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Climate change, grass invasions, and woody plant
dynamics in semi-arid savannas
Jake Weltzin
Department of Ecology and Evolutionary BiologyUniversity of Tennessee
NASA
Clay - Pleistocene Sand - Holocene
-20
-40
-60
-100
-80
20
40
60
>100
80
0 %
CGCM1 HadCM2
Precipitation regimes: wither the future?
NAST 2000
Seed
Seedlingemergence
Seedlingsurvival
Mature plant
• Climate-precipitation-temperature-[CO2]
• Microclimate
• Soil-water-texture-nutrients
• Fire
• Grass neighbors-identity-interactions-density
• Herbivory-direct-indirect
• Seed availability-production-predation-dispersal
Recruitment of woody plants
Recruitment of woody plants
Seed
Seedlingemergence
Seedlingsurvival
Mature plant
• Climate-precipitation-temperature-[CO2]
• Microclimate
• Soil-water-texture-nutrients
• Fire
• Grass neighbors-identity-interactions-density
• Herbivory-direct-indirect
• Seed availability-production-predation-dispersal
Experimental design
• Grass neighborhood- No grass (bare)- Native grass (Heteropogon contortus)- Non-native, invasive grass (Eragrostis lehmanniana)
• Summer precipitation- Wet (LTM + 50%)- Dry (LTM - 50%)
• Soil texture- Sand (Holocene)- Clay (Pleistocene)
Mesquite (Prosopis velutina) demography- Seeds planted August 2002- Seedlings monitored through June 2004
Annual409 mm
34 events
Irrigation mimics intra-annual variation (daily, seasonal)
Soil types respond differently to irrigation
Clay
Sand
Irrigation controls seasonal soil moisture
Clay
Grass presence and identity control soil moisture
Clay
Eragrostis depletes soil moisture faster than Heteropogon
Time since pulse (days)
June 2002
Huxman et al.
ClaySand
Emergence facilitated by grasses, esp. in wet plots
Sand & Clay
Under grasses, soil moisture inversely related to emergence
Bare
Eragrostis
Heteropogon
Sand & Clay
Grasses do not reduce soil temperature
English et al.
Sand: grass identity not important, irrigation not important
ns 0
Sand
ns 0
Clay: grass identity not important, irrigation is important
WxP: P = 0.008
Clay
Seedlings in bare plots largest on wet, clay soil
ClaySand
Low
Wate
r stress H
igh
Seedlings in summer less water-stressed on clay than on sand
Fravolini et al.
ClaySand
Most summer rain events are smallS
um
mer
rai
n e
ven
ts
Summer rain event size class (mm)Fravolini et al.
Small rain events are transparent to mature mesquite on clay soil
Fravolini et al.
Use
of
iso
top
e-la
bel
ed w
ater
(%
)
Time since pulse (days)
Sand
Clay
Recruitment ultimately depends on soil moisture
• Seed-seedling conflict mediated by grassesFacilitation ---> Competition
• Presence/absence of grass most importantUltimately, identity of grass unimportant
• Importance of summer precipitation depends on soil type
• Seedling dem./phys. inconsistent with landscape patterns
• Seedling-adult conflict mediated by ppt regime and soil
Landscape pattern controlled by interactions between life-history stage and environment
AcknowledgmentsUSDANSFUniversity of Tennessee
Santa Rita Experimental RangeJosh AveyColeen BrownDeborah Angell
David WilliamsTravis Huxman
Nathan EnglishMike MasonBill Cable
Steve ArcherBrian EnquistMitch McClaranGuy McPhersonDon PostSusan Schwinning
NCEAS PrecipNet Participants
Alessandra FravoliniLeigh Thomas
Daniel Potts
Philip AllenMichael Andregg
Enrico BrugnoliDayna BurnsJessica Cable
Janet ChenAlex Eilts
Rico GazalRobbie Hannawacker
Kevin HultineDanielle Ignace
Dan KoepkeCharles Price
Josh PolacheckLara Souza
Lisa SturdivantSam Waskow
PrecipNet
Improving understanding of precipitation effectson ecosystems through cross-disciplinary research networks
http://precipnet.ucsc.edu/index.html
PrecipNet
Improving understanding of precipitation effectson ecosystems through cross-disciplinary research networks
•Research coordination, communication, and integration
•Regional comparisons of precipitation change and its effects
•Fostering multidisciplinary activities
•Promoting skill development and technology transfer
•Participants
PrecipNet Goals
Dennis Baldocchi UC Berkeley [email protected]
Dave Breshears University of Arizona [email protected]
Dave Evans Washington State University [email protected]
John Harte UC Berkeley [email protected]
Travis Huxman University of Arizona [email protected]
William Lauenroth Colorado State University [email protected]
Yiqi Luo University of Oklahoma [email protected]
Russ Monson Colorado State University [email protected]
Lindsey Rustad USDA Northeastern Lab. [email protected]
Whendee Silver UC Berkeley [email protected]
Stan Smith University Nevada, Las Vegas [email protected]
Jake Weltzin University Tennessee [email protected]
Claus Beier RISOE National Lab., Norway [email protected]
Todd Dawson UC Berkeley [email protected]
Philip Fay University of Minnesota, Delouth [email protected]
Claus Holzapfel University of Massachusetts [email protected]
Robert Jackson Duke University [email protected]
Michael Loik UC Santa Cruz [email protected]
Hafiz Maherali University of Guelph [email protected]
Ronald Neilson Pacific Northwest Forestry [email protected]
Osvaldo Sala University Buenos Aires, Argentina [email protected]
Marcelo Sternberg [email protected]
David Tissue Texas Tech University [email protected]
David Williams University of Wyoming [email protected]
Jayne Belnap USGS Moab, Utah [email protected]
Evan Delucia University of Illinois [email protected]
Brent Haddad UC Santa Cruz [email protected]
Bruce Hungate Northern Arizona University [email protected]
Alan Knapp Colorado State University [email protected]
Steve Long University of Illinois [email protected]
Greg Masters CABI Biosciences [email protected]
William Pockman University New Mexico [email protected]
Susan Schwinning Texas State University, San Marcos [email protected]
Eric Small University of Colorado [email protected]
Enrique Vivoni New Mexico Tech [email protected]
John Zak Texas Tech University [email protected]
Participants
25 sites
9 sites (VULCAN / CLIMOOR)
5 sites (GLOWNET Network)Osvaldo Sala
CABI Bioscience (3 sites)
Participating sites and networks
http://eeb.bio.utk.edu/weltzin/
