Seasonal changes in water sources used by woody species in a tropical coastal dune forest

Abstract

Our aim was to investigate the water sources used by woody species under contrasting water availability and the extent of water-sources-use differentiation among dominant woody species in a tropical coastal dune forest. We sampled 15 woody species in a Brazilian restinga forest and, through Bayesian isotope mixing models, we estimated the proportion of water sources used. We tested whether water-sources-use was (i) different between contrasting water availability conditions; (ii) dependent on growth form, plant size or crown illumination; and (iii) influenced by stand density, evenness or biomass. We found a seasonal variation in water-sources-use, but no vertical soil-water partitioning among woody species. In wetter periods, plants used mainly water from top-soil, as a shallow water table limited water uptake to top-soil layers recharged with rainwater. Contrastingly, during drier periods, with the absence of rain and a deeper water table, plants generally relied on deeper (50 cm) soil layers. Only under less-wet conditions, a greater evenness and density implied higher water-uptake depth heterogeneity among plants. However, changes in the main water-sources used by plants were neither evoked in more dense or diverse plots, nor induced by plant size. Our study shows that restinga species have dynamic shifts in water-uptake depth caused by seasonal water availability changes, influenced by the combined effect of insufficient moisture at shallow soil layers and water-table lowering in drier periods. These temporal shifts are common among species, implying that restinga woody community has a homogeneous strategy of water-resources acquisition. This study enhances our understanding of the effects that water variations can have on water-resource use in restinga forests.