CONTRIBUTION OF TRANSPIRATION TO FOREST AMBIENT VAPOR BASED ON ISOTOPIC MEASUREMENTS

Using a simple isotope mixing model, we evaluated the relative proport ion of water vapour generated by plant transpiration and by soil evapo ration at two sites in the Amazon basin. Sampling was carried out at t wo different soil covers (forest and pasture), in a seasonal tropical rainforest at eastern Amazon where major deforestation is the result o f land-use change, and compared to a less seasonal central Amazon fore st. In both forests, vapour from transpiration was responsible for mos t, if not all, of the water vapour generated in the forest, while it c ould not be detected above the grassy pastures. Thus the canopy transp iration may be a major source of water vapour to the forest and perhap s to the atmosphere during the dry season. The results are discussed i n relation to predictive models based on net radiation that usually ar e not able to distinguish between transpiration and evaporation.