Carbon Dioxide Sequestration Capability of the Botanical Garden of Rome: Environmental and Economic Benefits

Carbon dioxide (CO 2 ) is one of the most abundant anthropogenic greenhouse gases contributing to increase air temperature. Urban areas covered by parks, gardens, tree-lined avenues, sports fields, and hedges are important sinks for CO 2 . Urban green areas should include the Botanical Gardens, taking into consideration their key role in ex situ plant conservation as well as air quality amelioration and social benefits. In such context, the CO 2 sequestration capability of the most representative plant collections developing in the Botanical Garden of Rome and their influence on microclimate was analyzed. Our results highlight that plant collections have a CO 2 sequestration capability of 6947 Mg CO 2 year −1 . The CO 2 sequestration capability and air temperature lowering by plant collections growing in the Botanical Garden have positive effects (p ≤ 0.05) on the surrounding area resulting in 4% CO 2 concentration and 1˚C air temperature decreasing at 150 m from the centre of the Garden.


Introduction
Urban areas are rapidly expanding globally and it is expected that 60% of the world's population will be living in cities by 2030 [1]. Cities account for more than 70% of the energy-related to global greenhouse gases [2] [3] and carbon dioxide (CO 2 ) is one of the most abundant anthropogenic greenhouse gases contributing to increase air temperature [4]. The exchange of CO 2 over cities is mostly governed by anthropogenic emissions originating from road traffic and local heating with natural gas, oil and coal [5]. As urbanization increases global-sources) that meet and exceed the EU's 20% CO 2 emissions reduction objective.
In addition to the energy efficiency and renewable energy sources, CO 2 emissions reduction can be achieved by plants [3]. Plants remove CO 2 from the atmosphere through photosynthesis and storing the carbon excess as biomass in roots, stems, and branches [4]. Nevertheless, today the relationship between vegetated urban areas and CO 2 emissions reduction has not been clarified and recently only the use of plants to ameliorate urban air quality has become a focus of research [7] [8]. In particular, urban areas covered by parks, gardens, tree-lined avenues, sports fields, and hedges are important sinks for CO 2 [8]. The CO 2 sequestration capability is related to species, plant age and growing conditions [9]. Urban greening also contributes to decrease air temperature through shading, blocking wind and evapotranspiration, thus counteracting the urban heat island effects [10] and lowering building energy used for cooling [10].
Moreover, green spaces serve important social, psychological, health, aesthetic and ecological functions within urban areas [11]. When exposed to green areas, people show a greater well-being with physical and psychological benefits [12].
Among green areas, Botanical Gardens have a key role in plant conservation.

The Botanic Gardens Conservation International (BGCI) defined Botanical
Gardens as ''Institutions holding documented collections of living plants for the purposes of scientific research, conservation, display and education'' [13]. There are more than 1700 Botanical Gardens worldwide [14]. Europe has the highest number of Botanical Gardens (527): Germany (74), France (66), United Kingdom (61), Italy (48) and the Netherlands (39) [14]. Botanical Gardens have a significant role in plant ex situ conservation [15], taxonomic research [16], horticultural and economic Botany [17], public education and natural history appreciation [18]. Botanical Gardens also offer economic benefits associated with attracting tourists [19]. Some visitors appreciate the educational experiences and opportunities to view unusual or rare species, and others their role in maintaining local traditions and community identity [20]. In this context, we analyzed an additional role for the Botanical Gardens that should be considered, i.e. the contribution to environmental quality amelioration. In particular, the CO 2 sequestration capability of the most representative plant collections developing in the Botanical Garden of Rome (Italy) and their influence on microclimate was analyzed.

The Study Area
The study was carried out in the period January-May 2016 inside the Botanical American Journal of Plant Sciences Garden of Rome (41˚53'53'' N, 12˚28'46'' E; 53 m a.s.l.). The Botanical Garden covers an area of 12 ha in the city centre, between Lungara Street and the Gianicolo Hill. The plane area is enriched with tree species, the Palm Collection and meadows, while the hill area is occupied by Ferns, Eucalyptus collection, Bamboos, Rose Garden, Japanese Garden, Rock Garden and Geophytes, Mediterranean Wood and Gymnosperms [21].
The study area is under a Mediterranean type of climate. The average total annual rainfall is 848 mm, most of it distributed in autumn and winter. The average maximum air temperature of the hottest months (July and August) is 31.7˚C ± 0.1˚C and the average minimum air temperature of the coldest month (January) is 4.9˚C ± 0.9˚C. The mean yearly air temperature is 16.7˚C ± 6.5˚C

Carbon Dioxide Concentration and Microclimate Measurement
Atmospheric carbon dioxide concentration (CO 2 , ppm), air temperature (T a , ˚C) and air humidity (RH, %) were monitored simultaneously by handheld tools The sites along each transect were chosen on the base of a progressive traffic intensity decrease from street densely congested to the inner of the Botanical Garden.

Plant Collections
The

Plant Traits
Leaf

Carbon Dioxide Sequestration
The CO 2 sequestration capability for each plant collection was calculated multiplying TPS by the mean yearly net photosynthesis and the total yearly photosynthetic activity time (in hours), according to [22]. In order to compare CO 2 sequestration capability of the different plant collections, the CO 2 sequestration capacity per hectare (CS, Mg CO 2 ha −1 •year −1 ) was calculated. The total CO 2 sequestration capacity of the Botanical Garden was also calculated (CS Tot ).

Monetary Value of CO2 Sequestration
The monetary value of CO 2 sequestration for the collections growing in the Botanical Garden was estimated assuming a monetary value of $ 0.00334/lb (i.e. $0.00736/kg) for sequestered CO 2 , according to [23].

Statistical Analysis
Differences of the means were tested by one-way analysis of variance (ANOVA),

Carbon Dioxide Concentration and Microclimate
The mean CO 2 concentration during the study period was 485 ± 22 ppm (mean value of Transect 1 and Transect 2) peaking in January (522 ± 21 ppm, mean of A 1 and A 2 ) ( A similar trend was observed in T a decreasing, on average, 14% from the outside to the centre of the Botanical Garden. In particular, T a decreased 7% and 13% from A 1 to B 1 and from A 1 to C, respectively, along Transect 1, and 8% and 14% from A 2 to B 2 and from A 2 to C, respectively, along Transect 2. An opposite trend was observed for RH, increasing, on average, 7% from outside to inside the Botanical Garden (Table 1).  The PCA returned two axis of variations across plant collections. In particular, PC 1 was positively related to LAI and TPS accounting for 55% of the total variance. PC 2 was positively related to N P accounting for 37% of the total variance.

Carbon Dioxide Sequestration
There was a significant linear regression between PC 1 and CS. Nevertheless, the relationship did not hold for Gymnosperms and Bamboos collections which felt apart from the fitted line. When they were removed from the analysis, the performance of the linear model significantly increased ( Figure 5).

Monetary Value of CO2 Sequestration
The monetary value of CO 2 sequestered by the Botanical Garden was 59-56$ ha −1 •year −1 , to which Bamboos and Mediterranean Wood gave the highest contribution (26% and 16%, respectively), and Ferns, Rose Garden (1%) and Meadows (2%) the lowest.

Discussion
Quantifying CO 2 sequestration by urban vegetation is necessary for the development of low-neutral carbon cities or climate-friendly cities [24].