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The most rapid climate warming is occurring in northern, permafrost environments. Peatlands of these regions are particularly sensitive to climate warming, with the high ground ice content of peat plateaux resulting in the formation of collapse scars as ground temperatures warm.To quantify the rates of permafrost thaw and associated vegetation changes, we sampled the plant and lichen communities in transects spanning actively thawing collapse scar margins at sites from mid‐Boreal to Low Subarctic conditions, within Canada's Northwest Territories. Seventeen transects were sampled in 2007/08 and 14 of these were resampled 10 years later.The rate of lateral permafrost thaw of collapse scar margins ranged from −6 to 63 cm year−1 (mean: 22.0 cm ± 4.7 cm year−1 (SE)); variability was high and no trends with respect to latitude or temperature gradients were detected.Plant communities displayed a clear gradient from lichen‐ and ericaceous shrub‐dominated peat plateaux, to collapse scars primarily characterized by Sphagnum mosses and graminoids. Both space‐for‐time (distance from collapsing margin) and direct measurement of 10‐year changes showed a successional sequence following permafrost thaw; floating mat communities characterized by Sphagnum riparium or S. balticum proceeded to lawn communities of S. angustifolium and, finally, to hummock communities with Mylia anomala or S. fuscum. This successional sequence was associated with increased water table depth and lower soil water content in plant communities farther from the actively collapsing front, illustrating that peat growth above the water table was driving plant community successional changes.The most rapid plant community succession occurred in recently thawed environments as peat growth propelled the ground surface above the water table while the slowest succession occurred in the collapse scar hummock communities located farthest from the actively collapsing peat plateau margin.Synthesis. In just 10 years, significant vegetation change was detected, in association with both permafrost thaw and subsequent plant community succession. These changes occurred across a broad climatic and latitudinal gradient, from mid‐Boreal to Low Subarctic and have implications for wildlife, global C cycle and indigenous communities who depend on this landscape for harvesting, spiritual and cultural practices. [ABSTRACT FROM AUTHOR]

In the northern hemisphere, terrestrial ecosystems transition from net sources of CO2 to the atmosphere in winter to net ecosystem carbon sinks during spring. The timing (or phase) of this transition, determined by the balance between ecosystem respiration (RECO) and primary production, is key to estimating the amplitude of the terrestrial carbon sink. We diagnose an apparent phase bias in the RECO and net ecosystem exchange (NEE) seasonal cycles estimated by the terrestrial carbon flux (TCF) model framework and investigate its link to soil respiration mechanisms. Satellite observations of vegetation canopy conditions, surface meteorology, and soil moisture from the NASA SMAP Level 4 Soil Moisture product are used to model a daily carbon budget for a global network of eddy covariance flux towers. Proposed modifications to TCF include: the inhibition of foliar respiration in the light (the Kok effect); a seasonally varying litterfall phenology; an O2 diffusion limitation on heterotrophic respiration (RH); and a vertically resolved soil decomposition model. We find that RECO phase bias can result from bias in RECO magnitude and that mechanisms which reduce northern spring RECO, like substrate and O2 diffusion limitations, can mitigate the phase bias. A vertically resolved soil decomposition model mitigates this bias by temporally segmenting and lagging RH. Applying these model enhancements at continuous soil respiration (COSORE) sites verifies their improvement of RECO and NEE skill compared to in situ observations (up to ΔRMSE = −0.76 g C m−2 d−1). Ultimately, these mechanisms can improve prior estimates of NEE for atmospheric inversion studies. Plain Language Summary: In the northern hemisphere, the plants and the soil respond to warming temperatures and increasing day lengths in spring and begin to store more carbon than they release to the atmosphere, on average. The timing of this change is very important for accurately modeling how much carbon is stored or released to the atmosphere. We found that a commonly used model of plants and soil has delayed predictions of the timing of this seasonal cycle of carbon. We studied different potential changes to the model, including changes to: how carbon inputs to the soil from plant roots, dead leaves, and woody debris are added over time; how soil microbes respond to high levels of soil moisture; whether the soil is represented by a single layer or by multiple layers at different depths; and how the release of carbon by plants varies with solar radiation. We found that multiple different changes resulted in similar corrections to the seasonal cycle of carbon so long as they reduced or delayed the amount of carbon released during the spring season. We discuss why that is and how it impacts the model's performance and its importance for other modeling studies. Key Points: A bias in the seasonal cycle of net ecosystem exchange at high northern latitudes is identified in a first‐order soil decomposition modelSoil respiration processes that reduce or delay respiration during spring mitigate this phase bias and improve modeling skillIn situ chamber measurements of soil respiration provide validation and verification of model enhancements [ABSTRACT FROM AUTHOR]

Abstract In the northern hemisphere, terrestrial ecosystems transition from net sources of CO2 to the atmosphere in winter to net ecosystem carbon sinks during spring. The timing (or phase) of this transition, determined by the balance between ecosystem respiration (RECO) and primary production, is key to estimating the amplitude of the terrestrial carbon sink. We diagnose an apparent phase bias in the RECO and net ecosystem exchange (NEE) seasonal cycles estimated by the terrestrial carbon flux (TCF) model framework and investigate its link to soil respiration mechanisms. Satellite observations of vegetation canopy conditions, surface meteorology, and soil moisture from the NASA SMAP Level 4 Soil Moisture product are used to model a daily carbon budget for a global network of eddy covariance flux towers. Proposed modifications to TCF include: the inhibition of foliar respiration in the light (the Kok effect); a seasonally varying litterfall phenology; an O2 diffusion limitation on heterotrophic respiration (RH); and a vertically resolved soil decomposition model. We find that RECO phase bias can result from bias in RECO magnitude and that mechanisms which reduce northern spring RECO, like substrate and O2 diffusion limitations, can mitigate the phase bias. A vertically resolved soil decomposition model mitigates this bias by temporally segmenting and lagging RH. Applying these model enhancements at continuous soil respiration (COSORE) sites verifies their improvement of RECO and NEE skill compared to in situ observations (up to ΔRMSE = −0.76 g C m−2 d−1). Ultimately, these mechanisms can improve prior estimates of NEE for atmospheric inversion studies.

Soil moisture and evapotranspiration (ET) are important components of boreal forest hydrology that affect ecological processes and land‐atmosphere feedbacks. Future trends in soil moisture in particular are uncertain. Therefore, accurate modeling of these dynamics and understanding of concomitant sources of uncertainty are critical. Here, we conduct a global sensitivity analysis, Monte Carlo parameterization, and analysis of parameter uncertainty and its contribution to future soil moisture and ET uncertainty using a physically based ecohydrologic model in multiple boreal forest types. Soil and plant hydraulic parameters and LAI have the largest effects on simulated summer soil moisture at two contrasting sites. In future scenario simulations, the selection of parameters and global climate model (GCM) choice between two GCMs influence projected changes in soil moisture and ET about as much as the projected effects of climate change in the less sensitive GCM with a late‐century, high‐emissions scenario, though the relative effects of parameters, GCM, and climate vary among hydrologic variables and study sites. Saturated volumetric water content and sensitivity of stomatal conductance to vapor pressure deficit have the most statistically significant effects on change in ET and soil moisture, though there is considerable variability between sites and GCMs. The results of this study provide estimates of: (a) parameter importance and statistical significance for soil moisture modeling, (b) parameter values for physically based soil‐vegetation‐atmosphere transfer models in multiple boreal forest types, and (c) the contributions of uncertainty in these parameters to soil moisture and ET uncertainty in future climates. Plain Language Summary: Future changes in soil water content and evapotranspiration (ET) in boreal regions will influence forest dynamics, permafrost thaw rates and have the potential to alter feedbacks between the land surface and the atmosphere, potentially influencing regional climate dynamics. These changes are uncertain and difficult to model because of the complexity of the boreal ecological and hydrological systems. In this study, we conduct experiments with a hydrological model to better understand which parameters most influence soil water content and ET, how much uncertainty these parameters contribute, and how specific parameter values affect whether soils become wetter or drier and by how much. We find that the parameters that control how tightly soils retain water and how much plants close their stomata and hence reduce transpiration in response to dry conditions, have the largest influences. Interactions between parameters are also important. Even when these parameter values are constrained by model performance in comparison with observations, parameter choice can determine whether soils become wetter or drier in future conditions. These findings suggest that modelers should carefully consider how parameter selection influences simulated future hydrologic conditions and highlight specific parameters that are worthy of further attention. Key Points: Modeled summer soil moisture at rooting depth is most sensitive to soil and plant hydraulic parameters and parameter interactions.Parameter uncertainty affects projected change in evapotranspiration and θVWC about as much as climate change or choice between two global climate models.We present parameter values useful for hydrologic modeling in multiple boreal forest types while quantifying the impacts of equifinality. [ABSTRACT FROM AUTHOR]

Obligatory mine reclamation efforts in the Athabasca oil sands region require leased land to be returned to pre-disturbance function. This entails the re-establishment of key ecosystem services such as carbon sequestration and water and nutrient cycling. Surface-atmosphere exchanges of energy, heat and moisture are critical to ecohydrological processes and feedbacks that govern ecosystem functionality. Eddy covariance and remote sensing measurements were used to examine the temporal changes in energy partitioning of a reclaimed fen-upland watershed during the initial seven years post-construction (2013–2019). Surface conditions evolved from bare ground to robustly vegetated plant communities, altering albedo, surface roughness and plant-mediated shading. Initially in the fen, due to near-surface water table, high soil moisture content and the low albedo (0.09) of bare, wet peat, energy was predominantly partitioned to latent heat (Q E) and exhibited a high degree of decoupled surface-atmosphere conditions (surface-atmosphere decoupling factor (Ω) of 0.49). As the fen plant community developed and stabilized, Q E remained the dominant energy flux, but a larger proportion of available energy was partitioned to sensible heat flux (Q H). Plant establishment altered the biophysical surface through sheltering and shading affecting ground heat flux and supressing surface evaporative losses. Moreover, plant-mediated water loss through stomatal controls resulted in an increase in surface-atmosphere coupling (Ω = 0.32), particularly during drier periods. In the drier upland, plant emergence and establishment was slower than in the fen. Initially, bare ground in the upland resulted in higher albedo (0.20–0.25), and Q E and Q H were similar. Once understory and tree species matured, Q E increased with seasonal trends that mirrored plant phenology. Unlike the fen, the upland continually exhibited a higher degree of surface-atmosphere coupling (Ω ranging between 0.23 and 0.3). By year seven (2019) energy fluxes and intra- and inter-seasonal trends at the constructed watershed were comparable to natural and post-disturbance boreal landscapes indicating the system is evolving towards a functional, reclaimed ecosystem. [Display omitted] • Surface energy fluxes during the initial seven years following reclamation • During bare ground conditions edaphic and atmospheric controls drove energy fluxes • Plant development altered biophysical surface properties, changing albedo • Energy partitioning shifted and exhibited strong biotic controls • By year seven energy dynamics were comparable to that of natural boreal landscapes [ABSTRACT FROM AUTHOR]