Plant functional groups in a tropical dry forest managed for millenia: Winners and losers

Background/Question/Methods: Plant classification into functional groups assumes that species having particular traits also have particular functional effects on ecosystems. Alternatively, species can be classified as specialists or generalists according to their responseto environmental gradients. The functional approach is based on phenological, physiological and morphological traits enabling plants to cope with or modify environmental factors that act as filters on species, and hence influence community assembly; whereas the distinction between generalists and specialists hinges on the strength of species’ affinities for particular habitats, and is especially relevant in human-modified landscapes –which are becoming increasingly common. Here, we aimed to classify woody-plant species in a disturbed tropical dry forest in The Yucatan, considering both species’ responses to environmental changes associated with secondary succession after swidden agriculture (generalist versus second-/old-growth forest specialists), and according to their effect on the ecosystem through the use of functional traits associated with life history and nutrient assimilation and retention (plant functional groups). We also aimed to identify traits that allow generalist species to persist throughout the environmental gradient associated with secondary succession in a forest that has been managed for millennia.

Results/Conclusions: We classified 65 woody plant species as generalists or specialists using a multinomial model based on abundance in second- and old-growth forests, and into plant functional groups based on 13 functional traits associated with life history and nutrient assimilation/retention. We found that all species that could be confidently classified (41) responded similarly to the environment of early and late successional forests, and were consequently classified as generalists. However, these species belonged to three functional groups with different sets of functional traits: one with rapid acquisition of resources, which was also highly dominant (the winners) and two groups with slow acquisition and long retention of resources, which also showed low dominance (the losers). Our results suggest that millennial human activities have favored winner species by providing them with the micro-environmental conditions that enable them to thrive, while disfavoring loser species that require conditions associated with old-growth forests, which are rapidly diminishing under human pressure. We also found changes in dominance of functional groups on hills, likely in response to increased water deficit. Overall, our results suggest that human-modified environments act as new filters on species, promoting the proliferation of common, generalist species at the expense of rare, specialist species, ultimately leading to increased similarity among communities (biotic homogenization).