Fluxes of energy, water, and carbon dioxide from mountain ecosystems at Niwot Ridge, Colorado

Abstract

Background: The eddy covariance (EC) technique provides a direct measure of water vapour and carbon dioxide fluxes between ecosystems and the atmosphere.

Aims: This review article highlights the findings of various studies that have integrated EC observations into basic meteorological, hydrological and ecological research questions in two ecosystems near Niwot Ridge, Colorado, and synthesises these studies into a catchment-scale model of water and carbon cycling, within the context of regional disturbance and environmental change.

Methods: EC was implemented continuously over subalpine forest and alpine tundra vegetation at Niwot Ridge, and resulting data were compared with discrete measurements and modelling studies.

Results: Sensible heat fluxes were generally in excess of latent heat fluxes, indicating that the forest and tundra ecosystems were moisture limited. Snow cover regulated the annual sum of primary productivity in the forest, and beneath-snow respiration represented a significant portion of ecosystem respiration at both locations.

Conclusions: Changes in the magnitude, timing, or spatial distribution of snow are likely to have the greatest impact on ecological processes in these semi-arid mountain catchments, but possibly in compensatory ways. Ultimately, the degree to which net carbon losses from alpine tundra offset forest carbon sequestration will determine the future magnitude of the Western United States carbon sink.

Keywords:

• alpine tundra
• complex terrain
• ecosystem respiration
• eddy covariance
• energy balance
• evapotranspiration
• gross primary productivity
• net ecosystem exchange
• subalpine forest
• sublimation

Acknowledgements

The authors wish to acknowledge the University of Colorado Mountain Research Station, as well as the field technicians, graduate students, post-doctoral researchers, and principal investigators that made this work possible. The Niwot Ridge AmeriFlux tower was funded by grants from the United States Department of Energy NIGEC, NICCR and TCP, and NSF DEB grants to RKM and PDB. Russell Monson is especially grateful for the support provided by NSF Grant DEB 1256526. Support for the Niwot Ridge LTER comes from the NSF Long Term Ecological Research Program. John Knowles wishes to acknowledge support from an NSF Doctoral Dissertation Research Improvement Grant. Comments from three anonymous reviewers added considerable synthetic value to this manuscript.

Additional information

Notes on contributors

John F. Knowles

John F. Knowles is a Ph.D. candidate. His dissertation research addresses the spatio-temporal patterns of soil respiration and the age of respired carbon from high-elevation alpine tundra. His research interests can be found at http://instaar.colorado.edu/people/john-knowles/

Sean P. Burns

Sean P. Burns has worked at NCAR and managed the Niwot Ridge AmeriFlux tower for over 10 years. He is currently a graduate student with a focus on forest–atmosphere exchange processes. Recent work and research interests can be found on-line at http://www.mmm.ucar.edu/people/burns/

Peter D. Blanken

Peter D. Blanken is currently an associate professor of geography and also Chair of the Department. He studies energy, water and carbon exchange over a variety of surfaces including alpine tundra, forests, and lakes.

Russell K. Monson

Russell K. Monson is a professor of ecology with a special interest in the carbon cycling of forest ecosystems, especially at temperate latitudes and at high elevations. His research is focused in the Western United States where water is a primary limitation to forest productivity.