Research Interests

I am interested in ecosystem and plant ecology. Especially in questions that deal with how plant communities, and ecosystems, are structured by their environment and how plants in turn change their geochemical environment. Species matter, as grasses structure grasslands and trees forest. However, individual species, like a nitrogen-fixing legume or a tree invading a grassland, can change the functioning of entire ecosystems. I am interested in developing a mechanistic understanding in which factors control species abundances (e.g. fire, herbivory, soil fertility, dispersal, etc.) and how these factors and the species composition, structure ecosystems. Insights in these processes are especially important today, as major issues like global climate change are impacted by carbon and nitrogen cycling, both of which are controlled by terrestrial vegetation. I am also interested in the functioning of agricultural ecosystems, because they dominate most of the earth today and have a major impact on global and regional biodiversity and element cycling.

Specifically, I am working with a group of researchers on an experiment examining the interactions of biodiversity loss, increasing nitrogen deposition and increasing atmospheric CO2 (see http://www.lter.umn.edu/biocon). I am working on the invasion and decomposition part within this experiment.

I am also starting a new project in which I will examine the impact and the mechanisms by which species control nitrogen and carbon fluxes and pools.

Lastly, I am working with Dr. Koenig on a long-term project to determine which factors influence acorn productivity and oak masting. We are using a 20-year database of acorn productivity of 250 individual trees at a site to determine which factors influence the variability both in time and space that we observe. We recently expanded our monitoring to 14 additional sites within California. This will allow us to better address regional and statewide scaling issues.

Recent Publications

• Reich, P.B., C. Buschena, M.G. Tjoelker, K. Wrage, J. Knops, D. Tilman & J.L. Machado. 2003. Variation in growth rate and ecophysiology among 34 grassland and savanna species under contrasting N supply: a test of functional group differences. New Phytologist, in press.
• Bradley, K.L., E.I. Damschen, L.M. Young, S. Went, G. Wray, D. Kuefler, N.M. Haddad, J.M.H. Knops, S.M. Louda. 2003. The herbivory uncertainty principle revisited. Ecology, in press.
• Craine, J.M., D.G. Tilman, D.A. Wedin, P.B. Reich, M.J. Tjoelker & J.M.H. Knops. 2002. Functional traits, productivity and effects on nitrogen cycling of 33 grasslands species. Functional Ecology 16: 563-574
• Kennedy, T., S. Naeem, K. Howe, J.M.H. Knops, D. Tilman & P. Reich. 2002. Biodiversity as a barrier to ecological invasion. Nature 417: 636-639
• Knops, J.M.H., K.L. Bradley & D.A. Wedin. 2002. Mechanisms of species impacts on ecosystem nitrogen cycling. Ecology Letters 5: 454-466.
• Spotswood, E., K.L. Bradley & J.M.H. Knops. 2002. Herbivory, flowering and seed production in four prairie species. BMC Ecology 2:2.
• Haddad, N.M., D. Tilman & J.M.H. Knops. 2001. Long-term oscillations in grassland productivity induced by drought. Ecology Letters 5: 110-120.
• Tilman, D., P. Reich, J. Knops, D. Wedin, T. Mielke & C. Lehman. 2001. Diversity and productivity in a long-term grassland experiment. Science 294: 843-845.
• Reich, P., D. Tilman, J. Craine, D. Ellsworth, M. Tjoelker, J. Knops, D. Wedin, S. Naeem, D. Behauddin, J. Goth, W. Bengston & T. Lee. 2001. Do functional groups differ in acquisition and use of C, N, and water under varying atmospheric CO2 and N deposition regimes? A field test using 16 grassland species from 4 functional groups. New Phytologist 150: 435-448.
• Haddad, N.M., D. Tilman, J. Haarstad, M. Ritchie & J.M.H. Knops. 2001. Contrasting effects of plant diversity and composition on insect communities: a field experiment. American Naturalist 158: 17-35.