The influence of multiple interacting global changes on the structure and function of a California annual grassland ecosystem.

摘要

Global environmental changes expected over the coming century have the potential to alter the structure and function of the Earth's ecosystems. The research presented in this dissertation was performed as part of the Jasper Ridge Global Change Experiment (JRGCE), a multi-year, multi-collaborator research effort. Starting in November, 1998 the JRGCE simulated future environmental conditions by factorially applying the following treatments to plots of intact, natural annual grassland vegetation: elevated atmospheric carbon dioxide (CO 2), warming, nitrogen (N) deposition, and increased precipitation. This research sought to understand how these multiple, interacting global changes altered various aspects of plant ecology, including the timing of growth (phenology), plant-herbivore interaction, plant competition with soil microbes for nutrients, and nutrient limitation via decomposition feedbacks.; Species-level phenology in the JRGCE was shifted by the global changes in surprising ways. Warming accelerated development of all species, while elevated CO2 and N-deposition significantly delayed the development of the dominant grasses, and moderately accelerated development of forbs.; In addition to altering the timing of plant activity, global changes are expected to alter the availability of C and N, thus effecting patterns of plant growth and tissue chemistry. Shifting tissue chemistry has the potential to further alter interactions among plants and herbivores, soil microbes, and other decomposers. This research found that elevated CO2 caused generalist gastropod herbivores to shift their feeding preferences; however, shifting tissue chemistry did not predict these shifts in feeding preferences.; Numerous lines of evidence suggested that plant growth in this system was co-limited by N and phosphorus (P) under elevated CO2. N deposition was the only treatment that increased primary production, and in some years elevated CO2 suppressed this stimulatory effect. Comparisons of plant versus microbial N and P pools found that plants effectively competed for N, but not P, under some treatment combinations. In addition, soil P availability was decreased by elevated CO2, and was not influenced by other treatments. Finally, decomposition of senesced litter grown under elevated CO2 had a lower rate of P mineralization, suggesting that P limitation under elevated CO2 occurred via shifting tissue chemistry, and slowed decomposition.