Since 2018, there have been increasing reports of beech trees that are ailing or dying altogether. The focus of damage has emerged as being in Lower Franconia, Bavaria’s climatic “hot spot”. Meanwhile, isolated oak stands there are also affected. Symptoms of a lack of vitality include premature wilting and dropping of leaves, crown dieback, and even the death of the entire tree. In most cases, the affected trees have already shown signs of a reduction in growth in previous years. Damage occurs mainly in older forests where thinning measures have already been carried out, but younger stands have not been spared either. As beech and oak are important tree species for forest conversion, it is important to analyse and accurately assess the damage to them.
Fig. 1: Drought damage to beech trees at the Ebrach forest climate station. Photo: Hans-Peter Dietrich (LWF)
Forest environmental monitoring
Changes in the environment and the reactions of the trees are recorded in the course of forest environmental monitoring. With regard to the trees’ reactions, there are essentially two pillars on which the monitoring is based: the annual forest condition survey (“Level 1”) and intensive monitoring at the forest climate stations (“Level 2”). The forest condition survey, which has been carried out since 1983, records the level of vitality of trees at over 450 inventory points in Bavaria. These surveys are complemented by intensive measurements made at the forest climate stations, including meteorological measurements, as well as measurements on the water and nutrient balance and on tree growth. Thanks to the continuous time series, the data obtained is a valuable reference. In addition, the spatial distribution of the data throughout Bavaria makes them ideal for comparisons between different sites and tree species. Based on the tree species composition in the dominant layer, the forest climate stations of Bad Brückenau, Ebrach and Mitterfels were defined as beech stations, and the forest climate stations of Riedenburg, Rothenbuch and Würzburg as oak stations. Freising functions as both a beech and an oak station.
Figure 2: The studied forest climate stations.
When is dry “too dry”?
There is no generally accepted definition of the term “dry year”. In meteorology alone, there are various methods for assessing how dry a year was compared to others. One possible approximation is the climatic water balance (CWB): This is calculated from the difference between precipitation and potential evaporation. If, due to high temperatures, more water evaporates than is available from the precipitation that has fallen, the CWB of this site would be negative. For the plants, this means stress due to a shortfall in water. The soil conditions and the specific water requirements of the individual tree species are not initially taken into account in this calculation. This is precisely where the advantage of the CWB lies. It requires few input variables, is comparatively easy to calculate, and allows regional comparisons. More detailed considerations of the site-specific and tree species-specific water balance, on the other hand, require more complex water balance modelling, such as the LWF-Brook90 water balance model. If we first look at the progression of the CWB since 1995 for the selected stations in Figure 3, the years 2003, 2015, 2018 and 2022 stand out due to the significant downward trajectories of the curves. The climatic water balance in these years was therefore negative. The graph also shows that the paths of the southern Bavarian and northern Bavarian forest climate stations often take a similar course. In individual years, but especially in 2018, strong regional weather differences between southern Bavaria (FRE and RIE stations) and northern Bavaria can however also be recognised.
Fig. 3: The difference to the mean climatic water balance clearly shows the dry years (dashed lines), even if not all stations react in the same way.
Dependent on the environment: the growth
The annual increase in the diameter and basal area of a tree depends on many factors. As well as longer-term effects such as social position and age, these include short and medium-term factors, such as weather conditions, fructification and changes in the stand structure. At the forest climate stations, the growth of the dominant trees is recorded annually using circumference measuring tapes. If these data are adjusted for medium-term trends (e.g. thinning measures) and long-term trends (age), the reactions of the trees to the weather can be analysed. The trend-adjusted growth curves of beech and oak are shown in Figure 4. Values below 1 indicate years of below-average growth, while years of above-average growth show values above 1.
Fig. 4: Over the course of time since 2000, the analysed oaks fluctuate significantly less in basal area growth than the beech, but for both tree species, slumps in growth in the dry years themselves (dashed lines) cannot be clearly discerned.
Beech a “problem child”?
We were not able to detect a clear and consistent slump in growth across all sites at the forest climate stations in the dry years 2003, 2015, 2018 and 2022, as caused in the case of beech by the severe late frost of 2011 (Figure 4). On the contrary: in 2018, the beech trees at all climate stations in fact grew better than on average. This was followed by two years of below-average growth in 2019 and 2020 - the trees were presumably unable to recover due to the prolonged drought in these years. However, the wet year of 2021 then helped the trees to achieve above-average growth again. Evidence of a general downward trend in vitality lasting several years, as is often described, is not apparent at our beech stations.
A look into the crowns
Changes in growth behaviour initially remain hidden from forest owners and practitioners for a long time. Leaf loss and drought damage to the trees’ crowns are however visible more quickly and immediately. The data from the crown condition survey (WZE) is an important source of information here. The crown condition survey has for example shown that the loss of needles and leaves has recently increased to 26% across all tree species; along with the year 2021, this is the second highest value since 2020, clearly reflecting the influence of the dry years since 2018. As the field observations of the crown condition survey are carried out between July and early August, the premature wilting and shedding of leaves in the late summer of the dry years is not yet recorded here. At the intensively monitored forest climate stations, such developments are observed more closely, and every four weeks, the quantity of fallen leaves is recorded in litter collectors. On average in the years 1997–2018, around 13 % of the total litter volume fell in the months of June to August. At the forest climate station in Ebrach (EBR) in the dry year of 2003, on the other hand, around 22 % of the total litter volume was found in the litter collectors in this period, and in 2015 it was as much as 43 %. Also striking were the significantly sparser crowns of the beech trees in the year following the drought. This was observed at all four beech stations in 2015 (Figure 5).
Fig. 5: Across all stations, greater leaf loss is observed in beech trees the year following a dry year than in the dry year itself. This is also to be observed throughout Bavaria (horizontal lines in the respective colour).
The oak by comparison
Also in the case of oak, a lack of water is not directly reflected in the growth curve (Figure 4) in the dry year itself either. The oaks at the forest climate stations do however grow more consistently overall than the beech trees. In the dry years after 2018, the oak proved to be more resilient in terms of growth: unlike that of beech, the basal area growth of the oak was above average again by 2020 at three of the four stations. It is striking that in Freising, the only station where both tree species could be examined, the oak obviously had no advantage over the beech in the dry years. Here, the beech still seems to have optimum growing conditions. Across Bavaria, the oak reacted with higher leaf loss in the years 2004 and 2019 than in the preceding dry years themselves. For the beech this was also the case in 2015 (Figure 5).
Drought strategy of beech and oak
No reduction in growth in the dry year, but some losses in growth and generally sparser crowns in the following year - how can this behaviour be explained? A whole series of scientific studies show that beech and oak leave their stomata open even when water availability in the soil decreases (anisohydric behaviour). They thus continue to release water into the atmosphere and carry out photosynthesis. As long as water is still available in the soil, they can still grow, but at the same time there is a risk of embolisms - i.e. a collapse of the conductive tissue. These embolisms can lead to serious and even irreversible damage. A lack of water not only leads to premature leaf shedding in beech and oak – it also has consequences for the following year: if the leaves are shed prematurely while they are still green, the trees have fewer reserves available for the following year. In addition, the trees lose part of their water-conducting tissue as a result of embolisms. In terms of plant physiology, this means that in the year following the drought, they also develop less leaf mass, as is also confirmed by the results of the forest condition survey. Conifers such as spruce and pine, on the other hand, close their stomata very quickly in dry conditions, thus stopping transpiration and largely also growth (isohydric behaviour). This is why significant losses in growth are often seen in the same year in these species. In some studies, the oak is also categorised as an isohydric tree species, as it appears to be able to actively close its stomata towards the end of a dry period. Before this happens however, the oak may also suffer drought damage to its leaves, and entire shoots may die. This is why oak is said to be predominantly anisohydric.
Summary
The drought tolerance of beech and oak is a much-discussed topic. The data from forest environmental monitoring provides valuable insights into how trees react to weather conditions and other environmental influences. Individual dry years left no long-term traces in the growth of beech and oak trees at most forest climate stations. The years from 2018 to 2020, however, which were all dry in a long-term comparison, led to a slight depression in beech growth, followed by recovery in 2021. The results of the forest condition survey showed an increased loss of foliage in beech throughout Bavaria in the year following individual dry years. There is also evidence of this in oak, but it is less pronounced. The extent to which the tendency towards anisohydric behaviour leads to irreversible damage to the trees or whether a fundamental recovery is possible is currently a subject of debate among experts. Fortunately, the majority of the forest climate stations with beech and oak still display sufficient recovery mechanisms.
Fig. 6: Various techniques for recording growth at the forest climate station in Freising. Photo: Tobias Hase (StMELF)