Through enhanced tree dynamics carbon dioxide enrichment may cause tropical forests to lose carbon

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Abstract
The fixation and storage of C by tropical forests, which contain close to half of the globe's biomass C, may be affected by elevated atmospheric CO 2 concentration. Classical theoretical approaches assume a uniform stimulation of photosynthesis and growth across taxa. Direct assessments of the C balance either by flux studies or by repeated forest inventories also suggest a current net uptake, although magnitudes sometimes exceed those missing required to balance the global C cycle. Reasons for such discrepancies may lie in the nature of forest dynamics and in differential responses of taxa or plant functional types. In this contribution I argue that CO 2 enrichment may cause forests to become more dynamic and that faster tree turnover may in fact convert a stimulatory effect of elevated CO 2 on photosynthesis and growth into a long–term net biomass C loss by favouring shorter–lived trees of lower wood density. At the least, this is a scenario that deserves inclusion into long–term projections of the C relations of tropical forests. Species and plant functional type specific responses (‘biodiversity effects’) and forest dynamics need to be accounted for in projections of future C storage and cycling in tropical forests.