Grazing mediates microclimate effects on lichen performance near its warm-range margin

Climate influences species performance and distribution both directly, through physiological constraints, and indirectly, by modulating biotic interactions. However, the relative importance of these pathways remains poorly understood, limiting our ability to predict species responses to environmental change. Here, we investigated how microclimate influences the growth of a cold-adapted lichen both directly and indirectly, through effects on mollusc grazing, and whether these effects differ among populations of different origins.

We conducted a transplant experiment using lobes of Peltigera aphthosa collected from five geographically distant populations along a 1200-km latitudinal gradient in Sweden. The lobes were transplanted to 56 forest sites spanning a broad microclimatic gradient near the species' warm-range margin in Sweden, and we monitored their growth and grazing damage for one year. Using piecewise structural equation models, we quantified the direct and indirect effects of microclimate through grazing on lichen growth and assessed whether these effects differed among source populations.

We found no evidence for direct effects of microclimate on lichen growth. Instead, microclimate indirectly influenced growth through mollusc grazing. Grazing damage increased with warmer temperatures (more growing degree days) and higher air humidity (lower vapour pressure deficit) during the growing season and grazing reduced lichen growth. While population origin did not affect direct responses to microclimate, populations differed in their susceptibility to grazing.

Our study highlights the importance of indirect microclimatic effects mediated by biotic interactions in shaping species performance near its warm-range margin. The absence of direct microclimatic effects, combined with intraspecific variation in susceptibility to grazing, underscores the need to consider both biotic interactions and differences among populations when predicting species responses to climate change. Transplant experiments across microclimatic gradients offer a valuable method to gain insights into the complex interplay between local adaptation and abiotic and biotic factors of species performance.