Diploma Thesis
CO2-, Wasserdampf- und Wärmeaustausch eines durch Windwurf gestörten Waldökosystems in der west-russischen Taiga
Alexander Knohl (12/1998-05/1999)
Support: Thomas Foken
Co-support: Prof. Dr. E.-D. Schulze
The carbon dioxide, water vapour and heat exchange between the atmosphere and a windthrow area in the western Russian taiga was investigated by eddy covariance from July to October in 1998. The study area was a regeneration forest (400 m x 1000 m), where all trees were uplifted during a storm event in 1996. Additionally, carbon pools of decaying wood of a chronosequence of three different windthrows were analysed and the decay rate of coarse woody debris was determined. The goal of this work was to quantify fluxes in a forest after disturbance and to combine results from eddy covariance with decay of coarse woody debris. This work shall contribute to the understanding of the long term capability of terrestrial ecosystems to sequester carbon.
During the entire measuring period the closure of the energy balance reached 78% including heat storage in the air column beneath the measuring device and in the coarse woody debris. The Bowen-ratio was between 0.5 and 1.3 in July and exceeded 2.5 in October. The Evaporation fraction was 0.35 in average. The average daily sum of evaporation was 1.3 mm d-1. During all three months of measurements a total of 101.6 mm was lost by evaporation, while precipitation was 200.7 mm.
Over the course of the measurement period the windthrow was a net source of CO2. During the day the average CO2 uptake was -2 µmol m-2s-1, while during nights with well-mixed atmosphere about 5 µmol m-2s-1 were released. On average 164 mmol CO2 m-2d-1 were lost from the windthrow to the atmosphere per day. Suppression of turbulent fluxes was observed only under stable conditions with low friction velocity (u* £ 0.05 ms-1). During the whole field campaign from July 11th to October 10th a total of 14.9 mol CO2 m-2 was released to the atmosphere.
A chronosequence of dead woody debris on three different windthrow areas showed an exponential decay with a decay coefficient of -0.04 yr-1. From carbon pools and the decay rate it is concluded that decomposition of dead woody debris accounted for about 40% of total ecosystem respiration. Coarse woody debris may influence net ecosystem exchange of the windthrow area for more than 30 years. I therefore conclude that it is necessary to include early successional stages after natural disturbances like windthrow and the decay of coarse woody debris in the search for the long term capability of terrestrial ecosystems to uptake carbon.