Tree Role in Environmental Quality Amelioration of the Sapienza University of Rome Campus

The capability of trees growing in the Campus of Sapienza University of Rome to improve environmental quality was analyzed. Measurements of CO 2 concentration, air temperature and humidity, traffic density and noise level were carried out along a transect from streets outside the Campus to sites inside. Moreover, measurements were also carried out at the Experimental Garden placed inside the Campus. In each of the considered sites, diameter at breast height, plant height and carbon stored in the tree aboveground biomass were calculated. Air temperature in summer was 5% lower inside the Campus than in the surrounding streets, while relative humidity increased by 4%. CO 2 concentration in winter was 11% lower inside than in the surrounding streets. Carbon stored by trees was 374 Mg to which Pinus pinea, Cedrus deodara, Quercus ilex and Tilia × europaea contributed by 30%, 20%, 18% and 13%, respectively. Moreover, noise level was 36% lower inside than in the surrounding streets.

distance from the lowest branch insertion to the highest point of the trees, according to [34].

Aboveground Biomass and Carbon Storage
The aboveground biomass (AB) of the tree species was obtained by allometric equations [32] [35] [36] [37] using DBH and H for each species. If no allometric equations were found for a species, the mean value of the equations of the same genus was used. If no genus equations were found, the value from broadleaf or conifer general equations was used, according to [17].
The carbon (C) stored in the aboveground biomass (C A ) was calculated by multiplying AB by 0.5 [38]. values were monitored in winter (544 ± 23 ppm) when traffic peaked (31 ± 6 vehicles•min −1 , mean value of A and D) decreasing by 17% and 20% in spring and summer, respectively. The highest CO 2 concentration along the transect was monitored in A and in D (523 ± 60 ppm and 505 ± 52 ppm, respectively, mean value at 8.30 a.m. and 11.30 a.m., during the study period) and the lowest in B (448 ± 34, ppm). There was a significant positive correlation between CO 2 concentration and traffic density (y = 4.0083x + 409.61, R 2 = 0.4025, p < 0.05) showing that 40% of CO 2 concentration variations depended on traffic density variations.
The mean yearly T a was 20.1˚C ± 8.9˚C (mean value of A-B-C-D) at 8.30 a.m. increasing by 27% at 11.30 a.m. (mean value of A-B-C-D). The highest T a was monitored in summer (31.0˚C ± 1.8˚C, mean value of A-B-C-D) at 8.30 a.m. (Figure 3) decreasing by 47% and 73% in autumn and winter, respectively. Along the transect, the lowest T a was monitored in B (21.4˚C ± 8.8˚C, mean value at 8.30 a.m. and 11.30 a.m.) increasing by 10% in A, C and D (mean value). There was a significant negative correlation between CO 2 and T a (y = −4.7597x + 595.15, R 2 = 0.63, p < 0.05).
The mean yearly RH was 49% ± 14% (mean value of A-B-C-D) at 8.30 a.m. decreasing by 24% (mean value of A-B-C-D) at 11.30 a.m. The lowest RH was monitored in summer (33% ± 6%, mean value of A-B-C-D) at 8.30 a.m., and the highest in autumn (65% ± 7%, mean value of A-B-C-D). Along the transect, the highest RH was measured in B (46.0% ± 10.0%, mean value at 8.30 a.m. and 11.30 a.m.) and the lowest in C (38% ± 4%, mean value at 8.30 a.m. and 11.30 a.m.).
The highest noise level was monitored in A and D (80 ± 2 dB, mean yearly value) decreasing by 44% and 36% in B and C, respectively. During the year, the highest noise level was monitored in winter (84 ± 2 dB, mean value of A and D) when traffic peaked, decreasing by 8% and 14%, in spring and summer, respectively, according to the traffic density decreasing (26% and 39%, respectively).

Plant Traits
Structural traits of the considered tree species growing in the Campus and in the Garden, and the tree numbers for each species are shown in Table 1 and Table   2, respectively. The total number of trees in the Campus was 647 of which Q. ilex, T. × europaea and P. pinea were 33%, 15% and 13%, respectively. H ranged from 2.9 ± 0.3 m (Chamaerops humilis) to 29.0 ± 1.1 m (C. deodara). C. deodara

Carbon Storage
The total C stored by all the trees growing in the Campus was 372 Mg of C to which those growing in the Garden contributed by 9% (Table 3 and Table 4). P. pinea, C. deodara, Q. ilex and T. × europaea had the highest C storage (30%,   [44], which contribute to mitigate the urban "heat island" [45]. The trees growing in the Campus contribute to decrease air temperature in summer by 8% and 3% at the Garden (site B) and in the center of the Campus (site C) than the surrounding streets (sites A and D), while RH increases by 23% and by 9%, respectively. In Autumn T a decreases by 7% and 6% at B and C sites, while in winter by 7% and 10%, respectively. The urban C cycle has its own driving forces, significantly different from those of natural ecosystems [46]. Humans and automobile activity produced more than 80% input of CO 2 into the urban environment [46] and motor vehicles are significant sources of air pollution emissions [18]. C is stored in plant tissues at different quantities depending on factors such as age [47], growth rate and leaf life span [14] [16], thus contributing to decrease the atmospheric CO 2 concentration. Trees with a large crown store more C than trees with a small crown [15]. In particular, the total C stored by trees growing in the Campus is 374 Mg of C to which C. deodara, Q.  The results highlight also the role of trees in reducing noise level. Noise is considered the third most serious kind of pollution because it affects human health unfavourably both physically and psychologically. General annoyance, disturbance in psychosocial well-being and reduction in sleep quality are commonly reported effects of noise exposure [49]. The mechanism of noise attenuation by plants is due to the capability of leaves to absorb acoustic energy by transferring the kinetic energy which vibrates air molecules in a sound field to the vibration pattern of leaves [50] [51] [52]. In particular, B site (Garden) de-American Journal of Plant Sciences creases noise level by 44% compared to A site (81 ± 4db). Areas characterized by a noise level above 65 dB are considered "black areas", while a noise level between 55 and 65 dB are "grey areas" [53]. The noise level monitored in C (54 ± 2 dB, mean value during the study period) follows in the "grey areas", thus resulting in a more comfortable environment for people.

Conclusion
Despite the importance of green areas in improving urban air quality, up to date, few attention has been paid to the role of the greening in Universities. The planning of these areas has been considered only from an ornamental point of view with the aim to create a "beautiful and relaxing environment". In addition, the preservation of biodiversity has become an important driver in many contemporary landscapes. Thus, the conservation of tree species and spreading information on their capability of environmental quality amelioration contribute to sensitize the public and, in particular, young people on the importance of naturalistic conservation. Our research highlighted that trees inside the Campus contribute significantly to create a healthy environment where people can reach a satisfactory wellbeing. The results, including tree traits and their air amelioration capability, can be incorporated in a database to monitor plant response over time also in consideration of changing environmental conditions. The Campus of the Sapienza University of Rome through the conservation of its collections, supported by scientific research, is a preferential way to spread information not only on plant biodiversity but also on its environmental quality amelioration.