Multiscale Climatic, Topographic, and Biotic Controls of Tree Invasion in a Sub-Alpine Parkland Landscape, Jefferson Park,

Oregon Cascades, USA

Harold S.J. Zald, Oregon State University MTNCLIM 2010 | 06.09.2010 HJA | Blue River | OR

Why Study Tree Invasion in Mountain Ecotones?

• Globally, treeline positions related to thermal conditions• Treeline movement a highly variable response to climate across multiple climate regions, species, & land use histories

• Treeline movement and meadow encroachment may influence: ecosystem productivity, carbon balance, energy budget, hydrological processes, species distributions, and biodiversity

Harsch et al. (2009) Ecology Letters

Gentle Elevation Gradient Treeline

Denali National Park, Alaska

Multiscale Drivers of Tree Invasion

• Tree invasion fundamentally driven by regeneration processes

• Not just climate!

• Biophysical controls: topography, soils, disturbance, seed sources, facilitation, competition, etc.

• Biophysical factors can control spatial patterns & sensitivity to climate

Edaphic “Triple Treeline”

Banff National Park, Canada

Multiple Gradient Subalpine Parkland

Mount Hood, Oregon

Spatial Autocorrelation of Biophysical Controls

Modified from Brooke et al. 1970

• Traditionally, observations of treeline movement &meadow invasion along transects with limited environmental gradients

• Biophysical variablesspatially autocorrelated

• Difficult to untangledrivers

0 50 100 m

Applying New Technologies to Old Questions

• Light Detection and Ranging (LiDAR)

• Landscape characterization of fine-scale vegetation structure & topography

• LiDAR can be used to sample across multiple biophysical gradients at scales compatible regeneration processes

Research Questions

• How have climatic andbiophysical factors controlled recentrates & spatial patternsof tree invasion?

Background:

• PNW tree invasion driven bysnow depth and persistence, believed to determine growing season length(Franklin et al. 1971, Woodward et al. 1995, Rochefort & Peterson 1996)

• Mount Jefferson Wilderness Willamette NF

• Elevation: 1755-1840 m

• ~130 ha

• Tree islands of mountain hemlock & Pacific silver fir

• Geomorphology: glacial &

neoglacial debris flows

• No known fires

• Unknown grazing history

Study Area: Jefferson Park, OR

Jefferson Park

HJA

LiDAR Derived Biophysical Variables

Bio

Overstory canopy

Influences snow depth & persistence,seed sources

Physical

Topographic position, elevation, radiation

Influence snow depth & persistence

Landform (glacial v. debris flows)

Disturbance, but also influences other biophysical variables

LiDAR Driven Sampling

Sample along biophysical gradients believed to influence snow depth

& persistence

Topography (5 Classes)

Distance to overstory canopy(5 classes)

Combine grids (5 x 5 = 25 classes)

100 x 100 m moving window(20 clusters)

Stratified random sample(25 points per cluster)

100 x 100 m cluster

• Mapping of overstory canopy by species

(potential seed sources)

• Spatial autocorrelation between explanatory variables accounted for

• Landscape-level estimates of invasion possible

LiDAR Driven Sampling Continued

• Points located with sub-meter GPS

• 2 m diameter plots 390 on glacial landform 109 on debris flows

• Snow depth survey July 29- Aug 1, 2008

• All trees < 8m tall tallied by species& height (1620 trees)

• 505 trees aged

Field Data

Spatial Patterns

• Snow depth in relation tobiophysical controls

• Tree abundance in relation to biophysical controls

Temporal Patterns

• Tree invasion over time

• Tree invasion and climate

Interactions of Climate &Biophysical Controls

Flow of Results

More snow with less radiation, lower elevation, distanceMore linear, reduced interactions, less variance described

Mean:0.2m

95% CI:0.1-0.3m

Deb

ris F

low

sG

laci

al L

andf

orm

s Mean:0.67m

95%CI:0.6-0.8m

More snow in depressions, lower elevations, distance from overstory Nonlinear interactions between explanatory variables

Results: Snow Depth & Biophysical Controls

Results: Multi-Scale Controls of Snow

Landscape context is important

• Larger landforms influence both overall snow depth and micro site controls of snow

• Smoother surface on debris flows

• Greater windredistribution of snow on smoother debris flows

Results: Tree Abundance & Biophysical Controls

• Mountain hemlock associated with microtopography and distance to overstory canopy

• Silver fir strongly associated with distance to potential seed sources, followed by microtopography

• Relationships between tree invasion and biophysical variables much stronger on glacial landforms

Glacial landforms Debris flow

Results: Temporal Patterns of Tree Invasion

• Not just an increase in densities1950: 7.8% of meadow area with tree invasion 2008: 34.7% of meadow area with tree invasion

• Invasion dominated by Mountain hemlock, Pacific Silver Fir restricted to under existing trees

• Invasion rategreater ondebris flows(0.75% v. 0.57% Yr-1)

• Annual snowfallmost important, not temperature

• On debris flows tree invasion not associated with

annual snowfall

• On debris flows, positive association with spring snowfall!

Results: Climate, Landforms, & Invasion

Glacial Landforms

Adj R2 = 0.2887p ≤ 0.01

Debris Flows

Adj R2 = -0.0356p = 0.5

Only For Hemlock on GlacialLandforms

• Hemlock invasion largely in years with low snow on ridgetops & midslopes

• During high snow years, less invasion overall & constrained to ridgetops

Results: Climate, Microtopography, and Invasion

• Snow and tree invasion associated with multi-scale landscape controls

• Species matter

• Landforms & topography alter both the spatial patterns of tree invasion & response to climate

• Tree invasion on debris flow landforms

• Scale & landscape context matter

• Multiscale and context dependent responses pose problems for modeling future responses to climate at regional and global scales

• Need for experimentation (future climate now)

Conclusions

Acknowledgements

Funding provided by:

USDA Forest Service, Pacific Northwest Research StationUSDA Forest Service, Forest Inventory and Analysis ProgramThe Native Plant Society of OregonHoener Memorial Fellowship, OSUWaring Travel Grant, OSU

Thanks to field assistants:Dan IrvineAlex Gonsiewski

Questions?