Data from: Evolutionary relationships can be more important than abiotic conditions in predicting the outcome of plant-plant interactions

Positive and negative plant–plant interactions are major processes shaping plant communities. They are affected by environmental conditions and evolutionary relationships among the interacting plants. However, the generality of these factors as drivers of pairwise plant interactions and their combined effects remain virtually unknown. We conducted an observational study to assess how environmental conditions (altitude, temperature, irradiance and rainfall), the dispersal mechanism of beneficiary species and evolutionary relationships affected the co-occurrence of pairwise interactions in 11 Stipa tenacissima steppes located along an environmental gradient in Spain. We studied 197 pairwise plant–plant interactions involving the two major nurse plants (the resprouting shrub Quercus coccifera and the tussock grass S. tenacissima) found in these communities. The relative importance of the studied factors varied with the nurse species considered. None of the factors studied were good predictors of the co-ocurrence between S. tenacissima and its neighbours. However, both the dispersal mechanism of the beneficiary species and the phylogenetic distance between interacting species were crucial factors affecting the co-occurrence between Q. coccifera and its neighbours, while climatic conditions (irradiance) played a secondary role. Values of phylogenetic distance between 207–272.8 Myr led to competition, while values outside this range or fleshy-fruitness in the beneficiary species led to positive interactions. The low importance of environmental conditions as a general driver of pairwise interactions was caused by the species-specific response to changes in either rainfall or radiation. This result suggests that factors other than climatic conditions must be included in theoretical models aimed to generally predict the outcome of plant–plant interactions. Our study helps to improve current theory on plant–plant interactions and to understand how these interactions can respond to expected modifications in species composition and climate associated to ongoing global environmental change.dataThis dataset aimed to assess the relative importance of evolutionary distance, dispersal mechanism and environmental conditions as drivers of plant-plant interactions in semiarid environments. For doing this, we sampled 11 Stipa tenacissima grassland communities along a climatic gradient spanning from the center to the southeast of Spain. Our study sites have annual precipitation and temperature values ranging from 273 mm to 488 mm and from 13°C to 17°C, respectively. Four environmental variables were collected for each site: irradiance, mean temperature, annual rainfall (collected from available climatic models, Ninyerola et al. 2005-Atlas Climático Digital de la Península Ibérica. Metodología y aplicaciones en bioclimatología y geobotánica. Univ. Autónoma de Barcelona, Bellaterra, Spain), and altitude. At each site we also evaluated particular pairwise interactions for the two dominant nurses in the study area (Q. coccifera shrubs and S. tenacissima tussocks) by using 0.5 m x 0.5 m quadrats (see associated article for further details). With this data, we obtained two indices, the RII (Armas et al. 2004-Ecology 85: 2682–2686) to evaluate the intensity of the interaction, and the Iimp (Seifan et al. 2010-J. Ecol. 98: 356–361) to evaluate the importance of such pairwise interactions. To complement de dataset, phylogenetic distance between the two interacting species and the dispersal mechanism of the beneficiary species were also measured or obtained from available literature. More details from the dataset are given in the "metadata" spreadsheet and in the associated article). Related databases can be found in the dryad data package Data from: Environmental conditions and biotic interactions acting together promote phylogenetic randomness in semi-arid plant communities: new methods help to avoid misleading conclusions. Provisional DOI: doi:10.5061/dryad.d24c7. Gaps in the data are in blank. Field and dispersal mechanism data were collected by S. Soliveres and F.T. Maestre, the phylogenetic distances were calculated by R.Torices. Contact for further information: Dr. Santiago Soliveres (e-mail: santiagosoliverescodina@gmail.com