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

Abstract

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.

Recreational trails as a source of negative impacts on the persistence of keystone species and facilitation

“…The strong winds and effect of frost heave also result in thin alpine humus soil and metasediments that are mostly covered by loose sedimentary shales. As a result, vegetative cover and diversity within the community is limited, with E. gunnii occupying around 54% of the community and only 19-23 species found here in total (Costin et al, 2000;Ballantyne and Pickering, 2015). Much of the remaining area consists of bare, rocky ground and small scattered herbs, cushions and graminoids (Costin et al, 2000;McDougall and Wright, 2004;Ballantyne et al, 2014a).…”

“…In response to this, community composition appears to be segregated in the community into distinct assemblages associated with either the shrubs or bare ground areas (Ballantyne and Pickering, 2015). In and around the protective canopy of E. gunnii there are taller herbs and graminoids with larger leaves, while in exposed areas of bare shale between the shrubs, plants tend to be more stress-tolerant with short canopies and small leaves (Ballantyne and Pickering, 2015).…”

“…Facilitation often results in increased alpha (within habitat) and beta (between habitats) diversity (Whittaker, 1972;Krebs, 1994) in plant communities Cavieres et al, 2014;Ballantyne and Pickering, 2015) when plants that benefit from the nurse canopy include species that otherwise would be restricted to less extreme environments (Sch€ ob et al, 2012. For example, facilitation can allow species with functional traits better adapted for competition (larger, softer leaves and tall growth forms) to grow in these environments in addition to those with traits better adapted to high stress conditions (shorter with tough, small leaves) (Cornelissen et al, 2003;Sch€ ob et al, 2012Sch€ ob et al, , 2013Ballantyne and Pickering, 2015).…”

“…For example, facilitation can allow species with functional traits better adapted for competition (larger, softer leaves and tall growth forms) to grow in these environments in addition to those with traits better adapted to high stress conditions (shorter with tough, small leaves) (Cornelissen et al, 2003;Sch€ ob et al, 2012Sch€ ob et al, , 2013Ballantyne and Pickering, 2015).…”

“…The dominant shrub in the community, Epacris gunnii (Epacridaceae), appears to facilitate the establishment of other plants on the ridgeline, with numerous dwarf herbs and graminoids growing in its canopy (Edwards, 1977;McDougall and Wright, 2004;Ballantyne and Pickering, 2015). Unfortunately, the sclerophyllous traits, prostrate brittle form and slow growth of E. gunnii make it particularly sensitive to damage and slow to recover (Costin et al, 2000;McDougall and Wright, 2004;Ballantyne et al, 2014a).…”