The species-rich genus Eucalyptus dominates Australian overstorys, often with closely related species occurring in the same communities. How did Eucalyptus radiate so prolifically? How do closely related species coexist in communities despite the likely role of competitive interactions? Recent developments including finely resolved phylogenetic hypotheses, sophisticated distribution modelling tools, and improved understanding of plant functional traits have revived interest in such classic questions. We combined the following information in an attempt to reconstruct plant assembly for Eucalyptus species: a species phylogeny, species distribution models created with Boosted Regression Trees, and plant trait data. Data were collected from field plots spanning elevation gradients in the Grampians National Park, Victoria, Australia.
We found evidence that the relative contribution of processes controlling the distribution of plants varied along an environmental gradient. In higher elevation, exposed rocky outcrops, coexisting plants tend to be closely related and share similar functional trait values such as specific leaf area (SLA), tree height, and seed size. These traits are likely important in conferring the ability to thrive in harsh environments. In lowland areas with deep soils and more favorable temperatures, coexisting species are more distantly related. These species contain similar values for traits that are important for habitat partitioning (SLA and tree height), but may differ in traits that lessen the effects of competition (flowering time). Traits important for habitat partitioning are conserved and traits contributing to competitive interactions are more labile through evolutionary history. Our work provides a baseline model, which can be extended to additional eucalypt-dominated systems.