
When temperatures drop, the trunk and woody roots of trees stop growing. That was what everyone assumed, but research by scientists, led by the University of Antwerp involving WSL, now shows that there is indeed winter activity in wood underground.
The underground world of trees is largely unexplored because it is mostly invisible and hard to access without damaging the tree. Yet, roots are essential for the tree’s nutrition, water uptake, anchorage and play a crucial role in acting as reservoirs for reserve compounds. Roots are often as widespread as the tree’s branches and leaves. Those with a diameter greater than two millimeters, are made of wood, long-living and store up to one third of the tree biomass. Despite their importance, scientific understanding of woody root growth has been limited and based largely on assumptions-until now.
Textbooks generally assume that the growth of woody roots in deciduous temperate trees mirrors the seasonal growth of the tree trunk. In fact, it’s accepted that both stop growing in autumn because the temperature becomes too low, starting again in spring when weather conditions are more favorable. Still, this assumption was never rigorously tested. A new study, conducted by scientists from the University of Antwerp together with European partners, now challenges this long-held assumption, showing that woody roots continue to grow during the colder months, even when tree trunks have ceased their growth.
Lorčne Marchand (University of Antwerp), the main author of the published in Nature Ecology and Evolution: "We tested this idea, because all forest models rely on this assumption, which can trigger uncertainty and errors. We focused on key tree species found in temperate forests of Western Europe."
The researchers collected roots and trunk microcores weekly from August till March from adult beech and birch trees in forests around Brasschaat (in the North of Belgium) for two years. They also conducted an experiment with young of beech, birch, oak and aspen trees of about 1 meter tall, grown in pots in Brasschaat, near Barcelona (Spain), and near Oslo (Norway), to assess whether the findings remained consistent in the center and at the edges of the European temperate zone. In total 330 trees were studied, and more than 1000 roots samples were collected.
Lorčne: "What we found was surprising. While the wood in the trunk stopped growing in autumn when the leaves fall, the wood in the roots grew slowly throughout the winter and even into the next spring, when new leaves unfolded. The fact that roots keep growing in winter goes against the assumed idea that root growth should match trunk growth. Wood growth in the roots continues even when the soil temperature drops close to zero, as observed in Norway. The growth pattern in roots does not depend on the location, as we saw similar data in young trees sampled in autumn from Spain, Belgium, and Norway, even though the temperatures were very different. Our results suggest that, in absence of soil freezing, autumn and winter growth in woody roots is a common trait in temperate trees of the Western European zone."
This discovery reshapes our understanding of how trees grow and manage their carbon reserves. It demonstrates that woody tissues can grow continuously, even in temperate climates, and highlights the active role of forests during winter-particularly below ground. Matteo Campioli, senior co-author of the paper: "Our findings carry significant implications for forest models, which previously relied on outdated assumptions about root growth patterns, potentially leading to errors in predicting forest carbon storage and growth dynamics."