Climatic and structural controls on respiratory and photosynthetic carbon exchange processes at an encroaching savanna site.
Ann
Thijs, University of Texas at Austin, annthijs@mail.utexas.edu
(Presenting)
Chun-Ta
Lai, San Diego State University, lai@sciences.sdsu.edu
Marcy
Litvak, University of New Mexico, mlitvak@unm.edu
Woody plant encroachment in grassland and savanna ecosystems is a rapidly occurring global phenomenon. This widespread land cover change has been attributed to different human activities such as fire suppression, overgrazing and rising atmospheric CO2 concentrations. Despite the dramatic shift in ecosystem structure and plant functional type, the consequences for the carbon balance at local and regional scales remains poorly quantified. Here we present work performed at our Central Texas AmeriFlux site, established in 2004, where woody encroachment by Ashe juniper and Honey mesquite has increased the woody cover from 0 to ~50% over the last 30 years. To come to a process-based understanding of the carbon dynamics at the site, monthly small scale measurements of soil respiration and leaf level gas exchange on the dominant species have been performed during contrasting periods of temperature and water availability. The functional response of soil respiration to the main drivers, soil temperature and soil water content, is surprisingly similar for soils under different vegetation covers, but differences in respiration rates result from differences in microclimatic factors, specifically soil temperature, under the different vegetation covers. Monthly measurements of leaf gas exchange were made on 4 functionally different species, dominant at the site: an evergreen tree (Ashe juniper), a deciduous tree (Honey mesquite), a C4 grass (King Ranch Bluestem) and a C3 grass (Texas winter grass). Two photosynthetic models were fitted for the different species at different times: a rectangular hyperbolic function was fitted to the light response curves to determine Amax, dark respiration and quantum efficiency. The Ball-Berry model of stomatal conductance and the C3 and C4 biochemical models of photosynthesis (Collatz, 1991 Collatz 1992) were fitted to light and CO2 response curves. The parameter estimates were used in a scaling approach to estimate the contributions of the different plant functional types to the canopy photosynthetic flux. Results indicate that woody encroachment significantly alters carbon dynamics in this savanna ecosystem. The drought resistance of the encroaching trees and the year-round photosynthetic activity of Ashe juniper allows this ecosystem to maintain carbon neutral during adverse conditions (winter months, summer droughts), making this site an important carbon sink on annual timescales.
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