## 1 Introduction  
Trees evolved in arid and semi-arid environments, such as the Mediterranean basin, have developed several defense mechanisms to cope with drought, changing their physiology and metabolism. One of these mechanisms is the biosynthesis of Biogenic Volatile Organic Compounds (BVOCs), a large group of secondary metabolites, among which volatile isoprenoids (mono-, sesqui- and homo-terpenes) are the most important [1; 2]. These compounds show noticeable functions in protection, defense and communication among plants, as well as between plants and other organisms. Their emissions are largely controlled by genetic and environmental conditions [3; 4]. Most of the Mediterranean forest plants have been described as high BVOC emitters, in particular of monoterpenes, investing a great proportion of fresh assimilated carbon in their biosynthesis during the summer season [5; 6; 7; 8]. Several studies have reported that BVOC emission is enhanced under abiotic stresses, such as drought and heat [9; 10]. The estimated amount of isoprene and monoterpenes emitted by Mediterranean forests is about 4.5 kg km\({}^{-2}\) day\({}^{-1}\)[11], and it has a great impact on the atmospheric chemistry of this vulnerable habitat [11; 12]. These compounds have a great effect on biosphere-atmosphere interactions by altering aerosol growth processes, cloud formation and, in general, modifying atmospheric reactivity [13]. Indeed, BVOCs oxidation, especially of monoterpenes, plays an important role in the atmospheric chemistry as precursors of tropospheric ozone (O\({}_{3}\)) and secondary organic aerosol (SOA) [14; 15; 16; 17; 18]. In particular, ozone is a greenhouse gas with detrimental effects on plant and human health [19; 20]. In addition, monoterpenes have been shown to interact with human health in different ways, from stress relief to influencing immune function. In fact, recent studies have shown meaningful effects of BVOCs inhalation on the relief of stress-related mood disorders [21; 22; 23]. Terpenes, in addition to being largely utilized in the pharmaceutical industry, have recently been identified as the main chemical compounds responsible for the beneficial effects of the \"forest bathing\" therapy, an emerging popular practice consisting of immersing oneself in nature by mindfully using all five senses [24; 25]. Increasing global concerns about the effects of atmospheric pollutants on human health and forest functioning are leading researchers to look for novel methods to collect and measure BVOCs in the atmosphere [26]. Up to date, BVOCs emission have been mainly investigated using plant and leaf enclosures connected to adsorption tubes or fibers followed by Gas Chromatography-Mass Spectrometry (GC-MS) analysis [27]. In particular, Solid-Phase MicroExtraction (SPME) fibers is a solvent free sampling technique and possesses useful characteristics to collect BVOCs in field conditions, since it is easy-to-handle, durable and provides repeatable results [28; 29; 30]. SPME can be used both in static (Static-HeadSpace--S-HS) and dynamic (Dynamic HeadSpace--D-HS) sampling enclosures of leaves or branches [31]. Between these two sampling systems, the D-HS has been recognized to provide more accurate measurements [32]. Indeed, measuring plants placed in an enclosed system, as in the case of S-HS, can result in unrealistically physiological conditions affecting gas exchanges and potentially altering BVOC emission rates of the plants [31; 33]. SPME fibers can be also directly exposed to open air, a method largely applied to monitor and quantify volatile organic pollutants in urban environment through static sampling [34; 35]. Moreover, SPME fibers have also been utilized for dynamic sampling at canopy level, by coupling with a fan-sampler [36] and, more recently, with a drone system [37]).