Functional types in the Bromeliaceae: Relationships with drought-resistance traits and bioclimatic distributions
Neotropical Bromeliaceae occupy an exceptional diversity of habitats. The five principal functional types, which are defined by innovations such as Crassulacean acid metabolism (CAM), epiphytism, the tank growth form and neoteny, display distinct ecological water-use strategies. The contribution of putative drought-resistance traits to the ecological differentiation of functional types has not previously been assessed, despite growing interest in the importance of these traits in other plant groups. We formulated a set of hypotheses to be tested through a major survey of 376 bromeliad species (over 10% of the entire family) representing different functional types and ecologies. We quantified four drought-resistance traits: osmotic potential at full turgor (π o ), saturated water content (SWC), water mass per unit area (WMA) and dry leaf mass per unit area (LMA). For a subset of 308 species, relationships between drought-resistance traits and species bioclimatic envelopes were also analysed. Saturated water content, WMA and LMA were closely inter-correlated, and there was weaker coordination with π o , but the four traits differed significantly between functional types. Species of different functional types occupied distinct areas of bioclimatic space, and the relationships mapping drought-resistance trait values into bioclimatic space also varied between functional types. We conclude that divergences in drought-resistance trait values form an integral part of the evolution of functional type distinctiveness and climatic niche differentiation in this megadiverse tropical plant family. This study demonstrates how rapid, taxonomically extensive quantification of plant functional traits can provide important insights into the evolution of ecological diversity.