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Forests: how thinning influences soil biological activity

Research Article published on 16 February 2024 , Updated on 16 February 2024

Like all living things, forests are threatened by global warming and their exploitation by humans for commercial purposes. A recent study by scientists from the Laboratory of Ecology, Systematics and Evolution (ESE - Univ. Paris-Saclay, CNRS, AgroParisTech) in collaboration with the Forest Ecosystems Laboratory (EFNO) of the French National Research Institute for Agriculture, Food and Environment (INRAE), focuses on the consequences of thinning, a widespread silvicultural practice. Through soil analysis of different forest plots, the teams measured the biological activity present for a better understanding of its functioning.

Carbon sinks, sources of timber and havens for biodiversity, forests are key spaces for the living world and its interactions with humans. For most of us, a forest is simply a collection of trees. But for Stéphane Bazot, professor and deputy director of the ESE laboratory and his team, what happens at the foot of this vegetation is worthy of their attention: the forest floor is a sensitive place of transition between the Earth's crust and living organisms.

An experimental approach to studying silvicultural practices

Forests are subject to many external disturbances. In addition to the regular droughts that have struck them in recent years as a result of global warming, they have long been commercially exploited for their timber. Silviculture, the activity aimed at maintaining forests for this economic purpose, uses various forest management techniques. One such practice is thinning. This technique involves cutting down some of the trees in a forest plot. Access to light, water and nutrients is then more evenly distributed among the remaining trees, encouraging their growth. The trees form straight, thick trunks that have a higher market value. The practice of thinning is now widespread. It is sometimes carried out without an economic purpose but with the aim of preserving a plot affected by drought.

Its impact on biodiversity also needs to be studied. In 2015, Nathalie Korboulewsky's team from INRAE's Forest Ecosystem laboratory (EFNO) studied this issue in the Orléans forest, near Nogent-sur-Vernisson. They conducted thinning on some of the forest plots while preserving the density of others, to the extent that the site now provides an exact comparison between thinned and denser plots.

These plots in the Orléans forest have also been managed by the French National Forests Office (ONF) for over 70 years, and the tree species present on each of them are very well defined; there are plots with just oaks, others with just pines, and finally plots where both tree species are mixed.

The forest floor: a recycling factory for organic matter

Recently, Stéphane Bazot's team, experts in soil analysis, joined forces with Nathalie Korboulewsky's team to assess the impact of thinning on the composition and biological activity of the forest floor. When describing the forest floor, Stéphane Bazot draws layers on a sheet of paper: at the very top, he places vegetation, at the very bottom, bedrock (in other words, the upper part of the Earth's crust) and in between, a series of intermediate layers made up of soil. This soil is composed of both mineral elements and living and dead organic matter. Indeed, the development of soil from bedrock occurs through the weathering of rock, combined with the activity of the organisms living within it. These organisms contribute and decompose organic matter. As the process advances, the soil becomes more structured and the vegetation becomes increasingly dense and diverse. Eventually, the soil hosts the trees that form the forest.

However, this organic matter released by plants throughout their lives does not remain in this form. It becomes the subject of intense biological activity. First broken up by small insects or worms, it is then digested by progressively smaller living creatures. This is referred to as the mineralization of organic matter. The chemical elements that make it up are found in the simplest possible molecules: carbon in the form of CO2, nitrogen in ammonium, nitrite or nitrate ions. It is these chemical species that plants exploit for their growth. The intensity of the soil's biological activity is therefore directly linked to its fertility.

This mineralization process gives rise to a layered structure: these are the soil horizons. Each horizon is characterised by a given proportion of organic and mineral matter, weathering processes, accumulation phenomena, etc. The bedrock beneath the soil is purely mineral. On the surface, in contact with the atmosphere, the O (for organic) horizon contains, in particular, the litter that covers the forest floor and the humus. Stéphane Bazot's team focused on this horizon because, being totally organic, it represents significant pathway for the entry of organic matter into the soil via aerial litter inputs.

Measuring life

The organic horizon contains such a diversity of living organisms and mechanisms at work that its study is complex. The ESE laboratory team had to carry out a large number of characterisations. Measurements of the flow of CO2 released from the soil were used to evaluate the intensity of the soil's biological activity. In addition, the quantity of microorganisms present in the soil was assessed by measuring the carbon and nitrogen concentrations in a sample before and after the microorganisms have degraded, releasing their cellular content into the environment.

The team also carried out qualitative measurements, particularly concerning the thickness and appearance of the humus, noting variations between different plots. They also used the quantitative PCR (Polymerase Chain Reaction) technique, which is used to quantify a specific DNA fragment of bacteria, archaea or fungi, to evaluate the representativeness of these groups within the overall microbial population.

Biodiversity as a guarantee of life

Once all these indicators had been collated, the team used a statistical method to identify which plot density or type of tree population accounted for most of the differences in the parameters studied. They concluded that the choice of tree species plays a major role in soil activity. Plots with mixed oak and pine cover are much more biologically active than pure pine or oak plots.
On the other hand, by varying the density of trees, as in the case of thinning, "we don't see a very clear effect on soil function," notes Stéphane Bazot. However, this other conclusion must be qualified because the thinning was only completed five years ago. "For a forest, five years isn't long," confirms Stéphane Bazot. While thinning does not appear to affect forest soil activity in the short term, the scientists confirm that it is indeed diversity that strengthens life.