Forest damage due to drought and heat


German forests suffer under the weather conditions of the years 2018, 2019 und 2020. In the following, we provide answers to frequently asked questions about forests and climate.

Effects of drought and heat: Damaged and undamaged trees are often found in direct neighbourhood (here: a pine stand in Lower Saxony). (© Michael Welling/Thünen-Institut)

How is the situation of the German forests after the dry and hot periods in the summers of 2018 to 2020?

Throughout Germany, we observed damage and the dying off in old stands of spruce, but in some areas also of beech. Pines and oaks seemed overall to be less affected. This is partly due to a higher tolerance of the two species to drought and heat (Sessile Oak is particularly noteworthy here) but also due to the climatically less severe conditions in the regions of their distribution. However, in some regions both tree species suffered from the infestation of pests (among others nun moth, pine-tree lappet moth, pine beauty moth, Diplodia shoot dieback on pine, and oak processionary moth, gipsy moth, winter moth on oak).

Considerable damage also occurred in plantations and areas of natural regeneration. Important information on the vitality of forests is provided by the time series of the Crown Condition Survey (Waldzustandserhebung), which has been carried out since 1985.

After 2018, the situation escalated further by the hot and dry year of 2019. At least 180,000 hectares of forest died and more than 100 million cubic metres of damage wood accumulated. Including the damage for the year 2020, the Federal Ministry of Food and Agriculture (BMEL) assumes that 171 million cubic metres of damage wood (harvest timber without bark) and an area of 277,000 hectares will have to be reforested.

This data is difficult to imagine due to its unpreceded magnitude. Comparisons may help: The damaged area of 277.000 hectares is larger than the total area of the German federal state of Saarland. A solid footbridge of 1 m width and 45 cm thickness, built from the 171 million cubic metres of damaged wood, would reach from the earth to the moon.

Is this forest dieback 2.0?

The term "Waldsterben" (forest dieback) is misleading. The current damage affects individual trees, groups of trees and, in the case of spruce monocultures, whole stands. However, the forests are not disappearing, but rather trees of other species can use the resulting gaps to grow, so that forest can once again develop. In the case of larger dead areas, plantations of new trees are used to reforest the area. Coping with the current damage, however, requires a high logistical and economic effort, which poses great challenges for forest management.

The current damage is also not mainly caused by air pollutants and acid rain, as in the 1980s but by a changing climate and the increasingly extreme weather conditions. The long-term solution therefore is a drastic reduction of greenhouse gas emissions in order to limit warming.

The current damage has been caused by a combination of different factors, beginning with the storms in autumn 2017 and spring 2018, leading to a high deadwood occurrence. The very dry and hot summers of 2018, 2019 and 2020, accompanied by intense insect infestations, worsened the situation in some regions. 277,000 hectare of damage, accumulated by the end of 2020, represent about 2.4% of the total forest area in Germany.

What caused the damage?

Causes of damage in the forest (© Thünen-Institut)

Strong storms such as Vivian and Wiebke (1991), Lothar (1999/2000) and Kyrill (2007) were the main damage drivers in Germany's forests until 2018. Since 2019, insects in particular have been causing a record proportion of damaged wood of up to 75% of total logging in Germany (see graph).

The causes differ for each tree species. In the case of spruce, the tree species with the highest share of damage, bark beetles (Ips typographus, Pityogenes chalcographus) are mainly responsible for the damage. Due to the already considerable damage in 2018, these could spread further in the weakened spruce stands. Vital spruce can defend itself against initial infestation by producing resin that surrounds and kills beetles at the bore hole but not against mass infestation. Drought, however, causes a limited resin production.

Beech trees have been damaged by the direct impact of heat (leaf damage, sunburn) and drought (crown wilt). Pathogens like fungi and beech bark beetle can then finally kill the trees. Also in pines and oaks, drought and a variety of different pests act together.

Has the cool and moist weather averted the danger of damage?

The cool and moist weather in the late winter and spring/summer of 2021 has replenished the soil water reserves of the upper forest soil layers. This has a positive effect on the water balance of the trees, especially during leaf and needle shoots with high water demand. Young and newly planted trees in particular benefit from the current high topsoil moisture.

However, the deeper soil layers are still too dry in many regions after three hot summers and excessively dry winter periods in recent years. It will take a longer time and more precipitation before they return to the moisture levels of before 2018. Therefore, forests remain vulnerable to new droughts, which can develop quickly.

The cool weather has also inhibited the development of insect pests. The swarming season of bark beetles is delayed by several weeks this spring. However, the bark beetle population is currently so high that further damage can be expected this year. The delayed development offers the opportunity to better control the spread and mass reproduction through consistent protective measures, such as the rapid removal of freshly infested trees. The fact that spruces release more resin when more water is available and can defend themselves better against bark beetles has a relieving effect. However, further cool and humid growing seasons and effective protective measures are needed to reduce the still high infestation pressure.

What are the consequences of forest damages?

The damage for forestry is immense, especially in the case of spruce. The trees attacked by the bark beetle must be felled as quickly as possible and removed from the forests. Due to the current oversupply, the damaged timber can only be sold at a discount but at the same time reforestation must be financed.

The spruce damage will likely have negative consequences for the climate protection function of the forests. As a fast-growing native tree species, spruce provides 40% of the timber growth and 50% of the harvested timber in Germany. Spruce growth thus makes a very important contribution to carbon sequestration in forest wood and wood products. The loss of high-growth spruce stands can reduce the climate protection effect.

In the medium and long term, a change in the composition of tree species can be expected.

How can we adapt forests to climate change?

Risk sites in climate change for spruce (left, altitude above 600 m) and beech (right, dry sites with low water storage capacity) (© Bolte et al. 2021)

Climate change with its more frequent heat and drought extremes is expected to lead to changes in tree species composition. A risk analysis by the Thünen Institute indicates that on about 2.85 million hectares of forest area (25 % forest area share), the current forest stands are particularly sensitive to drought and subsequent pest infestation due to their tree species composition. These need to be actively converted and managed in such a way that they remain climate resilient in the future, i.e. are not acutely endangered by the advancing climate change. Stands that have already been converted must be managed in such a way that they remain climate resilient. A financial requirement of 13 to 43 billion Euros is estimated for the conversion needs over the next 30 years. The annual conversion area would have to quadruple to 95,000 hectares to achieve the conversion targets by 2050.

Mixed forests with different tree species can distribute and reduce the risks of damage, so that a better adapted tree species can take the place and function of a damaged tree species. Mixing in trees of our native tree species such as beech or oak, which originate from the dry-warm distribution edge in southern and south-eastern Europe, can also increase the adaptation of the forests.

Especially on drier and warmer locations it is recommended to extend the spectrum of tree species with both native tree species from drier or continental climates such as chestnut, winter linden, hornbeam, wild fruit species, downy oak, service tree as well as planting imported tree species from southern and south-eastern Europe such as Hungarian oak, Turkey oak, hop hornbeam, oriental beech or even with imported species from overseas with distribution in summer dry climates such as Douglas fir, coastal fir or red oak. It is an important task of science and practice to develop a tree species selection, tree species mixture and stand treatments optimised for the future climate.

How is the forest fire situation in Germany?

Drought and heat favour the development of forest fires. Every year, the Federal Office for Agriculture and Food (BLE) compiles data on forest fire frequency and burned area in Germany. According to this data, an average of about 800 forest fires have been recorded annually over the past ten years. Most forest fires occur in May and July. Compared to the other German states, Brandenburg is particularly susceptible to forest fires due to its rapidly drying, sandy soils and the highly combustible pine forests.

In some years, the damage caused by forest fires can easily run into the millions, e.g. 2000, 2003, 2008, 2010 and 2018. Again, the year 2019 was an above-average year for forest fires, with a major fire near Lübtheen (Mecklenburg-Western Pomerania) alone and several very large fires in Brandenburg.

The German Meteorological Service (DWD) publishes the so-called Forest Fire Hazard Index (Waldbrandgefahrenindex) on its website on a daily basis. It shows on a map the risk height for forest fires from a meteorological point of view in the individual regions of Germany. As a further development of this index, the Thünen Institute of Forest Ecosystems is compiling a data collection of past forest fires with information on location, time of origin, fire cause, spread dynamics and other information (work package 1.1, History of forest fires) as part of the ErWiN joint project, in order to analyse climatic conditions of origin for fires.

Several years ago, the Thünen Institute of Forest Ecosystems and the Humboldt University of Berlin have developed a practicable system for the early detection of forest fires. The core of the system is a hydrogen sensor. When burning organic material, the first thing that is produced is hydrogen. If the hydrogen concentration in the air exceeds a certain threshold value, the sensor reports the possible fire to a control centre. In this way, a smouldering fire can be detected even before an open flame is present. Valuable time is gained. Further information is provided by the Thünen Project INPRIWA and an interview in the Thünen Magazine "Wissenschaft erleben" (in German).

The video Boundless Forests informs about the problems that climate change causes for forests in Central Europe and shows options for action. It was produced within the framework of the EU joint project SUSTREE ("Sustainable use of forest tree genetic resources in climate change"). The 26-minute video was produced by the Czech University of Life Sciences Prague with the support of the project partners, including the Thünen Institute of Forest Ecosystems.