Carbon and water cycling in a Bornean tropical rainforest under current and future climate scenarios

We examined how the projected increase in atmospheric CO2 and concomitant shifts in air temperature and precipitation affect water and carbon fluxes in an Asian tropical rainforest, using a combination of field measurements, simplified hydrological and carbon models, and Global Climate Model (GCM) projections. The model links the canopy photosynthetic flux with transpiration via a bulk canopy conductance and semi-empirical models of intercellular CO2 concentration, with the transpiration rate determined from a hydrologic balance model. The primary forcing to the hydrologic model are current and projected rainfall statistics. A main novelty in this analysis is that the effect of increased air temperature on vapor pressure deficit (D) and the effects of shifts in precipitation statistics on net radiation are explicitly considered. The model is validated against field measurements conducted in a tropical rainforest in Sarawak, Malaysia under current climate conditions. On the basis of this model and projected shifts in climatic statistics by GCM, we compute the probability distribution of soil moisture and other hydrologic fluxes. Regardless of projected and computed shifts in soil moisture, radiation and mean air temperature, transpiration was not appreciably altered. Despite increases in atmospheric CO2 concentration (C-a) and unchanged transpiration, canopy photosynthesis does not significantly increase if C-i/C-n is assumed constant independent of D (where C-i is the bulk canopy intercellular CO2 concentration). However, photosynthesis increased by a factor of 1.5 if C-i/C-a decreased linearly with D as derived from Leuning stomatal conductance formulation [R. Leuning. Plant Cell Environ 1995;18:339-55]. How elevated atmospheric CO2 alters the relationship between C-i/C-a and D needs to be further investigated under elevated atmospheric CO2 given its consequence on photosynthesis (and concomitant carbon sink) projections. (C) 2004 Elsevier Ltd. All rights reserved.