Application of the eddy-covariance method under the canopy at a boreal forest site in central Sweden

Abstract

The eddy-covariance (EC) method was applied at 1.5 m height, for the period 2007-2010 at Norunda, central Sweden, for calculating energy (sensible and latent heat) and carbon dioxide (CO2) fluxes. The aim of this study was to assess the applicability of EC method above the forest floor. In 2007 an open-path EC system was used and in 2008, 2009 and 2010 a closed-path system was used. The energy and CO2 fluxes showed a clear annual pattern, with maximum values occurring in the summer season or at the beginning of September. The values of the sensible heat (H) flux were low, below 10 Wm-2, while the latent heat (LE) flux had annual daily average maximum values of 48 Wm-2 (2007), 18 Wm-2 (2008) and 27 Wm-2 (2009 and 2010). The mean annual CO2 flux values were 2.5 µmol m-2s-1 (2007 and 2008), 3 µmol m-2s-1 in 2009 and 2.8 µmol m-2s-1 in 2010. The fluxes measured in 2009 and 2010 were on average higher than the ones measured in 2008, a possible consequence of the forest thinning operation, which allowed more net radiation to reach the ground and caused an increase in turbulence above the forest floor. LE and CO2 fluxes measured with the open-path system had the biggest values and the largest variability under the entire study periods. The summer daily patterns of the energy fluxes showed higher LE than H throughout the entire 24 hours cycle. Daily summertime H flux was positive during 2009 and 2010, however a decrease was observed during afternoon, when the flux became negative. CO2 flux was very variable in time in 2007 and during nights, a consequence of both the use of an open-path system and the data selection turbulence criterion, linked to the standard deviation of the vertical wind velocity. A mid-day minimum was observed for the summer CO2 flux in 2009 and 2010, caused by photosynthesis inside the footprint area of the EC system. A comparison of EC and soil chamber data showed that CO2 fluxes measured by both methods followed in general the same patterns; however there was a discrepancy between the recorded values. On average, the EC fluxes accounted for 39 to 52% of the soil chamber fluxes. These results are consistent to other studies. In order to have a complete understanding of the applicability of the EC method above the forest floor at Norunda, further studies are necessary in order to analyze the energy balance closure, the spectral corrections for the raw EC data and the treatment of CO2 flux under very low turbulent conditions.

Abstract

Popular science
The eddy-covariance (EC) method is one of the most important methods to measure gas and energy fluxes between biosphere and atmosphere at ecosystem level. This method was applied above the floor of a boreal forest in Norunda, central Sweden, at 1.5 m height. The objective was to measure the energy and carbon dioxide (CO2) fluxes from the soil and the understory vegetation to the atmosphere. The energy fluxes measured were the sensible heat (H) and the latent heat (LE). The study period was from 2007 to 2010, with summertime data mostly. In the same time, the CO2 flux from the soil was also measured with another method, soil chambers.
Both the energy and the CO2 fluxes had a clear annual pattern. The maximum values of the fluxes were recorded at the end of the summer or in the early beginning of autumn. In the case of the energy fluxes there were differences in value between the four studied years. In 2007 the LE was higher than in the other years due to a different instrumentation setup used. In 2009 and 2010 the values of the energy fluxes were higher than in 2008 because the forest was thinned at the end of this year. As a result, the incoming radiation allowed a higher energy exchange at the forest floor. Overall, the measured values of H and LE with EC were lower compared to similar studies.
The CO2 flux measured with EC was rather constant throughout the studied years. The forest floor was a source of CO2 to the atmosphere. In the summers of 2009 and 2010, after the forest thinning, in the diurnal pattern of the CO2 flux a minimum was observed around noon. This minimum was due to photosynthesis of the understory vegetation, which was higher in these years because the more open canopy allowed more light to reach the ground and increase photosynthesis.
However, the CO2 fluxes measured with EC were lower than the fluxes measured with soil chambers. This finding is consistent to other studies that compared EC with soil chambers fluxes. The difference is explained by the fact that EC measures the flux for a wider area, while the soil chambers measure CO2 flux at a particular spot of measurement and show a large spatial variability.

• author: Pirvulescu, Vlad
• supervisor: Meelis Mölder; Patrik Vestin
• organization: Dept of Physical Geography and Ecosystem Science
• course: NGEM01 20131
• year: 2013
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: closed-path system, open-path system, soil chambers, latent heat flux, carbon dioxide flux, sensible heat flux, Physical Geography and Ecosystem Analysis, eddy-covariance, Norunda
• publication/series: Student thesis series INES
• report number: 275
• language: English
• additional info: Master degree thesis, 30 credits in Atmospheric Sciences & Biogeochemical Cycles. Department of Physical Geography and Ecosystems Science, Lund University.
• id: 3813417
• date added to LUP: 2013-06-14 15:35:10
• date last changed: 2014-06-18 11:42:45