The Assembly of Tropical Dry Forest Tree Communities in Anthropogenic Landscapes: The Role of Chemical Defenses

Abstract

The effect of anthropogenic disturbance on plant community traits and tradeoffs remains poorly explored in tropical forests. In this study, we aimed to identify tradeoffs between defense and other plant functions related to growth processes in order to detect potential aboveground and edaphic environmental conditions modulating traits variation on plant communities, and to find potential assembly rules underlying species coexistence in secondary (SEF) and old-growth forests (OGF). We measured the foliar content of defense phytochemicals and leaf traits related to fundamental functions on 77 species found in SEF and OGF sites in the Jalisco dry forest ecoregion, Mexico, and we explored (1) the trait-trait and trait-habitat associations, (2) the intra and interspecies trait variation, and (3) the traits-environment associations. We found that phytochemical content was associated with high leaf density and leaf fresh mass, resulting in leaves resistant to drought and high radiation, with chemical and physical defenses against herbivore/pathogen attack. The phytochemicals and chlorophyll concentrations were negatively related, matching the predictions of the Protein Competition Model. The phylogenetic signal in functional traits, suggests that abundant clades share the ability to resist the harsh biotic and abiotic conditions and face similar tradeoffs between productive and defensive functions. Environmental filters could modulate the enhanced expression of defensive phytochemicals in SEF, while, in OGFs, we found a stronger filtering effect driving community assembly. This could allow for the coexistence of different defensive strategies in OGFs, where a greater species richness could dilute the prevalence of pathogens/herbivores. Consequently, anthropogenic disturbance could alter TDF ecosystem properties/services and functioning.

Linking Anthropogenic Landscape Perturbation to Herbivory and Pathogen Leaf Damage in Tropical Tree Communities

“…Herbivores and pathogens differed in the degree of specialisation in foliage damage inflicted on vegetation (Figure S1). More specialisation in plant-herbivore interactions was usually associated with the most diverse tree communities, as a greater species richness presents a greater diversity of plant defensive strategies [55] and less optimum biotic and abiotic conditions for proliferation of generalist herbivores (e.g., stronger top-down control of herbivores by predators) [56]. In this sense, in the less diverse and more disturbed tree communities, the proliferation of generalist herbivores is expected with less specialisation in their interactions, as we found in this study.…”