Understanding the Benefits from Green Areas in Rome: The Role of Evergreen and Deciduous Species in Carbon Dioxide Sequestration Capability

Urban areas are a major source of anthropogenic carbon dioxide (CO 2 ) emissions because of road traffic and local heating with natural gas, oil or coal. Rome is among the largest European cities (129,000 ha) with a large volume of green areas (69.6% of the total Municipality area). The CO 2 sequestration (CS) capability for the greenery extending for about 300 km 2 inside the area delimited by the Great Ring Road (GRA) in Rome was calculated combining satellite data with CS data measured in the field. Data from Sentinel-2 were collected and the Normalized Difference Vegetation Index (NDVI) was computed on a pixel-base. Three plant classes homogeneous in terms of annual NDVI profile were identified: deciduous trees (DT), evergreen trees (ET) and meadows (M) covering an area of 14,142.027 ha within the GRA, of which M had the highest percentage (48%), followed by DT (27%) and ET (25%). CS ranged from 428,241,492.9 Tons CO 2 year −1 (ET) to 263,072,460.6 Tons CO 2 year −1 (M). The total CS of the greenery inside the GRA was 1049,490,355.4 Tons CO 2 year −1 resulting in an annual economic value of $772,424,901.6/ha. The CO 2 sequestration capability of the considered plant classes could be incorporated into the national greenhouse gas emission budget to calculate the contribution of CO 2 sequestration to the economy of Rome.


Introduction
Nowadays, a target of air quality monitoring is addressed to greenhouse gases concentration responsible for global air temperature increasing [1]. Among greenhouse gases, carbon dioxide (CO 2 ) is the most abundant owing to fossil fuel combustion and deforestation worldwide [2] [3]. The United Nations Framework Convention on Climate Change (UNFCCC) led to an agreement to reduce rising levels of CO 2 and other greenhouse gases in the atmosphere, and the Kyoto Protocol proposed carbon (C) reduction through decreasing fossil fuel emission or accumulating C in vegetation and soil [4]. Urban areas are a major source of anthropogenic CO 2 emissions [5] because of road traffic and local heating with natural gas, oil or coal [6]. CO 2 concentration in urban areas is more than 50% compared to extra-urban areas [7] [8] [9]. It has been hypothesized that CO 2 emissions from road traffic will increase worldwide by 92% between 1990 and 2020 [10] [11] [12]. As it is estimated that currently 55% of the world's population lives in urban areas, and this percentage will be around 70% in 2030 [13], the forecasted increase of CO 2 concentration in cities can be considered not only an environmental issue but also a social issue [14] [15]. However, the CO 2 atmospheric concentration mitigation requires a complex strategy involving multiple actions at political, economic, social and ecological level. Urban green areas have recently gained popularity as a climate change adaptation/mitigation measure, and many city governments have adopted policies promoting tree-planting, the preservation of urban green spaces and, more recently, green architecture (i.e. green roofs and facades) [15]. The research on urban vegetation over the past decades has advanced our understanding of this resource and its impact on the society, which includes many ecosystem services, such as lowering air temperature, reducing building energy use, improving air and water quality, lowering noise level and enhancing social well-being [16]- [21].
Much less evidence is available to demonstrate the direct removal of CO 2 from the atmosphere by urban vegetation [15]. From an ecological point of view, CO 2 sequestration by plants is considered an offset mechanism for CO 2 emissions [22]. However, the lack of data and models evaluated with observations, which covers the large variability among cities in term of plant species, urban morphology and climate setting impedes a proper assessment of current green programs [15]. Moreover, plants contribute differently to CO 2 sequestration according to their habitus. Few data are available for CO 2 sequestration capability by deciduous and evergreen species growing in the same area as those characterized by Mediterranean climate. It is important to consider that deciduous species have a CO 2 sequestration capability from spring to the beginning of autumn while evergreens all year long, due to their continuous photosynthetic activity [23]. Thus, urban greenery reveals the extent and variation of this resource across a city [24]. There is the need to increase knowledge on the role of urban greenery in environmental quality improvement to select the more suitable species which can be planted [18] [25] [26] [27]. In this context, a useful tool to ex- In such context, the main objective of this research was to map the CO 2 sequestration capability for the greenery developing inside the area delimited by the Great Ring Road (GRA) in Rome, by combining satellite data with CO 2 sequestration data measured in the field at plant level, and referred to the carbon uptake rate over the year through photosynthesis. In particular, the contribution of evergreen species, deciduous species and meadows to the total CO 2 sequestration capability of the greenery inside the GRA was calculated. The monetary value of the CO 2 sequestration capability for the greenery inside the GRA was also calculated.

The Study Area
The study was carried out in the city of Rome, in the area delimited by the Great Ring Road (GRA), which is part of the Rome Municipality area. This area extends for 300 km 2 (i.e. approximately one fifth of the total surface of the Municipality of Rome) ( Figure 1).
The climate of Rome is of Mediterranean type. The mean minimum air temperature (T min ) of the coldest months (January) was 4.72˚C ± 1.09˚C, the mean maximum air temperature (T max ) of the hottest months (July and August) was 31.85˚C ± 0.12˚C and the yearly mean air temperature (T m ) was 16.76˚C ±

CO2 Sequestration Map
The CO 2 sequestration map of Rome was developed by three steps. First, data from satellite images were used to identify the greenery developing inside the GRA. In particular, data from Sentinel-2 (10 m of pixel size and 5 days of revis-   Figure 2. Then, the obtained CS values were weighted for the total extension of each class in order to obtain the total CS (TCS) inside the GRA.

Monetary Value of CO2 Sequestration
The monetary value of CO 2 sequestration capability for DT, ET and M classes inside the GRA in Rome was estimated, assuming a monetary value of $0.00334/lb (i.e. $0.00736/kg) for sequestered CO 2 , according to [35]. The monetary value referred to TCS was also calculated.

CO2 Sequestration Map
The satellite images covered a total green area of 14, 142.027 ha within the GRA of which M had the highest percentage (48%) and ET the lowest (25%) ( Figure   2, Table 1).
As shown in Figure

Discussion
The high biodiversity of urban landscapes resulting from variable land use cre-American Journal of Plant Sciences ates a great variability of ecological conditions for plants [34] [36]. Nevertheless, the rapid expansion of cities affects urban species composition and functioning.
Urban areas are projected to more than double between 2010 and 2060, which will impact agricultural lands, as well as expand the importance of urban forests in relation to environmental quality and human well-being [37]. Literature [38] [39] provides valuable insights into how humans interact with urban greenery.
The benefits of the contact with nature concern mental and physical health. In particular, urban green areas play a key role from a social perspective by promoting physical activity and increasing people interaction [40]. Moreover, urban forests encompassing trees, shrubs, meadows and other vegetation types in cities provide a variety of ecosystem services to city-dwellers, such as air purification, temperature regulation, noise reduction, runoff mitigation and recreational opportunities [41] [42] [43]. In particular, urban plants contribute to decrease atmospheric CO 2 concentration, which has increased dramatically since the start of the industrial revolution. Close to 280 ppm in 1870, the average global concentration surpassed 400 ppm in 2015, and this acceleration is similar to the rise in fossil CO 2 emissions, due to the use of fossil fuels. According to [44], the CO 2 concentration significantly increased in Rome from 1995 (367 ± 29 ppm) to 2004 (477 ± 30 ppm) (data referred to daily peak in the early morning when traffic is the highest) and a further increase was monitored in 2016 (560 ± 27 ppm). Dur- ing the year, CO 2 concentration in Rome peaks in winter, 18% higher than in summer in relation to traffic density [17] [44].
Concerning the area inside the GRA in Rome, [34] highlighted that the most It is important to highlight that the evergreen species have a CO 2 sequestration capability all year long having, as a consequence, an important role especially in autumn and winter when CO 2 emissions from road traffic are the high-American Journal of Plant Sciences est in Rome [44]. Recognition of the need to stabilize the CO 2 concentration in the atmosphere has been manifested in a number of international and national agreements and policies, such as the Kyoto Protocol, the Paris Agreement, the EU climate policy (e.g., [1]) and the Cop 24 in Katowice (Polonia 2018). The results show that the total CS inside the GRA corresponds to 10.49% of the total greenhouse gases emission of Rome for 2010 [45]. These findings may be of relevance in an international discussion related to the ongoing rise in the CO 2 concentration and its implications in the context of the hypothesized global change. The evaluation of urban greenery in both aesthetic and monetary terms can be an important tool to assist planners in protecting this resource [46]. In the future, based on the remote sensing images, data, and use of large-scale quadrats, the existing database can be updated via GIS, and an inversion model suitable for the actual situation in each region will provide a direction for developing more accurate assessment of regional forest C sequestration [48], including greenery in the cities. Through proper planning and management, urban greenery can be sustained and environmental and human health values improved. Healthy urban vegetation and proper management can reduce some of the environmental issues associated with urbanization (e.g., increased air temperature and energy use, reduced air and water quality, increased human stress) and ultimately, help humans living within and around urban areas [37].
This methodology can be applied to other cities in Italy and in other countries, characterized by different plant species, according to the different geomorphology and climate, and incorporated in a geographic information system to monitor spatial changes of CO 2 sequestration over time.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.