Pulling on multiple threads of evidence to reveal tree species coexistence

Understanding drivers of species coexistence is a central challenge in ecology. Coexistence cannot be observed directly, and while species co-occurrence in time and space is necessary for coexistence, it is not sufficient to prove coexistence. Species exclusion from a region is potentially observable, but can take decades to occur, and still might occur stochastically. Thus, ecologists generally use theory to identify indirect observations that are indicative of mechanisms driving coexistence or exclusion. Various methods have been developed to indirectly infer coexistence, each of which requires different data, and none of which are usually conclusive on their own. Here, we demonstrate agreement using three different approaches examining coexistence of multiple hardwood species. First, in an experimental planting of three mature tree species we found no relationship between productivity and species diversity, which could be due to a lack of niche differences among species. Second, we used modern coexistence theory to calculate niche and fitness differences for each pair of species, which confirmed the lack of niche differences among species, and showed high fitness differences that could create a neutral distribution of species in nature. Third, we used the United States Department of Agriculture Forest Inventory and Analysis data to examine co-occurrence patterns of our species across thousands of natural forest stands and found that indeed, these three species were distributed randomly throughout the USA. Given that these independent methods agree, we take this as strong evidence about a lack of coexistence.