treeline dynamics and
climate change
MALANSON, G.P., BOURGERON, P., BUNN, A., BUTLER,
D.R., DANIELS, L., FAGRE, D., HIEMSTRA, C., LIPTZIN, D.,
MILLAR, C.I., PETERSON, D.L., RESLER, L., SHEN, Z.,
SMITH, W.K., TOMBACK, D.F., WALSH, S.J., WEISS, D.
USGS Western Mountain
Initiative Workshop:
Alpine Forest-Tundra
Ecotone
3-D:
elevational gradient
patchiness
heights
http://unrnet.seismo.unr.edu/Aerials/Mt-Hood.jpg
Alpine Treeline Ecotone
15
m
Timberline
Seedling/Sa
pling
Forest Tree
←Wind Direction
Treeline
Krumholz Mats
Intact Forest
Ribbon
Forest
Flagged trees Mats with few
with mats
flagged trees
Mats only
Bill Smith’s slide
State
Alpine treeline may respond nonlinearly
to climatic change
e
at
li m
C
Vegetation
Time
Where the critical points are is unknown
AFTE: change in any or
all dimensions
upper, lower limit
infilling, density
layering
leader growth
dieback
Purpose: assess ecotone change
from a Plant’s-Eye-View
Apply an environmental sieve
to examine roles of negative
& positive feedbacks
Individual
scale
What does a
seed experience?
What does it
need in tundra?
Germination &
establishment
microsites
soil
microclimate
snow burial
vs. disturbance (predation)
Pluses and minuses of solifluction:
Stony surface vs. Dense vegetation
Concentrated fines in turf exfoliation -
Differential success:
from Lynn Resler
Roles for birds, mammals
Geomorphic processes and
micro-landforms are important
sieves for seeds and germination
from a plant’s-eye-view
Occupation of Initial Position at High
Elevation/Highly Exposed Sites
from Lynn Resler
Pseudotsuga menziesii
2%
Pinus contorta
5%
Juniperus horizontalis
1%
Juniperus communis
6%
Pinus albicaulis
39%
Abies lasiocarpa
23%
Picea engelmannii
24%
Patch scale
vegetation pattern
≠
soil pattern
+ Feedbacks
Microclimate
lower albedo
warmer temperatures
less wind
snow accumulation
reduced PET
less sky exposure (UV, Q1)
Soil
trapping of fine sediment
input of organic matter, nutrients
Strong, directional winds:
Hourly Average Wind Direction and Temperature
Hourly Maximum and Average Wind Speed
Lee High Weather Station
Dec. 21, 2000 - March 21, 2001
150
360
135
270
90
225
75
60
180
45
135
30
15
90
0
45
-15
-30
0
355
356
357
358
359
360
361
362
363
364
365
366
1
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80
Degrees Celsius
Kilometers per Hour
105
Julian Day
Average Wind Speed KmH
Average Air Temperature
Max Wind Speed KmH
Average Wind Direction
Compass Degrees
315
120
Wind, and effects,
altered by trees
Average Wind Speed
(m/s)
Average Wind Speed, Tree & Tundra
7
6
5
4
3
2
1
0
1715
815 2315 1415
515 2015 1115
215 1715
815
Time of Day
2315 1415
515 2015
Landscape scale
Pattern and process
Pattern and process
Disturbance
Pattern and process
Predation/disease
Pattern and process
Wind
Can a simple CA capture the
process-pattern feedbacks at
AFTEs?
Could it then generally
indicate trends for climate
change?
Feedback distance decay:
+ feedback
- feedback
-feedback
decays faster than
+feedback
In addition to lower soil temperatures,
too much snow,
shorter season
indurated duff
P ro b a b ility
Tree Establishment Probability
1.2
1
0.8
0.6
0.4
0.2
0
Probability=
0
0.2
0.4
0.6
α
−γ (
1+ β e
0.8
2
x −x )
1
1.2
Average size index
- feedback cancels + feedback next to trees
A single, simple model produces
all (?) the patterns that we
observe at treeline
The model shows fractal dynamics
linking the rate of advance and spatial
pattern
Advance & Fractal D
160
1.5
140
120
100
1.4
80
1.35
60
1.3
Fractal D
40
Advance
20
1.25
0
1
101
201
301
Iteration
401
501
New Trees
Fractal D
1.45
Same general results obtain when the surface is
more variable – either randomly or fractally
With a rapid climate amelioration in the middle
of the time series, the rate of advance briefly
increases but then returns to self-organized
fractal dynamics
250
Advance Potential
200
150
100
50
0
1
101
201
301
401
501
601
701
801
Iterations
901 1001 1101 1201 1301
Regional scale
climate
geology
biology
Climate
maritime vs
continental
timberlinelodge.com
and aspect
Geology
substrate controls
albedo
texture
frost shattering
periglacial
Biology
pines?
krummholz?
‘Northern’ Rocky Mountains
Waterton - Glacier National Parks
Whitebark pine vs. subalpine fir
Upslope advance of tree species
since end of Little Ice Age
temperature effect
in recent decades
moisture effect
Increased density, growth
temperature effect
Southern Rocky Mountains
ROMO, Niwot Ridge, Medicine Bow
Upslope advance of tree species
New patches observed
Migrating tree islands
Increased density, growth?
Important aeolian inputs
Southern Sierra Nevada
Species differentiation
Growth in krummholz
Establishment in snowfields (& meadows)
since end of Little Ice Age
moisture effect
Dieback
moisture effect: drought decades
Pacific Northwest
Species: hemlock or fir
Changes in growth and establishment of
trees near treeline
strong PDO signal
Aspect and resulting moisture changes
most important
Olympic sites are wet
wetter – too much snow?
Other regional sites are dry
wetter is better
but
Biologically meaningful water:
seasonality is crucial
AET must be able to increase
P ro b a b ility
Tree Establishment Probability
1.2
1
0.8
0.6
0.4
0.2
0
Probability=
0
0.2
0.4
0.6
α
−γ (
1+ β e
0.8
2
x −x )
1
1.2
Average size index
Actual height and shape a function of climate,
geology, and biology (water, substrate, species)
Indicators?
too many directions
initial conditions
climate change
warmer/wetter/drier
cycles
seasonality
even direction
What are we doing in here?
USFS photo