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1. Rising plant‐mediated methane emissions from arctic wetlands

   https://doi.org/10.1111/gcb.13469  

   Andresen, Christian G. ; Lara, Mark J. ; Tweedie, Craig E. ; Lougheed, Vanessa L. ( September 2016 , Global Change Biology)

   Plant‐mediatedCH₄flux is an important pathway for land–atmosphereCH₄emissions, 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 measuredin situduring peak growing season for the dominant aquatic emergent plants in the Alaskan arctic coastal plain,Carex aquatilisandArctophila fulva, to assess the magnitude and species‐specific controls onCH₄flux. Plant biomass was a strong predictor ofA. fulvaCH₄flux while water depth and thaw depth were copredictors forC. aquatilisCH₄flux. 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 inCH₄fluxes. A ~60% increase inCH₄flux 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 aquaticC. aquatilisandA. fulvaaccount for two‐thirds of the total regionalCH₄flux 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–atmosphereCH₄emissions for this region, potentially acting as a positive feedback to climate warming.

    

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2. A decade of boreal rich fen greenhouse gas fluxes in response to natural and experimental water table variability

   https://doi.org/10.1111/gcb.13612  

   Olefeldt, David ; Euskirchen, Eugénie S. ; Harden, Jennifer ; Kane, Evan ; McGuire, A. David ; Waldrop, Mark P. ; Turetsky, Merritt R. ( January 2017 , Global Change Biology)

   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 (F_CH4), 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 seasonF_CH4,GPPandNEE, 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 seasonGPPin subsequent flood years by ~15% compared to control plots. Conversely, water table position had only a weak influence onER, but dominant contribution toERswitched from autotrophic respiration in wet years to heterotrophic in dry years. Droughts did not cause inter‐annual lag effects onERin this rich fen, as has been observed in several nutrient‐poor peatlands. WhileERwas dependent on soil temperatures at 2 cm depth,F_CH4was 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 higherF_CH4in 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 onF_CH4, but droughts caused no inter‐annual lag effects onF_CH4. 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.

    

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3. Using multirate rapid A / C i curves as a tool to explore new questions in the photosynthetic physiology of plants

   https://doi.org/10.1111/nph.15657  

   Stinziano, Joseph R. ; Adamson, Rachael K. ; Hanson, David T. ( January 2019 , New Phytologist)

   Steady‐state photosyntheticCO₂responses (A/C_icurves) are used to assess environmental responses of photosynthetic traits and to predict future vegetative carbon uptake through modeling. The recent development of rapidA/C_icurves (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.

   Gas exchange necessarily incorporates photosynthesis and (photo)respiration. Each process was expected to respond on different timescales due to differences in metabolite compartmentation, biochemistry and diffusive pathways. We hypothesized that metabolic lags in photorespiration relative to photosynthesis/respiration andCO₂diffusional limitations can be detected by varying the rate of change in [CO₂] duringRACiR assays. We tested these hypotheses through modeling and experiments at ambient and 2% oxygen.

   Our data show that photorespiratory delays cause offsets in predictedCO₂compensation 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 apparentRACiR slopes under highCO₂ramp rates.

   MultirateRACiRs may prove useful in assessing diffusional limitations to gas exchange and photorespiratory rates.

    

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4. The response of boreal peatland community composition and NDVI to hydrologic change, warming, and elevated carbon dioxide

   https://doi.org/10.1111/gcb.14465  

   McPartland, Mara Y. ; Kane, Evan S. ; Falkowski, Michael J. ; Kolka, Randy ; Turetsky, Merritt R. ; Palik, Brian ; Montgomery, Rebecca A. ( October 2018 , Global Change Biology)

   Widespread changes in arctic and boreal Normalized Difference Vegetation Index (NDVI) values captured by satellite platforms indicate that northern ecosystems are experiencing rapid ecological change in response to climate warming. Increasing temperatures and altered hydrology are driving shifts in ecosystem biophysical properties that, observed by satellites, manifest as long‐term changes in regionalNDVI. In an effort to examine the underlying ecological drivers of these changes, we used field‐scale remote sensing ofNDVIto track peatland vegetation in experiments that manipulated hydrology, temperature, and carbon dioxide (CO₂) levels. In addition toNDVI, we measured percent cover by species and leaf area index (LAI). We monitored two peatland types broadly representative of the boreal region. One site was a rich fen located near Fairbanks, Alaska, at the Alaska Peatland Experiment (APEX), and the second site was a nutrient‐poor bog located in Northern Minnesota within the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment. We found thatNDVIdecreased with long‐term reductions in soil moisture at theAPEXsite, coincident with a decrease in photosynthetic leaf area and the relative abundance of sedges. We observed increasingNDVIwith elevated temperature at theSPRUCEsite, associated with an increase in the relative abundance of shrubs and a decrease in forb cover. Warming treatments at theSPRUCEsite also led to increases in theLAIof the shrub layer. We found no strong effects of elevatedCO₂on community composition. Our findings support recent studies suggesting that changes inNDVIobserved from satellite platforms may be the result of changes in community composition and ecosystem structure in response to climate warming.

    

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5. Detecting the permafrost carbon feedback: talik formation and increased cold-season respiration as precursors to sink-to-source transitions

   https://doi.org/10.5194/tc-12-123-2018  

   Parazoo, Nicholas C. ; Koven, Charles D. ; Lawrence, David M. ; Romanovsky, Vladimir ; Miller, Charles E. ( January 2018 , The Cryosphere)

   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.

    

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