Impacts of Climate Change
Table of Contents
Drought and heat stress
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 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.
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 natural forest reserves - case study | Endangered spruce stands | Incremental growth in timber | Forest condition
Stress due to harmful organisms
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.
Indicators from monitoring on DAS: Extend of timber infested by spruce bark beetle – case study
Increased risk of forest fires
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.
Indicators from monitoring on DAS: Forest fire risk and forest fires
Other climate impacts
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
Adaptation to Climate Change
Measures for adaptation to heat and drought stress
In the course of climate change, the water balance plays a key role in the adaptation of forests to increasing dry periods and heat events. In order to buffer temperature extremes, all silvicultural measures must therefore pay particular attention to maintaining or improving the internal forest climate (including high humidity, low light intensity, low wind speeds) and the soil water supply.
Irrigation as an acute technological measure (e.g. with sprinkler systems) to limit drought stress, as on agricultural land, is not very practical, is not economically worthwhile (high investment costs) and can hardly be justified from an ecological point of view (high water consumption). In certain forests (e.g. floodplain forests), the water balance of soils can be stabilized by rewetting. Recharge of groundwater in the case of lowered water level in forests can also be useful. In response to drought and heat stress, therefore, silvicultural or ecosystem adaptation measures should be undertaken first and foremost. These can be applied to forest restructuring, tree species composition, forest regeneration methods, and the choice of origin and genetic diversity within tree species.
Forest restructuring is primarily concerned with transforming the spruce or pine monocultures that are widespread in Germany into species-rich, multi-layered and near-natural mixed forests with a broader structural and genetic diversity. The aim is to increase the resilience of forests used for forestry and thus their adaptability to drought and heat stress, while at the same time safeguarding the utilization, protection and recreational functions as well as the biological diversity of the forest in the long term. Such forest conversion is financially supported by the EU, federal and state governments. Thus, an average of 22000 hectares of forest were converted annually until 2017.
Natural regeneration, in which individual trees are removed from the stand to create clearings for seedlings of surrounding trees, represents the most favorable and natural form of forest renewal. Due to its high genetic diversity, it provides better conditions for the establishment of adapted tree individuals than artificial regeneration methods. However, this often does not result in a change of tree species, so that the stand remains sensitive to drought and heat.
In contrast, afforestation, a regrowth stock established by man through seeding or planting, allows the use and cultivation of drought- and heat-tolerant tree species. The adaptability of the different tree species varies. The spruce, which is widespread in Germany and which generally prefers cool and moist locations, isn’t very drought- and heat-tolerant. Since it is often cultivated outside its natural range, its adaptability will continue to decline in the future due to climate change. For dry to very dry soils, black pine, scots pine, sessile oak, Norway maple, field maple and small-leaved lime are considered very suitable. Larch, English oak, sycamore maple, large-leaved lime, and walnut are well suited. Primarily with a targeted cultivation of heat-tolerant species through "artificial" regeneration, the forest becomes more resilient.
Measures to increase structural diversity also help to increase the natural adaptive capacity of forests to heat and drought stress. This refers to both the mix of tree species and age classes. Deeper-rooted species, such as oak, can transport more water than they claim from deeper soil layers to upper soil layers through their root system. Trees, with a fibrous root system, such as beech, benefit here from the "neighborhood" with oak.
In the course of climate change, it is not only important to choose the right tree species, but also the right origin. An "origin" is defined as a population occurring in a limited part of the species' range. It is characterized by a certain endowment of genes that enables it to survive under certain environmental conditions (adaptation). However, it also has the ability to adapt to new conditions (adaptability) if its genetic diversity is sufficiently high. Large and genetically variable tree populations will certainly have the greatest chance of survival. In general, therefore, genetically more variable tree species, such as fir or Douglas fir, are characterized by a lower sensitivity to environmental changes than tend to be genetically less variable tree species, such as spruce.
In order to adapt forests to increasing periods of drought and heat, forestry and silviculture are resorting to introduced drought- and heat-tolerant tree species (e.g. red oak, Douglas fir, Japanese larch) in addition to previously rare native species. Their use is viewed critically from a conservation perspective, as these tree species generally provide habitat for less native species and some of them are classified as invasive (e.g. black cherry (Prunus serotina), red ash (Fraxinus pennsylvanica). While there is a potential invasiveness risk associated with planting Douglas-fir (Pseudotsuga menziesii), this is considered to be low and controllable through forest management. In silvicultural adaptation strategies, alien tree species should only be used in exceptional cases and very restrictively after a comprehensive ecological risk assessment has been carried out in advance. In protected areas (e.g. nature reserves and FFH areas), the introduction of alien tree species should generally be avoided. In addition, they should be observed through explicit monitoring in various inventories (e.g. federal forest inventory, biotope mapping). For tree species already introduced in the past and classified as invasive, management plans should be developed that are suitable for repressing these species or controlling and reducing further spread and negative impacts on ecosystems.
Indicators from monitoring on DAS: Mixed stands | Financial support for forest conversion | Conversion of endangered spruce stands | Conservation of forest-genetic resources | Humus reserves in forest soils | Forestry information on climate adaptation
Measures in dealing with harmful organisms
In view of climate change, disturbances by harmful organisms in German forests and woodlands may be expected to a greater extent than in the past. These disturbances can be accepted as long as they do not affect the forests to such an extent that relevant utility, protective and recreational functions are severely impaired. Otherwise, appropriate defense measures must be initiated. In this context, the creation of climate-robust mixed forests and the continuous monitoring of insect and fungus populations as well as the effects of damage on trees are regarded as preventive strategies.
While monocultures are considered very sensitive to infestation by harmful organisms, e.g. pine monocultures by the nun moth (Lymantria monacha), near-natural, site-appropriate mixed forests rich in tree species and structures, on the other hand, are considered more resistant to insect and fungal infestation. Due to the spatial distance of the host trees, harmful organisms do not have sufficient food available in the immediate vicinity. Damage to the host trees can thus be reduced and even a total loss of the entire stand can be avoided.
As part of forest protection management, monitoring, which includes all forms of systematic recording of processes related to harmful organisms in the forest, forms an essential basis for effective harmful organism management. Monitoring can be used to determine the intensity with which harmful organisms are to be expected (e.g. size of populations and their development potential), the potential threats to the forests, woodlands and trees concerned (e.g. loss of vitality, mortality) and, if necessary, the defensive measures to be taken. In forestry and silviculture, various monitoring methods are used that are adapted to the harmful organisms. For example, bark beetles are caught in pheromone traps in spruce stocks during the growing season. Based on the number of individuals found, statements are then made about the threat to forests from mass reproduction.
Direct control measures of harmful organisms are also conceivable under certain conditions, whereby the very different life cycles and preferences for certain habitats of the individual harmful organisms make spatially, temporally and technically much differentiated strategies necessary. In this context, the strategy of eliminating entire populations is only pursued for invasive species (e.g. Asian long-horned beetle). For native harmful organisms, strategies range from local reduction to protection of a group of trees (e.g. oak processionary) or a stock (e.g. gypsy moth). In critical situations, the use of plant protection products (insecticides, fungicides) can often be the last resort to prevent stock losses. Such use should generally follow the principles of integrated pest management and balance its negative effects on the natural balance (e.g. decimation of other species, groundwater pollution) with the benefits for the conservation of the respective tree populations.
Measures to reduce risks of forest fires
The risk of wildfire is determined in significant part by tree species composition, flammability and quantity of combustible biomass, and the potential for fire spread.
The risk of forest fires is particularly high in the case of resinous conifers (e.g. pine, spruce). Therefore, forest conversion from conifer monocultures to mixed forests with a high proportion of deciduous trees is an essential approach to preventive protection against forest fires. Silvicultural measures can also aim to grow tree species that are fire resistant, produce little highly flammable litter, and/or reduce the development of a highly flammable lower tree layer, for example, through shading. To date, absolute fire resistance of trees has not been demonstrated. Nevertheless, there are tree species that are adapted to fire exposure (e.g., cork oak, lodgepole pine, and cypress). In addition to forest conversion, forest firebreaks, forest fire wound strips and forest fire bars as classic precautionary measures help to reduce the risk of forest fires. This can prevent or reduce the horizontal spread of fire. Vertical rise of ground fires into the tree canopy can be achieved by branching or removing the shrub layer. A reduction of combustible biomass can also be achieved by controlled burning, but this is hardly practiced in Germany to date.
The more technical measures for forest fire prevention include early detection, i.e., surveys of current and future forests and woodlands at risk from forest fire, and forest fire monitoring. The latter is now done in many areas through surveillance flights and the use of camera-based forest fire monitoring systems, which give firefighters immediate access to digital images and maps of the burn area. Satellite-based systems can also assist in this regard. The German Meteorological Service (DWD) publishes the daily forest fire danger index (WBI) on its website. This shows on a map how high the forest fire risk is in individual regions of Germany from a meteorological point of view. Together with the Humboldt University in Berlin, the Thünen Institute for Forest Ecosystems has developed the INPRIWA forest fire early warning system, which uses a hydrogen sensor to detect a fire before it starts to spread.
The main extinguishing agent for forest fires is still water. For this reason, it is necessary to create or expand and maintain extinguishing water reserves in large forest areas at risk of fire at suitable water bodies or by creating artificial water extraction points. In the case of a new installation of such extraction points, this includes the preparation of water management concepts. They must be coordinated with forest owners, municipalities, landscape associations, forestry administration, fire departments and road construction at all planning levels in order to integrate the interests of all parties involved and avoid conflicts of use. One project that has developed and tested such prevention and coping measures is the KLIMWALD project documented in the UBA | Kompass database.
Since more than 50 percent of forest fires are caused by human negligence, educating the public to increase risk awareness is an important part of forest fire prevention. This involves targeting various target groups, such as kindergartens, schools, forestry operations, recreationists and tourists, for example through publications, leaflets, reports, social media, warning signs, training courses, seminars, educational programs, forest youth games, forest tours or excursions.
Finally, taking out forest fire insurance (e.g., from the German Forest Insurance Agency) is one way to mitigate the financial damage of a forest fire.