Environmental conditions and biotic interactions acting together promote phylogenetic randomness in semi-arid plant communities: new methods help to avoid misleading conclusions.

QUESTIONS Molecular phylogenies are being increasingly used to better understand the mechanisms structuring natural communities. The prevalent theory is that environmental factors and biotic interactions promote the phylogenetic clustering and overdispersion of plant communities, respectively. However, both environmental filtering and biotic interactions are very likely to interact in most natural communities, jointly affecting community phylogenetic structure. We asked the following question: How do environmental filters and biotic interactions jointly affect the phylogenetic structure of plant communities across environmental gradients? LOCATION Eleven Stipa tenacissima L. grasslands located along an environmental gradient from central to south-east Spain, which covers the core of the distribution area of this vegetation type in Europe. METHODS We jointly evaluated the effects of environmental conditions and plant-plant interactions on the phylogenetic structure -measured with the mean phylogenetic distance index- of the studied communities. As an indicator of environmental conditions, we used a PCA ordination including eight climatic variables. Different metrics were used to measure the following processes: i) competition/facilitation shifts at the entire community level (Species Combination index), and ii) the effect of the microclimatic amelioration provided by the two most important nurses on neighbours composition (similarity indices and comparison of the phylogenetic pattern between canopy patches and bare ground areas). RESULTS Biotic interactions and, to a less extent, environmental conditions, affected the phylogenetic pattern of the studied communities. While positive plant-plant interactions (both at the community and the scale of individual nurses) increased phylogenetic overdispersion, higher rainfall increased phylogenetic clustering. The opposed effects of environmental conditions and biotic interactions could be the main cause of the overall random phylogenetic structure found in most of these communities. CONCLUSIONS Our results illustrate, for the first time, how an overall random phylogenetic pattern may not be promoted only by the lack of influence of either environmental filtering or biotic interactions, but rather by their joint and opposing effects. They caution about making inferences about the underlying mechanisms shaping plant communities from the sole use of their phylogenetic pattern. We also provide a comprehensive set of easy-to-measure tools to avoid misleading conclusions when interpreting phylogenetic structure data obtained from observational studies.