# Experimental plantations of tree species adapted to future climates

Between 2020 and 2022 a network of experimental plantations of tree species adapted to future climate will be established throughout Switzerland as a joined effort of the Federal Institute for Forest, Snow and Landscape Research (WSL), the Federal Office for the Environment (FOEN), cantonal forest services, forest managers, professional organizations and forest owners. The experimental plantations will be monitored for several decades and will allow insights in the suitability of tree species across broad environmental gradients. In July 2019, the project team at WSL has selected 57 sites and communicated the selection to the cantonal forest services and to the local forest managers. Now last arrangements follow to prepare the first third of sites for the first plantation period in autumn 2020.

A warmer climate with drier summers will affect the climatic suitability of tree species in their current habitat and, subsequently, future ecosystem services. On many forest sites other tree species than those in place will grow better towards the end of the 21st century. While some tree species already grow on sites where the future climate is supposed to suit them, others are currently still absent.

In this context foresters are exposed to the following question:

Which of the tree species supposedly adapted to the climate expected on a given site towards the end of the 21st century can already establish and grow there today?

To find answers to this question the project ‘experimental plantations of tree species adapted to future climates’ was started. Within its scope a network of 50-60 test plantations will be established throughout Switzerland and 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?

Two kinds of experimental plantations can be established. Experimental plantations with fixed design build the core of the project and allow sound scientific analyses, based on a consistent and strict experimental design. Experimental plantations in the so-called flexible design allow testing additional tree species and provenances which may be of special interest to some forest owners and foresters.

## Tree species

The tree species to be tested in the fixed design were chosen taking into account the considerations of cantonal stakeholders and experts in a multistage procedure. While 9 tree species belong to the core set and will be planted in approximately 35 sites, another 9 tree species belong to the extension set and will be planted in only approximately 15 sites. The process of tree species selection is reported in ‘Baumartenwahl für Testpflanzungen’ (only available in German and French).

The 18 tree species under investigation comprise many of the species which are probably suitable in future climates. For tree species of the core set reliable results will be generated to analyze and compare their performance over a broad environmental gradient, while for tree species of the extension set the explanatory power will be smaller.

Core set (9 tree species)Extension set (9 tree species)
Abies alba (silver fir)
Acer pseudoplatanus (Sycamore maple)
Fagus sylvatica (European beech)
Larix decidua (European larch)
Picea abies (Norway spruce)
Pinus sylvestris (Scots pine)
Pseudotsuga menziesii (Douglas fir)
Quercus petraea (sessile oak)
Tilia cordata (small-leaved lime)
Acer opalus (Italian maple)
Acer platanoides (Norway maple)
Cedrus atlantica (Atlas cedar)
Corylus colurna (Turkish hazel)
Juglans regia (Persian walnut)
Prunus avium (wild cherry)
Quercus cerris (Turkey oak)
Quercus robur (pedunculate oak)
Sorbus torminalis (wild service tree)

## Design

In order to allow for reliable statistical analyses, all experimental plantations must be consistent in their design, not only when it comes to the distribution of the sites and to the attribution of the tree species to the sites, but also regarding the planting scheme within an individual plantation. The design has been worked out in collaboration with researchers and revised by external statisticians. 50-60 test plantations will be distributed over all ecoregions and altitudinal levels throughout Switzerland and monitored over 30-50 years. Every experimental plantation is split in 3 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. Each plot is split in 4 subplots to which 4 provenances per tree species are assigned randomly. Finally, every subplot contains 9 individual trees. The big advantage of this project is the coordinated approach which allows analysis of the tree species’ performance over broad environmental gradients.

The area needed for a test plantation is given by the number of tree species assigned to this site, as well as by the spacing between plants and between plots and further by the height of the neighboring stands, which determines the width of the buffer zone needed. The spacing between plants is 2 m, between plots 4 m. The approximate area needed for a test plantation with 4 tree species is approximately 0.75 ha. If 10 tree species are tested it amounts to approximately 1.3 ha.

For each tree species 7 provenances will be tested. In order to keep the size of the test plantations manageable only 4 of the 7 provenances are assigned to each experimental plantation. While 6 provenances will be tested in half of the experimental plantations only, one provenance will be tested as reference in all test plantations a species is assigned to.  This allows for better coverage of the genetic variability within the tree species, while it keeps experimental plantation sizes manageable.

For test plantations in the flexible design we suggest to use the same experimental design to enable comparisons with the tree species and provenances of the fixed design. This is, however, not mandatory.

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

## Conclusions and outlook

The big advantage of this project is its coordinated approach. Single plantations allow statements on the performance of tree species and provenances on the very site they are planted.  A network of 50-60 experimental plantations adhering to a consistent experimental design, in contrast, allows for insights on the long-term performance of tree species in different environments. This long-term project will therefore help us to better understand how the tested tree species and provenances respond to climatic factors, including extreme events.

In mid-March 2018 the cantons have been invited to propose suitable sites for the project. These sites should meet the minimal requirements described in the report ‘Auswahl der Versuchsflächen für Testpflanzungen’ (only available in German and French). From November 2018 to July, the WSL project team visited 125 of the 172 areas 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 Selection of sites for test plantations(only available in German and French).

In autumn 2019, the FOEN and the cantons will renegotiate the financing of the establishment and maintenance of the test plantations in accordance with the Handbook PV 2020-2024 LI 4.2 ‘Regeneration Observation Sites". In addition, the selected areas will be contractually secured so that the first third of the selected sites can be prepared for the first planting period in autumn 2020. The plants are currently being cultivated.

2017 - 2038