Climate uncertainty on leaf surfaces: the biophysics of leaf microclimates and their consequences for leaf-dwelling organisms

1. Significant deviations between macro- and microclimates are quite common in different ecosystems. Such deviations have also been observed between leaf and air temperatures. The surface of leaves hosts a huge diversity of organisms. Here, we point out the crucial role of leaf microclimates in the fate of leaf-dwelling organisms in a changing climate. 2. Leaf microclimate, which includes temperature and humidity at the leaf surface, results from the biophysical filtering of local macroclimates by the plants themselves through complex and nonlinear processes. However, because the processes contributing to leaf microclimate are poorly understood, we lack a strong basis for predicting the impacts of global warming on plants and their denizens. 3. We describe two mechanisms that generate climate uncertainty at the leaf surfaces. First, stomatal responses to the environment generate great complexity in the dynamics of leaf temperatures. Secondly, herbivores, by feeding on leaf tissues, modify their leaf microclimates. Little is known about how these modifications affect the ecophysiology of organisms at the leaf surface, an effect called physical feedback of herbivory. 4. Recent findings report a latitudinal gradient in the temperatures of tree leaves, which can be linked to gradients in plant structural traits. We propose two competing hypotheses to describe how the leaf microclimate will change with global warming across latitudes. These hypotheses predict opposite patterns of change in the leaf microclimate. 5. How can we reduce our uncertainty about what will happen at leaf surfaces? Recent advances in stomatal biology give cues regarding the direction and the speed at which plant stomata will influence the evolution of leaf microclimates. In addition, local heterogeneity in microclimatic conditions might help leaf-dwelling organisms to find suitable microhabitats, as long as they can migrate over short distances. 6. The challenges now are to understand whether leaf microclimates will buffer or magnify the amplitude of warming, and to determine how much the outcome will affect ecological processes within new microclimates. Leaf microclimates can provide suitable microhabitats in an unfavourable climate, and conversely, they can bring a species to local extinction in what would seem to be an otherwise favourable climate.