The Status of Aspen
in the Western U.S.
Toni Lyn Morelli
Pacific 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 SDRumble 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
C. Millar
• Slope
Shepperd et al. 2006
Aspen Reproduction
• Mostly clonal
C. Millar
How Aspens Grow
How Aspens Grow
D. Bartos
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
Conifers
Air temperature
Shade tolerance
Soil nutrients
Lush undergrowth
Soil temperature
Soil moisture
Humidity
Aspen Soil Conditions
• Proper soil conditions are critical:
–
–
–
–
–
Mesic
Well-drained
Not too sloped
Not too much light
Mineral soil
• Deteriorates over time
• More selective than conifers
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 CO
since 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)
Is Aspen Declining in the West?
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 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
Conifer Succession
• 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
Gallant et al. 2003
Fire Suppression
Conifer Succession
• 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
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
Climate Change & Aspen
Preferred Climate-Examples
• 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
Aspen Response to ClimateExamples
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
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 goal
Ex: 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