The subalpine forest ecosystem is a critical natural resource to urban and suburban centers in the western United States. This ecosystem is dominant in mountain regions and provides goods and services such as water, recreation and timber. It is also a critical component of the continental sink for atmospheric carbon dioxide. These forests have become increasingly stressed in recent decades due to regional climate warming, outbreaks of insect epidemics such as the mountain pine beetle, and exposure to pollutants such as ozone and nitrogen oxides from nearby population centers. The research proposed would continue a ten year program of monitoring a subalpine forest west of the Denver metropolitan corridor to assess growth, water budgets, and the potential for continued carbon absorption. It has become apparent that this forest is susceptible to absorbing less carbon dioxide from the atmosphere over time and that it is growing at slower rates due to warming and associated reduced snowpacks. Furthermore, there have been signs of incipient outbreak of the mountain pine beetle. Thus, the forest is at the cusp of exhibiting important and major responses to environmental change. This research includes measurements of carbon dioxide, water and energy exchange between the forest located near Nederland, Colorado, and the atmosphere over the next five years to assess its response to these stresses. In addition to conducting research, this proposal will have broader impacts to the scientific community through its continuation as a training facility for individual graduate student and post-doctoral student projects, continuation of a summer course for graduate students from across the world at the University of Colorado Mountain Research Station, and continuation as a primary data resource for global carbon budget modeling.
The subalpine forests of Colorado’s Front Range play an important role in the carbon and water cycling of the region. A better understanding of the magnitude and controls of carbon and water inputs and outputs helps us not only understand how the forest may change with climate change and disturbances, but also helps to better understand the global carbon and water cycles when we compare this forest to forest around the globe. To measure the rate, or flux, of carbon and water exchange between the forest and atmosphere, this project used a 25-m tall tower located in the subalpine forest near Niwot Ridge, Colorado, at an elevation of 3050 m above sea level. The forest consisted of 100-y-old stand of 46% Subalpine Fir, 28% Engelmann Spruce, 26% Lodgepole Pine, with a mean height of 11.4 m. The exchange of both carbon and water between the forest and atmosphere was continuously measured using the well-known eddy covariance method. This method calculated the exchange based on fast measurements of the vertical wind speed and the CO2 or H20 concentration densities. In addition, several other measurements were made to help interpret the flux measurements. Examples include soil moisture, soil temperature, profiles of CO2 in the canopy, and radiation. From these measurements, we were able to determine if the forest is a net source of CO2 to the atmosphere, or a net sink of CO2 from the atmosphere. Our results show that the net ecosystem exchange of CO2, the difference between carbon uptake through respiration and carbon release through respiration, averaged -217 g C m-2 per year. That means the forest was a net sink of carbon from the atmosphere. Interannual variation in net ecosystem exchanged depended on the winter snowpack conditions; years with a low snowpack had decreased soil respiration, hence a greater net annual uptake of carbon. Concerning water cycling, the forest was overall a dry site, with the sensible heat flux nearly twice the latent heat flux on an annual basis. The forest was "moisture limited", with most of the annual precipitation precipitation (800 mm) falling in the form of snow and therefore unavailable until spring. Sublimation of the snowpack can be significant, with up to 32% of the winter precipitation lost to sublimation. The spring melt provided the necessary water to trigger transpiration and carbon uptake, and infrequent, sporadic convective summer rains with a weak late-summer monsoon flow provided moisture during the summer. Overall, this study has shown the annual magnitudes, physical, and biological controls of the carbon and water cycles of a subalpine forest through the continuous measurements of forest-atmosphere fluxes. The longevity of the measurements permits this forest to be placed in the global context of other forests. The baseline measurements from this study have provided pre-disturbance reference values for other forests that have been disturbed by fire or insects. Lastly, the knowledge and understanding made possible by this study should help with better decisions and policies concerning carbon, water, and forest management along the Colorado Front Range.
