Biogeographic context mediates multifaceted diversity-productivity relationships in island and mainland forests

Abstract

Aim: Growing evidence suggests that impacts of biodiversity loss on ecosystem functioning and nature's contributions to people are usually negative, yet the magnitude and direction of these impacts can be variable across naturally-assembled ecosystems. A potential driver of variation in diversity-productivity relationships is the biogeographical context, which may alter these relationships via processes acting on the size and composition of the species pool like dispersal limitation, environmental filtering, speciation, and invasibility. However, the extent to which the relationships between biodiversity facets and forest productivity are shaped by the biogeographic context remains uncertain. Here, we examine the effects of taxonomic, phylogenetic, and functional tree diversity on aboveground productivity in climatically similar forests on islands and mainland. Location: Continental and insular Spain. Time period: 1997-2018. Major taxa studied: Trees. Methods: Using plot data from a national forest inventory, we assessed the influence of taxonomic, phylogenetic, and functional diversity on aboveground productivity using linear models and structural equation models, while accounting for environmental conditions, non-native species, and the number of trees. Results: We find that drier environmental conditions lead to a decrease in productivity and in the number of trees in both island and mainland forests. In island forests, non-native species increased productivity directly and via their effects on phylogenetic diversity. Main conclusions: Our results suggest that multifaceted diversity, by capturing the diversity of evolutionary history, contributes to elucidating diversity-productivity relationships in island forests that could not be detected otherwise by taxonomic diversity alone. By filling empty niches in island forests, we find that non-native species are fundamentally altering ecosystem functioning on islands.