You have surely seen dead trees during your walks in the forest and may have asked yourself why it is left there? Isn’t a forest with a well-cleared undergrowth preferable?
For generations, people have considered dead trees to be something that must be removed from forests, either to be used as fuel or simply as a necessary part of “proper” forest management.
But the truth is that what seems to be nothing more than a barren, decaying trunk, is teeming with life beneath the bark.
Dead wood plays a fundamental role in maintaining soil fertility and forest biodiversity, and its presence is necessary for many living organisms.
What is deadwood?
Dead wood refers to all or part of trees that have died, regardless of their size and quality. It results from the death and natural decomposition of wood or from the decomposition of wood left on site after logging, known as logging residues.
Regardless of its form (branches, cavities, standing dead tree or fallen tree, stump, crown left on the ground, etc.), deadwood evolves passing through different stages of decomposition orchestrated by various organisms:
- Stage 1: The first phase corresponds to a colonization phase in which the wood is invaded by wood-decaying insects and fungi. They attack fresh wood, opening the way for other organisms. Fungi soften the hard trunk making it porous and penetrable and birds, like woodpeckers, dig cavities in the wood that attract other species of birds and mammals. This phase generally lasts around 2 years.
- Stage 2: In this 2nd phase, lasting 10 to 20 years, new organisms join or replace the previous ones, using their work to feed themselves or preying on them. Those new colonizers also find shelter and a place to reproduce in these dead trees. Amphibians, beetles, bats, and owls are some of the species that appear at this stage.
- Stage 3: During this last phase, known as humification, the previous organisms are gradually replaced by organisms that incorporate the wood remains into the soil. Those new organisms (earthworms, woodlice, gastropods, nematodes, etc.) keep breaking down organic matter, returning the nutrients to the soil. Fungi, however, are the main agents responsible for transforming decomposed wood into humus.
What roles and functions dead trees play in the forest?
Dead wood is essential for the forest ecosystems and biodiversity for several reasons.
Nesting site or shelter
Dead trees provide a habitat in their own right for many species. Some species use it simply as shelter, while others use it as a nesting site. They build their nests behind loose bark or in dead branches. Bats, for example, are highly dependent on dead trees.
Food source
As we have seen previously, other organisms use dead trees directly as a food source. These are known as xylophagous organisms, which are essential for the decomposition of wood.
Many species follow one another throughout the decomposition process, with some depending on the presence of others, placing deadwood at the center of what is known as a food web.
Carbon storage
Dead wood allows large quantities of carbon to be stored in it until it decomposes. Some of the carbon stored is gradually incorporated into the soil during the decomposition process, while the rest is completely decomposed through a process called mineralization. During mineralization, the carbon becomes CO² and is released into the atmosphere, where it is recaptured by the trees through photosynthesis.
Nutrient supply
Dead trees also play an essential role in providing nutrients to forest soils. Nutrients extracted from the soil by trees during their growth are thus recycled, limiting soil depletion and maintaining its fertility. Deadwood therefore benefits living trees.
Water storage tank for the forest
Finally, dead trees on the ground act as a water reservoir for the forest and especially for various types of vegetation such as mosses and young trees. Due to its spongy structure, decomposing wood stores large amounts of water, moistening the humus and enabling the forest to better cope with droughts. In addition, amphibians and insects benefit from the constant humidity in and under large dead trees.
What are the risks posed by dead wood?
It is common for people to think that dead trees in forests promote fires, attract pests that attack living wood, or even pose a danger to hikers. Some of these points do not have a unanimous answer. These assertions are more about perception than based on tangible evidence.
Dead wood and fires
Deadwood appears to play an ambivalent role in forest fires. While it can represent a significant portion of the fuel available, especially after natural disturbances, it burns slowly and contributes little to the intensity of the fire (Larjavaara, M and al., 2023).
Although its systematic removal does not automatically reduce the risk of fire, the presence of large quantities of dead wood on the ground can hinder access to areas threatened by fire and reduce the effectiveness of emergency response efforts. For this reason, deadwood management should focus on avoiding excessive and poorly distributed accumulations.
These factors should not overshadow the fact that most fires are caused by humans, making prevention and individual responsibility essential in fire management strategies (C. Biache et al., 2017).
Risk of pest attack
A common idea is that dead trees would attract pests that can attack living wood. The truth is that deadwood is often home to the predators of these pests. Which would mean that dead wood actually contributes to protecting living wood.
In addition, insects that cause massive damage to living trees do not live and feed on dead trees but on weakened living trees (C. Biache et al., 2017).
Find out how thematic data layers on explorer.land can help monitor fire risks: https://openforests.com/media/articles/thematic-data-layers-impact-reporting/
Accidents
Finally, a commonly mentioned risk is that of falling trees and dead branches, could damage property or injure a passerby. Although it is possible, the natural fall of a dead tree is unlikely except in extreme weather conditions (storms, strong winds, thunderstorms). Nevertheless, with the increase in climate events, the risk could grow.
Concerning public areas, removing dead trees in the most frequented areas, along paths and roads, helps to reduce the risk of accidents. In addition, trees often show warning signs before falling, allowing for an assessment of whether or not they need to be pruned.
In conclusion, far from being useless, dead trees are essential to forest ecosystems. They contribute to biodiversity, whether as shelter or a source of food, for example, but also to the surrounding soil. The risks they pose in forest environments are largely offset by the benefits they offer.
Sources:
ONF (C. Biache et al.), 2017. – Vieux bois et bois morts. Guide technique. – Office national des forêts, Direction forêts et risques naturels (DFRN), 102 p.
https://www.onf.fr/onf/%2B/3c0::vieux-bois-et-bois-mort-guide-technique.html
Fabio Lombardi, Boštjan Mali, Deadwood as a driver of forest functions, Italian Journal of Agronomy, Volume 11, Supplement 1, 2016, Pages 23-26.
https://www.sciencedirect.com/science/article/pii/S112547182400464X
Larjavaara, M., Brotons, L., Corticeiro, S., Espelta, J.M., Gazzard, R., Leverkus, A., Lovric, N., Maia, P., Sanders, T., Svoboda, M., Thomaes, A. and Vandekerkhove, K., Deadwood and Fire Risk in Europe, Publications Office of the European Union, Luxembourg, 2023, doi:10.2760/553875, JRC134562.
https://publications.jrc.ec.europa.eu/repository/handle/JRC134562
Petrillo, Marta & Cherubini, Paolo & Sartori, Giacomo & Abiven, Samuel & Ascher-Jenull, Judith & Bertoldi, Daniela & Barbero, Alice & Larcher, Roberto & Egli, Markus. (2015). Biochemical characteristics of decomposing deadwood in an Alpine forest and their relation to climate. iForest. 9. 154-164.
WSL (FR), Institut fédéral de recherches WSL- Bois mort en forêt : Formation, importance et conservation.
https://www.waldwissen.net/fr/habitat-forestier/protection-de-la-nature/bois-mort-en-foret
