Prairie diversity: Relative roles of biotic and abiotic factors in a managed grassland system

Background/Question/Methods

The establishment and maintenance of diversity in grassland ecosystems is controlled by a variety of abiotic and biotic factors, including soil characteristics, community dynamics, and management. Despite growing efforts to restore and conserve native grasslands, the relative roles of these factors, and especially how they interact, are poorly understood. This study examines the relative roles of soil texture, soil fertility (total C and N), and management (seeding, fire, and grazing) as they pertain to diversity and productivity. In order to study the primary and interactive effects of environmental factors, our study site was established in an area with high soil variability (sand 30-99%, silt 0-52%, clay 0-20%) along the Platte River floodplain in Nebraska. Sites are owned by two separate conservation agencies, each with different management regimes. We hypothesized that soil factors constrain the impact of management on diversity and productivity in this system. We anticipated that sandy, infertile soils have lower productivity and support higher plant diversity, while high fertility soils have highly productivity and lower species diversity. To date, we have environmental data and two years of species cover for 638 plots (1 m² ) across 35 management units.

Results/Conclusions

Richness varied positively from 1 to 21 species across the sand gradient (p<.001, r²=.03), and negatively from 14 to 3 species across the silt gradient (p<.001, r²=.03). Richness did not correlate with soil nitrogen, carbon or C:N ratio. Although these results are highly significant, the amount of variance in the data is poorly explained by these factors alone. Thus, while soil characteristics may control some of the species richness, management and community dynamics likely dominate in influencing species richness. Initial results shows that the origin of the site, remnant prairie or restored, also weakly correlates with species richness but has strong effects on species composition.  Due to the co-occurrence of native and many non-native plants at the site, species richness is a poor indicator of prairie quality, where high quality prairie is defined (by conservation agencies in the area) as a diverse community with high numbers of endemic, desired species. Species composition of plots will serve as a clearer measure of quality. Future work will evaluate the importance of different management regimes and community dynamics on species composition.