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rustic and adapted to local conditions and also for interventions in pioneer and degraded
conditions;
•
Turkey oak (
Quercus cerris
), a long-lived and fast-growing indigenous species;
•
Lawson cypress (
Chamaecyparis lawsoniana
), introduced species from the United States, very
used for experimental forestation, tolerant to pollution and capable of retaining fine dust;
•
among shrubs, hawthorn (
Crataegus monogyna
), has a relative fast growth, is a species
resistant and needs of low-maintenance. However it is important to retain that it is a shrub
with thorns. It can also be a vehicle for the propagation of fire blight (caused by bacterium
Erwinia amylovora
), infection that affects the Rosaceae (including various fruit trees). Its use
should therefore be limited in the vicinity of orchards (in some regions of the North is
forbidden to implant new individuals).
•
1.4.2 Criteria for the selection of species recommended for mitigating air pollution and
noise
Urban forestry interventions targeted to reduce air pollution and acoustic fields require a careful
choice of the species or of the association of species best adapted, thanks to their autoecological
characteristics, as well as environmental ones such as phytoclimate, exposure, water availability and
other present vegetal species.
In particular, the creation of green belts is here considered, which can represent not only effective
barriers against noise (especially that produced in urban areas by road traffic and, in the periurban
areas, including rail and air traffic), but also have other functions such as aesthetic, reduction of light
pollution and ecological corridors for the current fauna.
Monospecific green strips ("fifth plant", systems hedges or rows that develop along the line of
communication) and bands shrub-tree ("fifth composite") are not considered here, although used,
because they are not very functional for the above-mentioned roles and mitigations, and, in addition,
they are less resistant to environmental factors and most sensitive to plant health infections.
Some criteria for the creation of mixed systems consisting of multiple species, particularly
advisable for shielding the noise and having a positive action in mitigating air pollution, as well as for
the supply of other environmental benefits and services, such as those previously defined, are therefore
recommended.
In relation to noise pollution, the efficiency of the control of the noise differs between the various
species on the basis of phenology, in particular: the foliage characteristics (persistence on the plan
t 37,
thickness of the leaf blad
e 38, presence of hair and waxes, size of the leaf surface, density and shape of
the foliage), the individual habit, the leaves orientation or phyllotaxis and crown density.
An example
of tree species recommended for the function of shielding the noise is the holm oak (
Quercus ilex
) as
evergreen species with thick leaves, dense foliage and compact habit.
Also in reference to air pollution abatement, some peculiarities make some species more effective
than others. Among these there are the morpho-anatomical characteristics of the leaves: leaf area, size
and shape of the leaves and the foliage, density and morphology of stomat
a 39, thickness and structure
of the cuticle, persistence, etc. Also in relation to the fine particles the greater/lesser ability to capture
them is linked to: leaves surface roughness, presence of waxy coatings, hairs, wax and other structures
of the epicuticle of the leave.
Different studies show how, on average, trees were more efficient in the capture of the dusts respect
to bushes, thanks to their wider
leaf area
and to the crown structure more articulate and complex.
Conifers, in particular the genera
Pinus
,
Picea
and
Cupressus
and in the North also
Abies
, result to be
efficient for air pollutants abatement, although they are susceptible to high concentrations of these
compounds; for this reason their use in highly polluted contexts is to avoid.
Following, some general criteria to be considered for
the choice of species useful to realize green
belts able to contribute to the interception of air pollutants and mitigation of noise pollution are shown.
The criteria listed are also applicable to the realization of a wider and more complex green system,
although with some limitations.
As mentioned in relation to the capture of CO
2
, it is essential for the success of the intervention of
forestation that species are primarily appropriate to the environment which characterized the site of
implantation; among these those with characteristics better suited to the interception and/or absorption
37
The evergreen species (thus with persistent leaf) perform better shielding function because they ensure continuously to this function.
However, deciduous species that during the period of dormancy keep their leaves withered (for example hornbeam,
Carpinus betulus
, and
beech,
Fagus sylvatica
) can also be used, to create a green belt aesthetically pleasant even in autumn and winter.
38
Leaves of greater thickness allow greater energy absorption.
39
The ability to absorb gaseous pollutants increases with the density of stomata and lesser thickness of the cuticle.