Surprising finding: roots keep growing in winter
International team of scientists challenge long-held assumption that roots stop growing in winter
When temperatures drop, the trunk and woody roots of trees stop growing. That was what everyone assumed anyway, but research by scientists from the University of Antwerp (and other institutions) now shows that underground wood does continue to display activity in winter after all.
The underground world of trees is largely unexplored, because it is mostly invisible to our eyes and hard to access without damaging the trees. Be that as it may, roots are essential for a tree's nutrition, water uptake, and 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. The big roots (above 0.2 cm in diameter) are made of wood, live for a long time, and store up to one third of the tree’s biomass. Despite its importance, scientific understanding of woody root growth has been limited and based largely on assumptions – until now.
New research shows that woody roots continue to grow during the colder months.
Textbooks generally assume that the growth of woody roots in deciduous temperate trees mirrors the seasonal growth of the tree trunk. In fact, it is assumed that both stop growing in fall because the weather becomes too cold, and start again in spring when the weather is 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 stopped doing so.
Lorène Marchand (UAntwerp), main author of the paper, which was 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.’
Experiment with forest trees and potted young trees
The researchers collected roots and trunk microcores from adult beech and birch trees in forests around Brasschaat (in the north of Belgium) every week for two years, from August until March. They also conducted an experiment with young trees (about 1 meter tall) of the beech, birch, oak, and aspen varieties grown in pots in Brasschaat, near Barcelona (Spain), and near Oslo (Norway), to assess whether the same findings held up in both 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.
A Cross section of a root observed with a microscope (left picture), root samples from the forest (middle picture), and beech forest studied in Belgium (right picture). B Schematic view of the wood growth in roots and trunk in beech trees in September, November, and March; yellow and orange colors indicate growing cells (Marchand et al. 2025).
Lorène: ‘What we found was surprising. While – as expected – the wood in the trunk stopped growing in fall when the leaves are shed, the wood in the roots grew (slowly but continuously) throughout winter and even into the following spring, when new leaves unfolded. The fact that roots keep growing in winter goes against the assumed idea that root growth matches 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 fall from Spain, Belgium, and Norway, even though the temperatures were very different. Our results suggest that, in absence of soil freezing, fall and winter growth in woody roots is a common trait in temperate trees in the Western European zone.’
Implications for forest models
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 have 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.’