Mixed forests outperform spruce plantations in carbon fight
A 24-year field experiment in Slovakia finds that Norway spruce plantations are more vulnerable to drought, pest outbreaks and soil carbon losses than neighbouring mixed forests.
A recently accepted study in Forest Ecology and Management provides strong evidence that mixed-species forests offer more climate-resilient carbon dynamics than single-species plantations dominated by Norway spruce (Picea abies). The findings are based on long-term measurements from two adjacent forest stands in central Slovakia: a 70-year-old managed spruce monoculture and a 150-year-old mixed forest of beech, fir and spruce, growing on identical soils.
The study, led by Dr Michal Bosela and colleagues at the Technical University in Zvolen, involved in the HoliSoils project, highlights important differences in how these forest types respond to drought, disturbance and pest pressure, as well as how they influence carbon cycling both above and below ground.
Key findings
1. Greater sensitivity of spruce to drought
In the very dry summer of 2022, the spruce-only forest grew much less—about half as much as usual—and the soil released much less carbon dioxide, a sign that the forest was under stress. The nearby mixed forest, with spruce, beech, and fir trees, kept growing normally and had only a small drop in soil activity.
One reason for this difference is how the trees handle water. Spruce trees closed tiny pores on their leaves called stomata to save water, but this also stopped them from taking in carbon dioxide, which they need to grow. Beech and fir trees in the mixed forest kept their stomata open longer, allowing them to keep growing even during the drought.
2. Higher mortality and carbon emissions after disturbance
In 2023, the spruce plantation experienced a complete bark beetle (Ips typographus) outbreak, resulting in 100% tree mortality and a full clear-cut harvest in September. Following this harvest, soil CO₂ emissions doubled compared to pre-disturbance levels, with an estimated additional 4 tonnes of carbon per hectare released in the first post-clear-cut autumn.
The mixed forest, despite being located directly adjacent to the affected spruce stand, did not experience a beetle outbreak, indicating greater ecological resistance.
3. Soil microbial communities are more robust in mixed forests
Soils in the mixed forest supported higher microbial biomass and a richer community of ectomycorrhizal fungi, which are important for nutrient cycling and carbon stabilisation. These microbial characteristics contributed to more stable carbon dynamics during and after climatic stress events.
4. Faster recovery of the methane sink function
Soil methane (CH₄) uptake, which briefly collapsed following the clear-cut in the spruce stand, fully recovered within 18 months, but only after diverse ground vegetation re-established. The mixed forest maintained high levels of CH₄ uptake throughout, providing evidence that ecosystem recovery depends not only on trees but also on understorey plant communities and soil conditions.
Soil carbon: An underestimated source of emissions
The study also highlights that soil carbon losses exceeded litter inputs by approximately 50 % in both forest types. This suggests that soils in temperate forest systems may be a more significant and dynamic source of CO₂ emissions than currently recognised in national greenhouse gas (GHG) inventories.
Despite their importance, soil fluxes associated with drought and disturbance are rarely monitored or reported in European GHG accounting frameworks, leading to an underestimation of total ecosystem emissions following harvests or pest outbreaks.
Contrasting forest responses to climate and disturbance
The long-term field experiment in central Slovakia offers a rare and valuable comparison between two neighbouring forest stands. One is a managed Norway spruce monoculture, while the other is a near-natural mixed-species forest. Both stands grow on identical soils and experience the same climate, making them ideal for assessing how forest composition influences ecological resilience. This side-by-side design allowed researchers to observe how each forest type responded to drought, pest outbreaks and disturbance over time.
The table below summarises these key findings, providing clear evidence of the climate resilience benefits associated with species diversity.
Implications for forest policy and management
The results of this study have important implications for how forests are managed in the face of climate change. Norway spruce monocultures, which have long been a staple of Central European forestry, are increasingly vulnerable to drought, pest outbreaks and large carbon losses after disturbance. These risks undermine their long-term role in carbon sequestration and ecosystem stability. By contrast, mixed-species forests demonstrated greater resilience and more stable carbon dynamics under the same conditions.
These findings support a shift in policy toward promoting species diversity as a way to enhance both climate adaptation and mitigation. They also highlight gaps in current greenhouse gas accounting, particularly the underreporting of soil carbon emissions after droughts and harvests.
Turning scientific evidence into action
The study’s conclusions point to an urgent need for strategic and coordinated policy responses that align forest management practices with emerging climate realities. As the risks to monoculture plantations become more evident, particularly in terms of carbon loss and ecological vulnerability, decision-makers are in a position to drive meaningful change through clear targets, funding mechanisms and regulatory updates.
These actions should not only address immediate threats such as pest outbreaks and soil degradation but also support long-term transitions toward more resilient forest landscapes. The recommendations below are grounded in empirical evidence and are intended to guide national and EU-level strategies aimed at enhancing forest resilience.
Recommended actions for decision-makers
- Set national targets to reduce spruce monocultures by at least 20 percent by 2035, especially in low-elevation areas most prone to drought and beetle outbreaks.
- Establish targeted funding schemes to support the transition to mixed forests, including seedling costs and compensation for short-term revenue loss.
- Mandate post-disturbance soil carbon monitoring in updated Forest Reference Levels under the EU Land Use, Land-Use Change and Forestry (LULUCF) regulation.
- Support long-term ecosystem research, including integration of eddy-covariance towers and automated soil gas measurements, to improve understanding of whole-system carbon budgets.
Study reference and contact
Full reference:
Michal Bosela, Peter Marcis, Dominik Polťák, Jerguš Rybár, Peter Fleischer sr, Peter Fleischer jr, Erika Gömöryová, Jaroslav Vido, Paulína Nalevanková, Jaroslav Škvarenina, Marián Homolák, Jozef Výbošťok, Boris Ťupek, Tijana Martinović, Petr Baldrian, Aleksi Lehtonen, Raisa Mäkipää. Norway spruce monoculture has lower resilience and carbon sequestration capacity than a more diverse broadleaved forest: A case study in Central Europe. Forest Ecology and Management, Volume 591, 2025, 122829, ISSN 0378-1127, https://doi.org/10.1016/j.foreco.2025.122829.
Lead author contact:
Dr Michal Bosela
Technical University in Zvolen, Slovakia
Note on site and methods:
The study was conducted in the buffer zone of the Dobroč primary forest in central Slovakia. Both stands were monitored using a combination of tree growth measurements, soil respiration chambers, gas flux analyses, and microbial community assessments. The site offers a unique opportunity to directly compare managed and near-natural forests under equivalent site conditions.
