Relative influence of landscape vs. local factors on plant community assembly in restored wetlands

Ecological restoration often involves only the manipulation of abiotic factors at the local scale. However, processes external to a restoration site determine the range of local conditions within the site, constraining the level of restoration progress that can be achieved by on-site manipulations. We examined the relationship of landscape and local explanatory variables to plant species composition in 28 restored wetlands in Illinois, USA. Using constrained ordination combined with variation partitioning, we determined the independent and joint effects of three spatially hierarchical sets of variables: (1) macroscale landscape features reflecting site setting within regional landscapes, (2) mesoscale landscape features reflecting nearby propagule sources and buffers from disturbances, and (3) local environmental factors. Because the relative influence of landscape-vs. local-scale factors on restoration success may depend on particular restoration goals, we repeated the analyses using three multivariate plant community responses that represented three frequently stated goals: (1) replicating species composition, (2) restoring a particular wetland community type, and (3) constructing sites with high value for plant conservation. Explanatory variables at landscape and local scales had independent and nearly equally strong relationships to plant species composition. In contrast, when species were aggregated based on plant traits, the independent contribution of local predictors was greater than the independent contributions of macroscale or mesoscale landscape predictors, reflecting convergence of plant trait composition in sites with similar local conditions. Local predictors explained a significant amount of variation in plant conservation value among sites, but much of the variation could be explained by large-scale landscape setting, indicating that landscape constraints on local environmental conditions limited the level of floristic conservation value achievable. The appropriate scale at which to focus restoration efforts will vary depending upon restoration objectives. Restoration of particular wetland community types might be successfully achieved through manipulation of local abiotic factors. In contrast, restoration of a particular species assemblage or reconstruction of wetlands with high value for conservation requires consideration of landscape processes and available species pools.