Interpreting variation in restoration outcomes : functional traits shape community assembly and ecosystem functioning

Making sense of the mechanisms by which communities assemble and provide ecosystem functions is central to ecological research. The recovery of diversity and ecosystem functioning are also primary objectives of ecological restoration, yet these outcomes are often unpredictable. Restoration ecology has typically focused on reinstating particular sets of species; however, this focus on taxonomic composition limits generalization between restorations. Traits, due to their mechanistic and generalizable nature, may provide insights into community assembly mechanisms that move beyond this idiosyncrasy. That is, a better understanding of how traits vary among species may predict how their dispersal, establishment, and persistence affect species’ distribution and abundance among sites that vary in abiotic and biotic conditions. Functional trait-based approaches may shed light on a second major goal of restoration: understanding the functioning of ecosystems and how this is related to the diversity and composition of communities. Thus, functional traits hold great promise for interpreting, predicting, and linking the assembly and functioning of communities. This promise remains poorly realized, however, as tests linking environmental conditions, functional traits, and ecosystem functioning in restoration are rare. In turn, restored systems offer a unique test of ecological theory at the scale of ecosystems. Here I use plant functional traits to study community assembly and ecosystem functioning in grasslands undergoing restoration. My first two chapters take a trait-based approach to studying processes such as invasion and species establishment that underlie community assembly using experimentally manipulated prairie restorations. I found that the extent to which a species’ traits are adapted to the local environment, but not how much their traits overlap with species already residing at a site, influence a species’ ability to invade a novel community. Likewise, trait-environment interactions play an important role in the invasion process, further supporting the idea that having traits that are adapted to a particular environment is important for invasion success. Furthermore, considering traits independent of their environmental context is inadequate for understanding community assembly processes and trait-environment interactions determine seedling establishment rates in recent prairie restorations. My last two chapters attempt to use functional traits to link community assembly and ecosystem functioning using a set of 29 restored prairies in southwestern Michigan. I found that environmental conditions predicted community weighted mean traits, showing the value of traits for studying community assembly. In addition, I found that both functional traits and environmental conditions play an important role in shaping ecosystem functioning during restoration, and the importance of both traits and environment on functioning depends on the function of interest. Because of this, variation in environmental conditions will be necessary to promote multiple ecosystem functions across restored landscapes through management, such as prescribed fire, and by installing restorations in at sites with different environmental conditions. These results highlight the utility of functional traits for connecting community assembly and ecosystem functioning during restoration. Within this same system I also asked how different aspects of diversity (taxonomic, functional, phylogenetic), beyond individual traits, influence ecosystem functioning and an ecosystem’s ability to produce multiple functions simultaneously (ecosystem multifunctionality). I found that phylogenetic diversity and the makeup of the landscape surrounding a restoration determine ecosystem multifunctionality, though the effect of landscape is much stronger than the effect of diversity. I also find no tradeoffs between ecosystem functions that contribute to multifunctionality across sites; instead, functions are independently affected by diversity, environmental, and landscape variables. In this case, the processes that increase many individual functions—increased phylogenetic diversity and more natural landscapes—will increase multifunctionality. All of this work demonstrates that trait-based approaches to restoration can help improve our understanding of community assembly and ecosystem functioning at the ecosystem scale, explain variation in restoration outcomes, and show how restored systems can offer a unique test of ecological theory at the scale of ecosystems.