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Abstract: Most plant‐based emissions of volatile organic compounds are considered mainly temperature dependent. However, certain oxygenated volatile organic compounds (OVOCs) have high water solubility; thus, also stomatal conductance could regulate their emissions from shoots. Due to their water solubility and sources in stem and roots, it has also been suggested that their emissions could be affected by transport in the xylem sap. Yet further understanding on the role of transport has been lacking until present. We used shoot‐scale long‐term dynamic flux data from Scots pines (Pinus sylvestris) to analyse the effects of transpiration and transport in xylem sap flow on emissions of 3 water‐soluble OVOCs: methanol, acetone, and acetaldehyde. We found a direct effect of transpiration on the shoot emissions of the 3 OVOCs. The emissions were best explained by a regression model that combined linear transpiration and exponential temperature effects. In addition, a structural equation model indicated that stomatal conductance affects emissions mainly indirectly, by regulating transpiration. A part of the temperature's effect is also indirect. The tight coupling of shoot emissions to transpiration clearly evidences that these OVOCs are transported in the xylem sap from their sources in roots and stem to leaves and to ambient air. [ABSTRACT FROM AUTHOR]

Tree canopies are known to emit large amounts of VOCs (volatile organic compounds) such as monoterpenes to the surrounding air. The main source for these is considered to be the green biomass, i.e. foliage, but emissions from the woody compartments have not been quantified. A VOC emission anomaly has been observed during transition from winter to summer activity. We analyzed if non-foliar components could partially explain the anomaly. We measured the VOC emissions from Scots pine (Pinus sylvestris L.) stems and shoots during the dehardening phase of trees in field conditions in two consecutive springs. We observed a large, transient monoterpene burst from stems, while the shoot monoterpene emissions and transpiration remained low. The burst lasted about 12 h. Simultaneously, an unusual night-time sap flow and an anomalous diurnal pattern of tree diameter were detected. Hence, we suggest that the monoterpene burst was a consequence of the recovery of the stem from winter-time. This indicates that the dominant processes and environmental drivers triggering the monoterpene emissions are different between stems and foliage. [ABSTRACT FROM AUTHOR]

Aims: The possibility of NOx (NO + NO2) emissions from plants and the underlying mechanisms are still under discussion. Excess NO created possibly as a result of nitrite accumulation in plant leaves has been suggested to result in emissions. Such emission has been observed in laboratory conditions due to nitrate fertilization. In this study, we tested whether nitrate fertilization of Scots pine seedlings growing outdoors leads to accumulation of NO3- or NO2- in the needles and subsequent NOx emission. Methods: The experiment was done at the SMEAR II station in Southern Finland. The seedlings received nitrate or ammonium fertilizer or neither. Shoot NOx emissions were measured with dynamic chambers. Total dissolved nitrogen, inorganic nitrogen, NO3-, NO2- and NH4+ concentrations in the soil and needles were determined. Results: NOx fluxes from the shoots were, on average, deposition. There was no indication of fertilization-induced NO or NOx emissions. The highest NO fluxes were observed at night and were humidity-related. Conclusions: It seems unlikely that additional nitrate in the soil could cause significant NOx emission from boreal Scots pine forests in field conditions, possibly because of soil chemistry. [ABSTRACT FROM AUTHOR]

Biogenic volatile organic compounds (BVOCs) are essential in atmospheric chemistry because of their chemical reactions that produce and destroy tropospheric ozone, their effects on aerosol formation and growth, and their potential influence on global warming. As one of the important BVOC groups, monoterpenes have been a focus of scientific attention in atmospheric research. Detailed regional measurements and model estimates are needed to study emission potential and the monoterpene budget on a global scale. Since the use of empirical measurements for upscaling is limited by many physical and biological factors such as genetic variation, temperature and light, water availability, seasonal changes, and environmental stresses, comprehensive inventories over larger areas are difficult to obtain. We applied the boundary layer-chemistry-transport model SOSA to investigate Scots pine (Pinus sylvestris) monoterpene emissions in a boreal coniferous forest at the SMEAR II site, Southern Finland. SOSA was applied to simulate monoterpene emissions with three different emission modules: the semi-empirical G95, MEGAN 2.04 with improved descriptions of temperature and light responses and including also carbonyl emissions, and a process-based model SIM-BIM. For the first time, the emission models included seasonal and diurnal variations in both quantity and chemical species of emitted monoterpenes, based on parameterizations obtained from field measurements. Results indicate that modelling and observations agreed reasonably well, and that the model can be used for investigating regional air chemistry questions related to monoterpenes. The predominant modelled monoterpene concentrations, α-pinene and Δ³-carene, are consistent with observations. [ABSTRACT FROM AUTHOR]

Estimates of volatile organic compound (VOC) emissions from forests are based on the assumption that foliage has a steady emission potential over its lifetime, and that emissions are mainly modified by short term variations in light and temperature. However, in many field studies this has been challenged, and high emissions and atmospheric concentrations have been measured during periods of low biological activity such as in springtime. We conducted measurements during three years, using an online gas-exchange monitoring system to observe volatile organic emissions from a mature (1 yr old) and a growing Scots pine shoot. The emission rates of organic vapours (monoterpenes, methyl butenol (MBO), acetone and methanol) from vegetative buds of Scots pine during the dehardening and rapid shoot growth stages were one to two orders of magnitude higher than those from mature foliage. The normally assumed temperature dependency was not sufficient to explain the variations in emission rates during spring. The diurnal emission pattern of growing shoots differed from the diurnal cycle in temperature as well as from the diurnal emission pattern of mature shoots, which may be related to processes involved in shoot or needle elongation. Our findings imply that global estimations of monoterpene emission rates from forests are in need of revision, and that the physiological state of the plants should be taken into account when emissions of the reactive gases such as monoterpenes are estimated. The significant interannual variation in emission rates, related to changes in plant metabolic activity, has important implications to the aerosol precursor concentrations and chemical reactions in atmosphere, and potentially offers an explanation for the frequent aerosol formation events in spring. [ABSTRACT FROM AUTHOR]

Background and Aims: Volatile organic compound (VOC) emissions from biogenic sources are important contributors to chemical reactions in the air. Soil/forest floor VOCs contribute significantly to the ecosystem scale emissions, however, these emissions and their temporal and spatial variations are poorly characterised. The below-canopy VOC emissions have been measured mainly in campaigns; continuous measurements over the whole growing season are rare. Methods: VOCs were measured from boreal forest floor over the snow-free season 2010 in southern Finland with automated flow-through chambers connected to proton transfer reaction-mass spectrometer (PTR-MS). We measured 10 masses in total, of which five quantitatively (M33, M45, M59, M69, M137). Results: All of the fluxes showed clear diurnal and seasonal variation, being at their highest in early summer. Spatial variation in the fluxes was great and the lowest rates were found in chambers with dense vegetation cover. Also, VOCs deposition was observed regularly. Monoterpene (M137) emissions were one magnitude higher (up to 264 ng m s) than other emissions. The VOC fluxes correlated positively with temperature and light, while relative humidity correlated negatively. Conclusions: Results indicated that forest floor plays a substantial role in the boreal forest total VOC emissions. Understanding the processes controlling VOC emissions requires more detailed analysis and long-time measurements with sufficient time resolution and analytical accuracy. [ABSTRACT FROM AUTHOR]

Branch enclosure based emission rates of monoterpenes and sesquiterpenes from four Scots pines (Pinus sylvestris) and one Norway spruce (Picea abies), as well as the ambient mixing ratios of monoterpenes were determined during the HUMPPA-COPEC 2010 summer campaign. Differences in chemical composition and in emission strength were observed between the different trees, which confirmed that they represented different chemotypes. The chemotypes of Scots pine can be classified according to species with high, no and intermediate content of Δ-3- carene. The "non-Δ-3-carene" chemotype was found to be the strongest emitter of monoterpenes. From this chemotype, β-myrcene, a very reactive monoterpene, was the dominant species accounting for more than 32% of the total emission rates of isoprenoids followed by β-phellandrene (∼27%). Myrcene fluxes ranged from 0.8 to 24 µg g-1 (dw) h-1. α- Farnesene was the dominant sesquiterpene species, with average emission rates of 318 ng g-1 (dw) h-1. In the high Δ- 3-carene chemotype, more than 48% of the total monoterpene emission was Δ-3-carene. The average Δ-3-carene emission rate (from chemotype 3), circa 609 ng g-1 (dw) h-1 reported here is consistent with the previously reported summer season value. Daily maximum temperatures varied between 20 and 35 °C during the measurements. The monoterpene emissions from spruce were dominated by limonene (35%), β-phellandrene (15%), α-pinene (14%) and eucalyptol (9%). Total spruce monoterpene emissions ranged from 0.55 up to 12.2 µg g-1 (dw) h-1. Overall the total terpene flux (monoterpenes + sesquiterpenes) from all studied tree species varied from 230 ng g-1 (dw) h-1 up to 66 µg g-1 (dw) h-1. Total ambient monoterpenes (including α-pinene, Δ-3-carene, β-pinene and β-myrcene) measured during the campaign varied in mixing ratio from a fewpptv to over one ppbv. The most abundant biogenic VOC measured above the canopy were α-pinene and Δ-3-carene, and these two compounds together contributed more than 50%of the total monoterpenes. The ambient data reflect the emission rate, atmospheric reactivity and tree type abundance. The diel cycles of isoprenoid mixing ratios showed high levels during the night-time which is consistent with continued low nocturnal emission and a low and stable boundary layer. The chirality of α-pinene was dominated by (+)-enantiomers both in the direct emission and in the atmosphere. The two highest emitters showed no enantiomeric preference for α-pinene emissions, whereas the two lowest emitting pines emitted more (+)-enantiomer. The spruce emissions were dominated by (--)-enantiomer. The exceptionally hot temperatures in the summer of 2010 led to relatively strong emissions of monoterpenes, greater diversity in chemical composition and high ambient mixing ratios. [ABSTRACT FROM AUTHOR]

Atmospheric chemistry in background areas is strongly influenced by natural vegetation. Coniferous forests are known to produce large quantities of volatile vapors, especially terpenes to the surrounding air. These compounds are reactive in the atmosphere, and contribute to the formation and growth of atmospheric new particles. Our aim was to analyze the variability of mono- and sesquiterpene emissions between Scots pine trees, in order to clarify the potential errors caused by using emission data obtained from only a few trees in atmospheric chemistry models. We also aimed at testing if stand history and seed origin has an influence on the chemotypic diversity. The inherited, chemotypic variability in mono- and sesquiterpene emission was studied in a seemingly homogeneous 47-yr-old stand in Southern Finland, where two areas differing in their stand regeneration history could be distinguished. Sampling was conducted in August 2009. Terpene concentrations in the air had been measured at the same site for seven years prior to branch sampling for chemotypes. Two main compounds, α-pinene and Δ³-carene formed together 40-97% of the monoterpene proportions in both the branch emissions and in the air concentrations. The data showed a bimodal distribution in emission composition, in particular in Δ³-carene emission within the studied population. 10% of the trees emitted mainly α-pinene and no Δ³-carene at all, whereas 20% of the trees where characterized as high Δ³-carene emitters (Δ³-carene forming > 80% of total emitted monoterpene spectrum). An intermediate group of trees emitted equal amounts of both α-pinene and Δ³-carene. The emission pattern of trees at the area established using seeding as the artificial regeneration method differed from the naturally regenerated or planted trees, being mainly high Δ³-carene emitters. Some differences were also seen in e.g. camphene and limonene emissions between chemotypes, but sesquiterpene emissions did not differ significantly between trees. The atmospheric concentrations at the site were found to reflect the species and/or chemodiversity rather than the emissions measured from any single tree, and were strongly dominated by α-pinene. We also tested the effect of chemodiversity on modeled monoterpene concentrations at the site and found out that since it significantly influences the distributions and hence the chemical reactions in the atmosphere, it should be taken into account in atmospheric modeling. [ABSTRACT FROM AUTHOR]

In recent years evidence has emerged that the amount of isoprene emitted from a leaf is affected by the CO2 growth environment. Many -- though not all -- laboratory experiments indicate that emissions increase significantly at below-ambient CO2 concentrations and decrease when concentrations are raised to above-ambient. A small number of process-based leaf isoprene emission models can reproduce this CO2 stimulation and inhibition. These models are briefly reviewed, and their performance in standard conditions compared with each other and to an empirical algorithm. One of the models was judged particularly useful for incorporation into a dynamic vegetation model framework, LPJ-GUESS, yielding a tool that allows the interactive effects of climate and increasing CO2 concentration on vegetation distribution, productivity, and leaf and ecosystem isoprene emissions to be explored. The coupled vegetation dynamics-isoprene model is described and used here in a mode particularly suited for the ecosystem scale, but it can be employed at the global level as well. Annual and/or daily isoprene emissions simulated by the model were evaluated against flux measurements (or model estimates that had previously been evaluated with flux data) from a wide range of environments, and agreement between modelled and simulated values was generally good. By using a dynamic vegetation model, effects of canopy composition, disturbance history, or trends in CO2 concentration can be assessed. We show here for five model test sites that the suggested CO2-inhibition of leaf-isoprene metabolism can be large enough to offset increases in emissions due to CO2-stimulation of vegetation productivity and leaf area growth. When effects of climate change are considered atop the effects of atmospheric composition the interactions between the relevant processes will become even more complex. The CO2-isoprene inhibition may have the potential to significantly dampen the expected steep increase of ecosystem isoprene emission in a future, warmer atmosphere with higher CO2 levels; this effect raises important questions for projections of future atmospheric chemistry, and its connection to the terrestrial vegetation and carbon cycle. [ABSTRACT FROM AUTHOR]

The seasonal variation of mono-and sesquiterpene emission rates of Scots pine was measured from April to October in 2004. The emission rates were measured daily in the afternoons with the exception of weekends. Emissions were measured from two branches; one of them was debudded in May (branch A), while the other was allowed to grow new needles (branch B). The monoterpene emission pattern remained almost constant throughout the measurement period, Δ³-carene being the dominant monoterpene (50-70% of the VOC emission). The standard monoterpene emission potential was highest during early summer in June (the average of the two branches 0.55 µg g(dw)-1 h-1) and lowest during early autumn in September (the average of the two branches 0.12 µg g(dw)-1 h-1). The monoterpene emission potential of branch A remained low also during October, whereas the emission potential of branch B was very high in October. The sesquiterpenes were mainly emitted during mid summer, the dominant sesquiterpene being β-caryophyllene. Branch A had a higher sesquiterpene emission potential than branch B and the emission maximum occurred concomitant with the high concentration of airborne pathogen spores indicating the defensive role of the sesquiterpene emissions. The sesquiterpene emissions were well correlated with linalool and 1,8-cineol emissions, but not with monoterpenes. Sesquiterpene and 1,8-cineol emissions were equally well described by the temperature dependent and the temperature and light dependent algorithms. This is due to the saturation of the 20 light algorithm as the measurements were always conducted during high light conditions. [ABSTRACT FROM AUTHOR]

The volatile organic compound (VOC) emission rates of Scots pine (Pinus sylvestris L.) were measured from trees growing in a natural forest environment at two locations in Finland. The observed total VOC emission rates varied between 21 and 874 ng g-1 h-1 and 268 and 1670 ng g-1 h-1 in southern and northern Finland, respectively. A clear seasonal cycle was detected with high emission rates in early spring, a decrease of the emissions in late spring and early summer, high emissions again in late summer, and a gradual decrease in autumn. The main emitted compounds were Δ³-carene (southern Finland) and α- and β-pinene (northern Finland), with approximate relative contributions of 60-70% and 60-85% of the total observed monoterpene emission rates, respectively. Sesquiterpene (β-caryophyllene) and 2-methyl-3-buten-2-ol (MBO) emissions were initiated in early summer at both sites. The observed MBO emission rates were between 1 and 3.5% of the total monoterpene emission rates. The sesquiterpene emission rates varied between 2 and 5% of the total monoterpene emission rates in southern Finland, but were high (40%) in northern Finland in spring. Most of the measured emission rates were found to be well described by the temperature dependent emission algorithm. The calculated standard emission potentials were high in spring and early summer, decreased somewhat in late summer, and were high again towards autumn. The experimental coefficient β ranged from 0.025 to 0.19 (average 0.10) in southern Finland, with strongest temperature dependence in spring and weakest in late summer. Only the emission rates of 1,8-cineole were found to be both light and temperature dependent. [ABSTRACT FROM AUTHOR]

Abstract: In boreal forests, canopy-scale emissions of biogenic volatile organic compounds (BVOCs) are rather well characterised, but knowledge of ecosystem-scale BVOC emissions is still inadequate. We used adsorbent tubes to measure BVOCs from a boreal Scots pine (Pinus sylvestris L.) forest floor in southern Finland and analysed the compounds with a gas chromatograph–mass spectrometer. The most abundant compound group was the monoterpenes (averaging 5.04μgm−2 h−1), in which α-pinene, Δ3-carene and camphene contributed over 90% of the emissions. Emissions of other terpenoids (isoprene and sesquiterpenes) were low (averaging 0.05 and 0.04μgm−2 h−1, respectively). BVOC emissions from the forest floor varied seasonally, peaking in early summer and autumn, with most of the compounds following similar patterns. The emission pattern was sustained throughout the measurement period, suggesting that the main sources of the emissions remained more or less stable. We compared the BVOC fluxes with environmental parameters such as temperature, precipitation and PAR, and with fluxes of other trace gases (CO2, CH4, N2O), as well as with ground vegetation photosynthesis and with litter input. Several of these parameters were correlated with the presence of BVOCs. The sources of soil BVOC emissions are very poorly understood, but our results suggest, that changes in litter quantity and quality, soil microbial activity and the physiological stages of plants are linked with changes in BVOC fluxes. [Copyright &y& Elsevier]