HR: 08:30h
AN: B21B-03    [Abstracts]
TI: Influence of Vegetation Cover on Rain Pulse Responses in Semi-Arid Savannas in Central Texas
AU: * Litvak, M
EM: mlitvak@unm.edu
AF: University of New Mexico, Biology Department, Albuquerque, NM 87131, United States
AU: Heilman, J
EM: j-heilman@tamu.edu
AF: Texas A&M University, Soil and Crop Sciences, College Station, TX 77843, United States
AU: McInnes, K
EM: k-mcinnes@tamu.edu
AF: Texas A&M University, Soil and Crop Sciences, College Station, TX 77843, United States
AU: Thijs, A
EM: annthijs@mail.utexas.edu
AF: University of Texas, Section of Integrative Biology, Austin, TX 78701, United States
AU: Kjelgaard, J
EM: jkjelgaard@ag.tamu.edu
AF: Texas A&M University, Soil and Crop Sciences, College Station, TX 77843, United States
AB: Savannas in central Texas are dominated by live oak (Quercus virginiana) and Ashe juniper (Juniperus asheii) underlain by perennial, C3/C4 grasslands, and are increasingly becoming juniper and mesquite dominated due to overgrazing and suppression of wildfires. Since 2004, we have been investigating how carbon, water and energy exchange in these rain-limited savannas respond to rainfall variability and this observed vegetation change. In semi-arid regions, rainfall pulses provide inputs of soil moisture and trigger biotic activity in the form of plant gas exchange and microbial metabolism as well as water dependent physical processes in the soil. Each of these components has a different characteristic response curve to soil moisture and integrates soil water content over a different range of depths. Here we focus on examining how the observed increase of woody species in central Texas savannas alters the response of net ecosystem exchange and its components, ecosystem respiration and gross ecosystem exchange, to rain pulses. Using data we have collected over the last three years from three Ameriflux tower sites at Freeman Ranch near San Marcos, TX (C3/C4 grassland, juniper/mesquite savanna with 50 percent woody cover, and oak/juniper woodland), we quantify the responses of both ecosystem respiration and daily carbon uptake to rainfall pulses throughout the year. Specifically, we look at the enhancement and persistence of ecosystem respiration and carbon uptake responses following a pulse, and isolate the main controlling factors on the observed response: seasonality, antecedent soil moisture and temperature, or previous pulses. In all three land covers, the general response to precipitation pulses is a respiration pulse followed by an increase in total carbon uptake. Differences in pulse responses observed at the savanna site compared to the grassland and woodland sites can be explained, in part, by the observed differences in rooting structure and photosynthetic capacity due to differences in plant functional groups and leaf area index. The woodland site is most sensitive to winter pulses in terms of enhanced sink strength following pulses and is dependent on both temperature and pulse size. Both the grassland and shrubland sites show greater sink strength following summer pulses, rather than winter pulses. Both the ecosystem respiration and net uptake responses in all three sites are dependent upon whether there was shallow versus deep soil moisture recharge following pulses. Implications for the influence of future climate change on carbon dynamics in these savanna ecosystems will be discussed.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0430 Computational methods and data processing
DE: 0480 Remote sensing
SC: Biogeosciences [B]
MN: 2007 Fall Meeting