Water economy of a neotropical savanna: from leaf to ecosystem

GOLDSTEIN G, MEINZER FC, BUCCI S.J, SCHOLZ FG AND GAMBELLUCA T. WATER ECONOMY OF A NEOTROPICAL SAVANNA: FROM LEAF TO ECOSYSTEM

Edinburgo

Conferencia; International Conference. Tropical Savannas & Seasonally dry forests; 2003

Neotropical savanna (Cerrado) woody plants are highly isohidric (seasonal differences in midday minimum leaf water potential (YL) and total sap flow per plant are small) despite marked seasonal variations in precipitation and air saturation deficit (D). Stomatal conductance (gs) in the woody species decreases sharply with increasing D, strongly limiting gs during the dry season. Despite a nearly three-fold increase in mean D between the wet and dry season, sap flows per plant are similar in both, the wet and  dry season. The leaf surface area per unit of sapwood area (LA/SA), an index of potential architectural constraints on water supply in relation to transpirational demand, is about 1.5 to 8 times greater in the wet season compared to the dry season for most of the species. All woody species during both seasons share a common negative exponential relationship between maximum leaf area-specific apparent hydraulic conductance of the soil-to leaf pathway (Gtmax) and LA/SA during the dry season maintain or increases Gt. Responses of gs to D and Gt are consistent with stomatal regulation of YL above a threshold minimum value, leading to seasonal homeostasis of minimum YL . Hydraulic lift and non-saturated water flow in the soil also contributes to the homeostatic behavior of woody plants, however its contribution to the water content of the upper soil layers is not large enough to prevent dye back of the herbaceous plants that have sallow root systems. Stand level measurements of evapotranspiration (ET) using eddy covariance systems is suppressed by reduced moisture availability in the upper soil layer, specially at low tree density cerrado sites. The reduction in ET during the dry season is mainly the result of the leaf area index decrease in the herbaceous layer because de ET contribution from the woody component of the savannas remain approximately constant throughout the year. Heat dissipation probes were used to measure sap flow from individual trees and the scaled up to stand level using information on tree density. ET rates for several savannas sites approach similar values as moisture content in the upper soil layers increase after the start of the rainy season.