The beech forest – an ailing patient!?

Periods of heat and drought cause latent pathogens living in the bark and wood of beech trees to realise their harmful potential. These pathogens include, among others, the fungus genus Biscogniauxia. The species Biscogniauxia nummularia (also known as beech tarcrust) is one of the most frequently isolated fungi from diseased beech tissue in Baden-Württemberg. The fungus causes bark necrosis and wood rot (Figs. 2a and b), which lead to tree branch breakage and loss of stability.

Therefore, the Forest Protection department of the FVA determines, under controlled conditions, the decay dynamics and rate of wood decomposition of two of these Biscogniauxia species in order to assess the development of damage to living trees (Fig. 2c). It can be assumed that such warmth-loving Biscogniauxia species could become a serious pest for existing stands of the European beech, Tree of the Year 2022, as climate change progresses.

Dying crowns, inadequately leafed branches, sunburn and peeling bark or slime flux – insect infestations can significantly affect the beech as well. Trees weakened by drought easily fall prey to the small beech bark beetle or beech splendour beetle, from which they often can hardly recover. In addition to drought, leaf feeding by caterpillars of various butterfly and moth species such as the gypsy moth and frost moth, for example, or by weevils such as the beech leaf-miner beetle, can also predispose beech trees to an infestation of bark-breeding beetles.

Overall, fungal and beetle infestations worsen the already drought-related damage to beech trees, so that they may not prove resilient.

In the summer of 2022, observations and reports of growth anomalies and symptoms of die-back due to drought also increased in naturally regenerated beech stands (Fig. 4a). Observations have shown that after a long summer drought, locally abundant rainfall from mid-August onwards has again led to favourable growing conditions and thus to renewed shoot formation. These anticipated bud shoots and shoot formations, which are actually not intended to sprout until the following spring (prolepsis), can be particularly susceptible to frost due to insufficient lignification (Fig. 4b).

The observed mortality can be explained by the persistent and recurrent climatic extremes of recent years and a location that is unfavourable for the beech. The data collected in this context and laboratory samples retained by the FVA will be beneficial for further research.

A deterioration in the branching structure of the crowns and thus in the long term also poorer foliage seem to affect the lower altitudes first. Beech sites at higher elevations react with a delay. Evaluations of the winter and summer ratings in the experimental areas in Baden-Württemberg show that damage in 2021 and 2022 is at a similar level and is predominantly class 2 (moderately damaged). Another level higher would lead to classification in damage level 3 with severe crown defoliation, leading to dieback of the beech trees in the long term. Only follow-up images can show whether this scenario will occur.

The salvaging of beech wood because of damage increased rapidly in recent years. It has remained at a very high level since and was never previously measured. However, the figures need to be interpreted cautiously: Not all the beech trees removed have died completely at the time of harvesting. This is often done before due to traffic safety and occupational safety. Since the dying process is more protracted compared to, for example, conifers infested by bark beetles, the high levels of damaged wood salvaging in 2020 and 2021 are mostly due to damage from previous years. Thus the actual mortality of beeches is difficult to estimate based on timber use data. Therefore, it was examined again in more detail under the state’s Forest Emergency Plan Project for Beech Resilience at the FVA. In order to describe the condition of unmanaged beech forests, four protective forests in Baden-Württemberg were examined with a drone in July 2021. The resulting orthophotos were evaluated for crown defoliation and mortality (Fig. 6).

Both collectives were examined to support statements on differences between managed and unmanaged beech forests. The result: In comparison, the differences in mortality are minor.

The recognised signature years for the beeches were identifiable on all the discs (Fig. 7). Losses in growth after unusually hot and dry years were noticeable to different degrees at all locations. In addition to drought and location, age and, to a certain extent, tree height and social position also played a role. A connection between fructification and reduced growth has been found as well.