Nurse plant effects on plant species richness in drylands: the role of grazing, rainfall and species specificity.

The outcome of plant-plant interactions depends on environmental (e.g. grazing, climatic conditions) and species-specific attributes (e.g. life strategy and dispersal mode of the species involved). However, the joint effects of such factors on pairwise plant-plant interactions, and how they modulate the role of these interactions at the community level, have not been addressed before. We assessed how these species-specific (life strategy and dispersal) and environmental (grazing and rainfall) factors affected the co-occurrence of 681 plant species pairs on open woodlands in south-eastern Australia. Species-specific attributes affected the co-occurrence of most species pairs, with higher co-occurrence levels dominating for drought-intolerant species. The dispersal mechanism only affected drought-tolerant beneficiaries, with more positive co-occurrences for vertebrate-dispersed species. Conversely, the percentage of facilitated species at the community scale declined under higher rainfall availabilities. A significant grazing × rainfall interaction on the percentage of facilitated species suggests that grazing-mediated protection was important under low to moderate, but not high, rainfall availabilities. This study improves our ability to predict changes in plant-plant interactions along environmental gradients, and their effect on community species richness, by highlighting that: 1) species-specific factors were more important than environmental conditions as drivers of a large amount (~30%) of the pairwise co-occurrences evaluated; 2) grazing and rainfall interaction drive the co-occurrence among different species in the studied communities, and 3) the effect of nurse plants on plant species richness will depend on the relative dominance of particular dispersal mechanisms or life strategies prone to be facilitated.