Rethinking common gardens: how a new European network will test forest trees for a changing climate
A new study sets out the design behind MOSAIC, the OptFORESTS project network of 29 common gardens across Europe, built to test how tree species and populations respond to future climates.
For decades, researchers have planted trees from different origins side by side in "common gardens" to compare how they perform. Most of these trials, however, focused on commercially important species such as beech, oaks and pines, and measured growth rather than climate adaptation and resilience. Few included stressful test environments. As climate change reshapes Europe's forests, how can common gardens tell us which trees will cope with tomorrow's conditions?
In a new OptFORESTS study, Rethinking common gardens for forest tree species in the face of climate change (Westergren et al., 2026), researchers propose a step-by-step framework for designing a new generation of common gardens. They then apply this framework to MOSAIC, the project's own common garden network, which includes:
- 29 common gardens in 17 European countries and seven biogeographic regions, from Boreal to Mediterranean.
- 12 forest tree species, selected through stakeholder consultation, and 90 provenances.
- Populations from both the core and the margins of each species' range, tested at sites that include suboptimal conditions near or beyond the species' current climatic limits.
To choose the layout, the researchers simulated three experimental designs and ran power analyses to check that the chosen design would deliver statistically robust results.
What did the study find?
A small plot of land can still produce reliable results, as long as the trees are arranged cleverly. Each MOSAIC garden holds 1,600 young trees from five species. Every species has its own patch, and within each patch the trees are planted in small groups of four, each group grown from seed collected in a different place. Each species comes from eight places: four with typical conditions at the heart of its range, and four with marginal conditions, mainly at the edges of its range, where growing conditions are tougher. This pattern is repeated ten times at every site to account for natural variation within the site. Scots pine and sessile oak are planted across the whole network, although at two sites in Finland a close relative, pedunculate oak, is used instead for legal reasons.
Because of this arrangement, many trees grow right next to trees of the same species but from a different origin. This lets researchers see how trees of different origins perform when grown together. Other networks, such as TREEDIVNET, study mixtures of different species, but not mixtures within a single species. The design is also sensitive enough to detect real differences: a single garden can reveal moderate differences between trees of different origins, and combining just six gardens can pick up even small ones.
The researchers also deliberately planted some gardens in tough places, near or beyond the limits of the conditions a species normally grows in. For Scots pine, these are warmer and drier sites. For sessile oak, they also include colder and wetter ones. Such sites have to be chosen carefully, but they reveal things that comfortable sites cannot. Even if every tree at a site dies, that shows where a species or population reaches its limits. Tough conditions can also bring out hidden genetic differences between trees that only appear in such environments and may matter for adapting to climate change.
Why does this matter?
Through frameworks such as the UN Sustainable Development Goals, policymakers in Europe and beyond are calling for forests that are more diverse and more resilient to climate change. Meeting this goal requires a thorough understanding of how species, populations and their mixtures will respond to changing climatic conditions.
Because trees respond to climate differently at different life stages, with seedlings and mature trees reacting in different ways, long-term experiments are needed to follow trees over many years, with consistent monitoring across all sites. Accordingly, while each MOSAIC partner manages its own gardens, the network is coordinated through a memorandum of understanding, common monitoring protocols and a shared central database.
Key take-home messages:
- Traditional common gardens have mostly focused on commercial species and growth traits rather than on climate resilience.
- New-generation networks should include understudied species, marginal populations and harsh sites.
- The MOSAIC network tests 12 species and 90 provenances in 29 gardens across Europe.
- A compact split-plot design delivers statistically robust results while remaining feasible in terms of land and cost.
- The trees planted today will show which forests can thrive tomorrow.
The study was published in Annals of Forest Science. You can read it in full here.
Full reference
Westergren, M., Streit, K., van Loo, M., Alía, R., González-Martínez, S. C., Benito-Garzón, M., Lario Leza, F. J., Climent Maldonado, J. M., Mihai, G., & Myking, T. 2026. Rethinking common gardens for forest tree species in the face of climate change. Annals of Forest Science, 83, 40.
