4
that trees help to lower air temperatures and
the urban heat island affect in urban areas.
This reduction of temperature not only lowers
energy use, it also improves air quality. It has
been found that large shade trees can reduce
local ambient temperatures by 3 to 5 °C and
the maximum mid-day temperature reductions
due to trees range from 0.04 °C to 0.2 °C per 1%
canopy cover increase. In Sacramento County,
California, it was estimated that doubling the
canopy cover to five million trees would reduce
summer temperatures by 3 degree
8
.
Air pollution reduction:
As cities struggle to
comply with air quality standards, the trees help
to clean air in more than one ways. The most
serious pollutants in the urban atmosphere are
Carbon gases, nitrogen oxides, sulfuric oxides
and particulate pollution. Ground-level smog is
created by chemical reactions between volatile
organic compounds (VOCs) in the presence of
sunlight. High temperatures increase the rate of
this reaction. Vehicle emissions, emissions from
industrial facilities, power plants and chemical
solvents are the major sources of oxides of
Nitrogen, volatile organic compounds and dust
particles. Particulate pollution or particulate
matter is made up of microscopic solids or liquid
droplets that can be inhaled and retained in lung
tissue causing serious health problems. Most
particulate pollution begins as smoke or diesel
soot and can cause serious health risk to people
with heart and lung diseases and irritation to
healthy citizens. Trees are important, cost-
effective solution for reducing pollution and
improving air quality. Leaf stomata, the pores
on the leaf surface, take in polluting gases which
are then absorbed by water inside the leaf. Some
species of trees are more susceptible to the
uptake of pollution, which can negatively affect
plant growth. Ideally, trees should be selected
that take in higher quantities of polluting gases
and are resistant to the negative effects they can
cause
1,3
. A study reveals that 14.7 lakh trees in
Portlands city, Oregon, USA remove 9 lakh kg of
pollutants annually
4
.
Trees intercept particulate matter:
In addition
to the uptake of harmful gases, trees also act
as filters intercepting airborne particles and
reducing the amount of harmful particulate
matter. The particles are captured by the
surface area of the tree and its foliage. These
particles temporarily rest on the surface of the
tree, as they can be washed off by rainwater,
blown off by high winds, or fall to the ground
with a dropped leaf. Although trees are only a
temporary host to particulate matter, if they did
not exist, the temporarily-housed particulate
matter would remain airborne and harmful
to humans. Increased tree cover will increase
the amount of particulate matter intercepted
from the air. Large evergreen trees with dense
foliage collect the most particulate matter.
Large healthy trees greater than 77 cm in
trunk diameter remove approximately 70
times more air pollution annually (1.4 kg/yr)
than small healthy trees less than 10 cm in
diameter (0.02 kg/yr)
9,10
. Based on a study, it
was found that 24 lakh trees in the central part
of Beijing removed 1261.4 tones of pollutants
from the air in 2002. The air pollutant that was
most reducedwas PM
10
(particulatematters with
an aerodynamic diameter smaller than 10 μm),
the reduction amounted to 772 tons
5
. Urban
forestry aims to address issue of particulate
matter by campaigning for a strategy of green
city
9, 10
.
Urban biodiversity conservation
: Trees in cities
and towns, whether on streets, in woodland or
in parks, gardens, schools or hospitals, provide
a wealth of benefits relating to biodiversity.
They are unique in their ability to support a
great variety of wildlife in some of the harshest
