The mixed forests in the steigerwald and the habbergen are currently offering a magnificent play of colors. Green, yellow, ocher, orange, red, brown: the treetops shine in many colors, they outshine the landscape everywhere. The eyes of the observer are pleased. The biologist is sorry. Not really, it was just because of the rhyme: the biologist answers the question why all the colors are like this. But he first has to make a long story short.
Limited water available
It is clear what follows after the colorful show. Most deciduous trees shed their leaves. Jurgen thein, biologist and nature paddagoge from habfurt, can explain it all in context. "First of all you have to ask yourself: why do they shed their leaves??", it begins. Most people know the answer from school: in winter, trees have only limited water available to them. When the ground freezes, they may have to survive days or weeks of drought. If they had (active) leaves in the geast in such a phase, they would not grab it. "They lose a lot of water through the leaves", explains thein.
No wonder, because the flatness of the leaves makes up a large part of the tree. A deciduous tree growing in these latitudes carries an average of 30,000 leaves. The older and coarser it gets, the more there are. When it is between 80 and 100 years old and 25 to 30 meters tall, it has about 800,000 leaves (depending on the species), which corresponds to a leaf surface of 1600 square meters. To take care of them would be impossible in the cold, central european winter. So trees that switch to low flame in winter "cut the connection to the leaves", says thein.
Get nutrients out
But before they do, they get a lot of nutrients and energy out of them. To simply discard the leaves would be wasteful, considering that "an enormous amount of energy" is required has been put into them.
This process is also responsible for the bright colors in the leaves: in addition to all kinds of nutrients that the tree looses from the leaves with the help of enzymes, the green, light-absorbing pigment chlorophyll is also partly broken down into various proteins and carbohydrates and transported to other regions of the tree. Chlorophyll is needed by the tree for photosynthesis: energy from (sun)light is converted into chemical energy and then transformed into glucose mainly with the help of carbon dioxide and water. The tree produces oxygen as a kind of waste product. But back to the color of the leaves: as the chlorophyll is gradually broken down, other pigments present in the leaves, such as carotenoids (yellow, orange), appear, according to thein. Meanwhile, the leaves are still active and contribute to the energy production of the tree. In the case of the red pigment anthocyanin, researchers suspect that it is produced specifically to protect the leaves from uv radiation while they are still photosynthesizing.
In the mixed forests of the habberge district, however, deep red tones are not the rule, as thein explains: "a yellow tone is characteristic of our beech and oak forests." The reason for this could be the sunshine, which is not too intense in autumn anyway, so that native trees are not so dependent on the production of anthocyanin.
Leaves dry out
According to thein, the indicator for the color change of the leaves is the shortening of the days: for the trees this is a sign that they have to adapt to the cold weather. Before the first frost comes, they want to be ready for the barren months. Finally, they cut the connection to the leaf stalks. The leaves dry out and turn brown. As thein explains, not every tree sheds its leaves: the oak, for example, only clogs the supply lines so that the stems remain connected to the branches. Oaks usually lose their inactive leaves with the first spring storms. And most needle trees don't lose them at all: although the needles are basically nothing more than leaves, says thein. "But they have a relatively low evaporation rate." That's why coniferous trees can afford to keep their clothes over winter.