Successional habitat filtering of rainforest trees is explained by potential growth more than by functional traits

Abstract

Species along successional gradients differ in functional traits, which may function as environmental filters and also explain differences in growth rates. Alternatively, species might be filtered by growth rates and these are explained by differences in other traits. We explored the relationships between leaf and wood traits, growth and successional habitat, and asked if growth is related to habitat because both are driven by a similar set of traits or if growth is the main trait explaining habitat and relationships with other traits are indirect. Tropical trees are often classified as second-growth (SG) or old-growth (OG) specialists, and generalists. Since succession is a continuous process and lumping species into three groups may not do justice to gradual differences, we tested if a continuous variable based on relative tree abundance in OG and SG forests might be a better predictor than classifying trees' habitat preferences into categories. We measured the height growth of 47 trees species planted in a reforestation trial in Costa Rica and evaluated size-standardized potential growth during the first years when there was little shading among trees. Growth and habitat were related to wood density (WD), theoretical hydraulic conductivity (Kh), specific leaf area (SLA), leaf dry matter content (LDMC) and leaf nitrogen content (N). Potential growth rates and Kh were significantly correlated with habitat measured on a continuous scale, while growth and LDMC differed among the three groups of habitat specialization. Growth was also correlated with WD and Kh. Habitat specialization was correlated with the first principal component of the functional traits space. However, structural equation models suggest that the relationship with leaf and wood traits is mostly indirect and potential growth is the most important trait directly related to environmental filtering along a successional gradient. In our dataset, classifiying the successional status of species using the proportion of records from OG forests explains traits about as well as a classification into three groups, but provides more information about the species' position along the successional gradient. A free Plain Language Summary can be found within the Supporting Information of this article.