In Milan the plane trees are retiring due to the climate.What are the most resistant trees for cities?

Designers are often instructed to use only native species.But, experts tell Open, urban greenery is only possible with biodiversity and care

«Healthy trees, young, old, very old, have fallen.The wind is responsible."This is how Milan Green Councilor Elena Grandi summarizes the situation scene that the inhabitants of the Lombardy capital found themselves facing on the morning of July 25th.It seems like years have passed, but just four months ago the roads were blocked, the cars destroyed, the city astonished in front of at least 5 thousand trunks uprooted by winds that exceeded 100 kilometers per hour.Events characterized by this violence are becoming and will become increasingly common.Cities must be ready.For this reason - Grandi announces - the tree cover of the city will have to be modified in order to better withstand extreme weather conditions.Among the species that will pay the price there is one that has been part of the arboreal panorama of Italian cities for centuries:the plane tree, which will be largely replaced by lime trees and leathery hackberry trees.Plane trees "are very resistant trees", but "they fell like skittles", explains Grandi, quoted in the Milanese edition of Corriere della Sera.

«The trees are not healthy»

Perhaps it is precisely from this last consideration of the councilor that we need to start to understand that the discussion on which trees to plant in our cities is just one of many to be addressed in the discussion on how urban greenery will have to change to adapt to a more extreme and unpredictable climate.The plane trees suffered from the strong winds, but many of them and the other trees in Italian cities were not and are not healthy plants in the beginning, due to a series of factors, due to planning, lack of biodiversity, and poor consideration of public green areas. .«Fewer plane trees and more hackberries and lime trees seems like a superficial choice to me.We replace an extremely used species but we put down highly used species and therefore we do not increase diversity", explains a Open the professor of arboriculture and science and technology of tree and forest systems Alessio Fini.

Biodiversity and care

Fini highlights a problem linked to biodiversity.«In some respects replacing plane trees with other trees can be advantageous.They are resistant trees but they are very present in our cities, and this exposes them to diseases and parasites."One of these diseases is "colored canker", which, despite its name, is not autoimmune, but caused by an extremely virulent fungus capable of killing trees in 3 or 4 years.Similar problems can occur to all tree species in the city, and are often exacerbated by climate stress.«Each species should not exceed 10% of the total», explains Fini, who points out that lime trees and hackberry trees are already very present species in the Italian urban landscape.The expert adds:«If the trees are pollarded during pruning they weaken», he adds, suggesting a possible solution already being evaluated at Palazzo Marino:"In some cases they should be kept smaller."

The planning

Thus we enter into a discussion of planning:«In Italy we have lost the production of plants in nurseries and are therefore forced to buy them abroad».But this prevents public green spaces from being designed correctly.«Today the space is designed and then we go to the nurseries to look for the plants.But the right thing to do would be to get the trees 3-5 years in advance.The plants could thus be prepared with preconditioning techniques, i.e. the imposition of controlled stress which can guarantee greater tolerance to transplantation.Today the big problem is this.We plant millions of trees by scraping the bottom of nurseries.Planting is not enough.They must be managed correctly and the necessary resources are needed", adds Fini.«Right from the planting phase, trees can suffer stress if the soil conditions are not suitable to encourage their growth», highlights Carlo Masera, architect responsible for the redevelopment of the Parco delle Cave and Boscoincittà in Milan.

Which species to use

Masera also talks about the choice of species:«Designers often receive instructions to use only native species, for example in the areas of the South Milan Agricultural Park.In these cases we know that some of these plants, such as the English oak, cannot tolerate increased temperatures and drought.They should therefore be replaced with more resistant varieties, such as Cerro, even if they are not native to the woods of the Lombardy plain."Fini is of the same opinion:«As far as species are concerned, we need to disconnect from the concept of native.The hornbeam, for example, is among those that have suffered despite being typical of the Po Valley.Ditto the beech.We must understand that no species is native to the city."In general, «it has been observed that the ecotypes [i.e. the typical populations of a certain area, ed] Southerners are usually better suited to the high temperatures we experience and will experience in cities.The plant material that comes from the South should therefore be used, helping to make it migrate."

The benefits of trees in the city

However, these solutions will have a limited impact until urban greenery is considered in all its importance.The summer air temperature in tree-lined areas reaches up to six degrees lower than those without vegetation.The one on the ground can have a difference close to 20 degrees, fundamental, in a world where temperatures are in constant growth.Trees absorb tons of CO2, and remove some polluting particles, help absorb water when it rains too much and generally bring measurable economic benefits.«Every euro invested in a plane tree, for example, has a return of 24 euros – highlights Fini -.It is one of the species with the highest returns.For hackberries the return is between 7 and 8 euros."Often, however, green spaces are the last to be thought of and not always in the correct way.«Managing a wooded area in a park costs 100 euros a year – highlights Masera – while managing a grassy one costs a thousand and has many fewer benefits».

It all starts from the ground

Finally, the space in which the trees are grown plays a fundamental role.«We're talking about trees but to have healthy plants and at the same time counteract the increasingly frequent flooding and flooding, we need to take care of the soil.As long as the soil conditions are not good, the trees will continue to die with increasing frequency."Another example:«We consider that many
urban transformation areas the surfaces dedicated to natural drainage do not exceed 10% or 30% of the building surfaces depending on the type of construction.Too little.The problem can be solved by building multi-storey underground car parks with a reduction of their footprint on the ground to the benefit of the extension of the drainage surfaces:it is a problem of balance between costs and benefits, between private and public interest."

Synergy (interests) between public and private

The intervention of the administration is fundamental, but citizens must also do their part:«As long as the street beds are used to park cars, the soil compacted in this way will not have the ability to drain the water or promote the growth and stability of the trees.So why not remove cars from the pavements by taking advantage of the thousands of spaces available in many underused garages, especially for overnight parking?I'm thinking of the car parks of many supermarkets, but also of those of offices and universities."In summary, «we return public space to people and create the conditions to have healthy, non-compacted soil where plants grow much better, and we will have healthier greenery and fewer trees fallen during extreme weather events», concludes Masera.