New study reveals how climate, competition and species mixtures drive conifer mortality
As climate change increases pressure on Europe’s forests, understanding why trees die is becoming increasingly important for maintaining forest productivity, timber supply and ecosystem services. A new study by David Lee and Sonja Vospernik of BOKU University, through OptiForValue, provides fresh insights into the mortality drivers of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris), two of Europe’s most economically important conifer species.
Using 40 years of Austrian National Forest Inventory data (1981-2021), combined with high-resolution climate information, the researchers analysed nearly 150,000 individual tree observations to identify how tree characteristics, stand structure, species mixtures, soils and climate influence mortality risk.
Spruce and pine respond differently to climate stress
The study found that Norway spruce and Scots pine respond very differently to climatic stress. Norway spruce was highly sensitive to climatic variability, with mortality increasing during unusually warm springs and hot summers. The research challenges the common assumption that drought alone is driving spruce decline. Instead, direct thermal stress, bark beetle-favourable temperatures, and poor site drainage where wet, stagnant summer conditions can create waterlogging and root stress emerged as key risk factors.
Scots pine, by contrast, showed greater resilience to short-term climatic shocks. Rather than responding strongly to individual weather events, pine mortality was associated with long-term exposure to warmer baseline temperatures. The results suggest that Scots pine approaches a physiological threshold in warmer lowland areas, with mortality risk rising sharply where average annual temperatures exceed approximately 8.5°C.
Species mixtures can increase or reduce mortality risk
One of the study’s most notable findings is the important role of species composition in determining forest resilience.
For Norway spruce, mortality probabilities increased when growing in stands with higher proportions of European beech (Fagus sylvatica), likely reflecting stronger competition for soil resources and water. In contrast, mortality decreased when spruce was mixed with light-demanding conifer species such as pine or larch, suggesting these mixtures may create more favourable stand structures and reduce competitive pressure.
Scots pine showed the opposite tendency. As a strongly light-demanding species, pine experienced higher mortality when mixed with broadleaved species or Norway spruce, which can increasingly outcompete pine as forest stands mature and succession progresses.
These contrasting responses highlight that species diversity alone does not automatically increase resilience. The specific combination of species present can have major implications for forest stability and long-term survival.
Supporting climate-smart forest management
The findings point towards the need for increasingly site-specific silvicultural strategies. For Norway spruce, careful consideration of soil conditions and species mixtures may help reduce vulnerability to climate extremes. For Scots pine, avoiding warmer lowland environments and limiting competitive pressure from overtopping species may become increasingly important.
For the OptiForValue project, the study provides valuable evidence on how climate, stand dynamics and species composition interact to influence forest health and future resource availability. Such knowledge can help support more resilient forest management and contribute to sustaining Europe’s forest-based value chains under a changing climate.