Effects of Biodiversity on Ecosystem Attributes and Processes

Biodiversity is undergoing major changes on Earth, both because of human-caused changes to the environment that differentially influence the performance of different species and because of deliberate management activities that favor some species over others. These changes in biodiversity are known to have consequences for ecological communities - their composition, functioning, and capacity to provide ecosystem services. Past studies have largely explored how variation in biodiversity affects ecosystem processes such as productivity, and many, but not all, of these studies have been conducted in grassland ecosystems. We are building upon past work to tease apart the importance of species richness, phylogenetic diversity, and functional diversity in influencing species interactions (plant-plant, plant-symbiont, plant-consumer) and ecosystem processes, and how the relative importance of different aspects of diversity change over temporal and spatial scales. We are addressing these questions in the Forest and Biodiversity (FAB) experiment, where 12 tree species are planted in different combinations, ranging from 1 to 12, and in communities where species differ in how closely related they are to the other species in the community, either evolutionarily or functionally.

Effects of Resources

Human activities such as the production and use of fertilizer, cultivation of legume crops, and fossil fuel combustion, are altering the global N cycle and greatly increasing the cycling and deposition of reactive N in ecosystems. We are determining how long-term nutrient enrichment alters plant and microbial communities and how such changes impact ecosystem processes such as soil carbon cycling. We are also determining the dynamics of recovery following cessation of nutrient addition, key to understanding the capacity of ecosystems to recover from chronic N deposition as practices are put into place to curb N pollution. Nutrient pollution often drives loss of plant diversity. We previously found that plant diversity can remain low for decades after nutrient inputs stop. We are working to understand the processes that keep formerly fertilized plant communities in a low-diversity state. This includes considering feedbacks between plants and soil organisms, competition between plant species, and nutrient cycling.

Effects of Biodiversity, Climate, and Resources on Ecosystems 

Humans are altering numerous aspects of the environment simultaneously, including climate, the chemical composition of the atmosphere, as well as the numbers and kinds of species growing in particular regions. Past research on the effects of such global change effects on ecosystems has focused largely on understanding the influence of one or two of these environmental changes in isolation. Yet theory and common sense suggest that these environmental changes might have different effects in combination. We are determining how climate variation influences grassland community and ecosystem change processes, and specifically, whether responses of such processes to variation in biodiversity, atmospheric CO2 concentrations, and atmospheric N deposition are contingent on temperature and moisture availability. To do this we use both the long-running TeRaCON and BioCON experiments that manipulate temperature x rainfall x CO2 x N, and biodiversity x CO2 x N, respectively. In both experiments we annually measure a suite of community and ecosystem processes, including plant richness, evenness, and composition; biomass production; soil CO2 flux, and net N mineralization rate. In TeRaCON, because we manipulate climate directly, we use both interannual and experimental temperature and rainfall variability to test for their interactions with CO2 and N. In BioCON, we use interannual climate variability; our >20 year record makes this a rich data infrastructure for this.

Effects of Disturbance and Biodiversity on Ecosystems 

Oak savannas are among the most threatened ecosystems in Minnesota and globally. In fact, less than 0.1% of the original oak savanna remains in Minnesota. Our past work has shown the importance of fire in maintaining oak savanna: with too little fire, the system becomes a forest, whereas with too much fire,the system becomes a grassland, suggesting that intermediate fire frequencies are key to maintaining this ecosystem. The highest diversity of species occurs in frequently burned areas, and most species in the prairie-savanna-forest mosaic appear to have adaptations to fire. However, even at intermediate fire frequencies where the plant community looks like classic oak savanna habitat, a closer look reveals that these fire frequencies will eventually exclude oak trees, as these fires are too frequent to allow oak trees to regenerate and recruit into the canopy. We have now reintroduced bison, extirpated from these ecosystems for hundreds of years, to determine whether the combination of bison grazing and fire might help oak trees regenerate. We suspect that bison grazing on grasses will reduce the fuel for fires, decreasing fire temperatures enough to allow young oak trees to grow big enough to survive subsequent fires. 

In addition to disturbances, like fire, and large herbivores, like bison, diseases have been, and are, a major force structuring the forests of North America. At Cedar Creek, adult oak survival is threatened by oak wilt disease, caused by the fungus Bretziella fagacearum. This disease likely arrived at Cedar Creek in the past several decades and in the past decade has revealed major impacts on ecosystem dynamics The nature of these  impacts  is an ongoing area of study. We are surveying tree mortality caused by oak wilt disease across sites that vary in frequency of prescribed burn. We expect that oak will be less prevalent in burned savannas, because fire kills the pathogen and because lower stem densities reduce the rate of disease spread.

Synthetic and Cross-site Studies

As global scale human impacts increase, it is increasingly important to develop a general understanding of how ecosystems will respond to human-induced changes in resources, disturbance, climate, and biodiversity. However, most ecological studies are conducted at a single site. Synthesis and Cross-ste studies use data from multiple experiments or sites to test whether results from a single site can be extended to larger scales of space and time. Similarly, distributed experiments can offer powerful tests of the generality of empirical and theoretical predictions using standard treatments and data collection methods. Below we highlight major synthesis and cross-site efforts for which CDR is providing data 0r intellectual leadership.