Boreal Environment Research Articles

Tracer‐based evidence of heterogeneity in subsurface flow and storage within a boreal hillslope

AbstractRunoff from boreal hillslopes is often affected by distinct soil boundaries, including the frozen boundary and the organic‐mineral boundary (OMB), where highly porous and hydraulically conductive organic material overlies fine‐grained mineral soils. Viewed from the surface, ground cover appears as a patchwork on sub‐meter scales, with thick, moss mats interspersed with lichen‐covered, silty soils with gravel inclusions. We conducted a decameter‐scale subsurface tracer test on a boreal forest hillslope in interior Alaska to quantify locations and mechanisms of transport and storage in these soils, focusing on the OMB. A sodium bromide tracer was added as a slug addition to a pit and sampled at 40 down‐gradient wells, screened primarily at the OMB and within a 7 × 12 m well field. We maintained an elevated head in the injection pit for 8.5 hr to simulate a storm. Tracer breakthrough velocities ranged from <0.12 to 0.93 m hr−1, with the highest velocities in lichen‐covered soils. After 12 hr and cessation of the elevated head, the tracer coalesced and was only detected in thick mosses at a trough in the OMB. By 24 hr, approximately 17% of the tracer mass could be accounted for. The majority of the mass loss occurred between 4 and 12 hr, while the tracer was in contact with lichen‐covered soils, which is consistent with tracer transport into deeper flow paths via preferential flow through discrete gravelly areas. Slow breakthroughs suggest that storage and exchange also occurred in shallow soils, likely related to saturation and drainage in fine‐grained mineral soils caused by the elevated hydraulic head. These findings highlight the complex nature of storage and transmission of water and solutes from boreal hillslopes to streams and are particularly relevant given rapid changes to boreal environments related to climate change, thawing permafrost and increasing fire severity.

Assessing the environmental controls on Scots pine transpiration and the implications for water partitioning in a boreal headwater catchment

Climate change projections indicate reduced summer precipitation and increased air temperature for the northern high latitudes. These climate changes are likely to alter forest water budgets of which plant transpiration (T) forms a significant component. Plant transpiration is regulated by stomata behavior of particular species, which is constrained by ambient air and soil conditions. Here, we measured sap flow in a Scots pine (Pinus Sylvestris) plantation in a low energy Scottish headwater catchment during the main summer growth period. Effects of rainfall on forest transpiration, as well as the response of T to four environmental variables were investigated at a daily scale. In this boreal environment, transpiration was mainly restricted by radiation and vapor pressure deficit. Air temperature was the least important controlling factor. Soil water became an important factor when rainfall was limited. Frequent but small rain events dictated that precipitation met short-term transpiration demand most of the time. The trees needed supplementary water from antecedent soil water stores when weekly rainfall was below ∼8mm, but such periods were rare. Water exchange mainly occurred in the canopy or upper 10cm of the soil, with 47% of rainfall transpired, 45% intercepted and <8% evaporated from the soil surface. Understanding interactions between forests and their hydroclimate, as well as the role of forests in water partitioning is crucial to assist a sustainable land and water management in a changing climate. Whilst such studies are common in semi-arid regions, they are limited in boreal zones, therefore, our findings are a valuable contribution to understanding plant-water relations in a changing environment.

Assessing post-industrial land cover change at the Pine Point Mine, NWT, Canada using multi-temporal Landsat analysis and landscape metrics

This study investigates land cover change near the abandoned Pine Point Mine in Canada's Northwest Territories. Industrial mineral development transforms local environments, and the effects of such disturbances are often long-lasting, particularly in subarctic, boreal environments where vegetation conversion can take decades. Located in the Boreal Plains Ecozone, the Pine Point Mine was an extensive open pit operation that underwent little reclamation when it shut down in 1988. We apply remote sensing and landscape ecology methods to quantify land cover change in the 20years following the mine's closure. Using a time series of near-anniversary Landsat images, we performed a supervised classification to differentiate seven land cover classes. We used raster algebra and landscape metrics to track changes in land cover composition and configuration in the 20years since the mine shut down. We compared our results with a site in Wood Buffalo National Park that was never subjected to extensive anthropogenic disturbance. This space-for-time substitution provided an analog for how the ecosystem in the Pine Point region might have developed in the absence of industrial mineral development. We found that the dense conifer class was dominant in the park and exhibited larger and more contiguous patches than at the mine site. Bare land at the mine site showed little conversion through time. While the combination of raster algebra and landscape metrics allowed us to track broad changes in land cover composition and configuration, improved access to affordable, high-resolution imagery is necessary to effectively monitor land cover dynamics at abandoned mines.

Atmospheric circulation patterns explaining climatological drought dynamics in the boreal environment of Finland, 1962–2011

ABSTRACTUnderstanding the atmospheric circulation patterns (ACPs) influencing drought development is important in management of this spatially extensive and recurrently prolonged natural hazard. Using the Standardized Precipitation Evapotranspiration Index (SPEI), this study evaluates the spatio‐temporal variability in climatological drought dynamics (in terms of intensity, frequency, duration, and extension) in Finland in 1962–2011. The SPEI values are calculated in 12‐, 3‐, and 1‐month time steps (SPEI‐12, ‐3, and ‐1, respectively) and the analyses are based on annual (SPEI‐12Ann), seasonal (e.g. SPEI‐3Win for winter), and monthly (e.g. SPEI‐1Dec for December) time scales. The relationships between SPEI values and well‐known ACPs of climate variability over Finland are also investigated. Our analysis shows that on national scale, the longest observed drought is 12 months (October 2002–September 2003) observed in SPEI‐12 values. Droughts intensify in April, August, and September, and largely occur over the south, centre, east, and west of Finland. December has the highest frequency of droughts, but with significant wetting trends. Analysis of annual SPEI‐12 indicates an increase in the probability of drought extent over time. In warm months (April–September), influential ACPs for temperature variability and drought conditions are mainly the Arctic Oscillation (AO) and the Scandinavia pattern. For the cold months (October–March), the AO, the North Atlantic Oscillation, the East Atlantic, and the Polar patterns influence precipitation deficit and SPEI. Although Finland has experienced wetting trends during the period 1962–2011, this study provides a detailed spatial assessment of climatological drought variability and the plausible atmospheric circulation drivers for the boreal environment of Finland in recent decades.

Combining geodiversity with climate and topography to account for threatened species richness

Understanding threatened species diversity is important for long-term conservation planning. Geodiversity-the diversity of Earth surface materials, forms, and processes-may be a useful biodiversity surrogate for conservation and have conservation value itself. Geodiversity and species richness relationships have been demonstrated; establishing whether geodiversity relates to threatened species' diversity and distribution pattern is a logical next step for conservation. We used 4 geodiversity variables (rock-type and soil-type richness, geomorphological diversity, and hydrological feature diversity) and 4 climatic and topographic variables to model threatened species diversity across 31 of Finland's national parks. We also analyzed rarity-weighted richness (a measure of site complementarity) of threatened vascular plants, fungi, bryophytes, and all species combined. Our 1-km2 resolution data set included 271 threatened species from 16 major taxa. We modeled threatened species richness (raw and rarity weighted) with boosted regression trees. Climatic variables, especially the annual temperature sum above 5 °C, dominated our models, which is consistent with the critical role of temperature in this boreal environment. Geodiversity added significant explanatory power. High geodiversity values were consistently associated with high threatened species richness across taxa. The combined effect of geodiversity variables was even more pronounced in the rarity-weighted richness analyses (except for fungi) than in those for species richness. Geodiversity measures correlated most strongly with species richness (raw and rarity weighted) of threatened vascular plants and bryophytes and were weakest for molluscs, lichens, and mammals. Although simple measures of topography improve biodiversity modeling, our results suggest that geodiversity data relating to geology, landforms, and hydrology are also worth including. This reinforces recent arguments that conserving nature's stage is an important principle in conservation.

Simple proxies for estimating the concentrations of monoterpenes and their oxidation products at a boreal forest site

Abstract. The oxidation products of monoterpenes likely have a crucial role in the formation and growth of aerosol particles in boreal forests. However, the continuous measurements of monoterpene concentrations are usually not available on decadal timescales, and the direct measurements of the concentrations of monoterpene oxidation product have so far been scarce. In this study we developed proxies for the concentrations of monoterpenes and their oxidation products at a boreal forest site in Hyytiälä, southern Finland. For deriving the proxies we used the monoterpene concentration measured with a proton transfer reaction mass spectrometer (PTR-MS) during 2006–2013. Our proxies for the monoterpene concentration take into account the temperature-controlled emissions from the forest ecosystem, the dilution caused by the mixing within the boundary layer and different oxidation processes. All the versions of our proxies captured the seasonal variation of the monoterpene concentration, the typical proxy-to-measurements ratios being between 0.8 and 1.3 in summer and between 0.6 and 2.6 in winter. In addition, the proxies were able to describe the diurnal variation of the monoterpene concentration rather well, especially in summer months. By utilizing one of the proxies, we calculated the concentration of oxidation products of monoterpenes by considering their production in the oxidation and their loss due to condensation on aerosol particles. The concentration of oxidation products was found to have a clear seasonal cycle, with a maximum in summer and a minimum in winter. The concentration of oxidation products was lowest in the morning or around noon and highest in the evening. In the future, our proxies for the monoterpene concentration and their oxidation products can be used, for example, in the analysis of new particle formation and growth in boreal environments.

BAECC: A Field Campaign to Elucidate the Impact of Biogenic Aerosols on Clouds and Climate

Abstract During Biogenic Aerosols—Effects on Clouds and Climate (BAECC), the U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Program deployed the Second ARM Mobile Facility (AMF2) to Hyytiälä, Finland, for an 8-month intensive measurement campaign from February to September 2014. The primary research goal is to understand the role of biogenic aerosols in cloud formation. Hyytiälä is host to the Station for Measuring Ecosystem–Atmosphere Relations II (SMEAR II), one of the world’s most comprehensive surface in situ observation sites in a boreal forest environment. The station has been measuring atmospheric aerosols, biogenic emissions, and an extensive suite of parameters relevant to atmosphere–biosphere interactions continuously since 1996. Combining vertical profiles from AMF2 with surface-based in situ SMEAR II observations allows the processes at the surface to be directly related to processes occurring throughout the entire tropospheric column. Together with the inclusion of extensive surface precipitation measurements and intensive observation periods involving aircraft flights and novel radiosonde launches, the complementary observations provide a unique opportunity for investigating aerosol–cloud interactions and cloud-to-precipitation processes in a boreal environment. The BAECC dataset provides opportunities for evaluating and improving models of aerosol sources and transport, cloud microphysical processes, and boundary layer structures. In addition, numerical models are being used to bridge the gap between surface-based and tropospheric observations.

Boreal Forest Low Antenna Height Propagation Measurements

This paper presents the results of the first propagation measurement campaign conducted in the 2.4 GHz industrial, scientific and medical (ISM) band for low antenna heights in a North American boreal forest. The $2 \times 1$ multiple-input, single output measurements capture propagation conditions for wireless peer-to-peer or sensor networks in the boreal environment. Unlike many other forests, it is shown that ground reflections strongly influence large scale propagation for the low antenna boreal case. Results also indicate that scattering is severe but that both frequency diversity and multiple antenna techniques will be effective for mitigating and/or exploiting the fading caused by this scattering.

Adult lifetime reproductive value in fish depends on size and fecundity type

In a stable population, the adult lifetime reproductive value must be balanced against early life survival. Although delaying maturity may increase fecundity, it also reduces survival. Larger size at maturity therefore not only allows for higher fecundity, but requires it. Using simple arguments from life history, we derive a direct proportionality relationship between the adult lifetime reproductive value and weight at maturation and find that this relationship is consistent with empirical evidence from 28 stocks and species of bony fish from temperate–boreal environments. However, the expected proportionality falls off if mortality increases to include fishing. Furthermore, we find that the fecundity type (determinate or indeterminate) affects the predicted adult reproductive value, which is significantly (10-fold) higher for an indeterminate spawner than for a determinate spawner of the same weight. These differences may relate to trade-offs in the adult life history traits and (or) to seasonality in the spawning environment, with subsequent consequences for early life stage survivorship.

Pool size structure indicates developmental stages of boreal fens

Several boreal fens belong to the patterned fen category, which is composed of two compartments, the pool (or hollow), and the string (or hummock) compartments. Both compartments are distributed perpendicular to the mire slope and drainage flow. The surface of patterned fens is repeatedly invaded by water, which is known to affect the original pool–string pattern through the creation and enlargement of pools, a process caused by the action of aqualysis. Aqualysis refers to the destruction of the vegetation cover by flooding of low-elevated strings or lawns causing the formation and enlargement of pools. It is argued here that the nature and composition of the pool structure (number and size) reveal the state of development of patterned fens of the boreal environment. Quantitative and spatial data for pool size structure in 24 fens of northern Quebec, including 4881 pools, were used for the assessment of patterned fen development. Among the main factors influencing patterned fen development, area and slope contribute to the construction of the peatland surface. Also, watershed area, alternating strings and hollows, and peat thickness all facilitate pool creation and expansion. The construction of the pool structure of most fens has been a long process of ecosystem building. This probably explains why most sloping fens, i.e, moderate- and steep-sloping fens, are today at an equilibrium stage characterized by the coexistence of pools of all sizes with the smallest pools being the most numerous. In contrast, most low-sloping fens have developed toward a lacustrine stage with a bimodal distribution of small and very large pools. Boreal patterned fens of all developmental stages will be subject to increased pool size and number in the near future if more humid climatic conditions prevail.

Microwave snow emission modeling uncertainties in boreal and subarctic environments

Abstract. This study aims to better understand and quantify the uncertainties in microwave snow emission models using the Dense Media Radiative Theory Multi-Layer model (DMRT-ML) with in situ measurements of snow properties. We use surface-based radiometric measurements at 10.67, 19 and 37 GHz in boreal forest and subarctic environments and a new in situ data set of measurements of snow properties (profiles of density, snow grain size and temperature, soil characterization and ice lens detection) acquired in the James Bay and Umiujaq regions of Northern Québec, Canada. A snow excavation experiment – where snow was removed from the ground to measure the microwave emission of bare frozen ground – shows that small-scale spatial variability (less than 1 km) in the emission of frozen soil is small. Hence, in our case of boreal organic soil, variability in the emission of frozen soil has a small effect on snow-covered brightness temperature (TB). Grain size and density measurement errors can explain the errors at 37 GHz, while the sensitivity of TB at 19 GHz to snow increases during the winter because of the snow grain growth that leads to scattering. Furthermore, the inclusion of observed ice lenses in DMRT-ML leads to significant improvements in the simulations at horizontal polarization (H-pol) for the three frequencies (up to 20 K of root mean square error). However, representation of the spatial variability of TB remains poor at 10.67 and 19 GHz at H-pol given the spatial variability of ice lens characteristics and the difficulty in simulating snowpack stratigraphy related to the snow crust. The results also show that, in our study with the given forest characteristics, forest emission reflected by the snow-covered surface can increase the TB up to 40 K. The forest contribution varies with vegetation characteristics and a relationship between the downwelling contribution of vegetation and the proportion of pixels occupied by vegetation (trees) in fisheye pictures was found. We perform a comprehensive analysis of the components that contribute to the snow-covered microwave signal, which will help to develop DMRT-ML and to improve the required field measurements. The analysis shows that a better consideration of ice lenses and snow crusts is essential to improve TB simulations in boreal forest and subarctic environments.

Seventeen-year trends in spring and autumn phenophases of Betula pubescens in a boreal environment.

Trends in the timing of spring and autumn phenophases of Betula pubescens were investigated in the southern, middle, and northern boreal zones in Finland. The field observations were carried out at 21 sites in the Finnish National Phenological Network in 1997-2013. The effective temperature sum of the thermal growth period, i.e. the sum of the positive differences between diurnal mean temperatures and 5°C (ETS1), increased annually on average by 6-7 degree day units. Timing of bud burst remained constant in the southern and middle boreal zones but advanced annually by 0.5day in the northern boreal zone. The effective temperature sum at bud burst (ETS2) showed no trend in the southern and middle boreal zones, whereas ETS2 increased on average from 20-30 to 50 degree day units in the northern boreal zone, almost to the same level as in the other zones. Increase in ETS2 indicates that the trees did not start their growth in very early spring despite warmer spring temperatures. The timing of leaf colouring and leaf fall remained almost constant in the southern boreal zones, whereas these advanced annually by 0.3 and 0.6day in the middle boreal zone and by 0.6 and 0.4day in the northern boreal zone, respectively. The duration of the growth period remained constant in all boreal zones. The results indicate high buffering capacity of B. pubescens against temperature changes. The study also shows the importance of the duration of phenological studies: some trends in spring phenophases had levelled out, while new trends in autumn phases had emerged after earlier studies in the same network for a shorter observation period.

A synthesis of cloud condensation nuclei counter (CCNC) measurements within the EUCAARI network

Abstract. Cloud condensation nuclei counter (CCNC) measurements performed at 14 locations around the world within the European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) framework have been analysed and discussed with respect to the cloud condensation nuclei (CCN) activation and hygroscopic properties of the atmospheric aerosol. The annual mean ratio of activated cloud condensation nuclei (NCCN) to the total number concentration of particles (NCN), known as the activated fraction A, shows a similar functional dependence on supersaturation S at many locations – exceptions to this being certain marine locations, a free troposphere site and background sites in south-west Germany and northern Finland. The use of total number concentration of particles above 50 and 100 nm diameter when calculating the activated fractions (A50 and A100, respectively) renders a much more stable dependence of A on S; A50 and A100 also reveal the effect of the size distribution on CCN activation. With respect to chemical composition, it was found that the hygroscopicity of aerosol particles as a function of size differs among locations. The hygroscopicity parameter κ decreased with an increasing size at a continental site in south-west Germany and fluctuated without any particular size dependence across the observed size range in the remote tropical North Atlantic and rural central Hungary. At all other locations κ increased with size. In fact, in Hyytiälä, Vavihill, Jungfraujoch and Pallas the difference in hygroscopicity between Aitken and accumulation mode aerosol was statistically significant at the 5 % significance level. In a boreal environment the assumption of a size-independent κ can lead to a potentially substantial overestimation of NCCN at S levels above 0.6 %. The same is true for other locations where κ was found to increase with size. While detailed information about aerosol hygroscopicity can significantly improve the prediction of NCCN, total aerosol number concentration and aerosol size distribution remain more important parameters. The seasonal and diurnal patterns of CCN activation and hygroscopic properties vary among three long-term locations, highlighting the spatial and temporal variability of potential aerosol–cloud interactions in various environments.

MIS 13–12 in Britain and the North Atlantic: understanding the palaeoclimatic context of the earliest Acheulean

ABSTRACTResearch over the last two decades has revealed a rich record of Lower Palaeolithic occupation in Britain before 450 000 years. Acheulean industries (Mode II) first appear in the later part of the early Middle Pleistocene [Marine Isotope Stage (MIS) 19–13]. This paper reviews: (i) the age of the earliest Acheulean in Britain; (ii) the climates under which the earliest Acheulean industries occur, as recorded by the palaeoecological assemblages that are found at key archaeological sites; and (iii) the spatial and temporal pattern of regional climate change, i.e. the magnitude of glacial/interglacial cycles that occurred as a backdrop to the first arrival of Mode II archaeology in Britain. This review suggests that in Britain, the earliest Acheulean populations arrived during MIS 13 and that early occupation occurred under a range of climatic/environmental settings but frequently under post‐temperate late interglacial or interstadial‐type cool boreal environments. Furthermore, this review shows that the pattern of climate forcing in Western Europe during MIS 13–12 was not analogous to the interglacial cycles of the past 450 000 years as latitudinal climate gradients appear to have been less pronounced. The paper concludes by discussing the significance of these observations for understanding the arrival of the earliest Acheulean in Britain.

Industrial road crossings drive changes in community structure and instream habitat for freshwater fishes in the boreal forest

Summary Stream crossing structures are an increasingly prevalent anthropogenic feature on North American riverscapes, particularly in watersheds affected by industrial resource development in sensitive boreal environments. If improperly managed, stream crossings have the potential to alter fish habitat and impede fish movement. This study assessed instream habitat characteristics and fish communities from 33 culverted, bridged and reference streams in an industrialising region of the boreal forest in west‐central Alberta. Mixed‐effects modelling and multivariate analysis were used to determine impacts of stream crossings at three scales: whole‐stream scale, within‐stream scale and the interaction of scales. Instream habitat characteristics such as mean depth, water velocity, percent fines, turbidity, water temperature and dissolved oxygen showed significant between‐stream as well as within‐stream differences among stream crossings. The majority of fish species exhibited significantly lower densities (n m−2) in upstream habitats as compared to downstream habitats, including a significant reduction in Slimy Sculpin densities in culverted streams. Multivariate tests corroborated these results, showing that fish assemblages differ as a function of stream type. This study suggests industrial stream crossings influence abiotic habitat characteristics in freshwater ecosystems, restrict biotic connectivity and impact fish community structure at the whole‐stream and within‐stream scales. Alterations to stream ecosystems associated with stream crossings may be driving large‐scale changes in stream fish communities in the boreal forest. With expanded development expected in much of North America's boreal region, mitigation measures which limit impacts from stream crossings are needed to ensure proper ecosystem function in freshwater systems.

Stream water age distributions controlled by storage dynamics and nonlinear hydrologic connectivity: Modeling with high-resolution isotope data.

To assess the influence of storage dynamics and nonlinearities in hydrological connectivity on time‐variant stream water ages, we used a new long‐term record of daily isotope measurements in precipitation and streamflow to calibrate and test a parsimonious tracer‐aided runoff model. This can track tracers and the ages of water fluxes through and between conceptual stores in steeper hillslopes, dynamically saturated riparian peatlands, and deeper groundwater; these represent the main landscape units involved in runoff generation. Storage volumes are largest in groundwater and on the hillslopes, though most dynamic mixing occurs in the smaller stores in riparian peat. Both streamflow and isotope variations are generally well captured by the model, and the simulated storage and tracer dynamics in the main landscape units are consistent with independent measurements. The model predicts that the average age of stream water is ∼1.8 years. On a daily basis, this varies between ∼1 month in storm events, when younger waters draining the hillslope and riparian peatland dominates, to around 4 years in dry periods when groundwater sustains flow. This variability reflects the integration of differently aged water fluxes from the main landscape units and their mixing in riparian wetlands. The connectivity between these spatial units varies in a nonlinear way with storage that depends upon precipitation characteristics and antecedent conditions. This, in turn, determines the spatial distribution of flow paths and the integration of their contrasting nonstationary ages. This approach is well suited for constraining process‐based modeling in a range of northern temperate and boreal environments.

Winter diets of reintroduced bison (Bison bison) in northwestern Canada

I examined the winter diets of a reintroduced population of bison (Bison bison) in a mountainous region of northwestern Canada. The impetus of this study was to examine the winter diets of bison in this ecological region and to test for sexual and seasonal variation in the winter diet. Microhistological analyses of fecal samples from animals of known sex- and age-classes were examined to determine winter diets. I evaluated the hypothesis that sexual segregation in bison may be due to differences in diet. In addition, diet composition between early and late winter was examined to test for seasonal differences. Overall, bison winter diets were diverse, including at least 31 genera of vascular plants, but dominated by sedges (Carex). Nine species of Carex were identified in the winter diet. Diets of adult males in early winter differed in percentages of 7 forage classes used from those in late winter. The proportion of shrubs increased in late-winter diets, while that of sedges decreased. No evidence was found to support hypotheses of sexual segregation in bison based on differences in diet, at least for this population during late winter. This study shows that bison diets in winter are more diverse than previously thought based on studies in lowland boreal environments, where dietary niche breadth was comparatively narrow. Dietary flexibility is important within the context of choosing new sites for reintroducing bison, or predicting colonization of vacant habitats as existing populations grow.

Eighteen Year Record of Forest Fire Effects on Ground Thermal Regimes and Permafrost in the Central Mackenzie Valley, NWT, Canada

AbstractGround thermal conditions following a forest fire in 1994 were investigated along a hillslope transect in the discontinuous permafrost zone adjacent to a pipeline right of way in the central Mackenzie Valley, NWT, Canada. The intensity of the burn and damage to the forest and organic layer were more severe in the upper part of the slope than the bottom. Analysis of ground temperature records between 1995 and 2012 indicates that post‐fire changes to the surface altered the ground temperature regime, likely by allowing more snow to accumulate on the ground in winter (as interception by coniferous trees ceased) and decreasing albedo and evapotranspiration in summer. Active layer thickness at the affected site doubled between 1995 and 1998, before stabilising. Permafrost degradation likely occurred at the top of the slope, where burning was most severe. Towards the end of the study period, ground surface temperatures in the upper portion of the slope were lower than they were post fire (1997), whereas the surface temperatures were higher in the lower portion of the slope. In contrast, little to no change in surface temperature was observed for the Unburnt site. These changes in the burned sites are attributed to the increased re‐establishment of aspen, willow and tamarack in the upper part of the transect. Permafrost appears to be recovering at all burned sites in response to ongoing vegetation succession (except at the top of the slope), which is important for maintaining permafrost under conditions of projected climate warming in this boreal environment. Copyright © 2015 John Wiley &amp; Sons, Ltd.

A biophysical approach to delineate a northern limit to commercial forestry: the case of Quebec’s boreal forest

The boreal forest ecosystem is one of the largest frontier forests of the world, providing many ecological services to society. Boreal forests are also economically important, but forest harvesting and management become increasingly difficult when one moves from south to north in boreal environments. An approach was thus developed to assess the suitability of land units for timber production in a sustainable forest management (SFM) context in the northern boreal forest of Quebec (Canada). This area includes all of Quebec’s spruce – feather moss bioclimatic domain (closed forest), as well as the southern portion of the spruce–lichen bioclimatic domain (open woodland). Four criteria specific to the biophysical aspects of SFM were evaluated in 1114 land districts: physical environment, timber production capacity, forest vulnerability to fire (e.g., probability that it reaches maturity), and conservation of biodiversity. Indicators and acceptability cutoff values were determined for each selected criterion, and a sequential analysis was developed to evaluate if a land district has the potential to be sustainably managed. This analytical process led to the classification of land districts into three categories: slightly sensitive (SFM possible); moderately sensitive (SFM possible under certain conditions); and highly sensitive (SFM not possible). The results show that 354 land districts were highly sensitive, 62 due to physical constraints (7.5% of the area), 130 due to insufficient potential productivity (15.4% of the area), 92 due to insufficient potential productivity to account for the fire risk (13.8% of the area), and 70 due to an insufficient proportion of tall and dense forest habitats (7.7% of the area — biodiversity criterion). This work provides scientific background to proposing a northern limit for forest management activities in Quebec. The developed approach could be useful in other jurisdictions to address similar issues.

Variation of xylem vessel diameters across a climate gradient: insight from a reciprocal transplant experiment with a widespread boreal tree

Summary Xylem vessel diameters represent an important plant hydraulic trait to ensure sufficient water supply from the roots to the leaves. The ability to adjust the hydraulic pathway to environmental cues is key in order to satisfy transpirational demands and maximize growth and survival. We evaluated the variability of vessel diameters in trembling aspen in a reciprocal transplant experiment. We tested six provenances from three ecological regions of North America planted at four test sites in western Canada. All test sites were established at the same time with the same provenances arranged in a randomized complete block design. Vessel diameter showed a strong interaction of population and test site suggesting a high degree of phenotypic plasticity in this trait. Gaussian kernel density estimates support plastic as well as genetic contributions in vessel diameter control trending from bimodal distributions at the most northern test site towards unimodal distributions at the warmest and mildest test site. Furthermore, we used test site‐specific climate data in form of a 2‐year, 5‐year and 10‐year average of 21 directly and derived climatic variables and found that average site‐specific vessel diameters correlated strongly with summer moisture availability. A previously found negative relationship with vessel diameter and tree height in central Alberta was also found at two other boreal test sites but reversed at a wetter and milder sub‐boreal test site. In summary, vessel diameters were highly plastic in response to different environments and varied with summer moisture availability. The variability of vessel diameter and tree height correlations suggests that vessel diameter alone cannot serve as a reliable proxy for long‐term growth performance beyond boreal environments. Instead, selecting aspen populations with a high degree of plasticity in this trait appears to be the safest option for assisted migration and seed transfer programmes under climate change.