Within‐species variability is the main driver of community‐level responses of traits of epiphytes across a long‐term chronosequence

Summary There has been growing recent interest in the relative importance of within‐species trait variation vs. across‐species trait variation in vascular plants in determining total community‐level trait variation across communities and environmental gradients. Recent studies on plant communities have generally found across‐species variation to be more important than within‐species variation, but comparable studies involving other functionally important biota, such as lichens, are largely lacking. We used a fire‐driven chronosequence involving 30 lake islands in northern Sweden to study how declining soil fertility during retrogression affects the functional traits of each of the dominant epiphytic lichen species growing on the trunks of the tree B etula pubescens . We measured several functional traits for the commonest lichen species on each island and used community‐weighted measures to study the community‐level responses of lichens to the gradient. We found that as retrogression proceeds and soil fertility declines, thallus N and P concentrations and specific thallus mass ( STM ) increase, both within species and at the community level. Lichen secondary compounds showed contrasting within‐species responses and were non‐responsive at the whole community level. By decomposing community‐level measures of these traits across the gradient, we showed that for the three most responsive traits (N, P and STM ), within‐species variation was substantially more important than across‐species variation. This emerges in part because lichen species composition was not very responsive to ecosystem retrogression, and because unlike vascular plants, lichens easily absorb elements over their entire surface, meaning that nutrient concentrations within lichen species are likely to more closely reflect nutrient availability. We found that within‐species variability drove the changes in community‐weighted measures of lichen traits across a strong environmental gradient, which contrasts strongly with what we know from studies of vascular plants where across‐species variation and species turnover is much more important. To understand how lichen functional traits at the community level respond to environmental factors, it is therefore essential to consider the responses of individual species, and the application of traits‐based approaches to lichen communities needs to account for their considerable intraspecific variability.