Discovering the rules of plant biogeography using a trait-based approach

Lineage-specific traits determine how plants interact with their surrounding environment. As different species may find similar phenotypic solutions through evolution to tolerate, persist in and invade environments with certain characteristics, some traits may become more common in certain types of habitats. These general patterns of geographical trait distribution point towards the existence of some rules in how plants diversify in space over time. Trait-environment correlation analyses are ways to discover general rules in plant biogeography by quantifying to what extent unrelated lineages have similar evolutionary responses to a given type of habitat. In this synthesis, I give a short historical overview on trait-environment correlation analyses, from some key observations from classic naturalists to modern approaches using trait evolution models, large phylogenies, and massive datasets of traits and distributions. I discuss some limitations of modern approaches, including the need for more realistic models, the lack of data from tropical areas, and the necessary focus on trait scoring that goes beyond macro-morphology. Overcoming these limitations will allow the field to explore new questions related to trait lability and niche evolution and to better set apart rules and exceptions in how plants diversify in space over time.