Long-term oscillations in grassland productivity induced by drought

Nick M. Haddad, David Tilman and Johannes M. H. Knops Department of Ecology, 1987 Upper Buford Circle, University of Minnesota, St. Paul, MN 55108, USA Correspondence and present address: Department of Zoology, Box 7617, North Carolina State University, Raleigh, NC 27695±7617. E-mail: [email protected]. Tel.:+1 919-515-4588, Fax: +1 919-515-5327 Department of Biological Sciences, University of Nebraska, Lincoln, NE 68588, USA Abstract Disturbances such as drought have immediate impacts on ecosystem functioning, but little is known about long-term dynamic consequences of disturbance. Here, we show that a major drought perturbed prairie grassland from nearly constant average annual production, and induced 9 years of oscillations with a 2-year period. This pattern occurred in unmanipulated plots in many widely separated ®elds that were part of two different long-term studies. Using direct and indirect tests, we could reject the hypothesis that the oscillations were externally imposed by climate or herbivores. Weak evidence supported the hypothesis that oscillations were internally generated, caused by a litter and nitrogen dependent feedback on productivity. This hypothesis was supported by the results of two other long-term studies, in which burning and, separately, nitrogen addition eliminated the 2-year oscillations in plant production. However, more direct evidence failed to support the litter hypothesis. A ®nal, but untested hypothesis was suggested by the observation that production tended to be synchronized amongst dominant plants. Drought may have synchronized production dynamics amongst plants with biennially greater allocation to above-ground growth. Regardless of the speci®c mechanism, our results show that a single disturbance may have long-lasting effects on the dynamics of plant production.