Ecosystem services provided by forests—including wildlife habitat, wood, fiber, and carbon storage—depend on forest productivity, the collective biomass growth of individual trees. Forest productivity is thought to increase with tree biodiversity, but there are many ways to quantify biodiversity, with little consensus on which measures are most important. Using >1.8 million tree measurements across eastern US forests, we show that the number of species (the most widely available biodiversity measure) is as good a predictor of forest productivity as more complex biodiversity measures that consider species properties and evolutionary history. This result suggests that conservation strategies maximizing the number of species may effectively conserve ecosystem functioning.

Despite experimental and observational studies demonstrating that biodiversity enhances primary productivity, the best metric for predicting productivity at broad geographic extents—functional trait diversity, phylogenetic diversity, or species richness—remains unknown. Using >1.8 million tree measurements from across eastern US forests, we quantified relationships among functional trait diversity, phylogenetic diversity, species richness, and productivity. Surprisingly, functional trait and phylogenetic diversity explained little variation in productivity that could not be explained by tree species richness. This result was consistent across the entire eastern United States, within ecoprovinces, and within data subsets that controlled for biomass or stand age. Metrics of functional trait and phylogenetic diversity that were independent of species richness were negatively correlated with productivity. This last result suggests that processes that determine species sorting and packing are likely important for the relationships between productivity and biodiversity. This result also demonstrates the potential confusion that can arise when interdependencies among different diversity metrics are ignored. Our findings show the value of species richness as a predictive tool and highlight gaps in knowledge about linkages between functional diversity and ecosystem functioning.