Tree Damage Risks Due to Warming Climate

As the climate warms, the growing season lengthens, spring arrives earlier, and winter starts later. Winters become milder, and dry and hot spells become more common in summer. Although a longer growing season enhances tree growth, excessive drought and heat can limit it. Many tree-damaging pathogens and animals benefit from warming conditions. They, in turn, weaken trees' growth and carbon sequestration capacity.

The ground freezes less often and for shorter periods than before. The winter-time anchoring of trees in the soil weakens, and the risk of overturning, especially for shallow-rooted spruce, increases. Fresh windfalls provide breeding grounds for bark beetles. Species like the European spruce bark beetle and the eight-toothed spruce bark beetle proliferate when given the opportunity. Simultaneous heat and drought periods increase the likelihood of bark beetle outbreaks and tree mortality.

Warm and dry growing seasons can reduce diseases that thrive in moist and cool conditions. On the other hand, the same conditions facilitate the spread of other diseases by, for example, accelerating fungal mycelium growth and prolonging sporulation time. Root rots, significant decomposers of conifers, are among those that benefit from warming conditions.

During the growing season, drought combined with heat increases tree transpiration. Trees close their stomata to curb water loss. At the same time, photosynthesis and carbon sequestration decline. Carbonbased resin is a defense substance for conifers: insects get stuck in it, and resin compounds repel insects and inhibit pathogen growth. Trees' defense capabilities weaken as photosynthesis declines, and trees stressed by water scarcity become more susceptible to insects and diseases.

Disturbances are interconnected

Wind is a significant cause of disturbances, ranking as the fourth largest contributor to damage in forestry. As the climate warms, extreme weather events are becoming more common, and storms are intensifying. Freshly wind-felled conifers cannot defend themselves against bark beetles and provide them with breeding grounds.

European spruce bark beetle (Ips typographus) is the most well-known and economically significant spruce bark beetle (Figure 1). When present in small numbers, these beetles only infest weakened trees. However, if windfalls are abundant, bark beetle populations can proliferate. With enough numbers, they can even infest vigorous standing trees.

In spring and summer, spruce trees infested by the European spruce bark beetle, which are still green, can be identified by brown, coffee-ground-like frass accumulating at the base of the tree (Figure 2). As the species reproduces, it leaves characteristic galleries under the bark (Figure 3).

Removing already dead and/or debarked spruce trees will not reduce the bark beetle population in the stand. These trees may host beneficial species that prey on or parasitize bark beetles. Instead, you should look for infested but still green trees within the stand (Figure 4). Individual trees or small groups of trees infested by the European spruce bark beetle usually do not develop into large outbreaks.

A close-up macro image of a European spruce bark beetle (Ips typographus) on a black background, seen directly from above.

Figure 1. The European spruce bark beetle (4-5 mm) is one of the Finnish bark beetle species. Photo: Erkki Oksanen, Natural Resources Institute Finland.

A close-up of a spruce tree base, showing brown bore dust (frass) caused by bark beetles, accumulated on the moss and in the bark crevices.

Figure 2. Brown frass collected at the base of the tree and in bark crevices is a sign of the European spruce bark beetle. Photo: Tiina Ylioja, Natural Resources Institute Finland.

A close-up of the typical feeding galleries of a European spruce bark beetle, revealed under a piece of bark, showing a vertical maternal gallery and branching larval tunnels.

Figure 3. European spruce bark beetle gallery. Photo: Erkki Oksanen, Natural Resources Institute Finland.

An infographic illustrating the six-stage life cycle of the European spruce bark beetle, from swarming and egg-laying to the adult beetle emerging from the tree.

Figure 4. European spruce bark beetle life cycle.

The impact of climate warming on the European spruce bark beetle

The European spruce bark beetle is found wherever spruce trees grow. It's less numerous in the north than in the south, but as the climate warms, its population is increasing further north than before. For timber production, the bark beetle is a pest, but in nature, its role is that of a forest regenerator. Older spruce trees die and become decaying wood used and broken down by other organisms, making space for new trees to grow.

Poikilothermic (cold-blooded) insects are sensitive to environmental temperature fluctuations. Insect development from egg to adult is regulated by temperature. Insects also have the capacity to produce multiple generations when the growing season warms and lengthens.

The European spruce bark beetle's development from egg to adult accelerates, allowing it to reproduce more than once during the summer (graph). Overwintered adults reproduce twice, producing what's known as a sister generation. If the late summer is favorable, the first generation that emerged during the summer can also reproduce and lay eggs for a second generation before winter. If this second generation doesn't reach adulthood before winter, the larval and pupal stages will die. In late summer, decreasing temperatures and shorter days signal to the beetle that winter is approaching and it's time to enter diapause.

A diagram showing the development of European spruce bark beetle generations from spring to winter, including the overwintered generation, the first generation, a sister generation, and the second generation.