Net ecosystem exchange of CO2 in a wind-throw-disturbed upland spruce forest ecosystem – first results

Within the ongoing broad interest about the carbon source/sink behavior of natural ecosystems, forests, as large and highly dynamic terrestrial carbon pools, attract special attention. As knowledge of the carbon balance of intact forests becomes increasingly established, studies of ecosystem disturbances like fire, harvest, or windstorms get more and more important. Areas with wind-throw, where dead-wood remains on the ground without being salvaged, may turn into a substantial carbon source for extended periods, in contrast to the general carbon sink behavior of even mature intact forest ecosystems. The dynamics and magnitude of those processes on terrestrial carbon turnover are largely unknown.

In a large wind-throw area (ca. 600 m diameter, due to cyclone Kyrill in January 2007) in a mature upland spruce forest, where dead-wood has not been removed, in the Bavarian Forest National Park (Lackenberg, 1308 m a.s.l., Bavaria, Germany), fluxes of CO₂, water vapor and energy have been measured with the Eddy Covariance (EC) method since 2009. Decomposition of the coarse woody debris remaining on the ground is expected to lead to substantial net carbon emission in this area. On the other hand, nutrients leached from the dead biomass likely accelerate plant re-growth, leading to an earlier turn of the disturbed area from a carbon source to a carbon sink, compared to a cleared wind-throw or a harvested area.

For 2009, 2010 and 2011 estimates of annual Net Ecosystem Exchange (NEE) showed that the wind-throw was a marked carbon source. However, during daytime, on sunny and warm summer days, the wind-throw already acts as a net carbon sink, indicating that photosynthesis by the few remaining trees and newly emerging vegetation (grass, sparse young spruce, etc.) already exceeds respiration two years after the wind-throw event, leading to an already strong Gross Ecosystem Production (GEP). The relatively high elevation of the site, with cold temperatures and long periods with a closed snow cover, may also affect this balance by slowing down heterotrophic respiration in winter and at night.

To our knowledge, we present the worldwide first long-term measurements of NEE within a non-cleared wind-throw-disturbed forest ecosystem.