Networks of epiphytic lichens and host trees along elevation gradients: climate change implications in mountain ranges

Several studies have evaluated lichen responses in terms of shifts in species climate suitability, species richness and community composition. In contrast, patterns of co-occurrence among species that could be related to complex species interactions have received less consideration. Biotic interactions play a major role in shaping species niches, fitness and adaptation to new environments. Therefore, considering the specific relationships among co-occurring species is essential to further deepen our knowledge of biodiversity response to climate change. In this perspective, the analysis of lichen ecological networks across elevational gradients may provide a powerful tool to understand how communities are structured and how biotic interactions are modulated by changing climatic conditions. We evaluated the contribution of environmental and species biological attributes to the structure of epiphytic lichen-host tree networks. Specifically, we studied lichen communities considering two different network levels: the whole lichen community, and groups of lichen species that presented similar biological traits. In this framework, we (a) characterized the structure of the epiphytic lichen-host tree networks; (b) assessed how network structure varied with climate, forest attributes and community trait diversity and (c) evaluated the role that biological traits played in the connections established between co-occurring lichens. On the one hand, results indicate that epiphytic lichen communities are dominated by local segregation, suggesting habitat specialization among lichens within their host tree, and that climatic conditions and, to a lesser extent, lichen diversity are the main drivers of community assemblage. On the other hand, the role of lichen species in the networks depends on their particular biological traits, supporting the hypothesis that biological traits contribute to shape network structure by influencing the ability of the species to interact between each other. These findings warn about the potential impact of climate change on epiphytic lichen communities. Synthesis. This study builds towards a better understanding of lichen community assembly and on biodiversity response to climate change in forest alpine ecosystems. In particular, our results highlight the value of lichen-tree networks to inform about assemblage processes acting at different organizational levels and indicate that lichens might become one of the most threatened groups under global change scenarios.