Consequences of nocturnal water loss: a synthesis of regulating factors and implications for capacitance, embolism and use in models

Total daily water use is a key factor influencing the growth of many terrestrial plants, and reflects both day-time and nocturnal water fluxes. However, while nocturnal sap flow (E_n) and stomatal conductance (g_s,n) have been reported across a range of species, ecosystems and microclimatic conditions, the regulation of these fluxes remains poorly understood. Here, we present a framework describing the role of abiotic and biotic factors in regulating E_nand g_s,nhighlighting recent developments in this field. Across ecosystems, E_nand g_s,ngenerally increased with increasing soil water content and vapor pressure deficit, but the interactive effects of these factors and the potential roles of wind speed and other abiotic factors remain unclear. On average, g_s,n and E_n are higher in broad-leaved compared with needle-leaved plants, in C₃compared with C₄ plants, and in tropical compared with temperate species. We discuss the impacts of leaf age, elevated [CO₂] and refilling of capacitance on night-time water loss, and how nocturnal g_s,nmay be included in vegetation models. Younger leaves may have higher g_s,n than older leaves. Embolism refilling and recharge of capacitance may affect sap flow such that total plant water loss at night may be less than estimated solely from E_n measurements. Our estimates of g_s,n for typical plant functional types, based on the published literature, suggest that nocturnal water loss may be a significant fraction (10-25%) of total daily water loss. Counter-intuitively, elevated [CO₂] may increase nocturnal water loss. Assumptions in process-based ecophysiological models and dynamic global vegetation models that g_s is zero when solar radiation is zero are likely to be incorrect. Consequently, failure to adequately consider nocturnal water loss may lead to substantial under-estimation of total plant water use and inaccurate estimation of ecosystem level water balance.