## 3 Discussion  
In our study, air samples collected by static and DBSS techniques were compared to evaluate the most efficient and sensitive methodology for BVOC collection under field conditions. It is important to notice that the sampling site at the Maremma National Reserve allowed us to test the DBSS technique in a complex environment, in which many plants that emit MTs were present. Indeed, in this sampling site, in addition to _Q.__ilex_, other species such as _Rubus_ spp., _Phillirea lafilia_, _Cistus salvifolia_, _Q.__cerris_, _Pistacia lenticus_, _Acer monspessulanum_, _A.__ampestre_ and _Frazinus orus_ were observed in a lower percentage. Finally, two additional sources of BVOCs in the forest could be soil microbes and litter. However, microbial BVOC emission rates are very low in Mediterranean shrulbands [51]. In addition, as recently observed by [PERSON] et al. [52], BVOCs emitted from _Q.__ilex_ litter do not include monoterpenses, thus representing a negligible source of these compounds. On this basis, we can suggest that the compounds found in our study derived from green leaves and not from litter or soil microbes. We used DVB/CAR/PDMS coating type fiber, and the choice was dependent on its technical characteristics [30]. Additionally, our choice was dependent on the fact that, in previous studies, the extraction efficiency of DVB/CAR/PDMS coating for terpenoid was better than that of other fibers on the market [53; 54]. [PERSON] et al. [45] showed that DVB/CAR/PDMS fibers might have competitive adsorption of isoprenoids and saturation condition of the coating. Nonetheless, in our study, DVB/CAR/PDMS fibers were the most appropriate choice for sampling BVOCs in field conditions because the exposure of the fibers directly to the atmosphere did not allow them to reach the saturation of the fiber coating. Additionally, the use of an open, dynamic headspace system removed the problems related to static headspace, such as increases in temperature and humidity [31]. Sampling parameters, such as time window (11 am-3 pm), fan height (40-45 cm) and sampling duration (4 h), were chosen following previous studies (see below). Several authors [55; 56; 57] have shown that the higher emissions of BVOCs occur between 9 am and  
Figure 5: The amount of total MTs collected using DBSS (light grey) and static technique (dark grey) at sampling point 1, sampling point 2 and sampling point 3. Data are means \(\pm\) standard deviation (\(n=3\)). Data were analyzed by one-way ANOVA test, and the letters indicate statistical differences between the two sampling techniques in each sampling point obtained from a Tukey post hoc test (\(p\leq 0.05\)). Results of statistical analysis are reported in Table S4. 5 pm, with a maximum peak around 1-2 pm [56]. Indeed, their emission is linked with the time when both temperatures and solar radiation were higher. The height of 45 cm from the ground was chosen since terpene concentrations have been demonstrated to be high at heights from the forest floor to 4 m. This height is optimal to limit wind action, as it allows for a more sheltered sampling condition [39]. Moreover, the choice of the height at which the fan sampler has been positioned was made to allow ease of operation during sampling (i.e., to expose and to retract the fiber; to turn on and off the fan). Lastly, the sampling duration was set to 4 h to have a long exposition time of SPME (to collect as many BVOCs as possible while in the pre-equilibrium case--[40]) and to allow a feasible sampling time in the field during every season. Indeed, longer acquisition times could be difficult to implement in winter and fall because unsuitable weather conditions are more frequent, such as strong winds and rain and shorter daylight availability. Our results showed that the DBSS was able to sample higher amounts of BVOCs in the two experimental sites (Sesto Fiorentino and Maremma National Reserve) at all sampling points (Figures 1-5).