Co-variation of leaf traits and microclimate across canopies: does it matter for forest function?

The ecological importance of variation in both leaf microclimates and leaf energy balance traits in determining leaf temperatures and transpiration rates has been recognized for over fifty years. However, although leaf temperature emerges at the leaf or branch scale, most studies rely on either macro- (regional) or meso- (plot) scale estimates of air temperature, which may meaningfully differ from leaf temperatures. Critically, feedbacks between microclimate and energy balance leaf traits (e.g. leaf width, stomatal regulation, leaf absorptance of shortwave radiation) on forest temperature responses are generally ignored. Here we investigate whether such feedbacks might be important by testing for covariance between leaf energy balance traits and microclimate in a tropical forest in central eastern Amazonia. We use a unique dataset of leaf traits (400+ leaves from 39 individual trees of 10 most abundant species) accessed via climbing techniques across height and light gradients from the bottom to the top of the canopy. We ask: (1) is there covariance of leaf traits with microclimate (e.g. are leaves in light gaps narrower, with smaller boundary layers, and hence more tightly coupled to air temperature, than shaded leaves)?; and (2) if so, what impact may this covariance have on the distribution of leaf temperatures in the forest canopy? Using generalized linear mixed models, we found substantial covariance of leaf widths with both height and light (proxies for microclimate variation), with height and light strongly interacting to affect leaf width (and so leaf temperature via boundary layer conductance). We then used energy balance modeling to compare simulated leaf temperatures with and without covariance of leaf width and microclimate. This work shows that leaf-environment interactions have significant effects on leaf temperatures with important implications for forest temperature sensitivity and function.