How do transpiration, growth and stem water reserves vary within and between trees across two stands of contrasting successional stage? Case study in a reforested area of the Atlantic forest in São Francisco Xavier, São Paulo, Brazil

Details

Supervisors

Javaux, Mathieu ; De Simone Borma, Laura

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences et technologies de l'environnement, à finalité spécialisée

Abstract

enTropical secondary forests store a large share of regrowing carbon and return much of the rain they receive to the atmosphere, yet how individual trees combine water use and wood production remains poorly resolved. Over the 2020 hydrological year, Granier sap flow probes and point dendrometers were combined on 22 trees of two Brazilian Atlantic Forest plots of contrasting successional stage, a young pioneer and an older secondary stand, at São Francisco Xavier. Transpiration, radial growth and stem water reserves, the latter captured through the tree water deficit, were analysed tree by tree, linked from daily to annual time scales, and scaled to the stand alongside FAO-56 reference evapotranspiration. At the individual scale, the two stands shared a single stomatal regulation rule, close to the theoretical Oren optimum, with pioneers opening their stomata slightly more per unit of sapwood. Radial growth stopped at a milder water deficit than the one that closed the stomata, and the tree water deficit emerged as the variable linking the water transpired, stored and built into wood. The stem reservoir contributed comparably to daily water use in both stands, but pioneers relied more on their internal reserve while secondary trees appeared more buffered by the soil. Variation between trees exceeded the contrast between stands. The lower stand-level transpiration of the pioneer plot was structural, reflecting almost three times less conductive sapwood per unit of ground rather than a different regulation, and its faster growth reflected lighter wood rather than a more efficient conversion of water into biomass. These conclusions rest on relative, intrinsic signals that proved robust to the methodological choices, whereas absolute magnitudes, including stand transpiration, remain constrained by the uncertainty of the thermal-dissipation method, a single year without extreme drought, the absence of leaf water potential, and a small pioneer sample. Within these limits, successional stage set the structural frame, while regulation and the fate of individual trees were set at the tree level, along a continuum rather than across a sharp pioneer-versus-secondary boundary.