The Status of Aspen in the Western U.S.

Toni Lyn MorelliPacific Southwest Research Station

USDA Forest Service

Aspen Basics

•

Populus

tremuloides

•

Most widespread in NA

•

Clonal–

Hardy

•

Require high resource environment

•

Seral

or stable–

1/3 of western aspen may be stable C. Millar

Aspen’s Value

•

Keystone species, biodiversity hotspot–

2nd

only to riparian areas

–

Plant diversity–

Bird diversity (no advantage of conifers in SD-

Rumble et al. 2000)

•

Structural diversity

•

Soil quality

Bill Banazewski - FWS

Aspen’s Value

•

Water conservation–

Greater snow accumulation

–

Understory–

Low water use efficiency

• wildfire risk

–

200x less likely

•

Forage

•

Aesthetic value

Bill Banazewski - FWS

Aspen Habitat Types Ryel

& Bartos

2008

•

Lithic

(including talus)

•

Riparian

•

Meadow fringe

•

Slope

C. Millar

Shepperd

et al. 2006

Aspen Reproduction

•

Mostly clonal

C. Millar

D. BartosHow Aspens Grow

How Aspens Grow

Aspen Reproduction

•

Mostly clonal

•

Rarely from seeds in the West–

short-lived

–

occurs after severe fires–

specific temperature and precipitation requirements (high P)

–

important in marginal stands or areas with suitable habitat but no roots

–

important in changing climates?

Aspen’s Resource Requirements

Aspen

Air temperature

Soil nutrients

Lush undergrowth

Soil temperature

Conifers

Shade tolerance

Soil moisture

Humidity

•

Proper soil conditions are critical:–

Mesic

–

Well-drained–

Not too sloped

–

Not too much light–

Mineral soil

•

Deteriorates over time

•

More selective than conifers

Aspen Soil Conditions

Verbyla - UAF

Sierra Nevada History

•

Mining and Logging in late 19th

century

•

Sheep grazing = most widespread disturbance

Fewer aspen(1%)++

Is Aspen Declining in the West?

Yes:

–

50-96% decline in aspen throughout the U.S.(Bartos

2001)

–

75% reduction of aspen/3x conifers in COsince 1850s (Gallant et al. 2004)

–

50% decrease in aspen 1947-1992 in MT, primarily from conifer invasion (Wirth et al. 1996)

–

Aspen used to be more extensive Sierra Nevada <1850 (tree-ring study-Potter 1998)

But…–

No large change in much of the GYE (Brown et al. 2006)

–

Positive change in some parts of CO since 1900, with some recent conifer succession (photographic analysis –

Manier

& Laven

2001)

–

HRV?

–

Unusual aspen extent since 1880s?(Kulakowski

and others)

Is Aspen Declining in the West?

Is Disturbance Necessary?

1) Yes:

Apical dominance prevents suckering-

Low elk herbivory

hypothesis (White et al. 2003):

2) No:

Disturbance is not necessary to trigger sprouting but eliminates competition-

Stands w/o conifers regenerate w/o disturbance

-

Replacement dieback hypothesis (Mueller-Dombois

1992)

+ =

Is Disturbance Necessary?

Maybe Yes and

No

Stable

Seral

Decadent

(Bartos

& Campbell 1998) C. Millar

•

As aspen age, soil–

loses its organic layer

–

leaches nutrients–

Becomes more acidic and thinner and colder

Conifers don’t mind•

Happens early and late

•

Once conifers establish, aspen decline–

Too much shade

–

Different soil

Conifer Succession

Conifer Succession

Gallant et al. 2003

•

Seral

aspen are fire-dependent

•

Rare fire events (1/50 y) can replace a grove–

Even if just a few aspen left!

•

In CA, conifers are replacing aspen groves due to less frequent fire (and grazing pressures)

•

In some cases, frequent fire historically has depleted the conifer seed bank, preventing succession even in the absence of fire

Fire Suppression Conifer Succession

General Aspen Decline

+

Sudden Aspen Decline

David Burton

Sudden Aspen Decline(Shepperd

2008, Worrall et al. 2008)

= the death of mature aspen with little or no regeneration

•

UT and AZ >2002

•

Rapid and simultaneous

•

1-3 years

•

White defoliated trees standing w/ bark

SAD-Numbers

•

SW CO, N Arizona, parts of UT and Canada, + ID, NV, MT, S WY

•

Average mortality of aspen in the IM region in 2006-2007 was 31%, 2/3 within 2 years

•

56,091 ha affected in CO (aerial survey) •

Mortality in 4 stands up to 566% in 3-4 yrs

•

~13% of CO aspen cover showed effects by 2007 –

140,000 acres lost in San Juan area alone by 2009

•

Outside of the inner West, effects are unclear:–

recent survey in E WA showed no sign of SAD in 2 NFs

Worrall et al. 2008

SAD-Characteristics

•

Large trees die first? -

does not affect young•

Starts at edge of grove

•

Complete defoliation + branch dieback•

Roots are dying first? no regeneration

(may see delay of effect for a season)•

Most affected:–

Low elevation •also, fewer conifers at higher elevations

–

S & SW aspects–

Lower slope areas?

SAD-Causes

•

Drought and high temperatures (Worrall et al. 2008)

–

Early 2000s in IM West and W Canada–

Similar effects from a 1961 drought in W Canada

•

Fine root damage caused by extreme winter freeze followed by drought could reduce water and nutrient uptake

Climate changes seem to be causing SAD

Frey et al. 2004

Frey et al. 2004

SAD-Causes

Decline Disease Hypothesis(Frey et al. 2004, Worrall et al. 2008)

•

Defoliation/severe drought + high temp during the growing season

•

Stand/site factors predispose

•

Certain insects/pathogens contribute

SAD

•

Cytospora

canker:–

the most common fungus found on aspen, particularly weak, injured, or stressed trees, is normally not serious

•

Poplar borer: –

infest all age-classes but especially disturbed stands or stressed trees and are apparent from signs of ejected frass

and dried sap that accumulate around

the hole •

Bark beetle: –

affect dead bark or stressed trees fairly commonly, mining the bark on the trunk and large branches

(These are species that do not normally kill aspen.)

Johnston et al. 1995

What’s Happening in Colorado’s Aspen Forests? 2007

Climate Change & Aspen Preferred Climate

•

Aspen are limited by–

Temperature

–

Precipitation, including snow–

Radiation

Bright, warm (especially soil), and wet

•

Better at southern aspects at high elevations (and vice versa)

•

Otherwise stress tolerant–

Leaves

–

Clonality

•

GYE: aspen = 1.4% of area, found at average:–

Elevation = 2300 m

–

Annual precipitation = 70.6 cm

–

Temperature = 2.1°

C

•

Western CO (similar to SN -

fire is hist. rare)–

Aspen better at mid-elevations (2200-3000m)

–

If solar radiation is high, aspen found where•growing season PET < 120 cm•Annual Tmax

>6.9°

C

•Slope >2.5°And vice versa

Climate Change & Aspen Preferred Climate-Examples

Aspen is water-limited, drought-intolerant–

moisture deficits in w. Canadian interior have a more negative effect on boreal aspen than insects, even severe forest tent caterpillar outbreaks (Hogg and colleagues)

–

Pathogens and herbivores interact

with environmental stress

–

e.g., SAD

Aspen Response to Climate- Examples

Climate Change & Aspen Response to future climate

•

Drier = bad•

Hotter = good and bad

•

More fire = good•

More CO2

= mixed: •

A modelling

example:

aspen in the Canadian boreal will increase productivity for the next 200 years, acting as a large carbon sink, as long as prolonged droughts did not occur (Grant et al. 2006)

Managing Aspen•

Healthy aspen stands should have multiple age classes mostly younger than 100 years, adequate sprouting, and an ample herbaceous layer underneath the canopy

•

5 main risk factors for aspen:–

conifer cover (understory and overstory) > 25%

–

aspen canopy cover < 40%

–

dominant aspen trees > 100 yrs old

–

aspen regeneration < 500 stems per acre (5-15 ft tall)

–

sagebrush cover > 10%

Shepperd

2001

Managing Aspen•

Prioritize treatments where conifers are taller and live aspen present in the overstory

•

Set a goalEx: 2000-5000 stems/acre in 10 yrs and annual height

increase

•

Set up an effective monitoring plan–

Random, non-permanent, circular-shaped plots

•

Reintroduce natural ecological processes (e.g., fire, carnivores) where possible

•

Reduce and rotate livestock

Management Solutions

Increase Disturbance•

Prescribed burns–

preferred treatment: returns nutrients to soil, especially important if conifers have been there for a while

–

Difficult–

Careful if there are a lot of conifers and high fuel loads: very

hot fires can damage roots and reduce reprouting

•

Logging/cutting–

Careful of soil compaction and deep cutting –

should all be done in 1 yr

–

Clearcutting

better than partial cutting (shading, suppression,etc)

•

Other options:–

Bulldozing–

Hand thinning–

Ripping (sever or scrape roots): particularly good method for meadow fringe aspen

Management Issues: Animal Impacts

•

Elk are the biggest problem for aspen•

Deer and cattle/sheep together might potentially devastate aspen

•

Livestock grazing can negatively affected aspen regeneration (White et al. 1998)

•

Effects of wild ungulates and livestock can be reduced by managing around seasonal use patterns and at local scales

•

Certain areas will be more highly impacted due to browse availability –

Consider snow depths

Kilpatrick & Abendroth 2001

Management Solutions

Protect•

Herbivore exclosures–

Smaller exclosures better

–

Fence outside of previous grove edge–

choice of exclosure

will depend on browsing

pressure, which varies by site and year –

important to limit use of treated areas by domestic animals for several years •Short-term issue, long-term gain

–

Could help to treat large area (but Yellowstone NP)

Aspen Management Tradeoffs

•

Birds need dead/dying aspen stems & conifer snag•

Too much harvesting, building of new roads

•

Conifer succession is a natural process and future natural disturbances will be sufficient

•

Since fire frequency will be increasing with climate change, no proactive measures should be taken

•

Fire and other disturbances may encourage invasives–

nonnative species were found in aspen ecosystems much more commonly than other vegetation types in a study in CO

•

In some cases, a solution could be to use targeted, smaller fires, which can still have desirable effects

Ongoing & Recent Aspen Projects

•PSW Lake Tahoe Basin Management Unit Aspen Community Restoration Project

•Evaluation of Pine Creek Aspen Stand •Ochoco

National Forest

•Aspen Dahlgreen

Prescribed Fire•Shoshone National Forest•Lassen National Forest•Aspen Delineation Project•Pike and San Isabel National Forests•Arapaho-Roosevelt National Forest (Northern CO)•PRBO Conservation Science Sierra Nevada Program

Aspen Web Resources

•

Aspen Delineation Project: www.aspensite.org/

•

Riparian Bird Conservation Plan: www.prbo.org/calpif/htmldocs/riparian.html

•

US Forest Service RMRS Aspen Restoration Site: www.fs.fed.us/rm/aspen/

•

US Forest Service PSW Sierra Nevada Research Center: www.fs.fed.us/psw/programs/snrc/

•

Northern Rockies Aspen Working Group: www.aspensite.org/NorthernRockiesAspen.html

•

Western Association of Fish and Wildlife Agencies (WAFWA) Habitat Committee: www.wafwa.org

•

Western Aspen Alliance (WAA): www.western-aspen-alliance.org