At the moment, many woody plants are once again showing their particularly beautiful side; the fall coloration is in full swing, but due to the warmest autumn since measurements began and the lack of frost in the swiss plateau, it is much later than in other years. But what processes lead to the discoloration and what exactly happens?
Autumn discoloration to avoid winter water shortage.
In summer, large trees evaporate several hundred liters of water per day. When the ground freezes in winter, the water supply stops, the trees and shrubs can no longer absorb enough water and would dry up. Therefore, they shed their leaves in the fall, stopping the release of water. For the shedding of leaves, it needs low sunshine and shorter days, nighttime temperatures in the single digits, and several very cool nights in a row. In addition, the process is also controlled by hormones. The maximum of fall coloration is in October, in higher altitudes a little earlier, in low altitudes a little later (cf. Fig. 1).
Fig. 1: Beautiful autumn foliage coloration on the shore of a lake; Source: pixabay
Processes during autumnal leaf coloration
The nitrogen-rich green leaf pigment (chlorophyll), with which plants use the energy of sunlight to form sugars (photosynthesis), is broken down into its components and stored during leaf discoloration. During this process, it becomes visible that the leaves also contain orange and yellow pigments: Carotenoids (yellow, orange, red), xanthophylls (yellow) and anthocyanins (often red). Poplar and maple leaves, for example, turn yellow, while red oaks and vinegar trees often turn orange to red (see Fig. 2).
Fig. 2: The vinegar tree, which originates from North America, also shows beautiful color plays here (picture from Sarganserland); Source: Roger Perret
The bright play of colors in autumn is primarily due to a change in the quantitative ratio of these leaf pigments. In spring and summer, the green pigment, chlorophyll, which is indispensable for photosynthesis, predominates. It covers all other pigments. With the disappearance of chlorophyll, the other pigments now come into full effect, causing the impressive autumnal – mostly yellowish – leaf coloration. The leaf shedding is not only an effective evaporation protection, but has other advantages for the plants: It disposes of toxic metabolic end products and stored environmental toxins. Also, bare trees can better withstand large snow loads in winter without branches or even trunks breaking. In addition, the buds that sprout in the spring receive sufficient light for their development due to the fall of leaves in the autumn.
While most deciduous trees shed their leaves in the fall, this is not the case with conifers, with the exception of the larch. This is because the leaves of conifers have a thick wax layer and a very firm skin, which inhibits evaporation. In addition, the so-called stomata, through which the tree evaporates water, are recessed in the leaf, so that evaporation is further slowed down. In addition, the small surface area of the needles further reduces evaporation. Therefore, autumn needle fall is not necessary. However, this does not apply to the larch, which often grows in very cold regions, so that the above mechanisms no longer work. To prevent it from evaporating water, it sheds its needles in autumn. Good for the Engadine and the Valais, this is the only way to get wonderful pictures of discolored larches in autumn (cf. Fig. 3)!
Fig. 3: Autumn larches in Engadine colored yellow; Source: Roger Perret
Why usually more intense discoloration in North America?
Autumnal coloration is particularly intense in North America, where it is known as "Indian Summer" (see Fig. 4). The reason is that our trees produce fewer pigments. These are also there to shield the effect of sunlight so that the light-sensitive chlorophyll is not attacked. Thus, the sunnier it is, the more intense the colors are. On average, it is more often cloudy in our region, so that more intensive sun protection is usually unnecessary.
Fig. 4: Particularly intense autumn foliage coloration in the USA (Lake Elmore in New England); Source: pixabay
This year very late autumn coloration
This year the coloration has difficulties to get going, both in the mountains and in the swiss plateau (see Fig. 5). The reason for this is the very warm autumn so far (on average almost 4 degrees too warm and thus by far the warmest autumn since the beginning of measurements, cf. fig. 6) as well as the lack of frost at low altitudes so far and the absence of a longer series of very cool nights.
Fig. 5: Current image from Sarganserland. While the oaks and lime trees in the foreground are hardly discolored yet, the discoloration of the beeches in the background is more advanced; Source: Roger Perret
Fig. 6: Temperature deviation so far in autumn (Sept./Oct) compared to long-term average; Source: MeteoNews, UBIMET
Summer leaf fall - stress response
Particularly during prolonged heat waves or dry periods, such as this summer or 2018, we also repeatedly experienced premature leaf fall due to drying of the leaves (cf. Fig. 7). Such events are to be expected more and more frequently in the future in the course of further climate warming with ever longer heat waves and summer dry phases.
Fig. 7: Premature leaf fall in August 2018 due to severe drought (picture from Sarganserland); Source: Roger Perret
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