Patterns of vegetation change in Yukon: recent findings and future research in dynamic subarctic ecosystems
Abstract
In Yukon, Canada, average air temperature has increased by 2 °C over the past 50 years and, by the end of the century up to 6.9 °C of further warming is predicted, along with increased climate variability. As a result of these and other changes, vegetation communities are predicted to shift in space and composition. Changes to the vegetation assemblages across multiple ecological units or bioclimate zones will impact carbon and nutrient cycling, animal habitat, biodiversity levels, and other ecosystem processes. Yukon has a wide variety of vegetation communities, and paleoecological evidence indicates that significant vegetation changes have occurred throughout the territory in the past. No documented synthesis of changes to vegetation assemblages exists, restricting predictions of their future likelihood, abundance, and influence. Here, we review the literature of documented examples of vegetation change throughout Yukon that have occurred (i) in different vegetation communities due to the persistent press of climate change and (ii) after natural disturbances. Future research into all vegetation responses under ongoing climate change is warranted. We identify critical research gaps for each vegetation community and disturbance type that should be addressed to produce a more encompassing understanding of the response of Yukon bioclimate zones and vegetation communities to future warming and disturbances.
Résumé
Au Yukon, au Canada, la température moyenne de l’air a augmenté de 2 °C au cours des 50 dernières années et, d’ici la fin du siècle, on prévoit un réchauffement supplémentaire de 6,9 °C, ainsi qu’une variabilité climatique accrue. En raison de ces changements et d’autres, les communautés végétales devraient se déplacer dans l’espace et changer en matière de composition. Les changements apportés aux assemblages végétaux à travers plusieurs unités écologiques ou zones bioclimatiques auront une incidence sur le cycle du carbone et des nutriments, l’habitat animal, les niveaux de biodiversité et d’autres processus écosystémiques. Le Yukon possède une grande variété de communautés végétales, et les preuves paléoécologiques indiquent que des changements importants de la végétation se sont produits sur tout le territoire par le passé. Il n’existe aucune synthèse documentée des changements survenus dans les assemblages végétaux, ce qui limite les prédictions sur leur probabilité, leur abondance et leur influence futures. Les auteurs passent ici en revue la littérature qui comporte des exemples documentés de changements de la végétation à travers le Yukon qui se sont produits (i) dans différentes communautés végétales en raison de la pression persistante des changements climatiques et (ii) après des perturbations naturelles. Des recherches futures sur toutes les réponses de la végétation dans le cadre des changements climatiques en cours sont justifiées. Les auteurs identifient les lacunes critiques en matière de recherche pour chaque communauté végétale et chaque type de perturbation qui devraient être comblées afin de conduire à une compréhension plus globale de la réponse des zones bioclimatiques et des communautés végétales du Yukon au réchauffement et aux perturbations à venir.
1. Background
Arctic and boreal regions are being significantly impacted by climate change. Between 1971 and 2017, pan-Arctic average annual air temperatures rose by 2.7 °C, a change 2.4-times greater than the Northern Hemisphere’s average (Box et al. 2019). Other widespread changes include increasing trends in NDVI (or greening), attributed to increases in plant growth and productivity (Arndt et al. 2019), and a reduction in snow cover by 2–4 days per decade (Box et al. 2019). The increase in the number of snow-free days contributes further to summer warming (Chapin et al. 2005).
Changes in dominant vegetation patterns, as well as the processes that affect vegetation, are some of the most likely outcomes of climate change in northwest North America. Well-documented examples of vegetation change in the Arctic include widespread expansion of shrub communities (Tape et al. 2006) and reduced tree growth (McGuire et al. 2010) both of which are directly attributable to warming climates. Regarding processes, drought is strongly implicated in reduced tree growth (Walker et al. 2015) and Boreal fires have increased in intensity (Flannigan et al. 2009) and area (Coops et al. 2018) in recent decades, with further increases in fire activity predicted by the end of the century (Balshi et al. 2009). Earlier and warmer springs are likely to contribute to extreme fire years and to increased incidence of lightning at treelines (Veraverbeke et al. 2017). Future changes to northern wildfire regimes will have a myriad of biophysical implications (e.g., carbon sequestration, range expansion in plants, successional pathways; Veraverbeke et al. 2017; Baltzer et al. 2021).
In addition to widespread climate change typical of northwest North America (Streicker 2016), Yukon is experiencing major social and policy changes through implementation of modern-day treaties with Indigenous peoples and revision of predevolution legislation and policy (Natcher and Davis 2007; Staples et al. 2013). These processes are, for example, mandated to lead to regional land-use planning, forest management planning, and wetland policy development (UFA 1993). Now is an ideal time to integrate a synthesis of science on the emerging effects of climate change on vegetation into these institutional processes that are reorienting ecosystem management and biodiversity conservation (e.g., ecosystem mapping, forest harvesting, wildlife habitat management, fire management) under the treaty-mandated rubric of sustainable development (UFA 1993). These processes have yet to seriously deal with climate change, so knowledge of the patterns is crucial for good governance. Furthermore, changes in distribution of vegetation communities directly affect peoples’ mental and physical well-being (e.g., Dodd et al. 2018), so an appreciation of emerging changes can help with planning for and accommodating change when it occurs.
The goals of this review are to (i) synthesize recent (1980 to present ) documented patterns of change in vegetation communities throughout Yukon and surrounding areas, and (ii) highlight Yukon-specific knowledge gaps and topics for future research.
2. Geography and ecology of Yukon
Located in northwestern Canada, Yukon Territory has been the traditional homeland of First Nations peoples for over 10 000 years and currently supports a population of 42 000 First Nations and other peoples (Government of Yukon 2021). Along with Alaska and northern British Columbia, Yukon forms a substantial part (∼483 450 km2) of North America’s northern boreal mountains. In this sparsely populated region, many inhabitants rely on subsistence resources that are directly affected by climate-driven vegetation change.
Yukon has a subarctic continental climate, being relatively dry with temperature and precipitation patterns strongly dictated by major orographic barriers and the Pacific Ocean (Wahl 2004). Bound by the 60th parallel in the south and the Arctic Ocean to the north, annual mean temperatures vary from −2 °C in southern Yukon to −10 °C in coastal Arctic regions, with seasonal temperatures varying greatly from −60 °C in the St. Elias Mountains to >30 °C in the central interior. Annual precipitation ranges from ∼250 mm in valley bottoms up to 1000 mm in parts of the St. Elias Mountains (Wahl 2004). Over the past 50 years, the average annual air temperature in Yukon has risen by 2 °C (Streicker 2016); by the end of the 21st century, temperatures could rise by 3.9 °C–6.9 °C compared with the 1961–1990 average (SNAP-EWHALE 2012). In general, climate variability is predicted to increase, manifesting as increasing incidence and severity of wildfires, permafrost thaw and degradation, flooding, and forest insect infestations (Streicker 2016). Previous shifts in Yukon’s climate have resulted in significant changes to vegetation communities. For example, throughout the Holocene, there was conversion from balsam poplar forests to tundra (Cwynar and Spear 1991, 1995), and in the 19th century spruce stands in southern Yukon were more abundant than they are now (Strong 2017, 2020).
Vegetation composition in Yukon ranges from closed- to open-canopy forests in plateaus and valleys; shrub-dominated communities within open canopy forests, in disturbed areas, wetlands, and at treeline; sedge and cottongrass wet tundra on poorly drained sites; alpine tundra at high elevations; and grasslands mostly on steep, south-facing slopes. The dominant tree and shrub species in Yukon are white spruce (Picea glauca), black spruce (P. mariana), lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), balsam poplar (Populus balsamifera), trembling aspen (Populus tremuloides), birches (Betula spp.), and willows (Salix spp.) (McKenna et al. 2004). Two unusual features of Yukon vegetation communities are a high number of endemic species, and grasslands. Yukon has three national hotspots of endemicity (Ogilvie Mountains, Kluane, and Central Yukon Plateau), and ranks among the top locations for number of national (n = 43) and subnational (n = 20) endemic species (Enns et al. 2020). In Yukon, grasslands likely developed during the Holocene Thermal Maximum (∼11 000–9000 years ago; Strong 2018), with considerable species persistence and exchange as Beringian steppe evolved into Holocene grasslands (Vetter 2000). These grasslands provide particularly unusual habitats and act as refugia for species that would otherwise not be present in boreal regions (Sanborn 2010).
Biome shifts are projected based on future distributions of climate envelopes that have supported those communities in recent history (Rowland et al. 2016). Through to 2090, projections across Yukon include shifts from boreal forest to grasslands, Arctic tundra to shrublands and forests, and alpine tundra to forest, with the incursion of North American “prairie-type grasslands” representing a climate regime that has had no analog in recent history. In addition, many established forest types are projected to shift latitudinally and elevationally, resulting in communities that closely resemble current areas of Alberta, British Columbia, and southern Yukon (Rowland et al. 2016). As the climate continues to change at a rapid pace, vegetation composition across Yukon is likely to change. Changes to the location and composition of vegetation communities will have ecosystem-level implications and consequences, including changes in carbon cycling (Mack et al. 2004), litter decomposition (Hobbie 1996), distribution of preferred habitats (Gustine et al. 2014; Lantz 2017), and migration patterns (Gustine et al. 2014), among others. These ecosystem consequences affect numerous governance and stewardship decisions ranging from calculations of carbon budgets, locations of new protected areas, fire suppression regimes, translocations of threatened species, and mitigation measures in environmental impact assessments.
3. Approach
We conducted an integrative literature review, an in-depth nonsystematic approach to assess and synthesize literature (sensu Torraco 2005). The causes, prevalence, and consequences of recent (1980 to present) climate- and disturbance-driven community level vegetation change throughout Yukon have not yet been synthesized. This synthesis is therefore warranted given the substantial body of new science and lack of any integration to date. We focus our review on vegetation, not other life forms, as understanding changes to vegetation communities in response to climate change will be important for key decision-making processes. These include regional land-use planning for food security based on habitat suitability for ungulates and harvestable plants; quantifying the carbon storage and sequestering capacities of ecosystems as we develop nature-based solutions to excessive atmospheric carbon; planning for fire mitigation and suppression based on deciduous vs. coniferous forests; and projecting the distributions of species at risk for conservation attention.
In our review, we include peer-reviewed literature found through database searches (Scopus and Web of Science) and gray literature found via government databases and snowball reading. The review is in two sections. The first (Section 4.1.) concentrates on the persistent changes in growing conditions resulting from the chronic press of climate change (sensu Harris et al. 2018). The second (Section 4.2.) concentrates on the abrupt, or pulsed, changes (Harris et al. 2018) in vegetation composition that appear to emerge following disturbances and extreme events, some of which may be exacerbated by the changing climate regimes. These sections observe the following structure: standard conceptual view and general knowledge of the topic, Yukon-specific studies, and other related information from the northwest boreal. Finally, we identify gaps in our knowledge that future studies will ideally fill to provide a more comprehensive picture of the emerging future (Tables 2 and 3).
Current land classification divides Yukon into nine bioclimate zones, based on regional climate, vegetation, and soil development (Fig. 1 and Table 1; Environment Yukon 2016). To organize a discussion of vegetation change, we used the bioclimate zone framework, rather than the National Ecosystem Framework (ESWG 1995). The National Ecological Framework uses physiography integrated with climate to classify regions (Smith et al. 2004). Bioclimate zonation relies more directly on climate, as expressed by elevation and latitude, and a focus on vegetation composition, to classify regions. It is also the dominant scheme used for land cover classification and mapping in Yukon’s land planning and management (Environment Yukon 2016). Herein, we summarize studies based on the specific zones (capitalized names) where they occurred.
Fig. 1.
Table 1.
We focus our review mainly on field studies of woody vegetation dynamics (forests, shrublands, etc.) as there is a scarcity of information about vegetation change in the Subarctic Subalpine, Arctic Tundra Low Shrub, Subarctic Alpine Tundra, and Arctic Alpine Tundra bioclimate zones (but see Section 4.1.5. for patterns of change in the Arctic Tundra Dwarf Shrub bioclimate zone). We are not discounting the inherent value of these bioclimate zones or the changes within them, but without baseline data and research, an investigation into changes in these bioclimate zones is beyond the scope of this review. Furthermore, we do not address changes with the Pacific Maritime Glacierized bioclimate zone, which is dominated by ice and snow (Environment Yukon 2016), as we found no studies from this zone. This is an important gap in our understanding of the response of northern ecosystems to change as the St. Elias Mountains (located with the Pacific Maritime Glacierized bioclimate zone) have experienced some of the most significant environmental change in all of Yukon. We also did not specifically include remote sensing studies of vegetation change, except where they were used to support findings from field studies.
4. Vegetation change
4.1. Changing growing conditions and vegetation responses
4.1.1. Forests
Trees respond directly to temperature and moisture regimes, so chronic forest stress and mortality are predicted when global air temperatures rise and result in long-term and widespread drying (Boyer 1982; Allen et al. 2010). Many perennial plants, such as canopy trees and woody shrubs, have lifespans lasting into future periods with means and extremes in temperature and precipitation outside their historical experience. These plants may therefore need to make physiological adjustments to survive (Crous 2019). Forest mortality may be particularly noticeable in regions that experience cold winters because, as winter temperatures rise, physiological activity may continue after the growing season has ended (Allen et al. 2010). Together, warming and drought are already responsible for rising mortality rates of boreal trees in some parts of northwestern North America (Johnstone et al. 2010b; Porter and Pisaric 2011; Way et al. 2013), and general patterns of browning, or decreasing NDVI (Sulla-Menashe et al. 2018).
In continuous forests, white spruce showed consistent growth declines across both latitude and elevation, while other species were more variable in their responses (Miyamoto et al. 2010). Across Yukon and northern British Columbia, the response of lodgepole pine to temperature and precipitation patterns differed across latitudes (Miyamoto et al. 2010), a pattern previously documented across North America (Wheeler and Guries 1982; Xie and Ying 1995). In Yukon, radial growth of lodgepole pine responded positively to summer precipitation, suggesting that warm summer temperatures may induce similar moisture stress as seen in white spruce (Miyamoto et al. 2010). Lodgepole pine is also demonstrating continued northward range expansion at the current lodgepole pine—white spruce interface (Johnstone and Chapin 2003). Patterns of subalpine fir growth differed across elevations; radial growth of lower elevation populations was negatively correlated with spring temperatures, while higher elevation populations responded positively to summer temperatures (Miyamoto et al. 2010). Further north, Griesbauer and Green (2012) and Walker and Johnstone (2014) demonstrated that, in the Subarctic Woodland, both white and black spruce experience declines in growth related to seasonal increases in temperature and drought.
There is widespread evidence of reduced white spruce growth in Yukon due to increasing air temperatures and drought conditions throughout the Boreal Low and Boreal High (Supplementary data, Fig. S1; Hogg and Wein 2005; Zalatan and Gajewski 2005; Miyamoto et al. 2010; Griesbauer and Green 2012; Chavardès et al. 2013; but see alsoYoungblut and Luckman 2008). These negative relationships likely result from warming exceeding a physiological threshold of water availability; this could result in widespread die-off across the north (D’Arrigo et al. 2004; Way and Oren 2010). Drought stress has been demonstrated in white spruce stands in interior Alaska; if widespread across white spruce’s range, this pattern could disrupt carbon sequestration throughout boreal North America (Barber et al. 2000). Furthermore, long-term predictions indicate that white spruce’s optimal growth conditions are likely to become increasingly rare, resulting in decreased productivity and possibly a range contraction of white spruce to cooler and moister locations (Lloyd et al. 2013). Alternatively, if warming does not induce growth limitation through drought, cold-adapted tree species can grow larger at temperatures that exceed their current realized niche (Way and Oren 2010). Northward range expansion of northern tree species may therefore be enhanced by warming if seedlings can become established under these new conditions. The resulting combination of native species and northward shifting novel competitors could produce new combinations of species (Allen et al. 2010). Should the climate become wetter, white spruce may be outcompeted by black spruce in some landscape positions; conversely widespread permafrost thaw may lead to expansion of white spruce over black spruce (Nicklen et al. 2021). Overall, there is a strong need to understand how changing temperature and precipitation patterns, and their extreme events, are interacting to affect drought risk and survival of forest trees across a range of growing conditions (Table 2).
Table 2.
4.1.2. Treeline
While widely perceived as thermally limited, the Arctic and alpine boundaries of forests at treeline show complicated responses to elevated temperatures. This treeline ecotone is the transition from continuous forest to nonforested vegetation. Treelines form due to growth limitation, seedling mortality, and dieback of trees (Bader et al. 2020), and the ecotones encompass ecologically distinct range-edge populations of trees. Since global responses of treelines to climate warming are not uniform (Harsch et al. 2009), regional assessments of treeline dynamics are warranted.
In Yukon, treeline research thus far has predominately occurred at elevational treeline in the Boreal Subalpine in the Kluane region, a transition zone between white spruce forests (Boreal High/Subalpine) and shrub tundra (Boreal Alpine Tundra). Although under-represented in the literature, treeline also occurs throughout the Subarctic Woodlands (e.g., Goodwin 2019; Brown et al. 2019; Brehaut 2021) and Subarctic Subalpine, as both latitudinal and elevational treelines.
In the Boreal Subalpine, white spruce treelines are responding to global changes in three distinct ways, all of which are attributed to increasing air temperatures (Danby and Hik 2007a–2007c). First, over the past four decades, individual trees within the treeline ecotone have significantly increased their height and cover. Given the slow growth rate of trees at this elevation and latitude, significant growth of individuals is notable (Danby and Hik 2007a). Second, individuals are establishing beyond the previous range edge on south-facing slopes, indicating treeline advance if those individuals survive and successfully reproduce (Danby and Hik 2007c). Third, high occurrence of successful germination within the treeline ecotone has resulted in increased density of well-established stems on north-facing slopes (Danby and Hik 2007c).
Continued expanding distribution, increased growth, and increased density of trees in this ecotone are likely as air temperatures continue to warm, and other conditions (e.g., moisture regimes, levels of seed mortality) remain favorable for recruitment and growth (Danby and Hik 2007c). Asynchronous treeline change within a region is expected, as treeline responses to global change differ with aspect (Danby and Hik 2007c; Dearborn and Danby 2020): treelines on south-facing slopes are more likely to experience upslope treeline advance (Dearborn and Danby 2020), while seedlings on north-facing slopes are more likely to experience increased growth (Danby and Hik 2007b). Furthermore, current and future increases in air temperature may lead to drought stress in altitudinal and elevational treelines, as demonstrated in Alaska (Wilmking et al. 2004). The asynchronicity of change is likely to lead to variable biotic communities across space as other species shift in concert with or independently of trees.
Treelines throughout the circumarctic forest-tundra ecotone are often seed limited, hindering the ability of a treeline to successfully expand its range farther northward or upslope (Brown et al. 2019). In masting years, high seed production levels and dispersal may reduce reproductive constraints on treeline expansion (Kambo and Danby 2018a). Masting tends to occur the year after summer drought (Ascoli et al. 2019); mast-dependent expansion of treelines may therefore become more common if drought years increase, but only up to the point that masting becomes limited by nutrients available for tree growth and cone production.
Disturbances can act as catalysts for change when other drivers act slowly (Turner 2010). In ecotones such as the forest-tundra treeline, where colonization and growth are slow processes (Danby and Hik 2007a), small-scale disturbances creating microsites that are superior seedbeds may be an essential abiotic process in seedling recruitment and establishment. Still, a large number of suitable conditions (e.g., optimal soil temperature and moisture) must occur synchronously before this can happen (Kambo and Danby 2018b). Experimental fine-scale disturbance (i.e., manual scarification) at treeline contributes positively to treeline advance, serving as a proxy for natural fine-scale disturbances such as denning and digging by grizzly bears (Ursus arctos), burrowing by Arctic ground squirrels (Urocitellus parryii), trampling by caribou (Rangifer tarandus; Dufour Tremblay and Boudreau 2011), and cryoturbation (Kambo and Danby 2018b). The best understood of these disturbances is cryoturbation, which results in patches of bare mineral soil representing easy habitats for vegetation to colonize (Frost et al. 2013). However, climate change will likely reduce the frequency of cryoturbation (Aalto and Luoto 2014) and thus the presence of these optimal germination substrates. Furthermore, a gradual shift of trees toward higher latitudes would likely further reduce the prominence of cryoturbation (Hjort and Luoto 2009).
Studies outside Yukon indicate that treelines are not advancing as fast as climate conditions might allow, and that disturbance is an important catalyst in overcoming inertia. At the boreal-alpine treeline in the Kenai and Chugach Mountains in Alaska, treeline did not advance as fast as the climate envelope, but shrubs successfully colonized available upslope niche space faster than trees, suggesting that dispersal ability of propagules and competition between trees and shrubs are affecting forest advance (Dial et al. 2016). At the taiga-tundra ecotone in Northwest Territories, cold temperatures and limited dispersal ability of trees continue to limit treeline advance into the tundra, and fire appears to facilitate novel establishment of trees and shrubs (Lantz et al. 2019; Travers-Smith and Lantz 2020), likely by providing suitable microsites for germination and seedling recruitment, which are generally limiting in this region (Walker et al. 2012 ). At the forest-tundra ecotone in southwest Alaska, abundance of white spruce seedlings, saplings and mature trees were positively correlated with temperature (Miller et al. 2017). However, further north in the Brooks Range, radial growth of mature white spruce at treeline responded positively to temperature in the west but not in the east (close to Yukon) where trees appeared to be drought stressed (Wilmking and Juday 2005).
Treelines are diverse in landscape position and species composition, so require more attention such as in the poorly studied Subarctic Woodland to Subarctic Subalpine ecotone. Substantial treeline advance at the expense of alpine tundra is unlikely to be realized at rates projected by Rowland et al. (2016). Although empirical evidence shows that some changes are happening, treeline advance may be hindered by insufficient supply of viable seeds in treeline stands (Brown et al. 2019), poor dispersal of seeds, insufficient microsites for recruitment, and competition with expanding shrub growth in the same ecotone (see Section 4.1.4. on Shrub Ecosystems). Furthermore, soil types and genesis in some landscapes currently above treeline may not support robust tree growth. Yet fire may facilitate treeline advance in some settings. We are left with many uncertainties about dominant mechanisms in different regions, and the need for more widely distributed field studies and monitoring (Table 2).
4.1.3. Grasslands
Worldwide, grasslands are associated with conditions unsuitable for forest growth, which may include insufficient moisture, herbivory of seedlings, frequent disturbance such as fire, or insufficient depth of soil. In northwest North America, grasslands are interspersed within boreal forests, generally in well-drained locations on south-facing slopes, having been described in both southern Yukon and Alaska (Vetter 2000). Soils in Yukon grasslands have properties akin to grasslands in more southern latitudes (i.e., chernozems in prairies; Sanborn 2010), indicating these habitats have a long paleoecological history without forests and an enhanced ability for colonization by other grassland species. Although previously proposed to be relict communities of Beringian origin (a region that was largely unglaciated during Pleistocene glacial periods; Conway and Danby 2014), differences between current and Beringian climate moisture balances suggest that this is unlikely (Strong 2018).
Yukon grasslands are edaphically limited, and therefore both climate and topographic parameters (notably the mix of aspect and temperature, both of which affect moisture availability) strongly define their presence (Sanborn 2010). Increasing temperatures and changing precipitation patterns may therefore affect the size of grassland patches as shrubs and trees in bordering communities experience changing growing conditions. At the grassland–forest ecotone in the Kluane region of southern Yukon (Boreal Low), increased establishment of aspen but not white spruce has occurred at the ecotone’s leading edge. This ecotone’s tree age structure, including the marked absence of older and dead individuals at the grassland edge, indicates progressive forest encroachment into the grassland, likely due to a lengthening growing season and heightened moisture availability (Conway and Danby 2014).
Grasslands likely expanded to their recent distribution during past periods of drought (Swanson 2006). Drought is projected to increase in some portions of the Boreal Low by 2050, producing growing conditions in which grasslands could occur over a much wider range of topographic circumstances than they currently cover (Rowland et al. 2016). Aspen stands currently persist in the most drought-prone growing conditions of any forest type in Yukon, often adjacent to grasslands. For grasslands to expand, aspen dieback will likely occur. Dieback is likely to be primarily driven by increasing drought levels, as documented further south (Hogg et al. 2002, 2008). Fire may play a role, including repeated anthropogenic fires started by Indigenous peoples to create good grassland conditions for grazers in northern British Columbia (Leverkus 2015), and a hypothesized role for fire in the expansion of grasslands in the aspen parkland zone of the Canadian prairies (Schwarz and Wein 1990). However, we find no documentation of wild or anthropogenic fires influencing the spatial extent of grasslands in Yukon.
Yukon grasslands’ response to climate change is unlikely to be homogenous as precipitation differs by region and with longer-term climate patterns such as the Pacific Decadal Oscillation and El Niño (Whitfield et al. 2010). Although in southwest Yukon (the lower elevations of the Ruby Ranges ecoregion), evidence indicates that grasslands are contracting with incursion of aspen (Conway and Danby 2014), aspen stands adjacent to grasslands near Whitehorse demonstrate considerable aspen dieback (Author, P ers. O bs.). Projections by Rowland et al. (2016) indicate a conversion of forest to grasslands throughout considerable portions (Southern Lakes ecoregion) of the Boreal Low.
Losing grasslands to forests could result in a reduction of biodiversity, especially if rare grassland species are lost (see Vetter 2000 for detailed species information). Grassland community composition may transition into communities analogous to aspen parkland (along the prairie edge of the western boreal forest), depending on the diversity and functional traits of species currently present. Alternatively, if aspen understory species and grassland species coexist, a novel community may emerge. Future research directions are listed in Table 2.
4.1.4. Shrub ecosystems
Shrub communities across boreal and Arctic biomes have responded dramatically to new climate conditions, often with expanded cover and increased vertical growth (Mekonnen et al. 2021). Shrubs are perennial plants that range in height from <0.1 to ∼4 m and take on three distinct statures: tall multistemmed, erect dwarf, and prostrate dwarf (Myers-Smith et al. 2011a; Götmark et al. 2016). Globally, shrubs are important for ecosystem water balance, carbon uptake and storage, climate control, and soil stabilization, among other functions (Götmark et al. 2016). With climate change, the relative abundance and distribution of shrubs within alpine and tundra environments are predicted to shift (Myers-Smith et al. 2011a). Globally, shrubification (increases in shrub biomass, cover, and abundance) in Arctic and alpine regions occurs via three mechanisms: infilling (an increase in cover of existing individuals as well as recruitment of new individuals), increased individual growth (increased individual height or cover), and colonization beyond current/previous range limit (advancing shrubline) (Myers-Smith et al. 2011a). Plant structural changes are already occurring across circumarctic tundra ecosystems, most rapidly demonstrated by an overall increase in plant height. While shifts in dominant structural traits (e.g., plant height) will be influenced by warming, large-scale changes in these traits are not predicted to occur until novel competitors arrive from warmer, more southern environments (Bjorkman et al. 2018).
In Yukon, shrub research has mainly focused on the Boreal Subalpine, with some work occurring in the Boreal High and Arctic Tundra Low Shrub. In the Boreal Subalpine, Myers–Smith and Hik (2018) demonstrated that infilling of tall willows results from a combination of increased recruitment rates and reduced mortality rates. Patterns of shrub advance were uniform across site conditions such as slope and aspect, suggesting that a regional factor such as climate warming, rather than site-specific factors, was the primary factor responsible for observed changes (Myers-Smith and Hik 2018). Continued favorable conditions for shrub expansion in the Boreal Subalpine could result in continued advance of shrubline into alpine tundra, increasing cover by upward of 20% (Myers-Smith and Hik 2018). The influence of shrubs’ shifting distribution on the wider biotic community will depend on the spatial coverage that shrubs attain. When shrubs are sparse, their influence on abiotic processes (e.g., shading and snow trapping) is negligible. As cover reaches ∼50% of the landscape, shrub canopies can increase soil temperature in the winter and decrease it in the summer (Myers-Smith and Hik 2013). Abiotic changes such as soil temperature may lead to changes in ecosystem functions such as nutrient cycling, decomposition, and plant growth (Myers-Smith and Hik 2013; Livensperger et al. 2016; Sullivan et al. 2020). In the Boreal Subalpine, research has been limited to willows. We lack information on other prominent species, notably birch shrubs. Studies from neighboring regions may indicate how we expect other species to respond in the Boreal Subalpine (Lantz et al. 2010; Rinas et al. 2017; Brodie et al. 2019). Detailed understanding of responses to warming of all major shrub species is required because the most prominent effects of climate change (e.g., changes in snow accumulation and albedo, soil temperatures, and community floristics) are driven by the full shrub community.
In the Ruby Range (Boreal Subalpine to the Boreal Alpine Tundra ecotone), Weijers et al. (2018) found consistent climate sensitivity for two structurally different shrub species: Cassiope tetragona (an evergreen dwarf shrub often found in depressions with long-lasting snow cover), and Salix pulchra (a tall deciduous shrub widespread throughout the subalpine). Found on the same mountain slopes but in different habitats, similar responses in annual growth variability to early summer temperatures by both species suggest that climatic influences on plant growth are remarkably consistent across species (Weijers et al. 2018). Growth patterns of willow in the same region support these findings (Dearborn and Danby 2018).
Greening via shrubification is generally ascribed to tall, deciduous shrubs colonizing beyond their current limit (Elmendorf et al. 2012a, 2012b). However, greening may also occur via colonization by evergreen shrubs: the positive growth response of C. tetragona to early summer temperatures suggests that it may be able to expand its range and (or) cover within the Boreal Alpine Tundra with continued warming temperatures (Weijers et al. 2018). Dense mats formed by C. tetragona may prevent other shrubs from expanding in these locations (Weijers et al. 2018). The ecological consequences of expanding evergreen (compared with deciduous) shrubs will likely differ due to the evergreens’ slower decomposition and lower nutrient mineralization rates, relationships with different mycorrhizal fungi, and reduced influence on snow cover and soil temperature as a result of their lower stature (Vowles and Björk 2019).
Shrub communities in the forested Boreal High are often extensive, effectively forming the vegetation canopy. They often have multiple species with similar structures in the same stands (e.g., Betula glandulosa and Salix glauca). In the Kluane region (Boreal High), relationships between growth and climate variables differ by species: radial growth was negatively correlated with summer air temperatures in B. glandulosa, but positively correlated with summer drought and negatively correlated with precipitation in Shepherdia canadensis. This suggests that species-specific projections of climate responses over time are warranted (Grabowski 2015). The degree to which understory shrubs are shaded by canopy trees is likely to be a strong influence on shrub growth. Canopy trees are more climate and nutrient-sensitive than understory shrubs, suggesting that where they coexist (e.g., Boreal High and especially Boreal Low), trees in the canopy may provide a buffering effect to shrubs in the understory, protecting them from environmental fluctuations (Grabowski 2015).
Studies from the Arctic Tundra Dwarf Shrub (Herschel Island–Qikiqtaruk) demonstrate increased canopy cover and height of willow but no corresponding increase in radial growth (Myers-Smith et al. 2011b). Willows were also demonstrated to preferentially establish on bare ground rather than vegetated ground (Angers-Blondin et al. 2018). The pattern and rate of change of willow growth were not determined (i.e., whether pulsed or continuous), meaning that the future growth trajectory cannot be confidently predicted (Myers-Smith et al. 2011b). Recruitment of willow was impeded chemically and (or) physically by the community in situ (Angers-Blondin et al. 2018), suggesting that successful range expansion of willow will depend on overcoming the inherent inertia of the tundra community.
Alaskan studies may provide insight into responses of shrub communities in Yukon. For example, long-term monitoring in the western Brooks Range suggests that the successful establishment and dominance of tall shrubs beyond their historical range may be an ephemeral process in the absence of any form of disturbance (Terskaia et al. 2020). Furthermore, at upper elevations in the Chugach–St. Elias Mountains ecoregion, where montane forests (including black and white spruce, paper birch, trembling aspen, and balsam poplar) and tall shrubs (green alder (Alnus viridis) and willow) coexist, only tall shrubs were able to expand their range to higher elevations (Dial et al. 2016). Competitive range expansion by both shrubs and trees has not yet been explored in Yukon, although the presence of shrubs has been demonstrated to facilitate tree-seedling establishment and growth in the Boreal Subalpine (Kambo and Danby 2018c).
The shrub component of ecosystems is clearly changing in response to climate, especially at ecotones, and unlikely in concert with all other members of plant communities (Angers-Blondin et al. 2018), suggesting a variety of future research directions (Table 2).
4.1.5. Arctic tundra
Yukon’s Arctic tundra communities are diverse, ranging from drier prostrate shrub—herb landscapes, through various graminoids, to erect shrub types (Smith et al. 2004), yet they have not been well studied. In the Arctic Tundra Dwarf Shrub bioclimate zone, an early study (Kennedy et al. 2001) documented shifting abundances of nonshrub species, correlated with warming, in upland tundra on Herschel Island–Qikiqtaruk. The cover of polargrass (Arctagrostis latifolia) and Arctic lupine (Lupinus arcticus) increased over a 13-year period, mainly by expanding into cryogenic frost-boils and mud hummocks. The opportunity for this expansion is attributed to drying of the active layer of frozen ground, and consequent reduction in cryogenic disturbance (Kennedy et al. 2001). This work complements both the documented expansion of dwarf and willow shrubs (Section 4.1.4. above) and the changes driven by permafrost thaw slumping also at this site (Section 4.2.2. below). Becher et al. (2018) proposed reduced cryogenic soil disturbance as a general pathway for vegetation in-filling on Arctic tundra. More recent vegetation monitoring on Herschel Island has revealed continuation of some of these patterns in upland tundra, with a decadal doubling of shrub (especially deciduous) and graminoid abundance, coupled with substantial decreases in extent of bare ground. All functional groups, except lichens, showed increased cover (Myers-Smith et al. 2019). Factors driving these and other changes, such as increased shrub height, appeared to be increased length of the growing season and warmer autumn temperatures, with resultant increases in depth of the active layer (Myers-Smith et al. 2019).
Patterns in neighboring regions highlight the propensity for these communities to change. For example, over three decades in the Tuktoyaktuk Coastlands (Northwest Territories), shrubs proliferated at the expense of other terrain types (mainly tussock tundra and polygonal terrain) as a result of increased temperature and precipitation (Moffat et al. 2016). In Alaska, rapid climate change has reduced the fitness of Eriophorum vaginatum tussocks range-wide, thereby reducing their competitive success (Souther et al. 2014; Parker et al. 2017). The ongoing and diverse influences of climate warming on growing season length, temperature regimes, soil moisture, and active layer depth (among others) warrant further investigation in conjunction with studies of tundra vegetation response (Table 2).
4.2. Disturbance-induced changes to vegetation patterns
Large-scale disturbances, discrete events that alter community and population structures as well as substrates, resources, and the physical environment (Pickett and White 1985), take place over a relatively short period of time (e.g., hours to days) but have large influences on ecological heterogeneity. Disturbances can be biotic (pests or pathogens), abiotic (permafrost thaw and subsidence), or a combination of both (e.g., wildfire, which requires biotic components such as fuel in combination with suitable abiotic conditions, such as weather). The characteristics of disturbances (e.g., size, frequency, and severity) over an extended period of time are referred to as a disturbance regime (Turner 2010). Since many disturbances have a strong and significant climate forcing, disturbance regimes are likely to change with climate change. Understanding the ecosystem consequences from these changes is critical for future research (Turner 2010).
4.2.1. Fire
Climate-induced changes to the wildfire regime are significantly altering the distribution and composition of northern forests (Baltzer et al. 2021). Wildfire is the dominant disturbance throughout the boreal forest of North America and influences characteristics such as forest structure and carbon cycling (Bond-Lamberty et al. 2007; Beck et al. 2011; Walker et al. 2019). Three components of wildfire control post-disturbance regeneration, and all three are likely to be amplified with further warming. Fire frequency, the mean time between consecutive fires, varies across the dominant ecozones of Yukon (Boreal and Taiga Cordillera) from 439 to 709 years, respectively (Coops et al. 2018; Fig. 2A). There is further variation in fire risk within ecozones and therefore the demonstrated fire return interval varies from these means (Fig. 2A). Fire severity, the amount of aboveground and surface organic matter consumed by wildfire (Bonan and Shugart 1989), influences the quantity (or availability) of reproductive plant stages (seed and regenerative roots) available for regeneration and the seedbed quality. Fire intensity, the amount of energy (heat) released from fire (Bonan and Shugart 1989), influences the amount of viable seed available for reproduction when embryos within seeds cannot survive the heat from a fire.
Fig. 2.
A majority of postfire tree establishment occurs within 3–10 years after fire (Johnstone et al. 2004). Boreal forests have tended to experience cycles of self-replacement and, therefore, have been considered resilient to regular wildfires (Johnstone et al. 2010a). When fires are not severe enough to consume the entire organic soil layer, deciduous seeds from local trees demonstrate low recruitment (Johnstone and Chapin 2006a). In contrast, relatively large-seeded conifers can successfully recruit (Hesketh et al. 2009). However, the severity of fires throughout the Canadian boreal forest is predicted to increase throughout the 21st century (Wotton et al. 2017), a phenomenon already observed across northwestern North America, including Yukon (Kasischke and Turetsky 2006). Such changes to fire severity are likely to have significant regeneration implications.
As air temperatures in Yukon continue to rise, the number of lightning-ignited fires and the annual area burned are likely to continue to rise; in central Yukon, the number of fires per year may increase by up to 60% by 2039 (McCoy and Burn 2005). Furthermore, landscapes where fire has been suppressed by humans are increasingly susceptible to wildfire risk (Prince et al. 2018). Weather has exerted significant control over the amount of fuel (i.e., organic material) burned in the past, and future warming, especially extreme heating events, is predicted to increase risk, leading to shorter intervals of time between fires.
A majority of fire-vegetation research in Yukon has focused on the postfire regeneration patterns of black spruce, a semiserotinous conifer that releases most of its seeds after extreme heat, usually from wildfire. Black spruce is a dominant conifer throughout considerable portions of Yukon and is present in many different ecosystem types (McKenna et al. 2004). In Eagle Plains (Subarctic Woodland), fires that burned 14–15 years after the previous fire resulted in reduced recruitment of black spruce seedlings compared with fires that burned after longer time intervals; the parent trees were not reproductively mature when they burned the second time, severely depleting the seed bank (Brown and Johnstone 2012). Also in Eagle Plains, black spruce had a 50% chance of producing cones at 30 years old, increasing to 90% at 100 years (Viglas et al. 2013). Johnstone and Chapin (2006b) demonstrated similar patterns of low black spruce regeneration in young burned stands near Watson Lake (Boreal Low). Reduced recruitment, and possibly regeneration failure, leave opportunities for other species to colonize. Near Pelly Crossing (Boreal Low), black spruce stands that burned at a young age differed in their postfire understory community from stands that burned when mature (Johnstone 2006). Aspen regeneration was not affected by altered fire return intervals, suggesting that where conifers struggle to regenerate, aspen may successfully achieve dominance given sufficient survival or ingress of propagules such as roots and seeds (Johnstone and Chapin 2006b). In areas where alternate tree species, like aspen, are absent, regeneration failure of black spruce stands may result in postfire grass- or shrub-dominated communities (Brown and Johnstone 2012). The ecological repercussions of this magnitude of community shift are unknown yet are likely to have wide-reaching influence across different patterns and processes.
White spruce is also a dominant conifer through much of Yukon but, unlike black spruce, it has pulses of recruitment after masting events, which vary in local intensity (Lamontagne and Boutin 2007). After a fire, white spruce relies on dispersal of seeds from adjacent, unburned stands as its dominant regeneration strategy. When a masting seeder such as white spruce co-occurs with fire-adapted species such as black spruce and lodgepole pine, the masting species only experiences reproductive success if dispersal occurs within a short time frame after the fire, before the burned forest floor becomes too competitive for seedling establishment (as reviewed by Ascoli et al. 2019).
After an extensive fire in the Fox Lake area of the Boreal Low, a majority of sites switched from white spruce to aspen dominance. This shift was notable since, prefire, aspen was only present in ∼15% of stands and before this most recent fire, stands experienced cycles of white spruce self-replacement after disturbance (Johnstone et al. 2010b). Regeneration data from across the boreal forest indicate that at the time of this Fox Lake study (7–10 years after fire), a large majority of successful recruitment would have already taken place (Charron and Greene 2002; Gutsell and Johnson 2002; Johnstone et al. 2004; Lavoie and Sirois 1998), meaning species present at the time of the study were those expected to continue to dominate in the next succession cycle. The shift toward aspen dominance and a new vegetation community suggests the system's resilience threshold had been exceeded (Johnstone et al. 2010b).
Increased fire severity in the Boreal Low has reduced recruitment constraints in white spruce stands (i.e., thick layers of ground cover and organic soil), thereby increasing the number of potential establishment outcomes. Specifically, after high combustion, small-seeded deciduous species (e.g., aspen) have successfully established in high numbers (Johnstone and Chapin 2006a). Like black spruce stands, white spruce stands in the Boreal Low regenerate with different understories after short (40 years) and long (80–250 years) fire-free intervals. Instances of short fire return intervals tend to be dominated by woody shrub species whose regeneration strategy relies on resprouting from roots in mineral soil (e.g., aspen and willow). Conversely, sites with long fire return intervals are dominated by black spruce as well as species that regenerate via roots in the organic soil (e.g., Ribes hudsonianum, Chamerion angustifolium), and those found in mesic site conditions (e.g., Ceratodon-type moss), both of which are more typical of late succession stands (Johnstone 2006).
Throughout the Holocene, wildfires facilitated the expansion of lodgepole pine, a fully serotinous species, into existing spruce forests in eastern Yukon (Edwards et al. 2015; McKenna et al. 2004). At its current northern distributional limit, the proportion of lodgepole pine consistently increased, demonstrating strong evidence of nonequilibrium succession dynamics and a continuation of the species’ early Holocene range expansion (Johnstone and Chapin 2003). Expansion of lodgepole pine into regions previously dominated by black or white spruce will likely alter understory community composition. Even in low abundance, the shadows cast by white spruce play a dominant role in creating understory heterogeneity. Loss of these trees (to fire, increased dominance of lodgepole pine, or some other disturbance) will decrease understory heterogeneity (Strong 2011).
The potential future distributions of Yukon vegetation communities projected by Rowland et al. (2016) would entail shifts in forest canopy composition and structure within regions currently forested. Projected shifts included expansion of mixed boreal forests and aspen parklands from limited distribution in the south to cover the majority of southern and central Yukon, conversion of subarctic woodland to a variety of more closed-canopy boreal forests, and conversion of higher elevation forests in the south to closed canopy boreal forests with more southerly affinities (Rowland et al. 2016). To occur, such changes will require significant range expansion of lodgepole pine (and to a lesser extent, aspen and white spruce), plus shifts in the relative dominance of canopy species. Their likelihood would seem to depend on disturbances, particularly fires, because these provide the particular conditions for colonization (e.g., germination beds) and the opportunities for shifts in species dominance through succession. Substantial terrain variability in the Yukon and the variation of fire behavior at a landscape scale (e.g., with respect to aspect, and local winds) will add a further level of complexity to these patterns. The integrated effects of warming and higher precipitation (i.e., drought risk), plus extreme events, will affect the relative recruitment of tree species to the canopy through succession.
Fire significantly influences the composition of vegetation communities, and, with global warming, can drive substantive changes in the distribution of those communities, so further investigation is needed (Table 3).
Table 3.
4.2.2. Permafrost thaw
Increasing rates of permafrost thaw events throughout the Arctic play a significant role in the establishment and successional trajectories of vegetation communities. Permafrost (perennially frozen ground that remains at or below 0 °C for >2 years) underlies 10%–50% of land in southern Yukon and 90%–100% of land in northern Yukon (Smith et al. 2004). Globally, continued air temperature increases are predicted to lead to permafrost warming and thaw (Biskaborn et al. 2019) that may lead to large-scale collapses of ice-rich land surfaces (thermokarsts) with influences on both above- and below-surface processes. For example, in interior Alaska, widespread thawing of permafrost is predicted to lead to a range expansion of white spruce into landscape positions typically dominated by black spruce (Nicklen et al. 2021). Permafrost-elevation relations in Yukon are nonlinear and are significantly impacted by continentality: in areas with a strong maritime climate influence, the probability of permafrost increases with elevation, while in areas with continental climate influence, permafrost is commonly found in valley bottoms but less frequently at higher elevations (Bonnaventure et al. 2012).
In Yukon, studies of permafrost-vegetation relationships are uncommon (but see Price 1971). However, permafrost distribution, thermal patterns, and degradation are generally well explored through much of the territory, including ice-wedge polygon development, coastal erosion, and thaw slumping in the Arctic Tundra Low Shrub (Lantuit and Pollard 2008; Lantuit et al. 2012; Fritz et al. 2016); permafrost vulnerability in thermokarst lakes and the influence of ground-ice on lake shores in the Subarctic Woodland (Roy-Léveillée and Burn 2010, 2017); the influence of fire on permafrost degradation in the Boreal Low (Burn 1998); and permafrost mapping in the Boreal High and Boreal Subalpine (Lewkowicz and Ednie 2004) and regionally in southern Yukon (Bonnaventure et al. 2012).
In the Arctic Tundra Low Shrub, permafrost thaw occurs on a continuum of scales from 0.4 m2 (Wolter et al. 2016) to 24 400 m2 (Cray and Pollard 2015). At smaller scales, elongated ice wedges often delimit polygons on the tundra surface, called ice wedge polygons (IWPs). Widespread degradation of IWPs has occurred in recent decades, with vegetation changes (e.g., reduction in lichen and moss cover, and changes in the distribution of species) often the first sign of degradation. Intensified summer warming may promote ice-wedge degradation more rapidly than background-level climate change (Liljedahl et al. 2016). In the Arctic Tundra Low Shrub, draining IWPs may transition the vegetation toward greater shrub dominance with reduced vascular plant diversity, tipping the landscape’s equilibrium from circumneutral graminoids to acidic shrub tundra with possible effects for land surface properties (Wolter et al. 2016). Shifts in community composition rather than a decrease in overall community diversity are likely as different microhabitats and microtopographies still exist once these features drain (Wolter et al. 2016).
At larger scales on Arctic tundra, thaw characteristically results in retrogressive thaw slumps, where water-laden upper layers of soil slide downhill exposing bare mineral soil (Cray and Pollard 2015). These exposures are ideal microhabitats for germination of many functional groups. Willows and grasses rapidly establish, likely as they both produce high quantities of seed with high dispersal potential that can germinate on postdisturbance substrates and can survive with fluctuating soil moisture. In addition, willows can survive as a “vegetation island” of pre-existing tundra, moving from the slump headwall downslope as an intact vegetation unit (Cray and Pollard 2015). Differences between disturbed and undisturbed substrates as well as their associated vegetation communities likely remain for ∼250 years (Cray and Pollard 2015). As environmental conditions (e.g., ground thermal regime, slope, soil acidity, etc.) also continue to change, restructuring of vegetation communities and trajectories may be irreversible (Cray and Pollard 2015).
In the Subarctic Woodland, Lantz (2017) observed that catastrophic drainages of thermokarst lakes led to two distinct regeneration trajectories, dictated by moisture conditions. In wet areas, the vegetation became dominated by sedges, while in drier areas (which composed a majority of the drainage basin), tall willow shrubs dominated, reaching upward of two times the size of willows in undisturbed control sites, and often competitively excluding other species. Based on vegetation communities in older drained basins, the willow’s overpowering dominance is probably a seral stage that will later transition to communities of similar composition as those in older drained basins (e.g., dwarf shrub and tussock tundra). Transitioning to a community that is more representative of the current climate in the region (i.e., warmer than historical average) is also possible; however, it is not clear what that community would look like (Lantz 2017).
After retrogressive thaw slumps in the Boreal Low, vegetation propagules and surficial soil moisture are the two main determinants of community composition. Burn and Friele (1989) identified two distinct vegetation communities postdisturbance, separated based on their distance from the slump headwall and, therefore, meltwater supply. Closer to the headwall and in areas of higher soil moisture, willows and horsetails (Equisetum spp.) dominated. In drier areas further away from the headwall, the predisturbance forest community started to establish with fast-growing and wind-dispersed herbs establishing first, followed by tree saplings 10–15 years postdisturbance; re-establishment of the original community was predicted to begin 35–50 years after disturbance (Burn and Friele 1989). Similarly, Bartleman et al. (2001) predicted that the transition from shrub birch to spruce forests would occur once the surface began to dry out (i.e., with increasing time since disturbance). Longer-term monitoring of sites such as these would be valuable for a detailed understanding of the recovery trajectories.
Under continued climate change, permafrost becomes increasingly vulnerable to degradation by fire; thermal changes initiated by fire can cause surface subsidence and the development of thermokarst features in boreal and tundra landscapes (as reviewed by Holloway et al. 2020). In the last century, permafrost demonstrated resilience to fire, recovering after several decades in most situations. However, a combination of year-round warming and more frequent and severe fires will likely cause slower or no recovery of permafrost to its prefire state (Holloway et al. 2020). At its southern extent, permafrost tends to be thermally protected by forest cover. In these regions, fires that cause shifts in the dominant vegetation patterns (such as shifts from conifer to deciduous dominated forests) act as a destabilizing influence, and the permafrost is likely to degrade entirely (Jafarov et al. 2013; Holloway et al. 2020). For example, discontinuous permafrost in the Boreal Low maintained its integrity under unburned white spruce forest but started an ongoing process of progressive degradation in the burned areas, which were regenerating primarily as deciduous species (Burn 1998). Modelling efforts from Alaska suggest that when the postfire organic layer is <30 cm thick, permafrost is increasingly vulnerable to disturbance (Jafarov et al. 2013). When combined with the trend toward more severe wildfires completely combusting the organic layer, this suggests that there could be widespread degradation of boreal forest permafrost. The frequency of permafrost failures triggered by fire (e.g., active layer detachments and permafrost-related landslides) is predicted to increase over time in central Yukon (Boreal Low; Lipovsky and Huscroft 2006).
The current increasing rates of permafrost thaw in the boreal and Arctic are unprecedented and leave many unanswered questions as to how vegetation communities will respond and develop after colonization (Table 3).
4.2.3. Insects and pathogens
Insects and pathogens that consume trees in the forest canopy (or other characterizing forest components) can change forest composition and structure over large areas (Forest Management Branch 2020). In this regard, they can be thought of as agents of natural disturbance, much like wildfire. While insects and pathogens are currently limited in their spatial influence in Yukon, and thus poorly studied, the declining vigour of trees in many landscapes may increase the possibility that they become more significant agents of change. Consequently, we do not review all insects and pathogens present; here, we focus on the bark beetles (Dendroctonus spp.) because they have, or could have, by way of their outbreak dynamics, the most widespread influence on Yukon’s forests and the trajectories of future vegetation communities. In our recommendations for future work, we suggest locations where some of the other insects require attention because they could also be prominent agents of change in vegetation.
Spruce bark beetle (Dendroctonus rufipennis) is perhaps the most studied forest insect in Yukon, having killed over 400 000 ha of white spruce in southwest Yukon since an outbreak that began in 1990. Spruce bark beetle is present throughout the range of white spruce and generally occurs at low densities killing individual trees sporadically. As a result of drought stress, trees emit specific chemical(s), beetles respond to the chemical cues and attack the stressed trees, and the stressed trees have limited resources with which to attack the beetle, causing most trees to be killed, including healthy, seed-bearing trees (Garbutt et al. 2006). The beetle attacks semimature to mature trees, with a preference for larger individuals that can least defend themselves. Cold winter temperatures limit overwinter beetle survival, and beetle densities can increase rapidly and irrupt following unusually warm winters. These irruptions or outbreaks are responsible for the most intensive changes to mature spruce forests of any insect in Yukon (Government of Yukon 2019; Forest Management Branch 2020).
Spruce bark beetle outbreaks have been detected in Yukon since the 1930s, generally in years of above-average temperatures (Forest Management Branch 2020). Compared with infestations in Alaska (the Kenai Peninsula), Kluane does not have a long history of spruce bark beetle infestations, likely due to its lower winter temperatures and different wildfire regime (Berg et al. 2006). In the late 1980s, warm temperatures in the Boreal Low, Boreal High, and Boreal Subalpine (mostly within the Ruby Range ecoregion) caused drought stress in white spruce stands and increased overwinter survival of beetles (Garbutt et al. 2006; Forest Management Branch 2020). Following this, a 12-year beetle outbreak occurred between 1994 and 2006 (unprecedented in its spatial scale). In the Boreal Subalpine, larger trees and trees at relatively lower elevations were more susceptible to attack (Mazzocato 2015). Larger trees were also selected for by beetles in lower elevation bioclimate zones (Boreal High, Boreal Low; Garbutt et al. 2006). In Yukon, infestations have only been actively studied in association with the 1990s Kluane region outbreak (Berg et al. 2006; Garbutt et al. 2006; Randall et al. 2011; Chavardés et al. 2012; Hawkes et al. 2014; Mazzocato 2015; Paudel et al. 2015; Campbell et al. 2019).
White spruce continues to be the dominant tree in the postbeetle outbreak forest, albeit with a different age class distribution. In most cases, the growth trajectory of saplings establishing after the disturbance determines forest recovery (Campbell et al. 2019). In some landscapes (notably in the Boreal High), young trees survived the outbreak and quickly became dominant within the canopy. Repeated site surveys show that most white spruce seedlings/saplings established 6–14 years after the outbreak started (Hawkes et al. 2014). Patterns of increasing summer and winter temperatures already well established in Yukon (Streicker 2016) will probably increase the likelihood of drought stress to spruce and the overwinter survival of spruce bark beetles, possibly leading to more frequent and widespread outbreaks (Forest Management Branch 2020).
Mountain Pine Beetle (Dendroctonus ponderosae) is currently not present in Yukon, but is found in the Liard Basin of British Columbia, within 20 km of the Yukon border. (Forest Management Branch 2020). Mountain Pine Beetle is considered the most extensive forest health concern in western Canada and can turn forests from carbon sinks to large carbon sources (Kurz et al. 2008; Forest Management Branch 2020). Northward migration of the mountain pine beetle from BC (likely the Rocky Mountain Trench) into Yukon [likely the Liard Basin (Boreal Low)] is possible (Forest Management Branch 2020). Mountain pine beetles attack and kill mature and old stands of lodgepole pine, placing ecosystem values associated with these old-growth stands at risk, notably caribou winter range (Cichowski and Williston 2005). However, depending on the species composition and disturbance history of stands before beetle outbreak (i.e., fire and (or) fire-suppression impacts), stands can have divergent postbeetle recovery trajectories (Axelson et al. 2009), which might include novel forest types in particular landscapes.
The Yukon Forest Management Branch maintains an annual monitoring regime for forest health, with a priority focus on mapping the stand- and landscape-scale distributions of (i) eight insects or groups of insects that consume canopy trees, (ii) a rust-induced disease, and (iii) aspen dieback due to drought (Forest Management Branch 2020). In addition to monitoring initiated by the Forest Management Branch, we recommend future monitoring and research of a variety of pests across bioclimate zones (Table 3).
5. Major findings
Despite dramatic-projected changes in climate envelopes and the forest and vegetation types they support, there is relatively little published evidence of changes so far, reflecting (i) too little on-the-ground attention toward ecosystem changes; and (ii) the strong inertia of many forests in the face of the chronic press of incremental change until a disturbance event can induce a relatively abrupt shift in existing or emerging canopies. Trends in some climate parameters are prominent, but there is high variability between years and within bioclimate zones. Furthermore, changes in vegetation assemblages in response to climatic trends and disturbances vary subregionally. Findings synthesized here are likely relevant to similar communities in the Northwest Territories and interior Alaska.
We have identified knowledge gaps in the response of different vegetation assemblages to persistent trends in climate parameters and to disturbances influenced by climate parameters (Tables 2 and 3). Most documented changes have taken place in the Boreal Low bioclimate zone (Table 1), suggesting it needs ongoing attention. However, the Subarctic Subalpine and Arctic Tundra Dwarf Shrub zones have received little or no attention, suggesting another set of priorities. Ongoing research in Yukon is beginning to address these questions.
Two monitoring approaches — interpretation of remotely sensed imagery and permanent ground plots sampling stand composition — would ideally be systematically expanded and implemented to start to fill many knowledge gaps. These approaches work best in combination, with on-the-ground field plots providing training data, and validation data, in the development of interpretative and projection models that rely on remotely sensed inputs. Ideally, they would be designed to test hypothesized changes, and will require hypothesis-specific protocols and analyses that can be automated and replicated (in time and space).
Early investigation into changing ecological communities throughout Yukon will enable more proactive management and conservation policies and practices. It is our hope that the research gaps identified in this paper can help guide future research within Yukon and elsewhere in the North as rapid climate change threatens to change ecosystems, habitats, and subsistence lifestyles. Identifying these research gaps is timely as northern science and conservation are in a period of change. For example, the newly created Yukon University, Canada’s First Northern University, emphasizes northern science and resource management in the context of equitable partnerships with First Nations; and the Yukon Government Science Strategy is planned for re-evaluation by 2025, with goals of establishing long-term science capacity and approach throughout Yukon and supporting evidence-based decision making through gathering and storing scientific and Indigenous Knowledge (Qatalyst Research Group 2020).
Acknowledgements
The authors thank Wayne Strong, Jill Johnstone, and John Meikle for valuable discussions early in the study, and members of the Northern EDGE Lab at Memorial University for their helpful feedback throughout the writing process.
Author contributions
All the authors contributed to the design of the study; K.A.R. completed the literature review and wrote the first draft of the manuscript. All of the authors contributed to further drafts and approved the final version of the manuscript.
Funding Information
This research was supported by Yukon Government, Canadian Mountain Network (University of Alberta), Mitacs through the Mitacs Accelerate Program, the Wilburforce Foundation, and the Weston Family Foundation.
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Published online: 27 July 2022
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© 2022 The Author(s). This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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This research was supported by Yukon Government, Canadian Mountain Network (University of Alberta), Mitacs through the Mitacs Accelerate Program, the Wilburforce Foundation, and the Weston Family Foundation.
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Kirsten A. Reid, Donald G. Reid, and Carissa D. Brown. Patterns of vegetation change in Yukon: recent findings and future research in dynamic subarctic ecosystems. Environmental Reviews.
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https://doi.org/10.1139/er-2021-0110
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