Assisted migration is essential for future European forests
Europe’s forests are already suffering the impacts of the climate emergency. A new FORGENIUS-funded paper from a group of forestry researchers predicts that assisted migration — moving species into areas newly suitable for them — will be essential to reduce the decline in forest ecosystem services. Without such help, the future looks even more challenging.
This paper, published in Global Environmental Change, built a new modelling framework from location data for 66 species in the EU-Trees4F database, together with information on forest ecosystem services derived largely from the European Atlas of Forest Tree Species. This offered values for each species under seven different services: construction, culture, food, fuelwood, medicine, pulpwood paper, and soil protection, and could be used to construct an average index of services. These were assessed under 11 different regional climate models under two conditions.
Natural dispersal
By 2100, the suitable range for most species will have shifted, but without human intervention most species will be unable to colonise new areas because the pace of climate change outstrips the speed of dispersal of the species. Overall, tree species diversity will decline throughout Europe, apart from a few restricted areas in the Boreal region and in some parts of the Alps and Pyrenees.
As a result of the general decline, the provision of ecosystem services will also drop, on average by 15%. Across the area under study, about 12% would suffer a loss of 50% or more in forest ecosystem services. Most of this area is in the Mediterranean region, where average losses will be around 52%. The Mediterranean region will be most affected by the loss of fuelwood and soil protection, while in the Boreal region all services will increase, and especially construction, cultural services and soil protection.
Given the generally pessimistic predictions, it makes sense to consider assisted migration as an essential part of forest management for the future.
Assisted migration
The model indicates that the predicted loss of local diversity and services is driven not by climate but by dispersal. Indeed, most areas will have no shortage of well-adapted potential candidate species to replace those lost. With that in mind, the researchers examined three different strategies to select appropriate candidate species. The most realistic they call multi optimisation, where replacements are chosen by an algorithm that searches all possible species combinations to optimise provision of all services while also trying to preserve structural consistency and functional traits of the existing forest.
This multi optimised strategic approach reduces the expected losses across much of Europe, even reversing them in some places. On average, losses will be reduced by 10%, but distribution is uneven. In some sense the Mediterranean benefits most, with losses reduced by 19%. Nevertheless, the researchers caution that even so, the region is looking at a decline of 33% over the century compared to current forests. In the Alps, a multi optimisation strategy converts an 8% loss into a 1% gain, while in the Boreal region the 6% gain expected even with only natural dispersal is increased by another 1% with assisted migration.
Towards 2100
The model’s predictions are clear: only assisted migration, and especially with a strategy to optimise for multiple services, can mitigate the effects of rapid climate change on European forests. This, the researchers say, raises further questions and should guide policy. Will there be enough planting material of suitable species? And how will trans-border trade be adjusted to control and track increased flows of diverse planting material?
Europe is committed to planting three billion trees by 2030. These results could help to decide which species should be planted where, with an eye on environmental suitability some time hence. Other policies, such as the EU Bio-Economy strategy, the EU Biodiversity strategy 2030, the EU Forest Strategy, and goals for carbon sequestration, all depend on future reforestation and afforestation.
The scientists recognise that this research is, at heart, a modelling exercise, and as such is intended largely to inform future choices. Furthermore, the model’s predictions are consistent under the various climate models and future scenarios, making the advice more valuable.
“The purpose of our work is not to design tree species communities that are realistic,” said Alessandro Cescatti of the European Commission Joint Research Centre in Italy, one of the authors of the study. “However, our analysis does reveal the critical role that assisted migration can play in maintaining forest ecosystem services in the future, and the risk that discarding assisted migration as a management option holds.”
Some people object to assisted migration because they believe it interferes with natural ecosystems. The researchers point out, however, that only about 2% of European forests are classified as undisturbed. The vast majority have some sort of human management in their history. Assisted migration offers the best hope to enable that history to continue.
Full reference
Achille Mauri, Marco Girardello, Giovanni Forzieri, Federica Manca, Pieter S.A. Beck, Alessandro Cescatti, Giovanni Strona, Assisted tree migration can reduce but not avert the decline of forest ecosystem services in Europe, Global Environmental Change, Volume 80, 2023, 102676, ISSN 0959-3780, https://doi.org/10.1016/j.gloenvcha.2023.102676