Changes in Biomass, Aboveground Net Primary Production, and Peat Accumulation following Permafrost Thaw in the Boreal Peatlands of Manitoba, Canada

Permafrost thaw resulting from climate warming may dramatically change the succession and carbon dynamics of northern ecosystems. To examine the joint effects of regional temperature and local species changes on peat accumulation following thaw, we studied peat accumulation across a regional gradient of mean annual temperature (MAT). We measured aboveground net primary production (AGNPP) and decomposition over 2 years for major functional groups and used these data to calculate a simple index of net annual aboveground peat accumulation. In addition, we collected cores from six adjacent frozen and thawed bog sites to document peat accumulation changes following thaw over the past 200 years. Aboveground biomass and decomposition were more strongly controlled by local succession than regional climate. AGNPP for some species differed between collapse scars and associated permafrost plateaus and was influenced by regional MAT. A few species, such as Picea mariana trees on frozen bogs and Sphagnum mosses in thawed bogs, sequestered a disproportionate amount of peat; in addition, changes in their abundance following thaw changed peat accumulation. ²¹⁰Pb-dated cores indicated that peat accumulation doubles following thaw and that the accumulation rate is affected by historical changes in species during succession. Peat accumulation in boreal peatlands following thaw was controlled by a complex mix of local vegetation changes, regional climate, and history. These results suggest that northern ecosystems may show responses more complex than large releases of carbon during transient warming. Received 8 August 2000; accepted 12 January 2001.     

Long-term Perspectives on Lagged Ecosystem Responses to Climate Change: Permafrost in Boreal Peatlands and the Grassland/Woodland Boundary

Changes in climate could have far-reaching consequences for ecosystems sensitive to changes in temperature and precipitation, such as boreal permafrost peatlands and grassland/woodland boundaries. The long-term data from our studies in these ecosystems suggest that transient responses of permafrost and vegetation to climate change may be difficult to predict due to lags and positive feedbacks related to vegetation and disturbance. Boreal permafrost peatlands comprise an ecosystem with strong local controls on microclimate that influence the formation and thaw of permafrost. These local controls may preserve permafrost during the transient stages of climate warming, producing lagged responses. The prairie–forest border region of the northern Great Plains has experienced frequent change and has complex dynamics involving transitions in the grassland composition of prairie and in the degree of woodiness in bordering forests. Fire frequency interacts with fuel loading and tree recruitment in ways that affect the timing and direction of change. Lags and thresholds could lead to sudden large responses to future climate change that are not readily apparent from current vegetation. The creation of adequate models to characterize transient ecosystem changes will require an understanding of the linkages among processes operating at the scale of 10s of meters and over long time periods. Received 14 December 1999; accepted 7 July 2000.     

Decomposition and Peat Accumulation in Rich Fens of Boreal Alberta, Canada

Fens are important components of Canada’s western boreal forests, occupying about 63% of the total peatland area and storing about 65% of the peatland carbon. Rich fens, dominated by true moss-dominated ground layers, make up more than half of the fens in the region. We studied organic matter accumulation in three rich fens that represent the diversity in structural types. We used in situ decomposition socks, a new method that examines actual decomposition throughout the upper peat profile over an extended period of time. We coupled our carbon loss data with macrofossil analyses and dated peat profiles using ²¹⁰Pb. Across the three rich fens and in the top 39 cm of the peat column, dry mass increases on average 3.1 times. From our dry mass loss measurements, we calculate that annual mass loss from the top 39 cm varies from 0.52 to 1.08 kg m². Vertical accumulation during the past 50 years has varied from 16 to 32 cm and during these 50 years, organic matter accumulation has averaged 174 g m⁻² y⁻¹ compared to 527 g m² y⁻¹ dry mass loss, with additional mass losses of 306 g m² y⁻¹ from peat between 50 and 150 years of age. Organic matter accumulation from our rich fens compares well with literature values from boreal bogs, whereas peat bulk densities increase about three times within the uppermost 40 cm, much more than in bogs. Hence, rich fens accumulate peat not because the plant material is especially hard to decompose, is acidic, or has the catotelm especially close to the surface, but because dense, rapidly produced inputs outweigh the relatively rapid decomposition process of the upper peat column. Author Contributions: DHV conceived study; KS, KW, SF, & DHV performed research; DHV, KW analyzed data; DHV, KW contributed new methods; DHV, KW wrote the paper.     

Response of Net Ecosystem Productivity of Three Boreal Forest Stands to Drought

In 2000–03, continuous eddy covariance measurements of carbon dioxide (CO₂) flux were made above mature boreal aspen, black spruce, and jack pine forests in Saskatchewan, Canada, prior to and during a 3-year drought. During the 1st drought year, ecosystem respiration (R) was reduced at the aspen site due to the drying of surface soil layers. Gross ecosystem photosynthesis (GEP) increased as a result of a warm spring and a slow decrease of deep soil moisture. These conditions resulted in the highest annual net ecosystem productivity (NEP) in the 9 years of flux measurements at this site. During 2002 and 2003, a reduction of 6% and 34% in NEP, respectively, compared to 2000 was observed as the result of reductions in both R and GEP, indicating a conservative response to the drought. Although the drought affected most of western Canada, there was considerable spatial variability in summer rainfall over the 100-km extent of the study area; summer rainfalls in 2001 and 2002 at the two conifer sites minimized the impact of the drought. In 2003, however, precipitation was similarly low at all three sites. Due to low topographic position and consequent poor drainage at the black spruce site and the coarse soil with low water-holding capacity at the jack pine site almost no reduction in R, GEP, and NEP was observed at these two sites. This study shows that the impact of drought on carbon sequestration by boreal forest ecosystems strongly depends on rainfall distribution, soil characteristics, topography, and the presence of vegetation that is well adapted to these conditions. The online version of the original article can be found under doi:     

Response of Net Ecosystem Productivity of Three Boreal Forest Stands to Drought

In 2001–03, continuous eddy covariance measurements of carbon dioxide (CO₂) flux were made above mature boreal aspen, black spruce, and jack pine forests in Saskatchewan, Canada, prior to and during a 3−year drought. During the 1st drought year, ecosystem respiration (R) was reduced at the aspen site due to the drying of surface soil layers. Gross ecosystem photosynthesis (GEP) increased as a result of a warm spring and a slow decrease of deep soil moisture. These conditions resulted in the highest annual net ecosystem productivity (NEP) in the 9 years of flux measurements at this site. During 2002 and 2003, a reduction of 6% and 34% in NEP, respectively, compared to 2000 was observed as the result of reductions in both R and GEP, indicating a conservative response to the drought. Although the drought affected most of western Canada, there was considerable spatial variability in summer rainfall over the 100−km extent of the study area; summer rainfalls in 2001 and 2002 at the two conifer sites minimized the impact of the drought. In 2003, however, precipitation was similarly low at all three sites. Due to low topographic position and consequent poor drainage at the black spruce site and the coarse soil with low water-holding capacity at the jack pine site almost no reduction in R, GEP, and NEP was observed at these two sites. This study shows that the impact of drought on carbon sequestration by boreal forest ecosystems strongly depends on rainfall distribution, soil characteristics, topography, and the presence of vegetation that is well adapted to these conditions.     

A Qualitative Ecosystem Assessment for Different Shrublands in Western Europe under Impact of Climate Change

Climate change may affect the dynamics of ecosystems and the goods and services they provide. To investigate the consequences of warming and drought for the goods and services provided by different shrublands in various western European countries, an assessment was carried out using results of field manipulation experiments of the CLIMOOR and VULCAN projects. Goods and services of these shrublands mainly encompass biodiversity, various forms of recreation, conservation of culturally and historically important landscapes, groundwater as a drinking water source, and carbon sequestration. Warming of dry lowland heathlands in The Netherlands and Denmark increases nutrient availability, which may lead to grass encroachment reducing biodiversity and decreasing recreational values. Drought may reduce the chances of grass encroachment but increase the chances of disturbances to heather vegetation. Similarly, warming increases and drought decreases the chances of nitrate pollution to the groundwater, which is often used as a drinking water source. Warming of the upland heathland in the UK increases its productivity, which might enable higher grazing densities leading to improved agricultural production. However, complex interactions between heather and invading species may be affected. Furthermore, nitrate production is increased, which may lead to groundwater pollution. Under drought conditions, productivity decreases and agricultural production capacity drops. In the Mediterranean shrubland in Spain, both warming and drought led to a shift in the species composition of seedlings and recruitment, which might lead to a change in the plant community and a reduction in biodiversity. In the drought treatment, a decreasing soil carbon content may lead to a loss of biodiversity, recreational possibilities, and an increased threat of wildfires and erosion.     

1981～2000年中国陆地生态系统碳通量的年际变化

应用一个生物地球化学模型(CEVSA)估算了中国陆地净初级生产力 (NPP)、土壤异养呼吸(HR)和净生态系统生产力 (NEP) 在1981～1998年期间对气候和大气CO2浓度变化的动态响应.结果显示,全国NPP总量波动于2.89～3.37 Gt C/a之间,平均值为3.09 Gt C/a,年平均增长趋势约为0.32%.HR总量变化范围为2.89～3.21 Gt C/a,平均值为3.02 Gt C/a, 年均增长0.40%.NEP总量变动于 -0.32和0.25 Gt C/a之间,在统计上没有明显的年际变化趋势.在研究时段内,年平均NEP约为0.07 Gt C/a,表明中国陆地生态系统在气候与大气CO2浓度变化的条件下吸收了碳,为碳汇,总的吸收量为1.22 Gt C,约占全球碳吸收总量的10%,与同期内美国由大气CO2和气候变化所产生的碳吸收量大致相当.尽管由于较高的年际变率,NEP在统计上没有明显的变化趋势,但NPP的增长率低于HR的增长率,说明在研究时段内,中国陆地生态系统的吸碳能力由于气候变化降低了.全国大多数地区年平均NEP接近零,明显的NEP正值区(即碳汇)出现在东北平原、西藏东南部和黄淮平原等地区,而大小兴安岭、黄土高原和云贵高原等地区NEP为负值(即碳源).研究认为,1981～1998年期间中国气候温暖、干旱,因此估算的NEP可能低于其他时段.如果气候进入一个比较湿润的时期,碳吸收量可显著增加,但若当前干旱和暖化趋势以此为继,中国的NEP可能会变成一个负值.     

1981—2000年中国陆地生态系统碳通量的年际变化

应用一个生物地球化学模型(CEVSA)估算了中国陆地净初级生产力(NPP)、土壤异养呼吸(HR)和净生态系统生产力(NEP)在1981—1998年期间对气候和大气CO2浓度变化的动态响应。结果显示,全国NPP总量波动于2．89—3．37Gt／a之间,平均值为3．09Gt C／a,年平均增长趋势约为0．32％。HR总量变化范围为2．89—3．21Gt C／a,平均值为3．02Gt C／a,年均增长0．40％。NEP总量变动于-0．32和0．25Gt C／a之间,在统计上没有明显的年际变化趋势。在研究时段内,年平均NEP约为0．07Gt C／a,表明中国陆地生态系统在气候与大气CO2浓度变化的条件下吸收了碳,为碳汇,总的吸收量为1．22Gt C,约占全球碳吸收总量的10％,与同期内美国由大气CO2和气候变化所产生的碳吸收量大致相当。尽管由于较高的年际变率,NEP在统计上没有明显的变化趋势,但NPP的增长率低于HR的增长率,说明在研究时段内,中国陆地生态系统的吸碳能力由于气候变化降低了。全国大多数地区年平均NEP接近零,明显的NEP正值区(即碳汇)出现在东北平原、西藏东南部和黄淮平原等地区,而大小兴安岭、黄土高原和云贵高原等地区NEP为负值(即碳源)。研究认为,1981～1998年期间中国气候温暖、干旱,因此估算的NEP可能低于其他时段。如果气候进入一个比较湿润的时期,碳吸收量可显著增加,但若当前干旱和暖化趋势以此为继,中国的NEP可能会变成一个负值。     

Impacts of Elevated Carbon Dioxide and Temperature on a Boreal Forest Ecosystem (CLIMEX Project)

To evaluate the effects of climate change on boreal forest ecosystems, both atmospheric CO₂ (to 560 ppmv) and air temperature (by 3°–5°C above ambient) were increased at a forested headwater catchment in southern Norway. The entire catchment (860 m²) is enclosed within a transparent greenhouse, and the upper 20% of the catchment area is partitioned such that it receives no climate treatment and serves as an untreated control. Both the control and treatment areas inside the greenhouse receive deacidified rain. Within 3 years, soil nitrogen (N) mineralization has increased and the growing season has been prolonged relative to the control area. This has helped to sustain an increase in plant growth relative to the control and has also promoted increased N export in stream water. Photosynthetic capacity and carbon–nitrogen ratio of new leaves of most plant species did not change. While the ecosystem now loses N, the long-term fate of soil N is a key uncertainty in predicting the future response of boreal ecosystems to climate change. Received 18 November 1997; accepted 13 April 1998.     

Recovery of Ecosystem Carbon Stocks in Young Boreal Forests: A Comparison of Harvesting and Wildfire Disturbance

Corresponding with the increasing global resource demand, harvesting now affects millions of hectares of boreal forest each year, and yet our understanding of harvesting impacts on boreal carbon (C) dynamics relative to wildfire remains unclear. We provide a direct comparison of C stocks following clearcut harvesting and fire over a 27-year chronosequence in the boreal forest of central Canada. Whereas many past studies have lacked measurement of all major C pools, we attempt to provide complete C pool coverage, including live biomass, deadwood, forest floor, and mineral soil C pools. The relative contribution of each C pool to total ecosystem C varied considerably between disturbance types. Live biomass C was significantly higher following harvesting compared with fire because of residual live trees and advanced regeneration. Conversely, most live biomass was killed following fire, and thus post-fire stands contained higher stocks of deadwood C. Snag and stump C mass peaked immediately following fire, but dramatically decreased 8 years after fire as dead trees began to fall over, contributing to the downed woody debris C pool. Forest floor C mass was substantially lower shortly after fire than harvesting, but this pool converged 8 years after fire and harvesting. When total ecosystem C stocks were examined, we found no significant difference during early stand development between harvesting and fire. Maximum total ecosystem C occurred at age 27 years, 185.1 ± 18.2 and 163.6 ± 8.0 Mg C ha^?1 for harvesting and fire, respectively. Our results indicate strong differences in individual C pools, but similar total ecosystem C after fire and clearcutting in boreal forests, and shall help improve modeling terrestrial C flux after stand-replacing disturbances.     

Methanogenesis and Methanogen Diversity in Three Peatland Types of the Discontinuous Permafrost Zone,Boreal Western Continental Canada

Because recent patterns of permafrost collapse in boreal peatlands appear to enhance emissions of CH ₄ to the atmosphere, we examined methanogenesis and methanogen diversity in peat soil from peatlands with and without permafrost in two peatland complexes situated in continental western Canada. Peat soil from the active layer of permafrost bogs had very low rates of CH ₄ production (ca. 10 nmol g ^?1 day ^?1 ), and we were unable to PCR-amplify 16s rRNA gene sequences using Archaea-specific primers in four peat samples. Surface peat soil from continental bogs with no permafrost supported moderate rates of CH ₄ production (20–600 nmol g ^?1 day ^?1 ), with maximum rates in soil located close to the mean water table level. Additions of ethanol stimulated CH ₄ production rates, suggesting metabolic substrate limitations. Peat from internal lawns, which have experienced surface permafrost degradation in the past 150 years, had very rapid rates of CH₄ production (up to 800 nmol g ^?1 day ^?1 ) occurring within the soil profile. Concomitant rates of anaerobic CO ₂ production were greater in continental bogs (ca. 6 μmol g ^?1 day ^?1 ) than in internal lawns (ca. 4 μ mol g ^?1 day ^?1 ) or in permafrost bogs (2.8 μ mol g ^?1 day ^?1 ). Analysis of the 16s rRNA gene for Archaea in the continental bog indicated mostly sequences associate with Methanobacteriales and RC-I with a Methanosarcinaceae sequence in the deepest peat soil. In the internal lawn, Methanosarcinaceae were most common in peat soil with a Methanosaetaceae sequence in the deepest peat soil. This study showed that patterns of discontinuous permafrost and ongoing permafrost degradation in boreal regions create patchy soil environments for methanogens and rates of methanogenesis.     

Carbon sequestration in peatland: patterns and mechanisms of response to climate change

The response of peatlands to changes in the climatic water budget is crucial to predicting potential feedbacks on the global carbon (C) cycle. To gain insight on the patterns and mechanisms of response, we linked a model of peat accumulation to a model of peatland hydrology, then applied these models to empirical data spanning the past 5000 years for the large mire Store Mosse in southern Sweden. We estimated parameters for C sequestration and height growth by fitting the peat accumulation model to two age profiles. Then, we used independent reconstruction of climate wetness and model reconstruction of bog height to examine changes in peatland hydrology. Reconstructions of C sequestration showed two distinct patterns of behaviour: abrupt increases associated with major transitions in vegetation and dominant Sphagnum species (fuscum, rubellum–fuscum and magellanicum stages), and gradual decreases associated with increasing humification of newly formed peat. Carbon sequestration rate ranged from a minimum of 14 to a maximum of 72 g m^?2 yr^?1, with the most rapid changes occurring in the past 1000 years. Vegetation transitions were associated with periods of increasing climate wetness during which the hydrological requirement for increased seepage loss was met by rise of the water table closer to the peatland surface, with the indirect result of enhancing peat formation. Gradual decline in C sequestration within each vegetation stage resulted from enhanced litter decay losses from the near‐surface layer. In the first two vegetation stages, peatland development (i.e., increasing surface gradient) and decreasing climate wetness drove a gradual increase in thickness of the unsaturated, near‐surface layer, reducing seepage water loss and peat formation. In the most recent vegetation stage, the surface diverged into a mosaic of wet and dry microsites. Despite a steady increase in climate wetness, C sequestration declined rapidly. The complexity of response to climate change cautions against use of past rates to estimate current or to predict future rates of peatland C sequestration. Understanding interactions among hydrology, surface structure and peat formation are essential to predicting potential feedback on the global C cycle.     

Sensitivity of CO<Subscript>2</Subscript> Exchange of Fen Ecosystem Components to Water Level Variation

Abstract Climate change is predicted to bring about a water level (WL) draw-down in boreal peatlands. This study aimed to assess the effect of WL on the carbon dioxide (CO₂) dynamics of a boreal oligotrophic fen ecosystem and its components; Sphagnum mosses, sedges, dwarf shrubs and the underlying peat. We measured CO₂ exchange with closed chambers during four growing seasons in a study site that comprised different vegetation treatments. WL gradient in the site was broadened by surrounding half of the site with a shallow ditch that lowered the WL by 10–25 cm. We modeled gross photosynthesis (P _G) and ecosystem respiration (R _ECO) and simulated the CO₂ exchange in three WL conditions: prevailing and WL draw-down scenarios of 14 and 22 cm. WL draw-down both reduced the P _G and increased the R _ECO, thus leading to a smaller net CO₂ uptake in the ecosystem. Of the different components, Sphagnum mosses were most sensitive to WL draw-down and their physiological activities almost ceased. Vascular plant CO₂ exchange, en bloc, hardly changed but whereas sedges contributed twice as much to the CO₂ exchange as shrubs in the prevailing conditions, the situation was reversed in the WL draw-down scenarios. Peat respiration was the biggest component in R _ECO in all WL conditions and the increase in R _ECO following the WL draw-down was due to the increase in peat respiration. The results imply that functional diversity buffers the ecosystem against environmental variability and that in the long term, WL draw-down changes the vegetation composition of boreal fens.     

Decomposition and Organic Matter Quality in Continental Peatlands: The Ghost of Permafrost Past

Permafrost patterning in boreal peatlands contributes to landscape heterogeneity, as peat plateaus, palsas, and localized permafrost mounds are interspersed among unfrozen bogs and fens. The degradation of localized permafrost in peatlands alters local topography, hydrology, thermal regimes, and plant communities, and creates unique peatland features called internal lawns. I used laboratory incubations to quantify carbon dioxide (CO₂) production in peat formed under different permafrost regimes (with permafrost, without permafrost, melted permafrost), and explored the relationships among proximate organic matter fractions, nutrient concentrations, and decomposition. Peat within each feature (internal lawn, bog, permafrost mound) is more chemically similar than peat collected within the same province (Alberta, Saskatchewan) or within depth intervals (surface, deep). Internal lawn peat produces more CO₂ than the other peatland types. Across peatland features, acid-insoluble material (AIM) and AIM/nitrogen are significant predictors of decomposition. However, within each peatland feature, soluble proximate fractions are better predictors of CO₂ production. Permafrost stability in peatlands influences plant and soil environments, which control litter inputs, organic matter quality, and decomposition rates. Spatial patterns of permafrost, as well as ecosystem processes within various permafrost features, should be considered when assessing the fate of soil carbon in northern ecosystems.     

Warmer and drier conditions stimulate respiration more than photosynthesis in a boreal peatland ecosystem: Analysis of automatic chambers and eddy covariance measurements

Continuous half‐hourly net CO₂ exchange measurements were made using nine automatic chambers in a treed fen in northern Alberta, Canada from June–October in 2005 and from May–October in 2006. The 2006 growing season was warmer and drier than in 2005. The average chamber respiration rates normalized to 10 °C were much higher in 2006 than in 2005, while calculations of the temperature sensitivity (Q₁₀) values were similar in the two years. Daytime average respiration values were lower than the corresponding, temperature‐corrected respiration rates calculated from night‐time chamber measurements. From June to September, the season‐integrated estimates of chamber photosynthesis and respiration were 384 and 590 g C m^?2, respectively in 2006, an increase of 100 and 203 g C m^?2 over the corresponding values in 2005. The season‐integrated photosynthesis and respiration rates obtained using the eddy covariance technique, which included trees and a tall shrub not present in the chambers, were 720 and 513 g C m^?2, respectively, in 2006, an increase of 50 and 125 g C m^?2 over the corresponding values in 2005. While both photosynthesis and respiration rates were higher in the warmer and drier conditions of 2006, the increase in respiration was more than twice the increase in photosynthesis.     

Potential Changes in the Distributions of Western North America Tree and Shrub Taxa under Future Climate Scenarios

Increases in atmospheric greenhouse gases are driving significant changes in global climate. To project potential vegetation response to future climate change, this study uses response surfaces to describe the relationship between bioclimatic variables and the distribution of tree and shrub taxa in western North America. The response surfaces illustrate the probability of the occurrence of a taxon at particular points in climate space. Climate space was defined using three bioclimatic variables: mean temperature of the coldest month, growing degree days, and a moisture index. Species distributions were simulated under present climate using observed data (1951–80, 30-year mean) and under future climate (2090–99, 10-year mean) using scenarios generated by three general circulation models—HADCM2, CGCM1, and CSIRO. The scenarios assume a 1% per year compound increase in greenhouse gases and changes in sulfate (SO₄) aerosols based on the Intergovernmental Panel on Climate Change (IPCC) IS92a scenario. The results indicate that under future climate conditions, potential range changes could be large for many tree and shrub taxa. Shifts in the potential ranges of species are simulated to occur not only northward but in all directions, including southward of the existing ranges of certain species. The simulated potential distributions of some species become increasingly fragmented under the future climate scenarios, while the simulated potential distributions of other species expand. The magnitudes of the simulated range changes imply significant impacts to ecosystems and shifts in patterns of species diversity in western North America. Received 12 May 2000; accepted 20 December 2000.     

Ecosystem responses to increased precipitation and permafrost decay in subarctic Sweden inferred from peat and lake sediments

Recent accelerated decay of discontinuous permafrost at the Stordalen Mire in northern Sweden has been attributed to increased temperature and snow depth, and has caused expansion of wet minerotrophic areas leading to significant changes in carbon cycling in the mire. In order to track these changes through time and evaluate potential forcing mechanisms, this paper analyses a peat succession and a lake sediment sequence from within the mire, providing a record for the last 100 years, and compares these with monitored climate and active layer thickness data. The peat core was analysed for testate amoebae to reconstruct changes in peatland surface moisture conditions and water table fluctuations. The lake sediment core was analysed by near infrared spectroscopy to infer changes in the total organic carbon (TOC) concentration of the lake‐water, and changes in δ¹³C and C, N and δ¹⁵N to track changes in the dissolved inorganic carbon (DIC) pool and the influence of diagenetic effects on sediment organic matter, respectively. Results showed that major shifts towards increased peat surface moisture and TOC concentration of the lake‐water occurred around 1980, one to two decades earlier than a temperature driven increase in active layer thickness. Comparison with monitored temperature and precipitation from a nearby climate station indicates that this change in peat surface moisture is related to June–September (JJAS) precipitation and that the increase in lake‐water TOC concentration reflects an increase in total annual precipitation. A significant depletion in ¹³C of sediment organic matter in the early 1980s probably reflects the effect of a single or a few consecutive years with anomalously high summer precipitation, resulting in elevated DIC content of the lake water, predominantly originating from increased export and subsequent respiration of organic carbon from the mire. Based on these results, it was not possible to link proxy data obtained on peat and lake‐sediment records directly to permafrost decay. Instead our data indicate that increased precipitation and anomalously high rainfall during summers had a significant impact on the mire and the adjacent lake ecosystem. We therefore propose that effects of increased precipitation should be considered when evaluating potential forcing mechanisms of recent changes in carbon cycling in the subarctic.     

Stimulation of both photosynthesis and respiration in response to warmer and drier conditions in a boreal peatland ecosystem

Peatland ecosystems have been consistent carbon (C) sinks for millennia, but it has been predicted that exposure to warmer temperatures and drier conditions associated with climate change will shift the balance between ecosystem photosynthesis and respiration providing a positive feedback to atmospheric CO₂ concentration. Our main objective was to determine the sensitivity of ecosystem photosynthesis, respiration and net ecosystem production (NEP) measured by eddy covariance, to variation in temperature and water table depth associated with interannual shifts in weather during 2004–2009. Our study was conducted in a moderately rich treed fen, the most abundant peatland type in western Canada, in a region (northern Alberta) where peatland ecosystems are a significant landscape component. During the study, the average growing season (May–October) water depth declined approximately 38 cm, and temperature [expressed as cumulative growing degree days (GDD, March–October)] varied approximately 370 GDD. Contrary to previous predictions, both ecosystem photosynthesis and respiration showed similar increases in response to warmer and drier conditions. The ecosystem remained a strong net sink for CO₂ with an average NEP (± SD) of 189 ± 47 g C m^?2 yr^?1. The current net CO₂ uptake rates were much higher than C accumulation in peat determined from analyses of the relationship between peat age and cumulative C stock. The balance between C addition to, and total loss from, the top 0–30 cm depth (peat age range 0–70 years) of shallow peat cores averaged 43 ± 12 g C m^?2 yr^?1. The apparent long‐term average rate of net C accumulation in basal peat samples was 19–24 g C m^?2 yr^?1. The difference between current rates of net C uptake and historical rates of peat accumulation is likely a result of vegetation succession and recent increases in tree establishment and productivity.