Functional diversity and redundancy of tropical forests shift with elevation and forest-use intensity.
Change and intensification of forest use alter tropical ecosystems, influencing biodiversity and, subsequently, ecosystem functioning. The implications of eroding biodiversity may go beyond decreases in species diversity, resulting in changes of functional diversity, that is, the diversity of ecological strategies present in the community, and functional redundancy, that is, how redundant these strategies are to biodiversity loss. However, how environmental conditions and anthropogenic influences shape functional diversity and redundancy in tropical forests remains poorly understood. Here, we examine how tropical forests respond to forest-use intensity along an extensive elevational gradient in Mexico from the tropical lowlands to high-elevational mountain forests (0-3,500 m), in terms of functional diversity and functional redundancy, and how these biodiversity facets are related to forest structure. In our study, elevation was crossed with three levels of forest-use intensity: old-growth, degraded and secondary forests. At eight elevational sites, five replicate plots were inventoried for each level of forest-use intensity (total n = 120 plots). Functional diversity and redundancy were calculated using leaf and wood traits of 144 tree species for Hill numbers zero, one and two. Interactive effects between elevation and forest-use intensity significantly affected biodiversity facets. However, interactive effects resulted from forest-use intensity influencing biodiversity facets at only a few elevations, and not from a consistent, negative impact of forest-use intensity. Forest structure, specifically stem density and the Gini coefficient, explained variation across biodiversity facets when these facets gave equal weight to common and rare species. Synthesis and applications. High functional diversity and functional redundancy from lowland to mid-elevation tropical forests suggest that these ecosystems can be resilient to future disturbances. Our results indicate that the ability of high-elevation forests may be particularly susceptible to climate change and increasing forest-use intensity. For these high-elevation forests, we recommend that forest managers implement enrichment plantings with native species that can tolerate future environmental conditions. Finally, our study shows that efforts to conserve structurally heterogeneous forests and actively managing forests to increase structural heterogeneity will enhance the resilience of tropical forests while conserving their biodiversity.