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Heat waves are making U.S. cities like New York unbearably hot. Here’s a technology-free way to cool them down.

On a hot summer day in New York City, I have one need: shade.

That’s one reason I appreciate trees, especially those with big leaves. But there are many others: Trees absorb carbon dioxide and other pollutants, helping to clean the air and offset emissions that warm the earth. Not only do they block the sun and provide shade, but their leaves also pump water vapor out, cooling the air. Their roots, in turn, help rainwater that might otherwise flood the streets to seep into the ground.

Plus, trees literally make us less anxious. People who live in neighborhoods with more trees tend to be less stressed.

My point: trees are good.

While that’s not exactly a revelation, it’s a point worth making, since many neighborhoods — in New York, Phoenix, and elsewhere — don’t have enough trees. And these treeless neighborhoods are often disproportionately populated by non-whites and home to residents with less money. Planting more trees in these neighborhoods is far more than just aesthetic. It can literally help keep people alive.

But how do you plant trees in a place like New York and fill in those life-threatening gaps in the tree population? The garbage-strewn, concrete-covered blocks are not ideal for growing trees. Add to that climate change, which is causing temperatures to rise and bringing more salt water onto the land, changing growing conditions.

To understand how major cities think about the role of trees and where they are located, I spoke with Jessica Einhorn, director of the New York City Department of Parks & Recreation’s forestry program. Einhorn, a trained forester, oversees tree conservation and planting in the city. Our conversation has been condensed for length and clarity.

A tree planted by NYC Parks.

A tree planted by NYC Parks.
Deb Cohn-Orbach/UCG/Universal Images Group via Getty Images

Shadow is super important, but it is not evenly distributed

Trees offer an obvious benefit in warmer city air – they provide shade. But can you explain exactly how big this benefit is?

On a hot summer day, surface temperatures in shaded areas can be 20 to 45 degrees cooler than in unshaded areas. That’s wild.

How do you decide which type of tree to plant?

We have a list of more than 130 different species. We have tried to make our planting palette more diverse. By making the urban forest more diverse, we prevent the spread of various diseases. If we only had five different trees in the whole city, a pest like the Asian ash borer could get through and destroy a large part of it.

We consider the use of the site – whether it is commercial or residential, industrial or parkland – and whether there are overhead power lines. We don’t want a tree growing into the lines and causing power outages. In those cases, we might plant smaller species, such as a plum tree or a lilac tree. Smaller trees are usually flowering species, but often don’t have the same longevity as larger trees.

On heavily trafficked commercial sites, we may want to opt for a particularly robust species such as the Robinia. It can tolerate a lot of compacted soil and can survive a considerable period of drought.

The environment is another important aspect we consider. We have changed our approach to planting significantly since Superstorm Sandy, when we experienced significant saltwater flooding and a sharp decline in tree health. Across the city, many trees have died – especially sycamores, which are not salt-tolerant. In saltwater flood zones, we definitely look for salt-tolerant tree species like hawthorn and hackberry.

Does the amount of shade the tree provides matter?

Our goal is to maximize tree canopy space, so we want to plant the largest tree possible. If we have a site along the road with plenty of room to grow, we want to plant our larger canopy trees, such as oak species. Other trees, such as lime trees, have very broad leaves and can provide really dense shade.

How do you determine where to plant them?

Currently, we are prioritizing areas for tree planting based on the Heat Vulnerability Index (HVI). This is created by the city’s Health Department and is based on a number of factors including surface temperature, green space, access to air conditioning in homes, percentage of low-income residents, and percentage of non-Latino residents. We are focusing on these historically disadvantaged communities to increase tree canopy in areas that historically haven’t had as many trees.

We are committed to fully planting HVI-4 and HVI-5 areas – the regions with the highest heat risk – by 2027. This means that a forester will walk each block, looking for available planting areas and marking them.

Only a small percentage of the city’s trees are in predominantly black areas. Will these tree plantings help close this gap?

It will help, but we still have a long way to go. Historically, affluent communities have been planned with trees in mind, while disadvantaged communities have not. Even if we fully plant the areas that have been historically underserved, we will not be able to achieve the tree canopy cover of the historically served areas. The blocks cannot accommodate as many trees because there is other infrastructure that creates conflict.

Bringing heat-loving trees to the north

Are all trees native species?

About half of the more than 130 trees are not native to the United States. The landscape and streetscape of New York City do not resemble the trees native to this area. Many native trees are not robust enough; they cannot withstand many of the typical conditions in such a dense urban area.

You can define “native” in a few different ways: native to the US or native to this particular region? We introduce trees from warmer climates because our climate is trending in that direction, like crepe myrtles or southern magnolias. Historically, these trees weren’t in NYC, but we’re considering planting them because of the changing climate and the impact of the urban heat island effect. Some may consider them native, others may not.

To what extent do you take allergies into account? Are you thinking about planting trees that produce less pollen?

No. Shade and winter hardiness take priority over pollen dispersal.

Why are there so many ginkgo trees in cities?

This is another tree that can be added to the “very hardy” list. They can really take a lot. And they are beautiful; their leaves shimmer in the wind.

How are you preparing for an even warmer future?

We are introducing tree species from more southern climates. We are not focusing on the tropics here, but we are looking south. We can now plant crepe myrtles very successfully here, something we would never have thought of doing 15 to 20 years ago.

Then we also have to sort out other tree species that are no longer suitable here. We don’t actually plant one of my favorite tree species, the sugar maple, anymore because the urban climate is no longer suitable.

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