HR: 11:35h
AN: B52B-06    [Abstracts]
TI: Seasonal Controls on Water and Carbon Fluxes Responding to Pulse Precipitation Events in Dryland Systems: Examples from Southern African Savannas
AU: * Williams, C A
EM: caw@nrel.colostate.edu
AF: Natural Resource Ecology Laboratory, Colorado State University Campus Delivery 1499, Fort Collins, CO 80523 United States
AU: Hanan, N P
EM: niall@nrel.colostate.edu
AF: Natural Resource Ecology Laboratory, Colorado State University Campus Delivery 1499, Fort Collins, CO 80523 United States
AU: Scholes, R J
EM: bscholes@csir.co.za
AF: Division of Forest Science and Technology, CSIR, Pretoria, 001 South Africa
AB: Water and carbon fluxes from savanna landscapes are tightly coupled to soil water availability through physiological limitation from plant water stress. This general principle has been used to broadly characterize savanna vegetation distributions based almost solely on rainfall or soil moisture. However, a number of other physical and biotic drivers vary seasonally and interannually, including radiation, humidity, leaf area, and plant functional type. It remains unclear to what degree these other drivers limit our ability to accurately predict vegetation distributions in water-limited systems. In this study, we analyze five years of eddy flux data collected at Kruger National Park, South Africa, to investigate the degree to which these other drivers modulate soil moisture control of water and carbon fluxes. Our analysis focuses on what controls seasonal variation in the response of canopy-scale fluxes to pulse precipitation events and subsequent drydown. From more than thirty drydown response curves, we find pronounced seasonal variation in the time rate of decay of soil moisture and evapotranspiration, which are both well represented as either a logarithm or power of time since a rainfall pulse. Radiation and humidity explain most of the residuals in the response of evapotranspiration to soil moisture, with only weak explanatory power of leaf area. We also find little difference in the drydown responses of Combretum versus Acacia dominated savannas. Marked seasonal shifts in canopy-scale water use efficiency (carbon / water fluxes) documents transitions from early wet season greening, to dry season moisture stress, to dormancy and decay prior to first rains. These results suggest that generalized relations between soil moisture, evapotranspiration, and carbon exchange are robust when adjusted to incorporate seasonal dependence on radiation and humidty. Broader implications for modeling savanna vegetation distributions will be discussed.
DE: 0400 BIOGEOSCIENCES
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 1655 Water cycles (1836)
DE: 1818 Evapotranspiration
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
SC: Biogeosciences [B]
MN: Fall Meeting 2005