Temporal Variability in Growing Season CO2 Exchange of a High Arctic Ecosystem.

Within the ZERO (Zackenberg Ecological Research Operations) programme, continuous micrometeorological measurements of CO₂ exchange in a high arctic ecosystem have been conducted during summer seasons in NE Greenland since 2000. In addition to these measurements, there has previous been measured CO₂ from other dominant surfaces in the valley ecosystem. Here we present the results of the measurements carried out during the years 2000-2003 at a high arctic heath site. In addition measurements conducted in 1997 from one other dominant ecosystem is also presented. The study is an intercomparison of Net Ecosystem Exchange (NEE) among the 4 years and the environmental parameters governing the exchange. In Zackenberg NEE shows seasonal as well as inter annual variation, following the seasonal change in the environmental controls. At the heath site NEE ranged from a source of 0,8 gC/m²day to a sink of -1,1 gC/m²day in the 4 years of continuous measurements. The measurements show that start and length of the snow-free season is an important variable to explain the inter annual variability. Onset of the growing season is governed by the extent and magnitude of the snow cover in the valley. Once the snow disappears the vegetation start developing and the photosynthesis can take place, turning the C source into a net sink. Maximum CO₂ uptake occurs at the time of maximum leaf area index. The end of the sink season is marked by the photosynthetic uptake of CO₂ being balanced by the respiratory loss of CO₂. The ecosystem turns into a source again by the end of the growing season. The end of the growing season differs only by a few days during the years of measurements at the heath site, whereas the onset of the growing season differs with almost 2 weeks during the 4 years. The results of the NEE illustrate that the magnitude of the fluxes during the growing season differ through the 4 years, which indicate that the heath ecosystem is sensitive to changes in environmental factors. Thus when the growing season starts late, NEE is affected since the growing season ends almost at the same time each year. Leading to the conclusion: late snow melt results in delayed growing season and therefore decreased seasonal CO₂ uptake.