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
- NSF-PAR ID:
- Publisher / Repository:
- Date Published:
- Journal Name:
- Global Change Biology
- Page Range / eLocation ID:
- p. 3416-3435
- Medium: X
- Sponsoring Org:
- National Science Foundation
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CH4flux is an important pathway for land–atmosphere CH4emissions, but the magnitude, timing, and environmental controls, spanning scales of space and time, remain poorly understood in arctic tundra wetlands, particularly under the long‐term effects of climate change. CH4fluxes were measured in situduring peak growing season for the dominant aquatic emergent plants in the Alaskan arctic coastal plain, Carex aquatilisand Arctophila fulva, to assess the magnitude and species‐specific controls on CH4flux. Plant biomass was a strong predictor of A. fulva CH4flux while water depth and thaw depth were copredictors for C. aquatilis CH4flux. We used plant and environmental data from 1971 to 1972 from the historic International Biological Program ( IBP) research site near Barrow, Alaska, which we resampled in 2010–2013, to quantify changes in plant biomass and thaw depth, and used these to estimate species‐specific decadal‐scale changes in CH4fluxes. A ~60% increase in CH4flux was estimated from the observed plant biomass and thaw depth increases in tundra ponds over the past 40 years. Despite covering only ~5% of the landscape, we estimate that aquatic C. aquatilisand A. fulvaaccount for two‐thirds of the total regional CH4flux of the Barrow Peninsula. The regionally observed increases in plant biomass and active layer thickening over the past 40 years not only have major implications for energy and water balance, but also have significantly altered land–atmosphere CH4emissions for this region, potentially acting as a positive feedback to climate warming.
Rich fens are common boreal ecosystems with distinct hydrology, biogeochemistry and ecology that influence their carbon (C) balance. We present growing season soil chamber methane emission (
FCH4), ecosystem respiration ( ER), net ecosystem exchange ( NEE) and gross primary production ( GPP) fluxes from a 9‐years water table manipulation experiment in an Alaskan rich fen. The study included major flood and drought years, where wetting and drying treatments further modified the severity of droughts. Results support previous findings from peatlands that drought causes reduced magnitude of growing season FCH4, GPPand NEE, thus reducing or reversing their C sink function. Experimentally exacerbated droughts further reduced the capacity for the fen to act as a C sink by causing shifts in vegetation and thus reducing magnitude of maximum growing season GPPin subsequent flood years by ~15% compared to control plots. Conversely, water table position had only a weak influence on ER, but dominant contribution to ERswitched from autotrophic respiration in wet years to heterotrophic in dry years. Droughts did not cause inter‐annual lag effects on ERin this rich fen, as has been observed in several nutrient‐poor peatlands. While ERwas dependent on soil temperatures at 2 cm depth, FCH4was linked to soil temperatures at 25 cm. Inter‐annual variability of deep soil temperatures was in turn dependent on wetness rather than air temperature, and higher FCH4in flooded years was thus equally due to increased methane production at depth and decreased methane oxidation near the surface. Short‐term fluctuations in wetness caused significant lag effects on FCH4, but droughts caused no inter‐annual lag effects on FCH4. Our results show that frequency and severity of droughts and floods can have characteristic effects on the exchange of greenhouse gases, and emphasize the need to project future hydrological regimes in rich fens.
CO2responses ( A/ Cicurves) are used to assess environmental responses of photosynthetic traits and to predict future vegetative carbon uptake through modeling. The recent development of rapid A/ Cicurves ( RACiRs) permits faster assessment of these traits by continuously changing [ CO2] around the leaf, and may reveal additional photosynthetic properties beyond what is practical or possible with steady‐state methods.
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CO2diffusional limitations can be detected by varying the rate of change in [ CO2] during RACiR assays. We tested these hypotheses through modeling and experiments at ambient and 2% oxygen.
Our data show that photorespiratory delays cause offsets in predicted
CO2compensation points that are dependent on the rate of change in [ CO2]. Diffusional limitations may reduce the rate of change in chloroplastic [ CO2], causing a reduction in apparent RACiR slopes under high CO2ramp rates.
RACiRs may prove useful in assessing diffusional limitations to gas exchange and photorespiratory rates.
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Abstract. Thaw and release of permafrost carbon (C) due to climate change is likely tooffset increased vegetation C uptake in northern high-latitude (NHL)terrestrial ecosystems. Models project that this permafrost C feedback mayact as a slow leak, in which case detection and attribution of the feedbackmay be difficult. The formation of talik, a subsurface layer of perenniallythawed soil, can accelerate permafrost degradation and soil respiration,ultimately shifting the C balance of permafrost-affected ecosystems fromlong-term C sinks to long-term C sources. It is imperative to understand andcharacterize mechanistic links between talik, permafrost thaw, andrespiration of deep soil C to detect and quantify the permafrost C feedback.Here, we use the Community Land Model (CLM) version 4.5, a permafrost andbiogeochemistry model, in comparison to long-term deep borehole data alongNorth American and Siberian transects, to investigate thaw-driven C sourcesin NHL (>55∘N) from 2000 to 2300. Widespread talik at depth isprojected across most of the NHL permafrost region(14million km2) by 2300, 6.2million km2 of which isprojected to become a long-term C source, emitting 10Pg C by 2100,50Pg C by 2200, and 120Pg C by 2300, with few signs ofslowing. Roughly half of the projected C source region is in predominantlywarm sub-Arctic permafrost following talik onset. This region emits only20Pg C by 2300, but the CLM4.5 estimate may be biased low by notaccounting for deep C in yedoma. Accelerated decomposition of deep soilC following talik onset shifts the ecosystem C balance away from surfacedominant processes (photosynthesis and litter respiration), butsink-to-source transition dates are delayed by 20–200 years by highecosystem productivity, such that talik peaks early (∼2050s, although boreholedata suggest sooner) and C source transition peaks late(∼2150–2200). The remaining C source region in cold northern Arcticpermafrost, which shifts to a net source early (late 21st century), emits5 times more C (95Pg C) by 2300, and prior to talik formation dueto the high decomposition rates of shallow, young C in organic-rich soilscoupled with low productivity. Our results provide important clues signalingimminent talik onset and C source transition, including (1) late cold-season(January–February) soil warming at depth (∼2m),(2) increasing cold-season emissions (November–April), and (3) enhancedrespiration of deep, old C in warm permafrost and young, shallow C in organic-rich cold permafrost soils. Our results suggest a mosaic of processes thatgovern carbon source-to-sink transitions at high latitudes and emphasize theurgency of monitoring soil thermal profiles, organic C age and content, cold-season CO2 emissions, andatmospheric 14CO2 as key indicatorsof the permafrost C feedback.