Different Scales, Different Rules: How Do Temperature and Radiation Across Macro- and Microclimate Shape Forest...

Ecophysiological rules such as the Heat Conservation and Thermal Melanism Hypotheses, the Heat Transfer Theory or Bogert's Rule predict different relationships between insect body size, colour lightness and thermal environments. Forests provide scale-dependent thermal conditions - strong macroclimatic gradients across regions and microclimatic buffering depending on canopy density, with dense canopies producing cooler, more stable conditions and open canopies allowing higher light and heat input. Consequently, trait-environment relationships may vary accordingly. Here we test across both macro- and microclimatic scales whether temperature and solar radiation shape insect communities across different taxa with distinct activity periods. Location: Five forest regions across Germany. Time period: 2007-2018, non-continuous sampling. Major taxa studied: Predominantly diurnal Coleoptera and Heteroptera and nocturnal Lepidoptera. We used linear mixed effect models to test the response of mean community values to macro- and micro-climatic variables. We found smaller Coleoptera and smaller and lighter-coloured Lepidoptera with increasing seasonal mean temperature, in line with the Heat Conservation and Thermal Melanism Hypotheses. However, increasing solar radiation became increasingly important at microclimatic scales, with consistently smaller and darker-coloured Coleoptera and darker-coloured Lepidoptera in forests with open canopies, in line with the Heat Transfer Theory and Bogert's Rule. Heteroptera showed only a response in body size at the macroclimatic scale. Our findings suggest that cross-taxon community responses to macroclimate follow different mechanisms than those to microclimate and might be determined by the life history traits of the selected taxa. Further studies on arthropods with different activity periods in forests along microclimate gradients will help to further disentangle these mechanisms. Changes in macroclimate caused by climate change, along with changes in canopy cover at the microclimatic level, make understanding the mechanism how this leads to scale-dependent environmental trait filtering and consequently affects insect communities increasingly important.