Document: MAT-3-32-28

Evidence of soil moisture stress in an Amazonian rain forest and implications for carbon uptake.

WILLIAMS, M.* ¹, Y.SHIMABOKURO ², D.AHERBERT ¹, S.PARDI LACRUZ ², C.RENNO ² and E.BRASTETTER ¹

Marine Biological Laboratory, Woods Hole, MA 02543, USA ¹
Instituto Nacional de Pesquisas Espaciais, Sao Jose dos Campos, SP, Brazil ²

Abstract:
Even in well-watered tropical rain forests, soil moisture can limit primary production. Because of high evapotranspiration rates, a few weeks of low precipitation can draw down soil moisture. This, coupled with the often low available water capacity of tropical soils, means that moisture stress may play a significant role in constraining the carbon uptake of tropical forests, especially in El Nio years with enhanced dry seasons. However, data on carbon-water relations in tropical forests are scarce; in surveys at three Brazilian sites, the ABRACOS project found no indications of low dawn leaf water potentials (LWP), an indicator of moisture-stressed conditions. In November 1999, we sampled 13 primary forest sites within the Tapajos National Forest in Para, Brazil, recording stem size distributions, leaf area index (LAI), vegetation and soil carbon-nitrogen stocks, and dawn LWP. Leaves were collected by shotgun from the upper canopy, and potentials were determined by scholander pressure bomb and corrected for height. While soil texture varied from sandy to clayey among sites, LAI showed little variation (mean 6.2, SE 0.14). At three sites the leaves of one or more species had water potentials more negative than -1 MPa, an indicator of low available soil moisture. But at all sites at least one species had water potentials more positive than -0.3 MPa. These data suggest both a spatial variation in the occurrence of soil moisture stress, but also a variation in the susceptibility of individual species. This susceptibility may depend upon access to deep soil moisture. We investigate the implications of these data on carbon uptake and biomass allocation, via simulations and sensitivity analyses using a process-based model of the soil-plant-atmosphere continuum.

Keywords: leaf water potential, rain forest, LAI, model, vegetation-atmosphere exchange

This abstract is being presented at: 2:45 PM in session:
Oral Session #34: Water Relations in Trees.