The Spessart is a low mountain range covering over 200 square kilometres, predominantly forested with beech and oak trees, which is surrounded by the River Main in a quadrangle. The forest mountain range owes its name to the woodpeckers: "Spechts-Hardt", the old name for the area, means a wooded area covered with hardwood and inhabited by many woodpeckers. Its name was first mentioned in the Song of the Nibelungs in the 13th century.
It is mainly thanks to the hunting interests of the Archbishops of Mainz that the Spessart has remained a large area of deciduous forest to this day. They promoted the oak as a fattening tree for game and prevented the forest from becoming more heavily colonised. Another reason is undoubtedly the poverty of the coloured sandstone, which produces soils with low yields. But how and when did these deciduous forests develop?
The tree as a strategy
Most herbs and grasses can only position their leaves a few metres above the ground. Ferns were already much more successful and long-lived. But it was the trees that first formed the stem into a massive trunk - a development that began around 300 million years ago with the evolution of conifers. This made it possible to lift the leaf mass up to around 40 metres from the ground and open up an enormous three-dimensional space in order to absorb maximum solar energy. In this way, trees and forests determine the light regime, the temperature and the microclimate beneath them. The fact that this is physically possible is due to a building material, lignin. Similar to the concrete that reinforces a steel frame, lignin is deposited on the cell wall. Together with cellulose, it is a main component of wood.
Not only the trunk and branches are made of this basic building material, but also the roots. Their stable anchoring in the soil is a prerequisite for the formation of the enormous trunks, which often weigh many tonnes. Through them, the vital water is channelled from the roots to the leaves and part of the sugar-rich sap produced there is returned to the roots.
Trees have been forming forests ever since, and with them they shape entire landscapes. They tap into water reservoirs in deep layers of soil and release it via a gigantic leaf surface. An old beech tree evaporates around 300 litres of water every day, an old beech stand as much as 60,000 litres. They therefore play a decisive role in our water and temperature balance. Compared to open land, forests are around 4°C cooler. Due to the evaporation flow, forests transport water into the otherwise dry interior, far away from coasts. In this way, they also shape the relief through water erosion, create rivers and play a decisive role in shaping the appearance of the earth.
Without the influence of humans, Central Europe would be largely characterised by forests and trees. Only the particularly moist, dry or steep areas would be treeless. This forest would be largely dominated by beech. Depending on humidity, nutrient availability and vegetation length, other tree species would be added. In low mountain areas, the fir is often its direct companion, as it follows the beech into its shady realm and utilises a slightly different light spectrum than the beech. Otherwise, the other tree species only have a chance where the beech collapses, creating a gap in the canopy. Coniferous forests, on the other hand, are located in naturally harsh mountainous areas, adapted to high rainfall with low average temperatures and a short growing season.
Adaptations to the shadow realm
Other plant species have adapted to this shade. Spring geophytes, for example, have developed the ability to utilise the short phase in early spring until the leaves of deciduous trees emerge to store so much solar energy in their underground roots that they can survive the rest of the year. As the first greenery of the year, however, they are a favourite with herbivores. To evade them, spring-flowering plants are usually slightly poisonous. Substances such as anemonin protect them and their bulbs from being eaten by mammals.
The niche of the oak next to the beech
Let us now look at the two most important tree species in the Spessart and the temperate Central European deciduous forests in general.
First and foremost is the aforementioned beech, followed at a considerable distance by oak.
As a shade tree species, beech is the dominant tree species. As a competitive strategist, it is able to invest all its energy in its growth. As a result, it creates its own cool, moist microclimate without investing in particularly elaborate protective mechanisms (no thick bark, hardly any chemical protection).
This puts all other tree species in the shade. Only the fir can follow it by utilising its own light spectrum. The oak, on the other hand, as a light tree species, favours longevity and gaps in the canopy. Individual specimens can live up to 1000 years. The protection of the leaves by tannins and the protection of the wood against fungal attack make the oak so durable. It is therefore rightly our symbol of stability. In the course of its long life, it has many opportunities to accumulate near a gap in the light or to spread its heavy fruit. With its nutrient-rich acorns, it has found a special propagator in the jay. This enabled it to spread considerably faster than the beech in the post-glacial period.
As a light tree species, the oak is correspondingly more attractive to insects and provides four times more insect biomass than the beech, which is considered one of the least nutritious deciduous trees, especially during the bird breeding season. Correspondingly few insects can be found feeding on its leaves. One example of one of the few exceptions is the beech redstart with its distinctive caterpillar.
Disruption and regeneration
But how does regeneration or disturbance and regeneration take place in the beech forest? As a rule, only individual trees die, often in interaction with wind and tinder fungus. This results in small gaps that often only comprise one or a few tree crowns. Analyses in the Semenic primeval beech forest in Romania showed that the majority of gaps were less than 500 m2. This means that beech trees rejuvenate on their own in the stand depth and disturbances usually only occur in small areas.
If you take a closer look at these gaps, you can recognise clear structures such as deadwood. In primeval forests, deadwood often accounts for 10 % of the total biomass. Around one third of this is standing material and two thirds is lying material. This deadwood forms numerous niches and is of great importance for the accumulation of humus as a water reservoir, but also as a habitat for numerous xylobiont species. For example, around 1300 beetles and around 1500 species of large fungi depend on deadwood. In addition, rotten wood and mulm is an important seedbed for forest regeneration. This is particularly important in the calcium- and magnesium-poor Spessart. This is the only place where you can find shell snails, which need these nutrients to build their calcareous shells. These calcareous shells are in turn important for many bird species, which use them to form the egg shells for their eggs.
The cave as a habitat
In addition to deadwood, there are also old trees that have not died but are already colonised by fungi due to wounds. Woodpeckers prefer to make their breeding holes in these trees. This is particularly striking in the case of our largest native woodpecker, the black woodpecker.
There are few decisions that are as important to them as the choice of nest site. Whether the young will grow up and thus successfully pass on the genes depends largely on this. Woodpeckers have optimised nest building with their cavities. The cavities are safer and better protected from adverse weather conditions and have a more favourable microclimate. However, the larger cavity also provides easier access for predators.
The pine marten is the most important selection factor and is the focus of enemy avoidance. Where beech is found, it is by far the most important cavity tree for black woodpeckers and pine martens. Here, the largest native woodpecker species creates the cavity high up on the trunk, at a height of around 10-12 metres. Up there, the cavities are more difficult to discover, but in any case more difficult to climb on the smooth beech bark and therefore easier to defend. Particularly high dens (over 15 metres) were never visited by pine martens in our studies. Another safety factor is the distance between the natural regeneration and the cave entrance. If this is less than half the distance to the cave, the cave is no longer attractive to the black woodpecker. This means that a 12 metre high cavity will no longer be used for breeding once the natural regeneration has reached a height of six metres. Otherwise, the cavities in the beech are repeatedly used for breeding over long periods of time, often five to seven years in a row. Wilhelm Meyer in particular has carried out long-term studies on the utilisation of black woodpecker cavities over decades. He discovered that these cavities can easily stand for 30 years and be used as habitat trees. Another passive anti-predation strategy that plays a role against attacks by hawks, buzzards and Ural owls is the depth of the cavity. This is the only way to explain why black woodpeckers spend energy and time carving out a 40 cm deep cavity in beech trees, one of the hardest native tree species - just deep enough to prevent their claws from reaching the young birds.
In Scandinavia, the black woodpecker has a different strategy: it likes to build its den in the soft aspen. As the trees are lower and usually weaker due to the shorter vegetation period, the cavity is built on average 4 metres lower down the trunk. The higher risk is compensated for by the fact that it creates new dens every year and has usually already finished breeding before the pine marten even discovers the den.
The stock dove's strategy is different. As a secondary cave dweller, it is dependent on the available supply and also uses caves as breeding sites, where the shelter extends right up to the cave entrance. One possible means of reducing the risk of attacks by hawks may be the low feeding frequency of only around twice a day. This low frequency is made possible by the extremely high-fat crop milk, which is similar to mammalian milk.
The Stock Dove, as the most important successor to the caves, is more plastic and, as a secondary user of caves, is inevitably more tolerant with regard to the quality of the caves. Rotten or slightly wet cavities can still be utilised thanks to the secondary nest it has built. As far as the height of the tolerated natural regeneration is concerned, cavities whose entrance has already been reached by the treetops are still used, whereas the black woodpecker avoids cavities with natural regeneration from half the height of the cavity.
Conclusion
In conclusion, it can be stated that black woodpecker cavities are key structures that have complex significance in the forest ecosystem. An essential factor for the current choice of cavity is apparently enemy avoidance. This seems to be re-evaluated before each brood. In principle, however, these structures can be utilised by a number of species over long periods of time (decades). To ensure that natural regeneration does not jeopardise cave use too quickly for the indicator species, cave complexes in old wood should be kept dark for a particularly long time. Accordingly, these stepping stones should not be too small (approx. 0.5 to 1 ha) so that no side light promotes growth.
These caves are subject to succession. Initially woodpeckers use the self-made caves, followed by stock doves, great horned owls and later, when the cave roof rots out, bats such as the common noctule and later, when the floor slowly fills with mulch, faeces, nesting material and insect remains, it becomes the habitat of numerous insects with special biologies.
Mulm bodies of over 100 litres in size can be created in this way. They are the habitat of a particularly charismatic species of rose chafer: the hermit beetle, a priority species under European law. There are also antagonists in the mulm, such as the fire beetle, a species of click beetle that hunts the larvae of the hermit beetle and other rose chafer beetles.
Other caves fill with water after the black woodpecker has left, because it no longer pecks away the wound callus and water therefore flows into the cave. These water-filled caves are in turn a special microcosm. This is where, for example, the larvae of hoverflies, so-called rat-tail larvae, develop; these are species that have a kind of proboscis on their abdomen for the absorption of oxygen in the air. Black and yellow coloured death's head hoverflies or marsh hoverflies are among these special colonisers.
To summarise, forests are of paramount importance to us. Be it as a cooling landscape element, as a water reservoir and evaporator, as a haven of biodiversity or as a psychotope. In my opinion, climate change does not threaten the forest as a whole, but it does require us to adapt the way we deal with it. Future forests will probably grow less tall, be thinner and less productive and contain mainly deciduous trees. The priority here must be that we prioritise the resilience of the ecosystem in all our demands on the forest. And this resilience must be reassessed today in the light of climate change. 
