetles X air pollution

C L

I M

A T

E

Moser et al. 2009

The future: modeling projects continued warming and continued transition from snow to rain…

…as well as continued increases in fire activity…

State of California 2009

NRC 2011

Winter minimum temperature °C

High Suitability

Low Suitability

Current LTB

Future LTB

Modeling suggests that climatic warming will bring the Lake Tahoe Basin right into the center of

cheatgrass’ climatic niche by mid-century

…and range expansion of invasive species, pests, and diseases

Velo

z et al. 20

09

Cheatgrass • White pine blister

rust is spreading south

• Sudden oak death is spreading north (and predicted to move east)

• Golden spotted oak borer is spreading north…

• ?

Lenihan et al. 2008

0

5

10

15

20

25

30

35

Current

(1961-

1990)

GFDL-B1

(2071-

2100)

PCM-A2

(2071-

2100)

GFDL-A2

(2071-

2100)

% o

f la

nd

sc

ap

e

subalpine forest and

alpine

evergreen conifer forest

mixed evergreen forest

mixed evergreen

woodland

shrubland

grassland

arid lands

Much drier & much warmer

Same ppt. & warmer

Slightly drier & warmer

Together, these trends will likely lead to major changes in Sierra Nevada ecosystems

Increase in hardwood types, loss in conifer forest;

increase in grassland; major loss of subalpine forest

Summary II

• To this point, low and high elevation forests have experienced similar changes in temperatures and overall meteoric water input

• But rate and nature of forest change have been very different

- Much more relative change at low elevations (forest composition, density, fire frequency, fire severity, etc.)

- High elevation forests much closer to HRV

• Drivers of change - Fire suppression - Climate change (especially snow to rain ratios) - Invasive species, insects, disease

• The effects of all of these are expanding uphill over time…

Are there simple things we can do to increase forest resilience to these interacting

stressors?

General strategies

Reduce forest densities to reduce moisture stress and moderate threat of severe fire

• Strategic use of mechanical thinning and rx fire necessary in areas where human management has greatly changed forest structure (i.e., low elevation forest)…

• but very little of the ground is actually accessible and treatable, so major expansion of “wildland fire use” (WFU; use of wildfires under moderate burning conditions for resource benefit) is likely the only way to treat large areas

• WFU can most easily be implemented in and upwind of large wilderness areas, in higher elevation forests

Use the physical habitat template as a guide to the types and intensities of treatment

• Warm slopes should be thinned heavier than cool slopes (but not overthinned! Understory veg and seedlings need moderating effect of tree canopy)

• Maintain riparian corridors (but don’t ignore the fuels there!)

• Canyons and lower north-facing slopes will likely maintain cooler conditions and should be managed as microclimatic refugia

• Where fire activity is likely, don’t expect upper S and SW facing, windward slopes to sustain forest

General strategies

Expand genetic heterogeneity in the future forest, both within and among species

• Where planting is undertaken, select from a wider genetic pool (e.g., seeds from warmer, drier seed zones)

• Design planting so as to incorporate trials of seedlings of different provenance

• Be careful with assisted migration! Conduct full and open assessment of the likely consequences. High elevation forest trees are probably among the species least likely to cause problems though…

• Consider stressors beyond climate when considering changing forest composition (fire, insects/disease, pollution)

• Remember that the key life stage for successful recruitment is the juvenile stage! Models based on long-lived adult trees may not be as useful as we think

General strategies

Adopt an experimental approach to implementing management actions to respond to global change

• Let’s face it: we don’t know what is going to happen… • …but we do have some reasonable hypotheses • These should be turned into implementable

management actions, developed collaboratively, and tested on reasonably large landscapes

• Example: expand some forest thinning practices from low elevation forests to lower portions of red fir forest

• If we don’t monitor, we learn nothing, and the effort is wasted

• We will make mistakes, but human learning is experiential and mostly fed by unexpected outcomes

General strategies

Don’t give up on restoration!

• “Restoration” = static replication of snapshot photos of 1653: focus on restoring ecosystem processes that are necessary for sustainability with (ultimately) a minimum of human input

• Historical reference conditions may not be appropriate endpoints, but they will often be useful waypoints

• History is only source of information for processes and trends that happen over time-scales longer than the human life; focus should be on understanding mechanics of change

• Focus on restoration of ecological function: single-species and preservation-based strategies have higher probability of failure

• Humans are part of ecosystems and their numbers are increasing: they can’t be removed from the equation

General strategies

THANK YOU