Tree Water Use Strategies in a Neotropical Dry Forest

Abstract

Trees in tropical dry forests (TDFs) developed different drought coping strategies including e.g. succulence of different plant organs, wood anatomical traits and leaf phenology. Apart from the fact that water availability limits plant physiological activity our understanding of the eco-physiological mechanisms underlying species' responses to water stress remains still limited. Varying temporal and spatial precipitation patterns which are assumed to influence tree phenology, growth and water turnover make it a complex task to predict global and local change scenarios. The objective of this study was to 1) assess patterns in leaf phenology, radial stem circumference changes, sap flux responses and evaluate their potential suitability as climate change indicators, 2) to delineate the influence of phenology, fluctuating moisture regimes and site conditions on species specific water use responses, 3) and to analyze stem water residence time in the different functional species. The study was implemented along an elevational and moisture gradient in a sub montane dry forest of southern Ecuador. Methods include sap flux measurement techniques (thermal dissipation probes and heat field deformation), phenological observations and stable isotope tracing (Deuterium Oxide). Tree species of four different phenological types (after Borchert, 1996) were assessed at 670 m, 860 m, and 1100 m asl.In search of a potential climate change indicator, three leaf deciduous tree species were studied: Ceiba trichistandra (leaf deciduous, stem succulent), Eriotheca ruizii (deciduous) and Erythrina velutina (brevi-deciduous). At three altitudes 36 trees were equipped with digital band dendrometers and thermal dissipation probes . Phenology was observed during the whole study period. Basically all species reacted to the environmental drivers (vapor pressure deficit, VPD and soil water content, SWC) with shrinking and swelling of the stem circumference and particular sap flux patterns during the whole study period at all elevations. Stem succulent Ceiba responded most sensitive to drivers; even after shedding leaves it was the first species able to re-flush some canopy, triggered by an intermittent rain event. Stem water in Ceiba presumably serves to flush leaves one month before onset of the first rains in the wet season, marked in the dendrometer readings by a strong decrease of circumference. This extraordinary sensitivity toward changing moisture regimes makes Ceiba a useful indicator species for assessing climatic variations. Furthermore, the study setup revealed differences among species, tree water use patterns, radial growth and topographic position; therefore raising the question of species' specific drought coping mechanism.We subsequently focused on the influence of variation in SWC (supposed to control water use), VPD, species (representing phenology), elevation, and tree diameter on water use to derive species' specific responses. The evergreen (broad-leaved) Capparis scabrida was added to the study setup to cover all functiona...