Effects of climate and soil conditions on the productivity and defence capacity of Picea abies in Sweden-An ecosystem model assessment

Climate change can lead to an increased frequency of extreme weather events, which may induce a decline in tree vitality, rendering the forest more vulnerable to other stress factors. In this study we used the ecosystem model LPJ-GUESS to compare the NPP of boreal conifers on till soil and sandy soil, simulating the growth of Norway spruce during the 21st century using climate model projections corresponding to RCP 8.5. Model runs with and without nitrogen cycling enabled, in combination with two different approaches to implement the dynamics of non-structural carbohydrates (NSC) were compared. The simulations showed that the forest productivity in Sweden is influenced by an interplay between site-specific temperature conditions and precipitation. Local differences in soil conditions can have an impact on the productivity, mediated by soil water and nutrient content. A warmer climate will lead to a longer growing season, but the productivity increase will partly be offset by a higher frequency of drought stress in summer and higher autotrophic respiration in winter. The ecosystem model runs indicated that periods with drought stress can cause low NSC levels, leading to time periods with decreased defence capacity against attacks by secondary agents such as bark beetles. The risks may however not become apparent until the forest stands are middle-aged. The model results are discussed in relation to current forestry practice, with Norway spruce (Picea abies) commonly being planted on dry soils better suited for Scots pine (Pinus sylvestris) in order to reduce moose browsing damage of seedlings, which in the longer term may create a forest landscape more sensitive to spruce bark beetle attacks.