The Natural Reserve System exists, in large part, to provide researchers with sites where long-term field studies can be conducted undisturbed. A ground-breaking article in the February 2, 2007, issue of Science dramatically confirms the importance of such long-term studies. The article, “Species Interactions Reverse Grassland Responses to Climate Change,” written by Blake Suttle, Mary Power, and Meredith Thomsen, is based on a five-year research project conducted at the NRS’s Angelo Coast Range Reserve in Mendocino County.

The goal of their project was to assess the impact of changing rainfall regimes predicted by different global climate models on grassland ecosystems. To do this, the team, subjected 18 study plots to three different rainfall patterns: six plots received additional water during the area’s regular winter rainy season, six received additional water that extended the rainy season into late spring, and six control plots received no additional rainfall.

The team’s first discovery was that the timing of the additional water was crucial. Plots that received additional water during the already-wet winter period experienced moderate increases in plant production growth, but these effects did not alter the higher trophic levels of invertebrate herbivores and predators. When watering was used to extend the rainy season, however, the changes were much more pronounced. Plant production in those plots tripled in the first year compared to the control plots, and then doubled in the second year.

The most dramatic findings, however, came only after the study had been continued for several years. Although many native wildflowers prospered during the first years of extended watering, they were gradually pushed out by exotic grasses, whose litter suppressed regrowth of the forbs and drove steep declines in species richness. This shift in plant composition, in turn, had important consequences for biodiversity and food-web structure. By the end of the fifth year, the plots with extended rainfall had an almost 50 percent reduction in plant species richness, a 20 percent reduction in invertebrate richness, and herbivore and predator abundances that were each nearly 50 percent lower than the control plots.

While there remains great uncertainty about how global rainfall patterns will develop, this study conducted at the Angelo Reserve emphasizes the importance of carefully-designed, long-term manipulative studies conducted in areas where they can continue without interruption. Gian-Reto Walther of the University of Bayreuth makes this clear in a commentary that appears in the same issue of Science:.

The study by Suttle et al. shows how one can tackle ecological complexity in manipulative experiments: The 5-year duration of the experiment has revealed reversal trends of initial effects; different temporal patterns of treatment highlight the importance of the timing of rainfall compared to the annual amount; and feedback processes through higher trophic levels may overturn direct climatic effects on the species level and reverse community trajectories. These aspects make this work particularly valuable for ecological experiments and for global change research in general. It is a strong reference for the importance of field-based long-term monitoring and experiments for climate impact studies.

Blake Suttle recently received his PhD at UC Berkeley. His doctoral research was sponsored in part by an NRS Mildred Mathias grant, Mary Power is a UCB professor and faculty director of the Angelo Reserve, and Meredith Thomsen, who earned a doctorate from UCB, is now on the faculty of the University of Wisconsin.

Subscribers of Science online can read the full article at: http://www.sciencemag.org/

Hard copy access is universally available with this citation:

Suttle, K.B., M. A. Thomsen, and M. E. Power. 2007. Species interactions reverse grassland responses to changing climate. Science, Volume 315 (February 2, 2007), pp. 640-2.