Climate Change Effects on Cascade Mountain Vegetation Distribution and Carbon Storage

Because of steep topographical, edaphic, and meteorological gradients, minor changes in climate can have significant effects on the distribution of mountain ecosystems. This is of particular concern given the magnitude of predicted climate changes during the 21st century. To assess the sensitivity of mountain ecosystems to climate change, we downscaled data from nine general circulation model simulations (three GCMs each run at three CO2 emission scenarios) to an 800-meter resolution grid spanning a region of the Oregon and Washington Cascade mountain range. We then used MC1, a dynamic vegetation model, to simulate changes in the Cascade forest vegetation types and carbon loads during the projected 21st century climates. MC1 simulates competition for water, nutrients, and light between plant functional types and uses a biogeographical rule base to classify vegetation type. It also contains a detailed and interactive fire module. Changes in ecotone elevation, ecosystem spatial extent, and carbon storage are shown to be non- linear responses of temperature and precipitation forecasts over the 100-year time frame. Two hydrologic factors associated with higher temperatures interact to be dominant mechanisms of change: (1) an increasing synchrony between precipitation and the beginning of the Pacific Northwest growing season, and (2) earlier snowmelt. Both act to increase the available water content at the start of the growing season but also increase drought stress later in the summer.