HR: 1340h
AN: B23C-1499    [Abstracts]
TI: Insights from Tower-Flux, Physiological and Stable Isotope Measurements to Elucidate Climatic and Structural Controls on CO2 Exchange in an Encroaching Savanna
AU: * Thijs, A
EM: annthijs@mail.utexas.edu
AF: Integrative Biology University of Texas at Austin, 1 University Station C0930, Austin, TX 78712, United States
AU: Litvak, M
EM: mlitvak@unm.edu
AF: Department of Biology University of New Mexico, MSC03 2020, Albuquerque, NM 87131, United States
AU: Lai, C
EM: lai@sciences.sdsu.edu
AF: Department of Biology San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, United States
AB: Woody encroachment, a land cover change occurring in savanna ecosystems around the globe, remains a large uncertainty in regional carbon budgets. In this study, we investigate the structural and climatic controls on the interannual and seasonal variability of net CO₂ exchange in an encroaching savanna. At our study site, located on the Edwards Plateau in Central Texas, two woody plant species have encroached a former grassland over the past 20 years. Continuous eddy covariance and related meteorological measurements began in July 2004. In 2006 and 2007, two years with markedly different precipitation inputs, monthly leaf-level gas exchange on four functionally different plant species, soil respiration and chamber net ecosystem exchange (NEE) measurements were made to give an insight in the structural controls on the carbon balance. In March 2007, weekly measurements of carbon isotope ratios of ecosystem-respired CO₂ (δ¹³C_R) started as part of a national AmeriFlux-isotope network. This data provides an independent assessment of the structural controls (C3 vs. C4) on the carbon exchange in this savanna ecosystem. The physiological and eddy flux measurements indicate that woody encroachment significantly alters carbon dynamics in this savanna ecosystem. The drought resistance of juniper trees and their year-round photosynthetic activity allows this ecosystem to maintain carbon neutral in a dry year (2006). This structural control however is only second to the climatic control, as shown in the contrasting pattern of NEE fluxes between a dry (2006) and an extremely wet year (2007). Preliminary results of the δ¹³C_R measurements reflect a shift from C3 to C3 + C4 photosynthetic activity in the spring of 2007 and a sustained C3 + C4 activity throughout the wet summer of 2007, corroborating the results obtained from flux and physiological measurements.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 1851 Plant ecology (0476)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting