Long-term global change impacts on grasslands
Global changes are complex in nature: they involve many factors that occur simultaneously, but usually studied in isolation. Beginning in 1998, the Jasper Ridge Global Change Experiment at Stanford University has been altering four aspects of global change—temperature, precipitation, atmospheric composition (carbon dioxide concentration), and atmospheric deposition (nitrogen pollution)—both independently and in combination for a typical California grassland. This grassland ecosystem is ideal for experiments because it has many species, even small plots express a wide range of ecosystem processes, and the short lifetime of most species means that an experiment can encompass many generations of the most important organisms.
Combining 17 years’ direct observations and powerful models, we found that grassland production reached its maximum near present values of temperature and precipitation, with decreases at higher or lower values (nonlinear response); that effects of nitrogen addition were large, but effects of elevated carbon dioxide were subtle. These nonlinear results suggest that grassland production was finely tuned to average conditions, and could decrease if the future departures substantially from the long-term average climate. The finding also alarms that grassland ecosystems showed little tendency to sequester additional carbon in a changing climate, emphasizing the importance of reducing energy-systems emissions in tackling climate change (Zhu et al. 2016).