What makes a good neighbour? Drivers of facilitation in alpine cushion plant communities

Abstract

Species interactions, whether facilitative or competitive, play key roles in structuring plant communities. Research into these associations has focused on competitive interactions, however recently, facilitation research has increased in popularity. Using cushion plants as a model, the objective of this thesis is to determine the effect of abiotic drivers on species interactions as well as on the morphology and reproduction of a potential facilitator. Results from a New Zealand cushion species, Donatia novae-zelandiae found in mosaic alpine environments and oceanic sea-level sites were compared to those from a widespread Northern Hemisphere cushion species, Silene acaulis. Alpine (~1000m) D. novae-zelandiae cushions produced three times as many flowers and seven times as many seeds per capsule than at sea-level, but leaves were larger at sea-level. Cushion compactness was greatest at alpine sites. After two seasons of artificially warming 1.8°C, significant decreases in seed production (35%), leaf length (5%), and width (13%) were observed in the cushions. Donatia novae-zelandiae modestly increased species richness at the sea-level sites (1.4 ± 0.5 more species), but no species was specifically limited to growing within the cushions. Most species showed no significant association, although Dracophyllum longifolium, D. prostratum, Phyllachne colensoi, Rhacomitrium pruinosum, and Coprosma cheesemanii showed significant negative associations with Donatia novae-zelandiae at the alpine sites. Donatia novae-zelandiae may marginally increase species diversity locally, but diversity is not affected at the community level. At alpine sites, D. novae-zelandiae decreased species richness (2.5 ± 0.8 fewer species) compared to open areas. Removing neighbours growing within the D. novae-zelandiae resulted in significant heat stress to the cushions, reducing seed production, compactness, and leaf size. Donatia novae-zelandiae, unlike other cushion species, does not appear to be acting as key a facilitator regardless of whether they grow in an alpine community, or in a more mesic, low altitude habitat. D. novae-zelandiae may in fact benefit from their association with neighbouring plants. Unlike D. novae-zelandiae, Silene acaulis fitness at a high elevation site (2560 m) was reduced compared to the lower site (2317 m); female flower production decreased by 40%, seeds per fruit by 11.6%, and leaf size by 24% at the high site. Strong facilitators, hermaphroditic Silene acaulis individuals supported a greater number of plant species than females (hermaphrodites: 4.2 ± 0.3, females: 3.5 ± 0.2). Facilitative effects also significantly increased with elevation (2560 m: 2.1 ± 1.6, 2317 m: 3.2 ± 1.8). The typical ‘cushion model’ so often reported in the facilitation literature does not accurately represent the cushion plant functional type as a whole. D. novae-zelandiae is not a facilitator, but may rather be acting as a competitor, particularly in alpine cushion mosaic communities. This is likely driven by the peat substrate, typical of cushion mosaic communities. In comparison, cushions growing on more skeletal substrates, such as Silene acaulis are important facilitators. Results of this thesis stress the importance of studying cushion facilitation over a broader spectrum of cushion life forms and habitats