In autumn, tree leaves take on a reddish glow for the human eye. "What we see is just the visual side of very complex processes inside plants as they prepare for winter," biologist Arne Sellin said on the morning show "Terevisioon."

Plants, he explained, "know" how to get ready for winter by responding to signals from their environment. The most important cues are changes in temperature and in the photoperiod. "Simply put, the length of daylight decreases while the duration of darkness increases," Sellin said. A third signal is that autumn light itself shifts toward red.

These signals trigger several changes in plants. "Leaves turn yellow or orange because the plant begins to break down substances normally present in the leaf during summer," he said. In summer, chlorophyll makes leaves green, but maintaining it is energy-intensive. Since leaves are destined to die in autumn, the plant starts moving chlorophyll out of the leaves and into the roots and trunk for storage.

So far this year, there are still few leaves changing color in nature or in the city. One reason, according to Sellin, is the warm start to autumn: nights have been mild and daily minimum temperatures high. "Another important factor is that the summer was rainy and cool. As a result, the entire aging process of the leaves has slowed down and that also affects their autumnal senescence," the biologist explained.

Red and purple tones, he added, form in leaves under strong light. On the one hand, recent afternoons have been sunny. "At the same time, the high humidity and persistent morning or daytime fog prevent radiation levels from reaching the intensity needed for leaves to turn red or purple," he noted.

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