Functional and numerical responses of four lemming predators in high arctic Greenland

The high‐arctic tundra ecosystem has the world's simplest vertebrate predator–prey community, with only four predators preying upon one rodent species, the collared lemming (Dicrostonyx groenlandicus). We document the functional and numerical responses of all the four predators in NE Greenland. Using these data, we assess the impact of predation on the dynamics of the collared lemming with a 4 yr cycle and >100‐fold difference between maximum and minimum densities. All predator species feed mostly (>90%) on lemmings when lemming density is >1 ha^?1, but the shapes of the predators’ responses vary greatly. The snowy owl (Nyctea scandiaca) is present and breeds only when lemming densities at snowmelt are >2 ha^?1, giving rise to a step‐like numerical response. The long‐tailed skua (Stercorarius longicaudus) has a type III functional response and shifts from alternate food (mainly berries and insects) to lemmings with increasing lemming density. The skua surpasses all the other predators in summer by its total response. The type III functional response of the Arctic fox (Alopex lagopus) starts to increase at much lower lemming densities than the responses of the avian predators, but it has only a weak numerical response. Finally, the stoat (Mustela erminea) is the most specialized predator and the only one with a clearly delayed numerical response. According to their specific functional and numerical responses, each predator plays a key role at some point of the lemming cycle, but only the stoat has the potential to drive the lemming cycle. Stoat predation is greatly reduced in the winter preceding the lemming peak, and it reaches a maximum in the winter preceding the lowest lemming summer density. Stoat predation appears to maintain low lemming densities for at least two successive years. Our study provides empirical support for the specialist predator hypothesis about small mammal population cycles.     

Physiological and physicochemical controls on foliar volatile organic compound emissions

Plant leaves emit a broad spectrum of organic compounds that typically play multiple roles in plant protection. Furthermore, most of these compounds actively participate in tropospheric chemistry. There has been rapid progress in understanding how the emission of volatiles is regulated, mostly focusing on the biochemical controls over compound production. However, physicochemical characteristics such as low volatility or diffusion can also control the emissions and interact with physiological limitations. In particular, non-specific leaf storage of less volatile compounds smooths the emission responses to fluctuating environmental conditions, and diffusion through stomata leads to conspicuous emission bursts after stomatal opening and modifications of diurnal emission time courses. Because natural conditions always fluctuate, both physiological and physicochemical controls exert a major influence over plant volatile emissions.     

Climate change‐induced vegetation change as a driver of increased subarctic biogenic volatile organic compound emissions

Emissions of biogenic volatile organic compounds (BVOCs) have been earlier shown to be highly temperature sensitive in subarctic ecosystems. As these ecosystems experience rapidly advancing pronounced climate warming, we aimed to investigate how warming affects the BVOC emissions in the long term (up to 13 treatment years). We also aimed to assess whether the increased litterfall resulting from the vegetation changes in the warming subarctic would affect the emissions. The study was conducted in a field experiment with factorial open‐top chamber warming and annual litter addition treatments on subarctic heath in Abisko, northern Sweden. After 11 and 13 treatment years, BVOCs were sampled from plant communities in the experimental plots using a push–pull enclosure technique and collection into adsorbent cartridges during the growing season and analyzed with gas chromatography–mass spectrometry. Plant species coverage in the plots was analyzed by the point intercept method. Warming by 2 °C caused a 2‐fold increase in monoterpene and 5‐fold increase in sesquiterpene emissions, averaged over all measurements. When the momentary effect of temperature was diminished by standardization of emissions to a fixed temperature, warming still had a significant effect suggesting that emissions were also indirectly increased. This indirect increase appeared to result from increased plant coverage and changes in vegetation composition. The litter addition treatment also caused significant increases in the emission rates of some BVOC groups, especially when combined with warming. The combined treatment had both the largest vegetation changes and the highest BVOC emissions. The increased emissions under litter addition were probably a result of a changed vegetation composition due to alleviated nutrient limitation and stimulated microbial production of BVOCs. We suggest that the changes in the subarctic vegetation composition induced by climate warming will be the major factor indirectly affecting the BVOC emission potentials and composition.     

Biogenic volatile organic compound and respiratory CO2 emissions after 13C-labeling: online tracing of C translocation dynamics in poplar plants

Background

Globally plants are the primary sink of atmospheric CO₂, but are also the major contributor of a large spectrum of atmospheric reactive hydrocarbons such as terpenes (e.g. isoprene) and other biogenic volatile organic compounds (BVOC). The prediction of plant carbon (C) uptake and atmospheric oxidation capacity are crucial to define the trajectory and consequences of global environmental changes. To achieve this, the biosynthesis of BVOC and the dynamics of C allocation and translocation in both plants and ecosystems are important.

Methodology

We combined tunable diode laser absorption spectrometry (TDLAS) and proton transfer reaction mass spectrometry (PTR-MS) for studying isoprene biosynthesis and following C fluxes within grey poplar (Populus x canescens) saplings. This was achieved by feeding either ¹³CO₂ to leaves or ¹³C-glucose to shoots via xylem uptake. The translocation of ¹³CO₂ from the source to other plant parts could be traced by ¹³C-labeled isoprene and respiratory ¹³CO₂ emission.

Principal Finding

In intact plants, assimilated ¹³CO₂ was rapidly translocated via the phloem to the roots within 1 hour, with an average phloem transport velocity of 20.3±2.5 cm h⁻¹. ¹³C label was stored in the roots and partially reallocated to the plants'' apical part one day after labeling, particularly in the absence of photosynthesis. The daily C loss as BVOC ranged between 1.6% in mature leaves and 7.0% in young leaves. Non-isoprene BVOC accounted under light conditions for half of the BVOC C loss in young leaves and one-third in mature leaves. The C loss as isoprene originated mainly (76–78%) from recently fixed CO₂, to a minor extent from xylem-transported sugars (7–11%) and from photosynthetic intermediates with slower turnover rates (8–11%).

Conclusion

We quantified the plants'' C loss as respiratory CO₂ and BVOC emissions, allowing in tandem with metabolic analysis to deepen our understanding of ecosystem C flux.     

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.     

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.     

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.     

Microbial volatile organic compound emissions from Stachybotrys chartarumgrowing on gypsum wallboard and ceiling tile

In neutralophilic bacteria, monovalent metal cation/H+ antiporters play a key role in pH homeostasis. In Escherichia coli, only four antiporters (NhaA, NhaB, MdfA and ChaA) are identified to function in maintenance of a stable cytoplasmic pH under conditions of alkaline stress. We hypothesised that the multidrug resistance protein MdtM, a recently characterised homologue of MdfA and a member of the major facilitator superfamily, also functions in alkaline pH homeostasis. Assays that compared the growth of an E. coli ΔmdtM deletion mutant transformed with a plasmid encoding wild-type MdtM or the dysfunctional MdtM D22A mutant at different external alkaline pH values (ranging from pH 8.5 to 10) revealed a potential contribution by MdtM to alkaline pH tolerance, but only when millimolar concentrations of sodium or potassium was present in the growth medium. Fluorescence-based activity assays using inverted vesicles generated from transformants of antiporter-deficient (ΔnhaA, ΔnhaB, ΔchaA) E. coli TO114 cells defined MdtM as a low-affinity antiporter that catalysed electrogenic exchange of Na+, K+, Rb+ or Li+ for H+. The K+/H+ antiport reaction had a pH optimum at 9.0, whereas the Na+/H+ exchange activity was optimum at pH 9.25. Measurement of internal cellular pH confirmed MdtM as contributing to maintenance of a stable cytoplasmic pH, acid relative to the external pH, under conditions of alkaline stress. Taken together, the results support a role for MdtM in alkaline pH tolerance. MdtM can therefore be added to the currently limited list of antiporters known to function in pH homeostasis in the model organism E. coli.     

Diet selection by vertebrate herbivores in the high arctic of Greenland

Microhistological analysis of fecal droppings indicates that graminoids (sedges, grasses, and rushes) and the only willow present, Salix arctica , are primary food items of the muskox Ovibos moschatus , arctic hare Lepus arcticus , and collared lemming Dicrostonyx groenlandicus in northern Greenland (79°-83°N). Plant biomass available during the peak growing season varied with vegetation types from < 2 g m⁻² in polar barrens to 34 g m⁻² in sedge fens. The clustered distribution of vegetation minimizes competition among vertebrate herbivores.     

Human-induced changes in US biogenic volatile organic compound emissions: evidence from long-term forest inventory data

Volatile organic compounds (VOCs) emitted by woody vegetation influence global climate forcing and the formation of tropospheric ozone. We use data from over 250 000 re‐surveyed forest plots in the eastern US to estimate emission rates for the two most important biogenic VOCs (isoprene and monoterpenes) in the 1980s and 1990s, and then compare these estimates to give a decadal change in emission rate. Over much of the region, particularly the southeast, we estimate that there were large changes in biogenic VOC emissions: half of the grid cells (1°× 1°) had decadal changes in emission rate outside the range ?2.3% to +16.8% for isoprene, and outside the range 0.2–17.1% for monoterpenes. For an average grid cell the estimated decadal change in heatwave biogenic VOC emissions (usually an increase) was three times greater than the decadal change in heatwave anthropogenic VOC emissions (usually a decrease, caused by legislation). Leaf‐area increases in forests, caused by anthropogenic disturbance, were the most important process increasing biogenic VOC emissions. However, in the southeast, which had the largest estimated changes, there were substantial effects of ecological succession (which decreased monoterpene emissions and had location‐specific effects on isoprene emissions), harvesting (which decreased monoterpene emissions and increased isoprene emissions) and plantation management (which increased isoprene emissions, and decreased monoterpene emissions in some states but increased monoterpene emissions in others). In any given region, changes in a very few tree species caused most of the changes in emissions: the rapid changes in the southeast were caused almost entirely by increases in sweetgum (Liquidambar styraciflua) and a few pine species. Therefore, in these regions, a more detailed ecological understanding of just a few species could greatly improve our understanding of the relationship between natural ecological processes, forest management, and biogenic VOC emissions.     

Thermogenic respiratory processes drive the exponential increase of volatile organic compound emissions in Macrozamia cycad cones

An important outcome of plant thermogenesis is increased emissions of volatiles that mediate pollinator behaviour. We investigated whether the large increase in emissions, mainly the monoterpene ß‐myrcene (>90%), during daily thermogenic events of Macrozamia macleayi and lucida cycad cones are due solely to the influence of high cone temperatures or are, instead, a result of increased respiratory rates during thermogenesis. We concurrently measured temperature, oxygen consumption and ß‐myrcene emission profiles during thermogenesis of pollen cones under typical environmental temperatures and during experimental manipulations of cone temperatures and aerobic conditions, all in the dark. The exponential rise in ß‐myrcene emissions never occurred without a prior, large increase in respiration, whereas an increase in cone temperature alone did not increase emissions. When respiration during thermogenesis was interrupted by anoxic conditions, ß‐myrcene emissions decreased. The increased emission rates are not a result of increased cone temperature per se (through increased enzyme activity or volatilization of stored volatiles) but are dependent on biosynthetic pathways associated with increased respiration during thermogenesis that provide the carbon, energy (ATP) and reducing compounds (NADPH) required for ß‐myrcene production through the methylerythritol phosphate (MEP) pathway. These findings establish the significant contribution of respiration to volatile production during thermogenesis.     

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.     

The planktonic diatom flora of a high arctic lake in East Greenland

The physico-chemical characteristics and the planktonic diatom flora were studied in high arctic Loon Lake (Geographical Society Ø, East Greenland). Loon Lake is a cold, monomictic, alkaline, low conductive and likely oligotrophic lake with a maximum water depth of 11.70 m and a low diversity of planktonic diatoms. In summer 2003, the lake-water diatom community consisted of three taxa: frustules belonging to the morphologically highly variable Cyclotella ocellata-polymorpharossii-tripartita-complex were most abundant, and both Discostella pseudostelligera and Fragilaria tenera were of minor importance. The morphological variability of the three diatom taxa is documented by light micrographs.     

Non-methane biogenic volatile organic compound emissions from boreal peatland microcosms under warming and water table drawdown

Boreal peatlands have significant emissions of non-methane biogenic volatile organic compounds (BVOCs). Climate warming is expected to affect these ecosystems both directly, with increasing temperature, and indirectly, through water table drawdown following increased evapotranspiration. We assessed the combined effect of warming and water table drawdown on the BVOC emissions from boreal peatland microcosms. We also assessed the treatment effects on the BVOC emissions from the peat soil after the 7-week long experiment. Emissions of isoprene, monoterpenes, sesquiterpenes, other reactive VOCs and other VOCs were sampled using a conventional chamber technique, collected on adsorbent and analyzed by GC–MS. Carbon emitted as BVOCs was less than 1% of the CO₂ uptake and up to 3% of CH₄ emission. Water table drawdown surpassed the direct warming effect and significantly decreased the emissions of all BVOC groups. Only isoprene emission was significantly increased by warming, parallel to the increased leaf number of the dominant sedge Eriophorum vaginatum. BVOC emissions from peat soil were higher under the control and warming treatments than water table drawdown, suggesting an increased activity of anaerobic microbial community. Our results suggest that boreal peatlands could have concomitant negative and positive radiative forcing effects on climate warming following the effect of water table drawdown. The observed decrease in CH₄ emission causes a negative radiative forcing while the increase in CO₂ emission and decrease in reactive BVOC emissions, which could reduce the cooling effect induced by the lower formation rate of secondary organic aerosols, both contribute to increased radiative forcing.     

The Holocene development of some low and high arctic Greenland lakes

During the Holocene most West Greenland lakes passed from an early eutrophic stage, rich in both flora and fauna, through a mesotrophic to an oligotrophic stage with very low productivity. Temperature conditions were limiting factors only in the very beginning, whereas chemical factors alone were decisive later on.