Swiss common garden network of future tree species

To find answers to this question the project ‘Swiss common garden network of tree species adapted to future climates’ was started. Within its scope a network of 57 common gardens was established throughout Switzerland. Planted trees will be monitored over 30-50 years to find answers to pressing issues in climate change adaptation. The project will allow verification of results of the research program ‘Forests and climate change’. Its results will help the validation of site-specific tree species recommendations for forest practitioners, and it will create a long-term infrastructure for applied research. The scientific question behind the project is: which factors determine mortality, health and growth of tree species and provenances along large environmental gradients?

The great advantage of this project is the coordinated approach. While a single plantation provides only answers for the specific site, the network allows comprehensive and robust statements on the performance of tree species along large environmental gradients. Thus, this long-term trial will improve our understanding of effects of climate and climatic extremes on the trees species and provenances under investigation. The project is a follow-up activity of the research program "Forest and Climate Change", which FOEN and WSL conducted from 2009 to 2018.

For each tree species seven provenances are tested. In order to keep the size of the common gardens manageable only four out of the seven provenances are assigned to each plantation. While six provenances will be tested in half of the common gardens only, one provenance will be tested as reference in all common gardens a species is assigned to.  This allows for better coverage of the genetic variability within the tree species, while it keeps site sizes manageable.

The provenances were selected in an elaborate process (report ‘Provenance selection and seedling provisioning’). The plant material was procured in cooperation with several Swiss nurseries and foreign partners. Most of the plants are grown at the Emme AG nursery, a few tree species are also grown at the nursery of the Canton of Ticino (Morbio Superiore) and at the WSL experimental nursery.

All common gardens are consistent in their design which allows reliable statistical analyses. The experimental design includes both the distribution of tree species over the common gardens and the arrangement of plants within each site. The design has been worked out in collaboration with researchers and was revised by external statisticians. The 57 common gardens are distributed over all biogeographic regions and altitudinal belts occurring in Switzerland. Every common garden is split into three blocks. The blocks are split in as many plots as there are tree species assigned to a specific site. All tree species will be planted once in every block and assigned randomly to a plot within the block. The number of tree species tested per site and therefore the number of plots and the size of the plantation is variable. Each plot is split into four subplots to which four provenances per tree species are assigned randomly. Finally, every subplot contains nine individual trees.

The area needed for a common garden is given by the number of tree species assigned to this site and by the height of the neighboring stands, which determines the width of the buffer zone needed. The spacing between plants is 2 m, and between plots 4 m. The area needed for a common garden with 4 tree species is approximately 0.75 ha. A plantation with 10 tree species requires approximately 1.3 ha.

To record the local climate, an automated climate station is operated on each experimental site. It measures among other things soil moisture.

Detailed information about the experimental design can be found in the report ‘Versuchsdesign für Testpflanzungen’ (available in German and French only).

In mid-March 2018, the cantons have been invited to propose experimental sites which are meeting the requirements described in the report ‘Site selection for common gardens’ (only available in German and French). From November 2018 to July 2019, the WSL project team visited 125 of the 172 sites proposed by the cantonal forest services. In July 2019, 57 sites were selected and the tree species to be tested on each site were assigned (report `Site selection for common gardens’ (only available in German and French). Later, minor changes were made to the selection of sites and tree species allocation, and two sites were added in the canton of Baselland, while two sites were cancelled in the canton of Bern.

All common gardens are contractually secured in the long term.

The 57 common gardens are distributed over all biogeographic regions and altitudinal belts occurring in Switzerland. Depending on site size, four to 18 tree species are tested in a common garden. On six sites (‘supersites’), all 18 tree species are tested.

Table of all common gardens sorted by number.

Number TS = Number of tree species tested on site

Planting period = Planting period in which the site is established

1 means fall 20 to spring 21, 2 means fall 21 to spring 22, 3 means fall 22 to spring 23.