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In the face of accelerating global warming and the intensification of urban heat islands, trees should no longer be perceived as mere elements of landscape decoration. They represent a genuine natural infrastructure, capable of regulating temperature, filtering the air, and sequestering carbon. Here is a breakdown of the scientific mechanisms that make trees, and plants in general, indispensable allies.

1. The Natural Air Conditioner: The Science of Evapotranspiration

One of the most immediate benefits of trees is their ability to lower the ambient temperature. This phenomenon does not rely solely on the shade they cast, but on a complex physical process: evapotranspiration.

The Mechanism of Latent Heat

Unlike a parasol that merely blocks solar radiation, a tree acts like a giant mister. Through its leaves, the tree draws water from the soil and releases it into the atmosphere as vapor via tiny pores called stomata.

This transition from a liquid to a gaseous state is an endothermic process: it consumes a significant amount of energy in the form of heat. By “stealing” this heat from the surrounding air to transform water into vapor, the tree actively reduces the air temperature.

Concrete Figures

Research in microclimatology demonstrates the formidable effectiveness of this system:

Thermal Reduction: According to a baseline meta-analysis (Bowler et al., 2010), the presence of tree canopy can decrease air temperature by nearly 2°C.
Urban Impact: Data from the EPICEA study conducted in Paris showed even more spectacular results during heatwaves, with a temperature reduction of up to 5°C during the day in greened neighborhoods.

Cautionary Note: As highlighted by INRAE, in the event of a prolonged drought, trees close their stomata to conserve water. When this happens, the cooling mechanism stops, underscoring the crucial importance of proper water management.

The tree - a biological technology — The tree – a biological technology

2. The Carbon Sink: From Photosynthesis to Sustainable Storage

Trees are one of the few natural mechanisms capable of transforming a greenhouse gas (CO₂) into stable, solid matter.

The Carbon Pump

Through photosynthesis, the tree captures carbon dioxide present in the atmosphere. Utilizing light energy, it converts this carbon into carbohydrates for its growth. This process makes it possible to:

Sequester carbon in biomass (trunk, branches, roots).
Store carbon in the soil through the addition of organic matter and exchanges with fungal networks (mycorrhizae).

This storage capacity makes the tree a major lever in decarbonization strategy, transforming our gardens into true carbon reservoirs.

3. The Air Purifier: A Precision Biological Filter

Air quality in residential areas is often degraded by fine particles and exhaust gases. Here, the tree plays the role of a passive filter.

Particulate Capture (PM2.5 and PM10): The complex structure of the foliage (roughness, presence of hairs or waxes) allows it to trap airborne fine particles. These are subsequently washed down to the ground during rainfall.
Absorption of Gaseous Pollutants: Trees can also absorb certain gaseous pollutants (such as nitrogen dioxide NO₂ or ozone O₃) directly through their stomata during gas exchanges.

4. The Biodiversity Reservoir: A Vital Ecological Network

A wooded garden is not an isolated space; it is a biodiversity hub. By creating habitats (cavities for birds, nesting areas, food sources), trees promote ecological connectivity.

In the fight against the collapse of biodiversity, trees in private gardens constitute essential “stepping stones” (pas japonais), allowing species to move between major natural spaces and maintain the genetic diversity vital to the resilience of our ecosystems.

Practical Guide: How to Choose Your Trees Wisely?

Main Objective	Key Characteristics	Examples of Species
Maximum Cooling	Large canopy, high transpiration	Maples, willows, plane trees…
Biodiversity & Pollinators	Abundant flowering, fruits	Fruit trees, Seven-son flower (Heptacodium), linden trees…
Small Space / Resilience	Controlled growth, non-invasive roots	Japanese maples, Persian ironwoods (Parrotia), dwarf horse-chestnuts, flowering cherry trees…

Important Note: “Vigorously” growing trees (such as certain conifers) store carbon more rapidly.

In Conclusion: Let’s Cultivate the Future Today!

Making room for trees and plants in general within our living spaces is the greatest gift we can give to our environment and future generations. And the great news is, there is no wrong season to get started!

Since all of our plants are grown in containers, you benefit from total flexibility: you can plant your trees all year round, without having to wait for the traditional winter dormancy period.

To give your new green companions the best possible start, just keep one golden rule in mind: ensure they receive regular watering during the first two years following their planting. Be particularly vigilant and generous during dry spells. Of course, as nature takes care of things, this extra watering boost is not necessary during the winter period.

It’s up to you: plant, water (a little), and soon you’ll be enjoying an oasis of freshness at home!

Scientific Sources and References

INRAE (Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement) : Régulation du climat thermique urbain par les arbres. [https://hal.inrae.fr]
Bowler, D. E., et al. (2010): Urban greening to cool towns and cities: A systematic review of the empirical evidence. (Meta-analysis on temperature reduction).
The Conversation / Tania Landes: Where does the cooling power of trees in cities come from? (Studies on evapotranspiration).
ONF (Office National des Forêts) : The power of trees: evapotranspiration.
EPICEA Study (Paris): Data on thermal reduction in urban environments during heatwave events.

Water a garden like pros

When we started to create the Vallonchêne Garden, almost 20 years ago, our land was devoid of shrubs and trees. There was only the big bicentenary oak and a few horn beams and ash trees. So, we had to install complete vegetation.
In addition, at the nursery, we had to learn to administer water resources sparingly for the production of our plants. We would like to share with you our experiences, which have allowed us to install a garden of more than five hectares today.

How to save irrigation water

First, you must know the techniques to save the increasingly scarce water resource:

Mulch
Originally, mulch consisted of a mixture of dead leaves and pine needles. Therfore, remember not to throw dead leaves in the green waste in fall!
If you do not have these ingredients, use bark mulches, for example.
Cover your plantations with a thick layer of 15 to 20 cm.
Mulching preserves the soil moist longer, since it greatly reduces water evaporation by creating a cover. In addition, it keeps soil temperature lower by avoiding direct sunlight.
Furthermore, it also avoids the formation of crusts which prevent rain or irrigation water from penetrating the soil and reaching the roots.
Finally, mulch not only lowers the frequency of watering, but actually limits the development of weeds! So, you will have less to weed.

Do not water during the day, except in extreme cases. Water in the evening so that the soil can soak up the water. Then, it can be released the next day when the sun heats up.
At the same time, this allows plants to store moisture to better withstand daytime heat.

Use a drip system to better target watering locally and adapt the quantities of water according to the plant. For this, use drippers with a more or less important flow.

Avoid peat and heather soil
Peat and heather earth are often recommended in horticultural books and television programs to lighten the soil. To lighten the soil, you can also use compost. Thus, you will simultaneously provide nutrients for the plants.
But, why not use peat or heather earth? Indeed, heather soil contains a high percentage of peat. The problem with peat is its extreme difficulty in being moistened again once it is dry. Especially since this happens easily and frequently in dry periods.
Finally, scientific studies have proven that peat carries certain diseases such as verticillium wilt, which is fatal to Japanese maples.

When should you water the garden?

Often, gardening beginners wait for plants to show the first signs of lack of water. This can be manifested by the leaves starting to wilt. This is a terrible practice, as it creates considerable stress for the plant and does not work for plants with tougher foliage (usually evergreens). Such repeated stress can even lead to the death of the plant. Every so often, the watering also comes too late then.

To know easily if the plants in full ground need watering, buy a rain gauge for a few euros. It will tell you how much rain has fallen. If you do not register at least 15 millimeters of water over the week, you must water. One millimeter corresponds to 1 liter of water per square meter.
Finally, for potted plants, just feel with your finger to see if the substrate is moist. Weigh the pot if necessary.

How much water do you need to water the garden?

While it is easy to determine how much to water a potted plant, it is very complicated to determine how much to water a plant in full ground:

For perennials
Water at least 5 liters per week and at once.

For shrubs
Water at least 10 liters per week and at once.

For the trees
Water at least 20 to 50 liters of water per week and at once.

TIP: We use garden hoses frequently, or we would end up with monkey arms. However, it is then difficult to know how much water the plant has received. This is very variable depending on the flow of the pump or the water network of the house.
To know how to dose with a hose, start by filling a garden watering can with your garden hose. Count all this time until filled. This way, you get an idea how far to count to administer a full watering can.

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Category: Posts

The tree: A biological technology serving climate resilience