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Increasing fire and the decline of fire adapted black spruce in the boreal forest

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... 333 In contrast, conifer recruitment may be severely impeded or entirely absent following wildfires 334 burning in close succession (Splawinski et al. 2019), often resulting in long-lasting shifts in 335 forest structure (Johnstone et al. 2004). This phenomenon may also instigate a change in species 336 composition (shifts from conifer to broadleaf dominance or from Picea to Pinus) (Lavoie and 337 Sirois 1998;Baltzer et al. 2021;Dawe et al. 2022). That vigorous post-fire recruitment of trees-338 coniferous or broadleaf-is still observed following wildfires across the boreal biome is 339 indicative of sufficient moisture. ...
... For example, if timber resources are a priority, a 526 regeneration metric based on stem density may be the most appropriate (Pinno et al. 2013, 527 Rother andVeblen 2016). If vegetation type transition is the focus of study, the metric could 528 shift to whether there are low densities, or no (zero) tree regeneration (Kemp et al. 2016, Andrus 529 et al. 2018, Baltzer et al. 2021. Alternatively, if one species or functional group is targeted, 530 perhaps because of previous dominance, explicit focus on individual species dynamics and 531 limitations is needed (Littlefield et al. 2019). ...
... Some of the local regeneration "failures" 589 may actually represent a restoration of the shifting mosaic of forest and non-forest vegetation 590 that existed under historical fire regimes and will be essential to create more fire-resilient 591 landscapes under a warmer, often drier future in these forests(White and Long 2018, Hessburg 592 et al. 2019). Conversely, large-scale studies that demonstrate consistent and widespread 593 regeneration failure(Stevens-Rumann et al. 2018, Davis et al. 2019, Baltzer et al., 2021 594 highlight landscapes experiencing unprecedented change that will likely continue with a 595 continued warming and drying climate(Coop et al. 2020).598Although we do not advocate a single metric describing 'regeneration failure' across 599 western North America, we encourage more uniformity in the nature of the measurements and 600 spatio-temporal frame of observation. ...
Article
Wildfire-mediated changes to forests have prompted numerous studies on post-fire forest recovery of coniferous forests. Given climate change, a growing body of work demonstrates that conifer regeneration in temperate and boreal forests is declining, a phenomenon often termed “regeneration failure.” However, the definition and parameters are numerous and variable. Characterization of drought also varies greatly, thus hindering the ability to compare results among areas. This review discusses new perspectives on conifer regeneration failure and places these studies into the context of drought and fire activity. We focus this review on three forest types where conifer regeneration failure is well documented: western boreal forests, cold mixed-conifer, and dry pine forests. To place the challenges to conifer tree regeneration in the context of regional climate trends, we present a novel regional analysis that summarizes drought conditions prior, during, and following the year of a large wildfire. We demonstrate the need to assess failure in the context of specific forest dynamics and well-defined metrics. For example, tree establishment may historically occur over longer periods, and current and future climate may exacerbate this and not promote pre-fire forest structure and composition. Many forests are undergoing rapid change and the type, magnitude, and causes of changes need to be compared among areas. Thus, we should be cautious of quantifying “regeneration failure” and drought without providing spatial and temporal context.
... In northwestern North America, there has been particular interest in detecting changes to tree and tall shrub extent near forest-tundra boundaries along gradients of climate, latitude, and elevation (Brodie et al., 2019;Danby & Hik, 2007;Dial et al., 2007;Roland et al., 2016;Terskaia et al., 2020). Boreal forest wildfire and successional processes have also been the subject of numerous investigations in the subarctic (Baltzer et al., 2021;Kasischke et al., 2010;Mack et al., 2021;Walker et al., 2021). However, comparatively little is known about vegetation dynamics in subarctic riparian zones, despite their disproportionately high importance as "hotspots" of hydrological processes (Ploum et al., 2021), biogeochemical cycling (Blackburn et al., 2017), species diversity (Andersson et al., 2000;Johansson et al., 1996;Johnson & Almlöf, 2016;Nilsson & Svedmark, 2002), wildlife habitat use (Cooke & Tauzer, 2020), and ecological disturbance (Scrimgeour et al., 1994). ...
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Boreal forest and tundra ecosystems are undergoing rapid climatic and environmental changes with consequences for ecosystem structure, function, and services. Although riparian zones occupy a small footprint within subarctic landscapes, they have disproportionately high value as foci of hydrological processes, biogeochemical cycling, ecological disturbance, biodiversity, and wildlife activity. Recent observations of increased winter discharge, reduced peak flows, and increased connectivity between catchments, streams, and groundwater in subarctic riparian zones have prompted predictions of altered riverine disturbance regimes, increased channelization, and a decline in the extent of active floodplains. However, few observational data exist concerning the spatiotemporal dynamics of subarctic floodplain vegetation, which can serve as a bioindicator to corroborate such predictions. We analyzed the distribution and extent of riparian ecotypes across a network of streams in 12 Interior Alaska watersheds using high‐resolution image pairs from circa 1981–2010. All stream reaches encompassed pronounced elevational gradients and included elevational forest–tundra ecotones. We classified riparian ecotypes using an image‐based point‐intercept sampling approach, calculated the probability of ecotype transitions, and evaluated relationships between ecotype transitions and environmental covariates. Our results reveal widespread increases in the stature, density, and extent of riparian vegetation spanning gradients of elevation, floodplain morphology, and climate. Ecotype transitions occurred at >20% of sample points, and there was a strong imbalance toward forward successional transitions (16.5%) versus backward transitions (4.0%). That is, we observed a strong tendency toward increasing cover, stature, and density of vegetation communities across our extensive sampling domain across our approximately 30‐year sample period. This relatively rapid riparian “greening” signal tended to be most pronounced in our glaciated watersheds. Although the streams we studied displayed high local variability in ecotype transitions, our results support hypotheses of increasing channelization and reduced extent of unvegetated surfaces on subarctic floodplains. They also likely reflect a trend toward more rapid and extensive plant recruitment and growth due to processes associated with conspicuous warming in northern ecosystems, consistent with greening documented in other subarctic landscape segments along gradients of elevation and latitude.
... During the last two centuries, both sites were characterized by a decreasing resilience of black spruce to fire regimes, induced by regeneration accidents, as recently underligned byBaltzer et al., 2021. ...
Poster
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Representing 30% of the global forest, boreal biomes play a key role for human activities and climate regulation. Their dynamics are intrinsically linked to fire, developing mainly in mature stands of black spruce (Picea mariana (Mill.) B.S.P.). Recent studies project an increase of fire regimes in Canadian forests under climate change; however, eastern Quebec and Labrador remain poorly studied on a millennial timescale. The objective of our study istherefore to better characterize the mechanisms by which climate change affects disturbance regimes, and consequently forests structure and functioning in these regions. Our study is based on a north-south transect of holocene lacustrine sediment cores in eastern Quebec and Labrador, on which we study macrocharcoal, pollen grains and chironomid assemblages to reconstruct fire, vegetation and climate dynamics, respectively. Chronologies were based on 210Pb/137Cs and radiocarbon dating. On our study sites, the warmest and driest periods were characterized by frequent but small fires - depending on the amount of biomass available - favoring pioneer species such as jack pine (Pinus banksiana Lamb.). Conversely, wetter and colder periods show a decrease of fire occurrence, but an increase in their size due to a greater biomass accumulation. Black spruce has been the dominant species on most of the transect for the last 6,000 years. However, since the beginning of the industrial revolution - our study reveals an abrupt decrease in its abundance and an increase in pioneer taxa. In parallel, macrocharcoal analysis shows an increase in fire occurrence and size, leading to a decrease in black spruce populations resilience, and consequently to an increasing openness of forest ecosystems. In summary, our results suggest that recent and repeated large fires have already altered the vegetation composition and adaptation cycle of eastern Canadian boreal forests, and may radically alter the carbon sink function in the future.
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Future changes in climate are widely anticipated to increase fire frequency, particularly in boreal forests where extreme warming is expected to occur. Feedbacks between vegetation and fire may modify the direct effects of warming on fire activity and shape ecological responses to changing fire frequency. We investigate these interactions using extensive field data from the Boreal Shield of Saskatchewan, Canada, a region where >40% of the forest has burned in the past 30 years. We use geospatial and field data to assess the resistance and resilience of eight common vegetation states to frequent fire by quantifying the occurrence of short‐interval fires and their effect on recovery to a similar vegetation state. These empirical relationships are combined with data from published literature to parameterize a spatially explicit, state‐and‐transition simulation model of fire and forest succession. We use this model to ask if and how: (1) feedbacks between vegetation and wildfire may modify fire activity on the landscape and (2) more frequent fire may affect landscape forest composition and age structure. Both field and GIS data suggest the probability of fire is low in the initial decades after fire, supporting the hypothesis that fuel accumulation may exert a negative feedback on fire frequency. Field observations of pre‐ and post‐fire composition indicate that switches in forest state are more likely in conifer stands that burn at a young age, supporting the hypothesis that resilience is lower in immature stands. Stands dominated by deciduous trees or jack pine were generally resilient to fire, while mixed conifer and well‐drained spruce forests were less resilient. However, simulation modeling suggests increased fire activity may result in large changes in forest age structure and composition, despite the feedbacks between vegetation‐fire likely to occur with increased fire activity. This article is protected by copyright. All rights reserved.
Article
Increased fire frequency, extent and severity are expected to strongly affect the structure and function of boreal forest ecosystems. In this study, we examined 213 plots in boreal forests dominated by black spruce (Picea mariana) or jack pine (Pinus banksiana) of the Northwest Territories, Canada, after an unprecedentedly large area burned in 2014. Large fire size is associated with high fire intensity and severity, which would manifest as areas with deep burning of the soil organic layer (SOL). Our primary objectives were to estimate burn depth in these fires and then to characterise landscapes vulnerable to deep burning throughout this region. Here we quantify burn depth in black spruce stands using the position of adventitious roots within the soil column, and in jack pine stands using measurements of burned and unburned SOL depths. Using these estimates, we then evaluate how burn depth and the proportion of SOL combusted varies among forest type, ecozone, plot-level moisture and stand density. Our results suggest that most of the SOL was combusted in jack pine stands regardless of plot moisture class, but that black spruce forests experience complete combustion of the SOL only in dry and moderately well-drained landscape positions. The models and calibrations we present in this study should allow future research to more accurately estimate burn depth in Canadian boreal forests.
Chapter
Ecosystem theories must encompass the conclusions now emerging from studies of fire and vegetation in fire-dependent northern conifer forests. Such forests comprise more than half the present forest area of North America, including most of the forests that have never been altered through logging or land clearing. Furthermore, vast areas are still influenced by natural lightning-fire regimes, and it is possible to study directly the role of fire in controlling vegetation mosaics and ecosystem function.
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Complex nonparametric models-like neural networks, random forests, and support vector machines-are more common than ever in predictive analytics, especially when dealing with large observational databases that don't adhere to the strict assumptions imposed by traditional statistical techniques (e.g., multiple linear regression which assumes linearity, homoscedasticity, and normality). Unfortunately, it can be challenging to understand the results of such models and explain them to management. Partial dependence plots offer a simple solution. Partial dependence plots are lowdimensional graphical renderings of the prediction function so that the relationship between the outcome and predictors of interest can be more easily understood. These plots are especially useful in explaining the output from black box models. In this paper, we introduce pdp, a general R package for constructing partial dependence plots.
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Changes in climate and fire regimes are transforming the boreal forest, the world’s largest biome. Boreal North America recently experienced two years with large burned area: 2014 in the Northwest Territories and 2015 in Alaska. Here we use climate, lightning, fire and vegetation data sets to assess the mechanisms contributing to large fire years. We find that lightning ignitions have increased since 1975, and that the 2014 and 2015 events coincided with a record number of lightning ignitions and exceptionally high levels of burning near the northern treeline. Lightning ignition explained more than 55% of the interannual variability in burned area, and was correlated with temperature and precipitation, which are projected to increase by mid-century. The analysis shows that lightning drives interannual and long-term ignition and burned area dynamics in boreal North America, and implies future ignition increases may increase carbon loss while accelerating the northward expansion of boreal forest.
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Ecological memory is central to how ecosystems respond to disturbance and is maintained by two types of legacies – information and material. Species life-history traits represent an adaptive response to disturbance and are an information legacy; in contrast, the abiotic and biotic structures (such as seeds or nutrients) produced by single disturbance events are material legacies. Disturbance characteristics that support or maintain these legacies enhance ecological resilience and maintain a “safe operating space” for ecosystem recovery. However, legacies can be lost or diminished as disturbance regimes and environmental conditions change, generating a “resilience debt” that manifests only after the system is disturbed. Strong effects of ecological memory on post-disturbance dynamics imply that contingencies (effects that cannot be predicted with certainty) of individual disturbances, interactions among disturbances, and climate variability combine to affect ecosystem resilience. We illustrate these concepts and introduce a novel ecosystem resilience framework with examples of forest disturbances, primarily from North America. Identifying legacies that support resilience in a particular ecosystem can help scientists and resource managers anticipate when disturbances may trigger abrupt shifts in forest ecosystems, and when forests are likely to be resilient.
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Boreal forests and arctic tundra cover 33% of global land area and store an estimated 50% of total soil carbon. Because wildfire is a key driver of terrestrial carbon cycling, increasing fire activity in these ecosystems would likely have global implications. To anticipate potential spatiotemporal variability in fire-regime shifts, we modeled the spatially explicit 30-yr probability of fire occurrence as a function of climate and landscape features (i.e. vegetation and topography) across Alaska. Boosted regression tree (BRT) models captured the spatial distribution of fire across boreal forest and tundra ecoregions (AUC from 0.63-0.78 and Pearson correlations between predicted and observed data from 0.54-0.71), highlighting summer temperature and annual moisture availability as the most influential controls of historical fire regimes. Modeled fire-climate relationships revealed distinct thresholds to fire occurrence, with a nonlinear increase in the probability of fire above an average July temperature of 13.4°C and below an annual moisture availability (i.e. P-PET) of approximately 150 mm. To anticipate potential fire-regime responses to 21st-century climate change, we informed our BRTs with Coupled Model Intercomparison Project Phase 5 climate projections under the RCP 6.0 scenario. Based on these projected climatic changes alone (i.e. not accounting for potential changes in vegetation), our results suggest an increasing probability of wildfire in Alaskan boreal forest and tundra ecosystems, but of varying magnitude across space and throughout the 21st century. Regions with historically low flammability, including tundra and the forest-tundra boundary, are particularly vulnerable to climatically induced changes in fire activity, with up to a fourfold increase in the 30-yr probability of fire occurrence by 2100. Our results underscore the climatic potential for novel fire regimes to develop in these ecosystems, relative to the past 6000-35 000 yr, and spatial variability in the vulnerability of wildfire regimes and associated ecological processes to 21st-century climate change.
Chapter
The successional sequences described in this chapter were located in the central part of Alaska, in the general vicinity of Fairbanks, but the general trends and conclusions derived from our studies should have broader application to most of the North American taiga.
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Disturbance plays an important role in the distributional range of species by affecting their colonization potential and persistence. Short disturbance intervals have been linked to reduced seedbank sizes of some species, but the effects of long intervals are largely unknown. To explore the potential existence of seedbank sizes that may also be limited by long disturbance intervals, we studied an area in boreal eastern North America where time since fire (TSF) coincides with an increase in environmental stress (accumulating organic matter measured as depth of the soil organic layer (SOL)). Along a chronosequence dating back about 710 years, we counted the number of seeds cone-1 of black spruce (Picea mariana) and then estimated the number of seeds tree-1 and site-1 by upscaling. Younger sites [TSF 60–150 years] with mature first regeneration trees had average-sized seedbanks for black spruce [12.0–17.9 (105) seeds ha-1], whereas subsequent pulse trees that established in SOL depths greater than 35 cm showed highly reduced seed numbers. Sites with second- to fourth-regeneration pulse individuals [TSF c. 350–710 years] had exceptionally small seedbanks of 0.90 (105) and 0.46 (105) seeds ha-1, respectively. Radial tree growth rate showed a similarly negative response to SOL depth and could potentially be used as an indicator of seed output in plant species. Because the decline in seedbank size was possibly caused by more general environmental stress factors such as reduced nutrient availability, we suggest exploring whether other examples of ecosystems exist where long time since disturbance may lead to reduced seedbank sizes.
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Two determinants of fire behavior are fire weather and spatial variation in fuels. Their relative importance in boreal forests has been unclear. I evaluated the effect of fuels on a ∼74 000-km2 landscape in the boreal mixedwood region of western Canada. My data were the compositions, or the proportional areas of different forest types, of 48 mapped lightning fires and of their immediate surroundings. I measured areal compositions from forest inventory maps, using a five-way classification representing deciduous forest, three types of coniferous forest, and wetlands. The fires burned between 1980 and 1993. Fire sizes ranged from 70 ha to 70 000 ha. By multivariate linear regression, fire surroundings explain 57% of the variation in forest types within mapped fires. Fire compositions are not representative of the study area as a whole, or of a fire's surroundings, and are unrelated to fire size and location within the study area. Using the model, I predicted the areas of the five types burned within all oth...
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The circumpolar boreal zone is one of the world’s major biogeoclimatic zones, covering much of North America and Eurasia with forests, woodlands, wetlands, and lakes. It regulates climate, acts as a reservoir for biological and genetic diversity, plays a key role in biogeochemical cycles, and provides renewable resources, habitat, and recreational opportunities. Poor agreement exists amongst scientists regarding this zone’s delimitation and the areal extent of boreal forests, even though the zone has been well-studied. This paper reviews the literature on the phytogeography of the zone and makes use of a geographic information system (GIS) and published maps to delineate a current map of the North American boreal zone and the hemiboreal subzone, which is a transitional area lying immediately to the south of the boreal zone that is usually included in the boreal zone by Europeans but excluded by North Americans. On the basis of the map described here, the boreal zone covers about 627 million ha, or 29% of the North American continent north of Mexico. If the hemiboreal subzone, at 116 million ha, is included, then 34% of the same area is covered. Forests and other wooded land (362 million ha) cover 58% of the North American boreal zone on the basis of current forest inventory data. With forests and other wooded land of the hemiboreal subzone (68 million ha) factored in, this percentage remains basically unchanged. Values reported in this paper are compared with other published statistics. Important sources of error contributing to differences in areal statistics are discussed.
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1. The Standardised Major Axis Tests and Routines (SMATR) software provides tools for estimation and inference about allometric lines, currently widely used in ecology and evolution. 2. This paper describes some significant improvements to the functionality of the package, now available on R in smatr version 3. 3. New inclusions in the package include sma and ma functions that accept formula input and perform the key inference tasks; multiple comparisons; graphical methods for visualising data and checking (S)MA assumptions; robust (S)MA estimation and inference tools.
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Predicting plant community responses to changing environmental conditions is a key element of forecasting and mitigating the effects of global change. Disturbance can play an important role in these dynamics, by initiating cycles of secondary succession and generating opportunities for communities of long-lived organisms to reorganize in alternative configurations. This study used landscape-scale variations in environmental conditions, stand structure, and disturbance from an extreme fire year in Alaska to examine how these factors affected successional trajectories in boreal forests dominated by black spruce. Because fire intervals in interior Alaska are typically too short to allow relay succession, the initial cohorts of seedlings that recruit after fire largely determine future canopy composition. Consequently, in a dynamically stable landscape, postfire tree seedling composition should resemble that of the prefire forest stands, with little net change in tree composition after fire. Seedling recruitment data from 90 burned stands indicated that postfire establishment of black spruce was strongly linked to environmental conditions and was highest at sites that were moist and had high densities of prefire spruce. Although deciduous broadleaf trees were absent from most prefire stands, deciduous trees recruited from seed at many sites and were most abundant at sites where the fires burned severely, consuming much of the surface organic layer. Comparison of pre- and postfire tree composition in the burned stands indicated that the expected trajectory of black spruce self-replacement was typical only at moist sites that burned with low fire severity. At severely burned sites, deciduous trees dominated the postfire tree seedling community, suggesting these sites will follow alternative, deciduous-dominated trajectories of succession. Increases in the severity of boreal fires with climate warming may catalyze shifts to an increasingly deciduous-dominated landscape, substantially altering landscape dynamics and ecosystem services in this part of the boreal forest.