Residual stomatal conductance: an underestimated parameter of global significance

Background/Question/Methods

There has been a long standing assumption that stomatal conductance (g_s) drops to zero under nocturnal conditions.  However, recent reports show that significant ‘residual’ g_s may be present during nighttime periods (ranging from 20-400 mmol m^-2 s^-1).  The empirical g_s models of Ball-Woodrow-Berry and the later modified Ball-Berry-Leuning (BBL) model account for residual g_s by the inclusion of the parameter g₀ - g_s 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 g_s.  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. 

Results/Conclusions

Measurements of g₀ varied significantly between species and ranged from 10-220 mmol m^-2 s^-1 (a range of 5-50% of daytime g_s). 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 g_s 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.