Climate-induced Threshold Responses
in Rangelands
David D. Breshears1,2,3 and Steve Archer1,2
1School of Natural Resources, 2Institute for the Study Planet Earth,
and 3Department of Ecology & Evolutionary Biology
The University of Arizona
daveb@email.arizona.edu and sarcher@ag.arizona.edu
Beyond Boxes and Arrows Assessing Climate Change/Variability and Ecosystem Impacts/Responses
in Southwestern Rangelands
San Carlos - January 25, 2006
How does vegetation change?
How does vegetation change?
By a simple progression
Succession
How does vegetation change?
By a simple progression
Disturbance
Succession
How does vegetation change?
By a shifting from state to state
How does vegetation change?
By a shifting from state to state
- with variation within a state
Why is this stuff so tough to predict?
“Erasure fight!”
Larson - The Far Side
Biotic
Processes
Abiotic
Processes
Feedbacks
Biotic
Processes
Abiotic
Processes
Feedbacks
External Abiotic
Forcings
Biotic
Processes
Abiotic
Processes
Feedbacks
External Abiotic
Forcings
Biotic
Processes
Abiotic
Processes
Biotic Inertia
Abiotic Inertia
Biotic Inertia
Under
Disturbance
Biological Inertia
External Abiotic
Forcings
Biotic
Processes
Abiotic
Processes
A Growth Threshold at ~40 Years
Juniper Age x Canopy Area
250
Canopy Area (m2)
Miller et al.In prep
200
Rapid Canopy Closure
150
100
50
0
0
20
40
60
80
100
120
Tree Age (y)
Miller & Tausch 2001
Growth threshold dependent on land management
Plant height (cm)
60
Prosopis growth
40
grazed
site
20
N=800
0
2
protected
site
4
Plant Age (y)
6
8
from Archer 1995
Biological Inertia
External Abiotic
Forcings
Biotic
Processes
Abiotic
Processes
Shading Patterns
26%
34%
I TOP
Path Length, P
Foliar Density, F
I BOTTOM = I TOP e - F P
Martens et al. (2000) – Eco. Model.
42%
Shading Effects on Soil Water
Near ground differences in incoming solar radiation
produce greater soil evaporation rates in intercanopy
July
Soil water potential Ψ (MPa)
0.0
35 %
25 %
15 %
-0.5
Canopy
-1.0
-1.5
Intercanopy
-2.0
0
4
8
12
16
20
Time of day (hour)
Martens et al. (2000)
Ecol. Model.
Breshears et al. (1998)
Int. J. Plant Sci.
24
Martens et al. (2000) - Ecol. Model.
Number of HOT
locations
sensitive to
changes in
canopy cover
Martens et al. (2000) - Ecol. Model.
Abiotic Inertia
External Abiotic
Forcings
Biotic
Processes
Abiotic
Processes
Runoff and Runon
Runoff from bare patches becomes
runon to herbaceous patches
Runon
( mm )
10
9
8
7
6
5
4
3
2
1
0
0
10
20
30
40
50
Precipitation ( mm)
Reid et al. 1999
60
70
Canopy patch with storage.
Intercanopy patch with storage.
Intercanopy patch with no storage.
Davenport et al. 1998 – J. Range Mgmt.
Percolation Rules
Downslope
Patch-scale Runoff :
• generated on bare cells.
• redistributed to neighboring lateral or downslope cells.
Hillslope-scale Runoff:
• function of clusters connected to the bottom of slope.
Davenport et al. 1998 – J. Range Mgmt.
Low connectivity
Downslope
Hillslope runoff
Canopy patch with storage.
Intercanopy patch with storage.
Intercanopy patch with no storage: no contribution to hillslope runoff.
Intercanopy patch with no storage: contributes to hillslope runoff.
Davenport et al. 1998 – J. Range Mgmt.
Low connectivity
High connectivity
Downslope
Hillslope runoff
Hillslope runoff
Critical threshold of
bare ground cover
exceeded
Davenport et al. 1998 – J. Range Manage
Biological Inertia Under Disturbance
External Abiotic
Forcings
Biotic
Processes
Abiotic
Processes
Biological Inertia Under Disturbance
External Abiotic
Forcings
Biotic
Processes
Abiotic
Processes
Drought-induced Ecotone Shift
Forest Area (%)
50
40
30
20
10
0
1930
1940
Allen & Breshears (1998) – Proc. Natl. Acad. Sci.
1950 1960
Year
1970
1980
Drought-Induced Increases in Erosion
Allen & Breshears 1998 • Allen, Wilcox & Breshears – In prep.
Drought-triggered Tree Mortality
May 23
Breshears, Myers & Barnes – in prep.
Drought-triggered Tree Mortality
May 23
Aug 19
Breshears, Myers & Barnes – in prep.
Drought-triggered Tree Mortality
May 23
Aug 19
Breshears, Myers & Barnes – in prep.
Oct 29
October 2002
Photo: C. D. Allen
May 2004
Photo: C. D. Allen
Photo: C. D. Allen
Site-specific Die-off
Warmer
Temperatures
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Site-specific Die-off
Reduced
Precipitation
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Site-specific Die-off
Persistent
Low
Soil Moisture
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Site-specific Die-off
More than
90%
Tree Mortality
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Die-off and NDVI
Increase
in
Mortality
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Die-off and NDVI
Reduced
NDVI
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Die-off and NDVI
Reduced
Average NDVI
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Near Flagstaff
Photo: N. S. Cobb
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Near Flagstaff
Photo: N. S. Cobb
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Regional NDVI Change
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Die-off and NDVI
Reduced
Regional
Average
NDVI
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Verification
Aerial surveys
by USGS
and
research plots
Breshears et al. (2005) Proc. Natl. Acad. Sci.
2000s vs. 1950s Drought
2000s:
Not drier
but
warmer
Breshears et al. (2005) Proc. Natl. Acad. Sci.
2000s vs. 1950s Drought
2000s:
Not drier
but
warmer
Breshears et al. (2005) Proc. Natl. Acad. Sci.
2000s vs. 1950s Drought
2000s:
Warmer
Breshears et al. (2005) Proc. Natl. Acad. Sci.
2000s vs. 1950s Drought
2000s:
Warmer
Breshears et al. (2005) Proc. Natl. Acad. Sci.
Global-change-type Drought
Drought under warmer conditions
Global-change-type Drought
Drought under warmer conditions
Mortality through wetter sites rather
than just at drier ecotones
Global-change-type Drought
Drought under warmer conditions
Mortality through wetter sites rather
than just at drier ecotones
Regional-scale threshold response
Ecosystem Cascades
Ponderosa Pine
Forest
Ecosystem Cascades
Ponderosa Pine
Forest
Pinyon-Juniper
Woodland
Ecosystem Cascades
Ponderosa Pine
Forest
Pinyon-Juniper
Woodland
Juniper
Savanna
Ecosystem Cascades
Ponderosa Pine
Forest
Pinyon-Juniper
Woodland
Juniper
Savanna
Reduced canopy cover
Ecosystem Cascades
Ponderosa Pine
Forest
Reduced canopy cover
Reduced herbaceous cover
Pinyon-Juniper
Woodland
Juniper
Savanna
Ecosystem Cascades
Ponderosa Pine
Forest
Reduced canopy cover
Reduced herbaceous cover
Pinyon-Juniper
Woodland
Juniper
Savanna
Increased soil
temperatures and
evaporation
Ecosystem Cascades
Ponderosa Pine
Forest
Reduced canopy cover
Reduced herbaceous cover
Pinyon-Juniper
Woodland
Juniper
Savanna
Increased soil
temperatures and
evaporation
Increased erosion
How does vegetation change?
Early Warning
Feedbacks
Threshold
A
External Abiotic
Forcings
Axis II
B
C
D
Axis I
(from NRC 1994)
Biotic
Processes
Abiotic
Processes