Canopy height explains species richness in the largest clade of Neotropical lianas

Meyer et al. (2019) – Global Ecology and Biogeography

Tall and structurally complex forests can provide ample habitat and niche space for climbing plants, supporting high liana species richness. We test to what extent canopy height (as proxy of 3D habitat structure), climate and soil interact to determine species richness in the largest clade of Neotropical lianas. We expect that the effect of canopy height on species richness is higher for lianas from closed tropical rainforests compared to riparian and savanna habitats. We used structural equation models to evaluate direct and indirect effects of canopy height, climate (temperature, precipitation and precipitation seasonality), and soil (cation exchange capacity and soil types) on overall Bignonieae species richness (339 liana species), as well as on species richness of lianas from forest, riparian and savanna habitats, respectively. We further performed multiple regression models with Moran’s eigenvector maps to account for spatial autocorrelation. Canopy height was a key driver of liana species richness, in addition to climate and soil. Both, overall species richness and forest species richness showed a strong positive relationship with canopy height whereas the relationship was less pronounced for riparian species. Richness of savanna species even decreased with increasing canopy height. Climate also explained a substantial proportion of variation in liana species richness whereas soil variables showed little explanatory power. The relationship between canopy height and liana species richness differs among habitats. While forest and riparian lianas probably benefit from physical support to reach the forest canopy to escape low light availability in the understory, a high light availability in open habitats and an increased risk of embolism of conductive vessels for lianas with long stems living in areas with high seasonality might explain the inverse relationship between species richness and canopy height in savannas.