THE ROLE OF DEEP ROOTS IN THE HYDROLOGICAL AND CARBON CYCLES OF AMAZONIAN FORESTS AND PASTURES

DEFORESTATIONS and logging transform more forest in eastern and southe rn Amazonia than in any other region of the world(1-3). This forest al teration affects regional hydrology(4-11) and the global carbon cycle( 12-14), but current analyses of these effects neglect an important dee p-soil link between the water and carbon cycles. Using rainfall data, satellite imagery and field studies, we estimate here that half of the closed forests of Brazilian Amazonia depend on deep root systems to m aintain green canopies during the dry season. Evergreen forests in nor theastern Para state maintain evapotranspiration during five-month dry periods by absorbing water from the soil to depths of more than 8 m. In contrast, although the degraded pastures of this region also contai n deep-rooted woody plants, most pasture plants substantially reduce t heir leaf canopy in response to seasonal drought, thus reducing dry-se ason evapotranspiration and increasing potential subsurface runoff rel ative to the forests they replace. Deep roots that extract water also provide carbon to the soil. The forest soil below Im depth contains mo re carbon than does above-ground biomass, and as much as 15% of this d eep-soil carbon turns over on annual or decadal timescales. Thus, fore st alteration that affects depth distributions of carbon inputs from r oots mag also affect net carbon storage in the soil.