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Alpine treeline ecotones act as early warning systems for detecting the impact of climate change on terrestrial ecosystems. The physiognomic traits of the treeline ecotones, such as tree height, canopy cover, and leaf area, are expected to change in response to ongoing climate warming. These traits contain valuable information about the processes that govern treeline dynamics, and thus, it is important to describe them consistently along ecologically meaningful dimensions. However, conventional approaches to evaluating vegetation structure, such as those that rely on optical and synthetic aperture radar (SAR) imaging satellites, have significant limitations that limit their utility in mountainous terrain. The use of SAR imagery is constrained by shadowing and geometric distortion caused by steep terrain, while optical imaging systems are insensitive to changes in vegetation vertical structure. In contrast, Light Detection and Ranging (LiDAR) technology, offers an accurate means of estimating the three-dimensional physiognomic traits of vegetation in a mountainous alpine environment. Due to the obvious advantages of LiDAR over conventional multi-spectral imaging and radar imaging sensors, the number of LiDAR-based treeline studies has increased in recent years. In this review, we examined how LiDAR remote sensing is utilized in the alpine treeline research and evaluated the associated challenges and opportunities. Further, this study concludes by discussing the promising future of LiDAR technology, and accordingly, some recommendations are put forward. [ABSTRACT FROM AUTHOR]

Due to land use change, green alder (Alnus alnobetula), formerly restricted to moist slopes, is now expanding to drier sun-exposed sites at the alpine treeline. The highly productive shrub is forming closed thickets, establishing nitrogen-saturated species poor shrublands. To evaluate wood anatomical adaptations to changing environmental conditions, we analyzed vessel characteristics (mean vessel area, MVA; vessel density, VD; and theoretic conductive area, TCA) and axial parenchyma abundance, as well as their distribution in the annual ring at a moist north-facing and a dry south-facing site at the alpine treeline on Mt. Patscherkofel (Central European Alps, Austria). Results revealed that lower soil water availability and enhanced evaporative demand did not affect MVA while VD and TCA were significantly reduced at the dry south-facing site. This suggests that in green alder, vessel size is a static trait whereas vessel number responds plastic. Limited water availability also triggered a significant increase in axial parenchyma, confirming the important role of xylem parenchyma in water relations. Harsh environmental conditions at the distributional limit of green alder may have affected xylogenesis, leading to a near semi-ring-porous distribution of vessels and an accumulation of parenchyma in the late growing season. We conclude that in a warmer and drier climate, growth limitation and physiological stress may set limits to the distribution of Alnus alnobetula at drought-prone sites in the alpine treeline ecotone. [ABSTRACT FROM AUTHOR]

Context: Treeline-ecotone spatial patterns and their dynamics reflect underlying processes. Changes in ecotone pattern may reflect changes in natural drivers or land-use practices. However, characterizing these dynamics presents a major challenge, limiting our ability to map, understand and predict changes in the upper limits of mountain forests. Objective: This paper proposes a new method using multiple pattern dimensions to describe treeline-ecotone spatial pattern shifts. This standardized protocol should be able to (i) distinguish different types of treeline-ecotone patterns within a large study area, (ii) characterize temporal pattern shifts in spatial pattern between two or more dates. Method: We mapped alpine treeline ecotones (ATE) at 648 sites in the eastern French Pyrenees using aerial images from ~ 1955 and ~ 2015, identifying forest and non-forest areas at the hillslope scale. Extracted patch metrics were summarized using a Principle Component Analysis (PCA) and spatial pattern change was quantified from the shift in the PCA space and compared to elevational shifts. Results: Three clusters of patterns were distinguished: diffuse, discrete and island-forming ATEs. Between 1955 and 2015, about half of the sites changed from one pattern cluster to another. Shifts into discrete ATEs were associated with smaller and negative elevational shifts, while shifts into diffuse ATEs coincided with the highest positive elevational shifts. Conclusion: The proposed method allows a standardized and repeatable quantification of vegetation pattern change in alpine treeline ecotones based on historical aerial imagery. Seeing the importance of treeline-ecotone shifts for alpine biodiversity, we encourage the use of this protocol to better understand treeline dynamics at treelines globally. [ABSTRACT FROM AUTHOR]

Green alder (Alnus alnobetula) is currently the most expanding shrub species in the Alps. Because dense thickets impair tree establishment, understanding how climate affects shrub growth is essential for predictions of treeline dynamics. We evaluated ring width data from >50 A. alnobetula stems sampled at treeline on Mt. Patscherkofel (Central European Alps, Austria) to identify main climatic drivers and influence of climate warming on radial stem growth (RG). We also compared RG of A. alnobetula with RG of the co-occurring treeline conifer Swiss stone pine (Pinus cembra). We addressed our questions through calculation of response functions and evaluation of climate in years showing exceptional growth deviations. Response function analyses and evaluation of growth trends during 1991–2020 revealed that RG of A. alnobetula is significantly and directly related to summer temperatures. Precipitation in January also showed a direct relationship to RG, indicating effects of frost drought on RG. Surprisingly, nitrogen fixing A. alnobetula showed strikingly lower RG compared to P. cembra, and the latter also responded more strongly to the increase in summer temperature in the course of climate warming. We explain these findings by different carbon allocation strategies, i.e., preference of "vertical" stem growth in late successional P. cembra vs. favoring "horizontal" spread in the pioneer shrub A. alnobetula. [ABSTRACT FROM AUTHOR]

Because climate change is predicted to have a strong impact on high-altitude ecosystems, a better knowledge of litter decomposition in alpine ecosystems is critical to improve our predictions of the effect of climate change on ecosystem processes and services such as nutrient cycling, carbon sequestration, and below-ground biodiversity. To evaluate the effects of vegetation types [alpine shrubland (AS) and alpine meadow (AM)] and litter quality on litter decomposition and related biochemical processes, the decomposition of leaf litter of two dominant shrub species, Sorbus rufopilosa (SR, high quality) and Rhododendron lapponicum (RL, low quality), was studied using the litterbag method in an alpine treeline ecotone on the eastern Tibetan Plateau. After 1 year of decomposition, cellulolytic enzyme activities and gram-negative bacterial biomass were higher in shrubland than in meadow. However, higher fungal biomass, fungal/bacteria ratio and ligninolytic activity were observed in meadow than in shrubland after 2 years of decomposition. During the first year of decomposition, litter decomposition was faster in shrubland than in meadow probably due to the home-field advantage (HFA) effect and the bacteria-dominated decomposition, whereas in later decomposition stages, litter decomposition was faster in meadow than in shrubland, as the HFA effect diminished and fungal-dominated decomposition of recalcitrant components took over. These results indicated that litter quality effects were generally strongest in the first year and diminished in later stages when the effect of vegetation type in incubation sites developed. [ABSTRACT FROM AUTHOR]

Globally, the increase in the climatic variability has led to adverse effects on the treeline species in the high-elevation mountain landscapes. Identifying the geographical space that supports the treeline species survival over time is essential for conservation biogeography. Increase in the global warming and snowmelt has made available the treeline species favourable niches in the higher elevations. Random Forest algorithm assuming non-parametric distribution was employed to predict the potential distribution of Betula utilis niche in the Hindu-Kush Himalayan (HKH) region. The potential distributions were simulated in the Last Inter-Glaciation (LIG), present (the year 1970–2000) and future (the year 2061–2080) environmental conditions. The actual distribution of the species in the current time was modelled and evaluated. The model sensitivity with reference to independent evaluation dataset for highly suitable B. utilis niche was 0.78. The model statistics of the current time was further applied to both the LIG and future (2061–2080) scenarios in order to get a fundamental niche of B. utilis. The treeline species, B. utilis was projected to become vulnerable to 21st century climate changes. The high suitability of B. utilis occurrence in the LIG, current and the future scenario were more likely in the elevation ranges 2601–2800 m, 3801–4000 m, and 4201–4400 m, respectively. The magnitude of advancement was relatively more along elevation and longitude, compared to the latitudinal gradient. The present study provides scientific evidence to conclude that the treeline species potential distribution in HKH is climate driven. [ABSTRACT FROM AUTHOR]

Although climate warming is generally expected to facilitate upward advance of forests, conifer seedling regeneration and survival may be hindered by low substrate moisture, high radiation, and both low and high snow accumulation. To better understand substrate-related factors promoting regeneration in the alpine treeline ecotone, this study compared 2 substrates supporting conifer seedlings: rotten downed wood and adjacent soil. Study locations, each with 3 levels of incoming radiation, were randomly selected at forest line–alpine meadow borders in Pacific Northwest wilderness areas extending along an east–west precipitation gradient. Associations among substrate type, seedling density, radiation, site moisture, site temperature, plant water potential, and plant stomatal conductance were assessed. Wood microsites, flush with the ground and supporting Abies spp conifer seedlings, extended up to 20 m into alpine meadows from the forest line. Although wood microsites thawed later in the spring and froze earlier in the fall, they had warmer summer temperatures, greater volumetric water content, and more growing degree hours, and seedlings growing on wood had higher water potentials than seedlings growing on adjacent soil. At drier eastern sites, there was a positive relationship between seedling density and volumetric water content. Further, there was a positive relationship between seedling stomatal conductance and volumetric water content. Our study indicates that in the Pacific Northwest. and likely elsewhere, seedlings benefit from wood microsites, which provide greater water content. Given predictions of increased summer drought in some locations globally, wood microsites at forest line–alpine meadows and forest line–grasslands borders may become increasingly important for successful conifer regeneration.

Alpine Treeline Ecotone (ATE), the typically gradual transition zone between closed canopy forest and alpine tundra vegetation in mountain regions, displays an elevational range that is generally constrained by thermal deficits. At landscape scales, precipitation and moisture regimes can suppress ATE elevation below thermal limits, causing variability in ATE position. Recent studies have investigated the relative effects of hydroclimatic variables on ATE position at multiple scales, but less attention has been given to interactions between hydroclimatic variables and disturbance agents, such as fire. Advances in monoplotting have enabled the extraction of canopy cover information from oblique photography. Using airborne lidar, and repeat photography from the Mountain Legacy Project, we observed canopy cover change in West Castle Watershed (Alberta, Canada; ~103 km2; 49.3° N, 114.4° W) over a 92-year period (1914–2006). Two wildfires, occurring 1934 and 1936, provided an opportunity to compare topographic patterns of mortality and succession in the ATE, while factoring by exposure to fire. Aspect was a strong predictor of mortality and succession. Fire-exposed areas accounted for 83.6% of all mortality, with 72.1% of mortality occurring on south- and east-facing slope aspects. Succession was balanced between fire-exposed and unburned areas, with 62.0% of all succession occurring on north- and east-facing slope aspects. The mean elevation increase in closed canopy forest (i.e., the lower boundary of ATE) on north- and east-facing undisturbed slopes was estimated to be 0.44 m per year, or ~44 m per century. The observed retardation of treeline advance on south-facing slopes is likely due to moisture limitation.

The high-altitude treeline is known to be sensitive to climate variability, and is thus considered as a bio-monitoring indicator of climate change. However, our understanding of the population dynamics and the cumulative climate-change effects on the alpine treeline ecotone in recent decades is limited. Here, we investigated the population dynamics of Larix potainii on the southand north-facing slopes in the alpine treeline ecotone in the eastern margin of the Tibetan Plateau, China, including treeline position, population density, and tree recruitment. Results showed that on both south- and north-facing slopes, the treeline did not show a significant advancement in the past four decades. The population was dominated by young individuals, which tend to be established in the lower areas. Larix, here, tends to be clustered, especially in the upper areas. However, population density increased dramatically only on north-facing slopes. Larix here suffer from the stressful environment, but the warmer winter due to climate warming could facilitate the vertical growth of seedlings and saplings. Aggregated spatial patterns also provide a positive feedback in ameliorating the harsh environment. The slope-climate-moisture interactions have a pronounced impact on tree recruitment, including snow-limited tree establishment on the north-facing slopes and moisture-limited tree establishment on the south-facing slopes. [ABSTRACT FROM AUTHOR]

Alpine treeline ecotones represent ecosystems that are vulnerable to climate change. We investigated the ectomycorrhizal (ECM) community, which has potential to stabilize alpine ecosystems. ECM communities associated with Larix chinensis were studied in four zones along a natural ecotone from a mixed forest stand over pure forest stands, the timberline, and eventually, the treeline (3050-3450 m) in Tabai Mountain, China. Sixty operational taxonomic units (OTUs) of ECM fungi were identified by sequencing the rDNA internal transcribed spacer of ECM tips. The richness of ECM species increased with elevation. The soil C/N ratio was the most important factor explaining ECM species richness. The treeline zone harbored some unique ECM fungi whereas no unique genera were observed in the timberline and pure forest zone. Elevation and topography were equally important factors influencing ECM communities in the alpine region. We suggest that a higher diversity of the ECM fungal community associated with L. chinensis in the treeline zone could result from niche differentiation. [ABSTRACT FROM AUTHOR]

Alpine treelines are expected to advance to higher elevations in conjunction with global warming. Nevertheless, the importance of treeline reproductive patterns and seed dispersal within the alpine treeline ecotone remains unresolved. In this study, we address two research questions at mountain hemlock treelines on the Kenai Peninsula, Alaska: (1) What is the primary mode of reproduction and (2) are seeds leading to recruitment derived from within the local treeline populations or are they arriving from more distant seed sources? To answer these questions, we exhaustively sampled mountain hemlock individuals along a single mountain slope, and genotyped single nucleotide polymorphisms using doubledigest restriction site-associated DNA sequencing. First, we assessed mode of reproduction by determining the proportion of sampled individuals with identical multilocus genotypes that are the product of clonal reproduction. Second, we used a categorical parentage analysis to identify parent–offspring pairs, so that the proportion of treeline reproduction events could be spatially quantified and dispersal distance measured. We identified sexual reproduction as the primary mode of reproduction at our study site. Seedling establishment was characterized by extensive seed immigration and gene flow into the ecotone. The average dispersal distance was 73 m with long-distance dispersal identified as dispersal occurring at distances greater than 450 m. We found that production of seeds within the alpine treeline ecotone is not necessarily a requirement for treelines to advance to higher elevations in response to climate change. The extensive seed dispersal and gene flow into the alpine treeline ecotone is likely sufficient to propel the ecotone higher under more favorable climate. [ABSTRACT FROM AUTHOR]

Although climate warming is generally expected to facilitate upward advance of forests, conifer seedling regeneration and survival may be hindered by low substrate moisture, high radiation, and both low and high snow accumulation. To better understand substrate-related factors promoting regeneration in the alpine treeline ecotone, this study compared 2 substrates supporting conifer seedlings: rotten downed wood and adjacent soil. Study locations, each with 3 levels of incoming radiation, were randomly selected at forest line-alpine meadow borders in Pacific Northwest wilderness areas extending along an east-west precipitation gradient. Associations among substrate type, seedling density, radiation, site moisture, site temperature, plant water potential, and plant stomatal conductance were assessed. Wood microsites, flush with the ground and supporting Abies spp conifer seedlings, extended up to 20 m into alpine meadows from the forest line. Although wood microsites thawed later in the spring and froze earlier in the fall, they had warmer summer temperatures, greater volumetric water content, and more growing degree hours, and seedlings growing on wood had higher water potentials than seedlings growing on adjacent soil. At drier eastern sites, there was a positive relationship between seedling density and volumetric water content. Further, there was a positive relationship between seedling stomatal conductance and volumetric water content. Our study indicates that in the Pacific Northwest. and likely elsewhere, seedlings benefit from wood microsites, which provide greater water content. Given predictions of increased summer drought in some locations globally, wood microsites at forest line-alpine meadows and forest line-grasslands borders may become increasingly important for successful conifer regeneration. [ABSTRACT FROM AUTHOR]

Biogeomorphology is an increasingly popular field of study, but the approaches to biogeomorphic research and methods are not yet well developed. This research evaluated ecologic and geomorphic interactions after fire within the alpine treeline ecotone of Glacier National Park, Montana. Associations between soil conditions and vegetation, the effect of vegetation edges on soil characteristics, and the influence of quadrat size on select vegetation and soil variables were evaluated with the use of fieldwork and statistics. Of the 11 ecologic/soil variables compared, three were significantly correlated – soil compaction and krummholz density, soil loss and percent vegetation, and soil loss and burn severity. The ecologic edge between burned and unburned krummholz had significant influence on soil compaction, clast size, and effective soil depth patterns. Quadrat size did not have much influence on average results for either soil or vegetation variables. These findings highlight both the potential biogeomorphic interactions as well as interesting results that were not significant, and will contribute to the advancement of biogeomorphology. The results also provide novel information on alpine treeline ecotone response after fire. [ABSTRACT FROM AUTHOR]

Aims: While treeline positions are globally correlated to growing season temperatures, seedling establishment, an important process of alpine treeline dynamics, is additionally controlled by regional-scale factors such as snow cover duration, desiccating winds and biotic interactions. Knowing that alpine treelines have shown contrasting responses to climate change, we determined the relative importance of key abiotic and biotic factors involved in seedling survival and growth. Location: McGerrigle Mountains, Parc National de la Gaspésie, Appalachian Range, eastern Quebec, Canada. Methods: In two white spruce (Picea glauca) treeline sites, we used the microclimate in the vicinity of tree islands, densely packed clusters of trees isolated from each other by alpine tundra vegetation, to assess the effects of abiotic variables (sum of degree days [DD], snowpack duration and a wind exposure index) as well as the effects of biotic interactions with neighbouring vegetation on the survival and growth of transplanted white spruce seedlings. For 3 yr, we surveyed seedling survival twice a year to discriminate between winter and summer survival, and measured seedling growth at the end of each growing season. We used Bayesian hierarchical models to estimate the relative effects of covariates on survival and growth. Results: Survival probability decreased in microsites wherewinter DD was high, and increased in microsites with longer snowpack duration. In wind-exposed microsites, seedling survival increased when neighbouring vegetation was present, indicating facilitative mechanisms. Seedling growth was positively affected by the duration of snow cover and tended to increase with higher DD during the previous year. In wind-sheltered microsites, seedling growth tended to be negatively affected by neighbouring vegetation, indicating competitive mechanisms. Conclusions: Our study demonstrates that seedling establishment is more sensitive to winter conditions, notably to the length of snow cover (which protects seedlings from frost and desiccation), than to summer temperature. Biotic interactions increased seedling establishment when environmental stresses were higher. We suggest that regional-scale factors such as winter climate and biotic interactions should be included inmodelling exercises to improve future treeline location forecasts. [ABSTRACT FROM AUTHOR]

This paper evaluates the geomorphic effects of fire on the alpine treeline ecotone and how those effects influence post-fire seedling establishment conditions. Forest fires are becoming more common in high elevations, and ecosystem responses in these areas are not well studied. Results indicated that burned areas experienced increased soil loss, complete loss of the duff layer, less compact soils, larger clast sizes, and increased rock spalling, and altered seedling site conditions. Seedlings showed distinct preferences for micro-sites with deeper, less compact soils, and a greater cover of rock. The effects of fire within the alpine treeline ecotone will affect treeline position and geomorphic processes within these higher elevations. [ABSTRACT FROM AUTHOR]

Multiple environmental factors contribute to the spatial and compositional character and elevational patterns of alpine treeline ecotones (ATEs), and the relative influence of these factors is scale dependent and spatially variable. Frameworks detailing the hierarchical structure of the ATE have been developed to characterize scale dependencies of the pattern and controls of treeline, but this topic has not been studied across a broad range of scales (e.g., from the hillslope to the region). This research directly examines scaling by comparing relationships among treeline elevations and a set of possible controls as geographic extent is varied. The data set used for this research consists of elevational data at the ATE and a set of hypothesized controls for 1,006 sites in twenty-six mountain ranges across the Western United States. The response and predictor variables are quantified from digital data sets using geographic information systems and remote sensing methodologies and then analyzed using a Mantel test framework. Results generally support, and add empirically derived detail to, existing theoretical frameworks, with climatic controls (i.e., variables characterizing temperature and precipitation) having higher correlations with ATE elevation at coarser scales and topographic variables having higher correlations at finer scales. These scale relations support the conceptual hierarchical frameworks that have been proposed, and they are useful guides of covariate selection for future ATE modeling endeavors. [ABSTRACT FROM PUBLISHER]

Our study focused on the functional aspects of plant species and vegetation at the transition from larch ( Larix decidua Mill.) forest to mountain pine ( Pinus mugo Turra) stands on the alpine treeline ecotone. With increasing elevation, living conditions grow harsher, which is reflected in the plant species and functional trait composition of plant communities. At four different localities in the Slovenian Alps, relevés along an altitudinal gradient and according to vegetation type were made (European larch forests, larch trees-mountain pine shrubs, mountain pine shrubs), using standard Central European phytocoenological method. In the upper mountain pine belt, few differential species were found, since most species represented in mountain pine stands also occurred in the lower two vegetation belts, while there were many differential species in the lower forest belt. Species with considerable competitive ability and moderate stress tolerance dominated the upper treeline ecotone, whereas ruderality is poorly expressed. The importance of stress tolerance in plant strategies increased slightly in the mountain pine belt. Changes in the representation of some functional traits attributes were detected by vegetation belts, but complete species turnover did not occur. Changes in dominant life form involved greater cover of chamaephytes and nanophanerophytes in the upper mountain pine belt. Species with evergreen leaves dominated mountain pine stands and deciduous forest stands. The share of species with scleromorphic leaves increased in the direction of the mountain pine belt while the share of species with mesomorphic and hygromorphic leaves declined. Mountain pine stands create good conditions for the regeneration of tree species and colonisation by ecologically more demanding forest species while species of alpine grasslands withdraw on open areas. Since today's treeline is lowered due to past human activity, an upward shift is expected. Also, considering the great importance of competition strategy, current conditions in mountain pine stands are favourable for future forest succession. [ABSTRACT FROM AUTHOR]

The goal of the study was to derive up-to-date and complex information on the current vegetation cover of the Praděd Reserve (Hrubý Jeseník Mountains, Czech Republic), with special regard to the unique alpine treeline ecotone formed by krummholz of Norway spruce. We argue that the data of remote sensing and automated techniques of image processing should be preferably used. Accordingly, a colorinfrared orthophoto map was classified in a land cover map employing maximum likelihood spectral classifier, ancillary data, texture analysis, and a knowledge base classification technique. The overall classification accuracy was about 78%, distinguishing 7 land cover classes. Using a reclassified land cover map and the moving window mean filter, a spruce canopy closure map was calculated. The continuous map of the canopy closure was subsequently reclassified in predefined intervals that were used for an automated delimitation and mapping of complex transitional borders of the alpine treeline ecotone. The proposed method can serve for objectified mapping of gradual transitions between any land cover or vegetation classes. [ABSTRACT FROM AUTHOR]

Spatially explicit digital technologies are integrated within a geographic information science (GISc) context to map, model, and visualize selected direct and indirect geomorphic processes that influence the spatial organization of the alpine treeline ecotone (ATE) in Glacier National Park (GNP), MT. GISc is used to examine alpine treeline and its biotic and abiotic controls through the application of multi-resolution remote sensing systems, geospatial information and product derivatives, and simulations of treeline spatial organization. Three geomorphic features are examined: relict solifluction terraces, evidence of nonlinearity in the development of a catena, and the locations of isolated boulders. The significance of these features is in constraining subsequent geomorphic and biogeographic processes, thus leading to disequilibrium. Exploration of these features though GISc indicates that visualizations for characterizing the relations of geomorphic patterns and processes within a three-dimensional context show promise for improved alpine slope models in the future by defining landscape attributes within a spatially and temporally explicit context. [Copyright &y& Elsevier]

Alpine treelines are projected to shift upslope in response to climate warming, but empirical studies have yielded inconsistent results, with both upshifted and stable alpine treelines. Additionally, treelines on different slope aspects of the same mountain can differ. Thus, for a better understanding of the mechanisms of treeline formation and treeline responses to climate change, we need to elucidate the population dynamics at treelines on different slope aspects. Here, we quantified the population dynamics of Balfour spruce (Picea likiangensis var. rubescens) at treeline ecotones on contrasting north- and east-facing slopes on the eastern Tibetan Plateau based on field surveys. The alpine treeline positions of Balfour spruce have not advanced toward higher altitudes on the contrasting slopes in recent decades. Compared with the east-facing slope, more recruits occurred on the north-facing slope above the present treeline, indicating a more favorable regeneration condition. However, on the north-facing slope, the individual growth rate of Balfour spruce was lower, and the number of adult trees above the present treeline was higher than that on the east-facing slope. Thus, slope aspects mediate a trade-off between the growth and survival of treeline species, explaining the absence of an impact of slope aspects on treeline responses to climate change. Our results highlight the importance of considering the effect of topography on population dynamics in predicting alpine treeline dynamics under the scenario of climate change. [ABSTRACT FROM AUTHOR]

The decomposition of litter carbon (C) fraction is a major determinant of soil organic matter pool and nutrient cycling. However, knowledge of litter chemical traits regulate C fractions release is still relatively limited. A litterbag experiment was conducted using six plant functional litter types at two vegetation type (coniferous forest and alpine shrubland) in a treeline ecotone. We evaluated the relative importance of litter chemistry (i.e. Nutrient, C quality, and stoichiometry) on the loss of litter mass, non-polar extractables (NPE), water-soluble extractables (WSE), acid-hydrolyzable carbohydrates (ACID), and acid-unhydrolyzable residue (AUR) during decomposition. Litter nutrients contain nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sodium (Na), magnesium (Mg), aluminium (Al), manganese (Mn), zinc (Zn), iron (Fe) and copper (Cu), litter C quality contains C, WSE, NPE, ACID, and AUR, and stoichiometry was defined by C:N, C:P; N:P, ACID:N, and AUR:N. The results showed single exponential model fitted decomposition rates of litter mass and C fractions better than double exponential or asymptotic decomposition, and the decomposition rates of C fractions were strongly correlated with initial litter nutrients, especially K, Na, Ca. Furthermore, the temporal dynamics of litter nutrients (Ca, Mg, Na, K, Zn, and Fe) strongly regulated C fractions loss during the decomposition process. Changes in litter C quality had an evident effect on the degradation of ACID and AUR, supporting the concept of "priming effect" of soluble carbon fraction. The significant differences were found in the release of NPE, WSE, and ACID rather than AUR among coniferous forest and alpine shrubland, and the vegetation type effects largely depend on the changes in litter stoichiometry, which is an important implication for the change in plant community abundance regulate decay. Collectively, elucidating the hierarchical drivers of litter chemistry on decomposition is critical to soil C sequestration in alpine ecosystems. Unlabelled Image • Single-exponential model fitted the litter carbon fraction loss best. • Initial litter nutrient elements were critical for litter decomposition rate. • Nutrient elements strongly regulated the loss of litter C fractions over time. • Temporal dynamics of litter C quality induced more recalcitrant C fractions loss. • Vegetation type preferentially altered the loss of more labile litter C fractions. [ABSTRACT FROM AUTHOR]

Research Highlights: Limber pine (Pinus flexilis) is abundant in some alpine treeline ecotone (ATE) communities east of the Continental Divide in Rocky Mountain National Park (RMNP) and the Colorado Front Range. Limber pine may be able to colonize the ATE under changing climate aided by directed seed dispersal by Clark's nutcrackers (Nucifraga columbiana). Cronartium ribicola, white pine blister rust, is a growing threat to limber pine and may affect its functional role within the ATE. Background and Objectives: The ATE is sensitive, worldwide, to increasing temperature. However, the predicted advance of treeline under a changing climate may be modified by tree species composition and interactions. We aimed to (1) examine the conifer species composition and relative abundances in treeline communities with limber pine; (2) assess which functional roles limber pine assumes in these communities—tree island initiator, tree island component, and/or solitary tree; and (3) determine whether limber pine's occurrence as a tree island initiator can be predicted by its relative abundance as a solitary tree. Materials and Methods: We selected four study sites in RMNP above subalpine forest limber pine stands. We sampled the nearest tree island to each of forty random points in each study site as well as solitary tree plots. Results: Across study sites, limber pine comprised, on average, 76% of solitary trees and was significantly more abundant as a solitary tree than Engelmann spruce (Picea engelmannii) or subalpine fir (Abies lasiocarpa). Limber pine was a frequent component of multi-tree islands in three study sites, the major component in one study site, and dominated single-tree islands at two study sites. At three of four study sites, no species had significantly greater odds of being a tree island initiator. Limber pine was found less often as a tree island initiator than predicted from its relative abundance as a solitary tree, given the likely role of solitary trees in tree island formation. [ABSTRACT FROM AUTHOR]

Alpine Treeline Ecotone (ATE), the typically gradual transition zone between closed canopy forest and alpine tundra vegetation in mountain regions, displays an elevational range that is generally constrained by thermal deficits. At landscape scales, precipitation and moisture regimes can suppress ATE elevation below thermal limits, causing variability in ATE position. Recent studies have investigated the relative effects of hydroclimatic variables on ATE position at multiple scales, but less attention has been given to interactions between hydroclimatic variables and disturbance agents, such as fire. Advances in monoplotting have enabled the extraction of canopy cover information from oblique photography. Using airborne lidar, and repeat photography from the Mountain Legacy Project, we observed canopy cover change in West Castle Watershed (Alberta, Canada; ~103 km2; 49.3° N, 114.4° W) over a 92-year period (1914–2006). Two wildfires, occurring 1934 and 1936, provided an opportunity to compare topographic patterns of mortality and succession in the ATE, while factoring by exposure to fire. Aspect was a strong predictor of mortality and succession. Fire-exposed areas accounted for 83.6% of all mortality, with 72.1% of mortality occurring on south- and east-facing slope aspects. Succession was balanced between fire-exposed and unburned areas, with 62.0% of all succession occurring on north- and east-facing slope aspects. The mean elevation increase in closed canopy forest (i.e., the lower boundary of ATE) on north- and east-facing undisturbed slopes was estimated to be 0.44 m per year, or ~44 m per century. The observed retardation of treeline advance on south-facing slopes is likely due to moisture limitation. [ABSTRACT FROM AUTHOR]