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Hardened soil makes city’s tree roots struggle

Want your egg sunny-side up, or your steak medium-rare? Just place them on the oversized sizzling pan that is Shanghai.

The city is experiencing a once-in-a-century heat wave that has scorched it for 25 days and claimed more than 10 lives.

According to a meteorologist quoted in Wenhui Daily on Tuesday, the marked increase of carbon emissions from high-rises, cars and air-conditioners are the culprits causing a spike in ozone, which sequesters the heat and causes the temperature to rise.

Days ago, I walked on an unshaded street to run some errands for my wife. That 20-minute walk left me sweating buckets. That night, when I tossed in bed, unable to sleep in the muggy heat Ñ the central air-conditioning was not much respite Ñ I kept recalling my summertime memories from childhood.

Two decades ago, there was hardly a day in summer that saw the mercury soaring to above 37 degrees Celsius, much less consecutive days of high temperatures. When we ventured outdoors, children of my age could seek a real solace in the shade provided by a lush canopy of plane trees and other trees. Summer was never as unforgiving and intimidating as it is now.

Plane trees are still there, but there are much fewer since many have been uprooted to make way for wider streets and construction of metro stations. Today it is a tall order to expect trees, shrubs and other urban greenery to cool us down as they themselves are parched for water in a struggle for survival in this heat.

According to Hu Yonghong, deputy director of Shanghai’s Chenshan Botanical Garden, over the years the garden has submitted a list of more than 10,000 types of plants fit for landscaping, but it turns out that most cannot survive the city’s so-called “hardened” earth, since their root systems lack space to expand. The soil in cities like Shanghai is heavily polluted and lacks nutrients and organic matter that are necessary for their growth, said Hu.

One and a half meters to two meters beneath Shanghai’s surface spreads the enormous network of pipelines that transport water, gas and sewage. Trees’ roots are allowed to stretch to a maximum depth of 30 centimeters underground so they do not get entangled with utility pipes. This means trees are left to compete against each other for nutrients, Hu said, adding that the root systems of 99 percent of Shanghai’s street trees are concentrated in this space.

For healthy growth, the ideal proportion between the size of a tree’s root system and its canopy is 3 to 1. But in cities, the ratio is even below 1 to 1. A consequence of this cramped living space is that trees are not firmly established in the ground and easily succumb to typhoons, Hu said.

In recent years, widespread use of cement instead of water-permeable paving stones has turned Shanghai’s sidewalks into a blockage in an ecological cycle.

As the ancient Chinese saying goes, leaves fall back to their roots and enrich the soil. Nowadays fallen foliage on sidewalks ends up in dust bins.

To provide some greenery, there’s a tendency for building owners to cover their rooftops with artistically designed and manicured lawns, shrubs and small trees. The greening of Shanghai’s skyline is progressing at 300,000 to 500,000 square meters a year, and will likely gain momentum.

These patches of green oasis in a concrete jungle convey the inviting impression that tenants close to the roof will benefit from the cooling effect afforded by rooftop gardens.

In fact, as Hu pointed out, if the texture of the soil is not right, radiation, pollution and soil erosion will combine to reduce the life span of rooftop vegetation, making the gardens typical vanity projects.

Restoration of urban soil fertility is not just the work of gardeners and greenery authorities, because to some extent, what doesn’t suit the plants doesn’t suit humans either, said Hu.

In the coming week, the temperature is predicted to climb to 40 degrees Celsius. If there is no mercy of precipitation, Shanghai will be hot as hell. In a climate unfit for most species, it certainly doesn’t qualify as a livable city.

 


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