Downscaled Climate Change Projections for the Southern Colorado Plateau: Variability and Implications for Vegetation Changes

Recent and rapid forest mortality in western North America and associated changes in fire frequency and area burned are among the chief concerns of ecosystem managers. These examples of climate change surprises demonstrate nonlinear and threshold ecosystem responses to increased temperatures and severe drought. A consistent management request from climate change adaptation workshops held during the last four years in the southwest U.S. is for region-specific estimates of climate and vegetation change, in order to provide guidance for management of federal and state forest, range, and riparian preserves and land holdings. Partly in response to these concerns, and partly in the interest of improving knowledge of potential ecosystem changes and their relationships with observed changes and changes demonstrated in the paleoecological record, we developed a set of integrated climate and ecosystem analyses. We selected five of twenty-two GCMs from the PCMDI archive of IPCC AR4 model runs, based on their approximations of observed critical seasonality for vegetation in the Southern Colorado Plateau (domain: 35°- 38°N, 114°-107°W), centered on the Four Corners states. We used three key seasons in our analysis, winter (November-March), pre-monsoon (May-June), and monsoon (July- September). Projections of monthly and seasonal temperature and precipitation from our five-model ensemble indicate steadily increasing temperatures in our region of interest during the twenty-first century. By 2050, the ensemble projects increases of 3.0°C during May and June, months critical for drought stress and tree mortality, and 4.5-5.0°C by 2090. Projected temperature changes for months during the heart of winter (December and January) are on the order of 2.5°C by 2050 and 3.0°C by 2090; such changes are likely to affect snow hydrology in middle to low elevations in the Southern Colorado Plateau. Summer temperature increases are on the order of 2.5°C (2050) and 4.0°C (2090). The most striking aspect of projections of future precipitation is steadily decreasing May-June precipitation during the twenty-first century. Though absolute precipitation during this season is small, declining moisture during the arid pre-monsoon will likely decrease soil moisture, and increase drought stress - consequently, increasing vegetation susceptibility the insect outbreaks and disease. Summer precipitation projections show considerable multi-decade variability, but no substantial trends. Winter precipitation shows little interannual variability and no strong trends. By 2090, annual precipitation is projected to decline by 1-5% across much of the region, with greater declines in the southern part of the domain and increases of 1-5% in the northwestern and northeastern parts of the domain. As part of a National Institute for Climate Change Research project, these projected changes will be input into a USDA-FS vegetation response model, in order to estimate species-specific responses to projected climate changes. We expect increasing temperatures, declining annual precipitation, and extreme declines in pre-monsoon season precipitation to generate significant redistribution of some plant species in the Southern Colorado Plateau.