A characteristic of alpine ecosystems is the high degree of spatial and temporal variation in the variables that regulate surface-atmosphere CO2 and H2O exchanges. A a result, it has been difficult to directly measure ecosystem-level exchanges, especially continuously throughout all seasons including winter. Therefore, estimates of the role of alpine tundra in the global carbon and water cycles have been lacking. To address this concern, we used the eddy covariance micrometeorological method since June 2007 to directly measure CO2 and H2O fluxes at 0.5-hr intervals continuously above alpine tundra. Measurements were made above tree line at an elevation of 3,480 m above sea level on a flat ridge top on Niwot Ridge, Colorado. Using a paired-tower approach, net ecosystem exchange of CO2 (NEE) was calculated using the measured eddy covariance flux, CO2 storage beneath the sensors, and horizontal advection. Footprint calculations showed that 80% of the turbulent flux measurements occurred roughly with 340-381 m upwind of the instruments. Several ancillary measurements were made and analyzed to develop the first-order controls on NEE.
Results/Conclusions
Persistent, down-sloping (west) strong winds with a high friction velocity minimized horizontal advection and likely aided in the overall quality of the eddy covariance-based flux measurements. For example, energy balance closure averaged 71% and 91% during winter and summer months, respectively, and the horizontal advection component averaged a maximum of 6% of the summer, nighttime NEE. Most of the available energy was partitioned as evaporation (59%) throughout the year, aided by a downward (negative) sensible heat flux during the winter. Based on the length of the period of negative NEE (net CO2 uptake by the surface), the alpine tundra’s growing season was roughly 100-days-long. Slow, but steady, respiratory CO2 released over the long winter season resulted in an annual net cumulative loss of CO2 from the surface. The switch from wintertime net CO2 release to spring-time uptake coincided with the date when the daily-average air temperature reached +10 deg C, and the switch back to net CO2 release at the end of the growing season coincided with the date when the daily average air temperature decreased below -10 deg C. Measurements are on-going, and additional measurements and modeling are planned to identify the specific controls on the alpine tundra’s long-term carbon and water budgets.