Shrub encroachment, i.e. grassland invasion by shrubs, alters community composition and ecosystem function, and has been related to desertification in southeastern USA and elsewhere. Conversely, Mediterranean steppes colonized by shrubs commonly show higher levels of soil fertility and biodiversity. Contrasting effects of shrub encroachment may be related to differences in functional traits of patch-forming species and the way they interact. However, information on biotic and physical structure of shrub patches in these steppes is scarce. Identifying patterns of species composition in woody patches and their drivers would help us understand patch dynamics and the interaction between patch forming species and the vegetated matrix around them. In this study, we describe the composition and structure of 450 woody patches in a semi-arid Mediterranean steppe and explore the main drivers of patch properties.

Results/Conclusions

Woody patches are dominated by 8 species; all of them are sprouting shrubs and none of them N-fixing leguminous. The presence of a founder species, identified by larger individuals, is not evident. Most patches (58%) are plurispecific, species richness ranging from 2 to 5. Rhamnus lycioides, Pistacia lentiscus and Ephedra fragilis, the most common species, are present in 80%, 34% and 19% of the patches, respectively. Maximum patch size is over 100 m², but most patches cover between 3 and 8 m². Co-occurrences analysis revealed that species are not distributed at random. Rhamnus alaternus and Olea europaea only appear where R. lycioides is present, and R. alaternus never occurs in patches where Juniperus oxycedrus is present. The most dominant species showed no pattern of co-occurrences, and thus may not be structured by competition. Abiotic factors such as slope, aspect, stoniness and location within the watershed, were not strongly related to patch physical and biotic structure. Our results suggest that the size and composition of woody patches should be taken into account to fully understand the effects of shrub encroachment.