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

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

In natural forest reserves where stronger climate-related changes with higher temperatures and drier conditions as well as more frequent and more pronounced weather extremes are expected, the stand density has declined, especially in the most recent monitoring period from 2019 to 2023; this is due to increased tree mortality. The proportion of beech trees has continued to rise. The development of pioneer species, in particular, has been subject to major fluctuations.

Adaptability of natural tree species

Woodland and forest ecosystems have great longevity. Accordingly, forestry management has to deal with extended production 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 incremental losses. In commercial forests the species composition is informed by silvicultural principles of utilisation and cultivation. These are forests marked by interactions of natural succession and the effects of forest management.

In natural forest reserves woodlands evolve free from direct human intervention. In 2023 Germany had 746 natural forest reserves covering a total of 36,000 ha. In most of these reserves, the autochthonous (locally indigenous) 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 in natural forest reserves. The findings resulting from 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 future, in order to safeguard the existence of our forests with their varied ecosystems alongside the continued 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 the exposure of stands 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 to drought is regarded as comparatively high in respect of sessile and pedunculate oak, ash, sycamore, Norway maple and so-called pioneer species such as sallow, aspen and rowan. However, it is assumed that the 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 owned by 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. The majority of stands are characterised by increasing density, which points to the continued predominance of vigorous trees. While beech trees were able to increase their share, the proportions 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 dieback 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 and third observation periods from 2014 to 2018 and from 2019 to 2023 respectively are concerned, the direction of development has changed 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. In this context, the density of stands has decreased, especially during the period from 2019 to 2023. Obviously, this development can be attributed to greater tree mortality owing to drought years occurring more frequently since 2018. Whilst major fluctuations were observed regarding the proportions of pioneer tree species and the group of maple / ash trees, the proportion of European beech (Fagus sylvatica) has continued to increase – albeit to a much lesser extent than prior to 2014. As far as oak species are concerned, a minor decrease, and from 2019 to 2023, a minor increase were observed.

Likewise, in natural forest reserves which are expected to benefit from more favourable conditions in terms of water supply, the period from 2019 to 2023 shows a decrease in the density of stands and a continued increase in the proportion of beech trees. In these areas, the stand density had still increased distinctly in the period of 2014 to 2018.