For the future of forests, it is necessary to have a knowledge about silver fir impact on soil

The silver fir is the most significant coniferous tree species in the natural composition of Central European forests. It plays a crucial role in land reclamation and strengthening efforts. Under the right conditions, it has the highest production potential among native conifers, including larch and spruce.

Photo: Silver fir forest in Central Bohemia, author Jan Řezáč

The decline of fir trees in our forests started in the 18th century and continued into the 20th century, coinciding with the phenomenon known as fir dieback. This decline is attributed to several factors, including the prevalence of clear-cutting practices, which favour artificial regeneration of pine and spruce over fir. Additionally, fir trees have shown increased sensitivity to industrial air pollution, attacks from pathogens such as the Caucasian bark beetle, and pressure from wildlife.

Silver fir is adversely affected by climate change, particularly by decreases in rainfall, changes in soil and air humidity, prolonged droughts, and severe winter frosts. It remains to be seen how the end of the so-called Little Ice Age at the turn of the 19th and 20th centuries, when the decline of fir trees became more pronounced, impacted their overall decline.

Photo: Norway spruce forest in Central Bohemia, author Jan Řezáč

There is a growing interest in silver fir, and its presence in forests is gradually increasing, from 0.9% in 2001 to 1.2% in 2020. However, this figure remains significantly below the recommended target of 4.4%.

One of the main reasons for the rise in silver fir representation in our forests is its incorporation into forest management plans as a suitable tree species for habitat improvement and strengthening. Forestry practices recognize the stabilizing role of silver fir, which is particularly effective compared to spruce.

There is limited knowledge about the role of silver fir in improving conditions within spruce stands, particularly regarding its impact on soil quality, including microbiological activity and soil chemistry. Previous research has not shown a significant influence of silver fir, only noting partial improvements in certain soil characteristics.

Understanding the effect of silver fir on soil health is crucial for the future of forests. To address this, a team of scientists from the Forestry and Game Management Research Institute and the Czech University of Life Sciences in Prague investigated this matter as part of the NAZV QK1910292 project titled “Procedures for Supporting Silver Fir in the Forest Management of the Czech Republic.” (Postupy pro podporu jedle bělokoré v lesním hospodářství ČR).

Photo: Norway spruce forest with European beech gap, Central Bohemia, author Jan Řezáč

The results were published in the paper Srovnání stavu půd pod porostními skupinami jedle bělokoré a smrku ztepilého (Comparison of soil conditions under the growth groups of silver fir and Norway spruce), which was published in the journal Reports of Forest Research 1/2024.

The aim of the research was to document the influence of silver fir on the state of humus forms in a mixed spruce-fir stand located in the area of ČZU Forests in Kostelec nad Černými lesy. It was also tested the hypothesis that fir would significantly contribute to the improvement of pedochemical soil characteristics and demonstrate higher activity in cellulose decomposition.

The scientists compared three types of forest stands: a mature spruce stand that is 102 years old, a mature fir stand that is 105 years old, and a beech gap, that is 50 years old. The results highlighted not only the differences between fir and spruce but also the impact of establishing broadleaf stands.

In conclusion, the scientists state that their research has confirmed the limited impact of silver fir on soil characteristics compared to spruce. However, there is a slight influence on the improvement of humus forms, particularly due to the fir’s more efficient use of root space and the creation of slightly more favourable litter. This is reflected in the enhanced soil chemistry, notably the increase in base content under groups of silver fir.

Photo: Silver fir forest in Central Bohemia, author Jan Řezáč

The scientists summarized their findings as follows:

• The stock of holorganic horizons decreased in the order of spruce, fir, and beech, indicating an increased intensity of decomposition and transformation activities.
• In the beech stand, there was a noticeable but insignificant decrease in the thickness of the F2+H and Ah horizons.
• The soil reaction in both the spruce and fir stands was similar, while it was more favourable in the beech stand.
• Fir did not significantly improve the state of the soil sorption complex, unlike beech.
• The content of total (oxydizable) carbon was slightly higher in the mineral soil horizons under fir, and the content of total nitrogen also showed a slight increase in these horizons.
• The carbon-to-nitrogen ratio was only slightly more favourable under fir.
• The levels of accessible phosphorus and potassium were more significantly increased under fir compared to spruce, while the content of accessible calcium remained the same.
• The values of exchangeable acidity and exchangeable aluminium were more favourable under fir in comparison to spruce.
• Significant differences in the content of total nutrients were not documented, except for higher levels of total phosphorus under fir.
• The potential for cellulose decomposition was unaffected by the change in species composition.

Photo: Norway spruce forest with European beech gap, Central Bohemia, author Jan Řezáč

In the studied forest stands, no significant land reclamation effects of silver fir were observed. Additionally, the comparison between coniferous tree species and beech did not yield clear results, although the land reclamation effect of beech was evident. The habitat is nutrient-stressed, suggesting that individual trees likely exhibit selective uptake of nutrients.

The paper Srovnání stavu půd pod porostními skupinami jedle bělokoré a smrku ztepilého (Comparison of soil conditions under the growth groups of silver fir and Norway spruce) can be downloaded here.

Authors: Vilém Podrázský, Ivo Kupka, Jitka Staňová; ČZU v Praze, e-mail: podrazsky@fld.czu.cz

Prepared according original by Jan Řezáč, VÚLHM, v. v. i., e-mail: rezac@vulhm.cz