Guidelines suistainable urban forestry ROMA_Eng

21 / 55

19
A wood area requires wider spaces to be created and generally several belts are realized (such as a
tree and a shrub belt). Furthermore, the ecological service for which they were created could be not
immediately appreciated (for example the shading can require a few years). Finally, if the forestry
intervention is placed in a more natural context (such as protected areas and agricultural areas), there
are some more advantages such as less stress for new plants (less atmospheric pollution) and the new
forested area can perform several environmental (carbon sequestration, mitigation of the urban heat
island, habitat for the animals, etc.) and social functions (for example creating shaded areas suitable
for sports and other activitie ). When designing recreational and social sites (from scratch or
requalification), especially if used by children (e.g. playgrounds), it will be appropriate to take account
of the potential allergenicity of the species to be introduced in order to avoid the occurrence and the
burden of symptoms related to allergies and asthma, diseases that already have a high incidence in
urban areas (see below the box “Urban green spaces and allergies to pollens: recommendations”).
Green belts have a linear shape and generally a limited width. Thus, before their realization it is
important to consider distances from streets and privately proprieties. Especially in case of tree-lined
rows it is essential to assess the space available for plant growing and the constraints , and to choose
the most suitable species (for example avoiding species more sensitive to atmospheric pollution or
species which can represent a threat for citizen ). Moreover new trees should not be an impediment
to vehicular traffic and road sign. For a better interception of atmospheric pollutants it is important to
choose mainly species with an high breadth growing, so to create a green belt with optimal
characteristics to perform this function. As wood areas, also green belts and tree-lined rows can
perform different functions, such as anti-noise and windbreak barriers, heat regulation, ecological
connectivity, aesthetic values, shading, dust filters, etc.
Finally, for the realization of any forestry interventions it is necessary to consider the species
temperament to sunlight (heliophilous vs sciophilous species) to allow a correct growing.
1.4 Species selection
The choice of species is crucial for the success of forestry projects; their effectiveness in
performing the functions for which they are realized depends mainly on ecological characteristics of
the species.
In fact a wrong choice can involve a series of problems ranging from not achieving the initial
objectives, the failure of the intervention and high costs of management and maintenance. It is
therefore essential to consider first of all the previous points (purposes of the area, site characteristics,
planning addresses), and then analyze the aspects explained below.
In the selection of species to be planted in the forestation interventions, native species must be
favorite within the constraints of urban planning and landscape regulations. Only in some cases (such
as the tree-lined or where ornamental species are particularly necessary) is conceivable the use of
exotic species (not invasive), as should always favored indigenous species.
The main aspects to be considered in the choice of species are listed below, in addition to those
discussed in previous paragraphs (aim of the intervention, soil type, microclimate, etc.):
• the speed of growth, which is important in particular for the capture of CO 2;
• root development (especially important for interventions next to roads because roots must not
damage the sediment road);
• the bearing and the size of the species to the adult stage (height, shape of the foliage, etc.);
• persistence of the leaves (deciduous versus evergreen), a feature of particular interest in
relation to the mitigation of air pollution and noise;
• characteristics of flowering and fruiting and any unwanted elements (such as the plugs, the
presence of resins), important especially to increase biodiversity, but also for the safety (for
example in the case of tree lined roads);
• the strength of the wood and the propensity to rupture (both branches of the whole plant);
23 For example Parco Nord in Milan, Parco di Monza and Pratone delle Valli in Rome (here some trees have been planted in the dog area on
request of citizens with the aim to make the area more usable and pleasant).
24 Generally on pavements less than 3 meters it is possible to plant only trees of 3 rd size, on pavements between 3 and 4 meters trees of 2nd
size and on pavements wider than 4 meters trees of 1st size. Furthermore it is necessary to respect the distances from constrains for the correct
development of roots (Vittadini M.R., Bolla D. e Barp A. (a cura di), 2015. Spazi verdi da vivere. Il verde fa bene alla salute. Il prato
Editore.
).
25 See “Linee guida per la gestione delle alberature di proprietà comunale nel territorio di Roma Capitale” (in Italian).