Raoulia albosericea as a Facilitator in Tongariro National Park, North Island, New Zealand: Revised Version

In alpine ecosystems, cushion plants often facilitate the growth and survival of other plants by ameliorating harsh abiotic conditions. However, our knowledge of these interactions is largely derived from studies conducted across a narrow range of environmental contexts, mostly in the Northern Hemisphere. The few studies that have been conducted in other contexts have found either no facilitation provided by cushions or that their interspecific interactions are highly variable. This study evaluates the role of the cushion plant Raoulia albosericea in facilitating other plant species in the unique environment along the eastern slope of Mt. Ruapehu, North Island, New Zealand. The region's arid conditions and unstable volcanic sands provide an opportunity to assess how these abiotic factors influence cushion species interactions. At both high and low elevations we tested 1) whether the presence of R. albosericea altered soil temperature and compaction under its canopy; 2) whether overall plant density and cover differed between cushion and noncushion environments; and 3) whether plant species were preferentially spatially associated within R. albosericea canopies compared to open ground. The presence of R. albosericea lowered soil temperature and increased soil compaction. Overall plant density and cover were significantly greater within R. albosericea canopies than in adjacent open areas. These effects were more pronounced at the high elevation site compared to the low elevation site. In addition, 10 of the 14 species we encountered in our surveys were significantly associated with R. albosericea . These findings suggest that R. albosericea facilitates the growth and establishment of several species on Mt. Ruapehu. This study contributes to our understanding of how facilitation varies across different environmental contexts, and it suggests that cushion plants could play a role in influencing diversity patterns in this unique alpine environment.