Why the trees are already autumn yellow

Sweden is experiencing an unusual phenomenon as trees across the country are displaying vibrant autumn colors much earlier than expected, with some showing signs of fall since mid-August. However, this premature display is not an indication of the season's arrival but rather a direct response to the summer's persistent drought.

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The trees are not showing the colors out of joy and saying 'thanks for this season,' but they are acting to manage a stress situation.

Henrik Sjöman, a researcher at SLU Alnarp and scientific curator at the Gothenburg Botanical Garden, explains that trees are not signaling the end of the season with joy. Instead, they are reacting to environmental stress. "The trees are not showing the colors out of joy and saying 'thanks for this season,' but they are acting to manage a stress situation," Sjöman stated.

Trees lose water through their leaves, and when the ground is dry, they must quickly shed their foliage to prevent further dehydration and more severe damage. During this process, the trees reabsorb nutrient-rich chlorophyll, revealing other pigments like yellows and oranges that were already present in the leaves. Sjöman noted that different tree species react differently, with birches often being among the first to change color, while oaks tend to hold onto their green leaves longer.

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When the tree decides to shed, it absorbs the green chlorophyll which is very nutritious. Then other pigments appear that have always been in the leaves, but are not visible due to the green chlorophyll.

Forest researcher Ola Langvall from SLU concurs that drought is the primary cause but also points to other potential factors. He suggests that some trees might be affected by fungal infections on their leaves, or that leaves simply have a predetermined lifespan, leading to earlier yellowing in some species, a trend observed consistently in birch trees over 20 years of observation.

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For some of the trees, it can be about fungal-infected leaves.

Sjöman recalled the extreme summer of 2018, which saw similarly defoliated trees and birches losing their leaves by late July. He emphasized that the timing of leaf drop is highly localized, influenced by microclimates. A short car journey can reveal stark differences, with areas experiencing more heat and dryness showing significantly more yellow foliage compared to greener, less affected regions.

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You can take a short car trip from western Skåne, where it is relatively green and then you come to the eastern side and see that the trees are much yellower because it has been drier and warmer there.