Plants emit biogenic volatile organic compounds (BVOCs) causing transcriptomic, metabolomic and behavioral responses in receiver organisms. Volatiles involved in such responses are often called "plant language". Arthropods having sensitive chemoreceptors can recognize language released by plants. Insect herbivores, pollinators and natural enemies respond to composition of volatiles from plants with specialized receptors responding to different types of compounds. In contrast, the mechanism of how plants "hear" volatiles has remained obscured. In a plant-plant communication, several individually emitted compounds are known to prime defense response in receiver plants with a specific manner according to the chemical structure of each volatile compound. Further, composition and ratio of volatile compounds in the plant-released plume is important in plantinsect and plant-plant interactions mediated by plant volatiles. Studies on volatile-mediated plant-plant signaling indicate that the signaling distances are rather short, usually not longer than one meter. Volatile communication from plants to insects such as pollinators could be across distances of hundreds of meters. As many of the herbivore induced VOCs have rather short atmospheric life times, we suggest that in long-distant communications with plant volatiles, reaction products in the original emitted compounds may have additional information value of the distance to emission source together with the original plant-emitted compounds.

Living trees are the main source of biogenic volatile organic compounds (BVOCs) in forest ecosystems, but substantial emissions originate from leaf and wood litter, the rhizosphere and from microorganisms. This review focuses on temperate and boreal forest ecosystems and the roles of BVOCs in ecosystem function, from the leaf to the forest canopy and from the forest soil to the atmosphere level. Moreover, emphasis is given to the question of how BVOCs will help forests adapt to environmental stress, particularly biotic stress related to climate change. Trees use their vascular system and emissions of BVOCs in internal communication, but emitted BVOCs have extended the communication to tree population and whole community levels and beyond. Future forestry practices should consider the importance of BVOCs in attraction and repulsion of attacking bark beetles, but also take an advantage of herbivore-induced BVOCs to improve the efficiency of natural enemies of herbivores. BVOCs are extensively involved in ecosystem services provided by forests including the positive effects on human health. BVOCs have a key role in ozone formation but also in ozone quenching. Oxidation products form secondary organic aerosols that disperse sunlight deeper into the forest canopy, and affect cloud formation and ultimately the climate. We also discuss the technical side of reliable BVOC sampling of forest trees for future interdisciplinary studies that should bridge the gaps between the forest sciences, health sciences, chemical ecology, conservation biology, tree physiology and atmospheric science.

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Isoprene (IP) and monoterpenes (MTP) are the most abundant biogenic volatile organic compounds (BVOCs) emitted by terrestrial vegetation, particularly by forests. Global IP and MTP emissions are estimated to be 460 TgC yr −1 and 117 TgC yr −1 , respectively, representing 80% of the total BVOC emissions. These components have a significant impact on atmospheric chemistry and physics.Long term ecosystem-scale BVOC flux measurement is the sole method allowing quantification and description of BVOC emission responses to episodic events like budburst or stress; follow up of emission during vegetation growth and analysis of interaction with climate and environment. We will analyse the IP and MTP fluxes measured above a mixed temperate forest in order to illustrate the interest of long-term flux measurements by investigating the main driving variables and the underlying mechanisms of emission, how de novo carbon allocation to the isoprene/monoterpenes skeleton structure is altered through time. A disjunct eddy covariance system was installed at the forested site of Vielsalm (Belgium) from July to October 2009 and from April to October 2010 covering thus most of the vegetation season (spring, summer and first part of autumn). The system was completed by micrometeorological measurements and an eddy covariance system measuring continuously CO 2 and H 2 O fluxes.During the day, IP and MTP fluxes were mainly controlled by air temperature and light. This behavior resulted largely from a response of IP and MTP flux to photosynthesis itself. Indeed, a strong linear relation was found between these fluxes and the Gross Primary Production. In addition to these responses, a flux seasonal evolution was observed: a decrease in the standard emission factor was observed, probably due to acclimation or senescence. The standard emission factor (30˚C, 1000 µmol m −2 s −1 ) varied from 0.91 ± 0.01 to 0.56 ± 0.02 µg m −2 s −1 for IP fluxes and from 0.74 ± 0.03 to 0.27 ± 0.03 µg m −2 s −1 for MTP fluxes.During the night, IP flux was zero but a slight positive MTP flux was observed that seemed to be driven by air temperature. These night emissions were probably due to the volatility of monoterpenes stored in the needle resin ducts of coniferous species. There could also be a contribution from the soil through litter decomposition, from roots or from micro-organisms. The standard emission factor (30˚C) for night-time MTP fluxes was equal to 0.093 ± 0.019 µg m −2 s −1 .

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a b s t r a c tMonoterpenoid emissions from Fagus sylvatica L. trees have been measured at light-and temperaturecontrolled conditions in a growth chamber, using Proton Transfer Reaction Mass Spectrometry (PTR-MS) and the dynamic branch enclosure technique.De novo synthesized monoterpenoid Standard Emission Factors, obtained by applying the G97 algorithm (Guenther, 1997), varied between 2 and 32 mg g DW À1 h À1 and showed a strong decline in late August and September, probably due to senescence. The response of monoterpenoid emissions to temperature variations at a constant daily light pattern could be well reproduced with a modified version of the MEGAN algorithm (Guenther et al., 2006), with a typical dependence on the average temperature over the past five days.The diurnal emissions at constant temperature showed a typical hysteretic behaviour, which could also be adequately described with the modified MEGAN algorithm by taking into account a dependence on the average light levels experienced by the trees during the past 10e13 h.The impact of the past light and temperature conditions on the monoterpenoid emissions from F. sylvatica L. was found to be much stronger than assumed in previous algorithms.Since our experiments were conducted under low light intensity, future studies should aim at confirming and completing the proposed algorithm updates in sunny conditions and natural environments.

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