Effects of Experimental Water Table and Temperature Manipulations on Ecosystem CO2 Fluxes in an Alaskan Rich Fen

Peatlands store 30% of the world’s terrestrial soil carbon (C) and those located at northern latitudes are expected to experience rapid climate warming. We monitored growing season carbon dioxide (CO₂) fluxes across a factorial design of in situ water table (control, drought, and flooded plots) and soil warming (control vs. warming via open top chambers) treatments for 2 years in a rich fen located just outside the Bonanza Creek Experimental Forest in interior Alaska. The drought (lowered water table position) treatment was a weak sink or small source of atmospheric CO₂ compared to the moderate atmospheric CO₂ sink at our control. This change in net ecosystem exchange was due to lower gross primary production and light-saturated photosynthesis rather than increased ecosystem respiration. The flooded (raised water table position) treatment was a greater CO₂ sink in 2006 due largely to increased early season gross primary production and higher light-saturated photosynthesis. Although flooding did not have substantial effects on rates of ecosystem respiration, this water table treatment had lower maximum respiration rates and a higher temperature sensitivity of ecosystem respiration than the control plot. Surface soil warming increased both ecosystem respiration and gross primary production by approximately 16% compared to control (ambient temperature) plots, with no net effect on net ecosystem exchange. Results from this rich fen manipulation suggest that fast responses to drought will include reduced ecosystem C storage driven by plant stress, whereas inundation will increase ecosystem C storage by stimulating plant growth.     

Modelling the interannual variability of net ecosystem CO2 exchange at a subarctic sedge fen

This paper presents an empirical model of net ecosystem CO₂ exchange (NEE) developed for a subarctic fen near Churchill, Manitoba. The model with observed data helps explain the interannual variability in growing season NEE. Five years of tower‐flux data are used to test and examine the seasonal behaviour of the model simulations. Processes controlling the observed interannual variability of CO₂ exchange at the fen are examined by exploring the sensitivity of the model to changes in air temperature, precipitation and leaf area index. Results indicate that the sensitivity of NEE to changing environmental controls is complex and varies interannually depending on the initial conditions of the wetland. Changes in air temperature and the timing of precipitation events have a strong influence on NEE, which is largely manifest in gross ecosystem photosynthesis (GEP). Climate change scenarios indicate that warmer air temperatures will increase carbon acquisition during wet years but may act to reduce wetland carbon storage in years that experience a large water deficit early in the growing season. Model simulations for this subarctic sedge fen indicate that carbon acquisition is greatest during wet and warm conditions. This suggests therefore that carbon accumulation was greatest at this subarctic fen during its early developmental stages when hydroclimatic conditions were relatively wet and warm at approximately 2500 years before present.     

Seasonal Net Ecosystem Carbon Exchange of a Regenerating Cutaway Bog: How Long Does it Take to Restore the C‐Sequestration Function?

We measured the net ecosystem exchange (NEE) and respiration rates and modeled the photosynthesis and respiration dynamics in a cutover bog in the Swiss Jura Mountains during one growing season at three stages of regeneration (29, 42, and 51 years after peat cutting; coded sites A, B, and C) to determine if reestablishment of Sphagnum suffices to restore the C‐sequestration function. From the younger to the older stage Sphagnum cover increased, while net primary Sphagnum production over the growing season decreased (139, 82, and, 67 g m^?2 y^?1 for A, B, and C respectively), and fen plant species were replaced by bog species. According to our NEE estimations, over the vegetation period site A was a net CO₂‐C source emitting 40 g CO₂‐C/m² while sites B and C were accumulating CO₂‐C, on average 222 and 209 g CO₂‐C/m², respectively. These differences are due to the higher respiration in site A during the summer, suggesting that early regeneration stages may be more sensitive to a warmer climate. Methane fluxes increased from site A to C in parallel with Eriophorum vaginatum cover and vascular plant leaf area. Our results show that reestablishing a Sphagnum cover is not sufficient to restore a CO₂‐sequestrating function but that after circa 50 years the ecosystem may naturally regain this function over the growing season.     

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.     

Net ecosystem exchange of CO2 with rapidly changing high Arctic landscapes

High Arctic landscapes are expansive and changing rapidly. However, our understanding of their functional responses and potential to mitigate or enhance anthropogenic climate change is limited by few measurements. We collected eddy covariance measurements to quantify the net ecosystem exchange (NEE) of CO₂ with polar semidesert and meadow wetland landscapes at the highest latitude location measured to date (82°N). We coupled these rare data with ground and satellite vegetation production measurements (Normalized Difference Vegetation Index; NDVI) to evaluate the effectiveness of upscaling local to regional NEE. During the growing season, the dry polar semidesert landscape was a near‐zero sink of atmospheric CO₂ (NEE: ?0.3 ± 13.5 g C m^?2). A nearby meadow wetland accumulated over 300 times more carbon (NEE: ?79.3 ± 20.0 g C m^?2) than the polar semidesert landscape, and was similar to meadow wetland NEE at much more southerly latitudes. Polar semidesert NEE was most influenced by moisture, with wetter surface soils resulting in greater soil respiration and CO₂ emissions. At the meadow wetland, soil heating enhanced plant growth, which in turn increased CO₂ uptake. Our upscaling assessment found that polar semidesert NDVI measured on‐site was low (mean: 0.120–0.157) and similar to satellite measurements (mean: 0.155–0.163). However, weak plant growth resulted in poor satellite NDVI–NEE relationships and created challenges for remotely detecting changes in the cycling of carbon on the polar semidesert landscape. The meadow wetland appeared more suitable to assess plant production and NEE via remote sensing; however, high Arctic wetland extent is constrained by topography to small areas that may be difficult to resolve with large satellite pixels. We predict that until summer precipitation and humidity increases enough to offset poor soil moisture retention, climate‐related changes to productivity on polar semideserts may be restricted.     

Vegetation feedbacks of nutrient addition lead to a weaker carbon sink in an ombrotrophic bog

To study vegetation feedbacks of nutrient addition on carbon sequestration capacity, we investigated vegetation and ecosystem CO₂ exchange at Mer Bleue Bog, Canada in plots that had been fertilized with nitrogen (N) or with N plus phosphorus (P) and potassium (K) for 7–12 years. Gross photosynthesis, ecosystem respiration, and net CO₂ exchange were measured weekly during May–September 2011 using climate‐controlled chambers. A substrate‐induced respiration technique was used to determine the functional ability of the microbial community. The highest N and NPK additions were associated with 40% less net CO₂ uptake than the control. In the NPK additions, a diminished C sink potential was due to a 20–30% increase in ecosystem respiration, while gross photosynthesis rates did not change as greater vascular plant biomass compensated for the decrease in Sphagnum mosses. In the highest N‐only treatment, small reductions in gross photosynthesis and no change in ecosystem respiration led to the reduced C sink. Substrate‐induced microbial respiration was significantly higher in all levels of NPK additions compared with control. The temperature sensitivity of respiration in the plots was lower with increasing cumulative N load, suggesting more labile sources of respired CO₂. The weaker C sink potential could be explained by changes in nutrient availability, higher woody : foliar ratio, moss loss, and enhanced decomposition. Stronger responses to NPK fertilization than to N‐only fertilization for both shrub biomass production and decomposition suggest that the bog ecosystem is N‐P/K colimited rather than N‐limited. Negative effects of further N‐only deposition were indicated by delayed spring CO₂ uptake. In contrast to forests, increased wood formation and surface litter accumulation in bogs seem to reduce the C sink potential owing to the loss of peat‐forming Sphagnum.     

Landscape and Ecosystem-Level Controls on Net Carbon Dioxide Exchange along a Natural Moisture Gradient in Canadian Low Arctic Tundra

Our understanding of the controls and magnitudes of regional CO₂ exchanges in the Arctic are limited by uncertainties due to spatial heterogeneity in vegetation across the landscape and temporal variation in environmental conditions through the seasons. We measured daytime net ecosystem CO₂ exchange and each of its component fluxes in the three major tundra ecosystem-types that typically occur along natural moisture gradients in the Canadian Low Arctic biweekly during the full snow-free season of 2004. In addition, we used a plant-removal treatment to compare the contribution of bulk soil organic matter to total respiratory CO₂ loss among these ecosystems. Net CO₂ exchange rates varied strongly, but not consistently, among ecosystems in the spring and summer phases as a result of ecosystem-specific and differing responses of gross photosynthesis and respiration to temporal variation in environmental conditions. Overall, net carbon gain was largest in the wet sedge ecosystem and smallest in the dry heath. Our measures of CO₂ flux variation within each ecosystem were frequently most closely correlated with air or soil temperatures during each seasonal phase. Nevertheless, a particularly large rainfall event in early August rapidly decreased respiration rates and stimulated gross photosynthetic rates, resulting in peak rates of net carbon gain in all ecosystems. Finally, the bulk soil carbon contribution to total respiration was relatively high in the birch hummock ecosystem. Together, these results demonstrate that the relative influences of moisture and temperature as primary controls on daytime net ecosystem CO₂ exchange and its component fluxes differ in fundamental ways between the landscape and ecosystem scales. Furthermore, they strongly suggest that carbon cycling responses to environmental change are likely to be highly ecosystem-specific, and thus to vary substantially across the low arctic landscape. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

Warming impacts on boreal fen CO2 exchange under wet and dry conditions

Northern peatlands form a major soil carbon (C) stock. With climate change, peatland C mineralization is expected to increase, which in turn would accelerate climate change. A particularity of peatlands is the importance of soil aeration, which regulates peatland functioning and likely modulates the responses to warming climate. Our aim is to assess the impacts of warming on a southern boreal and a sub‐arctic sedge fen carbon dioxide (CO₂) exchange under two plausible water table regimes: wet and moderately dry. We focused this study on minerotrophic treeless sedge fens, as they are common peatland types at boreal and (sub)arctic areas, which are expected to face the highest rates of climate warming. In addition, fens are expected to respond to environmental changes faster than the nutrient poor bogs. Our study confirmed that CO₂ exchange is more strongly affected by drying than warming. Experimental water level draw‐down (WLD) significantly increased gross photosynthesis and ecosystem respiration. Warming alone had insignificant impacts on the CO₂ exchange components, but when combined with WLD it further increased ecosystem respiration. In the southern fen, CO₂ uptake decreased due to WLD, which was amplified by warming, while at northern fen it remained stable. As a conclusion, our results suggest that a very small difference in the WLD may be decisive, whether the C sink of a fen decreases, or whether the system is able to adapt within its regime and maintain its functions. Moreover, the water table has a role in determining how much the increased temperature impacts the CO₂ exchange.     

The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest‐wetland landscape

At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus (‘forest’) lead to expansion of permafrost‐free wetlands (‘wetland’). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH₄) emissions. Here, we quantify the thaw‐induced increase in CH₄ emissions for a boreal forest‐wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long‐term net carbon dioxide (CO₂) exchange. Using nested wetland and landscape eddy covariance net CH₄ flux measurements in combination with flux footprint modeling, we find that landscape CH₄ emissions increase with increasing wetland‐to‐forest ratio. Landscape CH₄ emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May–October) wetland CH₄ emission of ~13 g CH₄ m^?2 is the dominating contribution to the landscape CH₄ emission of ~7 g CH₄ m^?2. In contrast, forest contributions to landscape CH₄ emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr^?1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH₄ m^?2 yr^?1 in landscape CH₄ emissions. A long‐term net CO₂ uptake of >200 g CO₂ m^?2 yr^?1 is required to offset the positive radiative forcing of increasing CH₄ emissions until the end of the 21st century as indicated by an atmospheric CH₄ and CO₂ concentration model. However, long‐term apparent carbon accumulation rates in similar boreal forest‐wetland landscapes and eddy covariance landscape net CO₂ flux measurements suggest a long‐term net CO₂ uptake between 49 and 157 g CO₂ m^?2 yr^?1. Thus, thaw‐induced CH₄ emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.     

Temperature and Microtopography Interact to Control Carbon Cycling in a High Arctic Fen

High arctic wetlands hold large stores of soil carbon (C). The fate of these C stores in a changing climate is uncertain, as rising air temperatures may differentially affect photosynthesis and ecosystem respiration (ER). In this study, open-top warming chambers were used to increase air and soil temperatures in contrasting microtopographic positions of a high arctic fen in NW Greenland. CO₂ exchange between the ecosystem and the atmosphere was measured on 28 dates over a 3-year period. Measurements of the normalized difference vegetation index, leaf and stem growth, leaf-level gas exchange, leaf nitrogen, leaf δ¹³C, and fine root production were made to investigate the mechanisms and consequences of observed changes in CO₂ exchange. Gross ecosystem photosynthesis (GEP) increased with chamber warming in hollows, which are characterized by standing water, and in hummocks, which extend above the water table. ER, however, increased only in hummocks, such that net ecosystem exchange (NEE) increased in hollows, but did not change in hummocks with chamber warming. Complementary measurements of plant growth revealed that increases in GEP corresponded with increases in C allocation to aboveground biomass in hummocks and belowground biomass in hollows. Our results and those of several recent studies clearly demonstrate that effects of climate change on the C balance of northern wetlands will depend upon microtopography which, in turn, may be sensitive to climate change.     

Vascular plant‐mediated controls on atmospheric carbon assimilation and peat carbon decomposition under climate change

Climate change can alter peatland plant community composition by promoting the growth of vascular plants. How such vegetation change affects peatland carbon dynamics remains, however, unclear. In order to assess the effect of vegetation change on carbon uptake and release, we performed a vascular plant‐removal experiment in two Sphagnum‐dominated peatlands that represent contrasting stages of natural vegetation succession along a climatic gradient. Periodic measurements of net ecosystem CO₂ exchange revealed that vascular plants play a crucial role in assuring the potential for net carbon uptake, particularly with a warmer climate. The presence of vascular plants, however, also increased ecosystem respiration, and by using the seasonal variation of respired CO₂ radiocarbon (bomb‐¹⁴C) signature we demonstrate an enhanced heterotrophic decomposition of peat carbon due to rhizosphere priming. The observed rhizosphere priming of peat carbon decomposition was matched by more advanced humification of dissolved organic matter, which remained apparent beyond the plant growing season. Our results underline the relevance of rhizosphere priming in peatlands, especially when assessing the future carbon sink function of peatlands undergoing a shift in vegetation community composition in association with climate change.     

Soil CO2 efflux in contrasting boreal deciduous and coniferous stands and its contribution to the ecosystem carbon balance

Similar nonsteady‐state automated chamber systems were used to measure and partition soil CO₂ efflux in contrasting deciduous (trembling aspen) and coniferous (black spruce and jack pine) stands located within 100 km of each other near the southern edge of the Boreal forest in Canada. The stands were exposed to similar climate forcing in 2003, including marked seasonal variations in soil water availability, which provided a unique opportunity to investigate the influence of climate and stand characteristics on soil CO₂ efflux and to quantify its contribution to the net ecosystem CO₂ exchange (NEE) as measured with the eddy‐covariance technique. Partitioning of soil CO₂ efflux between soil respiration (including forest‐floor vegetation) and forest‐floor photosynthesis showed that short‐ and long‐term temporal variations of soil CO₂ efflux were related to the influence of (1) soil temperature and water content on soil respiration and (2) below‐canopy light availability, plant water status and forest‐floor plant species composition on forest‐floor photosynthesis. Overall, the three stands were weak to moderate sinks for CO₂ in 2003 (NEE of ?103, ?80 and ?28 g C m^?2 yr^?1 for aspen, black spruce and jack pine, respectively). Forest‐floor respiration accounted for 86%, 73% and 75% of annual ecosystem respiration, in the three respective stands, while forest‐floor photosynthesis contributed to 11% and 14% of annual gross ecosystem photosynthesis in the black spruce and jack pine stands, respectively. The results emphasize the need to perform concomitant measurements of NEE and soil CO₂ efflux at longer time scales in different ecosystems in order to better understand the impacts of future interannual climate variability and vegetation dynamics associated with climate change on each component of the carbon balance.     

The contribution of bryophytes to the carbon exchange for a temperate rainforest

Bryophytes blanket the floor of temperate rainforests in New Zealand and may influence a number of important ecosystem processes, including carbon cycling. Their contribution to forest floor carbon exchange was determined in a mature, undisturbed podocarp‐broadleaved forest in New Zealand, dominated by 100–400‐year‐old rimu (Dacrydium cupressimum) trees. Eight species of mosses and 13 species of liverworts contributed to the 62% cover of the diverse forest floor community. The bryophyte community developed a relatively thin (depth <30 mm), but dense, canopy that experienced elevated CO₂ partial pressures (median 46.6 Pa immediately below the bryophyte canopy) relative to the surrounding air (median 37.6 Pa at 100 mm above the canopy). Light‐saturated rates of net CO₂ exchange from 14 microcosms collected from the forest floor were highly variable; the maximum rate of net uptake (bryophyte photosynthesis – whole‐plant respiration) per unit ground area at saturating irradiance was 1.9 μmol m^?2 s^?1 and in one microcosm, the net rate of CO₂ exchange was negative (respiration). CO₂ exchange for all microcosms was strongly dependent on water content. The average water content in the microcosms ranged from 1375% when fully saturated to 250% when air‐dried. Reduction in water content across this range resulted in an average decrease of 85% in net CO₂ uptake per unit ground area. The results from the microcosms were used in a model to estimate annual carbon exchange for the forest floor. This model incorporated hourly variability in average irradiance reaching the forest floor, water content of the bryophyte layer, and air and soil temperature. The annual net carbon uptake by forest floor bryophytes was 103 g m^?2, compared to annual carbon efflux from the forest floor (bryophyte and soil respiration) of ?1010 g m^?2. To put this in perspective of the magnitude of the components of CO₂ exchange for the forest floor, the bryophyte layer reclaimed an amount of CO₂ equivalent to only about 10% of forest floor respiration (bryophyte plus soil) or ～11% of soil respiration. The contribution of forest floor bryophytes to productivity in this temperate rainforest was much smaller than in boreal forests, possibly because of differences in species composition and environmental limitations to photosynthesis. Because of their close dependence on water table depth, the contribution of the bryophyte community to ecosystem CO₂ exchange may be highly responsive to rapid changes in climate.     

Arctic browning: Impacts of extreme climatic events on heathland ecosystem CO2 fluxes

Extreme climatic events are among the drivers of recent declines in plant biomass and productivity observed across Arctic ecosystems, known as “Arctic browning.” These events can cause landscape‐scale vegetation damage and so are likely to have major impacts on ecosystem CO₂ balance. However, there is little understanding of the impacts on CO₂ fluxes, especially across the growing season. Furthermore, while widespread shoot mortality is commonly observed with browning events, recent observations show that shoot stress responses are also common, and manifest as high levels of persistent anthocyanin pigmentation. Whether or how this response impacts ecosystem CO₂ fluxes is not known. To address these research needs, a growing season assessment of browning impacts following frost drought and extreme winter warming (both extreme climatic events) on the key ecosystem CO₂ fluxes Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), ecosystem respiration (R_eco) and soil respiration (R_soil) was carried out in widespread sub‐Arctic dwarf shrub heathland, incorporating both mortality and stress responses. Browning (mortality and stress responses combined) caused considerable site‐level reductions in GPP and NEE (of up to 44%), with greatest impacts occurring at early and late season. Furthermore, impacts on CO₂ fluxes associated with stress often equalled or exceeded those resulting from vegetation mortality. This demonstrates that extreme events can have major impacts on ecosystem CO₂ balance, considerably reducing the carbon sink capacity of the ecosystem, even where vegetation is not killed. Structural Equation Modelling and additional measurements, including decomposition rates and leaf respiration, provided further insight into mechanisms underlying impacts of mortality and stress on CO₂ fluxes. The scale of reductions in ecosystem CO₂ uptake highlights the need for a process‐based understanding of Arctic browning in order to predict how vegetation and CO₂ balance will respond to continuing climate change.     

Changes in vegetation phenology are not reflected in atmospheric CO2 and 13C/12C seasonality

Northern terrestrial ecosystems have shown global warming‐induced advances in start, delays in end, and thus increased lengths of growing season and gross photosynthesis in recent decades. The tradeoffs between seasonal dynamics of two opposing fluxes, CO₂ uptake through photosynthesis and release through respiration, determine the influence of the terrestrial ecosystem on the atmospheric CO₂ and ¹³C/¹²C seasonality. Here, we use four CO₂ observation stations in the Northern Hemisphere, namely Alert, La Jolla, Point Barrow, and Mauna Loa Observatory, to determine how changes in vegetation productivity and phenology, respiration, and air temperature affect both the atmospheric CO₂ and ¹³C/¹²C seasonality. Since the 1960s, the only significant long‐term trend of CO₂ and ¹³C/¹²C seasonality was observed at the northern most station, Alert, where the spring CO₂ drawdown dates advanced by 0.65 ± 0.55 days yr^?1, contributing to a nonsignificant increase in length of the CO₂ uptake period (0.74 ± 0.67 days yr^?1). For Point Barrow station, vegetation phenology changes in well‐watered ecosystems such as the Canadian and western Siberian wetlands contributed the most to ¹³C/¹²C seasonality while the CO₂ seasonality was primarily linked to nontree vegetation. Our results indicate significant increase in the Northern Hemisphere soil respiration. This means, increased respiration of ¹³C depleted plant materials cancels out the ¹²C gain from enhanced vegetation activities during the start and end of growing season. These findings suggest therefore that parallel warming‐induced increases both in photosynthesis and respiration contribute to the long‐term stability of CO₂ and ¹³C/¹²C seasonality under changing climate and vegetation activity. The summer photosynthesis and the soil respiration in the dormant seasons have become more vigorous which lead to increased peak‐to‐through CO₂ amplitude. As the relative magnitude of the increased photosynthesis in summer months is more than the increased respiration in dormant months, we have the increased overall carbon uptake rates in the northern ecosystems.     

Spring photosynthetic onset and net CO2 uptake in Alaska triggered by landscape thawing

The springtime transition to regional‐scale onset of photosynthesis and net ecosystem carbon uptake in boreal and tundra ecosystems are linked to the soil freeze–thaw state. We present evidence from diagnostic and inversion models constrained by satellite fluorescence and airborne CO₂ from 2012 to 2014 indicating the timing and magnitude of spring carbon uptake in Alaska correlates with landscape thaw and ecoregion. Landscape thaw in boreal forests typically occurs in late April (DOY 111 ± 7) with a 29 ± 6 day lag until photosynthetic onset. North Slope tundra thaws 3 weeks later (DOY 133 ± 5) but experiences only a 20 ± 5 day lag until photosynthetic onset. These time lag differences reflect efficient cold season adaptation in tundra shrub and the longer dehardening period for boreal evergreens. Despite the short transition from thaw to photosynthetic onset in tundra, synchrony of tundra respiration with snow melt and landscape thaw delays the transition from net carbon loss (at photosynthetic onset) to net uptake by 13 ± 7 days, thus reducing the tundra net carbon uptake period. Two global CO₂ inversions using a CASA‐GFED model prior estimate earlier northern high latitude net carbon uptake compared to our regional inversion, which we attribute to (i) early photosynthetic‐onset model prior bias, (ii) inverse method (scaling factor + optimization window), and (iii) sparsity of available Alaskan CO₂ observations. Another global inversion with zero prior estimates the same timing for net carbon uptake as the regional model but smaller seasonal amplitude. The analysis of Alaskan eddy covariance observations confirms regional scale findings for tundra, but indicates that photosynthesis and net carbon uptake occur up to 1 month earlier in evergreens than captured by models or CO₂ inversions, with better correlation to above‐freezing air temperature than date of primary thaw. Further collection and analysis of boreal evergreen species over multiple years and at additional subarctic flux towers are critically needed.     

Informing climate models with rapid chamber measurements of forest carbon uptake

Models predicting ecosystem carbon dioxide (CO₂) exchange under future climate change rely on relatively few real‐world tests of their assumptions and outputs. Here, we demonstrate a rapid and cost‐effective method to estimate CO₂ exchange from intact vegetation patches under varying atmospheric CO₂ concentrations_. We find that net ecosystem CO₂ uptake (NEE) in a boreal forest rose linearly by 4.7 ± 0.2% of the current ambient rate for every 10 ppm CO₂ increase, with no detectable influence of foliar biomass, season, or nitrogen (N) fertilization. The lack of any clear short‐term NEE response to fertilization in such an N‐limited system is inconsistent with the instantaneous downregulation of photosynthesis formalized in many global models. Incorporating an alternative mechanism with considerable empirical support – diversion of excess carbon to storage compounds – into an existing earth system model brings the model output into closer agreement with our field measurements. A global simulation incorporating this modified model reduces a long‐standing mismatch between the modeled and observed seasonal amplitude of atmospheric CO₂. Wider application of this chamber approach would provide critical data needed to further improve modeled projections of biosphere–atmosphere CO₂ exchange in a changing climate.     

A scaling approach for quantifying the net CO2 flux of the Kuparuk River Basin,Alaska

Net CO₂ flux measurements conducted during the summer and winter of 1994–96 were scaled in space and time to provide estimates of net CO₂ exchange during the 1995–96 (9 May 1995–8 May 1996) annual cycle for the Kuparuk River Basin, a 9200 km² watershed located in NE Alaska. Net CO₂ flux was measured using dynamic chambers and eddy covariance in moist‐acidic, nonacidic, wet‐sedge, and shrub tundra, which comprise 95% of the terrestrial landscape of the Kuparuk Basin. CO₂ flux data were used as input to multivariate models that calculated instantaneous and daily rates of gross primary production (GPP) and whole‐ecosystem respiration (R) as a function of meteorology and ecosystem development. Net CO₂ flux was scaled up to the Kuparuk Basin using a geographical information system (GIS) consisting of a vegetation map, digital terrain map, dynamic temperature and radiation fields, and the models of GPP and R. Basin‐wide estimates of net CO₂ exchange for the summer growing season (9 May?5 September 1995) indicate that nonacidic tundra was a net sink of ?31.7 ± 21.3 GgC (1 Gg = 10⁹ g), while shrub tundra lost 32.5 ± 6.3 GgC to the atmosphere (negative values denote net ecosystem CO₂ uptake). Acidic and wet sedge tundra were in balance, and when integrated for the entire Kuparuk River Basin (including aquatic surfaces), whole basin summer net CO₂ exchange was estimated to be in balance (?0.9 ± 50.3 GgC). Autumn to winter (6 September 1995–8 May 1996) estimates of net CO₂ flux indicate that acidic, nonacidic, and shrub tundra landforms were all large sources of CO₂ to the atmosphere (75.5 ± 8.3, 96.4 ± 11.4, and 43.3 ± 4.7 GgC for acidic, nonacidic, and shrub tundra, respectively). CO₂ loss from wet sedge surfaces was not substantially different from zero, but the large losses from the other terrestrial landforms resulted in a whole basin net CO₂ loss of 217.2 ± 24.1 GgC during the 1995–96 cold season. When integrated for the 1995–96 annual cycle, acidic (66.4 + 25.25 GgC), nonacidic (64.7 ± 29.2 GgC), and shrub tundra (75.8 ± 8.4 GgC) were substantial net sources of CO₂ to the atmosphere, while wet sedge tundra was in balance (0.4 + 0.8 GgC). The Kuparuk River Basin as a whole was estimated to be a net CO₂ source of 218.1 ± 60.6 GgC over the 1995–96 annual cycle. Compared to direct measurements of regional net CO₂ flux obtained from aircraft‐based eddy covariance, the scaling procedure provided realistic estimates of CO₂ exchange during the summer growing season. Although winter estimates could not be assessed directly using aircraft measurements of net CO₂ exchange, the estimates reported here are comparable to measured values reported in the literature. Thus, we have high confidence in the summer estimates of net CO₂ exchange and reasonable confidence in the winter net CO₂ flux estimates for terrestrial landforms of the Kuparuk river basin. Although there is larger uncertainty in the aquatic estimates, the small surface area of aquatic surfaces in the Kuparuk river basin (≈ 5%) presumably reduces the potential for this uncertainty to result in large errors in basin‐wide CO₂ flux estimates.     

Soil‐surface CO2 efflux and its spatial and temporal variations in a young ponderosa pine plantation in northern California

Soil‐surface CO₂ efflux and its spatial and temporal variations were examined in an 8‐y‐old ponderosa pine plantation in the Sierra Nevada Mountains in California from June 1998 to August 1999. Continuous measurements of soil CO₂ efflux, soil temperatures and moisture were conducted on two 20 × 20 m sampling plots. Microbial biomass, fine root biomass, and the physical and chemical properties of the soil were also measured at each of the 18 sampling locations on the plots. It was found that the mean soil CO₂ efflux in the plantation was 4.43 µmol m^?2 s^?1 in the growing season and 3.12 µmol m^?2 s^?1 in the nongrowing season. These values are in the upper part of the range of published soil‐surface CO₂ efflux data. The annual maximum and minimum CO₂ efflux were 5.87 and 1.67 µmol m^?2 s^?1, respectively, with the maximum occurring between the end of May and early June and the minimum in December. The diurnal fluctuation of CO₂ efflux was relatively small (< 20%) with the minimum appearing around 09.00 hours and the maximum around 14.00 hours. Using daytime measurements of soil CO₂ efflux tends to overestimate the daily mean soil CO₂ efflux by 4–6%. The measurements taken between 09.00 and 11.00 hours (local time) seem to better represent the daily mean with a reduced sampling error of 0.9–1.5%. The spatial variation of soil CO₂ efflux among the 18 sampling points was high, with a coefficient of variation of approximately 30%. Most (84%) of the spatial variation was explained by fine root biomass, microbial biomass, and soil physical and chemical properties. Although soil temperature and moisture explained most of the temporal variations (76–95%) of soil CO₂ efflux, the two variables together explained less than 34% of the spatial variation. Microbial biomass, fine root biomass, soil nitrogen content, organic matter content, and magnesium content were significantly and positively correlated with soil CO₂ efflux, whereas bulk density and pH value were negatively correlated with CO₂ efflux. The relationship between soil CO₂ efflux and soil temperature was significantly controlled by soil moisture with a Q₁₀ value of 1.4 when soil moisture was <14% and 1.8 when soil moisture was >14%. Understanding the spatial and temporal variations is essential to accurately assessment of carbon budget at whole ecosystem and landscape scales. Thus, this study bears important implications for the study of large‐scale ecosystem dynamics, particularly in response to climatic variations and management regimes.