Warming decreases thermal heterogeneity of leaf surfaces: implications for behavioural thermoregulation by arthropods

Ectotherms rely heavily on the spatial variance of environmental conditions to thermoregulate. Theoretically, their fitness is maximized when they can find suitable microhabitats by moving over short distances - this condition is met when spatial variance is high at fine spatial scales. Strikingly, despite the diversity of organisms living in leaf microhabitats, little is known about the impact of warming on the spatial variance of climatic conditions at the scale of individual leaf surfaces. Here, we used experimental manipulation of ambient conditions to quantify the effects of environmental change on the thermal heterogeneity within individual leaf surfaces. We also explored the implications for behavioural thermoregulation by arthropods at a single leaf surface. Using thermography, we characterized the apple leaf microclimate in terms of span and spatial aggregation of surface temperatures across a range of air temperatures and relative humidities. Then, we assessed how thermal heterogeneity within individual leaves affected behavioural thermoregulation by the two-spotted spider mite (Tetranychus urticae Koch) under both near-optimal and sublethal conditions in this microhabitat. We measured the upper lethal temperature threshold of the mite to define sublethal exposure. Thermal heterogeneity of individual leaves was driven mainly by ambient air temperature. Higher air temperatures gave both smaller ranges and higher spatial aggregation of temperatures at the leaf surface, such that the leaf microclimate was homogenized.Tetranychus urticae used behavioural thermoregulation at moderate air temperature, when thermal heterogeneity was high at the leaf surface. At higher air temperature, however, heterogeneity declined and spider mites did not perform behavioural thermoregulation. Warming decreases thermal heterogeneity of leaf surfaces with critical implications for arthropods - behavioural thermoregulation alone is not sufficient to escape the heat in the leaf microhabitat. Information on spatial variance of microclimatic conditions is critical for estimating how readily organisms can buffer global warming by moving.