A modelling study of the effects of species composition changes on radiation, H2O and CO2 fluxes in a boreal forest ecosystem

Climatic changes may have significant impacts on forest ecosystems. They can result in changes of tree species composition, forest productivity and soil carbon sequestration. A global warming which is expected to be particularly large at higher latitudes will affect boreal forests probably more strongly than forests in other latitudinal zones (e.g. IPCC 2007). It can be expected that the greatest changes may occur at the southern boundary of the boreal forest zone, where the boreal coniferous forest is likely to give way to broadleaf species. How such vegetation changes will affect water and CO2 budgets of land surface at this area is not known yet. Within the framework of this study effects of species composition (coniferous and broadleaf species) on evapotranspiration and Net Ecosystem Exchange (NEE) of CO2 in a boreal forest ecosystem were determined using an one- and three-dimensional SVAT (Soil - Vegetation - Atmosphere Transfer) models (Mixfor-SVAT, Mixfor-3D). Both models were developed to describe the energy, water and CO2 exchange within and above vertically structured mixed or mono-specific forest stands. They allow simulating realistically the H2O and CO2 exchange between forest ecosystems and the atmosphere taking into account transpiration, water uptake, photosynthesis and respiration of different tree species in the forest overstorey and understorey, as well as evaporation and respiration of the soil and dead biomass respirations. Modelling experiments were carried out for five different scenarios of changes of tree species composition. Each scenario assumes different admixture of spruce and broadleaf species in a forest stand. As input meteorological parameters in our modelling experiments the measured meteorological air temperature, humidity, wind speed, precipitation rate and global solar radiation data for one year test period were used. Results of modelling experiments showed a relatively high dependence of forest radiation regime, evapotranspiration, photosynthesis and respiration on species composition. This effect is strongly depended on environmental and soil moisture conditions and it has a clear seasonal trend.