FW-I-1 Tree species composition in natural forest reserves - case study

The picture shows a sparse near-natural forest with deciduous trees. A tree has broken off at a great height. A large broken trunk lies on the ground.  Click to enlarge
Which tree species are more competitive at the site can be observed in natural forest reserves.
Source: Photograph: © Peter Meyer / NWFVA

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

Table of Contents

 

FW-I-1: Tree species composition in designated forest nature reserves – case study

In natural forest reserves where more distinct climate-related changes with higher temperatures and drier conditions as well as more frequent and more pronounced weather extremes are expected, the share of beech trees has declined

The bar chart shows the annual change in tree species composition in natural forest reserves with expected strong climate change in percent and differentiated for the two periods 1971 to 2014 and 2014 to 2018.
FW-I-1 Tree species composition in natural forest reserves - case study

The bar chart shows the annual change in tree species composition in natural forest reserves with expected strong climate change in percent and differentiated for the two periods 1971 to 2014 and 2014 to 2018. The overall stand density increased by about one per cent in the former period and decreased slightly in the latter period. The area stocked with the tree species has changed as follows. Beech, which increased by almost 0.4 percent between 1971 and 2014, decreased by almost one percent between 2014 and 2018. For oak, the decrease of more than 0.5 percent turned into an increase of around 0.3 percent, for ash and maple from minus 0.2 to plus 2.7 percent, and for pioneers from minus 0.8 to plus 1.8 percent.

Source: Nordwestdeutsche Forstliche Versuchsanstalt / Projektgruppe Naturwälder (federal state data from research and monitoring in natural forest reserves)
 

Adaptability of natural tree species

Woodland and forest ecosystems have great longevity. Accordingly, forestry management has to deal with extended rotation cycles. Planning must be for the long term and has to take future changes in growth conditions into account. Tree species which hitherto coped well with the climatic conditions prevailing at their location may, in coming decades, become more vulnerable to damage and incremental loss. In commercial forests the species composition is informed by forestry principles of utilisation and cultivation. This is an area marked by interactions of natural succession and anthropomorphic intervention.

In natural forest reserves woodlands evolve free from direct human intervention. In late 2017 Germany had 742 natural forest reserves covering a total of 35,500 hectares. In most of these reserves, the autochthonous tree species predominate. Whether the limits of their adaptability are exceeded as a result of climate-related changes is an issue that is being examined by means of ongoing observation of natural woodland dynamics. The outcomes of these observations will make it easier for woodland and forest managers to make decisions on the extent to which warmth- and drought-tolerant tree species in stands used for forestry purposes should be incorporated in order to safeguard the continued future use of timber.

Regional projections of future climate change make it possible to differentiate between natural forest reserves on one hand where future changes in water supply are expected to be rather negligible and on the other, forests where the water regime will presumably be affected quite badly thus threatening stands to be exposed to drought stress. By categorising the prevailing tree species in groups characterised by specific adaptability properties and by monitoring their long-term development, it is possible to infer statements on the development of adaptation processes in forest ecosystems. Adaptability is regarded as comparatively high in respect of sessile and pedunculate oak, ash, sycamore, Norway maple and so-called pioneer species such as silver birch and downy birch, sallow, aspen and rowan. However, it is to be expected that European beech will be rather vulnerable to drought stress. Besides, it should be remembered that also other factors such as pest infestation, wind throw or competition for light, water and nutrients will influence the development of individual categories of tree species.

In the natural forest reserves of Länder such as Baden-Württemberg, Bavaria, Brandenburg, Hesse, Lower Saxony, North-Rhine Westphalia, Rhineland- Palatinate, Saxony-Anhalt and Schleswig-Holstein there have been indications of shifts in tree species compositions up to 2014, which cannot be attributed to climate-related changes. In most cases, stands were characterised by increases in stand density which was taken as an indication that the vitality of trees has not been visibly affected. While beech trees were able to increase their share, the proportion of oak species and the ash / maple group had declined. This development can be explained on one hand by the competitiveness of beech trees, and on the other, by disease-related die-back affecting oak and ash trees. The group of pioneer tree species showed only negligible change. Overall, events did not indicate that developments were strongly influenced by climate change.

As far as the second observation period from 2014 to 2017 is concerned, the direction of development was reversed in those natural forest reserves where more distinct climate-related changes with higher temperatures and drier conditions as well as more frequent and more pronounced weather extremes are expected. The stand density increased only negligibly while the share of beech trees declined, compared to increases in the proportion of all other groups of tree species. It is true that the rather short observation period requires caution in respect of interpreting outcomes; however, the outcomes do confirm the expected developments under climate change. This interpretation is further supported by the fact that in natural forest reserves where rather negligible changes in water supply are expected, the development which occurred during the first observation period is still ongoing. In this category the stand density and proportion of beech trees continue to increase. In other words, for the first time it is possible to discern differences in the development of the two groups.

 

Objectives

Examination of the climate adaptation potential of stands free from tending or utilisation operations in existing woodlands where no utilisation takes place. On this basis, pointers are given that are intended to assist forestry decisions. (Waldstrategie 2020, p. 11)

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