The length of the day is currently decreasing by 3 minutes, and the equinox will be on September 22nd. In addition, temperatures will drop well below ten degrees by the end of the week. It is time for the deciduous tree to become winter hardy before the first frosts arrive. The winter buds are already forming here at the end of August.
SRF Meteo calculates a forecast for leaf coloring based on the current weather. When and where will your forest change color?
Who decides when to inspect trees in winter?
In short: day length, weather and the genetic conditions of each tree species. It is an “internal clock” that controls the start of the foliage coloring.
Internal clock
Open box
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- Professor Dr. Christoph Körner: Over thousands of years of evolution, trees in our latitudes have “developed” an internal clock that can measure the length of the day – regardless of whether it is cloudy or sunny. The clock triggers developmental rhythms and functions in a similar way to an hourglass, in which the sand flows from one half to the other at a constant speed. The amount of sand in the two halves in the morning and evening tells the plant how long the day is compared to the night. The sand is actually a protein molecule in the plant cells that can assume two states depending on the light. The plant’s genetic material can read these states and thus knows the probable date. Since the length of the day is the same in spring and autumn, the tree must know whether winter is coming or is already over. It does this by adding up the cold hours.
How does the splendor of colors come about?
The process of leaf discoloration is related to the tree breaking down the chlorophyll and withdrawing all the valuable building blocks and nutrients from the leaves. The yellow and orange tones that have always been there now appear. In addition, many species produce anthocyanin (red pigment) to protect these degradation and withdrawal processes from sunlight. In Switzerland, many tree species change color from golden yellow to brown (birch, linden, maple) or to yellow, orange and brown-red tones (beech). Intense red colors, like the oak and maple species in North America, are rare here (dogwood, viburnum), according to phenology expert Eric Wyss from PhaenoNet.
Landscape in autumn dress
First brown leaves fall to the ground…
A corking tissue forms at the base of the leaf stalk and the leaves fall off when the wind blows. This is very important because it can significantly reduce the snow load in winter.
Mainly brown leaves lie on the ground
Ascona/TI
Norma Widmer
Does global warming influence the timing of leaf coloration?
The temperature in Switzerland has increased by about 2.8 degrees since pre-industrialization. Winters have become milder on average, there have been more hot and dry periods and heavy rainfall events have increased. Plants are also feeling the effects of this change.
According to expert Eric Wyss, the influence of global warming on flowering in spring is particularly well known. In the case of hazel, global warming has led to an earlier flowering by three to four weeks over the last 70 years. But the autumnal leaf discoloration is also linked to global warming.
Development of leaf discoloration (Source: Meteo Switzerland)
The graphics from the Federal Office of Meteorology and Climatology show a significant shift backwards, especially for the large-leaved lime, the sycamore maple and the silver birch. In the case of the horse chestnut, on the other hand, the leaf coloring is nine days earlier. But here the chestnut leaf miner (a pest) and other plant diseases play a decisive role – says Eric Wyss.
According to experts from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), the timing of the foliage change depends on other factors, especially the number and length of drought and heat periods. Stressed plants can therefore have premature leaf shedding or are particularly sensitive to pests.
Since the end of August, past phenological data can be displayed in the PhaenoNet app. This makes it clear that different plant species react very differently to climate warming.
