The influence of climate change and climate-influenced disturbances on streamflow and carbon cycling in the Rocky Mountains

Background/Question/Methods

Global scale climate model projections show increases in temperatures, climate extremes, and inter-annual variability. Our understanding of the response to these changes in mountain environments is confounded by strong seasonal and altitudinal gradients in temperature, precipitation, and  snowpack; diverse vegetation species and densities; and complex geology. An ecohydrologic perspective on climate change in the Colorado Rockies asks two inter-related questions. How do hydrologic changes associated with warming, including increases in the frequency of temperature and precipitation anomalies, impact short- and long-term vegetation dynamics? Secondly, how do potential climate related changes in vegetation composition interact with climate driven changes in hydrology? We use eco-hydrologic simulations that combine data on climate trends, vegetation structure, and topographic controls on snow and water redistribution to explore where, and under what conditions, the biggest changes in streamflow and vegetation productivity occur and interact. We consider climate scenarios of increased mean annual temperatures, doubled frequency of temperature and precipitation anomalies, and higher seasonal temperatures. We also look at the implications of vegetation disturbance; specifically, change in species after widespread mortality due to disturbance. 

Results/Conclusions

For climate and vegetation change scenarios we examine direct effects on streamflow and the likelihood of significant declines or increases in forest productivity. Hydrologic effects are compared using 7-day minimum flows, timing of center of mass of streamflow, and changes in total annual flow.  Effects on forest productivity are analyzed using seasonal and annual estimates of net primary productivity and transpiration. We find a greater increase in total annual streamflow ( ~25%) from a conversion of 70% of forested lands to dead, standing biomass (e.g., post bark beetle attack) rather than from higher temperatures. Decreases in streamflow, however, do occur under some scenarios. We find the greatest carbon sequestration declines (up to 20%) are associated with scenarios that extend the growing season, which is strongly controlled by changes in snow water inputs.  A general finding is that the magnitude of relative changes in seasonal water availability and ecosystem productivity are tightly coupled and their interaction is an important control on climate responses in this region.