Tree species and size drive water consumption of beech/spruce forests - a simulation study highlighting growth under water limitation

By using a growth model, a simulation study was done to detect differing transpiration sums of an evergreen and a deciduous tree species within a mixed stand. The dependence of summer drought and transpiration on tree size and species, and the relationship of water use efficiency and tree growth was analyzed. The process-based growth model BALANCE was used to simulate the water balance and the growth of individual trees for the isohydric species Picea abies and the anisohydric species Fagus sylvatica within a mixed forest stand. The individual tree based model was able to realistically simulate the water balances at tree and stand level. Actual evapotranspiration and soil water content differed in species and was size-dependent. Spruce was more affected by drought than beech. Drought stress increased with tree size, an effect which was more pronounced for spruce than for beech. Wood productivity was positively correlated with water-use efficiency being more distinct in beech than spruce. Using individual tree based growth models effects of tree individuals in structured forest stands on water consumption, growth and productivity can be analyzed. The simulation results, i.e. the information of species-specific water consumption, growth rates and dependencies between water consumption and tree growth in stands of Norway spruce and European beech can help to mitigate effects of climate change on forest stand productivity and preserve an appropriate proportion of high quality timber mainly provided by spruce.