Are there major changes in forest soils following calamitous logging?

The bark beetle crisis that the Czech Republic and neighbouring countries experienced from 2018 to 2023 is now subsiding. Foresters and forest owners are now confronted with new challenges related to the reforestation of cleared areas. They must care for trees that have been established through both artificial and natural regeneration. A significant challenge they face is the cultivation of new saplings that are beginning to emerge in the areas affected by the disaster.

Photo: Clearing due to salvage felling in Horní Radouň, author Radek Novotný

Large-scale disasters have affected Czech forests in the past, particularly in the last two decades and throughout the 20th century. These forests have suffered damage from wind, snow, frost, and insect infestations, including bark and leaves damaging insects. In such cases, it has been necessary to reforest the cleared areas and focus on protecting and nurturing new stands. There is existing knowledge on establishing new stands in disaster-affected areas, which includes the use of both artificial and natural regeneration methods, as well as their combinations.

During the most recent large-scale disaster, scientists and foresters faced an important question regarding the condition of the forest soil. Concerns included the risk of rapid decomposition and loss of organic matter, which could lead to the release of carbon that was previously stored. There were also worries about the potential activation of harmful elements bound within organic matter, changes in the forest soil’s ability to retain water, and how these factors could affect the chemistry of the soil. Additionally, the impact of these changes on the nutrition of newly emerging and established stands with diverse species compositions was a significant topic of discussion.

Photo: Decomposition experiment, author Radek Novotný

A team of scientists from VÚLHM, v. v. i., the Czech University of Life Sciences (ČZU) in Prague, and Mendel University (MENDELU) in Brno focused on several questions in their project NAZV No. QK22020217, titled “Changes in forest soils after catastrophic logging: the impact of deforestation on carbon sequestration, nutrient balance, and mobility of risk elements” (Změny v lesních půdách po kalamitní těžbě – vliv odlesnění na sekvestraci uhlíku, bilanci živin a mobilitu rizikových prvků). Almost three years of field and laboratory work during the project solution from 2022 to 2024 have yielded valuable insights.

One fundamental finding is that the loss of organic matter caused by the more intensive decomposition (mineralization) of surface humus in open areas following catastrophic logging does not occur quickly enough to result in a harmful loss of the forest soil’s physical or chemical properties by the time the soil surface is re-covered. This re-covering is primarily by herbaceous and shrub vegetation or by woody plants through natural or artificial regeneration. Additionally, the presence of some logging residues in the surveyed areas helps maintain the necessary proportion of organic matter in the forest soil.

When converting coniferous forests to deciduous forests, we can expect some reduction in organic matter stock since the humus layer in deciduous forests is generally thinner than that in coniferous forests. However, the organic matter content may increase in the deeper parts of the soil profile because deciduous trees typically have a greater rooting depth.

Therefore, the total amount of carbon sequestered in the entire soil profile may be comparable to or even higher in mixed and deciduous forests than in coniferous forests, depending on the soil depth.

Photo: Samples collection in the clear-cutting area, author Radek Novotný

Deforested areas that are exposed to intense sunlight and heat can become sources of carbon dioxide (CO₂) due to increased microbial activity in the soil, known as soil biota respiration. In these areas, the balance shifts from carbon storage to carbon emissions, primarily in the form of CO₂.

While this shift is typically temporary and lasts from a few years to a few decades, it is crucial to shorten this period through active management. This approach can help the forest gradually resume its essential functions, including its capacity to remove carbon dioxide from the atmosphere.

Therefore, catastrophic clearings may not significantly harm soil carbon stock if: 1) enough organic material is left in the area for decomposition, and 2) these areas are reforested quickly enough.

With the changing climate and its anticipated negative impacts and potential risks, it is essential to focus on cultivating stable, healthy, and resilient mixed forests. Ideally, we should encourage more multi-storey stands that emphasize the diverse functions and benefits that forests provide. The priority function of a forest may shift depending on the region and habitat.

While conclusions from existing scientific studies are not always unanimous, the prevailing view is that active management—aimed at enhancing soil quality and the diversity of cultivated stands—tends to promote greater carbon sequestration compared to allowing commercial forests to develop spontaneously.

Photo: Clear cutting area afforested by oak, author Radek Novotný

The scientists summarized their findings as follows:

• There were no sudden changes in soil chemistry in the deforested areas. The pH values and the levels of basic nutrients (calcium, magnesium, potassium) in the humus layer and mineral soil, up to a depth of 20 cm, showed a slight increase. In the top 10 cm of the mineral soil, the concentrations of carbon and nitrogen also rose, primarily due to the mixing of the mineral layer with humus and an increase in organic material (such as mining residues and humus).
•  No significant changes were observed in the humus layer stock or the carbon or nitrogen stocks in the overlying horizon (FH).
• There was no significant mobilization of risk elements in most of the monitored areas; however, continued monitoring is necessary for areas heavily impacted by past mining and metallurgical activities.
• No changes in the proportions of risk elements in the soil (such as arsenic, cadmium, copper, lead, and zinc) were found between stands and clearings. Additionally, there was no observed effect from different clearing management methods (like stacking brushwood in piles versus milling it) on the distribution of these risk elements.
• Significant differences in the quality of organic matter were found only in the FH layer between stands and clearings. This could affect the physical properties of the soil, the sorption capacity of organic matter, and the mobility of nutrients and risk elements, which could lead to their easier transport to deeper soil layers.
• Accelerated mineralization of organic matter in disturbed areas is indicated by changes in the concentration of water-extractable organic carbon (DOC), which serves as an indicator of the mobile fraction of soil organic matter. The observed decrease in DOC in clearings, by about 30% compared to areas with standing vegetation, can be attributed to the increased mineralization of soil organic matter. DOC is the fraction of soil organic matter most susceptible to changes in the soil environment.

For details about the project and the results achieved, please visit the VÚLHM website here.

For more information, contact Ing. Radek Novotný, Ph.D., VÚLHM, v. v. i., email: novotny@vulhm.cz