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Summary: The representation of stomatal regulation of transpiration and CO2 assimilation is key to forecasting terrestrial ecosystem responses to global change. Given its importance in determining the relationship between forest productivity and climate, accurate and mechanistic model representation of the relationship between stomatal conductance (gs) and assimilation is crucial.We assess possible physiological and mechanistic controls on the estimation of the g1 (stomatal slope, inversely proportional to water use efficiency) and g0 (stomatal intercept) parameters, using diurnal gas exchange surveys and leaf‐level response curves of six tropical broadleaf evergreen tree species.g1 estimated from ex situ response curves averaged 50% less than g1 estimated from survey data. While g0 and g1 varied between leaves of different phenological stages, the trend was not consistent among species. We identified a diurnal trend associated with g1 and g0 that significantly improved model projections of diurnal trends in transpiration.The accuracy of modeled gs can be improved by accounting for variation in stomatal behavior across diurnal periods, and between measurement approaches, rather than focusing on phenological variation in stomatal behavior. Additional investigation into the primary mechanisms responsible for diurnal variation in g1 will be required to account for this phenomenon in land‐surface models. [ABSTRACT FROM AUTHOR]

A Thomson Web of Science search for the topic "leaf age" AND potassium AND tropic* identified just one similar study, which reported a similar strong decline in foliar K with leaf age ([16]). This scientific commentary refers to 'Changes in leaf functional traits with leaf age: When do leaves decrease their photosynthetic capacity in Amazonian forests? Google Scholar Crossref Search ADS WorldCat 6 Kitajima K, Mulkey SS, Wright SJ (1997 a) Decline of photosynthetic capacity with leaf age in relation to leaf longevities for five tropical canopy tree species. The LDO model integrates seasonality in the timing of leaf production and abscission, total leaf area present and the contribution of expanding, mature and old leaves to successfully explain seasonality in gross ecosystem production. [Extracted from the article]