Energy balance and canopy conductance of a tropical semi-deciduous forest of the southern Amazon Basin

Deforestation and climate change have the capacity to alter rainfall regimes, water availability, and surface-atmosphere flux of water and energy of tropical forests, especially in ecotonal, semi-deciduous tropical forests of the southern Amazon Basin, which have experienced rapid regional warming and deforestation over the last three decades. To reduce uncertainty regarding current and future energy and water flux, micrometeorological measurements of latent (Q(e)) and sensible heat flux (Q(h)) and canopy conductance (G(c)) were combined with measurements of sap flux density (F(d)) and maximum leaf conductance (g(smax)) to characterize the seasonal controls on mass (H2O) and energy exchange of an ecotonal, semi-deciduous forest in northern Mato Grosso, Brazil over the 2005-2006 annual cycle. Average diel patterns and daily rates of energy flux and conductance declined during the dry season; however, the decline in F(d) and Q(e) was smaller and/or more gradual than G(c) and g(smax). Weekly averages of transpiration calculated from sap flow measurements during the dry-wet season transition period were positively correlated (r2 = 0.47; p < 0.05; n = 11) with estimates of leaf area index (LAI) derived from the Modis-Aqua satellite platform while estimates of evapotranspiration ET derived from eddy covariance were not, presumably because these estimates also include an evaporation component. Overall, our results suggest that access to deep water reserves can support high rates of F(d) and Q(e) during the dry season, but because of high evaporative demand, declines in plant water potential lead to a corresponding decline in G(c). Furthermore, seasonal variations in LAI, that are likely to be controlled in part by plant water status and phenology, constrain tree and stand transpiration. Thus the consistency of Q(e) over the annual cycle appears to be the result of trade-offs between water availability (rainfall, soil moisture, water potential), canopy structural properties (LAI), and meteorological conditions including vapor pressure deficit and net radiation.