Adaptation: Field of Action Forestry

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: Financial support for forest transformation | Conservation of forest-genetic resources | Humus reserves in forest soils | Utilisation of raw timber | Timber construction quota | Silvicultural information on the theme of adaptation

Humus is essential for storing water and nutrients and for increasing the forests’ resilience.

Source: SoilPaparazzi / stock.adobe.com

Seed plantations serve the purpose of genetic conservation and of forest transformation.

Source: Andreas Meier-Dinkel

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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.

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.