Trees in urban environments - Problems and ways of improving night lighting

In recent years, national and local strategies have been put in place to reintegrate nature into the city. Numerous local authorities have embarked on massive tree and shrub plantations, particularly in the city centre. Almost all major cities have a "tree" plan, with dizzying planting targets. Toulouse, for example, has committed to planting 100,000 trees by 2030 as part of its territorial climate-air-energy plan (PCAET), of which 50,000 will already have been planted by spring 2024; Paris' tree plan calls for 170,000 trees to be planted between 2020 and 2026, of which 25,000 had already been planted by winter 2022; in Marseille, the city has given itself until 2029 to plant 308,000 trees, including 8,000 of mature size. Similar initiatives are also underway in medium-sized towns such as Châteauroux, Saint-Étienne and Metz. They can involve tree alignments, the creation of green spaces, the renaturation of wasteland or even "urban micro-forests".

In these virtuous and laudable approaches, however, the tree as an individual is often considered in an anthropocentric way, either as a landscape element (aesthetics) or as a "service provider" (reducing heat islands, creating shaded areas, improving air quality). More rarely, trees are considered for what they are: above all, living beings with their own circadian cycle*, metabolism, ecological requirements and sensory perception.

As a result, the living conditions of planted vegetation are only partially taken into account. Soil, exposure, water supply and climate are usually taken into account, but one key environmental factor seems to be systematically ignored: the nocturnal quality of the environment in which the trees are planted. Even today, trees are deliberately lit at night to "look good". In fact, it's not uncommon – - and the practice is perfectly authorized – - to see intentionally lit trees as urban furniture or built heritage. More generally, trees are exposed to ubiquitous artificial lights at night, which "inadvertently" spill over their target. Their crowns then encompass a lamppost, receive the ricochet of a building's lighting or the direct light from a shop window. Light pollution has become such a global phenomenon that virtually all plants in a city, and even outside it, are exposed to it in one way or another, either directly or diffusely. Animals react to this light, creating avoidance mechanisms in some cases. But trees, standing still, have no means of escape...

Yet respecting a circadian cycle, based on the day/night alternation that has rocked our planet for billions of years, has a direct influence on the life of a tree. The fact that plants are fixed and do not (for the most part) show changes between day and night gives the impression that they are not affected...