Do plant-plant interactions drive New Zealand's gravel beach plant community structure?

Abstract

Gravel beaches are a naturally rare ecosystem in New Zealand. Often poorly understood and managed, they also support a number of rare and threatened plant species. Extreme environmental conditions and high levels of disturbance combined with typically close proximity to highly modified landscapes make this a challenging environment for species survival. This study aims to understand some of the mechanisms structuring gravel beach plant communities. A survey of the plant community on the gravel beach at Birdlings Flat, Canterbury was carried out to identify plant co-occurrence patterns, the impact of invasion on these co-occurrence patterns and the spatial association between native and exotic species. This observational study demonstrated that species were segregated more than expected at random, indicating that spatial heterogeneity and competition are likely to be structuring the community. To investigate the factors influencing the recruitment of native woody shrub seedlings, in particular the role of facilitation, two experiments were established. The first experiment at Birdlings Flat, Canterbury involved planting native woody seedlings to assess the potential role of established native shrubs in facilitating seedling recruitment and whether either nutrient or water limitation had a significant impact on this interaction. The second planting experiment carried out at Tangoio, Napier was established to assess whether an exotic herb facilitates the recruitment of native plant seedlings. Environmental factors, specifically substrate composition, nutrient availability and distance from the sea, influence native seedling recruitment. Both experiments showed that facilitation is not actively promoting native woody seedling recruitment. The findings of this study contribute to the understanding of the importance of plant-plant interactions in highly stressed ecosystems and provide a basis for management decisions.