Why climate-stable mixed forests are the order of the day

Have you ever watched the trees die? I did in the summer of 2018. Every day on my way to work, I took the train through the forest - monoculture.

... Grown in 50 years, died of thirst in three months. Since then, I've been happy about every drop of rain that ruins a summer's day for me. Water is life and it is becoming scarce. If you want to survive, you have to adapt. That applies to us and to the German forest. Good bye monoculture, here comes the mixed forest!

May we introduce? The climate-stable mixed forest. An introduction to why animals like mixed forests.
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A conversation with our climate forester Jan Borchert 

1 Jan, what is a climate-stable mixed forest?

‍Climate-stable mixed forests are those that will be able to cope with the changing climate and survive in 80 to 100 years' time. Such forests contain trees that will not die like the coniferous monocultures of recent years. Not even in very dry and hot years. They can cope.

2. why do the animals like mixed forests?

In short: the more tree species, the more animal species. They live - like the city dwellers among us - on different levels: in the ground, on the ground, on the trunk, in the trunk, on the lower branches, in the treetop. Different levels mean: different animals, means: different possibilities. This is why animals feel at home in the climate-stable mixed forest. Insects, for example: They often specialize in one tree species, which means that the more different tree species grow in a forest, the more different insect species find shelter. Particularly in times of major insect extinction, the climate-stable mixed forest also plays a major role in this respect. More insects in turn means more birds and birds attract other animals.

And another thing: in a climate-stable mixed forest, the table is generously set: first the cherry blossoms and provides food for insects. Later, the oak produces acorns, which are popular with wild boar and squirrels. And under the hornbeam, mice rummage for its seeds. Deer and roe deer, on the other hand, like to eat the young deciduous trees and later the leaves of the mature trees. They also eat young spruce trees, but once they are big, the animals no longer like them. 

Also attractive for forest dwellers: mixed forest favors ground vegetation and thus additional habitat. In coniferous monocultures, it is too dark for this. There used to be a saying among foresters: "In a good pure spruce stand, you have to switch on the flashlight even in summer." It's pitch black there. Of course, nothing grows on the ground. In a mixed forest, on the other hand, the light falls all the way down. This makes natural regeneration possible: small trees grow under the large trees. Some are a little bushy, some high, some low, so that they provide cover for different animal species. 

3. does mixed forest have a positive effect on its immediate surroundings?

‍Yes, on the microclimate. Forests cool down their surroundings because they evaporate a lot of water and evaporating water has a cooling effect. Trees also provide shade, so it's more pleasant in the forest when it's hot. It can be up to 15 degrees cooler under a tree than in its surroundings. Another important factor: the climate-stable forest acts as a large filter so that we can breathe clean air.

4 Why is climate-stable mixed forest the forest of the future? 

‍It's quite simple. It defies the dangers of the future best: drought, wind and insects, diseases, fungi, bacteria. 

Against the pests:
If you mix up a forest, insects that tend to reproduce en masse have less of a chance. The reason: insects usually specialize in one type of tree. In the monocultures of the past, this had a huge impact. The bark beetle, which only ever attacks spruce, has eaten away entire forests. It had an easy time of it, the table was set and it was able to multiply (en masse). Of course, we don't want that, so it makes sense to plant different tree species together. 

Light and shade:
Another advantage of mixed cultivation: light-loving trees provide shade for others that like it darker. 

Balanced water balance:
Trees in the climate-stable mixed forest have roots at different depths. This means that they draw their water from different levels in the soil. As a result, they do not compete with each other; they can all quench their thirst. This is why, for example, a deciduous forest monoculture, such as an oak forest, is not necessarily climate-stable. All the trees are rooted at the same level. The water above or below them remains unused, while the trees almost die of thirst in hot, dry summers. 

No problem with wind:
In a climate-stable mixed forest, the risk of so-called windthrow is low: if there are trees at the edge and in between that are very stable because they have deep roots and are firmly anchored, they break the wind and protect the less well anchored trees. No one falls over. 

You can see: Different trees benefit each other and do not compete with each other. You could also say that together they are unbeatable. That is why a climate-stable mixed forest is the forest of the future.

5 Why is mixed forest so valuable for the climate?

‍Well, you can't put it quite like that. In terms of the climate, it would only be a question of which tree stores the most CO₂ in the shortest time. A conifer monoculture actually comes off better here. Spruces grow faster than others, so they store more CO₂ in a shorter time. In a climate-stable mixed forest, on the other hand, there are also slow-growing trees. 

So in the short term, monoculture is more valuable for the climate. BUT: As we have seen, in the worst case it only lives for 20 to 30 years because it falls victim to the bark beetle, heat and drought. 

Another threat to pure culture are those attackers that we are not even aware of at the moment. Globalization should be mentioned in this context. It has brought us humans the coronavirus and is constantly bringing insects and fungi from abroad to trees. This is why, for example, the elm is dying out. What if it had been the tree species that we had relied on throughout Germany in the form of monoculture? Then we would now have a real problem, namely no more forests.

You can compare it to the stock market: I can take the high-risk stock that makes me the most money. But that can also go really wrong: If it crashes, I'm broke. It's the same with monoculture: if I rely exclusively on it, the worst-case scenario is that I end up without a forest. That's why it makes more sense to plant mixed forest. In the long term, it is more valuable for the climate, even if it grows more slowly and does not store CO₂ as quickly as monoculture. 

6 How many tree species belong in a climate-stable mixed forest?

‍Justtwo different tree species make a mixed forest. But we usually plant three different varieties. We also rely on so-called natural regeneration. This means that we plant the main trees a little further apart and over the years other species grow in between, the seeds of which have flown into the forest by birds, for example. In this way, there are not just three types of tree. Nature determines how many there will be.

7 Which tree species belong in the climate-stable mixed forest?

‍Alltree species that can cope with heat and drought belong in a climate-stable mixed forest. These are tree species that have deep roots or can withstand a lot. 

Deep-rooted plants include oak, hornbeam and service tree. They reach the lower layers of soil, where water can still be found even when it is not raining. After rain, the upper layers are of course also well moistened, but dry out again quickly. You wouldn't believe how long it has to rain for them to become permanently soaked. Two weeks of uninterrupted drizzle would be good. This usually only happens in fall and winter. After that, the layers dry out from top to bottom and only deep-rooted plants get water even in summer. 

Trees that are tough, i.e. more stress-resistant than others, include fruit varieties such as cherries and certain pear varieties. Walnuts and black walnuts can also withstand a lot. You can't keep them down, so to speak. 

8. do nut and fruit trees belong in a climate-stable mixed forest?

‍Ofcourse! We've already planted some. Cherry; wild cherry, for example. Also wild pears. Walnuts and black walnuts will come in the fall. These trees are extremely stubborn. A walnut, for example, cannot be removed once it has taken hold. The cherry also grows incredibly well. That's why these trees definitely belong in a climate-stable mixed forest. 

9 What does a climate-stable mixed forest have to offer the soil on which it grows?

‍As a generalrule, coniferous forest makes the soil acidic and the more acidic it is, the lower the soil quality and the less life in the soil, because the nutrient content drops.

How does coniferous forest acidify the soil? Spruce trees filter fine dust from the air with their dense needles. When rain falls, it washes it from the trees into the soil. This becomes acidic. This cannot happen in climate-stable mixed forests, as there is no foliage on the trees to filter the air for half the year. 

Accordingly, the soil under a pure conifer stand is much more acidic than under deciduous forest. This affects soil life: Not only does the pH value change, but also the consistency and structure of the soil: it becomes sandier, which means it can no longer hold water as well. Sandy soil is also poorer in nutrients. As a result, fewer herbs and grasses grow, which means less food for animals living on the ground. There is also less going on in the soil: first the earthworm disappears, then the mole. Both are masters of soil aeration. They rummage through the soil and therefore microorganisms in the soil can also work better. So you could say: 

In a climate-stable mixed forest, there is not only huge diversity ABOVE the soil, but also IN the soil. This is because the different trees are rooted in all layers - in contrast to spruce monoculture, where the roots are only in the top layer. 

The soil is better aerated, the trees get enough water, the forest doesn't die, the animals have a habitat and the people have a climate hero. A win-win situation.