Seasonal variation in net carbon exchange and evapotranspiration in a Brazilian rain forest: a modelling analysis

Tropical rain forests account for a significant fraction of global net prim ary productivity, and are important latent energy (LE) sources, affecting e xtra-tropical atmospheric circulation. The influence of environmental facto rs on these fluxes has until recently been poorly understood, largely due t o a paucity of data, but in recent years the amount of available data has b een increased greatly by use of eddy covariance techniques. In this paper w e examine the factors that control daily and seasonal carbon (C) and LE flu xes, by comparing a detailed model of the soil-plant-atmosphere continuum a gainst a unique long-term data-set collected using eddy covariance at an un disturbed rain forest site north of Manaus, Brazil. Our initial application of the model was parametrized with simple measurements of canopy structure , and driven with local meteorological data. It made effective predictions of C and LE exchange during the wet season, but dry season predictions were overestimates in both cases. Sensitivity analyses indicated that the best explanation for this behaviour was a seasonal change in soil and root hydra ulic resistances (R-b). An optimization routine was then used to estimate t he increase in R-b during the dry season that would be required to explain the reduced dry season fluxes. The local soil, a clay latosol, is typical o f much of Amazonia, having very low available water and low hydraulic condu ctivity. We conclude that an increase in soil-root hydraulic resistance in the dry season introduces a significant seasonal cycle to carbon and water fluxes from this tropical forest. Furthermore, our model structure appears to be an effective tool for regional and temporal scaling of C and LE fluxe s, with primary data requirements being regional and temporal information o n meteorology, leaf area index (LAI), foliar N, critical leaf water potenti als, and plant and soil hydraulic characteristics.