Mind the tree. We call on them to save us from the heat, but we don’t know them

On the one hand, there isn’t a single mayor of a city, town or village who doesn’t call for ‘more trees, more trees!’. Not a single one who does not associate high-calibre cultural events with tree planting. On the other hand, however, there isn’t a single mayor of a city, town or village who, through an autumn or spring resolution, authorises – at the worst possible times of the year – the arrival of barbaric gangs of forced fellers who saw down trees as if they were gathering firewood, rather than pruning them sensibly. Furthermore, on the one hand there is always a speaker who argues for the need to fill the planet – as well as every nook and cranny of our towns – with every species of tree, so as to recreate the Garden of Eden and live happily in the shade. On the other hand, if you ask around a bit about what photosynthesis and respiration are, you’ll be met with a confused and horrifying stammer. The result? We want trees but we don’t know them; we imagine them as loyal soldiers at our command, in the trenches, ready to solve all the problems – the ones we have created, whether we like it or not. Yet trees, like nature itself, are completely indifferent to human destinies. Yet, since anthropogenic global warming is a reality, cities have become very hot; we suffer, we complain, and, often gripped by anxiety, we foresee the end of the world – and, given the progress of the noble but slow battle for the transition – we wonder whether trees can do anything for us. Can they do anything? The answer is not obvious; indeed, at first glance it might seem rather blunt: planting trees here and there, merely for the sake of beauty, serves no real purpose, other than to cause further distress to those who, loving trees, see them struggling for a drop of water. Just as green meadows serve no purpose.

I am aware that our generation was brought up on Heidi and that some high-profile architects are keen to produce renderings featuring lush flowerbeds, but if you then go and measure just how much the temperature actually drops, you realise the sad truth. Sad truths are always the result of measurements. Numbers are among the tools we use to understand the gap between our dreams and reality – that is, between election slogans and day-to-day administration. A study by the University of Manchester, published in "Landscape and Urban Planning", directly measured the temperatures of grass and concrete in an urban environment. Well, green grass – but only when it is well watered – reaches temperatures just 0.3 degrees lower than that of the air. But this effect disappears as soon as the grass is dry: which is to say, all summer long. If you go to a country house or a park right now, you’re more likely to be treading on dry vegetation rather than green lawns on which to play football or write poetry. With no moisture to evaporate, the sun heats the air instantly and creates heat peaks at face level. If you do not believe the measurements – because numbers are boring – take a walk across some stubble, that is, the crop residues left after harvesting cereals, and then tell me whether or not your face feels like it is on fire. When the lawn is dry, tarmac is actually better (in terms of temperature). At At it absorbs heat slowly and releases it over the course of several hours. To be sure, it is still heat, but at least it is spread out over time. However, this entire preamble is not meant to argue: no to trees. After all, how could I possibly maintain that? I am one of those speakers who always says: more trees, more trees. But the preamble serves to highlight a crucial issue. To date, the few studies carried out on the benefits of trees in cities tell us that it is not enough to plant those sad, solitary negundo maples to beautify certain suburban landscapes. The few studies available suggest we should assess the situation carefully before planting. It is both right and useful to know a tree’s physiology inside out: whether and how, and under what conditions, it can thrive in the city, lowering the temperature and performing numerous other – and necessary – functions. Conversely, if we fail to address the complexity of the tree-city relationship and do not study the interactions between the various dimensions, then we’d be better off leaving it at the rhetoric of trees. Which, incidentally, isn’t even supported by forest nurseries; we don’t know where to produce the trees, let alone grow them, maintain them and integrate them into the metropolis. Either we do it properly or we’re wasting money, and – not least – we’re making life difficult for these extraordinary plant organisms. So let’s plant them properly, or let’s not plant them at all. To understand what ‘planting properly’ means, we can start by looking at a few studies.

Let’s return to the meadows. It is true that, under certain conditions, placing trees amongst the grass results in a moderate cooling effect (compared to trees or grass on their own), and the combination, incidentally, also has its own beauty. But just to highlight the complexity, and to reiterate that what is beautiful to us is not necessarily beautiful to nature, in the driest regions – such as Piedmont at present – natural complexes of mixed vegetation – trees and grass – do not provide any significant cooling; indeed, the cooling effect is much lower than in areas consisting solely of forest. This is because it is often impossible to irrigate. If the tree-and-grassland system presents difficulties, let’s focus on the trees. What do trees do, amongst other things? They lower the temperature, both through shading and through evapotranspiration. Let’s look at the latter: under ideal conditions, water escapes through the stomata and cools the environment (stomata are microscopic pores on leaves through which plants absorb and release gases, as well as regulate water vapour). But we are not always in these ideal conditions; after all, it is also a metaphor for life. A 2025 study, published in "Urban Sustainability" and conducted in Zurich, found that when soil moisture is insufficient – meaning the soil is not properly irrigated – trees stop cooling the air. Why? Because plants reduce evapotranspiration. Trees try to conserve water by closing their stomata; therefore, from a cooling perspective, plants perform less effectively precisely when we need them most. That is to say, it’s worse for us, but from their point of view they’re functioning very well – succulents thrive by closing their stomata during the day. How much less do they cool the air? A 2024 study published in Nature analysed 182 research papers on various cities around the world and found that, during heatwaves, trees lose up to 30 per cent of their cooling capacity. Then there is the issue of temperature. In many cases, temperatures are measured by satellite, which generally has a fixed pass time in the mid- to late morning. This data does not accurately reflect temperature variations within cities, which, as we know from experience, vary from neighbourhood to neighbourhood. However, measurements taken using sensors mounted on bicycles, motorbikes and cars are becoming more widespread and are more effective. In this way, air temperature can be recorded at any time of day or several times a day, and measured every 5 metres over a distance of 7 km.

Using this methodology, it has been established (the study is published in PNAS) that the cooling effect in a neighbourhood is significant only when tree cover exceeds 40 per cent – other studies conducted by our CNR put the minimum tree cover at 30 per cent. Whether it’s 30 per cent or 40 per cent, it is clear that below that threshold, planting a few isolated trees serves no purpose. One might say: better than nothing; better a tree than a concrete pillar. However, to address a major issue such as urban heat islands – and given that we will soon be living in cities, unless some of us, the older generation, retreat to the famous, currently uninhabited ‘village hamlets’, repopulating them and giving rise to a host of problems linked to communal living – it is essential to think and act on a large scale. Speaking of measurements, how large should tree leaves be? Is a pine or a mulberry tree better? Leaf measurements lead to a paradox. Tree species with a high leaf area index – that is, large leaves – offer greater cooling capacity, both through shading and through evapotranspiration. Stand under a mulberry tree during the day and you’ll feel cooler. This isn’t just a fleeting impression. Apart from the mulberry, measurements show that the temperature drops – both at the soil surface and in the air. However, a global study (covering 596 cities) found that tall trees with dense canopies do indeed provide good cooling during the day, but can cause the temperature to rise at night. Those very same leaves that provide shade during the day become a barrier to the heat released by the ground. If you have curtains on your balcony, you will have noticed that it is best to draw them back at night, precisely to dispel that typical layer of dampness that has built up. The difference is that you can raise the curtains, but you can’t move the trees. This paradox has also been analysed at the ETH Zurich laboratories, where, using a wind tunnel, researchers sought to understand what happens in those narrow, tree-lined streets flanked by tall buildings – known in technical terms as ‘urban canyons’. It was found that medium-sized and large trees with large leaves (which occupy at least half the width of the streets) block the airflow at dusk and hinder its upward movement. Consequently, the heat accumulated in the ground struggles to rise: the temperature on this type of street is 1.5 degrees higher than on a street without trees.

So are trees with smaller, thinner leaves better (which, incidentally, require less water and can therefore be more efficient at cooling in the medium term)? Yes, because they limit the heat island effect. But during the day they provide less shade, so the ground absorbs more heat. Staying on the subject of shading: using trees to shade impermeable surfaces such as tarmac or concrete can reduce the amount of heat that would otherwise be absorbed. A 2019 study measured a temperature reduction of 6 degrees, but on the other hand, the extreme heat emanating from impervious surfaces can also trigger the closure of stomata, reducing evapotranspiration and limiting its cooling function. What’s more: trees planted in paved areas – concrete, tarmac – grow poorly; consequently, their reduced size means less shade is provided, and their evapotranspiration capacity is lowered. I realise these studies are contradictory, but not for the worse – quite the opposite. They are contradictory because trees are not neutral, nor are they soldiers in the trenches serving us. They need soil, water and care. Whilst in theory trees are essential for cooling, in practice we must enable them to do their job of cooling the air. Let’s return for a moment to concrete. A concrete surface, as well as absorbing heat, also acts as a hydraulic barrier. A Acer negundo, a Prunus, a ligustrum – in short, a tree planted in a hole in the pavement, surrounded by concrete – that tree risks being unable to benefit from rainwater, which ends up directly in the drains. So that tree, in search of water, sends its roots out in search of water, cracking concrete, tarmac and pavements. So, in essence, even though the tree balance (the difference between trees planted and felled) is required by law, and mayors are keen to boast about how many trees they have planted – even if they do not explain why they have felled trees or pruned them incorrectly – planting trees without an integrated scientific approach serves little purpose.

In addition to the contradictions mentioned above, there is a risk of planting thousands of trees on pavements without a sustainable urban drainage system – without a small basin comprising a layered bed of gravel, sand and compost, a system that filters water polluted by oils and microplastics, set slightly below ground level and capable, when a storm hits, of channelling and slowing the flood wave. What is needed is an integrated approach, drawing on agronomy, forestry, engineering, architecture and urban planning. Then, of course, above all, money is needed. That might well be found, as we are talking about necessary and welcome urban changes, but it must be spent wisely. A city that has spent its money wisely – though it may seem hard to believe, given the prevailing perception – is Medellín, with its Corredores Verdes. Since 2016, they have replaced kilometres of tarmac with vegetation on three levels: grass, shrubs and tall trees. In total, there are 2.5 million plants and 880,000 trees, well above the 40 per cent threshold.

Clearly, they did not simply pot the sapling, as happened in Bologna and, some time ago, in Milan, but combined the vegetation with high-albedo road surfacing materials, capable of reflecting the sun’s rays rather than absorbing them. The World Economic Forum has documented the results: over three years, the average temperature in the corridor areas fell from 31.6 degrees to 27.1. Surface temperatures dropped from 40.5 to 30.2. The cost? 16.3 million dollars, which works out at 6.50 dollars per inhabitant. It’s nothing really – just a seed to help us understand how to plant trees in future cities. Trees will need to be planted, but not merely as decoration; they are infrastructure that thrives better – and helps us live better – if we understand how they work. Let’s put an end to the rhetoric and cultivate a new, integrated culture.