Field measurements of atmosphere-biosphere interactions in a Danish beech forest

Abstract

A field station was established in a beech forest near Sorø, Denmark in the spring of 1996 to provide a platform for studies of atmosphere-biosphere interactions. The station is equipped with a 57-m mast and a 24-m scaffolding tower. The mast makes it possible to measure profiles of gaseous atmospheric compounds and meteorological variables and to undertake measurements of fluxes by the eddy covariance method. The tower gives access to the tree canopy where branch and leaf level exchange of water vapour and CO 2 is measured. Soil-atmosphere exchange of gaseous compounds is investigated with gas flux chambers on the soil surface. Water-mediated transport of C and N is measured in throughfall, stemflow and soil water. The paper provides information on the site, vegetation, climate and soil and gives a description of the instrumentation and other technical installations at the site. The paper also presents and discusses example results from the measurements such as meteorological variables (temperature, wind speed, wind direction, radiation, rainfall and relative humidity), gaseous concentrations (O3) and fluxes (CO2, CH4 and N2O), water mediated transport (NO3- and NH4+ in rainfall, throughfall, stemflow and soil water) and measurements on the trees (leaf area index, specific leaf area, litter fall, CO2 exchange at branch and leaf level, maximum photosynthetic capacity). The forest acted in all years as a carbon sink with an average uptake of 190 g C m-2 yr-1. Contributions from CH 4 and N2O in terms of global warming potential (GWP) were less than 10% of the CO2. CH4 was deposited and N 2O emitted, almost neutralising each other in terms of GWP. Leaves at the top of the canopy had the highest photosynthetic capacity (mid summer maximum of 50 μmol m-2 s-1) and leaves at the bottom the lowest (20 μmol m-2 s-1), indicating a clear acclimation to light. Sun leaves generally had significantly lower specific leaf area (118 cm2 g-1) and water content (42%) than shade leaves (specific leaf area: 282 cm2 g-1; water content: 53%). On an annual basis about 24% of the precipitation was lost as evaporation, 66% reached the forest floor as throughfall and 10% as stemflow. Due to leaching of NO3- from the trees, the forest floor received 30% more nitrogen than in precipitation, resulting in a total input of 2.9 g N m-2 yr-1). The soil water was almost completely depleted in NO3- with NH4+ constituting 97% of the total available inorganic nitrogen.