Wood density variation across an Andes-to-Amazon elevational gradient

Understanding how functional traits are related to species diversity and ecosystem properties is a central goal of ecology. Wood density is a trait that integrates many aspects of plant form and function and is highly variable among species. Previous studies of wood density across elevational gradients have been based on limited sampling and have reported declines with increasing elevation, though even this simple pattern remains unknown, much less its underlying functional and evolutionary relationships. Here, we use one of the longest and most speciose elevational gradients in the world, extending from the Andean tree line to the Amazon basin, to test the extent to which elevation, species composition, phylogenetic affinity and forest structure determine variation in wood density. Using field-collected wood samples and global databases, we assigned wood density to 1231 species and 31,330 stems across 41 (47.5 ha) mature forest plots arrayed across a 3500-m vertical gradient. Our results show that mean wood density, either weighted by abundance, basal area or species, was highly variable but tended to decline from low to middle elevations and increase again from mid-elevations to the tree line. As a result of this non-linearity, forests at the Andean tree line had higher wood density than their lowland Amazon counterparts. We observed an abrupt transition in wood density at the lower limit of persistent cloud formation (cloud base), where the lowest wood density values were found. The decline of wood density is attributed to a significant shift in life forms, with an abundance of tree ferns at middle elevations and a higher probability of landslides and disturbances favouring a suite of traits associated with low wood density, such as softer wood and higher elasticity. Species turnover explained most of the among-species variation across the gradient, with elevation having no consistent effect on within-species variation in wood density. Together, both gradual compositional changes and sharp local changes in the importance of non-dicot life forms, such as arborescent ferns and palms, define patterns of forest-level carbon density, with wood density per se controlling ecosystem properties, such as carbon flux, across the Andes-to-Amazon elevational gradient.