Response Manipulation vs Diurnal Observation: A Direct Comparison of Two Methods for Quantifying Stomatal Behavior in Tropical Forests

Abstract:

A primary source of uncertainty in terrestrial biosphere model (TBM) projection of carbon uptake and water cycling from ecosystems is the relationship between CO₂ assimilation (A) and water loss via stomatal conductance (g_s). A common mathematical framework for modeling this relationship is the “Unified Stomatal model”, which relates A to g_s over environmental conditions and is governed by two terms, the stomatal slope (g₁) and intercept (g₀). Given their importance in determining the relationship between forest productivity and climate, an accurate and mechanistic understanding of the g₁ and g₀ parameters is crucial, particularly in wet tropical broadleaf forests where changes in water cycling could impact global weather patterns. These stomatal parameters are estimated using leaf-level gas exchange by two alternative methods: (1) a response curve where the environmental conditions are modified for a single leaf, or (2) a survey approach, where repeated measurements are made on multiple leaves over a diurnal range of environmental conditions.

We compare the curve and survey approaches by conducting a comprehensive measurement campaign in which we paired diurnal gas exchange surveys with leaf level response curves for the estimation of g₁ and g₀ on six tropical species across a full range of leaf phenological stages. We examine how these different estimates impact model projection of g_s, and how the consideration of a diurnal effect on g₁ and g₀ can improve predictions relative to a model using parameter estimates which are fixed over the photoperiod.

Our results showed that age is an important factor to consider in estimates of g₀, however there was no effect of leaf age on estimates of g₁. The survey approach identified a diurnal trend associated with g₁ and g₀, which when accounted for improved model projections of diurnal trends in g_s. We found that while both approaches yield equally statistically valid estimates of g₁ and g₀ at a fixed point in time, they are not directly comparable across diurnal timescales, where shifting water supply and carbon demand lead to dynamic canopy scale water use efficiency (WUE). These results suggest that to improve the accuracy of modelled g_s in tropical forests, TBMs should recognize and implement diurnal variation in stomatal parameters which are associated with diurnal shifts in WUE.