Environmental filtering and competition are two fundamental processes that shape plant community assembly in terms of functional composition, that is, the distribution of trait values. Understanding the role of these two processes in the assembly of non-vascular vegetation, such as bryophytes and lichens, is important since these communities provide essential ecosystem services in many regions around the world, and these depend on functional composition. Responses of non-vascular communities to environmental selection pressures have been explored in a range of experimental and modelling studies. However, it is still largely unknown to what extent competition affects the distribution of functional traits of non-vascular communities. Moreover, it remains poorly explored which traits that are associated with competition are key for shaping community functional assembly.
Here, we integrated a field transplantation experiment with a process-based model to disentangle the effects of environmental filtering and competition on the assembly of non-vascular communities. Following the validation of environmental selection in the model using field observations, we performed a simulation experiment to understand the impacts of competition on trait distributions in non-vascular communities growing in two temperate locations that differ in microclimatic conditions (a shaded and an open location).
Our results suggest that the functional composition is likely a result of weak competition or may not depend on competitive exclusion at all while environmental filtering plays an essential role. Plant height seems to be a key trait for competition. However, no single-trait competition scheme could consistently explain the observed functional composition of the studied non-vascular communities.
Synthesis. The presented model provides a new trait-based approach for simulating the functional assembly of non-vascular communities. Environmental filtering appears to be more essential than competition for predicting trait distributions of non-vascular communities under temperate climatic conditions and we recommend caution in associating competition to a single trait while analysing community functional assembly.