Forest futures for the Netherlands: Climate change impacts on tree growth and species interactions

Climate change exerts effects on forests worldwide. More frequently occurring droughts, related to climate change, lead to reduced growth and loss of vitality of trees worldwide. After the recent exceptional 2018 drought, declined growth and vitality and increased mortality was observed in several European temperate forests and tree species. Mixed-species forests are gaining importance in the face of climate change. The species interactions in mixtures may vary in space or over time, and therefore understanding species interactions under various growing conditions is needed to be able to adapt management practices in the face of climate change.

This thesis analysed the current and future impacts of climate change on Dutch forests in relation to local site conditions, and evaluated possible adaptation through changes in forest management, specifically related to species selection and species composition. Therefore, (1) tree-ring analysis quantified drought responses at the individual tree-level and (2) process-based growth modelling projected future growth and species interactions in mixed forests under climate change on a range of edaphic conditions specific for the Dutch context.

Chapter 2 addressed the question how tree-growth responses to drought differed between nine tree species in relation to drought dimensions and edaphic conditions. It highlights major growth reductions during drought in both coniferous and broadleaved tree species. Drought impact varied across species and sites and was most prominent on the driest sandy soils. It remains unclear if the responses to severe droughts will result in continued growth decline and eventually higher mortality in the near future, but the immediate effects of drought on annual ring width are clear.

The effect of climate change on species interactions in mixtures of Scots pine (Pinus sylvestris L.) with pedunculate oak (Quercus robur L.) and non-native Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) with European beech (Fagus sylvatica L.) was investigated in chapter 3 and 4. Therefore, the 3-PGmix model was parameterized, calibrated and validated for growth and yield of these species under Dutch growth conditions and projections under future climate were made. The findings suggest that projected changes in climate will influence species interactions and result in increased Scots pine productivity, notably on resource-limited soils. For the mixture of Douglas-fir with beech, model projections showed that future climate negatively influenced beech growth, whereas Douglas-fir increased its competitive strength, even in the most extreme climatic scenario. This mixture is expected to maintain its productivity, while meeting the demand for a more species-diverse and climate-resilient forest.

Maintaining resilient and productive forests is based on an understanding of how individual species and species combinations are expected to perform under future climate conditions. This thesis shed light on the drought resilience of Dutch trees species and offered long-term projections of species interactions in mixtures under the specific Dutch circumstances. The insights in this thesis may support forest management to make informed decisions that account for site-specific conditions, in an increasingly uncertain future.