Effects of biodiversity and microsite heterogeneity on the restoration of quarries to alvars

Restoring ecosystem processes is challenging in severely degraded environments where stress is high and perturbations are common. Diversity-function and stress-gradient theories suggests that restoration may be improved by increasing the number of positively interacting species present in communities, through methods such as propagule introduction. An initiative to restore abandoned quarry floors in Ontario to globally rare, highly diverse rock outcrops called alvars provides a useful model system for testing this hypothesis; however, heterogeneity theory suggests that fine-scale spatial variability in these outcrops may interact with propagule diversity to influence the ultimate structure and functioning of restored communities. To investigate potential relationships among seed diversity, microsite heterogeneity, and restored vegetation cover, seeds of characteristic alvar species were planted on quarry floors across a natural gradient in within-community spatial heterogeneity, under five diversity treatments (involving 3, 6 or 12 species) designed to isolate effects of positive species interactions. Percent vegetation cover increased significantly with both seed richness and microsite diversity (estimated by the Shannon diversity of microsite types present in plots). Cover increased linearly with heterogeneity within all richness treatments, but at each level of heterogeneity seed-rich plots produced greater cover than seed-poor plots. The magnitude of the seed diversity effect increased with heterogeneity, probably because greater proportions of seeded species were able to germinate, establish and interact in more spatially diverse plots. These findings suggest that valued ecosystem properties can be more effectively restored in high-stress environments when land managers maximize or exploit interacting components of both biological and environmental diversity.