Facing water limitation: Hydraulic strategies of co-occurring tree species from semiarid subtropical forests and their intra-specific variation along precipitation gradients

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The distribution of trees along gradients of seasonality and precipitation is shaped by their drought sensitivity, i.e. their ability to efficiently utilize the resource water and to resist drought stress. Trees have developed a variety of strategies to cope with reductions in water availability, particularly in environments with irregular water supply like seasonal dry forests. The world’s largest continuous dry forest is the South American Chaco, which suffers under fragmentation by intensifying livestock grazing and farming practises and is further threatened by changing precipitation patterns and rising temperatures. Therefore, it is relevant to quantify the drought susceptibility of its keystone tree species and to select meaningful traits with mechanistic links to physiological processes that define the trees’ hydraulic strategies in order to facilitate sustainable land-use management and reforestation attempts. The productivity and drought vulnerability of trees has been linked to structural and morphological features, as well as anatomical properties of the water transport system such as the morphology of the pits between water-conducting cells, and vessel size. Yet, studies integrating physiological traits in combination with hydraulic measurements in an evolutionary context have rarely been conducted in subtropical trees along environmental gradients. Our current knowledge is mainly based on patterns observed in temperate species and within-species variability is often neglected. In collaboration with scientists from Argentina, we will study four dominant tree species of the semi-arid forests in the Gran Chaco Region across a precipitation gradient from 450 to 1400 mm of annual rainfall. By measuring a variety of relevant morphological (leaf morphology, inter-vessel pit morphology), physiological (photosynthetic activity, water use efficiency), wood anatomical (vessel dimensions, wood density), and hydraulic variables (potential conductivity, water uptake depth), we first aim to analyse the relationship between the water conducting system and productivity, and to characterize the hydraulic vulnerability of these important species. Second, we will examine to which extent the variability in selected traits can be related to phenotypic plasticity or local adaptation among different provenances, using existing progeny trials for the tree species Prosopis alba. Including data on the growth performance of different P. alba provenances will allow to evaluate their suitability for potential reforestation efforts. The project will improve our understanding of the physiological consequences of precipitation reduction for dominant dry forest tree species, and of the interactions between hydraulic efficiency and productivity, and will assess the adaptive potential of the species to increasing drought intensity.