Southwest Yukon Research Articles

Earthquake Relocations Delineate a Discrete Fault Network and Deformation Corridor Throughout Southeast Alaska and Southwest Yukon

AbstractDeformation in southeastern Alaska and southwest Yukon is governed by the subduction and translation of the Pacific‐Yakutat plates relative to the North American plate in the St. Elias region. Despite notable historical seismicity and major regional faults, studies of the region between the Fairweather and Denali faults are complicated by glacial coverage and the remote setting. In the last decade, significant improvements have been made to the density of regional broadband seismometer networks. We relocate more than 5,000 earthquakes between 2010 and 2021 in the region of southeastern Alaska and southwestern Yukon utilizing these improved seismic networks. With reductions in catalog uncertainty, particularly in depth, we quantify the thickness of the seismogenic layer in the crust throughout the region and locate seismicity on a shallow network of upper‐crustal faults. Relocated earthquakes, combined with an updated focal‐mechanism catalog, permit estimating and classifying motion of active faults. This includes mapping the Totschunda‐Fairweather “Connector” fault, which plays an important role in explaining regional deformation, and identifying new faults like the Kathleen Lake fault. We draw similarities between our seismic observations and simplified conceptual models of regional tectonics, which describe a dominant transpressional regime and localized slip partitioning. Our results support a hypothesis where current deformation is taking place on a well‐defined and evolved network of shallow faults in the corridor between the Totschunda‐Fairweather “Connector” and Denali faults.

Tectono-Metamorphic Evolution of the Cretaceous Kluane Schist, Southwest Yukon

Abstract A wealth of information regarding the Mesozoic evolution of the Northern Canadian and Alaskan Cordillera is held within a series of variably metamorphosed and deformed rocks that formed in Jura-Cretaceous basins. Located at the interface between the pericratonic Intermontane and exotic Insular terranes, these basinal rocks are key to understanding the timing and tectonic style of Insular terrane accretion, a topic of longstanding debate. This study unravels the structural and metamorphic evolution of one of these basins, the Kluane Basin, within southwest Yukon Territory. The Kluane Schist is the primary assemblage of the Kluane Basin. It consists of metamorphosed and deformed low-Al pelites that were intruded by granodioritic plutons of the Paleocene Ruby Range batholith. Previous workers have suggested the variable metamorphic character of the Kluane Schist represents an extensive and static thermal aureole related to Ruby Range batholith emplacement. Our work, however, indicates that the Kluane Schist experienced Buchan-style metamorphism coeval with protracted deformation and can be divided into seven distinct petrologic zones, which, based on their unique combination of mineral assemblage and structure, are incompatible with static thermal metamorphism. Instead, we propose the Kluane Schist experienced two distinct metamorphic phases: (1) an early greenschist-facies phase that resulted in the development of a bedding-parallel chlorite-muscovite-titanite fabric, preserved by its lowest-grade units, and (2) a later amphibolite-facies phase that manifests as the progressive transposition of the earlier chlorite-muscovite-titanite fabric into a penetrative biotite-rich schistosity that transitions upgrade into a segregated gneissic fabric comprised of biotite-cordierite and plagioclase-quartz (± sillimanite-K-feldspar-melt). By integrating the results of detailed petrography and petrological modeling, we demonstrate that the second main metamorphic phase experienced by the Kluane Schist preserves a record of pressures and temperatures that align with other Buchan-style terranes worldwide. Our data defines a field gradient across the Kluane Schist ranging from 3.0–3.5 kbar at 375–400 °C to 4–4.5 kbar at 700–750 °C. This record of a coupled Buchan-style metamorphic-deformational evolution and tops-to-the SW non-coaxial shear structures is consistent with the override of the thermally mature Yukon-Tanana terrane as the principal driver of Kluane Schist metamorphism, with some limited heat likely contributed by the late-syn- to post-tectonic intrusion of the Ruby Range batholith.

A record of Late Cretaceous to early Paleogene Insular terrane accretion within the northern Cordillera: Insights from monazite petrochronology across the Kluane Schist, southwest Yukon, Canada

The Kluane Schist is a metamorphosed package of siliciclastic and lesser calcareous rocks that lies between the inboard pericratonic Intermontane terranes and outboard Insular terranes of the North American Cordillera within Yukon, Canada. The metamorphic sequence of the Kluane Schist preserves a record of the tectono-thermal evolution and timing of Insular terrane accretion. Here we document the timing of metamorphism and deformation across the Kluane Schist using in situ laser ablation−inductively coupled plasma−mass spectrometry U-Th-Pb monazite petrochronology. Monazite-bearing samples collected across an inverted metamorphic sequence preserved in the northern regions of the Kluane Schist yield dates ranging from ca. 70 Ma to 55 Ma. Complementary phase equilibria modeling and thin section analysis indicate monazite grew between ∼450 °C and 3.0−3.5 kbar to ∼700−715 °C and 4.0−4.5 kbar, coeval with the development of the Kluane Schist’s inverted metamorphic sequence. Dating the four chemical zones preserved by monazite demonstrates its protracted growth during three distinct periods of garnet crystallization and breakdown, as well as coeval with melt generation. Our data illustrate that peak metamorphic conditions were reached at progressively younger ages with decreasing structural level within the Kluane Schist. Our results are consistent with Buchan-style metamorphism associated with the terminal accretion of the outboard Insular terranes and southwest-directed overriding of the inboard Yukon-Tanana terrane from ca. 70 Ma to 55 Ma. These findings are further congruous with a Late Cretaceous timing for the terminal accretion of the Insular terranes within southwest Yukon, facilitated by east-dipping subduction beneath a westward migrating North American continent.

How many worlds are there?

ABSTRACTQuestions about the nature of reality have lately become something of a preoccupation in anthropology. One prominent approach to such questions holds that different peoples inhabit distinct and incommensurable realities, or worlds. Although proponents of the “multiple‐worlds thesis” claim to be decolonizing social theory, their approach is beset by significant political and theoretical problems. I illustrate these problems by referring to my own ethnographic research in Canada's southwest Yukon. I then suggest an alternative approach: shifting the focus from multiplicity to indeterminacy. The indeterminacy framework can help us avoid the pitfalls of the multiple‐worlds thesis while leaving open the possibility that radically different understandings of the world might reveal something important about the nature of reality. Indeterminacy thus supports a more robust anti‐colonial politics than does the multiple‐worlds thesis. [ontology,indeterminacy,practice,indigeneity,Yukon]

Dating fault damage along the eastern Denali fault zone with hematite (U-Th)/He thermochronometry

Unraveling complex slip histories in fault damage zones to understand relations among deformation, hydrothermal alteration, and surface uplift remains a challenge. The dextral eastern Denali fault zone (EDFZ; southwest Yukon, Canada) bounds the Kluane Ranges and hosts a variety of fault-related rocks, including hematite fault surfaces, which have been exhumed through the brittle regime over a protracted period of geologic time. Scanning electron microscopy-based microtextural observations and hematite (U-Th)/He (hematite He) thermochronometry from these surfaces indicate multiple generations of foliated, high-aspect ratio hematite plates. Single-aliquot hematite He dates (n=38) from 11 samples range from 11.5 ± 3.2 Ma (2σ) to 3.4 ± 2.1 Ma and exhibit moderate inter- and intrasample dispersion. A subset of dates is 15-20 Myr younger than previously published apatite (U-Th)/He dates from collocated host rocks, despite similar closure temperatures, precluding a simple ambient cooling interpretation for our hematite He data. Mixture modeling defines hematite He date populations at ∼8 Ma, ∼6 Ma, and ∼4 Ma, and when combined with microtextural observations, supports episodes of hydrothermal alteration and fault reactivation at aseismic to subseismic slip rates. There is no evidence that hematite experienced deformation- or hydrothermal fluid-related He loss. Hematite He dates overlap previously documented Kluane Ranges surface uplift and shifting dynamics of the Yakutat microplate, pinpointing fault networks and deformation processes that accommodate regional deformation in response to far-field plate boundary processes.

Late Cretaceous to Paleocene Tectonometamorphic Evolution of the Blanchard River Assemblage, Southwest Yukon: New Insight into the Terminal Accretion of Insular Terranes in the Northern Cordillera

Abstract The Intermontane-Insular terrane boundary stretches over 2000 kilometers from British Columbia to Alaska in the western Cordillera. Juxtaposed between these terranes is a series of Jura-Cretaceous basinal and arc assemblages that record a complicated and contested tectonic evolution related to the Mesozoic-Paleocene accretionary history of northwestern North America. In southwest Yukon, west-verging thrust faults facilitated structural stacking of the Yukon-Tanana terrane over these basinal assemblages, including the Early Cretaceous Blanchard River assemblage. These previously undated compressional structures are thought to be related to the final collapse of the Jura-Cretaceous basins and the tectonic burial of the Blanchard River assemblage resulting in amphibolite facies metamorphism. New in situ U-Th-Pb monazite ages record at least three tectonic events: (1) the tectonic burial of the Blanchard River assemblage to amphibolite facies conditions between 83 and 76 Ma; (2) peak burial was followed by regional exhumation at ca. 70-68 Ma; and (3) intense heating and ca. 63-61 Ma low-pressure contact metamorphism attributed to the intrusion of the voluminous Ruby Range suite, which is part of the northern Coast Mountains batholith. The tectonometamorphic evolution recorded in the Blanchard River assemblage can be correlated to tectonism within southwest Yukon and along the length of the Insular-Intermontane boundary from western British Columbia through southwestern Yukon and Alaska. In southwest Yukon, these results suggest an asymmetric final collapse of Jura-Cretaceous basins during the Late Cretaceous, which relates to the terminal accretion of the Insular terranes as they moved northward.

Evidence for Large Holocene Earthquakes along the Denali Fault in Southwest Yukon, Canada

ABSTRACTThe Yukon–Alaska Highway corridor in southern Yukon is subject to geohazards ranging from landslides to floods and earthquakes on faults in the St. Elias Mountains and Shakwak Valley. Here we discuss the late Holocene seismic history of the Denali fault, located at the eastern front of the St. Elias Mountains and one of only a few known seismically active terrestrial faults in Canada. Holocene faulting is indicated by scarps and mounds on late Pleistocene drift and by tectonically deformed Pleistocene and Holocene sediments. Previous work on trenches excavated against the fault scarp near the Duke River reveals paleoseismic sediment disturbance dated to ∼300–1,200, 1,200–1,900, and 3,000 years ago. Re-excavation of the trenches indicates a fourth event dated to 6,000 years ago. The trenches are interpreted to show a negative flower structure produced by extension of sediments by dextral strike-slip fault movement. Nearby Crescent Lake is ponded against the fault scarp. Sediment cores reveal four abrupt sediment and diatom changes reflecting seismic shaking at ∼1,200–1,900, 1,900–5,900, 5,900–6,200, and 6,500–6,800 years ago. At the Duke River, the fault offsets sediments, including two White River tephra layers (∼1,900 and 1,200 years old). Late Pleistocene outwash gravel and overlying Holocene aeolian sediments show in cross section a positive flower structure indicative of post-glacial contraction of the sediments by dextral strike-slip movement. Based on the number of events reflecting ∼M6, we estimate the average recurrence of large earthquakes on the Yukon part of the Denali fault to be about 1,300 years in the past 6,500–6,800 years.

Evaluation of Maternal Penning to Improve Calf Survival in the Chisana Caribou Herd

Evaluation of Maternal Penning to Improve Calf Survival in the Chisana Caribou Herd

Picea (spruce) growth rate (mm year−1) changes in southwest Yukon (Canada) since the mid 19th century

Analyses of maximum early tree ring growth within a regional curve standardization (RCS) framework were used to construct ring width chronologies in mm year−1 to estimate historical growth rates in southwest Yukon (Canada), without the necessity of correcting for age related changes. The analysis relied on the use of the 11th–50th year growth rings to standardize chronology comparisons. This age range represents the period of peak radial growth for Picea albertiana and P. glauca (white spruces), which are common trees on upland boreal sites in the study area. Of 1,460 available Picea ring series, 1,029 fit the criteria for inclusion in the study and they represented five commonly occurring RCS curve types. When converted to chronologies, all curve types had year-to-year fluctuations in average ring widths (RW11–50), but nine of the 12 RCS types (four types in three study areas) also had long-term growth rate increases of 15–132% between ad 1850 and 2000, based on regression modeling (R/r = 0.54–0.76, p 0.94, n > 100) with conventional tree ring width index values derived from multiple RCS curves. This indicates that the two chronology construction approaches have very similar temporal patterns despite the maximum early growth analysis technique representing ring widths in mm.

Evolution of the 2014 Vulcan Creek landslide-dammed lake, Yukon, Canada, using field and remote survey techniques

The Vulcan Creek landslide is a 1 to 2 × 106 m3 landslide that occurred in southwest Yukon between 15 August and 9 September 2014. The elevation difference between the landslide headscarp and distal extent of the deposit is 370 m and the runout distance is approximately 1.2 km, which corresponds to a farboschung angle of 17°. The material in the headscarp of the landslide consists of highly fractured and disturbed rock mass, due to the presence of thrust faults near the headscarp and toe of the landslide. Overall, the total volume of the landslide is composed of approximately 50% surficial material (colluvium and till) mobilized from the middle and lower slope sections, and 50% of fractured bedrock from the middle and upper slope sections. No distinct trigger was identified for this landslide; however, contributing factors include the long-term seismicity of the area, 10 °C warmer than normal air temperature in January 2014, and approximately twice the normal rainfall for the month prior to the landslide. Long-term permafrost degradation could have also contributed to the slope instability. The landslide dammed Vulcan Creek, forming a lake with a maximum surface area of ~ 22,000 m2. The evolution of the lake and overflow channel was characterized using both field observations and satellite imagery. The lake area decreased between 2015 and 2017 as the overflow channel became more incised and due to sedimentation within the lake. Incision of the overflow channel on the order of 3 m was observed between June 2015 and June 2016. Four years of observations of changes in the lake area and overflow channel dimensions suggest that while the current configuration the landslide dam is likely stable, the associated hazard to recreational users in the area changes with time as the lake volume varies and the stability of the slope above the lake evolves.

Insect pollinators of haskap (Lonicera caerulea L.: Caprifoliaceae) in subarctic Canada

Abstract Recently, the Yukon has seen a large growth in agricultural activity. Crops of commercial interest for local consumption and the export market include domestic berries, especially haskap (Lonicera caerulea L.). However, information on the pollination of these crops in our northern climate is lacking. To begin addressing this knowledge gap, we characterized foraging habits of on-farm bees in southwest Yukon by: 1) identifying pollen collected by bees occupying solitary bee houses; and 2) identifying and counting insect visitors to haskap flowers. Results show that cavity-nesting bees collect a wide variety of pollen including pollen from haskap, and that bumble bees (Bombus spp.) were much more common on haskap flowers than domestic honey bees (Apis mellifera L.), other bee species, syrphid flies, and butterflies. The number of bumble bees per haskap flower was also higher than reported elsewhere in Canada. The ability of bumble bees to be active in cool temperatures and the proximity of the study farms to natural ecosystems likely explain the prevalence of bumble bees in this study. In Yukon, it is still possible to support insect pollinators by maintaining natural areas among agricultural lands. Such undeveloped lands are, at present, typical of agricultural landscapes in subarctic Canada.

Provenance analysis of the Dezadeash Formation (Jurassic–Cretaceous), Yukon, Canada: implications regarding a linkage between the Wrangellia composite terrane and the western margin of Laurasia

The Mesozoic convergence of the allochthonous Wrangellia composite terrane (WCT) with the western margin of Laurasia coincided with the construction of the Chitina magmatic arc on the WCT, and the dispersal of volcanic flows and sediment gravity flows into an adjacent flysch basin. The basin, preserved as the Gravina–Nutzotin belt, includes the Dezadeash Formation in southwest Yukon, the Nutzotin Mountains sequence in southern Alaska, and the Gravina belt in southeastern Alaska. The Dezadeash Formation is a submarine fan system comprising stacked channel-lobe transition and lobe deposits interposed with overbank deposits. Conglomerate pebble-counts, sandstone point-counts, detrital zircon ages, and major element, trace element, rare earth element, and Sm–Nd isotopic geochemistry of sandstone, mudstone, and hemipelagite beds suggests that the deposits consist mainly of first-cycle volcanogenic detritus shed from the undissected Chitina arc, in addition to material eroded from the WCT. The arc was constructed of undifferentiated magma sourced from the depleted mantel, as well as older crustal material attributed to the WCT proxying for continental crust. The compositional provenance results, together with published paleocurrent data for the Dezadeash Formation and compositional and directional provenance indicators from the Nutzotin Mountains sequence and Gravina belt, does not require a sediment source from Laurasia. The provenance record is compatible with deposition of the Gravina–Nutzotin belt in a convergent plate margin setting.

Postglacial Reconstruction of Fire History Using Sedimentary Charcoal and Pollen From a Small Lake in Southwest Yukon Territory, Canada

Funding was provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery and Discovery Northern Supplements programs to MFJP and KT, the Northern Scientific Training Program (NSTP) of Polar Knowledge Canada to TP,Ontario Graduate Scholarship (OGS) and Brock University Research Fellowship to TP.We acknowledge and thank the Brock University Library Open Access Publishing Fund for financial support.

Changes in the understory plant community and ecosystem properties along a shrub density gradient

Climate warming is projected to alter the vegetation community composition of arctic and alpine ecosystems including an increase in the relative abundance and cover of deciduous shrubs. This change in plant functional group dominance will likely alter tundra ecosystem structure and function. We conducted an observational study to quantify how the understory vegetation community and ecosystem properties varied along a shrub density and altitudinal gradient in a tundra alpine ecosystem in south-west Yukon. Although there was weak association between shrub density and species richness of understory community, there were large differences in functional group abundance between the different shrub densities; forb cover increased at lower elevations with higher shrub density at the expense of cryptogam and dwarf shrub cover. Litter mass, light interception, and soil carbon:nitrogen ratios all increased with shrub density. Sites with shrubs had higher summer soil temperatures, lower summer soil moisture, and lower percent soil nitrogen than the shrub-free site, although there was no difference in available nutrients among sites. This study presents findings from a nonmanipulated, model system where shrubification has been documented and suggests that direct and indirect effects of increasing shrub dominance are likely to affect the surrounding vegetation and abiotic environment controls.

Climatic drivers of tree growth at tree line in Southwest Yukon change over time and vary between landscapes

Growth of trees at their altitudinal and latitudinal range limits is expected to increase as climate warms, but trees often exhibit unexplained spatial and temporal variation in climate-growth responses, particularly in alpine regions. Until this variability is explained, predictions of future tree growth are unlikely to be accurate. We sampled Picea glauca (white spruce) growing at forest and tree line on north and south aspects in two mountain ranges of southwest Yukon to determine how and why ring-width patterns vary between topographic settings, and over time. We used multivariate statistical analysis to characterize variation in ring-width patterns between topographic factors and time periods, and calculated correlations between ring-width indices and climate variables to explain this variation. Ring-width patterns varied more between mountain ranges than elevations or aspects, particularly in recent decades when ring-widths increased in one mountain range but not the other. Growth responses to summer temperature were notably weaker during warmer time periods, but growth was not positively correlated to summer precipitation, suggesting trees may not be suffering from temperature-induced drought stress. Rather, ring-width indices began responding positively to spring snow depth after 1976. We conclude that tree growth is unlikely to increase in synchrony with rising air temperatures across subarctic tree lines in southwest Yukon. Instead, they may decline in areas that are prone to thin snowpacks or rapid spring runoff due to the negative influence warming springs will have on snow depth and, consequently, early growing season soil moisture.

Summer Climatic Moisture Balances for Yukon Xerophytic Grassland Slopes and Their Late-Wisconsinan Counterparts: Are Present-Day Grasslands Beringian Relicts?

Summer climatic moisture balances (precipitation vs. potential evapotranspiration) for topographic slopes favored by present-day xerophytic boreal grasslands (SSW aspects with 61% – 65% gradients) in southwest Yukon were compared to their unglaciated late-Wisconsinan (14 000 – 12 000 cal yr BP) Beringian counterparts. The purpose was to determine whether present-day grasslands are ecologically plausible Beringian relicts or analogues. The Beringian climate was represented by meteorological stations with average July temperatures 4˚C colder than those of present-day grassland areas; all were from Arctic locations. The most commonly occupied grassland slopes received maximum local solar radiation and likely represented the warmest topographic positions in the landscape. Their summer evapotranspiration demands exceeded the 200 mm moisture supply by 189 mm. The same slopes had late-Wisconsinan summer moisture balances of −57 to −101 mm because of colder and shorter growing seasons. Moisture balances for late-Wisconsinan SSW slopes were similar to those of present-day WNW – NW aspects, which do not support grasslands. Differences in moisture deficits indicated present-day xerophytic grasslands were improbable Beringian in situ relicts or analogues. Alternatively, it was hypothesized that present-day Calamagrostis purpurascens communities developed during the Holocene Thermal Maximum (11 000 – 9000 cal yr BP), whereas Hesperostipa comata communities formed less than 3000 years ago after postglacial migration of the species from central North America.

Inconsistent relationships between annual tree ring-widths and satellite-measured NDVI in a mountainous subarctic environment

Recent climate warming has intensified efforts to understand patterns of vegetation productivity at northern high latitudes. Remotely-sensed and dendrochronological measurements have indicated significant changes in aboveground vegetation production in these regions, but the relationship between the two indicators remains unclear. We examined the relationship between remotely-sensed data, using the normalized difference vegetation index (NDVI), and annual ring-width indices obtained from trees and shrubs in the boreal mountains of southwest Yukon. A 24-year (1990–2013) trend in annual maximum NDVI was generated from two Advanced Very High Resolution Radiometer (AVHRR) datasets: the Alaska Composite NDVI (1 km), and the third generation Global Inventory Modeling and Mapping Studies (GIMMS3g) NDVI (8 km). Twenty sites were systematically selected for dendrochronological sampling and chronologies were developed for the dominant coniferous (Picea glauca and Picea mariana) and deciduous (Salix glauca, Alnus crispa, and Populus tremuloides) species at each site. NDVI trends indicated low-level greening across much of the region during the 24-year period. However, ring-width trends varied in direction and magnitude and the agreement with annual maximum NDVI was low. Correlations between annual ring-width indices and biweekly NDVI values were also low, although eight coniferous chronologies did show significant positive correlations with GIMMS3g biweekly data, suggesting that radial growth may be reflected in NDVI at some point during the growing season. Reasons for the inconsistent relationships between ring-width and NDVI remain unclear. Future studies should examine the role of climatic variables on the ring-width-NDVI relationship, especially when multiple datasets and various resolutions are being used. Investigating the relative contributions of plant growth, establishment of new plants, and changes in plant resource allocation to NDVI trends are critical to achieve a more functional understanding of the changes occurring in mountainous boreal ecosystems.

Factors influencing the establishment and growth of tree seedlings at Subarctic alpine treelines

AbstractSignificant treeline advance can only occur with successful establishment, growth, and survival of new seedlings. Several studies have examined microsite factors at single locations to explain the presence/absence of seedlings at treeline. We conducted a much larger observational study and included multiple factors to determine (1) which variable(s) was/were most important, (2) whether their importance differed between aspects, and (3) whether the same variables explained variation in seedling growth and damage. We analyzed five biophysical and six shrub variables along four forest–tundra ecotones in southwest Yukon at 640 points. The model that best explained seedling occurrence was similar between north‐ and south‐facing slopes. Of all variables, seedling occurrence was best explained by the proximity, height, and upslope orientation of shrubs (relative to the seedling). The data indicate an optimal range of shrub cover, which differed with aspect. On north‐facing slopes, seedlings occurred most often when shrub cover exceeded 13%, while on south‐facing slopes seedlings occurred most when shrub cover was between 9% and 72%. We also found that with the exception of shrub‐related factors, very few biophysical variables explained size and growth characteristics and damage of tree seedlings, suggesting that the relative strength and importance of variables change depending on the life stage and size of the tree seedling. Collectively, our results demonstrate a non‐random distribution of seedlings in the forest–tundra ecotone, suggesting that as shrub distributions change with climate change, colonization sites for seedlings will also be influenced.

Contrasting shrub species respond to early summer temperatures leading to correspondence of shrub growth patterns

The Arctic-alpine biome is warming rapidly, resulting in a gradual replacement of low statured species by taller woody species in many tundra ecosystems. In northwest North America, the remotely sensed normalized difference vegetation index (NDVI), suggests an increase in productivity of the Arctic and alpine tundra and a decrease in productivity of boreal forests. However, the responses of contrasting shrub species growing at the same sites to climate drivers remain largely unexplored.Here, we test growth, climate, and NDVI relationships of two contrasting species: the expanding tall deciduous shrub Salix pulchra and the circumarctic evergreen dwarf shrub Cassiope tetragona from an alpine tundra site in the Pika valley in the Kluane Region, southwest Yukon Territories, Canada.We found that annual growth variability of both species at this site is strongly driven by early summer temperatures, despite their contrasting traits and habitats. Shrub growth chronologies for both species were correlated with the regional climate signal and showed spatial correspondence with interannual variation in NDVI in surrounding alpine and Arctic regions. Our results suggest that early summer warming represents a common driver of vegetation change for contrasting shrub species growing in different habitats in the same alpine environments.

Influences of fine-scale disturbance on germinant success in a treeline ecotone

ABSTRACTFine-scale disturbance can increase seed access to suitable substrates, facilitating germinant emergence and survival, which are necessary elements for treeline advance. We conducted an experiment to test this hypothesis in a white spruce (Picea glauca) treeline ecotone in southwest Yukon, Canada. Sixty seed germination quadrats were established at two elevations (treeline and alpine tundra) and subjected to three levels of simulated disturbance. We sowed 125 seeds in half of the quadrats (30) and measured their emergence and survival over 3 years. Soil temperature, moisture, and organic depth were recorded in all treatments. Treeline quadrats had significantly greater seedling emergence and survival than alpine tundra quadrats. Mean soil temperature, moisture, and organic layer depth were all greater in treeline quadrats. Partially scarified quadrats had the highest germinant emergence compared to unscarified and completely scarified quadrats. Completely scarified quadrats had the highest temperature range and the lowest soil moisture. The results indicate that moderate levels of disturbance can positively influence seedling emergence, while more severe disturbance can lead to high temperature ranges and moisture loss that negate the benefits of lower interspecific competition. Collectively, our findings suggest that fine-scale disturbance can play a significant role in influencing seedling presence in treeline ecotones.