Complex Interactions Shaping Aspen Dynamics in the Greater Yellowstone Ecosystem

1
Complex Interactions Shaping Aspen Dynamics in
the Greater Yellowstone Ecosystem

• Brown, K., A.J. Hansen, R.E. Keane, L.J.
  Graumlich. 2006. Complex interactions shaping
  aspen dynamics in the Greater Yellowstone
  Ecosystem. Landscape Ecology. Landscape Ecology
  21933951

2
Widespread Aspen Decline

• Aspen loss documented across Rocky Mountain
  ecosystems
• Fire exclusion
• Conifer encroachment
• Increased elk herbivory
• Climatic fluctuations

3
Regional-scale Studies Dispute Decline

• Regional-scale studies in Colorado actually show
  increasing/stable aspen
• -Kulakowski et al. In Press
• -Kaye et al. 2003
• -Manier Laven 2002
• -Suzuki et al. 1999
• Several small-scale studies in GYE show decline
• Lack a regional perspective for aspen in the GYE

4
Aspen in the Greater Yellowstone Ecosystem

• Small scale studies in areas with high elk
  densities show decline
• Jackson Hole valley
• YNP Northern Range
• Need for regional perspective
• 45 loss in Gravelly range
• 4,000 ha loss in YNP
• 75 loss in Centennials
• 4 loss in Northern Range
• Base-line needed to understand
• Fire
• Herbivory
• Competition

make a series of maps to bring in -plain study
area outline w/ hillshade, precip, dday, lith
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What are the Influences on Aspen?
Fire exclusion, herbivory, competition
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Objectives

• Distribution
• Does aspen occupy specific biophysical settings?

• Landscape Change
• Is aspen loss associated with particular
  biophysical settings?

• Growth Rates
• How do aspen growth rates (ANPP) vary along
  biophysical gradients?

7
Characterizing the Biophysical Environment
Climate Regime
Weather
?Tave ?TDay ?TNight ?Tsoil ---PPT ?DDay ?Dsr ?Dss
?Srad.tg ?Srad.fg ? Ppfd
?Tmax ? Tmin ? PPT ? VPD ? Rh ? SRAD
WXFIRE
Ecosystem
Polygon
? Elevation ? Aspect ? Slope ? Soil Depth ?
Sand ? Silt ? Clay ?TopoShading ? LAI ? site
type (pft)
? AET ? PET ? PSI ? PSI.MAX ? EVAP ? TRANS ?
VMC ? GSWS ? OUTFLOW ? SNOW
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Objectives

• Distribution
• Does aspen occupy specific biophysical settings?

• Landscape Change
• Is aspen loss associated with particular
  biophysical settings?

• Growth Rates
• How do aspen growth rates (ANPP) vary along
  biophysical gradients?

9
Aspen Distribution

• Compiled digital maps of aspen distribution
• (78 overall accuracy)
• Aspen occupy 1.4 of the mapped land area in GYE
• Aspen much more prevalent in the southern GYE

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Aspen Distribution
11
Aspen Distribution

• Built a classification and regression tree (CART)
  model
• Used the model to predict aspen presence for the
  validation datasets
• Accuracy assessment of the model

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Aspen Distribution CART Analyses
13
Aspen Distribution CART Analyses

• Aspen occupies a narrow range of available
  biophysical settings
• high radiation
• intermediate high PET
• intermediate snowfall, temperature
• Model performed well at classifying aspen where
  it was present (high producers accuracy)
• Model over-predicts the occurrence of aspen (low
  users accuracy)
• Aspen does not occur in all biophysically-favorabl
  e locations

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Aspen Distribution
15
Aspen Distribution
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Aspen Distribution
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Aspen Distribution
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Aspen Distribution
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Aspen Distribution -- Conclusions

• Aspen is rare in the GYE 1.4 of mapped land
  area
• Much more aspen in southern GYE seems to be
  related to higher GS radiation
• Fire?
• Aspen restricted to a narrow range of biophysical
  settings
• high radiation, intermediate snowfall,
  temperature, PET
• but not found in all suitable biophysical
  settings
• Other factors may restrict current range
• fire
• establishment events

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Objectives

• Distribution
• Does aspen occupy specific biophysical settings?

• Landscape Change
• Is aspen loss associated with particular
  biophysical settings?

• Growth Rates
• How do aspen growth rates (ANPP) vary along
  biophysical gradients?

21
Landscape Change

• Air photo interpretation from early (1955 1963)
  and current (1992 2001) time periods
• ANOVA, regressions

22
Landscape Change
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How Do Aspen Loss, Gain, Stable Plots Differ?
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Landscape Change

• Used AICc to select variables for a final
  regression model

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Landscape Change

• Selected Model
• Aspen Change
• -4.93 Snowfall
• -6.13 Conifer Change
• -9.22 (Intercept)

AICc 1414 AICc Weight 0.14 R2 0.18
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Where is Aspen Declining?

• Moisture
• Low summer precipitation and high winter
  precip./snowfall
• Temperature
• Cooler, shorter growing season
• Light
• Higher radiation
• Water Availability/Flow
• More arid sites with higher evapotranspiration,
  run-off, and lower water retention

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Landscape Change - Conclusions

• 34 of plots declined in aspen cover
• Averaged 10 decline overall
• Areas that gain aspen cover warmer and wetter
  during the growing season than loss areas
• Loss areas drier during the growing season but
  higher annual precip. and snowfall
• Lower radiation levels associated with aspen gain
• Other factors?
• insects/disease outbreaks defoliation
• fire suppression mature stems senesce w/ no
  replacement

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Objectives

• Distribution
• Does aspen occupy specific biophysical settings?

• Landscape Change
• Is aspen loss associated with particular
  biophysical settings?

• Growth Rates
• How do aspen growth rates (ANPP) vary along
  biophysical gradients?

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Aspen Growth Rates

• Used aspen ANPP as an index of aspen performance
• increment cores for growth rates
• density and size of aspen to estimate biomass

• Density and size class data for trees and shrubs
  used to estimate biomass of competing vegetation
• Soil characteristics
• soil moisture
• NH4
• NO3-

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Aspen Growth Rates

• Used AICc to select variables for the final
  regression model

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Aspen Growth Rates
Model selection results for ANPPN 107 sites

• Selected Model
• Aspen ANPP 2255 GS Precip. X Ann. Min.
  Temp
• 1902 Clay
• 1829 Annual Minimum Temperature
• -970 Conifer Biomass
• 378 GS Precipitation
• 4536 (Intercept)
• AICc 2130.37
• AICc Weight 0.71
• R2 0.37

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Growth is not Maximized in Current Distribution
33
Growth is not Maximized in Current Distribution
34
Aspen Growth Rates - Conclusions

• Growth rates are fastest where summer precip. is
  high, temperatures are warmer, and on clay soils.
  High conifer biomass is negatively associated
  with aspen growth.
• High ANPP at the tail edge of aspens
  distribution
• Competitive exclusion from favorable sites
• Possibly more susceptible to herbivory
• 37 of the variance explained with biophysical,
  other influences
• Competition
• Defoliation - insects

35
Conclusions
36
Conclusions

• Aspen is rare in the GYE, 1.4 of land area
• Aspen does not occupy all biophysically favorable
  settings
• Decline is less widespread than local-scale
  studies suggest.
• Loss of aspen cover appears to be localized to
  particular environmental condiditons.
• Possible constriction of aspen distribution
• fire
• herbivory
• competition
• Aspen may not currently occupy full range of
  abiotic tolerances and performance may not be
  optimized in current locations
• Aspen in the GYE likely more susceptible to
  herbivory, competition, fire exclusion as a
  result

37
Acknowledgements

• Committee
• Andrew Hansen
• Lisa Graumlich
• Robert Keane
• National Park Service
• Roy Renkin
• USGS-BRD
• Don Despain

• Bridger-Teton National Forest
• Tim Kaminski
• Field crew/Technicians
• Steve Wilcox
• Chris Brown
• Casey Sensesqua
• Funding
• Fire Sciences Lab, US Forest Service
• National Park Service
• University of Wyoming-NPS Co-op Unit
• Bridger-Teton National Forest