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The story behind Autumn

The 'r' is in the month again. September is on average warmer than May and can bring wonderful late summer weather. At the same time, the month is the beginning of a new season: autumn! In Dutch autumn is called ‘herfst’, which has Germanic roots and is derived from harbista, which was also the basis of the English harvest [1]. Indeed, ‘harvest’ used to be the term that referred to the season before the 16th century. It is the period when the harvest is brought in in the northern hemisphere. The English themselves use 'Autumn' (which has a Latin origin) and the Americans call it Fall.



A pleasant deviation

The fantastic alternation of the seasons in the Northern and Southern hemispheres is due to just one phenomenon: the position of the Earth's axis in relation to the Earth's orbit around the Sun: called obliquity. That deviation is 23.5 degrees, which means that day and night only last the same length throughout the year on the equator [2]. The further north or south, the greater the seasonal differences.



Color splendor

Trees ingeniously deal with the change of the seasons. In order to get through the cold and dark winter half of the year, a large part of the tree species drop their leaves in the autumn. During the growing season, new chlorophyll grains (chlorophyll) are continuously produced. Photosynthesis takes place in these mini-factories and the plant produces sugars. At the end of the growing season, the production of the green chlorophyll stops, allowing other colours in the leaf to take over. Depending on the species and stage of this process, leaves can take on a variety of beautiful colours, from bright yellow like Ginkgo to bright red like Japanese Maple.



Hibernation

The deciduous trees in our range also participate in this metamorphosis. Discoloured leaves at this time of year are therefore certainly not a cause for concern about the health of the tree. In the course of autumn, the new buds even become visible. When the temperature rises again in the spring, the tree will be full of leaves again in no time. The moment when a tree comes out of hibernation differs greatly per species and even between individuals of the same species. For example, the Birch starts early at the end of March, while the Albizia does not wake up until May.

A CO2 pump of extraordinary size

This special adaptation of trees to the seasons has a huge effect on a worldwide scale. It affects the level of CO2 concentration in the atmosphere every year. Since 1958, the CO2 concentration has been continuously measured on top of the volcano Mauna Loa (Hawaii). Since the industrial revolution, that concentration has increased from 280 ppm (parts per million) to 420 ppm. A clear sawtooth profile can be seen in this upward trend [3]. This is because there is more land mass in the Northern Hemisphere (and therefore more trees grow there) than in the Southern Hemisphere. In the summer six months the trees absorb CO2, causing the CO2 concentration to drop, and in the winter six months the fallen leaves are broken down by micro-organisms and CO2 is released again.



This annual pattern of CO2 absorption and release shows how important trees are in the fight against climate change. If we plant trees en masse and expand the forest area worldwide, the countless extra mini-factories will absorb more CO2 from the atmosphere and partly store it in wood. Wood that we can use in the future, or just wander through to enjoy the seasons.



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