Urban Forest: the urban forest, testing forest tree adaptation to climate change (UFO)

Context and challenges

Faced with the speed and intensity of current climate change, species must adapt, migrate or die. Trees are characterised by long adaptation times and low migration rates (IPCC 2014), which do not allow them to keep up with climate change. That is why many mortality phenomena are currently being observed in the world's forests. In France, massive mortalities have recently been observed following years of severe summer droughts (2018-2020, DSF), and white oaks (Quercus robur, Quercus petraea), a major species of French forests, both in forest, rural and urban environments, are waning.

Specific and genetic diversity is key to the adaptation of forests to climate change, and assisted migration has long been known to help achieve it. Since 2017, the ONF (French national forestry office) has been setting up "islands of the future", comparative trials of species and provenances aimed at identifying the forest species of the future. Moreover, the urban forest is characterised by a high specific diversity (e.g. 410 species in Bordeaux, 230 in Lyon, 707 in Paris) whose trees, at the adult or sapling stage, form a life-size test of adaptation to climate change. Indeed, in urban areas, the average temperature is already higher in city centres than in outlying rural areas (+0.7°C annual average, IPCC 2021), reaching or exceeding locally the climate change projections announced for the end of the 21^st century. At the same time, the water regime is strongly affected by high levels of runoff linked to soil artificialisation.

Goals

With a view to adapting to climate change through diversification in tree systems, the primary objective of UFO is therefore to characterise the resistance capacities of trees in urban areas to abiotic stress (water and heat), using an approach that combines tree ecophysiology and environmental physics, in order to complement and anticipate monitoring in forest environments.

The diversification of trees used in our territories raises intertwined socio-ecological questions. Moreover, territorial actors (be they urban, rural or forest-related, managers or researchers) sometimes have opposing views on diversification practices linked to assisted migration, emphasising the advantages but also the ecological risk (invasions, pathology, impacts on communities, etc.). Although the criteria for choosing plantation diversification vary greatly between stakeholders, availability for purchase is often a predominant criterion that impacts the biodiversity (inter- and intra-specific) present in tree plantations. Furthermore, although ecological criteria are emphasised, the selection of species often depends on regional contingencies and direct utilitarian criteria put forth by various interested parties.

Then there is the question of the acceptability of new species with poorly known appearance and uses in landscapes, and understanding the perceptions of these changes by territorial actors is underlined as a key element in their implementation. The second goal of the project is therefore to analyse tree diversity and tree diversification practices, past and present, in urban and forest areas, and to assess the acceptance by the various stakeholders of the changes that the use of assisted migration could bring.

The UFO project has two components aimed at assessing:

1. the resistance to water stress of key urban forest species as a test of forest tree adaptation to climate change and
2. ecological and sociological practices and constraints related to the diversification of trees used in plantations.

INRAE units involved

• BIOGECO, Bordeaux
• BioForA, Orléans
• ETTIS, Bordeaux
• ISPA, Bordeaux

External partner : Bordeaux Métropole