WHY IS THE RISK OF TREE MORTALITY HIGH IN SOUTHEASTERN FINLAND?

Climate stress is stronger in Southeast Finland than elsewhere

Tree mortality has increased significantly in Finland over the past few years. However, this increase is not evenly distributed across the country but is strongly concentrated in Southeast Finland. Why is this the case? The answer does not lie in a single factor. Instead, it results from the combined effects of climate, forest structure, and biological risks.

Southeast Finland belongs to the warmest and driest regions of the country. Continental climate influences are pronounced: summers are hotter, heat waves are longer, and precipitation is more irregular than, for example, along the western coast. The accumulated temperature sum in Southeast Finland is among the highest in Finland. Recent prolonged dry periods have caused long-term stress to forests. Drought does not necessarily kill trees immediately, but it weakens their resistance, making them more vulnerable to pests and diseases.

In many areas, forests in Southeast Finland are dominated by spruce growing on fertile mineral soils. Spruce is particularly sensitive to drought and high temperatures due to its shallow root system. In addition, a large proportion of spruce stands in Southeast Finland are mature and even-aged, which increases the risk of large-scale tree mortality. According to the most recent National Forest Inventory, Finland’s largest forest resources are located in Southeast Finland, which further increases the potential magnitude of forest damage.

Fewer forest disturbances in Western Finland

Western Finland has been more resilient to forest damage. The primary reason is its maritime climate: the Baltic Sea moderates temperatures, increases humidity, and shortens drought periods. In addition, Western Finland has a higher proportion of pine, which tolerates drought and heat better than spruce. However, this does not mean that large-scale tree mortality could not occur in Western Finland in the future, as drought may increasingly predispose pine forests on rocky sites to various pathogens and pests.

Pests and hotspot phenomena

Climate warming has increased the risks posed by forest insect pests. Bark beetles in particular benefit from warmer and drier conditions, and high temperature sums promote their reproduction. Research has shown that bark beetle-induced forest damage does not occur randomly but instead forms distinct hotspot areas, especially in southern and southeastern Finland. Certain forest structural characteristics, forest management choices, and climatic conditions facilitate the spread and recurrence of disturbance. The regional concentration of risk arises because these critical factors converge most strongly in Southeast Finland.

The key question for the future is not whether forest damage can be completely prevented, but how forests can adapt through species selection, diversification of forest structure, and proactive risk management.

References:

Junttila, S., Blomqvist, M., Laukkanen, V., Heinaro, E., Polvivaara, A., O’Sullivan, H., Yrttimaa, T., Vastaranta, M. ja Peltola, H., 2024. Significant increase in forest canopy mortality in boreal forests in Southeast Finland. Forest Ecology and Management, 565, p.122020.

Pulgarín Díaz, J.A., Pérez-Pérez, J., Melin, M., Peltola, H. ja Tikkanen, O.P., 2025. Assessing the impacts of forest stand structure and landscape on the formation of Ips typographus damage hotspots in Finland. Forestry: An International Journal of Forest Research, p.cpaf058.