Variability in exchange of CO2 across 12 northern peatland and tundra sites

Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized data on the carbon dioxide (CO₂) exchange obtained from eddy covariance measurements from 12 wetland sites, covering 1–7 years at each site, across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, spanning temperate to arctic climate zones. The average summertime net ecosystem exchange of CO₂ (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO₂. To evaluate the influence of gross primary production (GPP) and ecosystem respiration (R_eco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and R_eco. After this correction neither R_eco (P= 0.162) nor GPP (P= 0.110) correlated significantly with NEE on an annual basis. Spatial variation in annual and summertime R_eco was associated with growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. GPP showed weaker correlations with environmental variables as compared with R_eco, the exception being leaf area index (LAI), which correlated with both GPP and NEE, but not with R_eco. Length of growing season period was found to be the most important variable describing the spatial variation in summertime GPP and R_eco; global warming will thus cause these components to increase. Annual GPP and NEE correlated significantly with LAI and pH, thus, in order to predict wetland C exchange, differences in ecosystem structure such as leaf area and biomass as well as nutritional status must be taken into account.     

Holocene radiative forcing impact of northern peatland carbon accumulation and methane emissions

Throughout the Holocene, northern peatlands have both accumulated carbon and emitted methane. Their impact on climate radiative forcing has been the net of cooling (persistent CO₂ uptake) and warming (persistent CH₄ emission). We evaluated this by developing very simple Holocene peatland carbon flux trajectories, and using these as inputs to a simple atmospheric perturbation model. Flux trajectories are based on estimates of contemporary CH₄ flux (15–50 Tg CH₄ yr⁻¹), total accumulated peat C (250–450 Pg C), and peatland initiation dates. The contemporary perturbations to the atmosphere due to northern peatlands are an increase of ∼100 ppbv CH₄ and a decrease of ∼35 ppmv CO₂. The net radiative forcing impact northern peatlands is currently about −0.2 to −0.5 W m⁻² (a cooling). It is likely that peatlands initially caused a net warming of up to +0.1 W m⁻², but have been causing an increasing net cooling for the past 8000–11 000 years. A series of sensitivity simulations indicate that the current radiative forcing impact is determined primarily by the magnitude of the contemporary methane flux and the magnitude of the total C accumulated as peat, and that radiative forcing dynamics during the Holocene depended on flux trajectory, but the overall pattern was similar in all cases.     

Influence of Complexation on the Uptake by Plants of Iron, Manganese, Copper and Zinc: II. EFFECT OF DTPA IN A MULTI-METAL AND COMPUTER SIMULATION STUDY

Barley (Hordeum vulgare L.) plants were grown in nutrient solutionscontaining the chelating agent, DTPA. The experiments replicatedthose reported in the preceding paper in which EDTA was thechelating agent used. The concentrations of all the chemicalspecies of metals were stimulated using the program NUTRIENT.The concentrations of DTPA used were chosen to give a similarrange of complexation as used in the EDTA experiments. The effectof complexation by DTPA on the uptakes of the metal ions Fe³⁺,Mn²⁺, Cu²⁺, and Zn²⁺ and on plant growth were sufficiently differentfrom those with EDTA to indicate some dependence on the natureof the chelating agent used. The biggest difference betweenthe EDTA and DTPA experiments occurred in the solutions containingthe largest concentrations of these reagents. With DTPA, chlorosiswas seen only in the early stages; otherwise the plants showednormal growth. A simple chemical model for metal uptake is discussed. Key words: DTPA, EDTA, micronutrients, trace metals, computer simulation, plants, absorption, iron, manganese, copper, zinc     

Sources of variation in larval parasitism of two sympatrically outbreaking birch forest defoliators

1. Studies of insect communities rarely support the parasitoid–host regulation hypothesis. Spatio‐temporal variation in parasitoid prevalence due to complex food web interactions or abiotic factors may prevent parasitoids from regulating hosts. 2. We examined the relative contribution of spatial (altitude) and temporal (years) sources to total variation in parasitoid prevalence rates in outbreaks of Epirrita autumnata Borkhausen and Operophtera brumata Linnaeus populations. We tested whether prevalence rates of generalist parasitoids were correlated between sympatric host populations and to what extent any of the parasitoids were host density dependent. 3. Four larval parasitoids (two specialists and two generalists) exhibited significantly structured spatio‐temporal dynamics over years and altitudes. The prevalence rates of one of the generalists were spatio‐temporally correlated between the two host species, while for the other they were not. 4. Three parasitoids showed tendencies for direct or delayed positive density dependence as expected from numerical and functional responses to their hosts. However, the effects were weak and minute compared to the variation attributed to year and altitude. 5. We conclude that unknown aspects of the larval parasitoid ecology that co‐vary with altitude and year in the study system dominate their prevalence dynamics and thus act to hinder density‐dependent responses that could potentially regulate host populations.     

Wind structure in relation to odour plumes in tsetse fly habitats

Abstract. Key characteristics of airflow were measured in the African bush in a study of host odour plume structure. Wind speed, speed variance, direction, and directional variance were measured by conventional cup anemometers plus wind-vanes and by a solid state ultrasonic anemometer, on time scales from seconds to minutes. The two technologies gave opposite relationships between wind speed and turbulence measured as rate of angular direction change in the wind (° s^-1). A positive correlation between turbulence and wind speed was observed with mechanical anemometers and wind-vanes, evidently caused by their inherent hysteresis (stalling in weak wind, overswinging after gusts). The same correlation was negative with the solid-state anemometer which, being hysteresis free, should have measured the true directional turbulence more accurately. Such fine-scale turbulence at a fixed point in space (on a scale of about ∼15 cm diam.) decreased with wind speed up to ∼1.5 m s^-1, as does large-scale (∼1m diam.) turbulence of air moving through space (Brady et al. , 1989). This decrease occurred both within vegetation and out in the open, but the slope and intercepts of the relationship depended on vegetation and topography. Variables for describing wind speed and turbulence are considered in the context of odour plume structure.     

Identification of Genetically Induced Lesions and the Sites of Action of Inhibitors Affecting Photorespiration by Simple Tests on Leaf Discs

Turner, J. C. and Hall, N. P. 1988. Identification of geneticallyinduced lesions and sites of action of inhibitors affectingphotorespiration by simple tests on leaf discs.—J. exp,Bot. 39: 345-351. Six photorespiratory mutants of barley deficient in catalaseand two mutants lacking phospho-glycollate phosphatase wereidentified by a novel screening method using leaf discs. Leaf discs were punched directly into an appropriate bufferedreagent in which the enzymes diffused from the cut edges ofthe discs causing a change in the colour reagents. The reactionswere observed from 15 min onwards depending on which enzymeactivity was being followed. Hundreds of plants can be screenedrapidly for major differences in enzyme activity. The methods depend on the formation of a red product in thereaction of hydrogen peroxide (H₂O₂) with 4-aminoantipyrenein the presence of peroxidase. To detect P-glycollatc phosphataseand glycollate oxidase, the product of the linked reactions,H₂O₂, was measured. For catalase, the disappearance of addedH₂O₂ is followed. By omitting peroxidase from the colour reagentmixture, peroxidase activity in leaf discs can be measured. The method was evaluated by applying it to existing enzyme deficientmutants of barley lacking P-glycollate phosphatase and catalase.Further mutant plants were detected by this method. The techniquecould also be used to screen for inhibitors of the glycollatepathway for use as herbicides. Key words: Phosphoglycollate phosphatase, glycollate oxidase, catalase, peroxidase, hydrogen peroxide