Why lose water at night? : disentangling the mystery of nocturnal sap flow, transpiration and stomatal conductance - when, where, who?

Technological advancements have led to abundant evidence of nocturnal sap flow occurring in a range of species and ecosystems under varying microclimatic conditions. Here, we evaluate the physical factors that facilitate night-time water loss and explore potential advantages to this seemingly wasteful process. We discuss the microclimatic variables which affect nocturnal water fluxes, such as temperature, vapour pressure deficit, wind speed, and soil water content, as well as stem refilling, and the impact of phenology, or leaf age on nocturnal water loss. Rising CO2 and temperature affect nocturnal sap flux and stomatal conductance, with some counter-intuitive effects. Additionally, we discuss the possible role of embolism refilling, and recharge of capacitance when stomata are closed.