University of Minnesota
University of Minnesota
College of Biological Sciences
http://www.cbs.umn.edu/

Research at Cedar Creek

Research at Cedar Creek focuses on understanding the fundamental processes and principals that govern the dynamics and functioning of communities and ecosystems. We explore topics of fundamental scientific interest, as well as those relevant to current environmental change.

Our work synthesizes the sometimes disparate approaches of ecophysiology and population, community and ecosystem ecology, and exploits the interplay between experimental results, observational data, and theoretical prediction.

Research Themes
The goal of our research is to understand, via the interplay of long-term experiments, long-term observations, theory and models, the processes, mechanisms and feedbacks that control the dynamics and functioning of grassland, savanna, and forest ecosystems. The underlying processes we study include ecophysiology, competition, plant interactions with herbivores and disease, decomposition and nutrient cycling, controls of soil C and N, and the linkages and feedbacks among these and other processes. We are particularly interested in how and why human drivers of environmental change impact ecosystem structure and functioning. Anthropogenic drivers we study include the loss of biodiversity, changes in temperature and water availability, elevated atmospheric CO2, N deposition, fire suppression, and introduction of novel species. Together, these environmental changes drive fundamental shifts in the resource availability, the range of conditions, and the biological players experienced by and contributing to the composition and functioning of ecological communities and ecosystems. The mechanistic understanding of the underlying interactions, linkages, and long-term feedbacks would advance fundamental knowledge while also providing society with insights into managing and adapting to these challenges.  Our research at Cedar Creek falls into five categories:
 

Grassland and Forest Biodiversity Experiments: These experiments examine how and why changes in species, functional, and (with this proposal) phylogenetic diversity of plants affect foodweb structure and community and ecosystem dynamics and functioning. These include our existing grassland Biodiversity Experiment, ‘BigBio’, established in 1994; BioCON (described in 3.2); and a new forest biodiversity experiment.

Multiple Global Change Factors: These experiments examine the interactions among multiple environmental drivers. ‘BioCON’, established in 1998, manipulates CO2, N deposition, and biodiversity using a split-plot design. WWCON, a water x CO2 x N experiment nested into BioCON, has ambient or reduced rainfall at all combinations of ambient and enriched CO2 and N, and will incorporate a warming manipulation in 2012. The Biodiversity and Climate (BAC) experiment, nested within BigBio, has an unwarmed subplot, a subplot warmed ~1.5°C and one warmed ~3°C nested within 1, 4, and 16 species plots. We will add a reduced rainfall treatment in 2012 to create a diversity x warming x water factorial experiment.

Nutrient Addition Experiments: These experiments examine the community and ecosystem consequences of chronic nutrient loading. In an experiment begun in 1982, we impose various rates of N addition on 369 permanent plots in seven grassland sites. In 1993, one site was modified to become a full factorial N x fire experiment, and another site became a full factorial N cessation or N addition comparison. In 2005, a third site became an N x deer herbivory experiment. Two experiments initiated in 1999 and 2004 have explored N limitation of decomposition. New studies to explore multi-factor responses of aquatic communities are being added in this proposal.

Grassland-Forest Disturbance and Succession Experiments and Long-Term Observations: In our oldest CDR experiment, begun in 1964, we examine responses of microbial, plant, and ecosystem processes to direct and indirect effects of long-term disturbance by imposing different fire frequencies on larger (3 to 27 ha) naturally established savanna plots. We also are continuing long-term observational studies of the dynamics of plant species and soil C and N in numerous grassland fields, which recently were split in half, with a prescribed burning treatment (burned every other year) imposed on a randomly chosen half of each field.

Biotic Interactions and Feedbacks: These experiments examine the long-term feedbacks between plant diversity, microbes, invertebrates, and vertebrates and the effects of these feedbacks on grassland species abundances, productivity, and decomposition. In two experiments established in 2008, we independently reduce different components of the consumer foodweb via insecticide, foliar and soil fungicide, and vertebrate fences.  

Cedar Creek Transformational Science Bullets
  • “Loss of plant species after chronic low-level nitrogen deposition to prairie grasslands” (Clark & Tilman 2008 Nature). This was the first multi-decadal experiment to examine the impacts of chronic N addition at rates as low as 10 kg N ha-1 y-1 above ambient atmospheric N deposition, a rate comparable to terrestrial N deposition in many industrialized nations. This chronic low-level N addition rate reduced plant species numbers by 17% relative to controls receiving ambient N deposition. Moreover, plant species numbers were reduced more per unit of added N when N was added at lower rates, showing that chronic, lowlevel N deposition may have a greater impact on diversity than previously thought.
  • Nitrogen effects on decomposition: a five year experiment in eight temperate sites” (Hobbie 2008 Ecology). Although N deposition often increases primary production, its effect on global soil carbon (C) stores also depends on the impact of N deposition on litter decomposition. Long-term decomposition experiments showed that added N increased initial rates of decomposition, but led to shifts in extracellular enzyme activity that resulted in larger recalcitrant litter pools, slowing decomposition in the long term (Hobbie 2005, 2008, Hobbie et al. in revision). These results suggest that elevated atmospheric N deposition should contribute to increased rates of C sequestration, an insight that could only be gained by long-term decomposition studies (Adair et al. 2010).
  • “Biodiversity and ecosystem stability in a decade-long grassland experiment” (Tilman, Reich & Knops 2006 Nature). This paper tested, and found strong support for, our controversial conjecture (Tilman and Downing 1994) that greater numbers of plant species led to greater year-to-year temporal stability of aboveground net primary production. Ecosystem stability was also positively dependent on root mass, a measure of perenniating biomass. Temporal stability of the ecosystem increased with diversity, despite a lower temporal stability of individual species, because of both portfolio (statistical averaging) and overyielding effects.
  • “From selection to complementarity: Shifts in the causes of biodiversity-productivity relationships in a long-term biodiversity experiment” (Fargione, Tilman, Dybzinski, Hille-Ris-Lambers, Clark, Harpole, Knops, Reich & Loreau 2007 Proc. Roy. Soc. B). Using the method of Loreau and Hector (2001), this paper showed that the cause of the increasingly positive effect of biodiversity on plant biomass production shifted from sampling (increased likelihood of the presence of very productive species) to strong complementarity (complementary resource use among species) over time. Furthermore, complementarity was associated with the joint presence of legumes and C4 grasses.
  • “Nitrogen limitation constrains sustainability of ecosystem response to CO2” (Reich, Hobbie, Lee, Ellsworth, West, Tilman, Knops, Naeem & Trost 2006 Nature) This paper demonstrated a strong interaction between N supply and the capacity for plant growth rates to increase in response to elevated CO2. In particular, after a transient start-up period, low availability of soil N halved the positive response of plant biomass to elevated CO2 compared to that observed at higher N supply rates. This provided support for the hypothesis (Hungate et al. 2003) that past predictions of future atmospheric CO2 concentrations likely overestimated the extent that rising CO2 would stimulate net primary production and C sequestration across the earth’s terrestrial ecosystems.
  • “Plant species loss decreases arthropod diversity and shifts trophic structure” (Haddad, Crutsinger, Gross, Haarstad, Knops & Tilman 2009 Ecology Letters). We sampled arthropods for over a decade in an experiment that manipulated the number of grassland plant species. Herbivore and predator species richness were strongly, positively related to plant species richness. Moreover, there was a threefold increase, from low to high plant species richness, in abundances of predatory and parasitoid arthropods relative to their herbivorous prey. These results demonstrate that, over the long term, the loss of plant diversity can lead to decreased arthropod species richness and an increasingly herbivore-dominated foodweb, thereby potentially impacting ecosystem productivity.
  • “Elevated CO2 reduces losses of plant diversity caused by nitrogen deposition” (Reich 2009 Science). This paper addressed the interactive effects of rising atmospheric CO2 concentrations and N deposition on plant diversity in the BioCON experiment. Over 10 years, elevated N reduced species richness by 16% at ambient CO2 but by just 8% at elevated CO2 (Fig. 7). This resulted from multiple effects of CO2 and N on plant traits and soil resources that altered competitive interactions among species. Effects on species richness were the result of the aggregate effects of CO2 and N on soil water, tissue stoichiometry, and total biomass. As a result, elevated CO2 helped ameliorate the negative effects of N enrichment on species richness.
  • “Shocks to the system: Community assembly of the oak savanna in a 40-year fire frequency experiment” (Cavender-Bares & Reich 2012 Ecology) This paper showed that a major drought of the late 1980s caused temporary shifts in community traits (specific leaf area, leaf N, and leaf length), and that adaptive traits that evolved ~80 mya drove the assembly of communities across the fire gradient. Functional traits and the sorting of species into contrasting fire regimes revealed a clear signature of phylogenetic conservatism. The article is part of a special issue in Ecology initiated by a CDR-led LTER working group.
  •  “Productivity is a poor predictor of plant species richness” (Adler et al. 2011 Science) This paper used data from the Nutrient Network (NutNet) that comprises >70 grassland sites worldwide, including 6 LTER sites, to ask if there is a hump-shaped dependence of diversity on productivity, which has been a largely untested axiom in ecology for three decades (Al-Mufti et al. 1977). There was no evidence for a consistent dependence of diversity on productivity in this uniquely powerful data set).
  • “Land clearing and the biofuel carbon debt” (Fargione, Hill, Tilman, Polasky & Hawthorne 2008 Science) This paper addressed the potential for various biofuels to provide greenhouse gas (GHG) benefits relative to fossil fuels. Analyses showed that the land clearing associated with production of palm nut for biodiesel, corn for ethanol, soybean for biodiesel, and sugarcane for ethanol resulted in a “carbon debt”, the release of sufficient greenhouse gases such that all the biofuels analyzed would have a worse GHG signature than gasoline or conventional diesel for decades to centuries). This highlighted paper plus two related “broader impact” CDR papers (Tilman, Hill & Lehman 2006, Hill et al 2006) are the most cited of our recent papers. We presented their results in invited congressional testimony and in media interviews.