Carbon Dioxide and its Stable Isotopes at Niwot Ridge, Colorado

Abstract:

Recent studies highlight the importance of western mountain regions to the North American carbon sink, suggesting that an important part of the U.S. sink can be attributed to montane ecosystems. Isotopes of CO2 provide insight into land-atmosphere CO2 exchange, and are useful in integration of processes over multiple scales. CO2 isotopes have played a central role in our understanding of the magnitude and inter-annual dynamics of the terrestrial carbon sink at a variety of spatial and temporal scales, and will be crucial to understanding the carbon balance and carbon accounting of the North American continent. The carbon cycle has been studied for several decades at Niwot Ridge, a high-elevation site in the Rocky Mountains of Colorado, U.S.A. Here we will focus on observations over the last decade at a subalpine forest site (3050 m elevation, the Niwot Ridge AmeriFlux tower), and a tundra site 4 km away (3520 m, representing the overlying forest air). During this period, CO2 increased at an average rate of 18 ppm/decade, and d13C of CO2 decreased by 0.25 permil/decade. Variability in both quantities within a year was driven by processes at both the local forest and regional scales, and included anthropogenic processes. The monthly mean CO2 difference between tundra and forest sites was highly correlated with whole ecosystem respiration and gross primary productivity derived from forest flux tower measurements.