Graduate Thesis Or Dissertation
 

Simulated response of ecosystem processes to climate change in northern California and western Nevada

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  • In order to investigate potential climate impacts on landscape-scale ecosystem processes, I implemented a dynamic general vegetation model (DGVM) over a large domain in northern California and western Nevada on a rectangular grid of ca. 800-meter spatial resolution. I used 100 years of observed, monthly climate and nine future climate projections in an attempt to explore the range of possible climate futures in the region. I selected three general circulation models (MIROC3.2(medres), UKMO-HadCM3 and CSIRO-Mk3.0), incorporating a range of 2xCO₂ temperature sensitivity. Each GCM was run through three carbon dioxide emissions scenarios (SRES A2, A1B and B1). For this analysis, I focused the study on the simulated ecological impacts under the three A2 scenarios. Historical observations and future climate scenarios were interpolated to the 800-meter grid by the PRISM model. MC1, a systems-based DGVM, compared favorably to observed data for simulations of vegetation distribution and annual streamflow. MC1 slightly overestimated annual production in the Sierra Nevada and Klamath Mountains and underestimated it in the Coast Range and Eastern Cascades. MC1 displayed a low bias for annual area burned and high bias for pyrogenic emissions. Validation of simulated model output was complicated because MC1 does not consider the effects of land management on ecosystems and the study region is heavily-impacted by development, logging, fire suppression, grazing and pre-European, indigenous land-use and burning. Under all future climate projections, an increase in growing season length and temperature led to the replacement of tundra and subalpine vegetation types with temperate conifer forest. Increased winter minimum temperatures promoted the expansion of mixed needleleaf-broadleaf forest, particularly in the mid-elevations of the Sierra Nevada and in coastal forests. In the MIROC3.2 and HadCM3 scenarios, ecosystem-level net primary productivity (NPP) did not increase with enhanced CO₂ fertilization because production remained limited by water, even though both NPP and water-use-efficiency were increased at the leaf level in proportion to CO₂ concentration. Increases in NPP were projected in CSIRO-Mk3, but increased precipitation and warmer temperatures also increased rates of heterotrophic respiration for no net gain in net ecosystem productivity (NEP). Fire extent and severity increased in all scenarios, mostly driven by significant decreases in mountainous snowpack and earlier snowmelt. Thus, a relatively constant NEP and increased fire emissions produced decreases in total ecosystem carbon across all future scenarios. Projected annual streamflow varied between future climate scenarios and was highly influenced by projected precipitation. In all future simulations, high-elevation mountainous landscapes were highly sensitive to projected changes in climate, largely attributable to an increased growing season and temperature, decreased snowpack and reduced fire return interval. Coastal forests were also highly susceptible to changes in vegetation type and increases in fire. Several sources contribute to uncertainty in MC1, including input datasets, model assumptions, uncertainties in ecosystem science, and questions of scale. Therefore, these results should be considered preliminary, but useful in suggesting a range of plausible ecological futures as we continue to refine model capabilities.
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