A fundamental goal in ecology is to understand how species interactions influence patterns of biodiversity. Cushion plants are known to act as nurse plants for individuals germinating within their cushion and can increase diversity by ameliorating local environmental conditions, yet their impact on functional diversity patterns are not well understood. In a dry meadow tundra community at the Niwot Ridge LTER, we investigated 1) how an alpine cushion plant, Silene acaulis (Caryophylaceae), influences environmental conditions; and 2) how Silene influences patterns of functional diversity among coexisting tundra plants. We hypothesized that Silene will ameliorate environmental conditions and increase functional diversity as compared to the adjacent tundra. In 30 Silene cushions and 30 paired areas of adjacent tundra we measured species composition, functional traits (leaf area, leaf dry-matter content (LDMC), specific leaf area (SLA), plant height) and environmental conditions (soil moisture, temperature). We then calculated community-weighted mean (CWM) trait values and functional dispersion (FDis) for each trait to examine the influence of facilitation on patterns of functional diversity.
We found that Silene acaulis influences environmental conditions within dry meadow communities by buffering changes in moisture and temperature, providing a more stable but occasionally more stressful local environment compared to surrounding tundra communities. Contrary to our expectations, both species richness and species diversity were lower in cushions than in the adjacent tundra, and species within cushions were a subset of species found in the adjacent tundra. Interestingly, CWM SLA was lower and CWM LDMC and leaf area were higher for species growing in the cushions than in the adjacent tundra suggesting a more stress-tolerant functional strategy is required to grow inside the cushion, than outside the cushion. Similarly, FDis of leaf area was lower inside the cushion, suggesting selection towards a common leaf area inside rather than outside of the cushion. Functional diversity in the other three traits did not differ between species growing in the cushion and species growing in the adjacent tundra. Taken together, results suggest that facilitation may not be occurring within all cushion plants, and that whether a cushion has a positive or negative effect on species growing in its cushion may be dependent on the local environmental conditions.