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Climate change has increased the area affected by forest fires in boreal North America. An analysis of the depth of burning in forests and peatlands in Alaska indicates that ground-layer combustion has accelerated regional carbon losses. Climate change has increased the area affected by forest fires each year in boreal North America 1 , 2 . Increases in burned area and fire frequency are expected to stimulate boreal carbon losses 3 , 4 , 5 . However, the impact of wildfires on carbon emissions is also affected by the severity of burning. How climate change influences the severity of biomass burning has proved difficult to assess. Here, we examined the depth of ground-layer combustion in 178 sites dominated by black spruce in Alaska, using data collected from 31 fire events between 1983 and 2005. We show that the depth of burning increased as the fire season progressed when the annual area burned was small. However, deep burning occurred throughout the fire season when the annual area burned was large. Depth of burning increased late in the fire season in upland forests, but not in peatland and permafrost sites. Simulations of wildfire-induced carbon losses from Alaskan black spruce stands over the past 60 years suggest that ground-layer combustion has accelerated regional carbon losses over the past decade, owing to increases in burn area and late-season burning. As a result, soils in these black spruce stands have become a net source of carbon to the atmosphere, with carbon emissions far exceeding decadal uptake.

Increasing concentrations of dissolved organic carbon (DOC) have been identified in many freshwater systems over the last three decades. Studies have generally nominated atmospheric deposition as the key driver of this trend, with changes in climatic factors also contributing. However, there is still much uncertainty concerning net effects of these drivers on DOC concentrations and export dynamics. Changes in climate and climate mediated snowfall dynamics in northern latitudes have not been widely considered as causal factors of changes in long-term DOC trends, despite their disproportionate role in annual DOC export. We leveraged long-term datasets (1988-2013) from a first-order forested tributary of Lake Superior to understand causal factors of changes in DOC concentrations and exports from the watershed, by simultaneously evaluating atmospheric deposition, temperature, snowmelt timing, and runoff. We observed increases in DOC concentrations of approximately 0.14 mg C l−1 yr−1 (mean = 8.12 mg C l−1) that were related with declines in sulfate deposition (0.03 mg SO42− l−1 yr−1). Path analysis revealed that DOC exports were driven by runoff related to snowmelt, with peak snow water equivalences generally being lower and less variable in the 21st century, compared with the 1980s and 1990s. Mean temperatures were negatively related (direct effects) to maximum snow water equivalences (−0.71), and in turn had negative effects on DOC concentrations (−0.58), the timing of maximum discharge (−0.89) and DOC exports (indirect effect, −0.41). Based on these trends, any future changes in climate that lessen the dominance of snowmelt on annual runoff dynamics-including an earlier peak discharge-would decrease annual DOC export in snowmelt dominated systems. Together, these findings further illustrate complex interactions between climate and atmospheric deposition in carbon cycle processes, and highlight the importance of long-term monitoring efforts for understanding the consequences of a changing climate.

Sphagnum-dominated peatlands store more carbon than all of Earth’s forests, playing a large role in the balance of carbon dioxide. However, these carbon sinks face an uncertain future as the changing climate is likely to cause water stress, potentially reducing Sphagnum productivity and transitioning peatlands to carbon sources. A mesocosm experiment was performed on thirty-two peat cores collected from two peatland landforms: elevated mounds (hummocks) and lower, flat areas of the peatland (hollows). Both rainfall treatments and water tables were manipulated, and CO2 fluxes were measured. Other studies have observed peat subsiding and tracking the water table downward when experiencing water stress, thought to be a self-preservation technique termed ‘Mire-breathing’. However, we found that hummocks tended to compress inwards, rather than subsiding towards the lowered water table as significantly as hollows. Lower peat height was linearly associated with reduced gross primary production (GPP) in response to lowered water tables, indicating that peat subsidence did not significantly enhance the resistance of GPP to drought. Conversely, Sphagnum peat compression was found to stabilize GPP, indicating that this mechanism of resilience to drought may transmit across the landscape depending on which Sphagnum landform types are dominant. This study draws direct connections between Sphagnum traits and peatland hydrology and carbon cycling.

Peatlands, which account for approximately 15% of land surface across the arctic and boreal regions of the globe, are experiencing a range of ecological impacts as a result of climate change. Factors that include altered hydrology resulting from drought and permafrost thaw, rising temperatures, and elevated levels of atmospheric carbon dioxide have been shown to cause plant community compositional changes. Shifts in plant composition affect the productivity, species diversity, and carbon cycling of peatlands. We used hyperspectral remote sensing to characterize the response of boreal peatland plant composition and species diversity to warming, hydrologic change, and elevated CO2. Hyperspectral remote sensing techniques offer the ability to complete landscape-scale analyses of ecological responses to climate disturbance when paired with plot-level measurements that link ecosystem biophysical properties with spectral reflectance signatures. Working within two large ecosystem manipulation experiments, we examined climate controls on composition and diversity in two types of common boreal peatlands: a nutrient rich fen located at the Alaska Peatland Experiment (APEX) in central Alaska, and an ombrotrophic bog located in northern Minnesota at the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment. We found a strong effect of plant functional cover on spectral reflectance characteristics. We also found a positive relationship between species diversity and spectral variation at the APEX field site, which is consistent with other recently published findings. Based on the results of our field study, we performed a supervised land cover classification analysis on an aerial hyperspectral dataset to map peatland plant functional types (PFTs) across an area encompassing a range of different plant communities. Our results underscore recent advances in the application of remote sensing measurements to ecological research, particularly in far northern ecosystems.

While the ecological importance of bioturbation is well recognized and the prevalence of aquatic foraging by terrestrial ungulates is increasingly appreciated, research linking how terrestrial ungulates function as disturbance mechanisms via bioturbation in freshwater systems is lacking. The purpose of this study was to quantify potential nutrient pulses released from benthic sediments into the water column when moose Alces alces feed on aquatic plants. We also determined if we could experimentally mimic the benthic disturbance and the expected nutrient pulse created when moose feed aquatically. When moose foraged aquatically, significant releases of both total and dissolved phosphorus (P) and nitrogen (N) resulted in the waters that were disturbed in foraging areas compared to adjacent undisturbed waters. Nutrient concentrations for total P and N ranged from 42.5 × and 2.7 × greater in disturbed than undisturbed, respectively. Dissolved P and N were 26.8 × and 1.5 × greater, respectively, in disturbed versus undisturbed waters. Our experimental mimic created increases of total and dissolved P and N that were equivalent to pulses created by moose. This indicates that it is possible to experimentally test by proxy the potential impact of moose bioturbation on other ecosystem processes. This study is the first quantification of moose foraging as a consumer mechanism that influences the release of limiting nutrients in aquatic systems, thereby emphasizing the potential cascading importance for nutrient uptake and productivity of plants and microbes.

Dissolved organic matter (DOM) is a critical part of the global carbon cycle. Currently, it is understood that at least a portion of the chromophoric DOM (CDOM) character can be described through an electronic interaction of charge transfer (CT) complexes. While much work has been done to understand the influence of CT on soil and aquatic reference standard DOM, little is known about the influence of CT in fresh terrestrially derived DOM. In this study, leaf litter leachates from three tree species were treated (reduced) with sodium borohydride to determine the contribution of CT on a source of fresh terrestrial DOM. Leaf litter was sampled four times through decomposition under natural (field) conditions to determine the influence of degradation on response to borohydride treatment. Leaf litter CDOM displayed a unique loss of UVB absorption following borohydride treatment, as well as a homogenizing effect on fluorescence emission character. Humification index (HIX) differentiated Elliot Soil Humic Acid and Suwannee River Fulvic Acid from leaf litter leachates. However, biological index (BIX), and spectral slope metrics were not able to differentiate leaf leachates from these reference standards. Apparent quantum yields were similar in magnitude between leaf leachates and reference standards, although leaf leachate spectra displayed features not evident in reference standards. These results help understand the origins of DOM optical properties and associated quantitative indices in freshly sourced terrestrial material. Overall, these results suggest that even at the initial stages of decomposition, terrestrial CDOM exhibits optical characteristics and responses to removal of electron accepting ketones and aldehydes, through borohydride treatment, similar to more processed CDOM.

Permafrost is common to many northern wetlands given the insulation of thick organic soil layers, although soil saturation in wetlands can lead to warmer soils and increased thaw depth. We analyzed five years of soil CO2 fluxes along a wetland gradient that varied in permafrost and soil moisture conditions. We predicted that communities with permafrost would have reduced ecosystem respiration (ER) but greater temperature sensitivity than communities without permafrost. These predictions were partially supported. The colder communities underlain by shallow permafrost had lower ecosystem respiration (ER) than communities with greater active layer thickness. However, the apparent Q10 of monthly averaged ER was similar in most of the vegetation communities except the rich fen, which had smaller Q10 values. Across the gradient there was a negative relationship between water table position and apparent Q10, showing that ER was more temperature sensitive under drier soil conditions. We explored whether root respiration could account for differences in ER between two adjacent communities (sedge marsh and rich fen), which corresponded to the highest and lowest ER, respectively. Despite differences in root respiration rates, roots contributed equally (∼40%) to ER in both communities. Also, despite similar plant biomass, ER in the rich fen was positively related to root biomass, while ER in the sedge marsh appeared to be related more to vascular green area. Our results suggest that ER across this wetland gradient was temperature-limited, until conditions became so wet that respiration became oxygen-limited and influenced less by temperature. But even in sites with similar hydrology and thaw depth, ER varied significantly likely based on factors such as soil redox status and vegetation composition.

Increases in fire frequency, extent, and severity are expected to strongly impact the structure and function of boreal forest ecosystems. An important function of the boreal forest is its ability to sequester and store carbon (C). Increasing disturbance from wildfires, emitting large amounts of C to the atmosphere, may create a positive feedback to climate warming. Variation in ecosystem structure and function throughout the boreal forest is important for predicting the effects of climate warming and changing fire regimes on C dynamics. In this study, we compiled data on soil characteristics, stand structure, pre-fire C pools, C loss from fire, and the potential drivers of these C metrics from 527 sites distributed across six ecoregions of North America’s western boreal forests. We assessed structural and functional differences between these fire-prone ecoregions using data from 417 recently burned sites (2004–2015) and estimated ecoregion-specific relationships between soil characteristics and depth from 167 of these sites plus an additional 110 sites (27 burned, 83 unburned). We found that northern boreal ecoregions were generally older, stored and emitted proportionally more belowground than aboveground C, and exhibited lower rates of C accumulation over time than southern ecoregions. We present ecoregion-specific estimates of depth-wise soil characteristics that are important for predicting C combustion from fire. As climate continues to warm and disturbance from wildfires increases, the C dynamics of these fire-prone ecoregions are likely to change with significant implications for the global C cycle and its feedbacks to climate change.

Dominant plant functional groups (PFGs) found in boreal rich fens include sedges, grasses, horsetails, and cinquefoils (obligate wetland shrubs). Precipitation regime shift and permafrost thaw due to climate change will likely trigger changes in fen plant community structure through shifts in these PFGs, and it is thus crucial to understand how these PFGs will impact carbon cycling and greenhouse gas dynamics to predict and model peatland-climate feedbacks. In this study, we detail the above and belowground effects of these PFGs on aspects of carbon cycling using a mesocosm approach. We hypothesized that PFGs capable of aerating the rhizosphere (sedges, horsetails, and grasses) would oxidize the belowground environment supporting higher redox potentials, a favorable environment for decomposition, and higher CO₂:CH₄ in pore water and gas efflux measurements than PFGs lacking aerenchyma (cinquefoil, unplanted control). Overall, sedges, horsetail and grasses had an oxidizing effect on rhizosphere pore water chemistry, producing an environment more favorable for methanotrophy during the growing season, as supported by an approximate isotopic enrichment of pore water methane (δ¹³CH₄) by5‰, and isotopic depletion in pore water carbon dioxide (δ¹³CO₂) by 10‰, relative to cinquefoil treatments. Cinquefoil and unplanted control treatments fostered a reducing environment more favorable for methanogenesis. In addition, cinquefoil appeared to slow decomposition in comparison with the other PFGs. These findings, paired with PFG effects on oxidation–reduction potential and CO₂ and CH₄ production, point to the ability of rich fen plant communities to moderate biogeochemistry, specifically carbon cycling, in response to changing climatic conditions.

Great Lakes coastlines are mosaics of wetland, stream, and lake habitats, characterized by a high degree of spatial heterogeneity that may facilitate the co-occurrence of seemingly incompatible biogeochemical processes due to variation in environmental factors that favor each process. We measured nutrient limitation and rates of N2 fixation and denitrification along transects in 5 wetland–stream–lake ecotones with different nutrient loading in Lakes Superior and Huron. We hypothesized that rates of both processes would be related to nutrient limitation status, habitat type, and environmental characteristics including temperature, nutrient concentrations, and organic matter quality. We found that median denitrification rates (914 μg N m−2 h−1) were 166 × higher than N2 fixation rates (5.5 μg N m−2 h−1), but the processes co-occurred in 48% of 83 points measured across all 5 transects and habitat types. N2 fixation occurred on sediment and macrophyte substrate, while denitrification occurred mostly in sediment. Nutrient-diffusing substrate experiments indicated that biofilm chlorophyll-a was limited by N and/or P at 55% and biofilm AFDM was limited at 26% of sample points. N2 fixation and denitrification rates did not differ significantly with differing nutrient limitation. Predictive models for N2 fixation and denitrification rates both included variables related to the composition of dissolved organic matter, while the model for N2 fixation also included P concentrations. These results demonstrate the potential for heterogeneity in habitat characteristics, nutrient availability, and organic matter composition to lead to biogeochemical complexity at the local scale, despite overall N removal at broader scales.