Impacts of Climate Change
As a result of climate change, precipitation in Germany is increasingly shifting from the summer to the winter months. By 2050, precipitation is expected to be reduced by up to 40 percent in the summer months, while an increase of up to 30 percent is forecast for autumn and winter.
Changes in precipitation levels and the increasingly uneven distribution of precipitation over the seasons pose a risk to forest ecosystems.
The lower summer precipitation leads to drought stress. In addition, the increased temperatures increase the demand for water. If trees are insufficiently provided with water, the pressure with which the water is transported from the roots to the crowns drops. Hanging leaves are a first sign of this. If the drought stress persists, trees shed their leaves, fruits or even branches, their crowns become lighter.
The prolonged drought in the 2018 and 2019 growing seasons widely led to the premature fall of the leaves. According to the latest forest condition survey of the Federal Ministry of Food and Agriculture in 2019, the share of trees with significant crown defoliation rose from 29 percent in 2018 to 36 percent. Only about a fifth of the trees showed no damage.
Especially in deciduous trees the crown defoliation has increased significantly. The crown condition of conifers shows no trend. On average of all tree species, the crown condition has never been as bad as in 2019. Increasingly, the death of trees was observed.
The particularly severe drought since 2018 makes the trees vulnerable to pests and fungi. Spruce, the former "bread-tree" of forestry, is particularly affected by infestation with bark beetles. Because of the drought, the trees produce less resin with which they normally fend off the beetles. Other pests such as gypsy moth and oak processionary, which strip deciduous trees bare, are also spreading rapidly in some regions.
As reported by the Federal Statistical Office (Destatis), 32 million cubic meters of damaged wood due to insect damage were felled in 2019, almost three times as much as in the previous year with 11 million cubic meters. In 2017 it was still six million cubic meters.
If the damage caused by forest fires or storms is also included, by the end of 2020, 178 million cubic meters of damaged wood can be expected and an area of 285,000 hectares will have to be reforested.
Longer dry periods and warmer summers increase the risk of forest fires. The number of days with high forest fire warning levels rose from about 27 days per year in the period 1961 to 1990 to about 38 days in the period 1991 to 2019.
If cleared areas are created by fires or the removal of diseased and dead trees, the dessication may increase, because the affected areas are exposed to increased solar radiation.
This can further reduce the availability of water in the soil. In addition, the removal of dead wood is accompanied by the loss of nutrients and humus, which has a negative effect on water storage in the forest floor.
If there is not enough water available to trees, this reduces their evaporation capacity and growth. This also entails a reduced absorption of carbon dioxide, so that carbon storage is reduced. Drier climate conditions can increase the risk of forests becoming sources of carbon dioxide from sinks.
Annual average temperatures in Germany have already risen faster than the global average, and by 2019 they had already reached 1.6 °C. According to climate model calculations, by 2050 it will probably be 1.5 to 2.5 degrees warmer in summer than in 1990, and 1.5 to 3 degrees Celsius in winter.
Although the gradual heating leads to longer growing seasons, the monitoring report shows that temperature change and variability also have a negative impact on regional growth conditions for tree species. This is especially true in connection with a changed water supply of the forest soils. In addition, higher temperatures and longer growing seasons lead to an increased water demand of the trees, which further increases the risk of drought stress.
Spruce is particularly affected by the changing climatic conditions. With a share of 25 percent, it is the most common tree species in Germany, along with pine (23 percent), beech (16 percent) and oak (11 percent).
Due to its undemanding nature, robustness and easy propagation, spruce has long been regarded as an ideal, high-yield tree species. However, due to its mostly flat root system, the spruce is sensitive to storms and drought. As a consequence of the massive expansion of spruce cultivation, the tree species was also grown in locations that do not meet its requirements for rather cool and humid climate conditions and that will become even warmer and drier in the future according to climate change scenarios.
Increasing summer temperatures and drought severely restrict the spruce's growth, even dying off is possible. As a result, the forestry industry has suffered particularly high yield losses in spruce groves in recent years.
With rising summer temperatures and drought, the problem of spruce increases. The forestry industry can counteract this by converting spruce groves.
But also with oak, pine and beech, the proportion of significant crown defoliation has risen sharply.
As the canopy of the trees becomes thinner, the forest microclimate changes, as the cooling effect of a dense leaf canopy diminishes. This has an impact on the trees, but also on the animal and plant species that live in the undergrowth and also in the soil. Heat-loving species could benefit and drive other species adapted to cooler conditions out.
Extreme weather events
Climate change makes the occurrence of weather extremes such as droughts, heavy rain or storms more likely. Since the 1990s, forestry has been experiencing increasing economic damage from windthrows. They are due to strong storms with high wind speeds.
In comparison to gradual climate changes, extreme weather events often cause great damage within a short period of time, which can have serious consequences for humans and the environment.
In the mountain forests of the Alps, for example, the risk of heavy precipitation, rockfall or flooding is assumed to become significantly higher. The importance of the forest as a protection for infrastructures and settlements could therefore increase further in the future.
Adaptation to Climate Change
Ecosystemic adaptation measures are particularly important in forestry. Strategic planning on the part of politicians and the responsible forestry companies is important for this. This is because even though forest ecosystems have constantly adapted to changing environmental conditions in the past, the extent and rapid progress of climate change can overstretch the adaptive capacity of long-lived tree species.
In addition, the adaptability of the different tree species is pronounced differently. As an example, the spruce, which is widespread in Germany, can be mentioned here. It prefers cool and humid locations and is therefore not very tolerant of drought and heat. Since it is already frequently cultivated outside its natural range due to its rapid growth, its adaptability will continue to decline in the future.
The conversion of pure forest stands - such as the spruce or pine monocultures that are widespread in Germany - into mixed forests that are rich in structure and species, multi-layered and thus close to nature, is a necessary measure to secure the utilization, protection and recreational functions and the biological diversity of the forest in the long term. A broader structural and genetic diversity increases the resilience of forests used for forestry and thus their adaptability. Resilience means that an ecosystem is able to withstand ecological shocks and disturbances such as insect infestation or storms and maintain its basic organization and functionality.
The EU and federal and state governments support forest conversion with financial aid. By 2017, an average of 22,000 hectares of forest had been converted each year.hot p
Natural rejuvenation, in which individual trees are removed from the stand to create clearings for the seedlings of surrounding trees, is the most favorable and natural form of forest regeneration. Often, however, this does not result in a change of tree species, so that the stock is still susceptible to drought and heat. Primarily with the targeted cultivation of heat-tolerant species through artificial regeneration, the forest becomes more resilient.
Forest restructuring measures also help to protect against fires. The danger of forest fires has already increased due to longer and more frequent dry and hot spells. Mixed forests are less prone to forest fires, as they have a more humid interior forest climate. Species-rich mixed forests are also more resistant to insect pests and fungal attack.
The primary goal is to establish multi-level mixed forests, which are based on the tree species composition, dynamics and structure of natural forest communities. Non-native tree species should only be used in exceptional cases and very restrictively after a comprehensive ecological risk assessment has been conducted in advance. In protected areas (nature reserve and special areas of conservation), the introduction of non-native tree species should generally be avoided.
Significant parts of the currently created open spaces should not be cleared or should be cleared only to a limited extent, if this is justifiable due to the forest protection situation. Under protection of the dead, lying or still standing old trees, a diverse and often better adapted subsequent generation can establish itself in the course of natural succession. Due to their high genetic diversity, it can generally be assumed that natural rejuvenation provides better conditions for the establishment of adapted tree individuals than artificial rejuvenation methods.
The water balance plays a key role in the adaptation of forests in response to increasing dry and hot weather events. In all silvicultural interventions, special attention must therefore be paid to preserving or improving the interior forest climate and the soil water supply in order to buffer temperature extremes and reduce the competition for water.
Forest restructuring enables a diversification of the wood supply, which can also bring economic advantages. In addition, mountain forests in particular have the important function of protecting infrastructures and settlements from rockfall and flooding - a function that forest restructuring can also strengthen.
Especially in extremely dry regions, where water is only available to a limited extent during the summer months, forest ecosystems can be supported by technical measures that counteract the increasing water shortage and a drop in the ground-water level. For example, the water balance of soils in wetland forests can be stabilized by rewetting.
Dispensing with large harvester machines helps to reduce soil compaction.
Technical measures can also help against forest fires. Forest fire monitoring can be supported by additional video measures or satellite-based systems, for example.
The German Weather Service daily publishes the so-called Forest Fire Hazard Index on its website. Here, a map shows how high the risk of forest fire is from a meteorological point of view in the individual regions of Germany.
A few years ago, the Thünen Institute of Forest Ecosystems, together with the Humboldt University of Berlin, developed a system for early detection of forest fires.
Legal, political and management measures
As described above, adaptation of forestry to climate change requires consistent risk management to deal with damaging events such as forest fires, storm damage or pest infestation. This is the only way to identify risks in good time, ward them off and successfully manage them.
In addition to short-term crisis management, especially preventive measures such as coordinated water management concepts to reduce the risk of fire are important. They should be coordinated at all planning levels between forest owners, municipalities, landscape associations, forestry administration, fire department and road construction in order to integrate the interests of all parties involved and avoid conflicts of use.
Comprehensive knowledge is important as a basis for the described risk management plans and forest restructuring. Against this background, precise information is needed about which tree species are suitable for the respective location, taking into account future effects of climate change. In addition, planning maps or site mapping are required to provide information on the thriving of the tree species under the relevant environmental and climatic factors. The management of experimental plots, regional recommendations for cultivation by the federal and state governments, regular monitoring and research into the use of wood can also improve the information basis.
Natural forest reserves play a decisive role in this. In the existing non-utilization forests, the climate adaptation potential of tree stands without care and utilization is investigated. On this basis, advice for forestry is given. In mid-2018 a total of 742 natural forest reserves with an area of 35,500 hectares existed in Germany. This is only 0.3 percent of the forest area.
Forest and soil monitoring programs at European, national or country level already provide important information at present, but potential exists for expanding the knowledge base and better networking of actors.
With the Forest Climate Fund, the German government has also created an instrument that has provided over 65 million Euros for adaptation measures since 2013.
With the amendment of the Federal Hunting Act, game populations can be hunted more intensively, since deer eat the sprouts of young deciduous trees and thus prevent the conversion of forests into more natural mixed forests.