Denmark has set an ambitious goal of achieving net-zero greenhouse gas emissions by 2045. Afforestation, planting new forests, is one of the cheapest and most effective ways to get there. But what if we plant the wrong trees? What if climate extremes undo decades of progress? These are the questions that are driving a new research project led by Professor Klaus Butterbach-Bahl and Postdoctoral Researcher Xiaoyu Cen from the Land-CRAFT Pioneer Center at Aarhus University. 

Funded by the Independent Research Fund Denmark (DFF), the project seeks to predict the climate change mitigation potential of new forest landscapes. Its goal is simple yet profound: to ensure that every tree planted helps cool the climate instead of warming it. 

The challenge: more than just planting trees 

Forests are powerful carbon sinks. They absorb CO₂ and store carbon in woody biomass and soil while providing other ecosystem services such a recreation and clean water. Today, forests cover about 15% of Denmark’s land area, offsetting emissions from other land uses. As part of the Green Transition, an additional 250,000 hectares will be afforested over the next two decades. 

But the climate benefits of afforestation are not guaranteed. “The net greenhouse gas balance of a forest changes over time,” explains Xiaoyu Cen. “In the early years, forests can be net sources of CO2 as decomposition of soil organic matter may be higher as forest litter input. Later, they become strong carbon sinks, but only if species, soil, and management is right.” 

Legacy effects from agriculture complicate the picture. Former croplands often have high nitrogen levels, which can lead to nitrous oxide emissions: a potent greenhouse gas that offsets carbon gains. Add to this the uncertainty of future climate extremes, and the question becomes urgent: Which forests will thrive and deliver the greatest climate benefit under changing conditions? 

The research: modelling the future 

To answer this question, the team will gather information on the age and composition of forests in Denmark and neighboring countries, as well as regions earmarked for potential afforestation. They will then use this information to calibrate the LandscapeDNDC model, a sophisticated biogeochemical model, with decades of data from Danish tree species trials and long-term monitoring sites. Then, the model will be used to simulate the carbon and nitrogen dynamics of forests under varying climate and environmental conditions. 

The project will test three hypotheses:

1. Agricultural legacy has a decade-long impact on the greenhouse gas and nitrogen balances of new forests.
2. Climate-resilient species, those with high adaptability, minimise the risk of forest loss in extreme years.
3. Periodic harvesting, replanting and storing carbon in wood products can yield high ecological and economic benefits compared to no harvesting. 

Using these insights, researchers will run scenario analyses combining different tree species, management strategies, and climate projections ranging from low-emission futures to high-impact warming scenarios.

From research to policy 

This is not just an academic exercise. The findings will inform Denmark’s afforestation strategy and help policymakers make evidence-based decisions. “Our goal is to show to the public the benefits of converting farmland to forests,” says Klaus Butterbach-Bahl. “By demonstrating the climate and nitrogen benefits of different strategies, we can foster trust and guide the Green Transition.” 

The project’s outputs will include: 

• Spatially explicit recommendations for which species to plant and where.
• Science-based strategies for balancing ecological resilience and economic viability.
• Improved models for national greenhouse gas accounting. 

For farmers and landowners, this research could mean clearer incentives and better planning tools. For society, it means forests that effectively mitigate climate change in the long run. 

Voices from the Lab 

”Afforestation is often seen as a simple solution - just plant trees. But the reality is complex,” says Klaus Butterbach-Bahl. “Our job is to make that complexity manageable, so that Denmark can invest in forests that benefit the climate, biodiversity, and people.” 

Xiaoyu Cen adds: “We’re not only predicting numbers. We’re designing landscapes for the next century, landscapes that can withstand storms, droughts, and protect our soils.” 

Looking ahead 

By 2030, the team aims to deliver actionable recommendations for Denmark’s afforestation plans. By 2045, their work could help secure the country’s net-zero target. And by 2100? If they are successful, Denmark’s forests will stand as living proof that research and nature can work hand in hand to combat climate change. 

More information 

Project: Predicting the climate change mitigation potential of new forest landscapes in Denmark 

Funding: Independent Research Fund Denmark (DFF) Project period: April 2026 – March 2031 

Institution: Aarhus University, Department of Agroecology – Land-CRAFT Pioneer Center Project leader: Professor Klaus Butterbach-Bahl Participants: Xiaoyu Cen (postdoc), Jesper Riis Christiansen, Per Gundersen, Rüdiger Grote

Objective: To predict the climate and nitrogen benefits of different afforestation strategies in Denmark under future climate scenarios. 

Method: Combining field measurements, long-term tree species trial data, and advanced modeling using LandscapeDNDC. 

Contact: Professor Klaus Butterbach-Bahl, Department of Agroecology, Aarhus University. Tel.: 93508238 or mail: klaus.butterbach-Bahl@agro.au.dk 

Postdoctoral Researcher Xiaoyu Cen, Department of Agroecology, Aarhus University. Mail: xcen@agro.au.dk