FW-R-1: Mixed stands

The picture shows the colourful canopy of a forest composed of different deciduous and coniferous tree species.Click to enlarge
Climate forecasts are uncertain;by mixing tree species, foresters/forest owners are on the safe side
Source: Photograph: © RuZi / stock.adobe.com

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

FW-R-1: Mixed stands

In view of uncertain forecasts regarding future climate developments, the promotion of species and structural diversity in forests is a suitable strategy for spreading the risk. In 2002 mixed forests consisting of two or more tree species covered almost 54.9 %, compared to already 57.8 % of the forest area by 2012. Leaving the proportion of near-natural pure stands aside, the proportion of mixed stands could be increased in approximately a quarter of the entire forest area.

Two stacking columns are shown for the percentage area of mixed forms in the total forest area for the years 2002 and 2012. It is differentiated into the categories mixed forest with 2, 3, 4 and more tree species as well as into near-natural and non-near-natural pure stand. Furthermore, there is the category of forest without stocking information. There are minor changes between the years. Both types of pure stands have decreased. There was an increase  in the category of mixed forest with 3 tree species.
FW-R-1: Mixed stands

Two stacking columns are shown for the percentage area of mixed forms in the total forest area for the years 2002 and 2012. It is differentiated into the categories mixed forest with 2, 3, 4 and more tree species as well as into near-natural and non-near-natural pure stand. Furthermore, there is the category of forest without stocking information. There are minor changes between the years. Both types of pure stands have decreased. There was an increase above all in the category of mixed forest with 3 tree species.

Source: Thünen-Institut für Waldökosysteme (analysis based on the National Forest Inventory)
 

Mixed forests – diversity spreads the risk

Forest experts are united in the expectation that from the second half of the 21st century onwards, climate change will exert particularly strong influence on forests. However, the potential climate change scenarios differ a lot from each other. This means that forestry will have to deal with substantial uncertainties in planning. Nonetheless, it is possible to incorporate various probabilities of future climate change in the planning process. The tree species mix is an important starting point to make stands more stable and increase their vitality thus safeguarding the important functions of forests for the future. However, to control the tree species mix requires very far-sighted strategies in terms of forest restructuring.

Foresters have adopted the targeted promotion of mixed forests as a risk-spreading strategy according to the following motto: The more diverse the tree species that make up a forest community and the more diverse the forest structures, the lower the risk that in case of a gradual or sudden change in site conditions, the entire forest would be affected by damage or even collapse. There will always be some species which can handle the new conditions thus providing the foundation for a re-development of the stand concerned. Besides, tree species diversity is often linked with a high degree of structural and habitat diversity. This often includes the provision of diverse habitats for organisms which counteract pests thus subsequently reducing the distribution of pests and opportunities for infestation compared to homogeneous pure stands. This is why mixed stands are often less vulnerable to attacks by pests. This applies in particular to new harmful organisms some of which are specialised to prey on a either single or just a few tree species. From the point of view of production, diversification in the composition of tree stands can also be a suitable response to the uncertainty of future timber prices.

For reasons described above, uniform and species-poor stands are the particular focus of foresters in respect of forest restructuring efforts. This issue is considered as in greatest need of action. In principle, the objective is to convert these stands to diverse structures with predominantly native tree species thus spreading the risk as desired. At the same time, however, the specific site requirements, yield aspects and other forest functions have to be addressed. On some sites pure structures consisting of single-species trees such as beech will develop, while on others mixed stands of varied composition will develop. In principle it is true that there are natural limits to the proportion of mixed stands; however, German forests have a long way to go until the optimal mix is achieved.

The proportion of mixed stands is increasing in Germany’s forests, and as far as adaptation requirements are concerned, developments are heading in the right direction. The 2002 National Forest Inventory showed that just about 55 % of forest area contained mixed stands compared to 45 % pure stands. There are no stand statistics available for the remaining almost 5 %. The outcomes of the subsequent inventory in 2012 show that the proportion of mixed stand areas increased to almost 58 %. The proportion of areas with near-natural pure stands remained stable at 11 % until 2012. Pure stands which were not near-natural consisting mainly of spruce and pine trees and which are the focus of forest restructuring, declined by 3 % during the period in question.

For an evaluation of outcomes it is irrelevant whether the increase in mixed stands is ultimately a result of targeted forest conversion measures or whether these changes occurred spontaneously, i.e. as part of natural succession processes. The outcomes of the next National Forest Inventory will become available in 2022.

 

Interfaces

FW-R-2: Financial support for forest conversion

FW-R-4: Conservation of forest-genetic resources

FW-R-3: Conversion of endangered spruce stands

 

Objectives

Forest owners should drive forward the conversion of forests containing pure stands to mixed forests of site-appropriate trees which are low-risk (DAS, ch. 3.2.7)

Establishing diverse, stable and profitable mixed stands (Waldstrategie 2020, p. 23)

Maintaining the overall forest area in Germany and increasing the stability, diversity and near-natural character of forests. The cultivation of site-appropriate and predominantly native tree species makes an important contribution towards achieving this objective. (Waldstrategie 2020, p. 23)

Adaptation of forests to the challenges of climate change, e.g. by cultivating mixed forests of the greatest possible diversity (NBS, ch. B 1.2.1)

Continuous reduction of the proportion of non-native tree species (NBS, ch. B 1.2.1)

Conservation and evolution of natural and near-natural woodland communities (NBS, ch. B 1.2.1)