Urban and peri-urban forests in the metropolitan area of Rome: Ecophysiological response of Quercus ilex L. in two green infrastructures in an ecosystem services perspective
Introduction
Although hubs for development and human activities, urban areas threaten the environment and the ecosystem features in both urban and periurban areas (Elmqvist et al., 2013). The urban expansion has a consistent footprint, and it is a driver of land use and environmental changes (Churkina, 2008). Human settlements are a major cause for the extensive transformation and alteration of land covers, biogeochemical cycles, air quality and climate (Pataki et al., 2011, Janković and Hebbert, 2012). Several studies have reported that, owing to the higher connectivity among systems (Peters et al., 2008), environmental changes at local and regional scale may have a greater effect on vegetation than global change (Grimm et al., 2008a, Grimm et al., 2008b). The urban environment can be represented as an example of these phenomena. The change in surface energy balance, emission of greenhouse-gas, high concentration of gaseous pollutants and particulate matter (PM), are the main drivers for the local alteration of rainfall patterns (Cerveny and Balling, 1998) and promotion of the urban heat island (Hidalgo et al., 2008). These peculiarities of urban areas may enhance the stress pressure on the vegetation functionality in the form of abiotic and biotic factors (Dale and Frank, 2014), or specifically as water stress (Ugolini et al., 2015). Urban pollution is becoming a large concern for human health and thus lately the urban green is being appreciated for its ability to abate pollutants (Manes et al., 2012, Nowak et al., 2013), but also for regulating the urban microclimate (Sung, 2013). The concept of green infrastructure (GI) was introduced to deal with the high complexity and dynamism of urban areas. GI can be outlined as a network of natural and semi-natural areas (Tzoulas et al., 2007), where combinations of different functions (ecological, social and economics) are conserved and coexist, at different spatial scales, from urban centers to peri-urban areas. A functional understanding of vegetation responses to urbanization would promote the planning of multifunctional GI in order to enhance and better manage their capacity to provide the desired Ecosystem services (ESs). GI can give different contribution to the same ESs because of differences in functional vegetation types and different typology of green spaces as corridor (i.e. tree line or street trees), patch (i.e. grass field, isolated trees), matrix (i.e. urban and peri-urban forest) (Gill et al., 2007). Moreover the quality and quantity of the ESs provided by vegetation depend on the urban setting in which the GIs are located (i.e. built-up vicinity and non-urban counterparts, Janković and Hebbert, 2012). Describing the GI properties (structure, processes and ecophysiological functioning) is the first essential step to reach a reliable ESs estimation (Bastian et al., 2012). In the quantification of regulating services, the mass and energy exchange between vegetation and atmosphere has a pivotal role (Calfapietra et al., 2013), since pollutant removal or climate mitigation are underpinned by how these functions change during the season and along a small spatial scale (Escobedo et al., 2011). In Mediterranean urban areas, where stress factors related to the urban environment overlap with already existing limiting factors (summer drought, high irradiance and temperature), seasonal trend of gas exchange can have great relevance in order to seek synthetic functional index that could be useful to adjust the general models used to ESs estimation (i-Tree, CITYgreen) for a Mediterranean urban areas. In this perspective vegetation phenology can supply useful information for modeling purpose, being strictly related to air temperature and moisture availability. Moreover, phenological survey allows for the identification of urban heat island and how vegetative active phases, that have an influence on ESs provisioning, can change in response to urban environment versus natural forested areas (Jochner and Menzel, 2015). The potentiality of urban forests for mitigating the impact of urban sprawl have been accounted in minimum part for European cities (Roy et al., 2012). Among the available literature only few works considered cities located in the Mediterranean area (Manes et al., 2012, Manes et al., 2014, Soares et al., 2011, Morani et al., 2014, Baró et al., 2015; Silli et al., 2015), and a lack of information about how stressful conditions might impair urban trees and forest benefits in these climatic area is still missing (Roy et al., 2012). Quercus ilex L. (Holm oak) is largely distributed in natural ecosystems and it also is commonly used for urban green in Mediterranean. Although this species is notably resistant to summer drought and oxidative stresses in general (Baquedano and Castillo, 2006, Fusaro et al., 2014), the urban environment might represent a challenging environment for a species with a conservative use of resources such as Q. ilex (Valladares et al., 2000). Actually, the urban environment itself affects tree vitality (Ugolini et al., 2012, Savi et al., 2015). In this study we have compared the seasonal variation of key functional parameters between urban and periurban evergreen broadleaved forest, with the aim to quantify how the environmental differences and stress factors, that typify the two GI, affect the functionality of Q. ilex and consequently its capacity to provide ecosystem services such as the amelioration of air quality. We quantified (a) the magnitude of the seasonal variation of gas exchanges and photosystems functionality in each GI; (b) the environmental factors that act as major drivers of the GI functionality in the urban and periurban area. We tested the hypothesis that the periurban green infrastructure had higher functionality than the forest in the urban area, which has to cope with harsher environmental conditions, namely urban heat island and pollutants coming from different sources. The obtained results can give a contribution to implement the procedures currently used to quantify the Ecosystem Services provided by green infrastructures, and accordingly direct their management to best practices in a Mediterranean metropolitan area.
Section snippets
Studied sites
The studied sites were located inside an urban park in the northern part of Rome (Villa Ada), placed in the city centre and surrounded by an intense road traffic area, and in a periurban area (Presidential Estate of Castelporziano), located 25 km SW from the centre of Rome and just 1.5 km from the Tyrrhenian coast.
Villa Ada urban park (VA, urban forest), with an extension of 160 ha, is one of the largest urban parks in the Rome city center (Alessio et al., 2002). Artificial pastures, water bodies
Environmental condition
The analysis of the environmental parameters highlights that the differences between the two sites in terms of temperature, relative humidity and precipitation are more pronounced in 2013 than in 2014 (Fig. 1). In particular, in the urban area the temperature during the summer period rises up to 30 °C, whereas in CP the highest value of mean daily air temperature was around 26 °C. The relative humidity in the urban site decreases gradually during spring, reaching the lowest value of 40% in June
Discussion
In order to provide a reliable estimation of Ecosystem Services supply, a local scale analysis, as the one performed here in the metropolitan area of Rome, would be necessary, since the microclimatic variability and the spatial heterogeneity of the urban surface types are known to influence the energy balance of urban green, and thus the exchanges between vegetation and atmosphere. In particular, knowing the seasonal trend of gas exchange in urban and periurban vegetation, could a have great
Conclusions
Urban and Periurban Forests can contribute differently to the provision of those Ecosystem Services deriving from the exchanges between vegetation and atmosphere. Since the functional performance between the two green infrastructure changes during the seasons, it is necessary to pay attention in applying species generalized models to urban environment. In addition, the functions by which relationships between functional traits and regulating services are expressed could greatly change owing to
Acknowledgments
This research has been supported by the following grants: MIUR, Rome, Project PRIN 2010-2011 “TreeCity” (Prot. no. 20109E8F95); Ministero della Salute, Centro Nazionale per la Prevenzione ed il Controllo delle Malattie—CCM, Project: “VIIAS”; Program for Environmental Monitoring of the Castelporziano Presidential Estate, Accademia Nazionale delle Scienze detta dei XL (2010 and 2012 Grants); Sapienza Ateneo Research Project 2013 (Prot. No. C26A13E7JB). We thank the Scientific Commission of
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