Golden Leaves in Fall

Golden yellow leaves in autumn create a special mood in the third season. For some nature lovers, it is more beautiful than summer, because it is not that hot anymore and with a little luck you can still enjoy many sunny days in the garden. Tees and shrubs with intense autumn colors make nature walks a special experience.

Trees and shrubs, which have delighted us with their fresh leafy green for months, change in autumn to a sea of ​​yellow-reddish brown leaves that last until the first frost. What is the secret of this foliage change?

The autumn colors have different causes

With the foliage change, deciduous trees prepare for the cold and low-water season. The trigger for the plant hormone-controlled process is the decreasing length of the day in autumn, supported by falling temperatures. The shorter days and the first cool nights start a genetically controlled aging process in the leaves. The green dye chlorophyll is no longer needed because photosynthesis is “reduced” when the light diminishes – the plant metabolism switches to a low flame.

Color change by shifting leaf dyes

Poplar and maple leaves turn yellow, red oaks stand out due to their fiery red. The bright play of colors in autumn is primarily based on a change in the quantity ratio of the leaf dyes. In spring and summer, the green dye predominates, chlorophyll, which is essential for photosynthesis. It covers all other pigments. With the disappearance of chlorophyll, the other pigments come into their own: carotenoids (yellow, orange, red), xanthophylls (yellow) and anthocyanins (red, violet, blue) and cause the impressive autumnal – mostly yellowish – foliage color. Like chlorophyll, they were involved in photosynthesis and were present in the leaves all the time. Brown as a color only appears when the leaf dies. In addition, the color change is related to the fact that nutrients are broken down and important elements such as phosphorus, iron, potassium, and nitrogen are shifted to the living trunk, branch, and root areas. Nitrogen is a particularly scarce commodity for plants.

Important Nutrients are stored

In order to prevent a loss of the green dye, which is very important for the plant, it almost completely breaks it down into smaller components in autumn, which it pulls out of the leaf and into branches, stems or until the next spring, when the new leaf generation needs chlorophyll again Root deposited. The remaining pigments are not so valuable for the plant and are only partially broken down. This is the reason for the predominantly yellow coloring of the autumn leaves. Unlike the yellow and brown coloring, the impressive red foliage color is due to a group of substances that are newly formed in this transition phase. They are anthocyanins, which are also responsible for the red coloring of ripening fruit. In autumnal foliage coloring, they arise as a by-product of the very high metabolic activity.

Falling leaves as an adaptation to winter drought

The annual fall of leaves in autumn is an adaptation to the winter’s lack of water. Trees usually continuously evaporate a large part of the water absorbed by the roots through their leaf surfaces. A hundred-year-old beech tree evaporates around 400 liters per day, with a large birch it is between 40 and 150 liters per day. In order to prevent water loss through evaporation over the leaf surface, deciduous trees shed their leaves in times of low water supply. With falling temperatures, the roots absorb less and less water and completely stop water absorption near freezing. If the tree now had left, the water would continue to evaporate and the tree would slowly dry out.

What else is going on?

Since chlorophyll is missing, photosynthesis is stopped. In parallel to the leaf coloring, something happens at the connection between the petiole and branch. After all valuable substances have been removed, a separating fabric is formed at the base of the petiole. This prevents the water supply to the blade and at the same time creates a predetermined breaking point. A light breeze is enough to let the leaf fall off. With the cork fastener at the leaf attachment point, however, the tree also provides external protection that prevents pathogens such as bacteria or fungi from entering. The fall of leaves is not primarily caused by frost and strong winds but is the result of an active process to prepare for the unfavorable winter time, in which water supply is the biggest problem with persistently frozen soil.

Disposal of toxins and other advantages

The leaf fall is not only effective protection against evaporation but also a detoxification treatment. Together with the dropped leaves, the tree repels the toxins that have accumulated in the course of the summer. The plant, therefore, disposes of toxic metabolic end-products and stored environmental toxins. Bare trees are better able to withstand the snow load of winter. Leafy trees would collapse under the weight of the snow. In addition, the fall of leaves ensures that the budding buds in spring have sufficient light for their development. Last but not least, the open leaf canopy in early spring makes it possible for the next year to have enough light on the ground for early bloomers such as anemones, wild garlic or celandine.

Early leaf fall

Due to diverse environmental influences, early leaf fall occurs again and again. The falling leaves are still green, but often colored brown. The leaves dry up during longer heat periods in summer. Increased soil moisture also leads to withering because the roots rot and hardly absorb any water. Pollutants in the soil and in the air, such as chloride ions from road salt or nitrogen oxides from car exhaust gases, weaken the plants and let the leaves fall from the trees in summer. Trees weakened by extreme weather conditions and environmental toxins are particularly susceptible to fungal attack and insect pests such as leaf moths, which also causes the leaves to fall prematurely. If ash, alder and elderberry shed green leaves, this is normal because they do not change color.

Why don’t the conifers lose their needles in autumn?

The effective evaporation protection of the needles prevents conifers from being “leafless” in winter. The “leaves” of the evergreen conifers have a thick layer of wax and a very firm skin that inhibits evaporation. In addition, the so-called stomata, through which the tree evaporates water, are narrowed in the leaf, so that the evaporation is also slowed down here comes the small surface of the needles, which also reduces evaporation. The only conifer that does not keep its needles in autumn is the larch. It does not have needles as thick as spruce or pine and its wax layer is not so pronounced, so their protection against evaporation would not be enough in winter.

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