The assembly of plant–patch networks in Mediterranean alpine grasslands

Aims Harsh environmental conditions in alpine ecosystems shape vegetation structure into patches, where many different plant species cluster and grow together. Yet, which factors are important for the structure and dynamics of such plant-patch networks remains poorly understood. We aim to assess which and how environmental and biotic factors predict the assembly of plant patch networks along a mountain range. Methods We examined the distribution of plant species in more than 5500 vegetation patches in 37 Mediterranean alpine grasslands distributed along a 500 m altitudinal gradient (National Park of Sierra Guadarrama, Spain). We established a plant-patch network for each grassland community and analyzed how nestedness and modularity vary with environmental (altitude, insolation and soil conditions) and biotic factors (number of species per plot, mean patch area and total pasture area). Important Findings Plant-patch networks showed consistent, non-random patterns characterized by a nested, but not modular, structure, which suggests that positive associations among co-occurring specialists promote their growth within patches as subsets of a pool with more generalist species. Both nestedness and modularity of plant patch networks varied among grasslands. Specifically, nestedness decreased with increasing species per plot and increased with mean patch area, while it was independent of environmental variables; modularity increased with increasing pasture area and species per plot. The negative relationship between species per plot and nested patterns may be linked to the restricted number of species that can coexist within the same patch at a given size. Moreover, the positive relationship between patch size and nestedness indicates that the growth of rare plant species within vegetation patches occupied by more abundant species is facilitated in bigger rather than smaller patches. Furthermore, these results indicate that the nested assembly of vegetation patches may be independent of abiotic conditions. These findings suggest that large and unfragmented vegetation patches are fundamental for the maintenance of plant diversity in alpine grasslands. Looking at species distribution at fine spatial scales may shed new light on the biotic processes underlying plant network assembly and provide novel ways for conserving biodiversity.