Shrub facilitation is an important driver of alpine plant community diversity and functional composition

Facilitation often occurs in environments where extreme climatic factors favour positive rather than negative species interactions. This includes larger 'nurse' cushion plants in some alpine environments that shelter other species from abiotic filters such as abrasive winds and low temperatures. This type of facilitation can contribute to higher alpha (within habitat), beta (between habitats) and gamma diversity (across the community), by facilitating species with less stress tolerant but more competitive functional traits otherwise excluded from these communities. We assess whether shrubs play a similar role in facilitation, using the dominant prostrate shrub Epacris gunnii in a high conservation value plant community along the highest ridgeline in the Australian Alps. Differences in alpha and beta diversity, species and functional composition were compared in and out of shrubs using point sampling and quadrats. Shrub habitat enhanced alpha (species richness per plot) and beta diversity resulting in greater gamma diversity for the community as a whole, with twice as many species associated with the shrub than open quadrats. There were also differences in functional composition with vegetation in shrub quadrats being taller with larger leaves. It seems that E. gunnii creates micro-refugia that assist in the range expansion of more productive but less stress-tolerant species, including some endemics, in the community. Alpine shrubs can therefore occupy a similar facilitative role to that of cushion plants in alpine plant communities and therefore may also be important drivers of diversity and functional composition.