FLUXES OF CARBON-DIOXIDE AND WATER-VAPOR OVER A C-4 PASTURE IN SOUTH-WESTERN AMAZONIA (BRAZIL)

In Brazil, pastures for cattle ranching are being established in areas that were previously forested. To investigate some consequences of th is change in land use we measured fluxes of CO2 and water vapour over a typical pasture, dominated by the introduced C-4 grass Brachiaria br izantha. In addition, we compared the CO2, water vapour fluxes and can opy stomatal conductances observed with those obtained simultaneously over a nearby undisturbed rain forest. Measurements were made near the end of the wet season under conditions of ample soil moisture. Leaf a rea index of the pasture was 3.9. The pasture had a lower canopy stoma tal conductance than the forest (typically 0.2-0.3 mol m(-2) s(-1) ver sus 0.4-0.9 mol m(-2) s(-1) at high photon irradiance) and was less re sponsive to the canopy-to-air vapour pressure difference. As a consequ ence of these lower canopy stomatal conductances, the pasture used muc h less water than the forest with average values over the period exami ned being 153 mol H2O m(-2) d(-1) and 249 mol H2O m(-2) d(-1) for past ure and forest respectively (2.74 and 4.48 mm d(-1) respectively). Thi s was also reflected by differing fractions of the absorbed energy bei ng dissipated as evaporation. This proportion was typically 0.56 for t he pasture and 0.74 for the forest. After allowing for soil and plant respiration, average daily photosynthetic rates were 0.67 mol C m(-2) d(-1) for the pasture and 0.57 mol C m(-2) d(-1) for the forest (8.0 a nd 6.8 g C m(-2) d(-1), respectively). Thus, despite an appreciably lo wer rate of water use the pasture assimilated more carbon on a daily b asis. Nevertheless, Brachiaria displayed a somewhat lower rate of phot osynthesis than expected for a C-4 grass, perhaps because of a low nut rient status. Indeed, at low and medium photon irradiance the pasture and forest showed remarkably similar photosynthetic performance. There was, however, less tendency for CO2 assimilation rates of the pasture canopy to saturate at high photon irradiance. The respiratory fluxes from the two ecosystems at night were quite similar, 6-8 mu mol m(-2) s(-1). The ratio of intercellular CO2 concentration to ambient CO2 con centration was usually 0.4 to 0.6 for the pasture, a range which is hi gher than that often reported for C-4 plants but possibly not unusual for tropical grasses in their natural environment.