The relative importance of environmental drivers on the growth of Norway spruce depends on soil types: A case study from Saxony and Thuringia, Germany

In forest management and sience it is important to determine the drivers of tree growth and to quantify their relative importance with regard to forest site characteristics. The growth of individual trees depends on complex interactions between biotic and environmental drivers. Forest management can make use or buffer the effects of biotic drivers, e. g. through thinning strategies. However, large uncertainties emerge from environmental drivers and its effects on tree growth.

The aim of this study is to quantify the relative importance of environmental drivers (climate, soil, and terrain attributes) on the growth of Norway spruce trees (Picea abies (L.) Karst.). For that purpose we distinguished three common soil types of Saxony and Thuringia, Germany (Cambisols, Podzols and water-influenced soils, i.e. Gleysol, Planosol, Stagnosol). We used national forest inventory data, regionalized climate data and terrain inferred parameters with a Boosted Regression Tree (BRT) approach. The approach is particularly suitable, since BRT quantify the relative predictor importance, considering non-linearities and interactions among predictors.

The results of this study clearly demonstrate the importance of soil properties on the growth of Norway spruce trees. Terrain attributes and temperature are similarly important for Norway spruce growth on Cambisols and Podzols, whereas spruce growth is mainly influenced by the relative sand content of the soil, the available field capacity and terrain attributes on water-influenced soils. Interactions among environmental drivers are most relevant on Cambisols and Podzols but not on water-influenced soils. Thus, the implementation of the results in growth models of high spatial resolution will support decision making in forest management.