2024-07-31 22:00:00
In recent years, national and local strategies have been put in place to reintegrate nature into the city, and this primarily involves giving more space to plants. Many communities are thus committed to the mass planting of trees and shrubs, particularly in the heart of cities. Almost all major cities have adopted a “tree” plan with staggering planting targets. For example, the city of Toulouse has committed to planting 100,000 trees by 2030 as part of its territorial climate-air-energy plan (PCAET), 50,000 of which will already be planted by spring 2024; Paris plans in its “tree” plan to plant 170,000 trees between 2020 and 2026, 25,000 of which have already been planted by winter 2022; In Marseille, the city has given itself until 2029 to plant 308,000 trees, including 8,000 adult trees. Similar initiatives are also being initiated in medium-sized cities, such as Châteauroux, Saint-Étienne and Metz. These may involve tree alignments, the creation of green spaces, the renaturation of wasteland or even “urban microforests”.
In these virtuous and praiseworthy approaches, the tree as an individual is however often considered in an anthropocentric way, either as a landscape element (aesthetic) or as a “service provider” (reduction of heat islands, creation of shaded areas, improvement of air quality). More rarely, trees are considered for what they are, that is to say above all living beings which themselves have their own circadian cycle*, their metabolism, their ecological requirements and their sensory perception.
As a result, the conditions of habitability of the plants planted are ultimately only partially taken into account. Most of the time we will be interested in the soil, the exposure, the water supply and the climate, but a key environmental factor seems systematically ignored: that of the nighttime environmental quality where trees are planted. Trees are even deliberately lit up at night, to “look nice”. It is indeed very common – and this practice is perfectly permitted – to see trees intentionally lit up as street furniture or built heritage. More broadly, trees are exposed to omnipresent artificial lights at night which, “inadvertently”, overflow their target. Their crown then encompasses a street lamp, receives the ricochet of the lighting of a building or the direct light of a shop window. Light pollution has become such a global phenomenon that almost all the plants in a city, and even outside, are exposed to it in one way or another, either directly or diffusely. Animals react to this light, which creates avoidance mechanisms in some of them. But the trees, immobile, have no way of fleeing…
However, respecting a circadian cycle, based on the alternation of day and night that has rocked our planet for billions of years, directly influences 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 by this cycle. But this is not the case, a tree also “sleeps” and its activity is structured in time as in animals. Growth, flowering, fruiting and the storage of their sugars are closely linked to the alternation of day and night and its variations during the seasons.
Therefore, it seems crucial to better take into account the quality of the night for plants, including in a utilitarian vision of the tree, because a tree in poor health will not provide or will provide less well the expected services… Plants are also the basis of ecosystems, they provide food for herbivores and habitats for wildlife. If they are exposed to artificial light, their place in the ecosystem will change radically. They could suddenly become a hostile habitat for wildlife, open their flowers out of sync with pollinators, etc. Wanting to (re)create nature in the city while ignoring the exposure of trees to light pollution therefore shows a paradox in our way of understanding ecosystems.
This article therefore asks the question: how to design and implement a night lighting strategy without harming trees? It will take stock of the impacts of lighting and possible solutions. It will mainly focus on chlorophyll and woody plants, therefore shrubs and trees. We will distinguish between voluntary lighting (enhancement) which therefore raises political choices to be questioned and involuntary lighting which is sometimes difficult to avoid, but which can be improved.
* a glossary of terms and expressions is presented at the end of the article.
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