Phytochemical Screening of Volatile Organic Compounds in Three Common Coniferous Tree Species in Terms of Forest Ecosystem Services

: Multiple positive effects that forests have on human health and overall well-being have been reported widely in the literature. Still, multiple elements of this relationship remain unidentiﬁed and unexplained. In this study, the composition of leaf volatile organic compounds (BVOCs) content in three common coniferous species: the Austrian pine ( Pinus nigra ), Scots pine ( Pinus sylvestris ) and Spruce ( Picea abies ), was analyzed. The speciﬁcity of BVOCs content in the examined species and their genotypes is observed as a plant potential to evaporate these organic compounds and potentially improve human health and well-being. Principal component analysis applied on BVOCs content among species showed signiﬁcant differences between compounds that have previously been characterized as having positive effects on human health by acting as anticancer, anti-inﬂammatory, antiviral and antibacterial. Variations among genotypes of the investigated species were observed in the content of BVOCs relevant for human health improvement, such as limonene, terpinolene, β -pinene, linalool, camphene, camphor, citronellol and α -cadinol. The observed intra- and inter-species variations in the BVOCs content provide an appropriate base for further research on the forest–human health relationship, breeding and selection of the most suitable genotypes for human health improvement, and could I mpact the sustainable management of forests.


Introduction
In light of the on-going pandemic, a significant effort has been invested into finding the alternative strategies for human health improvement, among which nature visits and forest therapy or Shinrin Yoku showed the potential to enhance the immune response [1]. The basis of forest therapy lies in the relaxed time spent in forests as a means of restoring mental and physical balance or the act of engaging in various therapeutic activities while in forests, to improve health and well-being [2]. In certain countries, such as Korea, this term is defined by law as 'immune-strengthening and health-promoting activities utilizing various elements of the forest, such as fragrance and scenic view' [3]. Also, due to urbanization and the modern "sedentary" lifestyle, the overall societal need to maintain contact with nature is increasing, with forests playing an important role in this regard given their capacity to provide important recreational services [4].
A significant amount of medical research has shown the positive impact that trees and forests have on the human health, with studies proving its positive impact on the immune, Table 1. Positive impacts of trees and forests on human health.

Impact
Description of Improvement References The immune system Observed increment of natural killer (NK) cells [5][6][7]

Diabetes
Observed decrement of blood glucose values [13] Mental disorders Decreased symptoms of stress, depression, and anxiety [3,6,8,10,[14][15][16][17] Chronic pain Chronic pain decrement [18] A recent comprehensive investigation of global forest ecosystem services (ES) research has revealed that the published studies have assessed multiple forest functions, whereas the majority of research was focused on the regulating services, such as carbon storage and/or sequestration and climate regulation [19]. Since the general influence of the ecosystems on human well-being is well-recognized [20], further research on this subject would notably contribute to improving overall public awareness that ecosystems should be understood as an important determinant of human health [21].
Although the positive influence of trees and forests on human health is evident, the exact mechanism of human health improvement in these environments is still not fully explained [22]. A recently published review on the existing literature considering the forest-human health relationship highlights that it still remains largely unexplored [23] and more accurate descriptions of forest variables in future human health studies are needed in order to provide data for future changes in the forest management [24]. Some researchers suggest that the volatile organic compounds (VOCs) released from trees, also named phytoncides or biogenic volatile organic compounds (BVOCs), are the main factors for human health improvement [8,25]. In addition to the potential for direct human health improvement, these compounds also contribute to changes in the air quality [26] and therefore indirectly affect human health and well-being. The current literature on BVOCs analysis shows that the results describing content of these compounds in forest tree species were mostly used as chemotaxonomic indicators and biochemical markers of provenances in different species [27,28]. More recently, the role of the BVOCs composition in the air is being analyzed, especially in the urban areas trough impact on ozone formation [29][30][31] and the potential to reduce the content of harmful particulate matter (PM) in the urban air [32,33]. Further work on the ozone formation is highly valuable, especially in the field of forestry, since it is causing greater damage to the vegetation compared to other pollutant gases, which results in substantial economic loss for horticulture, agriculture and forestry [34].
BVOCs in the air include atmospheric trace gases of organic origin, such as terpenes (isoprenoids), alkanes, alkenes, aldehydes, ketones, alcohols, esters, ethers and acids [35]. Terpenes are the largest class of plant secondary metabolites, but still are largely unrecognized, although about 1000 new structures are being added every year [36]. The volatile terpenes mostly include classes of monoterpenes, sesquiterpenes and diterpenes, which differentiate based on the number of characteristic C5 units. Furthermore, monoterpenes (MT) with two C5 units and sesquiterpenes (SQT) with three C5 units are the most abundant in the air and are recognized to have an important role in forming secondary organic aerosols [37]. BVOCs, including terpenes, are described as highly reactive compounds when emitted in the air, and their lifetime is highly dependent on the environmental conditions [35,[38][39][40] that are characteristic to each specific time and place.
Terpenes that originate from plants are produced as secondary metabolites, whose primary role is protection from stress, herbivores and pathogenic microorganisms [41]. Forests are recognized as a major source of these compounds [42], and although the content and type of terpenes is characteristic for each species, certain authors [40] emphasize that their content can also differ between individual trees.
The aim of our research was to define and investigate differences in the terpene emission potential of the selected tree species and their superior genotypes. The use of this "screening" methodology, implemented in the presented research, means the results would not be limited and restricted to a certain time and place, keeping in mind high dependability of BVOCs in the air on the different factors of the environment. Our research provides insight on the qualitative and quantitative composition of the blends of BVOCs that were found in the needles of three common coniferous tree species, Austrian pine (Pinus nigra J. F. Arnold), Scots pine (Pinus sylvestris L.) and Spruce (Picea abies (L.) Karst.) and its genotypes from Fruška gora mountain, Vojvodina Province, Serbia. The main aim of this study is to determine species-and genotype-dependent differences in the BVOCs emission potential among investigated species as a platform for further research that will estimate BVOCs impact on human health and well-being. Furthermore, the knowledge gained through this research on qualitative and quantitative BVOCs content in the examined material, observed as the genetically determined emission potential and the ability to form compound-specific BVOCs storage, will contribute to future breeding and selection of species and genotypes more suitable for human health and well-being improvement.

Plant Material and Location
The content of BVOCs in the needles of three common coniferous species, growing in the Fruška gora mountain, Austrian pine, Scots pine and Spruce were analyzed to investigate their potential to evaporate BVOCs that can have a positive impact on human health and well-being.
Fruška gora mountain is located in the Vojvodina Province, Republic of Serbia near the second largest populated city in this country, Novi Sad. Vojvodina, characterized with the high agriculture production, and as one of the most deforested regions in Europe, with less than 6.00% forests coverage of its total area [43]. The vicinity of natural area to the highly populated cities and the existence of multiple hiking trails on this mountain makes it a valuable protected nature resource, providing multiple ES for over 500,000 people. The individuals of investigated species are growing in the areas of the most popular visitor sites near the hiking trails ( Figure 1).
Climate conditions in the sampling area are defined as temperate continental to modified continental, fully humid with warm summers [44]. The average altitude of the sampling area is 270 m. The ten-year average annual temperature is 11.2 • C, with an annual rank of 22.1 • C and average precipitation of 603.1 mm [45].

Plant Material Collection
Analyzed plant material was collected from National Park Fruška gora (Serbia) during June 2019. For each of three tree species (P. nigra, P. sylvestris and P. abies), three individual genotypes were investigated to define their potential for emission of different BVOCs. The genotypes of the investigated species were fully formed and medium aged, with characteristics representing their natural population. No visible signs of pathogens or mechanical injuries were present. A detailed description of the selected genotypes is shown in the Table 2.
Samples were taken as the tip of branches with fully formed needles. In order to ensure full plant examination, samples were taken from three different parts of the canopy of each genotype: lowest point, middle point and the top point. Collected plant parts were south and light exposed. Sampling was carried out during noon in specific environmental conditions as previously described [46].  Climate conditions in the sampling area are defined as temperate continental ified continental, fully humid with warm summers [44]. The average altitude of pling area is 270 m. The ten-year average annual temperature is 11.2 °C, with an rank of 22.1 °C and average precipitation of 603.1 mm [45].

Plant Material Collection
Analyzed plant material was collected from National Park Fruška gora (Ser ing June 2019. For each of three tree species (P. nigra, P. sylvestris and P. abies), th vidual genotypes were investigated to define their potential for emission of BVOCs. The genotypes of the investigated species were fully formed and mediu with characteristics representing their natural population. No visible signs of pa or mechanical injuries were present. A detailed description of the selected geno shown in the Table 2.   After cutting, samples were stored in paper bags, delivered to the lab and analyzed within 12 h. The voucher specimens were determined and deposited at the Herbarium of Department of Pharmacy, Faculty of Medicine, University of Novi Sad, under the following voucher numbers: PN-07/2019, PS-08/2019 and PA-09/2019.

BVOCs Analysis
The chemical profiling of Austrian pine, Scots pine and Spruce was performed by analyzing the content of primary BVOCs and essential oils. Primary BVOCs were investigated using the headspace sampling technique gas chromatography-mass spectrometry (Headspace-GC/MS), while the essential oils were characterized qualitatively and quantitatively by gas chromatography coupled to mass spectrometry and, flame ionization detection (GC-MS/FID).
The essential oils were isolated by hydrodistillation with a Clevenger apparatus for 2.5 h. After the solvent removal, the yield of extraction was determined gravimetrically. The qualitative and quantitative analyses of the essential oils were carried out on HP-5MS capillary column (30 m × 0.25 mm; film thickness 0.25 µm) on Agilent 6890B GC-FID instrument coupled to Agilent 5977 MSD. The samples were injected in splitless mode, at inlet temperature of 220 • C. The oven temperature was set at 60 • C and increased at a rate of 3 • C/min up to 246 • C. Helium was the carrier gas (1 mL/min) while the temperature of the MSD transfer line was set to 230 • C.
Prior to the analysis of primary BVOCs, dried samples of collected plant material were crushed and sealed in headspace vials (20 mL). Vials were incubated at 100 • C for 20 min in a headspace sampler (5977HSS, Agilent Technologies) and injected to gas chromatograph in splitless mode. The inlet temperature and oven heating program were the same as in the case of EO analysis. Mass spectral data were collected in scan mode (m/z = 50-550), while the identification of compounds was performed using the NIST (v14) mass spectral database and by comparison to relative retention indices (RT), as well as literature data [47].

Data Analysis
The two-way analysis of variance (ANOVA) has been carried out to conclude if there are significant differences among the mean values of BVOCs content in the examined species and among its genotypes. In order to observe clustering and variation of detected BVOCs among species, as well as between the examined genotypes of the species, the principal component analysis (PCA) was performed. PCA was carried out in the R environment [48]) with the use of R packages "prcomp" (Version 4.0.2) and "ggplot2" (Version 3.3.2).

BVOCs Composition in the Examined Species
The results of the data analysis confirmed the presence of 60 different compounds, mainly terpenes, in three examined species: Austrian pine, Scots pine and Spruce. Values were obtained with different types of the analysis: primary BVOCs (Headspace-GC/MS), while the essential oils were obtained by gas-chromatography coupled to mass spectrometry and flame ionization detection (GC-MS/FID).
Complete data of phytochemical screening of the examined species, together with the mean values (Mean%) with standard deviation (SD%) and coefficient of variance (CV%) for each detected compound is given in the Supplementary Materials.
Compounds classified as terpenes dominated in all of the examined material. Among them, monoterpenes (MT) were the most abundant. According to the mean values for BVOCs content in each of the examined species (Figure 2), the most abundant classes of analyzed species were monoterpene hydrocarbons, followed by sesquiterpene hydrocarbons in the Austrian and Scots pine. The second most represented terpene class in Spruce were oxygenated monoterpenes, while the least present were aliphatic compounds in all of the examined species. The most abundant individual compound in all of the examined species was α-pinene.
The results of the two-way ANOVA among individual BVOCs content showed statistically significant differences in the analyzed species, with the value of p = 5.2 × 10 11 . The content of limonene was the highest in Spruce (6.00%), followed by Austrian pine (1.83%), while Scots pine had the smallest amount (1.72%) of this particular compound. The results have shown the highest presence of α-terpineol in Spruce (1.87%), compared to Austrian pine (0.13%) and Scots pine (0.22%). The highest amount of camphor was detected in Spruce (0.56%), lower value was detected in Austrian pine (0.33%), while Scots pine was characterized with the lowest value of this terpene (0.05%). Isoborneol, citronellol and eugenol were only detected in Spruce in small amounts 0.04%, 0.25% and 0.07%, respectively. Terpinen-4-ol and linalool were the most present in Spruce in the concentrations of 0.38% and 0.17%, respectively. α-cadinol and camphene were the most present in Spruce (8.44% and 8.35%), followed by Scots pine (2.28% and 5.34%) and Austrian pine in small amount (0.92% and 1.38%).
The principal component analysis (PCA) in Figure 3 showed that the first two principal components (PC1 and PC2) explain more than 50% of the data variability considering BVOCs content in needles of the examined species.  The results of the two-way ANOVA among individual BVOCs content showed statistically significant differences in the analyzed species, with the value of p = 5.2 × 10 11 . The content of limonene was the highest in Spruce (6.00%), followed by Austrian pine (1.83%), while Scots pine had the smallest amount (1.72%) of this particular compound. The results have shown the highest presence of α-terpineol in Spruce (1.87%), compared to Austrian pine (0.13%) and Scots pine (0.22%). The highest amount of camphor was detected in Spruce (0.56%), lower value was detected in Austrian pine (0.33%), while Scots pine was characterized with the lowest value of this terpene (0.05%). Isoborneol, citronellol and eugenol were only detected in Spruce in small amounts 0.04%, 0.25% and 0.07%, respectively. Terpinen-4-ol and linalool were the most present in Spruce in the concentrations of 0.38% and 0.17%, respectively. α-cadinol and camphene were the most present in Spruce (8.44% and 8.35%), followed by Scots pine (2.28% and 5.34%) and Austrian pine in small amount (0.92% and 1.38%).
The principal component analysis (PCA) in Figure 3 showed that the first two principal components (PC1 and PC2) explain more than 50% of the data variability considering BVOCs content in needles of the examined species.   PCA analysis of the detected BVOCs in the analyzed species showed grouping based on the species, with the highest prevalence evident in Spruce, considering the detected number of different BVOCs. A small amount of compound content overlapping is evident in the examined Pinus species. PCA analysis of BVOCs content in the examined species showed clear separation of Spruce (P. abies) from Pinus species. Figure 4 shows the correlation of the detected BVOCs in the examined species. The first component (PC1), with 35.67% of the variance represents the negative correlation between monoterpenes (MT) (e.g., camphene, camphor, α-pinene) and sesquiterepenes (SQT) (e.g., α-copaene, epicubenol, α-farnesene) of the detected compounds in the examined species. The second component (PC2), with less variance (21.35%), shows negative correlation among certain aliphatic compounds (e.g., eicosane, heneicosane) with certain MT (β-ocimene, camphor, 3-carene).  Figure 4 shows the correlation of the detected BVOCs in the examined species. The first component (PC1), with 35.67% of the variance represents the negative correlation between monoterpenes (MT) (e.g., camphene, camphor, α-pinene) and sesquiterepenes (SQT) (e.g., α-copaene, epicubenol, α-farnesene) of the detected compounds in the examined species. The second component (PC2), with less variance (21.35%), shows negative number of different BVOCs. A small amount of compound content overlapping is evident in the examined Pinus species. PCA analysis of BVOCs content in the examined species showed clear separation of Spruce (P. abies) from Pinus species. Figure 4 shows the correlation of the detected BVOCs in the examined species. The first component (PC1), with 35.67% of the variance represents the negative correlation between monoterpenes (MT) (e.g., camphene, camphor, α-pinene) and sesquiterepenes (SQT) (e.g., α-copaene, epicubenol, α-farnesene) of the detected compounds in the examined species. The second component (PC2), with less variance (21.35%), shows negative correlation among certain aliphatic compounds (e.g., eicosane, heneicosane) with certain MT (β-ocimene, camphor, 3-carene).

Variation of BVOCs Content among Genotypes of the Investigated Species
The two-way analysis of variance (ANOVA) showed relevant statistical difference in the intra-species variations of BVOCs content in the investigated species, with p = 4.5 × 10 164 for Austrian pine, p = 6 × 10 163 for Scots pine, and p = 6.4 × 10 87

Variation of BVOCs Content among Genotypes of the Investigated Species
The two-way analysis of variance (ANOVA) showed relevant statistical difference in the intra-species variations of BVOCs content in the investigated species, with p = 4.5 × 10 164 for Austrian pine, p = 6 × 10 163 for Scots pine, and p = 6.4 × 10 87  The PCA performed on the BVOCs content in the tested genotypes of the investigated species showed that first two principal components (PC1 and PC2) explain 92.85% variability for Austrian pine (Figure 5a The loading plot of the PCA analysis performed on the genotypes of the Austran pine (Figure 6a) defines the major variability between the content of MT: limonene, terpinolene, β-pinene, 3-carene, linalool, β-myrcene and camphene. The loading plot of PCA analysis for Scots pine (Figure 6b) also showed high variability between the content of certain MT: terpinolene, camphene and limonene. The highest variations among Spruce genotypes were also observed within the detected BVOCs belonging to the MT: camphor, linalool, citronellol and SQT: α-cadinol, α-copaene and trans-caryophyllene (Figure 6c). The loading plot of the PCA analysis performed on the genotypes of the Austran pine (Figure 6a) defines the major variability between the content of MT: limonene, terpinolene, β-pinene, 3-carene, linalool, β-myrcene and camphene. The loading plot of PCA analysis for Scots pine (Figure 6b) also showed high variability between the content of certain MT: terpinolene, camphene and limonene. The highest variations among Spruce genotypes were also observed within the detected BVOCs belonging to the MT: camphor, linalool, citronellol and SQT: α-cadinol, α-copaene and trans-caryophyllene (Figure 6c). The loading plot of the PCA analysis performed on the genotypes of the Austran pine (Figure 6a) defines the major variability between the content of MT: limonene, terpinolene, β-pinene, 3-carene, linalool, β-myrcene and camphene. The loading plot of PCA analysis for Scots pine (Figure 6b) also showed high variability between the content of certain MT: terpinolene, camphene and limonene. The highest variations among Spruce genotypes were also observed within the detected BVOCs belonging to the MT: camphor, linalool, citronellol and SQT: α-cadinol, α-copaene and trans-caryophyllene (Figure 6c).

Discussion
A significant amount of research regarding ecosystem services (ES) has contributed to the acknowledgment of the importance of natural environments in terms of the human health and well-being in global society [49]. Previous research on the health-related ES has focused attempts on defining the benefits that humans gain from natural environments [21,50,51]. In recent years, a number of papers were published investigating the relation of BVOCs and positive impact of forests on human health [42,[52][53][54]. Furthermore, it was reported that certain VOCs can have long-and short-term negative effects on human health and the majority of them have anthropogenic origin (AVOCs) [55]. On the other hand, it is known that during the photochemical reactions with AVOCs, VOCs of organic origin (BVOCs), including terpenes, can contribute to the creation of new air compounds, of which some can harm human health [56]. Taking this into consideration, certain authors [57] are questioning, predominately, the undoubtable role of trees in urban air quality improvement and are calling for further research on this subject. Furthermore, many studies are reporting pharmacological activities of the terpenes providing new data and an insight about new chemotherapeutic characteristics of these compounds [52]. Human health and ecosystems are complex domains of research; therefore, some authors [23] suggest that an overly simplistic approach in the research of this forest-human health relationship should be avoided, whilst a multidisciplinary approach that considers all the variables affecting the final results should be used.

The BVOCs Content in the Investigated Species and Their Genotypes
Our research aim presented in this paper is to define and to investigate differences in the potential of the selected tree species and their genotypes to evaporate BVOCs. With the use of the "screening" methodology, the results would not be limited to a certain time and place, keeping in mind high dependability of BVOCs in the air on the environmental conditions.
The results of the presented research have shown the presence of 60 different BVOCs in three examined tree species, of which the majority belongs to the monoterpenes (MT) and sesquiterpenes (SQT) groups. The highest number of different BVOCs was observed in Spruce (47), followed by Austrian pine (46) and Scots pine (42). Although the number of different BVOCs among investigated species does not differ significantly, the two-way ANOVA revealed relevant differences between the mean values of the BVOCs content among the investigated species.
The most dominant terpenes in the Austrian pine were α-pinene (MT), germacrene D and trans-caryophyllene (SQT). Previous phytochemical analysis with similar methodology on the content of terpenes in Austrian pine matches with our results, reporting dominance of α-pinene and limonene as MT and trans-caryophyllene among SQT, as major detected compounds [58]. During research on the impact of herbivore on the BVOCs content and emission of Austrian pine, previously published research [59] reported 3-carene, α-pinene and myrcene as major emitted terpenes from the control group of plants. Our results have shown that the most abundant terpenes in the needles of Scots pine were monoterpenes: α-pinene, β-pinene and γ-muurolene, whilst others [40] reported α-pinene, β-pinene, camphene and limonene as major terpenes of this species. Considering Scots pine terpene emission measurements, it was reported that MT were the highest of the total terpene emission during long-term measurement, and that 3-carene, α-pinene and myrcene were dominant MT, while trans-caryophyllene was dominant among detected SQT [38]. The results of the analysis of Spruce terpenes showed dominance of SQT: bornyl acetate and MT: camphene and α-pinene. Our results match with previously reported α-pinene, β-pinene, camphene, myrcene, limonene and bornyl acetate as dominant terpenes in Spruce [40]. The measurements of the Spruce terpenes emission and their content in the air [38] showed prevalence of MT: α-pinene, limonene and camphene, while dominant SQT were transcaryophyllene and α-humulene.
Considering the obtained results, higher dominance of MT in the majority of the presented cases can be justified by higher reactivity of SQT in the air. Previously published results [35] reported that MT lifetime in the air during the day can last up to 3 h, while the lifetime of SQT is less than 4 min. During long-term measurements of air concentrations of MT and SQT, certain authors [56] reported higher concentrations of MT, as well as the significant amount of SQT. Their further research [60] showed higher content of SQT in the air compared to MT. The difference in the results of the terpene content in the air is a consequence of the multiple different factors that influence the concentration of BVOCs in the air. Their biosynthesis in plants is highly dependent on the genetic traits, as well on the impact of abiotic and biotic stressors, such as ozone or the presence of the insects, mites and pathogens [35,61,62]. Finally, the higher content of the other chemical compounds in the air that BVOCs could react with, mostly those with anthropogenic origin, such as nitrogen oxides and sulfur dioxide, would result with the formation of the secondary organic aerosols in the air [63] and decrement of the lifetime of certain BVOC.
Complex correlation among functional and genetical traits and BVOCs emission in tree species, as well as their pollution mitigation capacity and high reactivity with other air compounds has been previously described [61]. It was previously concluded that there are results claiming a significant difference in the emission of terpenes from trees, not only among different tree species, but also that different individuals within the same speciesgenotypes have their own terpene profile [40]. The results of our research also support this thesis. The results we have gained not only show significant differences in terpene composition within different coniferous species, they also show significant differences among the genotypes from the same species. Major intra-variations in Austrian pine genotypes were detected in the content of MT: limonene, terpinolene, β-pinene, linalool and camphene. Similar variability has been observed among the Scots pine genotypes and their BVOCs content. High inter-variability within this species has also been observed on the content of MT: terpinolene, camphene and limonene. High variations within BVOCs content in Spruce were recorded in the content among MT: camphor, linalool, citronellol and SQT: α-cadinol, α-copaene and trans-caryophyllene. The majority of these compounds were previously characterized to have a positive effect on human health (Table 3). Table 3. Literature evidence relative to biological activities of BVOCs detected in the investigated species.

Phytoncide
Effects References

Health Effects of the Detected Terpenes
The results of our research show that monoterpenes (MT) and sesquiterpenes (SQT) were characterized as major BVOCs components in all of the examined material. The PCA on the BVOCs content in the examined species showed grouping within species, and clear prevalence of the Spruce in comparison to Pinus species in considering the overall BVOCs content. The results also show the presence of terpenes previously characterized as health improving agents in Spruce that were not detected in the investigated Pinus species. Since Spruce has more versatile terpene composition compared to the Pinus species, it can be concluded that this species has more potential for human health improvement through the impact of individual terpenes, as well as through a greater possibility of creations of new blends that impact human health. Detected BVOCs characterized with the potential for human health and well-being improvement based on previously published results are listed in Table 3.
Based on the obtained results, the α-pinene is found to be the major BVOCs constituent of Pinus species, with measured mean values for Scots pine (47.58%) and Austrian pine (38.52%). Previously published results indicate that α-pinene has anticancer and neuroprotective impact on humans [64]. Other research also suggest that α-pinene and γ-terpinene have sedative and anesthetic properties [72]. Although the results have shown higher concentrations of α-pinene in the Pinus species, higher values of γ-terpinene were detected in Spruce.
Recent research presented the potential that BVOCs in general have for the treatment of viral infections [92], while a previous study showed the positive impact of these compounds, among which α-cadinol was found in the investigated species for antiviral therapy [90]. The highest content of this compound has been detected in Spruce. Additionally, high content of germacrene D, that has anticancer properties [93] in Austrian pine and β-pinene, characterized as anticancer and neuroprotective [64] in Scots pine, corresponds to the observation that natural environments with Scots pine and Austrian pine as dominant species could contribute to the upgrade of the therapeutic methods dealing with certain diseases and afflictions that significantly affect global society, such as cancer, depression and anxiety.
One of the major BVOCs constituent of Spruce is camphene, previously characterized with anticancer activity [85], together with α-pinene, as one of the three major BVOCs constitutes of this species. This indicates that Spruce volatiles could be beneficial for further research on prevention and therapeutic methods of treatment. The content of αterpineol, that was characterized as anticancer [94], antioxidant [95], anti-inflammatory [96] and antibacterial constituents [9] differed among species. The highest mean value of this compound was observed in Spruce. Interestingly, citronellol, described as an anticancer [88] and anti-inflammatory agent [89], was only detected in Spruce, although in small amount.

Health Forest Ecosystem Services and Terpenes
A recent review paper on health-related ES highlights that within the overall research on this subject, fourteen direct health-related ES were defined, while placing emphasis on the fact that the majority of these did not specify which human health outcomes are affected by these ES [21]. None of this research observed forest BVOCs as a potential for human health and well-being improvement. The influence of certain positive effects of forest BVOCs may be limited, since the content of these compounds in the air is highly dependent on the environmental conditions [61,97], but medical reports claim that even the mild interaction may be beneficial for individual human well-being [53], which created the need for further research.
Considering the research on natural environments and humans, certain authors [49] claim that the main challenge is to apply the knowledge on the nature and health relationship in such manner that a fuller potential of these relationships is realized. Although the link between the ES and social benefits is evident, especially in terms of the human health improvement and the current pandemic, there is still a major difference among the approaches and the methodologies between ecosystem and health researchers [21]. Considering current medical research on this subject, there is a need for more in-depth investigation of the relationship between environment, human health and detailed forest characteristics [98]. Still, the major constrain in the investigations that are considering the impact of forests on human health is the lack of the support of forestry research, which could contribute to the more accurately defined potential of specific habitats and species for preventive medicine and clinical practice guidelines [23,24].
An additional constraint in considering the final definition of the role of BVOCs in terms of health-related ES is the lack of the large studies that assess the influence of forest BVOCs on human health in real-life situations [53]. The results presented in this paper may prove to be a good basis upon which further research can be implemented.
By observing detected BVOCs content as plant potential to evaporate these compounds that act as potential health and well-being improvement agents, the presented research aims to investigate if there are BVOCs that could improve human health which are present in the investigated species and their inter-and intra-variations. While the investigated species show evident potential for human health improvement, there is a need for further research in order to provide enough information for future investigation and valuation of health-related forest ES.

Conclusions
Multiple different BVOCs, previously characterized as potential human health and well-being improving agents, were detected in the needles of the investigated species and their genotypes. Furthermore, the results of the conducted statistical analyses confirmed that there is a relevant statistical difference among the content of BVOCs in three investigated species: Austrian pine, Scots pine and Spruce. Moreover, high variations among genotypes of the investigated species were observed in the content of BVOCs relevant for human health improvement, such as limonene, terpinolene, β-pinene, linalool, camphene, camphor, citronellol and α-cadinol.
The high intra-and inter-variations of BVOCs content that have previously been characterized as having positive effects on human health by acting as anticancer, antiinflammatory, antiviral, antibacterial and neuroprotective agents, indicates that there is a basis for further research aimed at providing a better understanding of tree BVOCs in health-related ES.
Primarily, the presented research is intended to sharpen the role of ecosystem services, specifically forest health ES and forest BVOCs for human health improvement, by linking different disciplines and state of the art knowledge in order to provide a basis for further research and to remove current barriers in this research field.