What happens if you cut down all of a city's trees? - Stefan Al

View full lesson: https://ed.ted.com/lessons/what-happens-if-you-cut-down-all-of-a-city-s-trees-stefan-al

By 2050, it’s estimated that over 65% of the world will be living in cities. We may think of nature as being unconnected to our urban spaces, but trees have always been an essential part of successful cities. Humanity has been uncovering these arboreal benefits since the creation of our first cities thousands of years ago. So what makes trees so important to a city’s survival? Stefan Al explains.

Lesson by Stefan Al, directed by Mette Ilene Holmriis.

This is the tale of two ancient cities

and the trees
that determined their destinies.

In 3,000 BC Uruk was more densely
populated than modern day New York City.

This crowded capital had to continually
expand their irrigation system

to feed its growing population.

2,500 years later in Sri Lanka, the city
of Anuradhapura had a similar problem.

They were also growing constantly,

and like Uruk, their city relied heavily
on an elaborate irrigation system.

As Uruk grew, its farmers began chopping
down trees to make space for more crops.

In Anuradhapura, however,
trees were sacred.

Their city housed an offshoot
of the Bodhi tree

under which Buddha himself
was said to have attained enlightenment.

Religious reverence
slowed farmer’s axes

and even led the city to plant
additional trees in urban parks.

Initially, Uruk’s expansion worked well.

But without trees to filter
their water supply,

Uruk’s irrigation system
became contaminated.

Evaporating water left mineral deposits,

which rendered the soil
too salty for agriculture.

Conversely, Anuradhapura’s
irrigation system was designed to work

in concert with the surrounding forest.

Their city eventually grew to more than
twice Uruk’s population,

and today, Anuradhapura still cares
for a tree planted over 2,000 years ago.

We may think of nature as being
unconnected to our urban spaces,

but trees have always been
an essential part of successful cities.

Trees act like a natural sponge,
absorbing storm water runoff

before releasing it back
into the atmosphere.

The webs of their roots protect against
mudslides

while allowing soil to retain water
and filter out toxins.

Roots help prevent floods,

while reducing the need for storm
drains and water treatment plants.

Their porous leaves purify the air
by trapping carbon and other pollutants,

making them essential
in the fight against climate change.

Humanity has been uncovering
these arboreal benefits for centuries.

But trees aren’t just crucial
to the health of a city’s infrastructure;

they play a vital role in the health
of its citizens as well.

In the 1870’s, Manhattan had few trees
outside the island’s parks.

Without trees to provide shade,

buildings absorbed up to nine times
more solar radiation

during deadly summer heat waves.

Combined with the period’s
poor sanitation standards,

the oppressive heat made the city a
breeding ground for bacteria like cholera.

In modern day Hong Kong, tall skyscrapers
and underground infrastructure

make it difficult for trees to grow.

This contributes to the city’s
dangerously poor air quality,

which can cause bronchitis
and diminished lung function.

Trees affect our mental health as well.

Research indicates that the presence
of green foliage increases attention spans

and decreases stress levels.

It’s even been shown that hospital
patients with views of brick walls

recover more slowly than
those with views of trees.

Fortunately, many cities
are full of views like this—

and that’s no accident.

As early as the 18th century,

city planners began to embrace
the importance of urban trees.

In 1733, Colonel James Oglethorpe
planned the city of Savannah, Georgia

to ensure that no neighborhood
was more than a 2-minute walk from a park.

After World War II, Copenhagen directed
all new development along five arteries—

each sandwiched between a park.

This layout increased
the city’s resilience

to pollution and natural disasters.

And urban trees don’t just benefit people.

Portland’s Forest Park preserves
the region’s natural biodiversity,

making the city home to various
local plants,

112 bird species,
and 62 species of mammals.

No city is more committed to trees
than Singapore.

Since 1967, Singapore’s government
has planted over 1.2 million trees,

including those within 50-meter tall
vertical gardens called supertrees.

These structures sustain themselves
and nearby conservatories

with solar energy and collected rainwater.

Trees and vegetation currently cover
over 50% of Singapore’s landmass,

reducing the need for air conditioning

and encouraging low-pollution
transportation.

By 2050, it’s estimated that over 65% of
the world will be living in cities.

City planners can lay an eco-friendly
foundation,

but it’s up to the people who live
in these urban forests

to make them homes for more than humans.