Transpiration and canopy conductance of secondary vegetation in the eastern Amazon

Secondary woody vegetation in the Brazilian Amazon accounts for about 30% of the cleared area in this region, which exceeds 100,000 km2. Despite the relative predominance of secondary vegetation, the hydrological and climatic properties of these areas are not well documented. In an effort to address this, the evapotranspiration (E) of a 3.5-year-old secondary vegetation in the eastern Amazon of Brazil was measured over 1 year. The annual evapotranspiration according to the Bowen ratio energy balance (BREB) amounted to 1421 mm, which is equal to rates quoted for tropical primary forests. The secondary vegetation returned 73% of the annual rainfall (1954 mm) to the atmosphere. Evapotranspiration required 73% of the net-radiation (Rn) energy; this function remained fairly constant over the whole year. In order to estimate evapotranspiration with the Penman–Monteith (PM) method, the canopy conductance (gc) was determined using the BREB results. The monthly mean daily gc varied between 14 and 22 mm s−1 in the rainy season and the transitional period (January–August), and reached a minimum of 7 mm s−1 in the dry season in October. The hourly as well as daily mean canopy conductance were approximated by a multiple linear regression analysis incorporating hourly and daily averages of Rn and vapour pressure deficit, respectively. In addition, the Jarvis-type model, which is based on a set of environmental control functions, was applied to predict hourly gc. The multiple linear regression and the non-linear optimisation (Jarvis-type model) were equally suitable for gc prediction. The optimised environmental control functions were comparable to those predicted elsewhere for Amazonian primary forests. This underlines the similarity of secondary and primary forests with respect to hydrological characteristics as well as energy turnover.