We all know that trees help clean the air in cities, but which trees do so best? According to new research conducted in Sweden, a mixture of deciduous and coniferous trees will give you the most bang for your buck.
For the study, a University of Gothenburg team led by Prof. Håkan
Pleijel analyzed the leaves and needles of 11 types of trees growing in
the same area of the Gothenburg Botanical Garden. The deciduous (leafy)
trees consisted of aspen, cherry, rowan, beech, oak, walnut and birch,
while the coniferous (needle-bearing) trees consisted of larch, spruce,
fir and black pine.
Because much of the air pollution in urban areas comes
from cars, the scientists focused on polycyclic aromatic hydrocarbons
(PAHs), of which traffic is the largest source within cities. A total of
32 different PAHs were analyzed, some of which were bound to particles
and others of which were gaseous.
Of all the trees, it was found
that black pine and fir performed best at absorbing gaseous PAHs in
their needles. As an added bonus, unlike deciduous trees which lose
their leaves in the winter, most coniferous trees keep their needles
year-round, allowing them to trap PAHs in both the summer and winter.
A coniferous tree which does
shed its needles, the larch, was found to do best at absorbing
particle-bound PAHs. That said, all of the deciduous trees came in next
at doing so – this is likely because their broad leaves present more
particle-trapping surface area than the thin needles of spruce, fir and
pine trees.
The scientists noted that the pollutants didn't seem
to affect the trees' ability to perform photosynthesis, although they
added that such might not be the case in much more polluted cities. The
researchers also expressed concern over the possibility that as the
trees shed their PAH-containing leaves and needles, the pollutants may
accumulate in the soil as those leaves and needles decompose over time.
Additionally,
the team suggests that simply lining streets with trees may not always
be the best way to go. If a street is too narrow, for instance, the
overhanging trees could reduce air flow, trapping airborne pollutants
close to the road. In such a setting, using lower-growing plants such as
hedges may be more effective.