The GLEES AmeriFlux Site (US-GLE)

Climate change and disturbance are dramatically shaping our forests, especially in the western United States where insect outbreaks, forest fire, and rising temperatures have led to substantial tree mortality. Understanding the processes though which ecosystems cycle carbon and water will benefit our ability to anticipate, mitigate, and manage disturbance in forested ecosystems.

The AmeriFlux and FLUXNET networks were established to foster collaborative exchange of micrometeorological data among researchers from various ecosystems across continental to global scales. Among these is the Glacier Lakes Ecosystem Experiments Site (GLEES) AmeriFlux site, one of the highest-elevation and windiest sites in the world. The GLEES AmeriFlux site was established in 1999 to measure the exchange of energy, water vapor, and carbon dioxide between a subalpine spruce-fir forest and the atmosphere. In the late 2000s a spruce beetle outbreak caused the mortality of most of the mature Engelmann spruce trees, providing a unique opportunity to study the carbon and water response to substantial tree mortality following a spruce beetle outbreak. Since then, the site has been measuring the forest recovery as the surviving trees and understory plants have responded to the newly available resources.

Approach

The site focuses on the eddy-covariance technique to measure how the forest ecosystem uses the circulations in the wind to exchange energy, water vapor, and carbon dioxide. These measurements require the atmosphere to be well mixed and turbulent, for which GLEES is ideally suited. Over the years, complementary measurements have been conducted on respiration from the soil, snowpack, and canopy, the stable isotopes of carbon dioxide and water (lake and snow water, precipitation, water vapor), and forest biometric and stand characteristics.

A wintertime view from the top of the GLEES AmeriFlux scaffold. Forest Service photo by John Frank.

Key Findings

• Immediately following the spruce beetle outbreak, the amount of water lost by the forest ecosystem to the atmosphere during the growing season fell by ~1/3 due to hydraulic failure in the trees attacked by spruce beetle and their associated blue-stain fungus.
• Ecosystem photosynthesis fell by ~ .5% after beetle attack and by ~50% following tree mortality, causing the summer growing season at GLEES to become carbon neutral.
• There was minimal change in respiration from the soil or snowpack, except during the winter immediately after tree mortality caused considerable needle and litter fall.
• The Forested portion of GLEES shifted from an annual carbon sink to carbon neutral during the beetle attack and then to a carbon source following tree mortality.
• During winter, water flux to the atmosphere decreased by ~1/4 because recently dead trees could no long intercept snowfall, i.e., snow that falls on the snowpack is substantially harder to sublimate than fresh snow in the canopy.
• Considering winter snowpack persists for ~2/3 of the year at GLEES, the beetle caused reduction in sublimation is equivalent to the snowpack retaining 6% more precipitation every year.

Datasets

GLEES is an active site in the AmeriFlux network, who host the 30-minute micrometeorological and eddy flux data for GLEES and for the GLEES Brooklyn Tower on their website, with data and variables corresponding to the standardized AmeriFlux format. The data is also available via FLUXNET. Other data and resources from the GLEES AmeriFlux site can be found at:

Berryman, E.M., Frank, J.M., Massman, W.J., Ryan, M.G., Fahnestock, J., 2017. Advective and Diffusive Snowpack Carbon Dioxide Fluxes from 2009 to 2016 at GLEES. U.S. Geological Survey Data Release, Wyoming.

Frank, John M.; Massman, William J.; Ewers, Brent E.; Williams, David G. 2018. Data and source code for "Bayesian analyses of seventeen winters of water vapor fluxes show bark beetles reduce sublimation". Fort Collins, CO: Forest Service Research Data Archive.

Frank, John M.; Massman, William J.; Chan, W. Stephen; Dengel, Sigrid; Biraud, Sébastien C.; Billesbach, David P.; Hanson, Chad V. 2019. AmeriFlux site visit report and data for the Glacier Lakes Ecosystem Experiments Site (US-GLE). Fort Collins, CO: Forest Service Research Data Archive. 

An overview of the GLEES AmeriFlux data, including daily average air temperature, daily average snowpack height and cumulative precipitation for each water year, daily average friction velocity, daily total water flux, and daily total carbon flux.