Climate-induced induced Threshold Responses in Rangelands David D. Breshears 1,2,3 and Steve Archer 1,2 1 School of Natural Resources, 2 Institute for the Study Planet Earth, and 3 Department 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 Biotic Inertia Abiotic Inertia Biotic Inertia Under Disturbance Abiotic Processes
Biological Inertia External Abiotic Forcings Biotic Processes Abiotic Processes
A Growth Threshold at ~40 Years 250 Miller et al. In prep Juniper Age x Canopy Area Canopy Area (m 2 ) 200 150 100 50 Rapid Canopy Closure 0 0 20 40 60 80 100 120 Tree Age (y) Miller & Tausch 2001
Growth threshold dependent on land management Plant height (cm) 60 40 20 Prosopis growth N=800 grazed site protected site 0 2 4 6 8 Plant Age (y) from Archer 1995
Biological Inertia External Abiotic Forcings Biotic Processes Abiotic Processes
Shading Patterns 26% 34% I TOP Path Length, P Foliar Density, F 42% I BOTTOM = I TOP e - F P Martens et al. (2000) Eco. Model.
Shading Effects on Soil Water Near ground differences in incoming solar radiation produce greater soil evaporation rates in intercanopy Martens et al. (2000) Ecol. Model. Soil water potential Ψ (MPa) 0.0-0.5-1.0-1.5 July 35 % 25 % 15 % Intercanopy -2.0 0 4 8 12 16 20 24 Time of day (hour) Canopy Breshears et al. (1998) Int. J. Plant Sci.
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 60 70 Precipitation ( mm) Reid et al. 1999
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 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 50 Forest Area (%) 40 30 20 10 0 1930 1940 1950 1960 1970 1980 Year Allen & Breshears (1998) Proc. Natl. Acad. Sci.
Drought-Induced Increases in Erosion Allen & Breshears 1998 Allen, Wilcox & Breshears In prep.
Drought-triggered triggered Tree Mortality May 23 Breshears, Myers & Barnes in prep.
Drought-triggered triggered Tree Mortality May 23 Aug 19 Breshears, Myers & Barnes in prep.
Drought-triggered triggered Tree Mortality May 23 Aug 19 Oct 29 Breshears, Myers & Barnes in prep.
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 Breshears et al. (2005) Proc. Natl. Acad. Sci. Photo: N. S. Cobb
Near Flagstaff Breshears et al. (2005) Proc. Natl. Acad. Sci. Photo: N. S. Cobb
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 change-type Drought Drought under warmer conditions
Global-change change-type Drought Drought under warmer conditions Mortality through wetter sites rather than just at drier ecotones
Global-change 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 Reduced canopy cover Pinyon-Juniper Woodland Juniper Savanna
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 Increased soil temperatures and evaporation Juniper Savanna
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 A B Threshold Feedbacks External Abiotic Forcings Axis II D C Biotic Processes Axis I (from NRC 1994) Abiotic Processes