Graduate Thesis Or Dissertation
 

Potential impacts of climate change on vegetation distributions, carbon stocks, and fire regimes in the U.S. Pacific Northwest

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  • The U.S Pacific Northwest contains a wide variety of ecosystems, all subject to relatively dry summers and wet winters. As has been shown with paleoclimatic and paleoecological data, the region is vulnerable to changes in climate. We assessed the sensitivities of vegetation distributions, carbon stocks, and fire regimes to 21st century climate change by running MC1, a dynamic general vegetation model, over a large domain across Oregon and Washington at 800-meter resolution. During the historical period, MC1 generally overestimated carbon stocks in the Western Forests region and underestimated carbon stocks in the Eastern Forests and Columbia Plateau. MC1 displayed a strong bias in the seasonality of NPP towards decreased summer and increased winter production. This suggests the model’s productivity equations may be overly sensitive to low soil moisture and under-sensitive to low temperatures. We downscaled nine future climate projections from three General Circulation Models (CSIRO Mk3, MIROC 3.2 medres, and Hadley CM 3), each run through three CO2 emission scenarios (SRES B1, A1B, and A2). Temperatures increased ubiquitously and concurrently with increasing emission scenario, but precipitation was more varied. CSIRO climates were relatively cool and wet, MIROC climates were hot and wet, and Hadley climates were hot and dry. Precipitation generally increased in winter and decreased in summer, and temperature increases were highest in summer. Previous work showed that CSIRO performed poorly, MIROC moderately well, and Hadley very well in the Pacific Northwest for the historical period. Future climate projections amplified the seasonal trends in climatic variables, water stress, and productivity. MC1 simulated the Pacific Northwest’s western maritime forests as being vulnerable to large increases in fires, subsequent losses in carbon stocks, and encroachment from more southerly and/or easterly forest types. The arid, fire-adapted forests east of the Cascade appeared to be resilient to climate changes under MC1. With increasing precipitation, MC1 simulated vast expanses of shrublands in the Columbia Plateau and Northern Basin converting to grasslands or woodlands. Across the domain, MC1 runs under the CSIRO climate projections averaged 82% increases in biomass combusted and 1.2% (0.1 Pg C) decreases in ecosystem carbon, while those under MIROC averaged 22% increases in biomass combusted and 0.8% (0.07 Pg C) increases in ecosystem carbon. Climate projections from the Hadley model resulted in the most extreme changes, averaging 259% increases in biomass combusted and 15% (1.26 Pg C) decreases in ecosystem carbon. Our study suggests some areas within the Pacific Northwest may be vulnerable, and others resilient, to climate change, although this is highly dependent on model assumptions and uncertainties.
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