HR: 1330h
AN: B52A-1024    [PDF]
TI: How Plant Functional-Type, Weather, Seasonal Drought, and Soil Physical Properties Alter Water and Energy Fluxes of an Oak-Grass Savanna and an Annual Grassland
AU: * Baldocchi, D
EM: baldocchi@nature.berkeley
AF: University of California-Berkeley, Dept Environmental Science Policy and Mgt 151 Hilgard Hall, Berkeley, ca 94720
AU: Xu, L
EM: lkxu@nature.berkeley.edu
AF: University of California-Berkeley, Dept Environmental Science Policy and Mgt 151 Hilgard Hall, Berkeley, ca 94720
AB: Savannas and open grasslands often co-exist in semi-arid regions. How these contrasting landscapes affect the exchanges of energy remain to be quantified. Here we examine how a number of abiotic, biotic and edaphic factors modulate water and energy flux densities over an oak/grass savanna and an annual grassland that coexist in the same climate but on soils with different hydraulic properties. The net radiation balance was greater over the oak woodland than the grassland despite the fact that both canopies received similar sums of incoming short and long wave radiation. The lower albedo and lower surface temperature of the transpiring woodland caused it to intercept and retain more long and shortwave energy during the dry period. The observed differences in net energy exchange had profound impacts on canopy evaporation and sensible heat exchange. The woodland evaporated about 380 mm per year and the grassland evaporated about 300 mm per year. Differences in the water holding characteristics of the soils at the two sites account for this difference in evaporation, and provide a partial explanation why the vegetation differs at the two sites. The response of evaporation to diminishing soil moisture was quantified using information on volumetric water content, soil water potential of the root zone and predawn water potential. When soil moisture was ample, after recharging winter rains, values of latent heat flux density, normalized by the equilibrium evaporation rate, were greater for the grassland than for the oak savanna. The grassland died and quit evaporating when the water content of the soil dropped below the permanent wilting point (-1.5 MPa). The oak trees, on the other hand, were able to transpire, at low rates, under very dry soil conditions (soil water potentials down to -4.0 MPa). The trees were able to endure such low water potentials and maintain basal levels of metabolism because a few exploratory roots tapped deep water sources during the dry season and the density of the woodland remained low.
DE: 0315 Biosphere/atmosphere interactions
DE: 1818 Evapotranspiration
SC: Biogeosciences [B]
MN: 2003 Fall Meeting