Modelling changes in VOC emission in response to climate change in the continental United States

The alteration of climate is driven not only by anthropogenic activities, but also by biosphere processes that change in conjunction with climate. Emission of volatile organic compounds (VOCs) from vegetation may be particularly sensitive to changes in climate and may play an important role in climate forcing through their influence on the atmospheric oxidative balance, greenhouse gas concentration, and the formation of aerosols. Using the VEMAP vegetation database and associated vegetation responses to climate change, this study examined the independent and combined effects of simulated changes in temperature, CO₂ concentration, and vegetation distribution on annual emissions of isoprene, monoterpenes, and other reactive VOCs (ORVOCs) from potential vegetation of the continental United States. Temperature effects were modelled according to the direct influence of temperature on enzymatic isoprene production and the vapour pressure of monoterpenes and ORVOCs. The effect of elevated CO₂ concentration was modelled according to increases in foliar biomass per unit of emitting surface area. The effects of vegetation distribution reflects simulated changes in species spatial distribution and areal coverage by 21 different vegetation classes. Simulated climate warming associated with a doubled atmospheric CO₂ concentration enhanced total modelled VOC emission by 81.8% (isoprene + 82.1%, monoterpenes + 81.6%, ORVOC + 81.1%), whereas a simulated doubled CO₂ alone enhanced total modelled VOC emission by only + 11.8% (isoprene + 13.7%, monoterpenes + 4.1%, ORVOC + 11.7%). A simulated redistribution of vegetation in response to altered temperatures and precipitation patterns caused total modelled VOC emission to decline by 10.4% (isoprene – 11.7%, monoterpenes – 18.6%, ORVOC 0.0%) driven by a decline in area covered by vegetation classes emitting VOCs at high rates. Thus, the positive effect of leaf-level adjustments to elevated CO₂ (i.e. increases in foliar biomass) is balanced by the negative effect of ecosystem-level adjustments to climate (i.e. decreases in areal coverage of species emitting VOC at high rates).     

Release of lipoxygenase products and monoterpenes by tomato plants as an indicator of Botrytis cinerea-induced stress

Changes in emission of volatile organic compounds (VOCs) from tomato induced by the fungus Botrytis cinerea were studied in plants inoculated by spraying with suspensions containing B. cinerea spores. VOC emissions were analysed using on-line gas chromatography–mass spectrometry, with a time resolution of about 1 h, for up to 2 days after spraying. Four phases were delimited according to the starting point and the applied day/night rhythm of the experiments. These phases were used to demonstrate changes in VOC flux caused by B. cinerea infestation. Tomato plants inoculated with B. cinerea emitted a different number and amount of VOCs after inoculation compared to control plants that had been sprayed with a suspension without B. cinerea spores. The changes in emissions were dependent on time after inoculation as well as on the severity of infection. The predominant VOCs emitted after inoculation were volatile products from the lipoxygenase pathway (LOX products). The increased emission of LOX products proved to be a strong indicator of a stress response, indicating that VOC emissions can be used to detect plant stress at an early stage. Besides emission of LOX products, there were also increases in monoterpene emissions. However, neither increased emission of LOX products nor of monoterpenes is specific for B. cinerea attack. The emission of LOX products is also induced by other stresses, and increased emission of monoterpenes seems to be the result of mechanical damage induced by secondary stress impacts on leaves.     

Drought effects on volatile organic compound emissions from Scots pine stems

Tree stems have been identified as sources of volatile organic compounds (VOCs) that play important roles in tree defence and atmospheric chemistry. Yet, we lack understanding on the magnitude and environmental drivers of stem VOC emissions in various forest ecosystems. Due to the increasing importance of extreme drought, we studied drought effects on the VOC emissions from mature Scots pine (Pinus sylvestris L.) stems. We measured monoterpenes, acetone, acetaldehyde and methanol emissions with custom-made stem chambers, online PTR-MS and adsorbent sampling in a drought-prone forest over the hot-dry summer of 2018 and compared the emission rates and dynamics between trees in naturally dry conditions and under long-term irrigation (drought release). The pine stems were significant monoterpene sources. The stem monoterpene emissions potentially originated from resin, based on their similar monoterpene spectra. The emission dynamics of all VOCs followed temperature at a daily scale, but monoterpene and acetaldehyde emission rates decreased nonlinearly with drought over the summer. Despite the dry conditions, large peaks of monoterpene, acetaldehyde and acetone emissions occurred in late summer potentially due to abiotic or biotic stressors. Our results highlight the potential importance of stem emissions in the ecosystem VOC budget, encouraging further studies in diverse environments.     

Monoterpene emissions from rubber trees (Hevea brasiliensis) in a changing landscape and climate: chemical speciation and environmental control

Emissions of biogenic volatile organic compounds (VOCs) have important roles in ecophysiology and atmospheric chemistry at a wide range of spatial and temporal scales. Tropical regions are a major global source of VOC emissions and magnitude and chemical speciation of VOC emissions are highly plant-species specific. Therefore it is important to study emissions from dominant species in tropical regions undergoing large-scale land-use change, for example, rubber plantations in South East Asia. Rubber trees ( Hevea brasiliensis ) are strong emitters of light-dependent monoterpenes. Measurements of emissions from leaves were made in the dry season in February 2003 and at the beginning of the wet season in May 2005. Major emitted compounds were sabinene, α -pinene and β -pinene, but β -ocimene and linalool also contributed significantly at low temperature and light. Cis -ocimene was emitted with a circadian course independent of photosynthetic active radiation (PAR) and temperature changes with a maximum in the middle of the day. Total isoprenoid VOC emission potential at the beginning of the wet season (94 μg gdw⁻¹ h⁻¹) was almost two orders of magnitude higher than measured in the dry season (2 μg g dw⁻¹ h⁻¹). Composition of total emissions changed with increasing temperature or PAR ramps imposed throughout a day. As well as light and temperature, there was evidence that assimilation rate was also a factor contributing to seasonal regulating emission potential of monoterpenes from rubber trees. Results presented here contribute to a better understanding of an important source of biogenic VOC associated with land-use change in tropical South East Asia.     

Stem emissions of monoterpenes,acetaldehyde and methanol from Scots pine (Pinus sylvestris L.) affected by tree–water relations and cambial growth

Tree stems are an overlooked source of volatile organic compounds (VOCs). Their contribution to ecosystem processes and total VOC fluxes is not well studied, and assessing it requires better understanding of stem emission dynamics and their driving processes. To gain more mechanistic insight into stem emission patterns, we measured monoterpene, methanol and acetaldehyde emissions from the stems of mature Scots pines (Pinus sylvestris L.) in a boreal forest over three summers. We analysed the effects of temperature, soil water content, tree water status, transpiration and growth on the VOC emissions and used generalized linear models to test their relative importance in explaining the emissions. We show that Scots pine stems are considerable sources of monoterpenes, methanol and acetaldehyde, and their emissions are strongly regulated by temperature. However, even small changes in water availability affected the emission potentials: increased soil water content increased the monoterpene emissions within a day, whereas acetaldehyde and methanol emissions responded within 2–4 days. This lag corresponded to their transport time in the xylem sap from the roots to the stem. Moreover, the emissions of monoterpenes, methanol and acetaldehyde were influenced by the cambial growth rate of the stem with 6–10-day lags.     

Origin of volatile organic compound emissions from subarctic tundra under global warming

Warming occurs in the Arctic twice as fast as the global average, which in turn leads to a large enhancement in terpenoid emissions from vegetation. Volatile terpenoids are the main class of biogenic volatile organic compounds (VOCs) that play crucial roles in atmospheric chemistry and climate. However, the biochemical mechanisms behind the temperature‐dependent increase in VOC emissions from subarctic ecosystems are largely unexplored. Using ¹³CO₂‐labeling, we studied the origin of VOCs and the carbon (C) allocation under global warming in the soil–plant–atmosphere system of contrasting subarctic heath tundra vegetation communities characterized by dwarf shrubs of the genera Salix or Betula. The projected temperature rise of the subarctic summer by 5°C was realistically simulated in sophisticated climate chambers. VOC emissions strongly depended on the plant species composition of the heath tundra. Warming caused increased VOC emissions and significant changes in the pattern of volatiles toward more reactive hydrocarbons. The ¹³C was incorporated to varying degrees in different monoterpene and sesquiterpene isomers. We found that de novo monoterpene biosynthesis contributed to 40%–44% (Salix) and 60%–68% (Betula) of total monoterpene emissions under the current climate, and that warming increased the contribution to 50%–58% (Salix) and 87%–95% (Betula). Analyses of above‐ and belowground ^12/13C showed shifts of C allocation in the plant–soil systems and negative effects of warming on C sequestration by lowering net ecosystem exchange of CO₂ and increasing C loss as VOCs. This comprehensive analysis provides the scientific basis for mechanistically understanding the processes controlling terpenoid emissions, required for modeling VOC emissions from terrestrial ecosystems and predicting the future chemistry of the arctic atmosphere. By changing the chemical composition and loads of VOCs into the atmosphere, the current data indicate that global warming in the Arctic may have implications for regional and global climate and for the delicate tundra ecosystems.     

On the induction of volatile organic compound emissions by plants as consequence of wounding or fluctuations of light and temperature

Among the volatile organic compounds (VOCs) emitted by plants, some are characteristic of stress conditions, but their biosynthesis and the metabolic and environmental control over the emission are still unclear. We performed experiments to clarify whether (1) the emission following wounding can occur at distance from the wounding site, from VOC pools subjected to metabolic signals; and (2) the emission of biogenic VOCs generated by membrane damage (e.g. consequent to wounding or ozone exposure) can also be induced by exposure to high light and high temperature, recurrent in nature. In Phragmites australis, leaf cutting caused large and rapid bursts of acetaldehyde both at the cutting site and on parts of the cut leaf distant from the cutting site. This emission was preceded by a transient stomatal opening and did not occur in conditions preventing stomatal opening. This suggests the presence of a large pool of leaf acetaldehyde whose release is under stomatal control. VOCs other than isoprene, particularly acetaldehyde and (E)-2-hexenal, one of the C-6 compounds formed by the denaturation of membrane lipids, were released by leaves exposed to high temperature and high light. The high-temperature treatment (45 degrees C) also caused a rapid stimulation and then a decay of isoprene emission in Phragmites leaves. Isoprene recovered to the original emission level after suspending the high-temperature treatment, suggesting a temporary deficit of photosynthetically formed substrate under high temperature. Emission of C-6 compounds was slowly induced by high temperature, and remained high, indicating that membrane denaturation occurs also after suspending the high-temperature treatment. Conversely, the emission of C-6 compounds was limited to the high-light episode in Phragmites. This suggests that a membrane denaturation may also occur in conditions that do not damage other important plant processes such as the photochemistry of photosynthesis of photoinhibition-insensitive plants. In the photoinhibition-sensitive Arabidopsis thaliana mutant NPQ1, a large but transient emission of (E)-2-hexenal was also observed a few minutes after the high-light treatment, indicating extensive damage to the membranes. However, (E)-2-hexenal emission was not observed in Arabidopsis plants fumigated with isoprene during the high-light treatment. This confirms that isoprene can effectively protect cellular membranes from denaturation. Our study indicates that large, though often transient, VOC emissions by plants occur in nature. In particular, we demonstrate that VOCs can be released by much larger tissues than those wounded and that even fluctuations of light and temperature regularly observed in nature can induce their emissions. This knowledge adds information that is useful for the parameterization of the emissions and for the estimate of biogenic VOC load in the atmosphere.     

Sensitivity of Norway spruce physiology and terpenoid emission dynamics to elevated ozone and elevated temperature under open-field exposure

Volatile organic compounds (VOCs) emitted from vegetation to the atmosphere contribute to global climate change, but climate change factors also affect VOC emission from vegetation. Soil-grown Norway spruce seedlings were exposed to elevated ozone (1.4 × ambient ozone concentration) and elevated temperature (ambient + 1.3 °C) alone and in combination as well as to ambient ozone and temperature treatments under open-field conditions. VOC emissions (mainly terpenoids), genes involved in early steps of plastidial monoterpene and isoprene synthesis, photosynthetic parameters and growth were measured. In July, when daytime elevated ozone concentrations had been over 40 ppb, ozone doubled the total terpenoid emissions by increasing the emissions of many monoterpenes and sesquiterpenes. Elevated temperature changed the terpenoid profile by increasing the emissions of oxygenated monoterpenes, but did not influence total emissions. Terpenoid emission profiles also differed between elevated ozone alone and elevated ozone in combination with elevated temperature. In August, when daytime elevated ozone concentrations had been ca. 30 ppb, significant treatment effects were not found. Ozone and temperature reduced the expression of DXS2B (1-deoxy-d-xylulose 5-phosphate synthase type II), and ozone that of DXR (1-deoxy-d-xylulose 5-phosphate reductoisomerase) in August. Elevated temperature reduced the stem diameter growth, net photosynthesis and stomatal conductance, but elevated ozone did not have effects on these parameters. Results suggest that elevated temperature may not modify the ozone responses, or vice versa, in terms of gas exchange, growth or total terpenoid emission rates of young Norway spruces in a near-future climate. However, observed changes in terpenoid emission profiles may be important in the future climate, as reactivity in the troposphere differs between individual terpenoids.     

Strong correlation between isoprene emission and gross photosynthetic capacity during leaf phenology of the tropical tree species Hymenaea courbaril with fundamental changes in volatile organic compounds emission composition during early leaf development

Changes of the volatile organic compounds (VOC) emission capacity and composition of different developmental stages of the tropical tree species Hymenaea courbaril were investigated under field conditions at a remote Amazonian rainforest site. The basal emission capacity of isoprene changed considerably over the course of leaf development, from young to mature and to senescent leaves, ultimately spanning a wide range of observed isoprene basal emission capacities from 0.7 to 111.5 µg C g^?1 h^?1 during the course of the year. By adjusting the standard emission factors for individual days, the diel courses of instantaneous isoprene emission rates could nevertheless adequately be modelled by a current isoprene algorithm. The results demonstrate the inadequacy of using one single standard emission factor to represent the VOC emission capacity of tropical vegetation for an entire seasonal cycle. A strong linear correlation between the isoprene emission capacity and the gross photosynthetic capacity (GP_max) covering all developmental stages and seasons was observed. The present results provide evidence that leaf photosynthetic properties may confer a valuable basis to model the seasonal variation of isoprenoid emission capacity; especially in tropical regions where the environmental conditions vary less than in temperate regions. In addition to induction and variability of isoprene emission during early leaf development, considerable amounts of monoterpenes were emitted in a light‐dependent manner exclusively in the period between bud break and leaf maturity. The fundamental change in emission composition during this stage as a consequence of resource availability (supply side control) or as a plant's response to the higher defence demand of young emerging leaves (demand‐side control) is discussed. The finding of a temporary emergence of monoterpene emission may be of general interest in understanding both the ecological functions of isoprenoid production and the regulatory processes involved.     

Seasonal variation of monoterpene emission from Malus domestica and Prunus avium

Emission rates of monoterpenes released by apple (Malus domestica Borkh) and cherry (Prunus avium L.) were estimated at different phenological stages. These measurements employed a dynamic flow-through Teflon chamber, sample collection onto cartridges filled with graphitized carbon and thermal desorption gas chromatography-mass spectrometry (GC-MS) for identification and quantification of the emitted volatiles. At full bloom the release of monoterpene hydrocarbons from cherry flowers was 1213 ng g(-1) dry weight (DW) h(-1), exceeding by approximately three-fold the emission rate of apple flowers (366 ng g(-1) DW h(-1)). Observed seasonal variations in biogenic volatile organic compound (VOC) emissions ranged over several order of magnitudes. At fruit-set and ripening stages, in fact, the hydrocarbon emission dramatically decreased reaching the lowest values at harvest time when leaves were fully mature (3-9 ng g(-1) DW h(-1)). Wide diversity in the composition of compounds from the species studied was also recorded. At blooming, linalool contributed significantly to the monoterpene emission from apple (94% of the emitted carbon) while alpha-pinene and camphene represented on average more than 60% of the total emitted volatiles from cherry flowers. Among the monoterpenes identified in flowers, alpha-pinene, camphene and limonene were also found in the foliage emission of both species. Fruit trees are relevant monoterpene emitters only at blooming and thus for a short period of the vegetative cycle. When leaves are fully developed, the carbon loss due to monoterpene emissions related to the photosynthetically carbon gain is negligible.     

Isoprene is more affected by climate drivers than monoterpenes: A meta‐analytic review on plant isoprenoid emissions

Isoprene and monoterpenes (MTs) are among the most abundant and reactive volatile organic compounds produced by plants (biogenic volatile organic compounds). We conducted a meta‐analysis to quantify the mean effect of environmental factors associated to climate change (warming, drought, elevated CO₂, and O₃) on the emission of isoprene and MTs. Results indicated that all single factors except warming inhibited isoprene emission. When subsets of data collected in experiments run under similar change of a given environmental factor were compared, isoprene and photosynthesis responded negatively to elevated O₃ (?8% and ?10%, respectively) and drought (?15% and ?42%), and in opposite ways to elevated CO₂ (?23% and +55%) and warming (+53% and ?23%, respectively). Effects on MTs emission were usually not significant, with the exceptions of a significant stimulation caused by warming (+39%) and by elevated O₃ (limited to O₃‐insensitive plants, and evergreen species with storage organs). Our results clearly highlight individual effects of environmental factors on isoprene and MT emissions, and an overall uncoupling between these secondary metabolites produced by the same methylerythritol 4‐phosphate pathway. Future results from manipulative experiments and long‐term observations may help untangling the interactive effects of these factors and filling gaps featured in the current meta‐analysis.     

Variations in the Volatile Organic Compound Emission Potential of Plant Functional Groups in the Temperate Grassland Vegetation of Inner Mongolia, China

The biogenic volatile organic compounds (VOC) emitted by the vegetation of a terrestrial ecosystem play a key role in both regional air quality and tropospheric chemistry. To describe the general emission properties of VOC of different plant functional groups (PFG) in a typical temperate grassland in Inner Mongolia, China, we randomly selected 175 plant species and measured the quantities of isoprene and monoterpene in situ. Results showed that most plants had low VOC emission potential at the species level,especially for some dominant plants, such as Leymus chinensis Tzvel., Stipa grandis Smirn., and Agropyron cristatum Gaertn. At the PFG level, the lowest VOC emission potential was found for perennial rhizome grasses, a major PFG in a typical temperate grassland ecosystem. The effects of overgrazing and subsequent vegetation succession on the emission of VOC by different plant life form functional groups (PLFG)were also discussed.     

Modelling the effect of the 2018 summer heatwave and drought on isoprene emissions in a UK woodland

Projected future climatic extremes such as heatwaves and droughts are expected to have major impacts on emissions and concentrations of biogenic volatile organic compounds (bVOCs) with potential implications for air quality, climate and human health. While the effects of changing temperature and photosynthetically active radiation (PAR) on the synthesis and emission of isoprene, the most abundant of these bVOCs, are well known, the role of other environmental factors such as soil moisture stress are not fully understood and are therefore poorly represented in land surface models. As part of the Wytham Isoprene iDirac Oak Tree Measurements campaign, continuous measurements of isoprene mixing ratio were made throughout the summer of 2018 in Wytham Woods, a mixed deciduous woodland in southern England. During this time, the United Kingdom experienced a prolonged heatwave and drought, and isoprene mixing ratios were observed to increase by more than 400% at Wytham Woods under these conditions. We applied the state‐of‐the‐art FORest Canopy‐Atmosphere Transfer canopy exchange model to investigate the processes leading to these elevated concentrations. We found that although current isoprene emissions algorithms reproduced observed mixing ratios in the canopy before and after the heatwave, the model underestimated observations by ~40% during the heatwave–drought period implying that models may substantially underestimate the release of isoprene to the atmosphere in future cases of mild or moderate drought. Stress‐induced emissions of isoprene based on leaf temperature and soil water content (SWC) were incorporated into current emissions algorithms leading to significant improvements in model output. A combination of SWC, leaf temperature and rewetting emission bursts provided the best model‐measurement fit with a 50% improvement compared to the baseline model. Our results highlight the need for more long‐term ecosystem‐scale observations to enable improved model representation of atmosphere–biosphere interactions in a changing global climate.     

Seasonal differences in isoprene and light-dependent monoterpene emission by Amazonian tree species

Whereas for extra‐tropical regions model estimates of the emission of volatile organic compounds (VOC) predict strong responses to the strong annual cycles of foliar biomass, light intensity and temperature, the tropical regions stand out as a dominant source year round, with only little variability mainly due to the annual cycle of foliar biomass of drought‐deciduous trees. As part of the Large Scale Biosphere Atmosphere Experiment in Amazônia (LBA‐EUSTACH), a remote secondary tropical forest site was visited in the dry‐to‐wet season transition campaign, and the trace gas exchange of a strong isoprene emitter and a monoterpene emitter are compared to the wet‐to‐dry season transition investigations reported earlier. Strong seasonal differences of the emission capacity were observed. The standard emission factor for isoprene emission of young mature leaves of Hymenaea courbaril was about twofold in the end of the dry season (111.5 μgC g^?1 h^?1 or 41.2 nmol m^?2 s^?1) compared to old mature leaves investigated in the end of the wet season (45.4 μgC g^?1 h^?1 or 24.9 nmol m^?2 s^?1). Standardized monoterpene emission rate of Apeiba tibourbou were 2.1 and 3.6 μgC g^?1 h^?1 (or 0.3 and 0.8 nmol m^?2 s^‐1), respectively. This change in species‐specific VOC emission capacity was mirrored by a concurrent change in the ambient mixing ratios. The growth conditions vary less in tropical areas than in temperate regions of the world, and the seasonal differences in emission strength could not be reconciled solely with meteorological data of instantaneous light intensity and temperature. Hence the inadequacy of using a single standard emission factor to represent an entire seasonal cycle is apparent. Among a host of other potential factors, including the leaf developmental stage, water and nutrient status, and abiotic stresses like the oxidative capacity of the ambient air, predominantly the long‐term growth temperature may be applied to predict the seasonal variability of the isoprene emission capacity. The dry season isoprene emission rates of H. courbaril measured at the canopy top were also compared to isoprene emissions of the shade‐adapted species Sorocea guilleminiana growing in the understory. Despite the difference in VOC emission composition and canopy position, one common algorithm was able to predict the diel emission pattern of all three tree species.     

Hybridization of European oaks (Quercus ilex × Q. robur) results in a mixed isoprenoid emitter type

European oaks have been reported to emit isoprene or monoterpenes derived from recently fixed photosynthetic carbon. The emission type is plant species specific and can be used as chemo‐taxonomic marker. In the present article the isoprenoid biochemical properties of mature Quercus × turneri‘Pseudoturneri’ hybrids resulting from a crossing of a Mediterranean evergreen monoterpene‐emitting species (subgenus Sclerophyllodrys; Quercus ilex L.) and an isoprene‐emitting deciduous oak species (subgenus Lepidobalanus; Quercus robur L.) are described. Both species are compared with respect to the capacity for isoprenoid synthesis and the actual isoprenoid emission pattern of different tree‐types. The analysis showed that the oak hybrid combines properties of both parental species. Furthermore, it could be shown that the enzyme activities of isoprene synthase and monoterpene synthases are reflected in the isoprenoid emission pattern of the hybrids as well as in the observed emission rates.     

Biogenic VOC emissions from forested Amazonian landscapes

A tethered balloon‐sampling platform was used to study biogenic volatile organic compounds (BVOCs) in the atmospheric boundary layer in three distinct moist tropical forest ecoregions, as well as an extensive pasture area, in Amazonia. Approximately 24–40 soundings, including as many as four VOC samples collected simultaneously at various altitudes, were made at each site. Concentrations in the mixed layer increased during morning hours and were relatively constant midday through afternoon. Since most important meteorological and chemical parameters were very similar among the sites during the measurement periods, a BVOC canopy emission model was used with a model of the chemistry of the boundary layer to reproduce the atmospheric concentrations observed. The simulations indicated significantly different midday landscape isoprene and α‐pinene emission rates for the three forest ecoregions (2200, 5300, 9800 μg m^?2 h^?1 isoprene and 90, 120, and 180 μg m^?2 h^?1α‐pinene for the three moist forest ecoregions studied, respectively). The differences in emissions among the ecoregions may be attributed to the species composition, which were markedly different and in which the percentage of isoprene and terpene emitting species also differed significantly.     

Range size and growth temperature influence Eucalyptus species responses to an experimental heatwave

Understanding forest tree responses to climate warming and heatwaves is important for predicting changes in tree species diversity, forest C uptake, and vegetation–climate interactions. Yet, tree species differences in heatwave tolerance and their plasticity to growth temperature remain poorly understood. In this study, populations of four Eucalyptus species, two with large range sizes and two with comparatively small range sizes, were grown under two temperature treatments (cool and warm) before being exposed to an equivalent experimental heatwave. We tested whether the species with large and small range sizes differed in heatwave tolerance, and whether trees grown under warmer temperatures were more tolerant of heatwave conditions than trees grown under cooler temperatures. Visible heatwave damage was more common and severe in the species with small rather than large range sizes. In general, species that showed less tissue damage maintained higher stomatal conductance, lower leaf temperatures, larger increases in isoprene emissions, and less photosynthetic inhibition than species that showed more damage. Species exhibiting more severe visible damage had larger increases in heat shock proteins (HSPs) and respiratory thermotolerance (T_max). Thus, across species, increases in HSPs and T_max were positively correlated, but inversely related to increases in isoprene emissions. Integration of leaf gas‐exchange, isoprene emissions, proteomics, and respiratory thermotolerance measurements provided new insight into mechanisms underlying variability in tree species heatwave tolerance. Importantly, warm‐grown seedlings were, surprisingly, more susceptible to heatwave damage than cool‐grown seedlings, which could be associated with reduced enzyme concentrations in leaves. We conclude that species with restricted range sizes, along with trees growing under climate warming, may be more vulnerable to heatwaves of the future.     

Variations in the Volatile Organic Compound Emission Potential of Plant Functional Groups in the Temperate Grassland Vegetation of Inner Mongolia, China

The biogenic volatile organic compounds (VOC) emitted by the vegetation of a terrestrial ecosystem play a key role in both regional air quality and tropospheric chemistry. To describe the general emission properties of VOC of different plant functional groups (PFG) in a typical temperate grassland in Inner Mongolia, China, we randomly selected 175 plant species and measured the quantities of isoprene and monoterpene in situ. Results showed that most plants had low VOC emission potential at the species level,especially for some dominant plants, such as Leymus chinensis Tzvel., Stipa grandis Smirn., and Agropyron cristatum Gaertn. At the PFG level, the lowest VOC emission potential was found for perennial rhizome grasses, a major PFG in a typical temperate grassland ecosystem. The effects of overgrazing and subsequent vegetation succession on the emission of VOC by different plant life form functional groups (PLFG)were also discussed.     

Impacts of nitrogen fertilization on volatile organic compound emissions from decomposing plant litter

Nonmethane volatile organic compounds (VOCs) are reactive, low molecular weight gases that can have significant effects on soil and atmospheric processes. Research into biogenic VOC sources has primarily focused on plant emissions, with few studies on VOC emissions from decomposing plant litter, another potentially important source. Likewise, although there have been numerous studies examining how anthropogenic increases in nitrogen (N) availability can influence litter decomposition rates, we do not know how VOC emissions may be affected. In this study, we measured the relative contribution of VOCs to the total carbon (C) emitted from decomposing litter and how N amendments affected VOC emissions. We incubated decomposing litter from 12 plant species over 125 days, measuring both CO₂ and VOC emissions throughout the incubation. We found that VOCs represented a large portion of C emissions from a number of the litter types with C emissions as VOCs ranging from 0% to 88% of C emissions as CO₂. Methanol was the dominant VOC emitted, accounting for 28–99% of total VOC emissions over the incubation period. N additions increased CO₂ production in 7 of the 12 litter types by 5–180%. In contrast, N additions decreased VOC emissions in 8 of the 12 litter types, reducing net VOC emissions to near zero. The decrease in VOC emissions was occasionally large enough to account for the increased CO₂ emissions on a per unit C basis, suggesting that N additions may not necessarily accelerate C loss from decomposing litter but rather just switch the form of C emitted. Together these results suggest that, for certain litter types, failure to account for VOC emissions may lead to an underestimation of C losses from litter decomposition and an overestimation of the effects of N additions on rates of litter decomposition.     

Biodegradation of 2,4-dichlorophenol in the presence of volatile organic compounds in soils under different vegetation types

It has been suggested that monoterpenes emitted within the soil profile, either by roots or by decaying biomass, may enhance the biodegradation of organic pollutants. The aim of this study was to evaluate the effect of biogenic volatile organic compounds (VOCs) on the catabolism of 2,4-dichlorophenol in soils. Soils were collected from areas surrounding monoterpene (woodland) and nonmonoterpene (grassland)-emitting vegetation types. Soils were spiked with [UL-¹⁴C] 2,4-dichlorophenol at 10 mg kg⁻¹ and amended with α-pinene, p -cymene or a mix of monoterpenes (α-pinene, limonene and p -cymene in 1 : 1 : 1 ratio). The effects of monoterpene addition on the catabolism of [UL-¹⁴C] 2,4-dichlorophenol to ¹⁴CO₂ by indigenous soil microbial communities were assessed in freshly spiked and 4-week-aged soils. It was found that aged woodland soils exhibited a higher level of [UL-¹⁴C] 2,4-dichlorophenol degradation, which was subsequently enhanced by the addition of monoterpenes ( P <0.001), with the VOC mix and α-pinene amendments showing increased [UL-¹⁴C] 2,4-dichlorophenol catabolism. This study supports claims that the addition of biogenic VOCs to soils enhances the degradation of xenobiotic contaminants.