Spatial structure and interspecific interactions in Mediterranean high mountain vegetation (Sistema Central, Spain)

Interspecific interactions among species are generally accepted as important processes in communities. Positive interactions seem to be most common in environments with high stress, but negative effects also appear simultaneously. The mechanisms of these interactions are diverse and include shade, improvement of soil characteristics, or protection from animals in the case of facilitation, or allelopathy or direct contact, in the case of competition. We studied vegetation patches in two high areas of the Sistema Central (Spain): Sierra de Bejar and Sierra de Guadarrama. This vegetation is very rich in cushion chamaephytes such as Armeria caespitosa, Silene elegans, or Jasione centralis. The aim of this study was to determine whether interspecific associations exist in this Mediterranean high mountain vegetation and if such associations are important in plant communities. We also wanted to know if environmental features can explain spatial distribution of species on patches. For that purpose, we recorded species composition of patches and their cover, as well as the highest and lowest diameter of patches, altitude, and exposition. Successional changes have also been checked through the total species richness and size of patches. Results showed that patches dominated by different cushion plants did not differ substantially in species composition, although species affinity for patches is different depending on the number of species. Herbaceous species appear to act as late colonizers, mostly restricted to species-rich patches, although some species, such us Agrostis truncatula, act as early colonizers in Sierra de Bejar. Flat cushion plants could be considered nurse plants, as they can modify micro-environmental conditions and allow establishment and survival of associated species. However, in some cases they can also act in competition with other cushion plants or herbs for the same space.