Forest/Atmosphere Carbon Fluxes in a Colorado Subalpine Forest

P.I./Institution: Russell K. Monson, University of Colorado, Boulder

Summarize in two to four sentences what activities will be (are being) carried out in the project: Eddy covariance flux measurements, and a variety of environmental and biological measurements, will be conducted to quantify the magnitude of net ecosystem CO2 exchange (NEE), and the principal factors controlling NEE, at the Niwot Ridge Ameriflux site in the Front Range of Colorado. Wintertime transects and manipulated plots will be established to address the relationship between snow depth, soil temperature and soil respiration rate. Summertime transects will be established to measure differences in soil respiration, soil moisture content and the stable-isotope composition of respired air to address the causes of differences in NEE and ecosystem water-use efficiency between two contiguous forest communities.

Project Abstract: A recent modeling analysis of South Central and Western U.S. ecosystems has concluded that montane and subalpine forests are (1) among the most important carbon sinks of the region during years with normal climate, and (2) become the on/y significant carbon sinks during decadal-scale droughts on the west coast (Schimel et al., 2002, EOS, in press). The measurements proposed here will continue a long-term, continuous data set of carbon sequestration rates in a high-elevation, subalpine forest in the South Central U.S. It is only with long-term measurements that we can validate the models and evaluate significant interannual trends in carbon uptake and their coupling to climate variation. In addition to maintaining the long-term flux record at the Niwot Ridge site, we will address two specific hypotheses:

Hypothesis 1. Variation in the amount of snow and the timing of the spring snow melt exerts as much, or more, influence on variation in annual NEE as variation in growing season precipitation and temperature.

Hypothesis 2. Differences in NEE and ecosystem water use-efficiencies of the pine-dominated forest to the east of the tower, and the fir-dominated forest to the west of the tower, are due to soil processes (i.e., soil respiration rates and soil moisture content), not canopy structure or ecophysiological attributes of the foliage.

We will continue two lines of collaborative study. We will assess wind flows in complex terrain and their influence on NEE, including the influence of downs lope, gravitational drainage and mesoscale motions, such as mountain waves and rotor winds. We will observe the interactions between forest gas-exchange and the deposition of urban and suburban air pollutants, including (1) the role of the forest in emitting volatile organic compounds (VOCs) which act as ozone precursors, and (2) the subsequent effects of forest VOC emissions on the deposition of ozone back to the forest.

Summary of Proposed Work: The proposed studies will provide key information on the amount of atmospheric carbon sequestered by an important forest ecosystem in the South Central U.S. This information is crucial to understanding how ecosystems mitigate recent increases in atmospheric carbon dioxide concentration, and the potential of this mitigation to interannual variation in climate and future climate change. Our project will bring together observations with computer models to provide better tools for analyzing future trends in forest growth and carbon uptake. In particular, we will focus on how the winter snow regime influences carbon uptake and how processes in the forest soil affect the amount of carbon that is sequestered.