There has been a long standing assumption that stomatal conductance (gs) drops to zero under nocturnal conditions. However, recent reports show that significant ‘residual’ gs may be present during nighttime periods (ranging from 20-400 mmol m-2 s-1). The empirical gs models of Ball-Woodrow-Berry and the later modified Ball-Berry-Leuning (BBL) model account for residual gs by the inclusion of the parameter g0 - gs at zero net photosynthesis. Recently, the BBL model was further modified in an effort to connect it to the stomatal optimization theory. The combined optimization model almost removed the g0 parameter as the accompanying analysis pointed to another parameter (g1) as the primary driver of gs. Thus, recent efforts continue to overlook the importance of the g0 parameter. In the absence of measured values of g0, that show substantial nocturnal conductance , global vegetation models assume unrealistically low values (as low as 2 mmol m-2 s-1) across ‘plant functional types’. The objective of this study was to investigate the seasonal and drought response of g0 across four temperate broadleaved tree species and assess its species-specific impact on transpiration estimates.
Measurements of g0 varied significantly between species and ranged from 10-220 mmol m-2 s-1 (a range of 5-50% of daytime gs). Repeated measures analysis showed an inconsistent response to either growing season or drought. Two species (Betula nigra and Cercis canadensis) showed significant seasonal changes in g0, while only one species (Quercus rubra) responded to drought. The incorporation of g0 into a mechanistic, 3-dimensional canopy transpiration model (MAESTRA) significantly improved individual-tree and stand transpiration estimates. Subsequent canopy-level model investigations revealed g0 as the main driver (up to 80% of total) of water loss in the shaded lower-canopy layers, where the driving force of photosynthesis on gs is at its lowest. Seasonal water use could be underestimated by as much as 55% when MAESTRA was parameterized with values of g0 similar to those used in large scale process models (e.g. Community Land Model). Here, we show that g0 is a highly significant parameter whose influence spans multiple scales. Therefore, models should incorporate and carefully characterize g0 when estimating global-scale terrestrial water fluxes.