Fruit development, not GPP, drives seasonal variation in NPP in a tropical palm plantation

We monitored seasonal variations in net primary production (NPP), estimated by allometric equations from organ dimensions, gross primary production (GPP), estimated by the eddy covariance method, autotrophic respiration (Ra), estimated by a model, and fruit production in a coconut (Cocos nucifera L.) plantation located in the sub-tropical South Pacific archipelago of Vanuatu. Net primary production of the vegeta-tive compartments of the trees accumulated steadily through-out the year. Fruits accounted for 46% of tree NPP and showed large seasonal variations. On an annual basis, the sum of estimated NPP (16.1 Mg C ha-1 year-1) and Ra (24.0 Mg C ha-1 year-1) for the ecosystem (coconut trees and herbaceous understory) closely matched GPP (39.0 Mg C ha-1 year-1), suggesting adequate cross-validation of annual C budget methods. However, seasonal variations in NPP + Ra were smaller than the seasonal variations in GPP, and maximum tree NPP occurred 6 months after the midsummer peak in GPP and solar radiation. We propose that this discrepancy reflects seasonal variation in the allocation of dry mass to carbon reserves and new plant tissue, thus affecting the allometric relationships used for estimating NPP. (Résumé d'auteur