Effects of enhanced UV-B radiation on production-related properties of a Sphagnum fuscum dominated subarctic bog

1. The aim of the study was to investigate effects of enhanced UV-B radiation on the balance between biomass production and decay in an ombrotrophic bog which is dominated by one species of Sphagnum ( S. fuscum). This paper concerns production.
2. Enhanced UV-B radiation (simulating 15% ozone depletion under clear sky conditions) was applied by means of fluorescent tubes during two growing seasons.
3. In S. fuscum, shoot density, mass relations and length increment over time were measured and productivity was estimated. Pigment concentration, rates of dark respiration and maximum net photosynthesis were recorded.
4. Sphagnum fuscum productivity was not changed by enhanced UV-B radiation while properties determining production were highly influenced although in opposite directions.
5. Height increment was decreased by 20% in the first growing season and by 31% in the second growing season under enhanced UV-B radiation. After two growing seasons spatial shoot density was decreased by 8% by enhanced UV-B radiation. The shoots became stunted as capitulum dry mass and stem dry mass per unit length were increased by 21 and 17%, respectively, under enhanced UV-B radiation.
6. Dark respiration was significantly decreased by 31% after growth under enhanced UV-B radiation.
7. The UV-B induced change in shoot biometry together with the reduced spatial shoot density involve potential long-term effects on peat structure with possible feedback on productivity, decomposition and the strength of the system as a carbon sink.     

Warming effects on greenhouse gas fluxes in peatlands are modulated by vegetation composition

Understanding the effects of warming on greenhouse gas feedbacks to climate change represents a major global challenge. Most research has focused on direct effects of warming, without considering how concurrent changes in plant communities may alter such effects. Here, we combined vegetation manipulations with warming to investigate their interactive effects on greenhouse gas emissions from peatland. We found that although warming consistently increased respiration, the effect on net ecosystem CO₂ exchange depended on vegetation composition. The greatest increase in CO₂ sink strength after warming was when shrubs were present, and the greatest decrease when graminoids were present. CH₄ was more strongly controlled by vegetation composition than by warming, with largest emissions from graminoid communities. Our results show that plant community composition is a significant modulator of greenhouse gas emissions and their response to warming, and suggest that vegetation change could alter peatland carbon sink strength under future climate change.     

Divergent Pathways of Successional Recovery for In Situ Oil Sands Exploration Drilling Pads on Wooded Moderate‐Rich Fens in Alberta,Canada

Peatlands in northern Alberta, Canada, are being rapidly impacted by oil sands activities, with potentially long‐term consequences for their recovery. In situ oil sands extraction requires exploration of oil resources on a dense network of drilling pads across the landscape. This study examined the recovery of wooded moderate‐rich (WMR) fens 10 years after abandonment of these sites with minimal restorative measures. Bryophyte and vascular plant diversity, site microtopography, and water chemistry were assessed on drilling pads and in adjacent areas of undisturbed reference habitat. WMR fens affected by drilling activities were divided a priori into two groups based on strongly divergent trends in their successional development. One group represented the majority of WMR fens observed on the land base; at these sites hummock‐forming mosses including minerotrophic Sphagnum species were infrequent and tree recruitment was almost absent. The other group was dominated by Sphagnum species, had Picea mariana and Larix laricina recruitment, and appeared to recover more quickly. Both groups had high abundance of wetland sedges, notably Carex aquatilis. Further, drilling pads belonging to the first group had a high water table, limited elevated microsites, and had surface flooding over a portion of the growing season, in contrast to Sphagnum‐dominated sites. Development of the aquatic, bryophyte‐poor wetland type is comparable to early stages of wetland succession and these systems will recover relatively slowly, likely from decades to more than a century. Restoring part of the vertical distribution of microhabitats before abandonment of these pads could stimulate the successional recovery of vegetation.     

Amazonian peatlands: an ignored C sink and potential source

In tropical lowlands, peatlands are commonly reported from Southeast Asia, and especially Indonesian tropical peatlands are known as considerable C sinks and sources. In contrast, Amazonia has been clearly understudied in this context. In this study, based on field observations from 17 wetland sites in Peruvian lowland Amazonia, we report 0–5.9 m thick peat deposits from 16 sites. Only one of the studied sites did not contain any kind of peat deposit (considering pure peat and clayey peat). Historic yearly peat and C accumulation rates, based on radiocarbon dating of peat samples from five sites, varied from 0.94 ± 0.99 to 4.88 ± 1.65 mm, and from 26 ± 3 to 195 ± 70 g C m⁻², respectively. The long-term apparent peat and C accumulation rates varied from 1.69 ± 0.03 to 2.56 ± 0.12 mm yr⁻¹, and from 39 ± 10 to 85 ± 30 g C m⁻² yr⁻¹, respectively. These accumulation rates are comparable to those determined in the Indonesian tropical peatlands. Under altered conditions, Indonesian peatlands can release globally relevant amounts of C to the atmosphere. Considering the estimated total area of Amazonian peatlands (150 000 km²) close to that of the Indonesian ones (200 728 km²) as well as several factors threatening the Amazonian peatlands, we suggest that the total C stocks and fluxes associated with Amazonian peatlands may be of global significance.     

Photodegradation leads to increased carbon dioxide losses from terrestrial organic matter

CO₂ production in terrestrial ecosystems is generally assumed to be solely biologically driven while the role of abiotic processes has been largely overlooked. In addition to microbial decomposition, photodegradation – the direct breakdown of organic matter (OM) by solar irradiance – has been found to contribute to litter mass loss in dry ecosystems. Previous small‐scale studies have shown that litter degradation by irradiance is accompanied by emissions of CO₂. However, the contribution of photodegradation to total CO₂ losses at ecosystems scales is unknown. This study determined the proportion of the total CO₂ losses caused by photodegradation in two ecosystems: a bare peatland in New Zealand and a seasonally dry grassland in California. The direct effect of solar irradiance on CO₂ production was examined by comparing daytime CO₂ fluxes measured using eddy covariance (EC) systems with simultaneous measurements made using an opaque chamber and the soil CO₂ gradient technique, and with night‐time EC measurements under the same soil temperature and moisture conditions. In addition, a transparent chamber was used to directly measure CO₂ fluxes from OM caused by solar irradiance. Photodegradation contributed 19% of the annual CO₂ flux from the peatland and almost 60% of the dry season CO₂ flux from the grassland, and up to 62% and 92% of the summer mid‐day CO₂ fluxes, respectively. Our results suggest that photodegradation may be important in a wide range of ecosystems with exposed OM. Furthermore, the practice of partitioning daytime ecosystem CO₂ exchange into its gross components by assuming that total daytime CO₂ losses can be approximated using estimates of biological respiration alone may be in error. To obtain robust estimates of global ecosystem–atmosphere carbon transfers, the contribution of photodegradation to OM decomposition must be quantified for other ecosystems and the results incorporated into coupled carbon–climate models.     

Sphagnum establishment on bare peat: The importance of climatic variability and Sphagnum species richness

Question: What is the relative ability of four species of Sphagnum (S. fuscum, S. rubellum, S. magellanicum and S. angustifolium) to establish on bare peat substratum in the field when re‐introduced as single or multi‐species re‐introductions and in relation to interannual variations in climate? Location: Continental southeastern Canada. Methods: Diaspores (fragments) of four Sphagnum species alone or in combination were re‐introduced onto residual peat surfaces and were monitored to follow the development of the moss carpet over four growing seasons. In order to compare results under a variety of climatic conditions, this whole experimental setting was repeated four times (trials), with a four‐year follow‐up for each trial. Conclusions: The establishment rate of the moss carpet varied among years, in response to climatic variations between growing seasons. The relative success of different moss species and combinations of species, however, did not vary within or between trials. Thus, the species and combinations of species resulting in the highest short‐term or long‐term establishment rates remained the same for all trials, independent of the climatic conditions at the time of re‐introduction. Our results showed no link between the number of species in the diaspore mixture and successful establishment of the moss carpet. Yet successful regeneration was clearly influenced by the identity of species chosen for re‐introduction. S. fuscum, alone or in combination, was the species found to lead to the most extensive development of the moss carpet under the current test conditions.     

Sedge Succession and Peatland Methane Dynamics: A Potential Feedback to Climate Change

Under the warmer climate, predicted for the future, northern peatlands are expected to become drier. This drying will lower the water table and likely result in reduced emissions of methane (CH₄) from these ecosystems. However, the prediction of declining CH₄ fluxes does not consider the potential effects of ecological succession, particularly the invasion of sedges into currently wet sites (open water pools, low lawns). The goal of this study was to characterize the relationship between the presence of sedges in peatlands and CH₄ efflux under natural conditions and under a climate change simulation (drained peatland). Methane fluxes, gross ecosystem production, and dissolved pore water CH₄ concentrations were measured and a vegetation survey was conducted in a natural and drained peatland near St. Charles-de-Bellechasse, Quebec, Canada, in the summer of 2003. Each peatland also had plots where the sedges had been removed by clipping. Sedges were larger, more dominant, and more productive at the drained peatland site. The natural peatland had higher CH₄ fluxes than the drained peatland, indicating that drainage was a significant control on CH₄ flux. Methane flux was higher from plots with sedges than from plots where sedges had been removed at the natural peatland site, whereas the opposite case was observed at the drained peatland site. These results suggest that CH₄ flux was enhanced by sedges at the natural peatland site and attenuated by sedges at the drained peatland site. However, the attenuation of CH₄ flux due to sedges at the drained site was reduced in wetter periods. This finding suggests that CH₄ flux could be decreased in the event of climate warming due to the greater depth to the water table, and that sedges colonizing these areas could further attenuate CH₄ fluxes during dry periods. However, during wet periods, the sedges may cause CH₄ fluxes to be higher than is currently predicted for climate change scenarios.     

Atmospheric impact of bioenergy based on perennial crop (reed canary grass,Phalaris arundinaceae,L.) cultivation on a drained boreal organic soil

Marginal organic soils, abundant in the boreal region, are being increasingly used for bioenergy crop cultivation. Using long‐term field experimental data on greenhouse gas (GHG) balance from a perennial bioenergy crop [reed canary grass (RCG), Phalaris arundinaceae L.] cultivated on a drained organic soil as an example, we show here for the first time that, with a proper cultivation and land‐use practice, environmentally sound bioenergy production is possible on these problematic soil types. We performed a life cycle assessment (LCA) for RCG on this organic soil. We found that, on an average, this system produces 40% less CO₂‐equivalents per MWh of energy in comparison with a conventional energy source such as coal. Climatic conditions regulating the RCG carbon exchange processes have a high impact on the benefits from this bioenergy production system. Under appropriate hydrological conditions, this system can even be carbon‐negative. An LCA sensitivity analysis revealed that net ecosystem CO₂ exchange and crop yield are the major LCA components, while non‐CO₂ GHG emissions and costs associated with crop production are the minor ones. Net bioenergy GHG emissions resulting from restricted net CO₂ uptake and low crop yields, due to climatic and moisture stress during dry years, were comparable with coal emissions. However, net bioenergy emissions during wet years with high net uptake and crop yield were only a third of the coal emissions. As long‐term experimental data on GHG balance of bioenergy production are scarce, scientific data stemming from field experiments are needed in shaping renewable energy source policies.