Boreal Systems Research Articles

Wildfire influences species assemblage and habitat utilisation of boreal wildlife after more than a decade in northern Sweden

Fires can strongly change the vegetation structure and the availability of resources for wildlife, but fire suppression has long affected the natural role of fire in shaping boreal ecosystems in northern Europe. Recently, wildfires have increased in frequency, possibly due to global warming. In contrast to the boreal systems in North America, there have been few studies on responses of wildlife to wildfires in northern Europe. Based on the findings from North America, we predict that responses of wildlife to wildfire vary among wildlife species: where mammalian herbivores, such as moose Alces alces and mountain hare Lepus timidus, will be attracted to burnt areas following an increase in food availability, other species, such as reindeer Rangifer tarandus, are negatively impacted due to fire reducing their preferred food. We then tested our predictions by contrasting wildlife utilization of sites that burnt by wildfire in 2006 with nearby unburnt control sites in three areas in northern Sweden. To measure wildlife utilization, we used 72 camera traps, equally divided between the burnt and control sites, with two placement strategies: random and on wildlife trails. The cameras recorded 27 mammal and bird species during summer 2018. Species assemblage differed between burnt and control sites. Fieldfare Turdus pilaris used burnt sites more than control sites, while pine marten Martes martes and western capercaillie Tetrao urogallus used control sites more than burnt sites. We however did not find support for a positive effect of past forest fires on any of the observed wild mammals. We discuss how, due to the impact of forestry, forage‐rich habitat may not be as limiting in Scandinavia as in the North‐American context, potentially leading to recently burnt sites being less attractive to herbivores such as moose.

Soil carbon in the boreal region under climate and land use change

AbstractCarbon cycling in boreal regions is controlled naturally by fire disturbance, climate, hydrology, snowmelt, soil heterogeneity, acidity, low nitrogen and phosphorus availability, mycorrhizal distribution, plant cover, and cryoturbation/permafrost at the northern edge. It is primarily altered by human management via and following land clearance. Each factor interacts with others in complex ways, creating a system characterized by high carbon storage and shaped by natural and anthropogenic disturbance. This paper examines boreal research to answer the question: ‘what are the primary factors controlling boreal carbon cycling?’ Though climate change threatens boreal systems, they may yet be resilient given many natural feedback loops that resist massive carbon loss. Perhaps the greatest danger is land conversion, which causes permanent losses and drastic alteration of certain carbon pools. Accordingly, it might be argued that it would be overall better to implement policies that keep these lands as forests, including managed forests, rather than convert to agriculture. Conversely, farming of marginal boreal soils might be managed to enhance carbon storage while satisfying local food security needs. However, the impact of agriculture on soil carbon storage is yet to be effectively quantified and initial results offer inconsistent assessments, reflected in the as‐yet natural sciences data‐scarce policy development. Research targeting long‐term carbon cycling, land conversion practices, agroforestry, and communication between boreal scientists and other groups (climate scientists, policymakers, public, and farmers) must be addressed through creation of long‐term experiments.

Hydrobiogeochemical Controls on the Delivery of Dissolved Organic Matter to Boreal Headwater Streams

AbstractUnderstanding controls on the delivery of dissolved organic matter (DOM) from terrestrial to aquatic systems is key for constraining carbon balances and fluxes in boreal headwater catchments. The largest export of DOM generally occurs during intense periods of water delivery to the landscape (e.g., rain events); however, the timing and magnitude of DOM is complicated by geomorphological, hydrometeorological, and biogeochemical variability. To better assess mechanisms controlling the delivery of DOM to boreal streams, DOM was investigated in two morphologically distinct catchments of an experimental forest watershed in western Newfoundland, one dominated by low relief wetlands and the other by steep hillslopes. The DOM was compared during two fall storms of similar magnitude but contrasting moisture conditions during the autumn transition. Variable concentrations and optical character highlighted antecedent conditions are important for the delivery of DOM to boreal streams. Concentration‐discharge relationships with stream hydrograph separation analysis suggest preferential flowpaths through shallow mineral horizons as a key pathway for delivery of DOM in the hillslope dominated catchment compared to rapid input from near‐stream wetlands in the low relief catchment. Loss of DOM via preferential pathways suggests an additional mechanism for surface soil carbon loss not directly informed by lateral flow and soil hydrology, which has important implications for our conceptualization and modeling of carbon fluxes in boreal systems. These results suggest watershed scale responses of DOM in boreal headwater catchments are complex, and better linkages between catchment scale hydrology and landscape hydrobiogeochemistry are required to constrain terrestrial to aquatic carbon fluxes in boreal landscapes.

Mycorrhizal ecology would benefit from region-specific hypotheses

Most arbuscular mycorrhizal fungi exhibit global distributions but have been studied mostly in a specific region of the globe, mainly covering temperate and boreal habitats, where nitrogen is the most common nutrient limiting primary productivity. Yet, it is relatively common to extrapolate our understanding of temperate and boreal systems to other regions of the globe. While the physiology of mycorrhizal associations is unlikely to differ worldwide, there are good chances that environmental settings interact with the way mycorrhizas function across ecoregions. Here, we first argue that mycorrhizal ecologists should develop region-specific hypotheses. We subsequently identify likely differences in how mycorrhizas function in subtropical regions compared with better-studied temperate and boreal areas. We finally use the subtropical region of China to develop five geographically-focused hypotheses, envisaging that they will trigger the interest of the scientific community worldwide in understudied regions into studying mycorrhizas through a new lens. Expanding the range of hypotheses in mycorrhizal ecology to describe understudied regions of the world, has the potential to confer multifaceted benefits to both science and society. We advocate to do so, and present a roadmap on how to develop such hypotheses.

Long‐Term Changes in Concentration and Yield of Riverine Dissolved Silicon From the Poles to the Tropics

AbstractRiverine exports of silicon (Si) influence global carbon cycling through the growth of marine diatoms, which account for ∼25% of global primary production. Climate change will likely alter river Si exports in biome‐specific ways due to interacting shifts in chemical weathering rates, hydrologic connectivity, and metabolic processes in aquatic and terrestrial systems. Nonetheless, factors driving long‐term changes in Si exports remain unexplored at local, regional, and global scales. We evaluated how concentrations and yields of dissolved Si (DSi) changed over the last several decades of rapid climate warming using long‐term data sets from 60 rivers and streams spanning the globe (e.g., Antarctic, tropical, temperate, boreal, alpine, Arctic systems). We show that widespread changes in river DSi concentration and yield have occurred, with the most substantial shifts occurring in alpine and polar regions. The magnitude and direction of trends varied within and among biomes, were most strongly associated with differences in land cover, and were often independent of changes in river discharge. These findings indicate that there are likely diverse mechanisms driving change in river Si biogeochemistry that span the land‐water interface, which may include glacial melt, changes in terrestrial vegetation, and river productivity. Finally, trends were often stronger in months outside of the growing season, particularly in temperate and boreal systems, demonstrating a potentially important role of shifting seasonality for the flux of Si from rivers. Our results have implications for the timing and magnitude of silica processing in rivers and its delivery to global oceans.

Earthworm-invaded boreal forest soils harbour distinct microbial communities

Abstract. Earthworm invasion in North American forests has the potential to greatly impact soil microbial communities by altering soil physicochemical properties, including structure, pH, nutrient availability, and soil organic matter (SOM) dynamics. While most research on the topic has been carried out in northern temperate forests, little is known about the impact of invasive earthworms on soil microbial communities in hemiboreal and boreal forests, characterized by a slower decay of organic matter (OM). Earthworm activities can increase OM mineralization, altering nutrient cycling and biological activity in a biome where low carbon (C) and nitrogen (N) availability typically limits microbial and plant growth. Here, we characterized and compared microbial communities of earthworm-invaded and non-invaded soils in previously described sites across three major soil types found in the Canadian (hemi)boreal forest using a space-for-time approach. Microbial communities of forest floors and surface mineral soils were characterized using phospholipid fatty acid (PLFA) analysis and metabarcoding of the 16S rRNA gene for bacteria and archaea and of the internal-transcriber-spacer-2 (ITS2) region for fungi. In forest floors, the effects of earthworm invasion were minor. In mineral soil horizons, earthworm invasion was associated with higher fungal biomass and greater relative abundance of ectomycorrhizal fungi. Oligotrophic bacteria (Acidobacteriota and Chloroflexi) were less abundant in invaded mineral soils, where Gram(+) : Gram(−) ratios were also lower, while the opposite was observed for the copiotrophic Bacteroidota. Additionally, earthworm-invaded mineral soils harboured higher fungal and bacterial species diversity and richness. Considering the important role of soil microbial communities for ecosystem functioning, such earthworm-induced shifts in their community composition are likely to impact nutrient cycling, as well as vegetation development and forest productivity at a large scale, as the invasion progresses in these (hemi)boreal systems.

Fish behavior in response to an approaching underwater camera

Responses of fish to camera survey gear can influence abundance estimates, and fish behaviors such as attraction or avoidance of an approaching camera platform can bias fish counts. In addition, artificial lighting is generally required for camera-based surveys in boreal systems at > 30 m depth. This study recorded the responses of fish and their apparent orientation as seen in camera images taken in two different areas in Alaska. Stereo-image methods were used to estimate fish movement, range from camera, and the movement of the camera platform itself by estimating the spatial relationship between fish and the seafloor. Change in the apparent position of feature points along the seafloor and the fish was used to derive fish and camera speed. A total of 670 individual fish across 19 near-bottom species were analyzed from a dataset of 3463 image frames from 112 separate camera deployments. Individual fish species were combined into groups and analyzed for reactions, orientation toward the camera, nearest approach to the camera, and swimming speed. Most fish did not react strongly to the camera. A number of distinct patterns in reactions were observed, with roundfish exhibiting the highest level of attraction to the camera and flatfish showing the highest number of strong avoidance reactions. Significant differences in approach distance and fish swimming speed across analysis groups, reactions and regions were found, with highest fish speeds observed with roundfish and closest approach of the camera observed with demersal rockfish and flatfish. The findings demonstrate distinct differences in reactions among groups and suggest that the potential attraction response of roundfish might lead to positive biases for density. Flatfish were reactive mostly at close ranges, and rockfishes exhibited neutral and moderate reactions, which may signal reasonable estimates of abundance for these groups using camera-based surveys.

Catchment and lake network modify export of anaerobic oxidation capacity in boreal freshwaters

Anaerobic terminal electron acceptors (aTEAs, i.e. NO3, Fe, SO4) enable anaerobic respiration, and each has a specific ability to oxidize reduced compounds. However, little is known about how seasonal and lake-specific aTEA fluxes form anaerobic oxidation capacity (AOC) to oxidize organic carbon in boreal systems. We compiled 26 years of data from two interconnected semi-pristine boreal lakes and defined mean daily imports, pools, and exports of aTEAs. In both lakes, the export of NO3 formed 2 %–3 % of the total AOC in summer and autumn, and up to 11 % in winter and spring. In a predominantly monomictic humic lake surrounded by peatlands, Fe was responsible for 15 %–31 % of the seasonal export of AOC, with a large proportion of Fe originating from the lake bottom. A dimictic clear-water lake downstream retained Fe and exported 87 %–95 % of AOC as SO4. In the humic lake, the annual SO4:Fe:NO3 export ratio for AOC was 10:3:1 and in the clear-water lake 15:0.4:1. In the monomictic lake, exports were specifically regulated by stratification; in the dimictic lake, exports were more regulated by spring flooding and the ascending and descending side of the peak flood. These events modified lake dynamics and caused lake-specific NO3, Fe, and SO4 exports which continued for months. We conclude that a catchment and lake network can cause spatial and temporal variation in exports of NO3, Fe, and SO4 affecting AOC export. Such natural variations in exports have significant potential to modify the system's capacity to oxidize C and resist changes in oxidation-reduction reactions coupled to nutrient cycling and the formation of greenhouse gases in downstream water bodies.

Spatial and seasonal patterns of mercury concentrations, methylation and demethylation in central Canadian boreal soils and stream sediment

Terrestrial ecosystems store large amounts of mercury (Hg), which may be subject to methylation, mobilization and uptake into downstream aquatic ecosystems. Mercury concentrations, methylation and demethylation potentials are not well characterized simultaneously across different habitats in boreal forest ecosystems, particularly not so in stream sediment, leading to uncertainties about the importance of various habitats as primary production areas of the bioaccumulative neurotoxin methylmercury (MeHg). In this study, we collected soil and sediment samples from 17 undisturbed, central Canadian boreal forested watersheds during spring, summer and fall to robustly characterize the spatial (upland and riparian/wetland soils, and stream sediment) and seasonal patterns of total Hg (THg) and MeHg concentrations. Mercury methylation and MeHg demethylation potentials (Kmeth and Kdemeth) in the soils and sediment were also assessed using enriched stable Hg isotope assays. We found the highest Kmeth and %-MeHg in stream sediment. In both riparian and wetland soils, Hg methylation was lower and less seasonally variable compared to stream sediment, but had comparable MeHg concentrations, suggesting longer-term storage of MeHg produced in these soils. Soil and sediment carbon content, and THg and MeHg concentrations were strong covariates across habitats. Additionally, sediment carbon content was important for delineating between stream sediment with relatively high vs. relatively low Hg methylation potential, which generally separated between different landscape physiographies. Broadly, this large and spatiotemporally diverse dataset is an important baseline for understanding Hg biogeochemistry in boreal forests both in Canada and possibly in many other boreal systems globally. This work is particularly important with respect to future possible impacts from natural and anthropogenic perturbations, which are increasingly straining boreal ecosystems in various parts of the world.

Not just for the birds: Spiders as natural enemies of spruce budworm (Choristoneura fumiferana, Clem.)

Abstract(1) The eastern spruce budworm (Choristoneura fumiferana, Clem.) is a native irruptive forest pest that defoliates spruce‐fir forests throughout North America's boreal zone. (2) Past studies suggest that successful spruce budworm population control requires high natural mortality from a variety of sources, including predators, especially from parasitoids and birds. While well represented in many different ecosystems, the role of generalist predatory spiders in these boreal systems remains largely unstudied. (3) To determine the identity and percentage of spiders preying upon spruce budworm, we hand‐collected spiders from balsam fir (Abies balsamea) in stands with relatively high spruce budworm densities from forests in insular Newfoundland and Labrador, Canada. (4) Using a spruce budworm‐specific TaqMan real‐time PCR assay, we successfully amplified spruce budworm DNA in 32% of collected spiders. After spider molecular barcoding, we found the web builders Grammonota angusta Dondale, Pityohyphantes (aff. subarcticus), Dictyna brevitarsa Emerton and Estrandia grandaeva (Keyserling) represented 58% of the spiders feeding on spruce budworm, and the wandering hunter Philodromus rufus Dondale represented 11.8%. (5) Our molecular approach was an effective means with which to identify recently consumed prey and natural enemies in this boreal system.

Environmental Impact of Triclopyr on Habitat Quality in Boreal Rights-of-Way.

The indirect effects of herbicides on habitat quality in boreal ecoregions remain poorly understood. Herbicides are commonly applied on boreal rights-of-way to control vegetation below power lines, where they can indirectly enter the soil ecosystem after leaf abscission. Key soil processes such as litter decomposition and soil nutrient cycling can be influenced by altering litter chemistry and/or impacting decomposer species. Disruption of these soil processes could lead to changes in ecosystem health of boreal systems. The indirect impacts of triclopyr on habitat quality of treated boreal rights-of-way were examined through litter mass loss and quality (carbon-to-nitrogen ratios) and the response of boreal invertebrates (Folsomia candida and Oppia nitens) in microcosms and avoidance tests. Litter breakdown rates were not significantly different within a year of treatment. However, we did observe nitrogen profile differences between field-treated and untreated samples, which likely resulted from triclopyr-induced repression of natural leaf senescence processes. At field application rates, there were no differences in survival and reproduction rates of F. candida, which is key in litter breakdown. The triclopyr concentrations that caused 50% of tested F. candida and O. nitens to avoid treated litter were above field application rates. Therefore, field application rates of triclopyr are not expected to impair habitat quality and ecosystem services of boreal ecoregions based on the parameters we evaluated. Our study improves understanding of the effects of herbicide application on habitat quality and is critical for responsible herbicide use on boreal rights-of-way. Environ Toxicol Chem 2022;41:2955-2967. © 2022 SETAC.

Greenhouse gas emissions from African lakes are no longer a blind spot.

Natural lakes are thought to be globally important sources of greenhouse gases (CO2, CH4, and N2O) to the atmosphere although nearly no data have been previously reported from Africa. We collected CO2, CH4, and N2O data in 24 African lakes that accounted for 49% of total lacustrine surface area of the African continent and covered a wide range of morphology and productivity. The surface water concentrations of dissolved CO2 were much lower than values attributed in current literature to tropical lakes and lower than in boreal systems because of a higher productivity. In contrast, surface water–dissolved CH4 concentrations were generally higher than in boreal systems. The lowest CO2 and the highest CH4 concentrations were observed in the more shallow and productive lakes. Emissions of CO2 may likely have been substantially overestimated by a factor between 9 and 18 in African lakes and between 6 and 26 in pan-tropical lakes.

Use, calibration and verification of agroecological models for boreal environments: A review

AbstractPast assessments report negative impacts of the climate crisis in boreal areas; but milder and shorter winters and elevated atmospheric CO2may provide opportunities for agricultural productivity potentially playing a significant role in future food security. Arable cropping systems are expanding in boreal areas, but the regional mainstay will likely continue to be livestock production. Agroecological models can when appropriately calibrated and evaluated, facilitate improved productivity while minimising environmental impacts by identifying system interactions, and quantifying greenhouse gas emissions, soil carbon stocks and fertiliser use. While models designed for temperate and tropical zones abound, few are developed specifically for boreal zones, and there is uncertainty around the performance of existing models in boreal areas. We reviewed model performance across boreal environments and management systems. We identified a dearth of modelling studies in boreal regions, with the publication of three or less papers per year since the year 2000, constituting a significant research gap. Models IFSM and BASGRA_N performed best in grassland production, DNDC best in predicting soil N2O and NH3emissions. No model outperformed all others, strengthening the case for ensemble modelling. Existing agroecological models would be worthy of further evaluation, providing model improvements designed for boreal systems.

Winter thriving: on the role of a boreal city on bird communities

Abstract Information on how urban areas affect bird communities during winter in the boreal region is still scarce. With the aim of assessing such role of the urban habitats on over-wintering boreal birds, I focused on a city-wide approach in the city of Lahti (southern Finland) and two nearby forests (as controls). Point count surveys were conducted in 157 sites within the city and 30 in the control forests. In order to achieve comparable sets of data to contrast with the reference forests, I randomly selected five 30 point count sub-samples from the Lahti city-wide survey. Species richness was, in general, higher in the sub-samples from the city of Lahti. Such pattern did not show relationship with the built cover of the studied sites. Bird abundances were 3.3–5.9 times higher in the urban sub-samples when contrasted with the forest ones. Although results of this study are limited to a single city and consider one wintering season, they clearly illustrate the important role of urban systems as habitat for wintering birds in boreal systems.

Integrated surface-subsurface water and solute modeling of a reclaimed in-pit oil sands mine: Effects of ground freezing and thawing

Study regionAthabasca Oil Sands, Alberta, Canada. Study focusThe upland and wetlands substrate in reclaimed oil sands landforms will be constructed of post-mining materials with an objective of replicating the landscape and hydrology of the surrounding boreal systems. Porewater in these materials contain elevated levels of salts and other solutes. Water quality will govern the success and sustainability of the reclaimed landscape. Tightly coupled water and heat dynamics control water and solute movement in boreal systems. We used three-dimensional integrated surface-subsurface flow and chloride transport models with and without ground freezing-thawing to compare performance of an in-pit oil sands mine currently being reclaimed. New hydrological insights for the regionTransient simulations under wet/dry climate cycles suggest that the reclaimed landform will shed water only during wet years. Annual water balance with and without coupled heat dynamics is identical. However, the three-dimensional representation of ground freeze-thaw results in reduced snowmelt infiltration, summer groundwater table, and solute release during winter. The outcome is increased spring runoff, 20% decrease in chloride mass release over simulated eight-year wet climate cycle and relatively reduced summer runoff. The model results suggest that (1) coupled heat dynamics should be considered for detailed evaluation of reclaimed in-pit landforms at finer time scales, and (2) modeling reclaimed landforms without freeze-thaw provides conservative annual solute release estimates, which is appropriate for coarse site-wide models.

Technical note: CO<sub>2</sub> is not like CH<sub>4</sub> – limits of and corrections to the headspace method to analyse <i>p</i>CO<sub>2</sub> in fresh water

Abstract. Headspace analysis of CO2 frequently has been used to quantify the concentration of CO2 in fresh water. According to basic chemical theory, not considering chemical equilibration of the carbonate system in the sample vials will result in a systematic error. By analysing the potential error for different types of water and experimental conditions, we show that the error incurred by headspace analysis of CO2 is less than 5 % for typical samples from boreal systems which have low alkalinity (< 900 µmol L−1), with pH < 7.5, and high pCO2 (> 1000 µatm). However, the simple headspace calculation can lead to high error (up to −300 %) or even impossibly negative values in highly undersaturated samples equilibrated with ambient air, unless the shift in carbonate equilibrium is explicitly considered. The precision of the method can be improved by lowering the headspace ratio and/or the equilibration temperature. We provide a convenient and direct method implemented in an R script or a JMP add-in to correct CO2 headspace results using separately measured alkalinity.

Frugivore zoogeochemistry in tropical forest ecosystems

Frugivore zoogeochemistry in tropical forest ecosystems

Evaluating floodplain organic carbon across a gradient of human alteration in the boreal zone

River corridors play an important role in the carbon cycle as sites of carbon transport, storage, and transformation. Floodplain soil organic carbon (OC) and dead, downed large wood (LW) are two of the largest OC stocks in rivers. Human modifications of river corridors, such as damming and floodplain land-use change, have likely modified floodplain OC storage and retention through removal of LW and potential reduction of OC concentrations in soils. However, the effect of human alterations on floodplain OC storage is poorly understood. We measured floodplain soil OC and downed LW loads on three rivers in northern Sweden that display a gradient in the degree of human alteration. The Muddus River is located in a national park and is unaltered. The Vindel River is free-flowing but has been modified via logging and other land-use changes within the floodplain. The Ume River is dammed along its length and has also experienced floodplain logging and land-use change. We used statistical models to determine which factors are associated with differences in mineral soil OC and LW among rivers with different degrees of human alteration. We find the highest mineral soil OC concentrations on the unaltered Muddus River (mean ± standard error (SE) = 3.70 ± 0.59%; median = 3.81%), with lower soil OC along the Vindel (mean ± SE = 1.44 ± 0.22%; median = 0.72%) and Ume (mean ± SE = 2.47 ± 0.44%; median = 1.12%) Rivers. The Muddus River also has the highest downed LW loads (mean ± SE = 22.25 ± 6.99 m3 ha−1) compared to the Vindel (mean ± SE = 3.10 ± 1.26 m3 ha−1) and Ume (mean ± SE = 7.26 ± 3.53 m3 ha−1) Rivers. Variations in soil OC and downed large wood loads indicate that damming may reduce floodplain OC in these boreal systems through reducing lateral channel-floodplain connectivity and longitudinal connectivity. Logging and other land-use changes likely reduce OC inputs to the floodplain surface through removal of organic matter and LW. Further research is needed to elucidate the impact of human modifications on floodplain OC across diverse regions and to inform river restoration efforts to enhance floodplain OC storage.

Wildfire combustion and carbon stocks in the southern Canadian boreal forest: Implications for a warming world.

Boreal wildfires are increasing in intensity, extent, and frequency, potentially intensifying carbon emissions and transitioning the region from a globally significant carbon sink to a source. The productive southern boreal forests of central Canada already experience relatively high frequencies of fire, and as such may serve as an analog of future carbon dynamics for more northern forests. Fire-carbon dynamics in southern boreal systems are relatively understudied, with limited investigation into the drivers of pre-fire carbon stocks or subsequent combustion. As part of NASA's Arctic-Boreal Vulnerability Experiment, we sampled 79 stands (47 burned, 32 unburned) throughout central Saskatchewan to characterize above- and belowground carbon stocks and combustion rates in relation to historical land use, vegetation characteristics, and geophysical attributes. We found southern boreal forests emitted an average of 3.3±1.1kgC/m2 from field sites. The emissions from southern boreal stands varied as a function of stand age, fire weather conditions, ecozone, and soil moisture class. Sites affected by historical timber harvesting had greater combustion rates due to faster carbon stock recovery rates than sites recovering from wildfire events, indicating that different boreal forest land use practices can generate divergent carbon legacy effects. We estimate the 2015 fire season in Saskatchewan emitted a total of 36.3±15.0TgC, emphasizing the importance of southern boreal fires for regional carbon budgets. Using the southern boreal as an analog, the northern boreal may undergo fundamental shifts in forest structure and carbon dynamics, becoming dominated by stands <70years old that hold 2-7kgC/m2 less than current mature northern boreal stands. Our latitudinal approach reinforces previous studies showing that northern boreal stands are at a high risk of holding less carbon under changing disturbance conditions.

Arsenic, chromium, and other elements of concern in fish from remote boreal lakes and rivers: Drivers of variation and implications for subsistence consumption

Eating fish provides numerous health benefits, but it is also a dominant pathway for human exposure to contaminants. Many studies have examined mercury (Hg) accumulation in fish, but fewer have considered other elements, such as arsenic (As) and chromium (Cr). Recently, freshwater fish from several pristine boreal systems across northern Ontario, Canada, have been reported with elevated concentrations of As and Cr for reasons that are not well understood. Our goal was to investigate the ecological and environmental influences over concentrations of As, Cr, and other elements in these fish to better understand what affects metal uptake and the risk to consumers. We measured 10 elements (including As, Cr, Hg) as well as carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) stable isotopes in 388 fish from 25 lake and river sites across this remote region. These data were used to determine the effect of: 1) trophic ecology; and 2) watershed geology on piscine elemental content. Overall, most element concentrations were low, often below provincial advisory benchmarks (ABs). However, traces of Hg, As, Cr, and selenium (Se) were detected in most fish. Based on their exceedance of their respective ABs, the most restrictive elements on fish consumption in these boreal systems were Hg > As > Cr. Arsenic and Se, but not Cr concentrations were related to fish size and trophic ecology (inferred from δ13C and δ15N), suggesting bioaccumulation of the former elements. Fish with enriched δ34S values, suggestive of anadromous behaviour, had marginally lower Hg but higher Se concentrations. Modeling results suggested a strong effect of site-specific factors, though we found weak trends between piscine elemental content and geological features (e.g., mafic intrusions), potentially due to the broad spatial scale of this study. Results from this study address gaps in our understanding of As and Cr bioaccumulation and will help to inform fish consumption guidelines.