Climate Impacts: Field of Action Forestry

As a result of climate change, precipitation in Germany is increasingly shifting from summer to the winter months. By 2050, a reduction in precipitation of up to 40 percent is expected in the summer months. For autumn and winter, an increase of up to 30 percent is forecast. The changing precipitation amounts and the increasingly uneven distribution of precipitation across the seasons pose a risk to forest ecosystems.

Drought stress

Drought is considered one of the main abiotic stress factors for forestry. It can severely affect the vitality of trees. Although mature trees rarely die from the direct effects of drought, it can increase their sensitivity to other stressors, such as forest fire, windthrow, and pest infestation.
If trees are supplied with too little water, the pressure with which the water is transported from the plant roots to the tree crown drops. An initial symptom of this pressure drop is seen in drooping leaves. To prevent further drying and the associated drop in pressure, the trees close the stomata of their leaves. This causes them to lose less water, but at the same time they can absorb less carbon dioxide. This limits the performance of photosynthesis and thus the buildup of important plant substances. At the same time, this means that the storage of carbon decreases. If the drought stress persists, trees shed their leaves, fruit or even entire branches, and their crowns become thinner. The prolonged drought in the 2018 and 2019 growing seasons caused widespread premature leaf drop. According to the most recent 2019 Forest Damage Report from the Federal Ministry of Food and Agriculture, the percentage of trees with significant crown defoliation increased from 29 percent in 2018 to 36 percent. Only about one-fifth of the trees showed no damage. Crown defoliation increased significantly, especially among deciduous trees. The crown condition of conifers showed no trend. Increased dying of trees was observed.

If the canopy of the trees becomes thinner, the forest microclimate also changes, as the cooling effect of a dense leaf canopy diminishes. This affects the trees and the animal and plant species that live in the understory and soil, too. Heat-loving species could benefit and displace other species adapted to cooler conditions. If cleared areas result from wildfires or removal of diseased and dead trees, drying may increase because the affected areas are exposed to increased solar radiation. This can further reduce water availability in the soil. In addition, the removal of dead wood is accompanied by the loss of nutrients and humus, which negatively affects water storage in the forest floor. If trees do not have enough water available, this lowers their evaporative capacity and growth. This also results in a reduced uptake of carbon dioxide, so that carbon storage decreases. Drier climatic conditions can thus increase the risk of forests losing part of their function as carbon sinks.

For all deciduous trees and conifers, very young trees are susceptible to drought. Their root system is not yet sufficiently developed to tap water from deeper soil layers. The probability that young trees will die from drought is therefore significantly increased. As adult trees, however, they differ in their sensitivity to drought. For example, spruce and european beech are considered more sensitive to drought compared to oak and pine. For spruce (Picea abies) in particular, drought is a decisive factor. For a long time, spruce was considered the ideal, high-yielding tree species due to its undemanding nature, robustness and ease of propagation, and with a share of 25 percent, it is the most common tree species in Germany, along with pine (23 percent), copper beech (16 percent) and oak (11 percent). Due to its mostly shallow root system, spruce is very sensitive to drought and can be damaged much more easily by dryness in the topsoil. Its growth is severely restricted as a result and, in extreme cases, can lead to death. In addition, spruce has also been grown on sites that do not meet its requirements for more cool and moist climatic conditions. As a result of these climate effects, forestry has experienced particularly high yield losses in spruce stands in recent years. One reason for this is that dry periods reduce the flow of resin, which spruce trees use to defend themselves against bark beetles and other pests. This makes it easier for them to penetrate the bark and wood. Other tree species also experienced drought stress and damage in dry years such as 2003. European beech (Fagus sylvatica) suffered from growth collapses that persisted into the following year, especially on sites with poor water supply. In many places, the crowns of mostly older beech trees died or the branches did not have sufficient foliage. Scots pine (Pinus sylvestris) has the lowest water demand compared to the native tree species. Since it usually develops a deep taproot, it can fetch water from deep soil layers. Nevertheless, it too may show more severe failures as a result of dry years. Oaks (including Quercus robur) show drought damage to leaves (discoloration and necrosis) and increased root development in response to drought stress.

Heat stress

Average annual temperatures in Germany have already risen more than the global average, and by 2019 they had already risen by 1.6 degrees Celsius. By 2050, climate model calculations predict that summer temperatures will be 1.5 to 2.5 degrees warmer than in 1990, and winter temperatures 1.5 to 3 degrees Celsius. In general, plant life processes run faster as temperatures rise and can result in higher yields with increased growth. However, high temperature extremes can also lead to acute heat damage in trees, such as beech and spruce, both of which are characterized by comparatively thin bark. In the case of very high direct solar radiation, the bark can heat up to 50 degrees Celsius. Since directly under the bark lies the tree's cambium, which controls cell growth in the trunk (wood cells for water and nutrient transport, bast cells for assimilate transport), damage to the cambium is inevitable as a result of high solar radiation. Damage to the cambium can impair water transport, causing drought stress or exacerbating it. In addition to beech and spruce, thin-barked maples, lime trees, ashes, and alder are also at risk. If the trunk is damaged by strong sunlight, this can also be an entry point for harmful fungi.

Indicators from monitoring on DAS: Tree species composition in forest nature reserves - case study | Endangered spruce stands | Incremental growth in timber | Forest condition

When the timber increment decreases, the volume of timber harvested decreases too.

Source: Ansario / stock.adobe.com

With climate change, the risk of forest fires may increase.

Source: Photograph: © Butch / stock.adobe.com

Even drought-tolerant pines suffer from excessive drought like the black pines near Würzburg.

Source: Photograph: © Tanja Sanders / Thünen-Institut

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Part of the natural forest dynamics is that insects and fungi feed on forest trees. This natural process is problematic when forests and woodlands can no longer perform important functions or services for humans (e.g. water and soil conservation, timber production, recreation, carbon sink) due to an excessive spread of harmful organisms and damage caused by them. Warm temperatures interfere with the population dynamics of many pest organisms as a critical factor, because temperature directly affects many life functions and development stages of insects and other pest organisms.
In warm years, the bark beetle (Ips typographus) already occurs earlier in spruce stands, with higher reproduction rates, shorter development times, and the formation of additional generations and sibling broods. As climate change progresses, there may be an increase in infestation intensities and expansion of infestation areas. Bark beetles not only cause primary infestations, but also transmit wood-destroying fungi of the genus Ophiostoma, some species of which clog vessels and thus cut off the water supply in the tree crown (wilt disease). Other relevant forest pests that have occurred in recent years, especially dry years, and have caused damage in many places are the northern spruce bark beetle (Ips duplicatus) in spruce stands, the nun moth (Lymantria monacha) in pine stands, the pine-tree lappet (Dendrolimus pini) and the blue pine beetle (Phaenops cyanea), in beech stands the beech splendour beetle (Agrilus viridis), and in oak stands the oak processionary (Thaumetopoea processionea) and oak splendour beetle (Agrilus bigattus).

The spread of harmful organisms is also favored by the fact that the lack of water supply after long periods of drought weakens the trees and makes them susceptible to harmful insects and fungi. During a prolonged drought, spruce produce less resin, which they normally use to repel bark beetles.
Climate changes also enable the immigration and spread of "new" pests that encounter unadapted host trees in forests and woodlands. Examples include the Siberian North Asian timber bark beetle (Cyclorhipidion bodoanus), which occurs in oak forests, and the black timber bark beetle (Xyleborus germanus), which attacks deciduous trees and conifers. Invasive species also include phytopathogenic fungi, such as Chalara fraxinea, which lives parasitically in the tissues of leaves, shoots, and woody parts of ash trees and is thought to be involved in increased ash dying.

Damage caused by harmful organisms can have far-reaching consequences. As reported by the Federal Statistical Office (Destatis), 32 million cubic meters of damaged timber were felled in 2019 due to insect damage, almost three times as much as the previous year's figure of eleven million cubic meters. In 2017, it was still six million cubic meters. Massive losses of forest biodiversity can occur via regional or largely complete loss of tree species via damaging organisms. The habitat and ecosystem functions of a tree species cannot simply be taken over by another tree species, especially if this is the only representative of the genus in a forest community.

Indicator from monitoring on DAS: Volume of damaged timber due to spruce bark beetle – case study

Long dry periods with hot temperatures, especially in the summer months, are becoming more frequent with climate change. This also increases the risk of forest fires. The number of days with high forest fire level increased from about 27 days per year in the period 1961 to 1990 to about 38 days in the period 1991 to 2019. Factors in the sensitivity of forests to forest fires include tree species composition and forest construction type. In principle, coniferous forests are more sensitive to wildfire than deciduous or mixed forests. Monocultures also tend to be more sensitive to forest fires than mixed forests. Another important factor is the water storage capacity of the soil. Dry leaves or dry needles increase the risk of forest fires, as do lush ground vegetation and dense undergrowth, as well as tree residues left behind after timber harvesting. An increasing forest fire risk does not necessarily lead to more or larger forest fires, because half of the forest fires are currently still caused negligently or intentionally by humans. The direct triggers are manifold: a discarded cigarette, a campfire, a lightning strike or even arson.

The consequences of a forest fire are also diverse. The extent of the impact depends, among other things, on the duration, intensity, extent and type of the forest fire. Ground fires and smoldering fires in the soil are of high importance to the vitality of forest stands because of the frequent destruction or impairment of roots and seeds. Ground fires or wildfires often result in the burning of ground-level vegetation and litter cover, which accelerates the mineralization process of the litter cover and may result in increased leaching of nutrients. Crown fires and full fires often result in the loss of the entire stand. If cleared areas result from fires or the removal of diseased and dead trees, soil drying may be exacerbated because the affected areas are exposed to increased solar radiation. After the fire, the temporarily lighter forest structure and better short-term nutrient conditions provide good living conditions for many animals and plants. Many species return. In the case of frequent and intense fires, it is mainly species that have adapted to fire that survive in the long term. Settlements and traffic routes are exposed to increased erosion and rockfall hazards after a fire on steep slopes. Directly during the forest fire, as with any combustion process, emissions occur (e.g. fine dust) that can affect human health. Greenhouse gases such as carbon dioxide (CO2) nitrous oxide (N2O) and methane (CH4) are also emitted. In addition, forest fires impair the sink function for carbon.

Indicator from monitoring on DAS: Forest fire hazards and forest fires

Windthrow: Climate change also makes the occurrence of storms more likely. Already since the 1990s, the forestry industry has recorded increasing economic damage from windthrow due to strong storms with high wind speeds. On the coast (especially the North Sea coast) and at high altitudes in the mountains, storm gusts can become particularly strong.

Indicators from monitoring on DAS: Damaged timber – Extent of random use