Forest damage due to drought and heat


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

Effects of drought and heat: Damaged and undamaged trees can often be found in the direct vicinity (here a black pine stand in Franconia). (© Thünen-Institut/Tanja Sanders)

How is the situation of the German forests after the dry and hot periods in summer 2019?

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. Adding the expected damage for the current year 2020, the Federal Ministry of food and agriculture (BMEL) assumes that 178 million cubic metres of damage wood (harvest timber without bark) and an area of 285,000 hectares will have to be reforested.

This data is difficult to imagine due to its unpreceded magnitude. Comparisons may help: The damaged area of 285.000 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 178 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 and 2019 worsened the situation in some regions due to insect infestations fed on the large amount of dead wood and the heightened susceptibility due to the drought. 285,000 ha of damage, accumulated by the end of 2020, represent about 2.5% of the total forest area in Germany.

What caused the damage?

The causes differ for each tree species. In the case of spruce, 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.

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?

Climate change with more frequent heat and drought extremes is likely to change tree species composition. Mixed forests with different tree species can distribute and reduce the damage risk; better adapted tree species can take over the place and function of those endangered. In addition, the admixture of our native tree species such as beech or oak, with those originating from the dry-warm margins of their distribution in Southern (Eastern) Europe can increase the adaptive capacity of 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 (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.

The Thünen Institute of Forest Ecosystems is working together with the Humboldt University of Berlin on the development of 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.