Grassland conversion in relation to the onset of drought in a multi-factor global change experiment

Background/Question/Methods

Mediterranean environments provide a unique test of the integrating role of soil moisture in accounting for effects of climate change and climate variability on ecosystem processes. In these environments, soil moisture is out of phase with other resources­—it  strongly and predictably limits ecosystem processes when other resources tend to be most available, and it is often superabundant when light and temperature are limiting. Both the onset and the end of the summer drought, which are influenced by multiple global change factors within the Jasper Ridge Global Change Experiment (JRGCE), trigger important ecosystem dynamics. Recently, the JRGCE has focused on factors that initiate a conversion from annual grassland to another vegetation type, including conversion to shrubland, savanna, or stands of thistle. Some aspects of climate change could, however, also facilitate restoration of annual grassland to native perennial grassland. All these types of conversion involve increased success of plants that persist into or through the summer, so they should be favored when the summer drought is delayed or attenuated.

Results/Conclusions

In the JRGCE, the onset of summer drought can be delayed by three of the experiment’s single-factor treatments­—increased precipitation, elevated atmospheric CO₂, and, surprisingly, climate warming­—though not by the fourth factor, simulated nitrogen deposition. The JRGCE is a full-factorial experiment, and we have used all 16 treatment combinations to assess the integrating, mechanistic role of soil moisture in grassland conversion and to assess the additive or interactive roles of other global change factors.  We examined establishment and growth of woody species and non-native thistles that were deliberately introduced as seedlings in two successive years; natural establishment of native perennial grasses over 12 years; and deliberate addition of native perennial grasses as seeds. Across these three types of introductions, we compare the degree to which soil moisture at the time of grassland senescence accounts for the establishment and biomass of these added functional groups of plants, and we examine whether other global change factors contribute differently to the establishment of the various functional groups. This assessment can help identify the controls on possible conversions of grasslands in Mediterranean-type climate regions to other ecosystem types. It can also help reveal the aspects of climate change that facilitate or complicate restoration of native California grassland.