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Atmospheric science

Atmospheric science

Eddy-covariance flux tower in the alpine tundra on Niwot Ridge, photo credit: John Knowles

Research Spotlight

Surface-atmosphere fluxes of carbon and water

The annual cycles of CO2 and H2O between the surface and atmosphere of alpine tundra are poorly understood, especially during the winter season since most ecological measurements are made during the summer. The eddy covariance micrometeorological method offers the advantage of spatially integrating across the heterogeneous and complex alpine landscape. Several years of continuous measurements have now given us the opportunity to begin to understand the complex processes governing the exchange of CO2 and H2O from alpine tundra. Our primary finding is that annually, more CO2 is released from the tundra to the atmosphere than is sequestered from the atmosphere (Figure 1; Knowles et al. 2014).  

Not surprisingly, we find that the long winter season plays a dominant role in the annual carbon balance. In the framework of the water year starting 1-Oct, the average date when the eddy-covariance measured Net Ecosystem Exchange (NEE) switched from positive (source: winter respiration season) to negative (sink: summer growing season) was 15-June (± 7 days), with the 87-day-long (± 12 days) growing season ending 10-Sept (± 8 days).  An average of 27.2  (± 2.80) g C m-2 were released, 5.02 g C m-2 (± 1.33) absorbed, with a 24-mean cumulative NEE of 23.7 g C m-2  (± 4.01) released to the atmosphere (Figure 1b).

Our working hypothesis is that the high winter respiration is due to several factors possibly including changes in snow properties and the subsequent moisture and insulative properties it provides to the soil.  We find that years with low snowpack and snow water equivalents have lower cumulative NEE during the winter period (October to May 13 of the water year) prior to melt out. 2012 provides the one exception, and points to the additional contributions of warm soil temperatures in the fall (prior to snow accumulation) to winter respiration.

Figure 1. Net Ecosystem Exchange of CO2 (NEE) measured using a pair of eddy covariance towers located on the alpine tundra of Niwot Ridge suggests that the tundra loses more CO2 to the atmosphere than it sequesters annually. A) Loss of CO2 during the winter season and sequestering (negative NEE) during the growing season. B) Cumulative 24-hr mean (NEE) shows that the summer sink period does not make up for the losses in the winter, resulting in an average net carbon loss of 23.7 g C m-2  (± 4.01) released to the atmosphere annually. C) A positive association between cumulative snow water equivalent in May and the cumulative NEE over the same period suggests a link between snowpack and winter respiration. The  anomaly is water year 2012, where a warm fall influenced NEE (Knowles et al. 2014). 

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This material is based upon work supported by the National Science Foundation under Cooperative Agreement #DEB-1637686. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necesarily reflect the views of the National Science Foundation.

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