# Forest Volatile Organic Compounds and Their Effects on Human Health: A State-of-the-Art Review
## 1 Introduction
### Non-Tree-Derived Forest BVOCs  
In temperate forests, the canopy layer of trees acts as the largest emission source of BVOCs [19] and, in general, from a quantitative point of view, plant leaves with their canals, oil glands, and glandular trichomes are the major emitters of these volatile compounds [8; 10]. However, all plant organs and tissues produce BVOCs: leaves, flowers, and fruits release them into the atmosphere, whereas roots secrete them into the soil. Moreover, wood, phloem, and bark of trunks can serve as pools of stored BVOCs [19]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Constitutive Forest VOCs** & **Herbivore-Induced Plant Volatiles (HIPVs)** \\ \hline Reduction of abiotic stress. Isoprene and monoterepnes increase general thermal tolerance of photosynthesis, protect photosynthetic apparatus and its activity under high-temperature stress by stabilizing the thylakoid membranes and quenching Reactive Oxygen Species (ROS) & Reduction of abiotic stress. Isoprene and monoterepnes increase general thermal tolerance of photosynthesis, protect photosynthetic apparatus and its activity under high-temperature stress by stabilizing the thylakoid membranes and quenching Reactive Oxygen Species (ROS) & Reduction of abiotic stress. Isoprene and monoterepnes increase general thermal tolerance of photosynthesis, protect photosynthetic apparatus and its activity under high-temperature stress by stabilizing the thylakoid membranes and quenching Reactive Oxygen Species (ROS) \\ \hline Defense against herbivores. Comprises toxic, repellent, anti-nutritive constitutive BVOCs (biogenic Volatile Organic Compounds) or HIPVs, as well as growth and reproductive reducers & Defense against herbivores, mainly indirectly but also directly. HIPVs and volatile compounds that attract, nourish, or otherwise favor another organism that reduces herbivore pressure \\ \hline Inter-plant signaling. HIPVs, especially Green Leaf Volatiles (GLVs), and constitutive BVOCs can travel from a herbivore-damaged part to other plants (both conspecific and heterospecific), autocrine defense genes and priming a more vigorous response after an attack & Inter- and intra-plant signaling. HIPVs, especially GLVs, and constitutive BVOCs can travel from a herbivore-damaged part to an undamaged one, or to other plants (both conspecific and heterospecific), activating defense genes and priming a more vigorous response after an attack \\ \hline Defense against microbial pathogens & Defense against microbial pathogens \\ \hline Allelopathy. Inhibition of competing species’ seed germination and competition & \\ \hline Attraction of pollinators and seed dispersers & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Functions of constitutive and herbivore-induced forest Volatile Organic Compounds (VOCs). Figure 1: BVOCs’ functions in relation to biotic and abiotic stresses. Adapted from [PERSON] et al. (2009) [8]. Nevertheless, BVOCs are not exclusively tree-derived: soil bacteria, mycorrhizal fungi, and other rhizosphere microbes can also emit BVOCs, but it is not easy to evaluate the exact contribution of these non-plant sources of BVOCs in the soil or their role in the rhizosphere. This is because their emissions cannot be easily separated from those of the roots [19] and because of experimental difficulties [8]. These limits notwithstanding, one study showed that the emission of sesquiterpenes (in particular, (-)-thujopsen) by fungi (_Laccaria bicolor_) can interact with the ability of _Populus_ trees to develop their root system (in particular, with the proliferation of their lateral roots) [20]. One research article reported that, in the Amazonian rainforest, soil microorganisms can emit large amounts of sesquiterpenes, and these emissions strongly influence atmospheric chemistry near the soil surface and under the canopy, to an extent comparable with canopy emissions themselves [21].