The DroughtBox: A new tool for phenotyping residual branch conductance and its temperature dependence during drought.

Xylem hydraulic failure is a major driver of tree death during drought. However, to better understand mortality risk in trees, especially during hot-drought events, more information is required on both rates of residual water-loss from small branches (g ) after stomatal closure, as well as the phase transition temperature (T ), beyond which g significantly increases. Here, we describe and test a novel low-cost tool, the DroughtBox, for phenotyping g and T across species. The system consists of a programmable climatically controlled chamber in which branches dehydrate and changes in the mass recorded. Test measurements show that the DroughtBox maintains stable temperature and relative humidity across a range of set points, a prerequisite for getting accurate g and T values. Among a study group of four conifer and one angiosperm species, we observed a range of g (0.44-1.64 mmol H O m s ) and T (39.4-43.8°C) values. Furthermore, the measured time to hydraulic failure varied between two conifers species and was shortened in both species following a heatwave event. The DroughtBox is a reliable and customizable tool for phenotyping g and T , as well as for testing models of time to hydraulic failure that will improve our ability to assess climate change impacts on plants.