Net ecosystem carbon exchange and the greenhouse gas balance of tidal marshes along an estuarine salinity gradient

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO₂) and methane (CH₄) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide ( \( {\text{ER}}_{{{\text{CO}}_{2} }} \) ) or methane ( \( {\text{ER}}_{{{\text{CH}}_{4} }} \) ), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m^?2 year^?1) and ²¹⁰Pb radiodating (336 g C m^?2 year^?1), a minor sink for atmospheric CH₄, and a GHG sink (~620 g CO_2-eq m^?2 year^?1). The tidal freshwater marsh was a source of CH₄ to the atmosphere (~22 g C–CH₄ m^?2 year^?1). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though ²¹⁰Pb radiodating indicated modest C accretion (110 g C m^?2 year^?1). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm^?1) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH₄ at the oligohaline site (>80 g C–CH₄ m^?2 year^?1), making this site a significant GHG source (>2,000 g CO_2-eq m^?2 year^?1). The oligohaline site did not accumulate C over the 2 year study period, though ²¹⁰Pb dating indicated long term C accumulation (250 g C m^?2 year^?1), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.     

Pools and fluxes of carbon in three Norway spruce ecosystems along a climatic gradient in Sweden

This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001–2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190–233 g C m⁻² year⁻¹). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from −8 g C m⁻² year⁻¹ in the north to +9 g C m⁻² year⁻¹ in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m⁻² year⁻¹. An erratum to this article can be found at     

The impact of some environmental factors on the establishment of Plantago maritima seedlings and juveniles along a distributional gradient

The establishment of Plantago maritima seedlings and juveniles along a distributional gradient was investigated with pantograph technique. Mortality was closely dependent on vegetational characters such as cover, height of vegetation and amount of litter. Superimposed on these factors was mortality due to flooding. The density dependent mortality of seedlings was also investigated. The number of surrounding seedlings seemed to be of vital importance for survival, i.e. self thinning occurred most likely as a result of competition for light. The establishment and survival of seedlings cannot fully explain the distribution of adults.     

Shoots, roots and ectomycorrhiza formation of pine seedlings at elevated atmospheric carbon dioxide

The effect of elevated atmospheric CO₂ concentration on the growth of shoots, roots, mycorrhizas and extraradical mycorrhizal mycelia of pine (Pinus silvestris L.) was examined. Two and a half-month-old seedlings were inoculated axenically with the mycorrhizal fungus Pisolithus tincto-rius (Pers.) by a method allowing rapid mycorrhiza formation in Petri dishes. The plants were then cultivated for 3 months in growth chambers with daily concentrations of 350 and 600 μmol mol^?1 CO₂ during the day. Whereas plants harvested after 1 and 2 months did not differ appreciably between ambient and increased CO₂ concentrations, after 3 months they developed a considerably higher root biomass (%57%) at elevated CO₂, but did not increase significantly in root length. The mycorrhizal fungus Pisolithus tinctorius, which depended entirely on the plant assimilates in the model system, grew much faster at increased CO₂: 3 times more mycorrhizal root clusters were formed and the extraradical mycelium produced had twice the biomass at elevated as at ambient CO₂. No difference in shoot biomass was found between the two treatments after 91 d. However, since the total water consumption of seedlings was similar in the two treatments, the water use efficiency was appreciably higher for the seedlings at increased CO₂ because of the higher below-ground biomass.     

Interannual growth response of Norway spruce to climate along an altitudinal gradient in the Tatra Mountains, Poland

Ring-width chronologies of Picea abies (L.) Karst. from ten sites in the Tatra Mountains, Poland, were developed to explore growth/climate responses in stands along an altitudinal gradient ranging from 839 to 1468 m a.s.l. There were positive relationships between current-year radial growth and mean monthly temperatures in March, April, June and July, but with increasing elevation, the strength of this correlation declined for March–April and increased for June–July temperatures. The mean monthly temperature in October of the previous year positively influenced radial growth of trees at all sites. Lower mean temperatures in January negatively affected growth of trees at the high-elevation sites. Trees at the low-elevation sites responded positively to a warm early spring, whereas trees at the high-elevation sites showed positive growth responses to higher summer temperatures. Growth of trees at the high-elevation sites was better explained by the temperature regime than was growth at the lower-elevation sites. Therefore, it is likely that these sites may be particularly sensitive to potential effects of temperature change.     

Spatial relations and population structure of a dominant tree along a treeline ecotone in the Tropical Andes: interactions at gradient and plant-neighbourhood scales

Background: Studies in temperate mountains suggest that plant–plant interactions modulate tree establishment above the forest line. In tropical mountains worldwide this issue remains largely unexplored.

Aims: To analyse the population structure and local spatial relationships of a dominant tree at a species-rich tropical Andean forest line.

Methods: We determined changes in the population structure of Diplostephium venezuelense along an elevation gradient between continuous forest and open páramo and analysed plant community structure and superficial rock cover in the neighbourhood of saplings and adults at the upper forest line.

Results: Sapling and adult densities were highest in low-altitude páramos adjacent to the forest line and at the borders of small forest islands. Saplings showed local spatial aggregation, were positively associated with small boulders and low shrubs, and negatively associated with mosses and lichens. However, no spatial association was found between scattered adults in the páramo and saplings of other forest trees.

Conclusions: Complex species-specific local spatial interactions, suggesting both facilitative and antagonistic effects, seem to modulate the establishment of the dominant tree D. venezuelense at and above the upper forest line. Nevertheless, the establishment of other tree species above continuous forests does not appear to be facilitated by the canopy cover offered by the isolated D. venezuelense individuals established in open páramo environments.     

Structural and functional patterns of bacterial communities in response to protist predation along an experimental productivity gradient

Substrate supply and protist grazing are two of the most important forces that determine the composition and properties of bacterial assemblages. General ecological theory predicts that the relative importance of these factors is changing with the environmental productivity. In the present study, the interplay between bottom-up and top-down control was studied in a productivity gradient simulated in one-stage chemostats containing natural assemblages of freshwater bacteria and heterotrophic nanoflagellates. Bacterial assemblages in the chemostats differed strongly with respect to their morphological, physiological and compositional properties in the presence versus the absence of predators. However, theses differences were modified by the productivity gradient. Whereas in predator-free chemostats the mean abundance and biomass of bacteria increased proportionally with increasing substrate supply, in treatments that included flagellates bacterial production was largely channelled into predator biomass. The bacterial morphological diversity increased along the productivity gradient with increasing substrate input but even more so with predators. Proportional to the increasing substrate supply, predation shifted the remaining bacteria towards morphologically inedible forms. Predation also caused shifts in bacterial substrate-utilization profiles, and in bacterial community composition, as analysed by terminal restriction fragment length polymorphism of PCR-amplified 16S-rRNA genes. Without predators, bacterial richness increased along the productivity gradient whereas with predators bacterial richness was higher at intermediate substrate levels. In accordance with ecological theory, these results demonstrated that predators influence all of the major characteristics of bacterial assemblages but the magnitude of this effect is modulated by the productivity of the system.     

Host-use patterns of saproxylic phloeophagous and xylophagous Coleoptera adults and larvae along the decay gradient in standing dead black spruce and aspen

Wood-feeding insects play important functional roles in forest ecosystems, contributing significantly to wood decay processes. However, sampling these species in a direct and quantitative way is difficult because they live most of their lives as larvae deep into the wood; knowledge of species-specific host-use patterns along the decay gradient is thus lacking in this group. To cope with these difficulties, we used a novel approach, snag dissection, to investigate occurrence patterns of such Coleoptera adults and larvae. We selected 80 snags of both black spruce and aspen along four classes of decay in five different stands distributed over the tree species' ranges within the province of Quebec, Canada, and dissected a one-meter section of each. All adults and larvae of Buprestidae, Cerambycidae and Scolytinae (Coleoptera: Curculionidae) were collected and identified to the lowest taxonomical level possible. Wood density and snag age were also calculated for each sampled snag. In black spruce, host-use was mostly concentrated at the beginning of the decay gradient. Patterns observed in aspen were opposite, as few insects were found in fresh snags, while most snags in middle to late stages of decay contained insects, often in large numbers, in some reaching densities of over 1000 cerambycid larvae m⁻³. For both tree species, patterns observed were similar across regions sampled. Differences in host-use patterns between the coniferous and deciduous host species may be due to differences in secondary chemistry, mechanical defence mechanisms or the stand dynamics typically associated with each tree species.     

Contrasting patterns in leaf traits of Mediterranean shrub communities along an elevation gradient: measurements matter

Plant Ecology - We assessed the changes in community-weighted mean (CWM) and variability of specific leaf area (SLA) and leaf area (LA) of different Mediterranean shrub communities along an...     

Nitrogen and carbon isotope ratios in termites: an indicator of trophic habit along the gradient from wood-feeding to soil-feeding

1. Nitrogen and carbon stable-isotope ratios (δ¹⁵N and δ¹³C) of body tissues, mound/nest materials and dietary substrates were determined in termite species with differing trophic habits, sampled from the Mbalmayo Forest Reserve, southern Cameroon.
2. δ¹⁵N of termite tissues was enriched gradually along a spectrum of species representing a trophic gradient from wood- to soil-feeding. Species that could be identified from their general biology and from gut content analysis as feeding on well-rotted wood or as wood/soil interface feeders showed δ¹⁵N intermediate between sound-wood-feeders and soil-feeders. It is proposed that δ¹⁵N is therefore a possible indicator of the functional position of species in the humification process. Differences in δ¹³C were also observed between wood-feeding and soil-feeding forms.
3. High values of δ¹⁵N in soil-feeding termites suggest that nitrogen fixation is of little importance in these species.
4. A wide range of isotope effects (the difference in isotope ratios between termites and their diet) was observed for both nitrogen (Δδ¹⁵N = –1.6 to + 8.8‰) and carbon (Δδ¹³C = –2.2 to + 3.0‰), which suggests a diversity of nutrient acquisition mechanisms within termites and diverse relationships between termites and their intestinal micro-organisms.     

Effects of long-term open-air exposure to fluoride,nitrogen compounds and SO2 on visible symptoms,pollutant accumulation and ultrastructure of Scots pine and Norway spruce seedlings

 Effects of SO₂, aqueous fluoride (NaF) and a solution of nitrogen compounds (NH₄NO₃) on the visible symptoms, pollutant accumulation and ultrastructure of Scots pine (Pinus sylvestris L.) and Norway spruce [Picea abies (L.) Karst.] seedlings were studied in an open-air experiment lasting for 3 consecutive years. Visible injury symptoms were most pronounced in combination exposures and whenever F was applied. Visible symptoms correlated well with needle pollutant concentrations. Exposure to NaF increased needle F contents particularly when F was applied with SO₂ or NH₄NO₃. This suggests that a reduction in N or SO₂ emissions, in F polluted areas, could improve the condition of conifers via decreased accumulation of phytotoxic F in the needles. Norway spruce needles accumulated 2 – 10 times as much S and F as those of Scots pine. Microscopic observations showed various changes in the needle mesophyll cell ultrastructure. In both species, exposure to SO₂ increased significantly the amount of cytoplasmic vacuoles, suggesting detoxification of excess sulphate or low pH. F treatments resulted in a significant enlargement of plastoglobuli in Scots pine and a darkening of plastoglobuli in Norway spruce. All exposures enhanced the accumulation of lipid bodies. An increased portion of translucent plastoglobuli was most pronounced in N treatments. Many of the ultrastructural changes and visible symptoms appeared only as number of years exposed increased, indicating that long-term experiments are needed. Both visible symptoms and ultrastructural changes pointed to the more pronounced sensitivity of Norway spruce compared to Scots pine. Ultrastructural results mostly supported earlier qualitative observations of F, N and SO₂ effects on needle mesophyll cell ultrastructure. However, no reduction of thylakoids in SO₂ containing exposure or curling of thylakoids in F exposure could be detected in the present study. Received: 5 December 1994 / Accepted: 28 April 1995     

Effect of exposure to fluoride,nitrogen compounds and SO2 on the numbers of spruce shoot aphids on Norway spruce seedlings

Summary Development of spruce shoot aphid (Cinara pilicornis Hartig) populations was monitored in natural and artificial infestations of Norway spruce (Picea abies Karst.) seedlings, exposed to air pollutants in an experimental field. The pollutants, applied both singly and in mixtures, were gaseous sulphur dioxide, NaF (30 mg l^-1 F) and Ca(NO₃)₂ or (NH₄)₂SO₄ in aqueous solutions (200 mg l^-1 N). Aphid numbers on 10 seedlings in each treatment and two control plots were counted at 2-week intervals. At the beginning of the experiment aphid numbers did not differ between treatments. Aphid populations peaked in late June and early July. All the pollutants and their combinations significantly increased the numbers of aphids per seedling. Four apterous females were transferred to spruce seedlings which were growing in containers in the same plots. After 4–5 weeks aphid numbers were significantly higher in the fluoride treatment and in the combined treatment of fluoride, nitrogen and SO₂. The pollution treatments did not have a significant effect on shoot growth. Concentrations of F and S in needles were higher in treatments involving these pollutants. There were no significant differences in the concentrations of free amino acids in shoot stems between control and fluoride treatment. However, the relatively low concentration of arginine in the F treatment at the end of the growing season might indicate disturbances in the nitrogen metabolism of spruce seedlings.     

Impact of warming and drought on carbon balance related to wood formation in black spruce

Background and Aims

Wood formation in trees represents a carbon sink that can be modified in the case of stress. The way carbon metabolism constrains growth during stress periods (high temperature and water deficit) is now under debate. In this study, the amounts of non-structural carbohydrates (NSCs) for xylogenesis in black spruce, Picea mariana, saplings were assessed under high temperature and drought in order to determine the role of sugar mobilization for osmotic purposes and its consequences for secondary growth.

Methods

Four-year-old saplings of black spruce in a greenhouse were subjected to different thermal conditions with respect to the outside air temperature (T0) in 2010 (2 and 5 °C higher than T0) and 2011 (6 °C warmer than T0 during the day or night) with a dry period of about 1 month in June of each year. Wood formation together with starch, NSCs and leaf parameters (water potential and photosynthesis) were monitored from May to September.

Key Results

With the exception of raffinose, the amounts of soluble sugars were not modified in the cambium even if gas exchange and photosynthesis were greatly reduced during drought. Raffinose increased more than pinitol under a pre-dawn water potential of less than –1 Mpa, presumably because this compound is better suited than polyol for replacing water and capturing free radicals, and its degradation into simple sugar is easier. Warming decreased the starch storage in the xylem as well the available hexose pool in the cambium and the xylem, probably because of an increase in respiration.

Conclusions

Radial stem growth was reduced during drought due to the mobilization of NSCs for osmotic purposes and due to the lack of cell turgor. Thus plant water status during wood formation can influence the NSCs available for growth in the cambium and xylem.     

Storage and turnover of carbon in grassland soils along an elevation gradient in the Swiss Alps

Amount, composition, and rate of turnover of soil organic carbon (SOC) in mountainous cold regions is largely unknown, making predictions of future responses of this carbon (C) to changing environmental conditions uncertain. We hypothesized increasing amounts and declining turnover times of soil organic matter (SOM) under permanent grassland with increasing elevation and decreasing temperature. Samples from an irrigated transect in the Swiss Alps (880 to 2200 m elevation, mean annual temperatures +8.9 to +0.9 °C) were analyzed. Soil C stocks ranged from 49 to 96 t C ha⁻¹ (0–20 cm) and were not related to elevation, though the highest site stored least C. Particulate organic carbon (POC) increased significantly with elevation and accounted for > 80% of the total soil C at 2200 m (0–5 cm). Mean residence times (MRTs) of POC computed by means of radiocarbon dating were in the order of years to decades and were positively related to elevation in the topsoil. At higher elevations, the estimated total C flux through the soil profile mainly depended on this fraction. MRT of mineral-associated matter ranged from decades to centuries and was not systematically related to elevation, but positively related to the soil mineral surface area and it increased with soil depth. Turnover rates from simulations with the soil C model RothC exceeded those from ¹⁴C measurements by a factor of 1.7–3.3 which suggests that C dynamics at these sites is overestimated by the model. Size of model pools and amount of C in soil fractions were only weakly correlated, thereby challenging previously postulated hypotheses concerning the correspondence of pools and fractions for grasslands at higher elevations.     

Leaf lifespan and lifetime carbon balance of individual leaves in a stand of an annual herb, Xanthium canadense

Leaf lifespan in response to resource availability has been documented in many studies, but it still remains uncertain what determines the timing of leaf shedding. Here, we evaluate the lifetime carbon (C) balance of a leaf in a canopy as influenced by nitrogen (N) availability. Stands of Xanthium canadense were established with high-nitrogen (HN) and low-nitrogen (LN) treatments and temporal changes of C gain of individual leaves were investigated with a canopy photosynthesis model. Daily C gain of a leaf was maximal early in its development and subsequently declined. Daily C gain at shedding was nearly zero in HN, while it was still positive in LN. Sensitivity analyses showed that the decline in the daily C gain resulted primarily from the reduction in light level in HN and by the reduction in leaf N in LN. Smaller leaf size in LN than in HN led to higher light levels in the canopy, which helped leaves of the LN stand maintain for a longer period. These results suggest that the mechanism by which leaf lifespan is determined changes depending on the availability of the resource that is most limiting to plant growth.     

Carbon balance and allocation of assimilated CO2 in Scots pine, Norway spruce, and Silver birch seedlings determined with gas exchange measurements and 14C pulse labelling

Carbon dioxide is released from the soil to the atmosphere in heterotrophic respiration when the dead organic matter is used for substrates for soil micro-organisms and soil animals. Respiration of roots and mycorrhiza is another major source of carbon dioxide in soil CO₂ efflux. The partitioning of these two fluxes is essential for understanding the carbon balance of forest ecosystems and for modelling the carbon cycle within these ecosystems. In this study, we determined the carbon balance of three common tree species in boreal forest zone, Scots pine, Norway spruce, and Silver birch with gas exchange measurements conducted in laboratory in controlled temperature and light conditions. We also studied the allocation pattern of assimilated carbon with ¹⁴C pulse labelling experiment. The photosynthetic light responses of the tree species were substantially different. The maximum photosynthetic capacity (P _max) was 2.21 μg CO₂ s⁻¹ g⁻¹ in Scots pine, 1.22 μg CO₂ s⁻¹ g⁻¹ in Norway spruce and 3.01 μg CO₂ s⁻¹ g⁻¹ in Silver birch seedlings. According to the pulse labelling experiments, 43–75% of the assimilated carbon remained in the aboveground parts of the seedlings. The amount of carbon allocated to root and rhizosphere respiration was about 9–26%, and the amount of carbon allocated to root and ectomycorrhizal biomass about 13–21% of the total assimilated CO₂. The ¹⁴CO₂ pulse reached the root system within few hours after the labelling and most of the pulse had passed the root system after 48 h. The transport rate of carbon from shoot to roots was fastest in Silver birch seedlings.     

A comparison of soil climate and biological activity along an elevation gradient in the eastern Mojave Desert

Summary Soil temperature, moisture, and CO₂ were monitored at four sites along an elevation transect in the eastern Mojave Desert from January to October, 1987. Climate appeared to be the major factor controlling CO₂ partial pressures, primarily through its influence of rates of biological reactions, vegetation densities, and organic matter production. With increasing elevation, and increasing actual evapotranspiration, the organic C, plant density, and the CO₂ content of the soils increased. Between January and May, soil CO₂ concentrations at a given site were closely related to variations in soil temperature. In July and October, temperatures had little effect on CO₂, presumably due to low soil moisture levels. Up to 75% of litter placed in the field in March was lost by October whereas, for the 3 lower elevations, less than 10% of the litter placed in the field in April was lost through decomposition processes.