Soils and rainfall drive landscape-scale changes in the diversity and functional composition of tree communities in premontane tropical forest

Questions Premontane tropical forests harbour exceptionally high plant species diversity; understanding which factors influence their species composition is critical to conserving them, and to predicting how global environmental change will affect them. We asked: (1) How does alpha and beta diversity vary at the landscape scale? (2) How important is environmental filtering in structuring these communities? (3) Which soil and climate variables account for the most compositional variation? Location Old-growth premontane forest (850-1330 m a.s.l) in the Fortuna Forest Reserve, western Panama. Methods All trees ≥ 5 cm diameter at breast height (d.b.h) were censused in twelve 1 ha plots up to 13 km apart. For each plot, we measured soil properties (0-10 cm depth) at thirteen locations, and estimated or measured monthly rainfall. To evaluate how the environmental and spatial variables are associated with community composition, we used ordination and mantel tests. Results Alpha diversity varied nearly three-fold among plots (68 – 184 species ha−1). Beta diversity was also high, with only one of 364 species present in all plots. Turnover reflected distinct forest community types that have developed on different parent materials: forests on rhyolite had an abundance of either ectomycorrhizal-associated trees or canopy palms, while forests on the other rock types (andesite, dacite and basalt) were dominated by trees that form arbuscular mycorrhizal associations. While NMDS ordination showed that seasonality of rainfall was significantly correlated with species turnover, turnover also covaried with geographic distance. Nonetheless, large compositional differences were apparent among sites <2 km apart with similar rainfall but differing soils. Partial Mantel tests controlling for geographic distance highlighted the relationship between total phosphorus and species composition. Conclusions Soil nutrient availability and rainfall seasonality in premontane forests at Fortuna is associated with striking variation in the taxonomic and functional composition of nearby tree communities, and with plot differences in species richness comparable in magnitude to those reported over >1000 m of elevation in previous studies. Accounting for how local edaphic conditions structure premontane and montane tropical forests will be critical to predicting how tree communities will respond to climate change. This article is protected by copyright. All rights reserved.