Restoration ecology seeks to repair the diversity and dynamics of ecosystems degraded by human activities. Restoration studies in tallgrass prairie have become a core part of the Konza Prairie LTER (KNZ) program over the last decade, and are particularly timely because human activities have resulted in widespread loss and degradation of tallgrass prairie and other temperate grasslands. Here we present an overview of three ongoing restoration studies that aim to develop and test hypotheses relevant to key issues in basic ecology while also addressing applied questions related to improving the structure, function, and sustainability of prairie restorations.
The first restoration experiment at KNZ was initiated in 1998, with a focus on the role of soil resource availability and heterogeneity in the restoration of plant community structure and ecosystem processes. We manipulated levels and heterogeneity of two factors (i.e., soil depth and nitrogen availability) known to influence plant diversity in native prairie. This test of the ‘heterogeneity hypothesis’ demonstrated that nitrogen availability is a strong determinant of diversity in newly-restored grasslands, with high soil nitrogen increasing dominance of grasses at the expense of other species. In the early years of this experiment, reducing available nitrogen reduced non-native species and promoted diversity. Over a decade of study has revealed subsequent declines in plant diversity in all treatments (a common problem in restorations), but more species have been maintained in plots with the most soil heterogeneity. New LTREB funding for this project is enabling us to assess longer-term trajectories of community and ecosystem recovery and test whether spatial and temporal variability in community structure creates vacancies and resources that can promote biodiversity.
A second KNZ restoration experiment is part of a multi-site test of how different source populations of prairie grasses affect plant community composition and ecosystem functioning. We created restoration plots using locally-collected seeds of a dominant grass or seeds from commercial cultivars, and then assessed intraspecific variation in plant traits for the different source populations. Cultivars exhibit different root architecture (i.e., greater root length, surface area, and volume) than non-cultivars, and had equivalent or enhanced physiological performance (i.e., net photosynthesis rate, stomatal conductance, and water use efficiency). We are also evaluating how intraspecific variation in a dominant species affects subordinate species and ecosystem processes in developing communities. Our results to date indicate that biotic filters of sown diversity and the selection of subordinate species have stronger effects on plant community structure and ecosystem functioning than does the source population of the dominant grasses. These studies are providing important insights into how the choice of species and seed source can affect restoration outcomes.
Most recently, we initiated a long-term restoration chronosequence experiment as part of our current KNZ LTER grant. We are restoring prairie in an agricultural field using the same species, seeding rates, and restoration techniques to examine the extent to which inter-annual variation in environmental conditions at the onset of a restoration is a strong organizing force on community assembly. Controlling soil type, land-use history, seeding rates, species pools, and management will allow us to address the relative importance of stochastic events, such as climatic variability, which can affect initial seedling establishment and early community structure. The chronosequence will also allow us to more accurately quantify belowground recovery from disturbance and rates of carbon accrual over time.
Collectively, the restoration studies at KNZ are providing insights into the roles of abiotic and biotic variation on the restoration of tallgrass prairie communities and associated ecosystem functioning. These studies will aid in developing more effective restoration approaches and enable us to better forecast the responses of restored grasslands to environmental change.