Fine-scale population dynamics help to elucidate community assembly patterns of epiphytic lichens in alpine forests

Highlights

• Simultaneous processes drive the fine-scale distribution of epiphytic lichens.

• These processes determine how the single species are sorted into community assemblage.

• Habitat filtering is the main factor underpinning species sorting.

Abstract

We examined the main and interactive effects of factors related to habitat filtering, dispersal dynamics, and biotic interactions, on tree-level population dynamics of a subset of species composing the epiphytic lichen pool in an alpine forest. We tested these processes evaluating the population size of 14 lichen species on six hundred and sixty-five trees within a 2 ha plot located in a high elevation alpine forest of the eastern Italian Alps. Our results indicate that community assembly patterns at the tree-level are underpinned by the simultaneous effects of habitat filtering, dispersal, and biotic interactions on the fine-scale population dynamics. These processes determine how the single species are sorted into community assemblages, contributing to tree-level community diversity and composition patterns. This corroborates the view that the response of lichen communities to environmental gradients, in terms of compositional and diversity shifts, may reflect differential species responses to different drivers.

Introduction

Habitat filtering, dispersal dynamics and biotic interactions are the main processes that determine community assembly (Lortie et al., 2004). The importance of each process may, however, depend on environment type, organism traits, and spatial scale of analysis (e.g., Guisan and Thuiller, 2005). The niche-based model emphasizes the role of habitat filtering in determining species patterns, especially at the fine-scale, while the neutral model (Hubbell, 2001) stresses the importance of dispersal dynamics at both broad (Wisz et al., 2013) and fine (Hanski, 1999) spatial scales, in connection with habitat connectivity and dispersal ability of the species (Snäll et al., 2004, Snäll et al., 2005). Biotic interactions also contribute to determine community assembly at different spatial scales (Wisz et al., 2013), based on the concept that species are not independent entities and interact with other species.

Overall, these processes influence the co-occurrence of the species within communities, which in turn determines community diversity and composition. In this perspective, the analysis of population dynamics in terms of influence of environmental factors, dispersal, and biotic interactions on the performance of single components of the community, may contribute to elucidate the mechanisms behind community assembly patterns (Lortie et al., 2004).

Epiphytic lichens are a functionally important and species-rich component of alpine forests (e.g. Nascimbene et al., 2014). They contribute to the water and nutrient cycling by retaining precipitation in the canopy and fixing atmospheric nitrogen (cyanobacterial lichens), and increasing microhabitat complexity and the diversity of forest invertebrate fauna, which in turn serve as food for a variety of passerine bird species (Ellis, 2012). Moreover, several species are worthy of conservation being included in red-lists (e.g. Nascimbene et al., 2013). Several studies have already assessed the effects of different drivers of epiphytic lichen community diversity and composition. In particular, there is a wide consensus about the role of host tree features and stand conditions (Johansson et al., 2007, Jüriado et al., 2009, Mežaka et al., 2012, Nascimbene et al., 2012), as well as dispersal dynamics (Löbel et al., 2006a). Recent findings also support the role of species interactions in determining lichen distribution patterns (Maestre et al., 2008). For instance, the inter-specific competition in saturated epiphyte communities leads to dominant species out-competing pioneers (for an overview see Ellis, 2012), and photobiont sharing could facilitate lichen establishment in forests (Rikkinen et al., 2002). However, the exploration of the mechanisms determining the observed community patterns is still scanty, hindering a deeper understanding of the general rules driving lichen community assembly along environmental gradients.

In this study, we developed a fine resolution analysis to test the main and interactive effects of factors related to different processes, namely habitat filtering, dispersal, and biotic interactions, on tree-level population dynamics of a subset of species composing the lichen pool in an alpine forest. This approach is expected to help to elucidate how species are sorted into tree-level communities, contributing to a more mechanistic view of community assembly patterns of epiphytic lichens on their host trees.