A study reveals that deciduous trees’ roots remain active in winter

Deciduous trees — such as beeches and birches — change colour from season to season. Between autumn and winter, they lose their leaves and go into a state of dormancy, halting their above-ground growth to save energy for the coming spring, when they will sprout new leaves. Until now, their roots were also thought to become dormant, ‘hibernating’ along with the rest of the tree. However, a study published today in the journal Nature Ecology & Evolution shows that the roots of deciduous trees actually grow without pause all year round. The study was led by University of Antwerp researcher Lorène J. Marchand and involved CREAF and the Spanish National Research Council (CSIC). Its findings suggest that forest soils have a greater carbon sink capacity than was previously believed, as they accumulate carbon all year long rather than only when trees are more active and their leaves are photosynthesizing. This discovery alters our ideas about how deciduous trees might adapt to climate change in the future.

“Our results demonstrate that the above-ground and below-ground parts of a tree follow separate calendars, allowing for underground growth to be prioritized in winter and aerial growth at the beginning of spring,” says CREAF researcher Paolo Zuccarini, a co-author of the study. This divergence could offer a strategic advantage: in winter, while the rest of the tree is dormant, its roots store reserves of carbohydrates and nitrogen-rich substances, such as proteins and amino acids. Thus, when the tree ‘reawakens’, it will have enough nourishment for leaves to bud and photosynthesis to resume, which is essential for trunk and branch growth. “It’s kind of like having building blocks ready to use in the spring, when the priority is to produce leaves so that photosynthesis can start again,” explains Zuccarini.

This surprising research was conducted with four tree species — European beech (Fagus sylvatica), silver birch (Betula pendula), pedunculate oak (Quercus robur) and common aspen (Populus tremula) — in various temperate forests in Spain, Belgium and Norway. In total, the researchers studied 330 specimens, comprising both mature trees and three-year-old saplings. They collected a thousand samples of coarse roots, i.e. roots with a diameter of over 2 mm, which are woody organs and therefore undergo wood formation (referred to as xylogenesis). “Our data show that coarse roots remain active even at soil temperatures of under 3°C, refuting the theory that cold brings growth below ground to a halt,” remarks Marchand, the study’s lead author. 

Another noteworthy finding is that forest soils may be a greater carbon reservoir than was previously thought, given that the study establishes that deciduous trees fix carbon throughout the year. The researchers explain that trees store carbon in their structure and use it for wood growth in their roots and trunks. This stored carbon is highly stable, as it is incorporated into plant tissues in the form of lignified biomass, composed of cellulose, hemicellulose and lignin. “Coarse roots actually contribute significantly to trees acting as carbon reservoirs, as they account for up to 20 to 30% of a tree’s total biomass due to their high density,” points out Zuccarini. The carbon cycle in trees has traditionally been considered to be closely linked to the aerial growth cycle; that would entail carbon dioxide absorption peaking in spring, when trees have leaves, carry out photosynthesis, and capture carbon to use it for trunk and root growth. However, the new data from the study show that trees use carbon all year round.  

As the study’s results indicate that root growth does not stop in winter, the authors suggest that the biological clock of roots may be not so much influenced by external signals (e.g. temperature and light) as controlled by internal signals (e.g. the tree’s carbon reserves). “This is in contrast to the above-ground part of the tree, which is more affected by the external climate,” comments CREAF-based CSIC researcher Josep Peñuelas, another of the study’s co-authors. Preparing their roots in winter for use in spring could help trees adapt to changing climate conditions, such as warmer springs, premature thawing or irregular rainfall. 

The international study was led by Lorène J. Marchand and Matteo Campioli of the University of Antwerp’s PLECO research group, along with the University of Ghent in Belgium. Besides CREAF and the CSIC in Spain, various other European organizations and institutions also contributed to the study, including the Slovenian Forestry Institute, Umeå University in Sweden, the Norwegian Institute of Bioeconomy Research, and the Swiss Federal Institute for Forest, Snow and Landscape Research.

Referenced article: Marchand, L. J., Gričar, J., Zuccarini, P., Dox, I., Mariën, B., Verlinden, M., Heinecke, T., Prislan, P., Marie, G., Lange, H., Van den Bulcke, J., Peñuelas, J., Fonti, P., & Campioli, M. (2024). No winter halt in belowground wood growth of four angiosperm deciduous tree species. Nature Ecology & Evolution. DOI: https://www.nature.com/articles/s41559-024-02602-6