TRANSPIRATION AND STOMATAL CONDUCTANCE OF 2 RAIN-FOREST SPECIES GROWING IN PLANTATIONS (SIMAROUBA-AMARA AND GOUPIA-GLABRA) IN FRENCH-GUYANA

Water relations of 2 tree species from the tropical rain forest of French Guyana were studied in young plantations of Simarouba amara and Goupia glabra. Experiments took place in 1988 and 1989. Sap flow was recorded continuously for several months including a dry season. On bright days, sap flux densities (eg sap flow per unit of conducting area) exhibited high values of almost-equal-to 3.5 to 4.0 kg.dm-2.h-1. Total sap flow differed from one tree to another depending on individual sapwood areas. In spite of the increase of global radiation and of the vapour pressure deficit, sap flow remained constant for Simarouba and even decreased for Goupia between 10:00 and 15:00 h as a consequence of stomatal closure. Sap flow measurements allowed the calculation of stand transpiration, which for bright days represented only 50% of Penman potential evapotranspiration (PET). This low transpiration level was explained by incomplete canopy closure and hence a low LAI of the plots. Canopy conductances were calculated from the Penman-Monteith equation. They demonstrated the inhibiting effect of vapour pressure deficits > 4 hPa. These results confirm those of Huc and Guehl (1989), that for tropical rain forest species, transpiration may be limited by stomatal closure notwithstanding a high annual rainfall.