## 4 Discussion  
The volatile compounds emitted by infested trees have been studied in two main contexts: atmospheric chemistry processes, with a focus on reactive particle formation [50], and ecological roles related to bark beetles and their predators [27; 28; 32; 51]. Most reports in atmospheric chemistry measured volatiles at the canopy or twig level [52]. Emission measurements of biogenic volatile organic compounds (BVOC) from infested trees appeared in only a few studies investigating the potential use of these emissions as early attack detection markers [9; 10; 39]. Our study is unique as it examines emissions within a distance of 10 m of the infested tree, specifically at ground level and up to 5 m, where \(\alpha\)-pinene accumulates at an average infestation season temperature of 20 to 25 \({}^{\circ}\)C in central European forests [53]. This novel approach contributes new insights to our understanding of volatiles emitted by bark beetle-infested trees. Primarily, in our study, we employed laboratory optimization techniques to collect volatiles from freshly infested trees. The main emitted compound, \(\alpha\)-pinene, was selected as a representative of the primary monoterpenes. It reflects the trends observed in other monoterpenes emitted by spruce trees infested by bark beetles [54]. In the laboratory, where the temperature was 25 \({}^{\circ}\)C, we observed a higher abundance of \(\alpha\)-pinene in the open space in the closest proximity to logs, particularly in the lowest and highest positions. This increase was likely caused by the proximity of open log cuts and exposed bark edges, despite our attempts to prevent them by covering them with PE foil. The first detection of \(\alpha\)-pinene in the surroundings of a naturally infested tree in the field was conducted approximately two weeks after the beginning of a mass attack on the tree, corresponding with the time the monoterpene emission peaks [8; 50]. This detection also took place at a temperature of 25 \({}^{\circ}\)C, when, expectedly, \(\alpha\)-pinene vapors are heavier than air (with a vapor pressure of 4.9 mm Hg at 27 \({}^{\circ}\)C) [55]. Similar to our laboratory findings and reports from the literature [27; 28], we observed the highest abundance of \(\alpha\)-pinene in the immediate vicinity of the infested tree. Vertically, \(\alpha\)-pinene accumulated at the lowest level, approximately 1.3 m above the ground, which was confirmed by two different collection methods:  
The lower-level accumulation of the monoterpenes under similar conditions was previously observed as the output of the continual measuring campaign of BVOC in the forest. It has been reported that monoterpenes are emitted by healthy trees during the daytime, but they are also more susceptible to degradation in the atmosphere during the same period [53]. Monoterpenes are known to be unstable in the atmosphere, with a relatively short lifetime, often lasting only hours or even minutes, forming atmospheric secondary organic aerosol particles, followed by reactions with ozone and radicals like NO, OH, and NO\({}_{3}\)[56]. As a result of their decomposition, methanol and acetone, both of which are highly abundant in the forest atmosphere, are produced, alongside other products. Additionally, at higher elevations in the forest, strong air streams are present, resulting in monoterpenes having a higher'mixing ratio,' which implies a lower concentration. The monoterpenes accumulate at lower altitudes in the forest, particularly during the nighttime [57]. The abundance of \(\alpha\)-pinene in the controls placed at a further distance from the tree (9 m) exhibited greater variability [10]. This variability was most likely influenced by the surrounding trees. There were spruces, some of them in more progressed infestation stadia, or even dead and intact pines. In naturally infested trees, we also assessed an \(\alpha\)-pinene orientation related to the infested stem's cardinal direction.