Does Douglas fir improve the properties of forest soil?
The increased use of Douglas fir in European and Czech forestry has been a topic of discussion for a long time. This is due to its exceptional production potential, both in terms of volume and value, as well as its stabilising function in forest stands. Additionally, its cultivation method is relatively straightforward and comparable to that of Norway spruce.
Photo: Small Douglas fir tree in a sparse spruce stand; the anthill indicates a healthy environment. Author: Jan Řezáč
The potential benefits of introduced tree species, including Douglas fir, are currently a primary focus for European and Czech foresters. This interest is linked to global climate change and the necessity of enhancing the resilience and stability of forest stands.
Scientists from the Czech University of Life Sciences in Prague, specifically from the Faculty of Forestry and Wood Science, conducted a study on the relationship between Douglas fir and soil. They published their findings in the article titled „Zlepšuje douglaska tisolistá půdní podmínky? Srovnání humusových forem se smrkem ztepilým v jižních Čechách“ (Does Douglas fir improve soil conditions? A comparison of humus forms with Norway spruce in South Bohemia), which appeared in the journal Reports of Forestry Research , issue 1, in 2026.
The objective of their research was to evaluate the soil conditions beneath stands of Norway spruce and Douglas fir located on the property of the School Forest District Hůrky, affiliated with the Higher School of Forestry and the Bedřich Schwarzenberg Forestry School (Vyšší odborná škola lesnická a Střední lesnická škola Bedřicha Schwarzenberga). The scientists examined the differences in humus forms between Douglas fir and Norway spruce stands on the same site, which consisted of trees approximately 39 years old. The research site is situated at an altitude of 455 meters above sea level, with an average annual temperature of about 8.8 °C and average annual precipitation of 607 mm, based on data from the Czech Meteorological Institute for the period from 1989 to 2024.
Photo: Forest stand of Douglas fir in a mixture with spruce, oak and beech. Author Jan Řezáč
In the Douglas fir stand, a higher accumulation of overlying humus was recorded, measuring 67.82 t/ha, compared to the spruce stand, which had a lower accumulation of 53.36 t/ha. The research indicated that the soil reaction under the Douglas fir stand was significantly more favourable. Douglas fir plays a vital role in land reclamation in coniferous stands; however, in broadleaf stands, it tends to contribute to a slightly acidifying trend. Based on the results, scientists concluded that Douglas fir litter and its transformation have a positive impact on the chemistry of forest soils when compared to spruce litter.
The Douglas fir stand exhibited a significantly higher content of accessible phosphorus, suggesting richer litter and more rapid humus transformation and phosphorus uptake. Additionally, the content of accessible potassium was significantly higher under Douglas fir. In contrast, the total magnesium content was significantly lower in the spruce stand. Overall, forest soils beneath Douglas fir showed slightly more favourable conditions, characterised by lower acidity and higher macroelement content. However, these beneficial effects should be understood in relation to domestic coniferous tree species.
The risks associated with the cultivation of intensively grown Douglas fir stands include increased nutrient extraction from the soil and alterations in soil nitrogen dynamics. These effects are only slightly reflected in the characteristics of the plant communities (phytocenoses).
While Douglas fir has a more positive impact on the physical properties of the soil compared to spruce, it still has overall less favourable effects on soil health due to its rapid growth and the extensive use of root space for nutrition. However, the root system of Douglas fir significantly stabilises forest stands. Many of the associated risks can be mitigated by planting Douglas fir in mixed stands, with a recommended proportion of this species being between 20% and 40%.
Photo: A young Douglas fir stand mixed with beech. Author Jan Řezáč
In conclusion, the scientists summarised the findings of the case study conducted in South Bohemia, which indicates that Douglas fir has a more favourable effect on forest soils compared to spruce, exhibiting less acidification. Therefore, replacing spruce with Douglas fir can lead to a less negative impact on soil chemistry and macroelement cycles. Additionally, Douglas fir can be a suitable substitute for spruce in the production of high-quality coniferous wood at lower altitudes. Any potential negative effects related to nitrogen dynamics and higher nutrient requirements can be mitigated by cultivating Douglas fir alongside compatible tree species in mixed stands.
However, it is important to approach these conclusions cautiously, as they are drawn from a single investigation at a specific location with a limited number of repetitions. Thus, the results may not apply to other habitat conditions, age categories of stands, or soil types.
The paper „Zlepšuje douglaska tisolistá půdní podmínky? Srovnání humusových forem se smrkem ztepilým v jižních Čechách“ (Does Douglas fir improve soil conditions? A comparison of humus forms with Norway spruce in South Bohemia) can be downloaded here.
Authors: Václav Trojan, Josef Gallo, Ivo Kupka, Česká zemědělská univerzita v Praze, Fakulta lesnická a dřevařská, Katedra pěstování lesů; e-mail: trojanv@fld.czu.cz
Prepared according to the original by Jan Řezáč, VÚLHM, v. v. i., e-mail: rezac@vulhm.cz