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of pollutants and noise mitigation will be identified. Specifically, it should require to follow some
recommendations, if possible. In particular, it is recommended:
•
to choose
always native species, well suitable to the climate and environment of the implant
site;
•
to privilege tree species with large foliage and tall, preferably characterized by a dense system
of branching;
•
to prefer long-lived species, so as to ensure for a long time the effectiveness of the green
barrier; or varieties resistant to diseases and urban air pollution also because, often individuals
are located near roads, thus they could be subjected to the stresses caused by pollution which
can make plants potentially more susceptible to infections and parasitosis;
•
to choose species with low maintenance requirements, so to reduce the management costs and
of the intervention;
•
to privilege, as far as possible, evergreen species, while still maintaining a certain variety of
species by not incurring problems of green monospecific systems, more vulnerable and
critical;
•
to favour species with leaves equipped with trichomes, waxes, resins and rough surface,
irregularly shaped. Crowns equipped with numerous leaves and of small size are generally
more efficient in the interception of atmospheric pollutants;
•
another aspect to consider is the characteristic of some species to emit Volatile Organic
Compounds (VOC), particularly in urban areas (especially in sites located near sources of
pollution, such as roads with heavy traffic). VOC, especially in presence of high
concentrations of nitrogen oxides (NOx), may induce to an increase of the concentration of the
tropospheric ozone and of the so-called SOA (Secondary Organic Aerosol). It is so necessary
to give priority to low-emitting VOC species such as maples, hawthorns and limes;
•
near objects (roads, buildings, etc. ), it should be preferred species less susceptible to collapse
and structural failures and with not superficial root system, so that they can ensure a greater
stability and security for citizens and the environment;
•
in the creation of green belts, furthermore, it should be reminded the importance of evaluating
direction and intensity of the prevailing winds, so to maximise the efficacy of forestry
intervention;
•
finally, in the implementation of the forestry intervention is important that individuals are
arranged properly, in relation to the objective to reach. For example, to limit noise levels it is
desirable that plants were arranged continuously and neatly. Concerning to air pollutants
abatement, it is appropriate to remind that various studies have shown how important is to
maintain air streams inside green belts, so to increase turbulences and to improve interactions
between polluted air masses and vegetable surfaces;
•
it is therefore necessary to consider, for the choice of implantation system, sizes that the
different species (trees and shrubs) will take to maturity. To realize an intervention fully
functioning for acoustic and atmospheric pollution mitigation yet in first years after
implantation, it should be necessary to adopt preliminary planting patterns to replace later, but
this involves significant extra costs, due to different interventions of thinning and new
implantations. Therefore during planning phase it is necessary to evaluate if adopt
immediately the final planting pattern, taking into account that the full functionality of the
intervention will be reached at a later stage, or if, on the other hand, the green belt has to be
fully functioning, thanks to the plantation of adult individuals, but at a considerably higher
costs.
Some species are more recommended than others for these interventions, because their morpho-
functional characteristics make them more efficient in atmospheric pollutants and noise abatement.
Among these species, there are:
•
maples (
Acer campestre, A. platanoides
), which are particularly resistant to air pollutants, as
well as being efficient for the realization of noise barriers and for climate change mitigation
actions, such as forestry interventions aimed to the storage of atmospheric CO
2
;
•
some species of oaks (
Quercus cerris, Q. ilex, Q. robur, Q. frainetto, Q. pubescens
), which
represent big, long-lived trees, with large and generally dense foliage. Moreover they are
characterized by diversified auto-ecological features which make them suitable for different
environments and climates. For the sound-absorbing function can be used turkey oak, downy
oak and holm oak, as reminded. However, the use of some oaks must be carefully evaluated