Assessing the current status of urban forest resources in the context of Parco Nord, Milan, Italy
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- Sanesi, G., Lafortezza, R., Marziliano, P.A. et al. Landscape Ecol Eng (2007) 3: 187. doi:10.1007/s11355-007-0031-2
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During the early part of the 1980s, a major project called Parco Nord was undertaken by the Lombardia Region to establish forest resources within an industrial area located in the northern part of the city of Milan. Since 1983, more than 60 ha of formerly industrial land has been converted into urban forest plantations, thus creating large patches of trees with the potential to sustain a wide range of functions and services. This paper describes an integrative study aimed to assess the current status of forest resources in Parco Nord. It focuses on the actions taken to determine whether forest resources significantly changed their status 20 years after their establishment, considering historical field data and records of management practices. Analyses have been conducted at both stand and tree level by collecting quantitative and qualitative parameters. Stand-level analysis gave a quantitative estimation of the response of species to ecological conditions and management practices while tree-level analysis provided evidence of species renovation after thinning operations.
KeywordsUrban forestryIndustrial sitesForest assessmentBiotic and abiotic stressorsItaly
Marked changes in urbanization have occurred worldwide over the past decades. In some cases, these changes have been led by processes of deindustrialization and conversion of portions of urbanized areas into other land uses (Hobbs and Lambeck 2002; De Sousa 2003).
Where factories and industrial parks once stood in downtown areas of cities, residential and commercial areas are now taking over. At the same time, new areas and open spaces are becoming available for alternative developments, such as urban forest areas and green spaces (Rydberg and Falck 2000; Konijnendijk 2003; Nam-choon 2005; Yokohari and Amati 2005).
Converting industrial sites into other land uses is a critical issue in many developed countries of Europe, North America, and Eastern Asia where land use reconversion makes planning for new urban forest areas and green spaces practicable.
However, where industrial sites, especially soils and water, have been deeply modified by human activities, creating green spaces could be challenging and requires thoughtful understanding of the local environmental conditions and constraints (Rhoades and Stipes 1999; Close et al. 1996a, b; Iakovoglou et al. 2001; Quigley 2002, 2004; Sæbo et al. 2003). In addition, abiotic and biotic factors (e.g., climate conditions and plant diseases) have to be taken into account as they affect forest (health) conditions (Ragazzi 2004; Harris et al. 2006).
Assessing the current status of these resources in the context of highly dense urban areas becomes a priority as this could inform the process of planning, developing, and managing urban forests and green spaces (Ruiz-Jaén and Aide 2006).
Such assessment could be based on various methods and approaches, ranging from a mere qualitative description of forest stands and/or single tree to a more sophisticated analysis of dendrometric parameters and ecological indicators. Data can be used to compare the status of forest resources at different time scales considering additional variables such as climate conditions, soil quality, and water supply.
In this context, this paper describes an integrative study aimed at assessing forest conditions in Parco Nord, which is a large periurban park located in the northern part of the city of Milan, northern Italy. Specifically, this paper focuses on the actions taken to determine whether forest resources significantly changed their status 20 years after their establishment, considering historical field data and records of management practices.
These actions consisted of the collection and subsequent analysis of quantitative and qualitative parameters at the stand and tree level in conjunction with other data from the analysis of biotic and abiotic factors of stress.
The paper concludes by proposing ways of sustaining the long-term integrity of forests in Parco Nord as well as in other similar contexts by emphasizing the key role of urban forestry and ecological engineering as reference disciplines to assess the status of forest resources.
Urban forest resources in Parco Nord
Urban forest resources provide citizens with a range of services with ecological, social, and economic consequences (Nakamura et al. 2005; Sanesi et al. 2006). Forest areas, trees, and green spaces allow cities to fulfil their regulatory requirements for clean air, soil, and water (Nowak 1993; Nowak et al. 2006). Forest resources revitalize neighborhoods and conserve energy by shading buildings and paved surfaces (Picot 2004; Yu and Hien 2006).
The functional role of forest resources becomes critical in the case of large and densely populated urban areas, such as most of cities in northern Italy. An example is the city of Milan, which is one of the largest metropolitan areas in Italy by population and gross domestic product per capita (Agnew et al. 2002; Trono and Zerbi 2003). During the last three decades, the area of Milan has expanded dramatically with a sprawl effect that led to the conversion of large areas of rural and forest landscapes into highly modified residential or industrial areas. Despite this process, land-use polices and new regulations were implemented in order to (1) preserve the pattern of landscapes that have been changed by urban growth and sprawl, and (2) re-establish forest patches and green spaces in areas characterized by the presence of decommissioned industrial sites.
The area of Parco Nord (45°53′71″N, 9°20′97″E) is located in the northern part of Milan’s metropolitan area (about 9 km from the city center). The total area is more than 600 ha. Forty percent of the land is covered by forest trees and other green spaces whereas the largest part of the area is used for agricultural purposes and infrastructures.
In origin, the area belonged to industrial companies specialized in mechanical products. Production ceased during the late 1960s with the main industries moving far away from the metropolitan area of Milan. In the early 1970s, a large part of the area was bought by the Lombardia government through the Consortium Parco Nord of Milan (CPNM), which was entrusted with the task of building a park by removing industrial debris and planting forest trees. Only on a few occasions were public servants working for the CPNM supported by volunteers (Fig. 1).
The main difficulties of planting forest trees in this area were mainly due to soil conditions, which had (1) low levels of organic matter (2–4%), (2) limited depth (approx. 50 cm), and (3) industrial debris and inert materials.
According to Pignatti (1998), the area is characterized by the Querco-Carpinetum boreoitalicum, (i.e., Orniyhogalo-Carpinetum of Marincek) alliance. Along rivers and water bodies this vegetation is replaced by Popolus spp., Salix spp., and Querco-Ulmetum minoris. This type of vegetation is reflected in the variety of species that were used in Parco Nord since its establishment (1983). In addition, some coniferous species were introduced, such as Pinus sylvestris and Pinus wallichiana. Broadleaved and coniferous species were planted at a separation of about 3 m (1,110 trees/ha). Understory species were not part of the initial plan but were introduced through secondary successions or plantations of new species, such as Sambucus nigra, Crataegus monogyna, Cornus sanguinea, Viburnum lantana, and Corylus avellana.
Given the importance of Parco Nord for the overall metropolitan area of Milan, it is essential to determine the current status of forest resources in relation to some of the main biotic and abiotic stressors and management practices carried out over the last 20 years. Such an assessment would allow urban forest managers to gain a better understanding of the restoration process started in 1983 and its effects on forest trees and vegetation.
Abiotic and biotic stressors
In recent years, forest trees in Parco Nord have had to face severe stress conditions as a consequence of abiotic (e.g., frequent drought and heat stress periods) and biotic factors (e.g., fungal micro-organisms). These factors caused a widespread dieback of adult trees. It is therefore essential to illustrate some basic information about these factors by providing a general profile of climate conditions and fungal pathogens.
In terms of climate, the area of Parco Nord can be described as having mesothermal subcontinental climate conditions (transition between the Oceanic and Mediterranean climates).
Mean monthly data for Linate airport (1971–2000)
Maximum temperature (°C)
Minimum temperature (°C)
Mean temperature (°C)
Evapotranspiration from reference crops (mm)
As a general indication, the air temperature is relatively mild, with the warmest month (July) averaging between 17.6°C and 29.6°C and the coldest month (January) between −2°C and 4.2°C. The precipitation pattern is characterized by two pronounced maximums during May (78 mm) and August (84 mm) and two minimums in July (601 mm) and December (63 mm).
The average amount of evapotranspiration from reference crops (ET0) (Allen et al. 1998) is about 1,036 mm/year, while the amount of water in the soil reaches its highest level in October and the lowest in June. On a yearly basis, the weather in the area of Parco Nord can be described as having approximately: (1) 200 days/year of anticyclonic patterns; (2) 90 days/year of cyclonic patterns (mainly due to Atlantic troughs or Mediterranean cyclones triggered by outbreaks of Polar Maritime, Polar Continental or Arctic air); and (3) 75 days/year of transitional patterns, with 25 days characterized by the “foehn” catabatic wind (generated by the action of the Alps on northern air currents). The wind regime shows the prevalence of low-speed winds (breezes) that are typical of anticyclonic weather types.
In the recent past, forest resources in Parco Nord have been affected by various diseases and syndromes. In particular, species such as Acer pseudoplatanus, Alnus cordata, Quercus robur, and Quercus rubra showed symptoms of decline as a result of fungal endophytes (i.e., the “complex disease” syndrome). Symptoms included a progressive loss of vigour, leaf-wilt symptoms, chlorosis, bark cracks, necrosis of cambium and xylem, crown and/or tree dieback (Ragazzi et al. 2001; Turco et al. 2006).
Biscogniauxia mediterranea has already been associated with oak decline in Italy in relation to high temperatures and water stress (Anselmi et al. 2000). This endophyte causes charcoal cankers on stems and branches. It grows rapidly into wood tissues and spreads through xylem vessels, thus invading the adjacent parenchyma until the tree dies (Kowalsky 1991).
Botryosphaeria dothidea is a canker agent that causes diffuse bark ruptures and flakings, often extending from the base of the tree to the apical parts of the stem. Infected tissues desiccate within one year from the first appearance of visible symptoms. B. dothidea can survive for a long period on both dead plant materials and on dead woods. In Parco Nord, B. dothidea appeared with its anamorph Fusicoccum aesculi (Turco et al. 2006).
Materials and methods
Chronology of stand-level data collection
Chronology of interventions and management practices
Extraction of standing dead trees
Thinning and clear-cutting
Thinning operations were also implemented to remove plants of Populus nigra ‘Italica’ (located in the inner part of the forest area) that were perishing due to the high crown density and limited sun radiation. In 2005 minor cuttings took place for sanitary purposes.
Results and discussion
Results from the experimental analysis have been organized into two sections corresponding to the assessment of the status of forest resources in Parco Nord at stand and tree levels. Although complementary, the two levels of analysis provided different types of information that could be summarized as follows: stand level analysis gave quantitative estimation of species response to ecological conditions and management practices while tree level analysis provided evidence of species renovation after thinning operations.
Number of trees per hectare and main dendrometric parameters (2001)
Number of trees per hectare
Basal area (m2 ha−1)
Mean diameter (cm)
Mean height (m)
Acer pseudoplatanus L.
Alnus cordata Loisel.
Fraxinus ornus L.
Quercus rubra L.
Populus nigra ‘Italica’
Fraxinus excelsior L.
Quercus robur L.
Quercus cerris L.
Betula pendula L.
Celtis australis L.
Pinus wallichiana A.B. Jacks
Quercus pubescens Willd
Prunus avium L.
Carpinus betulus L.
Acer platanoides L.
Pinus sylvestris L.
Acer campestre L.
Acer pseudoplatanus, though the highest in number per hectare, showed a smaller diameter (mean 12.7 cm) than other species (Table 4 and Fig. 9). From field observations, this phenomenon could be interpreted by the presence of A. pseudoplatanus trees suffering from water sprouts and/or longitudinal bark scrapings or limited bark on the stem (especially along the southwest direction). In some cases, this species presented symptoms of crown dieback caused by fungal diseases (B. dothidea) and a high number of trees having several stems each. Low levels of DBH also emerged for F. ornus, probably due to severe girdling damage by voles and competition with more vigorous plants, as observed in the field. In the case of Ulmus spp., A. cordata, and Q. rubra the distribution of DBH revealed large mean diameters, ranging from 18.6 to 19.6 cm (Fig. 9).
In terms of the mean height of trees, the most relevant values were recorded for Q. rubra (17.4 m), P. nigra (16.8 m), and A. cordata (16.7 m). Acer campestre showed the lowest value (mean height = 4.3 m); see Table 4. Quercus cerris and Fraxinus excelsior, though limited in number, exhibited large heights (Table 4).
Analysis of variance (ANOVA)
Analysis of variance procedure, dependent variable: DBH
Sum of squares
A significant difference of mean values of DBH emerged when comparing all species together (Table 5). However, multiple range tests revealed cases in which differences between species were not significant. For example, Q. cerris and Q. robur did not differ significantly in terms of mean diameter and therefore their growing rate could be assumed to be equal (Fig. 10).
Comparison of forest trees before and after thinning
Number of trees per hectare
Basal area (m2 ha−1)
Number of trees per hectare
Rate of removal (%)
Basal area (m2 ha−1)
Mean diameter (cm)
Populus nigra ‘Italica’
Standing dead trees
A similar rate of removal occurred for A. cordata, F. excelsior, P. sylvestris, and Quercus pubescens, while A. campestre, A. platanoides, Carpinus betulus, Celtis australis, and Prunus avium were only partially thinned.
At the tree level, collection of dendrometric parameters aimed to assess species renovation after thinning operations.
Magnitude of natural regeneration after thinning
Number of trees per hectare
Diameter range (cm)
Height range (m)
Species with the greatest number of new plants per hectare were C. australis (258), F. ornus (242), Acer platanoides (208), Ulmus spp. (188), Q. rubra (121), and Prunus spp. (75). With the exception of Q. rubra, these species are characterized by seeds spreading through wind or birds.
Results at the stand level showed high levels of mortality for species such as Q. pubescens and P. sylvestris, which suffered from the strong competition (i.e., higher growing rate) of species such as Q. rubra, Ulmus spp., A. cordata, and F. excelsior.
Dendrometric parameters provided evidence of the different growing models of species in Parco Nord. Q. rubra, Ulmus spp., and A. cordata revealed the best performance in terms of mean diameter and height, followed by P. nigra ‘Italica’, P. wallichiana, Q. robur, F. excelsior, Q. cerris, B. pendula. On the other hand, A. pseudoplatanus and F. ornus showed only limited increases in these parameters, as did A. campestre, C. australis, C. betulus, P. avium, and A. platanoides.
Results at the tree level allowed the identification of gaps in the vegetation as a consequence of thinning and clear-cutting operations. Such operations caused a significant reduction of tree density, which in turn allowed processes of renovation, especially in the case of C. australis, F. ornus, A. platanoides, Ulmus minor, and Q. rubra.
Although at the time of plantation C. australis and F. ornus were limited in terms of number per hectare, these species exhibited strong recolonization of gaps in the forests. This process could be explained by considering the change in climate conditions and the subsequent increase of drought and heat stress during summer periods (see “Climate conditions”). Based on field observation and results from other studies (e.g., Mariani and Cola 2006) these processes likely correspond to a natural shift of forest vegetation from mesophytic to xerophytic species.
The current status of forest resources in Parco Nord can be summarized as follows: (1) co-occurrence of abiotic and biotic stressors causing complex diseases; (2) prevalence of renovation processes within forest’s gaps; (3) changing of forest structure and composition towards uneven-aged and mixed groups.
Another important consideration is that forest species in Parco Nord had different responses to local environmental conditions and management activities. Some species (e.g., F. excelsior and Q. cerris) were more successful than others as a consequence of their attitude to adapt and compete for the limited resources available. Although extensively used at the time of plantation, A. pseudoplatanus showed limited growing capacities and therefore its use is not recommended in similar contexts.
From the collected data, some species appeared to take advantage of the presence of mixed oak woodlands due to the lower competition for water resources, especially during periods of intense evapotranspiration.
These results confirm the need to develop new ways to sustain the long-term integrity of forest resources through sound scientific silviculture strategies and the principles of urban forestry and ecological engineering. In particular, species having a greater aptitude to grow in decommissioned industrial sites should be preferred and selected during planting and management activities. Such strategies should be supported by dendrometric parameters and ecological indicators, thus allowing the comparison of the status of forest resources on different time scales.
Further studies are needed in order to assess the ecological status of (urban) forest species in the context of environmental restoration projects. New research initiatives and research studies are advisable to determine the long-term response of forest resources to the change of climate conditions and the impact of fungal pathogens and other biotic stressors.
The authors wish to thank the staff of Parco Nord, Milano, Benedetto Selleri and all the people working for the project “Laboratori Boschi”, Milan.