Temperature governs the relative contributions of cuticle and stomata to leaf minimum conductance

During periods of stomatal closure, such as drought, plant leaves continue to lose water at a rate determined by the minimum leaf conductance, g_min. Although g_min varies with temperature, less is known about what drives this variation, including how the pathways of water loss (cuticle or stomata) vary with temperature. We used gas exchange and bench drying methods to measure g_min and cuticular conductance, g_cw, across a wide temperature range (20-50°C) in 11 broadleaf species. Vapour pressure deficit (VPD) covaried with temperature from 0.83 to 10.7 kPa. The dominant pathway of water loss for g_min shifted from stomatal transpiration towards cuticular transpiration as temperature increased. Leaf traits had variable, temperature-dependent relationships with g_min and g_cw, with trait-conductance relationships being generally stronger at higher temperatures. Cuticular thickness varied inversely with high-temperature g_cw. Simulation results showed that g_cw may impact photosynthetic capacity estimates, particularly in species with low stomatal conductance. The pathways of water loss in leaves during times of stomatal closure depend strongly on temperature. This effect may have large implications for landscape-scale water balance modelling and improving gas exchange measurements. We propose variation in VPD as a potential contributing factor in g_min and g_cw variation among studies.