Time to get serious about forest soil

Originally published in Publico.es in Spanish.

European legislation on forests and climate and the forthcoming legislation on soil quality place particular emphasis on the importance of the soils that support our forests. These soils store more carbon and biodiversity than the trees they feed and are full of water and nutrients that provide a myriad of services. The European Union warns that, in general, degraded soils reduce the provision of ecosystem services such as food, timber, nutrient cycling, carbon sequestration, pest control or water regulation – a loss that costs the EU billions of euros every year. After the degradation of urban, industrial and agricultural soils, forests can be a lifeline against impacts such as climate change.

In the forests of Gamiz, a council of the city of Vitoria (Álava) located in the foothills of the Vitoria Mountains, work is being carried out as part of what is considered “the largest research project in Europe on the central, though little understood, role of forest soils in the fight against climate change”. This is what is stated in HoliSoils, an acronym for Holistic management practices, modelling & monitoring for European forest soils, a project of the EU’s Horizon 2020 programme in which 20 European universities and research centres are participating, including four Spanish ones.

The work being carried out in Gamiz includes that done by Jorge Curiel, the principal researcher in Spain at HoliSoils, who also leads the working group dedicated to the vulnerability of soils to climate change. He stops for a moment to talk to us: “We have three plots with a common design where we test, on the one hand, how different types of wood extraction affect the soil, such as 50% thinning and clear-cutting – cutting down all the trees in a forest area – and, on the other, how we can accelerate soil recovery with two treatments: in one we leave the soil bare and in the other we use the remains of the cutting, which are broken up and spread to create a layer that protects it and accelerates its recovery”.

An important part of this work is to analyse the carbon sequestration achieved by forest management taking into account different soil treatments. Forest soils store more carbon than trees, thanks to the combined action of dead wood, litter and the innumerable associated fauna, flora, and fungal communities, including other important micro-organisms such as protists, bacteria, and archaea.

Forest soils: champions of carbon sequestration

Gonzalo Almendros, research professor at the Department of Biogeochemistry and Microbial Ecology of the National Museum of Natural Sciences (MNCN/CSIC), highlights this value in figures: “In general, soils contain 2,500 petagrams of carbon (one petagram is equivalent to one thousand trillion grams), that is, more than three times the amount of atmospheric carbon and four times the amount stored in the biomass of plants and animals. Therefore, the interest in forest soils lies in the fact that they constitute a reservoir of carbon in slowly biodegradable forms, which is not actively exchanged with CO2 in the atmosphere”.

But there is a risk that climate change will accelerate this loss of soil carbon, as Pablo García-Palacios, a scientist at the Consejo Superior de Investigaciones Científicas at the Instituto de Ciencias Agrarias (ICA-CSIC), pointed out in a study published in June 2021 in the journal Nature: “Until now, the size of the carbon pool has been balanced annually between carbon losses from soil respiration and gains from carbon fixation by plants. However, anthropogenic warming is disturbing this balance”.

HoliSoils, like other European projects working in the same direction (DrySom, Benchmark, Atlantis, etc.), seeks to halt the loss of valuable forest soil, which is why not only research centres are involved in it, but also associations of forest owners, the European Environment Agency, the United Nations Convention on Climate Change and the FSC forest certification seal. Dozens of forest plots in various countries with different climates, tree species and management are serving as test laboratories.

Undergrowth in Peñacaballera, Sierra de Béjar, Salamanca. Photo: Javier Rico.

Every soil is a world

Each soil is different depending on all these factors: climate, altitude, trees, type of management, slope… There is nothing like taking a look at some of the almost 2,900 entries on the blog Un universo bajo nuestros pies (A universe beneath our feet) to make sure of this. It is the work of Juan José Ibáñez, senior scientist at the Desertification Research Centre (CSIC-University of Valencia), who constantly repeats that “soil is a very fragile and non-renewable resource”. There are soils dominated by leaf litter, in others by the herbaceous stratum, in others by scrub, and in some there is more moss, or rocky and stony ground, or a bit of everything. Ibañez even claims the value of dead wood: “The fall of a tree is essential for the maintenance of the dynamics of forest ecosystems and their soils”.

In this variety of soils and their composition also lies the ability to adapt to impacts such as those induced by climate change. Ana Rey, MNCN/CSIC scientist and expert in forest ecology, says: “Preliminary results of a study we are carrying out in Mediterranean forests on different types of soils indicate that forests with soils that are in principle more unfavourable seem to be less susceptible to drought, because they have already developed an adaptation to extreme conditions. Therefore, although they grow less, they are more resilient”. According to Jorge Curiel, “in the Iberian Peninsula, the most fragile forests are on steeper slopes, with more erosion potential, which increases depending on the type of management they have”.


Curiel is also a research professor at one of the centres participating in HoliSoils, the BC3 (Basque Centre for Climate Change). He says that all the work carried out (types of felling, soil treatments, placement of sensors, analysis of carbon sequestration, inventory of organisms, measurement of soil health parameters) “serves, first, to generate evidence that what is being done so far is not correct and, second, to generate more responsible management practices focused on soil conservation”.

Time to get serious about forest soil

Raisa Mäkipää, coordinator of HoliSoils and research professor at the Natural Resources Institute of Finland, stresses: “Soils really matter, but they have not been studied enough, especially forest soils. By learning more about how they trap and release CO2, countries can help ensure that their forests can adapt to and mitigate climate change”.

In the meantime, the EU has also got down to work. On 5 July 2023, the European Commission (EC) presented the proposal for a Directive of the European Parliament and of the Council on soil monitoring and soil resilience. The main objective is to achieve a healthy state of EU soils by 2050. It recognises that soil degradation costs us tens of billions of euros per year by affecting basic ecosystem services that provide important economic benefits: food, timber, carbon sequestration, pest control, erosion control, etc.

The variety of soils and their composition also determines the capacity of forests to adapt to impacts such as those induced by climate change. Photo: Javier Rico.

Without departing from the economic line, the proposed directive mentions: “The availability of healthy and fertile soils and land is crucial for the transition to a sustainable bioeconomy and can therefore help to increase and preserve land values”. In an article in The Conversation, Jorge Curiel and four other BC3 researchers stress that “the bioeconomy should take into account forest soil conservation”, but he warns: “We cannot expect the bioeconomy to cover all the production and business niches that are currently covered by other materials such as concrete or petroleum derivatives because, like these, natural resources are not infinite”.

One of the mainstays of the forest bioeconomy is the extraction of biomass for energy production. The Spanish Biomass Association (Avebiom) they state: “The companies that carry out works of exploitation and extraction of this biomass must comply with the requirements imposed by the EU Renewable Energy Directive, and they are doing so, many of them with the help of the European certification system SURE (Sustainable Resources Verification Scheme)”. In addition, they consider that removing biomass from forests for energy recovery “contributes to increasing the resilience of our forests to increasingly dangerous forest fires and climate change. Keeping the destructive power of fires and forest stress at bay is also a way of caring for soils and biodiversity”.

No planting for planting’s sake

Ana Rey considers how we should deal with reforestation very important: “When we talk about reforestation, about planting trees, we must remember that a forest is not just trees, because sometimes we ignore the complexity of forests and the many ecosystem services they provide, including the great importance of the soil. We have to take into account the type of soil we plant in and the species we plant”. Almendros stresses: “Each situation has to be studied separately, taking into account climate, soil type and the quality of pre-existing organic matter.”

Much remains to be done. The EC itself, in its proposal for a directive on soil monitoring and resilience, estimates: “More than 60% of European soils are unhealthy and scientific evidence shows that they are being further degraded due to unsustainable land management, pollution and overexploitation, combined with the impact of climate change and extreme weather events”.