Climate variability and water supplies in Rocky Mountain ecosystems

Background/Question/Methods

Our two study areas include portions of the Niwot Ridge Long Term Ecological Research site. Boulder Creek is representative of the Colorado Front Range, where population increased by 30% from 1990-2000, reaching 3.5 million.  Population growth continued to exceed the national rate from 2000 to 2005.  The Upper Colorado River basin is representative of biophysical and social systems on the west side of the Continental Divide. While not as densely populated as the Front Range, the basin has experienced similar rates of population increase, leading to rapid exurbanization of rural areas.  The primary goal of this study is to characterize how human systems interact with three key structural drivers in Colorado:  climate, hydrology (streamflow), and landscape patterns of disturbances (fire and mountain pine beetle, MPB).  Three questions guide our analysis of long term climate, hydrology, and land use data: how do disturbances interact with each other to alter ecosystem structure and function (e.g., hydrological budget); how do altered ecosystem dynamics affect critical ecosystem services (including water quantity and quality); and which regional and global human actions and decisions on landscape management influence the disturbance regimes and their interactions, further affecting ecosystem services.

Results/Conclusions

Results indicate that climate variability, population increase, changing land use, and fire suppression combined to induce shifts in the structure and function in the two areas at all elevations and spatial scales.  Extreme droughts combined with fuel loading increase (due to fire suppression) and land-cover change (a function of land-use change, such as exurban development) to alter landscape patterns, stand structure, and hydrological budgets.  Changes in landscape patterns and stand structure in turn created a positive feedback to fuel loading accumulation and tree stress, and therefore likely influenced the threshold behavior of fire and MPB outbreak intensity and size. Changes in landscape patterns and hydrological budgets profoundly impacted all ecosystem services, their interactions, and trade-offs. For example, as water quantity and quality have increased as a function of managing for closed and dense forests, the capacity of landscapes to mitigate the size and intensity of disturbances (such as fires and insect outbreaks) has decreased. This suggests that trade-offs in ecosystem services often result in multiple ecosystem services being compromised for the benefit of a solitary ecosystem enhancement.