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Arctic warming is affecting snow cover and soil hydrology, with consequences for carbon sequestration in tundra ecosystems. The scarcity of observations in the Arctic has limited our understanding of the impact of covarying environmental drivers on the carbon balance of tundra ecosystems. In this study, we address some of these uncertainties through a novel record of 119 site-years of summer data from eddy covariance towers representing dominant tundra vegetation types located on continuous permafrost in the Arctic. Here we found that earlier snowmelt was associated with more tundra net CO2 sequestration and higher gross primary productivity (GPP) only in June and July, but with lower net carbon sequestration and lower GPP in August. Although higher evapotranspiration (ET) can result in soil drying with the progression of the summer, we did not find significantly lower soil moisture with earlier snowmelt, nor evidence that water stress affected GPP in the late growing season. Our results suggest that the expected increased CO2 sequestration arising from Arctic warming and the associated increase in growing season length may not materialize if tundra ecosystems are not able to continue sequestering CO2 later in the season.

Significant climate risks are associated with a positive carbon-temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence from natural to managed conditions to quantify the "cost" of CH4 emissions for the benefit of net carbon sequestration. With a sustained pulse-response radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new guidelines of the Intergovernmental Panel on Climate Change (IPCC) accounting for both sustained CH4 emissions and cumulative CO2 exchange.

Canada's forests play a critical role in the global carbon (C) cycle and are responding to unprecedented climate change as well as ongoing natural and anthropogenic disturbances. However, the representation of disturbance in boreal regions is limited in pre-existing land surface models (LSMs). Moreover, many LSMs do not explicitly represent subgrid-scale heterogeneity resulting from disturbance. To address these limitations, we implement harvest and wildfire forcings in the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC) land surface model alongside dynamic tiling that represents subgrid-scale heterogeneity due to disturbance. The disturbances are captured using 30-m spatial resolution satellite data (Landsat) on an annual basis for 33 years. Using the pan-Canadian domain (i.e. all of Canada south of 76° N) as our study area for demonstration, we determine the model setup that optimally balances detailed process representation and computational efficiency. We then demonstrate the impacts of subgrid-scale heterogeneity relative to standard average individual-based representations of disturbance and explore the resultant model biases. Our results indicate that the modeling approach implemented can balance model complexity and computational cost to represent the impacts of subgrid-scale heterogeneity resulting from disturbance. Subgrid-scale heterogeneity is shown to have impacts 1.5 to 4 times the impact of disturbance alone on gross primary productivity, autotrophic respiration, and surface energy balance processes in our simulations. These impacts are a result of subgrid-scale heterogeneity slowing vegetation re-growth and affecting surface energy balance in recently disturbed, sparsely vegetated, and often snow-covered fractions of the land surface. Representing subgrid-scale heterogeneity is key to more accurately representing timber harvest, which preferentially impacts larger trees on higher quality and more accessible sites. Our results show how different discretization schemes can impact model biases resulting from the representation of disturbance. These insights, along with our implementation of dynamic tiling may apply to other tile-based LSMs. Ultimately our results enhance our understanding of, and ability to, represent disturbance within Canada to facilitate a comprehensive process-based assessment of Canada's terrestrial C cycle. [ABSTRACT FROM AUTHOR]

Peatlands store disproportionally large amounts of carbon per unit area, a function that is dependent on maintaining high and stable water tables. Climate change is likely to negatively impact carbon storage in peatlands, in part due to increases in vapour pressure deficit (VPD) driving higher evaporation (E) rates. However, the response of E to increasing VPD depends on the dominant vegetation type within peatlands. In this study, we used multiple years of eddy covariance (EC) measurements to compare E regimes at two peatlands with contrasting vegetation types – Kopuatai bog in Aotearoa New Zealand, dominated by the vascular jointed wire rush Empodisma robustum , and Mer Bleue bog in Canada, a 'typical' shrub and moss-dominated Northern Hemisphere peatland. We examined seasonal variability in E and equilibrium E (E eq), energy balance partitioning, and the response of E, evaporative fraction (EF), and canopy conductance (g c) to VPD. Mean annual E was 45 % lower than mean annual E eq at Kopuatai, but only 16 % lower at Mer Bleue, demonstrating much greater limitations on E at Kopuatai. In addition, the mean midday (10:00–14:30) dry canopy Bowen ratio (β) at Kopuatai was 1.96, compared to 0.77 at Mer Bleue; therefore, the sensible heat flux (H) dominated over the latent heat flux (LE) at Kopuatai, and vice versa at Mer Bleue. The responses of E , EF, and g c to increasing VPD at Kopuatai differed from those at Mer Bleue in a way that resulted in stronger limitations on E above ~0.7 kPa at the former bog. The observed limitations at Kopuatai were attributed to strong stomatal control by E. robustum due to the rapid decrease in g c with increasing VPD, however surface E could also be limited by its dense standing litter. At Mer Bleue, however, E was only weakly limited at VPD > 2 kPa, likely due to weak stomatal control over transpiration by the sparse shrub canopy and relatively large surface E from Sphagnum carpets. As such, the results of this study suggest that E. robustum drives a greater "hydrological resistance" to increasing VPD than the vegetation at Mer Bleue, leading to greater water retention at Kopuatai. This may enable greater resilience of the carbon sink function at Kopuatai to climatic warming and drying than at Mer Bleue. [ABSTRACT FROM AUTHOR]

The theory of conductive heat transfer cannot explain different attenuations of the daily amplitude of peat temperatures (Ts) in hummocks (detectable below the 20-cm depth) and hollows (disappearing above the 10+ cm depth). Large readily drained macropores in the upper fibric peat determine a large air permeability and hence may enhance heat transfer by air convection in porous media, driven by temperature gradients between hummock sides and interiors. In this study, the ecosys model was used to simulate a peat thermal regime at Mer Bleue peatland, Ontario, Canada. It was hypothesized that adding the air-convective heat transfer to conductive plus water-convective heat transfers would improve simulations of [T.sub.S]. The results for [T.sub.S], ground heat fluxes, G, and sensible heat fluxes, H, modeled with and without air-convective heat transfer were tested with continuous hourly measurements from 2000 to 2004 using thermocouples, heat flux plates, and eddy covariance. Simulated air-convective heat transfer caused an average increase in 6' and a corresponding decrease in H of ~20 W [m.sup.-2] from the simulated conductive plus water-convective heat transfer. Hastened soil warming in hummocks resulted in better agreement between measured and simulated hummock [T.sub.S] values with (RMSD of 2.23[degrees]C) than without air-convective heat (RMSD of 2.54[degrees]C). Enhanced hummock [T.sub.S] caused an indirect increase in hollow Ts in the model with (RMSD of 1.68[degrees]C) compared to without airoconvective heat (RMSD of 1.82[degrees]C). Our results suggest that air convection is probably an important mechanism of heat transfer in peat hummocks and should be included in peatland biogeochemical models. doi: 10.2136/sssaj2009.0288

In this study, the ecosys model was used to simulate the hydrology of the Met Bleue bog, Ontario, Canada, with seasonally varying water tables in the upper 1 m. The soil profile was divided into three zones of peat (fibric, hemic, and sapric). In the model, large, readily drained macropore fractions in the fibric peat caused low water-holding capacity and high infiltration rates, in contrast to hemic and sapric peat, with small macropore fractions, high water-holding capacities, and low infiltration rates. Model results for peat water contents, [theta], and water table depths, Z, were tested with continuous hourly measurements from 2000 to 2004 using time domain reflectometry probes and piezometers. Within the zone of pronounced water table variation, the [theta] and Z modeled with the Hagen--Poiseuille equation (or macropore flow and Richards' equation for peat matrix flow corresponded better to the measured [theta] and Z (regression slopes between 0.62 and 1.03, intercepts between -0.05 and 0.02 [m.sup.3] [m.sup.- 3], and [R.sup.2] between 0.40 and 0.56), than did the modeled [theta] and Z with Richards' equation alone (regression slopes between 0.33 and 1.43, intercepts between 0.11 and 0.22 [m.sup.3] [m.sup.-3], and [R.sup.2] between 0.27 and 0.41). The Richards equation alone, even when parameterized with extremely high or low bulk saturated hydraulic conductivities of fibric peat, modeled slower infiltration, greater water retention, and lower Z than measured. The implications of macropore flow might be of great importance for peatland hydrology, therefore this experimental and modeling work should be extended to other wetlands as well. Abbreviations: DOY, day of the year; EC, eddy covariance; ET, evapotranspiration; TDR, time domain reflectometry; WT, water table. doi:10.2136/sssaj2009.0148

The carbon (C) dynamics of northern peatlands are sensitive to hydrological changes owing to ecohydrological feedback. We quantified and evaluated the impact of water level variations in a beaver pond (BP) on the CO2 flux dynamics of an adjacent, raised Sphagnum - shrub-dominated bog in southern Canada. We applied the CoupModel to the Mer Bleue bog, where the hydrological, energy and CO2 fluxes have been measured continuously for over 20 years. The lateral flow from the bog to the BP was estimated by the hydraulic gradient between the peatland and the BP's water level and the vertical profile of peat hydraulic conductivity. The model outputs were compared with the measured hydrological components, CO2 flux and energy flux data (1998-2019). CoupModel was able to reproduce the measured data well. The simulation shows that variation in the BP water level (naturally occurring or due to management) influenced the bog net ecosystem exchange of CO2 (NEE). Over 1998-2004, the BP water level was 0.75 to 1.0 m lower than during 2017-2019. Simulated net CO2 uptake was 55 g C m-2 yr-1 lower during 1998-2004 compared to 2017-2019 when there was no BP disturbance, which was similar to the differences in measured NEE between those periods. Peatland annual NEE was well correlated with water table depth within the bog, and NEE also shows a linear relation with the water level at the BP, with a slope of -120 g CO2-C m-2 yr-1 m-1. The current modelling predicts the bog may switch from CO2 sink to source when the BP water levels drop lower than ~ 1.7 m below the peat surface at the eddy covariance tower, 250 m from the BP. This study highlights the importance of natural and human disturbances to adjacent water bodies in regulating net CO2 uptake function of northern peatlands. [ABSTRACT FROM AUTHOR]

Arctic terrestrial ecosystems may be contributing to increasing atmospheric carbon dioxide (CO2) concentrations under amplified climate change at high‐latitudes. This research investigates how summer net ecosystem CO2 exchange (NEE) and its component fluxes, gross primary production (GPP), and ecosystem respiration (Reco) varied over five years (2008, 2009, 2010, 2012, and 2014) at the Cape Bounty Arctic Watershed Observatory (74.92°N, 109.58°W). The eddy covariance technique was used to measure NEE and a combined light and temperature response model was used to partition NEE into GPP and Reco. Total summer NEE varied from −19.8 to 7.9 g C m−2. Despite two summers with net CO2 uptake, this tundra was likely a source of CO2 on an annual basis. In most years, growing degree days with a base 0°C (GDD0) had more predictive power than other environmental variables in random forest analyses of daily NEE. Interannual variability in total summer NEE over the five study years was also attributed to greater variability in GPP than Reco (coefficient of variation: 62% and 27%, respectively). However, total summer GDD0 significantly correlated with total summer Reco but not NEE nor GPP. Instead, summer total NEE and GPP significantly correlated with satellite‐derived normalized difference vegetation index (NDVI), which may have exhibited carry‐over effects from one year to the next to limit the impact of current year GDD0 on total summer NEE at this tundra site. Plain Language Summary: Arctic ecosystems currently store a vast amount of carbon within frozen soils. Over the last two decades, Arctic surface air temperatures have increased at a rate twice the global average, which has the potential to shift terrestrial carbon cycling, ultimately impacting atmospheric carbon dioxide (CO2), a potent greenhouse gas. Whether Arctic terrestrial ecosystems are releasing more CO2 than they are absorbing is important, as this can impact the global climate system. This research measured CO2 exchange between the atmosphere and the tundra over five summers at the Cape Bounty Arctic Watershed Observatory (74.92°N, 109.58°W) on Melville Island, Nunavut, Canada. The ecosystem acted as a CO2 source for three out of five summers and is likely an annual net CO2 source to the atmosphere. Interannual differences in weather appeared to impact plant productivity and CO2 uptake more than decomposition and respiration and CO2 emissions to the atmosphere at this site. The amount of CO2 absorbed or lost from this tundra each summer was correlated to satellite observations, which could help us monitor the carbon balance of these remote but important ecosystems. This research highlights the importance of multiyear observations to constrain understanding of High Arctic tundra CO2 dynamics. Key Points: Over five summers, a High Arctic tundra site with low productivity varied from being a net source to a net sink for atmospheric CO2Summer net CO2 uptake depended more on photosynthesis than respiration and correlated well with satellite‐derived NDVIDaily net ecosystem CO2 exchange could be predicted using growing degree days and other environmental variables using random forests [ABSTRACT FROM AUTHOR]