Alpine Treeline Research Articles

Tree Line Identification and Dynamics under Climate Change in Wuyishan National Park Based on Landsat Images

The alpine tree line ecotone, reflecting interactions between climate and ecology, is very sensitive to climate change. To identify tree line responses to climate change, including intensity and local variations in tree line advancement, the use of Landsat images with long-term data series and fine spatial resolution is an option. However, it is a challenge to extract tree line data from Landsat images due to classification issues with outliers and temporal inconsistency. More importantly, direct classification results in sharp boundaries between forest and non-forest pixels/segments instead of representing the tree line ecotone (three ecological regions—tree species line, tree line, and timber line—are closely related to the tree line ecotone and are all significant for ecological processes). Therefore, it is important to develop a method that is able to accurately extract the tree line from Landsat images with a high temporal consistency and to identify the appropriate ecological boundary. In this study, a new methodology was developed based on the concept of a local indicator of spatial autocorrelation (LISA) to extract the tree line automatically from Landsat images. Tree line responses to climate change from 1987 to 2018 in Wuyishan National Park, China, were evaluated, and topographic effects on local variations in tree line advancement were explored. The findings supported the methodology based on the LISA concept as a valuable classifier for assessing the local spatial clusters of alpine meadows from images acquired in nongrowing seasons. The results showed that the automatically extracted line from Landsat images was the timber line due to the restriction in spatial autocorrelation. The results also indicate that parts of the tree line in the study area shifted upward vertically by 50 m under a 1 °C temperature increase during the period from 1987 to 2018, with local variations influenced by slope, elevation, and interactions with aspect. Our study contributes a novel result regarding the response of the alpine tree line to global warming in a subtropical region. Our method for automatic tree line extraction can provide fundamental information for ecosystem managers.

Correction to: Regional variability in the response of alpine treelines to climate change

The environmental lapse rate used to estimate historical treeline advance was incorrectly reported as 0.006 °C/1000 m; a value of 0.006 °C/m was used in the calculations.

Soil nitrogen pool drives plant tissue traits in alpine treeline ecotones

Alpine treeline ecotone is one of the most vulnerable ecosystems and considered an ideal “warning line” for the monitoring of global climate change. However, shifts in the nutrient traits of plant biomass and soils of alpine treeline ecotones along the elevation gradient are poorly investigated. This study aimed to quantify nitrogen (N) and phosphorus (P) traits of Qinghai spruce (Picea crassifolia) and their driving factors for seven typical treelines (timberline - treeline - tree species line) covering 800 km southeast-to-northwest transect within the northeastern Tibetan Plateau. The biomass of plant tissue (leaves, twigs, and fine roots) and the N and P contents in them and soil nutrients (total and available N and P) were determined. Two mechanisms, which can drive the nutrient content in plants, namely, temperature-plant physiology and the sink (growth) limitation hypotheses were checked here. The N and P contents in plant tissue increased with the elevation, and temperature negatively impacted tissue N and N:P in all of the seven typical treelines, which supported both of the hypotheses. Soil N availability increased along the altitudinal gradient in 1.23 times. Soil TN explained 13.4% (p < 0.01), NO3–-N 10.1% (p < 0.05), and NH4+-N 8.6% (p < 0.05) of the variations in plant tissue N and P stoichiometry and biomass. Thus, soil N pools stimulated Qinghai spruce growth and have a strong effect on treeline dynamics.

Community Structure and Functional Role of Limber Pine (Pinus flexilis) in Treeline Communities in Rocky Mountain National Park

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.

Mosses modify effects of warmer and wetter conditions on tree seedlings at the alpine treeline.

Climate warming enables tree seedling establishment beyond the current alpine treeline, but to achieve this, seedlings have to establish within existing tundra vegetation. In tundra, mosses are a prominent feature, known to regulate soil temperature and moisture through their physical structure and associated water retention capacity. Moss presence and species identity might therefore modify the impact of increases in temperature and precipitation on tree seedling establishment at the arctic-alpine treeline. We followed Betula pubescens and Pinus sylvestris seedling survival and growth during three growing seasons in the field. Tree seedlings were transplanted along a natural precipitation gradient at the subarctic-alpine treeline in northern Sweden, into plots dominated by each of three common moss species and exposed to combinations of moss removal and experimental warming by open-top chambers (OTCs). Independent of climate, the presence of feather moss, but not Sphagnum, strongly supressed survival of both tree species. Positive effects of warming and precipitation on survival and growth of B. pubescens seedlings occurred in the absence of mosses and as expected, this was partly dependent on moss species. P. sylvestris survival was greatest at high precipitation, and this effect was more pronounced in Sphagnum than in feather moss plots irrespective of whether the mosses had been removed or not. Moss presence did not reduce the effects of OTCs on soil temperature. Mosses therefore modified seedling response to climate through other mechanisms, such as altered competition or nutrient availability. We conclude that both moss presence and species identity pose a strong control on seedling establishment at the alpine treeline, and that in some cases mosses weaken climate-change effects on seedling establishment. Changes in moss abundance and species composition therefore have the potential to hamper treeline expansion induced by climate warming.

How many tree species are in the Himalayan treelines and how are they distributed?

Increase in endemism with elevation is a common feature in Himalayas because of increased isolation of higher areas like treelines. To protect species of treeline, we at least need to record their numbers and distribution. We estimated that the total number of tree species in Himalayan treeline is 58, and they belong to 14 genera and 8 families. It is impressive, given that globally alpine treeline species richness is unlikely to be considerably higher than 100. While estimating the species number, we confronted uncertainties in some cases concerning synonyms of species, weak demarcation between species and incorrect identification. Because of a high species differentiation in the Hengduan Mountains and Western Sichuan Province in Tibet, the eastern part of the Himalayan arc (88°E and more) has much higher species richness than the western part (less than 88°E), despite similar number of genera. These areas of high species differentiation accounted for nearly half of the total species. The dominant genera in terms of geographical distribution are Juniperus, Abies, Rhododendron, and Betula. Among the families, Pinaceae, Cupressaceae, Ericaceae, Betulaceae, and Rosaceae were prominent. Because of global climatic change, species shifts are predicted to occur along an altitudinal gradient (representing temperature gradient) as well as along the 31o longitudinal range, representing a moisture gradient. The Himalayan treelines are relatively species-rich, largely because of variations in precipitation, topography, continentality, and cultural factors, among others along the east-to-west arc. There is a need to undertake additional taxonomic studies, using tools such as molecular techniques to provide solution to confusion relating to incorrect identification of some species.

Litter stoichiometric traits have stronger impact on humification than environment conditions in an alpine treeline ecotone

Litter humification is vital for carbon sequestration in terrestrial ecosystems. Probing the litter humification of treeline ecotone will be helpful to understand soil carbon afflux in alpine regions under climate change. Foliar litter of six plant functional groups was chosen in an alpine treeline ecotone of the eastern Tibetan Plateau, and a field litterbag decomposition experiment (669 days) was conducted in an alpine shrubland (AS) and a coniferous forest (CF). Environmental factors, litter quality, humus concentrations (total humus, Huc; humic acid, HAc; and fulvic acid, FAc) and hue coefficient (ΔlogK and E4/E6) were measured to explore litter humification processes. Litter humification was controlled by both litter stoichiometric traits and local-environment conditions, while stoichiometric traits played a more obvious regulatory role. Significant discrepancies in litter humus were detected among six plant functional groups; more precisely, litter of evergreen conifer and shrubs showed a net accumulation of Huc and FAc during winter, whereas others experienced more mineralization than accumulation. Huc, HAc, and hue coefficient were mainly controlled by cellulose/N, cellulose/P, C/N, lignin/P, lignin/N, etc., yet FAc was more susceptible to local-environment conditions. Meanwhile, Huc, HAc and FAc, as well as humification degree and E4/E6 differed between AS and CF, with faster humification in AS. We suggest that litter stoichiometric traits are more responsible for regulating litter humification than environmental conditions in elevational gradients. Furthermore, potential upward shifts by plants may accelerate litter humification in alpine ecosystems.

Tree growth responses and resilience after the 1950-Zayu-Medog earthquake, southeast Tibetan Plateau

Understanding how severe disturbances affect forest dynamics is fundamental in ecology, conservation and management. Earthquakes of large magnitude severely impact mountain landscapes, causing strong disturbances on forests. However, it is unknown how the resilience of tree growth after strong earthquakes changes as a function of site factors as elevation. Herein, we investigated the radial-growth responses of surviving trees after the 1950-Zayu-Medog MW 8.6 earthquake which devastated the southeastern Tibetan Plateau. We reconstructed radial growth dynamics of Abies delavayi var. motuoensis after 1950 in six sites selected along an elevational transect located in the Medog valley. Post-earthquake growth responses were detected among 60% of sampled trees in the period 1950–1955, but these responses varied depending on site elevation. Trees at the alpine treeline were less disturbed compared to those located at mid and low elevations. Severe growth suppressions, including the formation of missing rings, occurred during the first three years after the earthquake, and were stronger at low elevations. Growth releases mostly occurred after 1954. Long-term growth release, lasting more than ten years, was mainly observed at low elevations, near talus fans prone to landslides and rock falls. Growth rates returned to pre-earthquake values 45 years after the earthquake occurrence. Our results evidence how tectonic influences on tree growth depend on local factors as elevation, and demonstrate that tree resilience after severe geo-hazards is contingent on site conditions.

Linking variability of tree water use and growth with species resilience to environmental changes

Tree growth is an indicator of tree vitality and its temporal variability is linked to species resilience to environmental changes. Second‐order statistics that quantify the cross‐scale temporal variability of ecophysiological time series (statistical memory) could provide novel insights into species resilience. Species with high statistical memory in their tree growth may be more affected by disturbances, resulting in lower overall resilience and higher vulnerability to environmental changes. Here, we assessed the statistical memory, as quantified with the decay in standard deviation with increasing time scale, in tree water use and growth of co‐occurring European larch Larix decidua and Norway spruce Picea abies along an elevational gradient in the Swiss Alps using measurements of stem radius changes, sap flow and tree‐ring widths. Local‐scale interspecific differences between the two conifers were further explored at the European scale using data from the International Tree‐Ring Data Bank. Across the analysed elevational gradient, tree water use showed steeper variability decay with increasing time scale than tree growth, with no significant interspecific differences, highlighting stronger statistical memory in tree growth processes. Moreover, Norway spruce displayed slower decay in growth variability with increasing time scale (higher statistical memory) than European larch; a pattern that was also consistent at the European scale. The higher statistical memory in tree growth of Norway spruce in comparison to European larch is indicative of lower resilience of the former in comparison to the latter, and could potentially explain the occurrence of European larch at higher elevations at the Alpine treeline. Single metrics of resilience cannot often summarize the multifaceted aspects of ecosystem functioning, thus, second‐order statistics that quantify the strength of statistical memory in ecophysiological time series could complement existing resilience indicators, facilitating the assessment of how environmental changes impact forest growth trajectories and ecosystem services.

Explaining the exceptional 4270m high elevation limit of an evergreen oak in the south-eastern Himalayas.

Unlike the well-understood alpine treeline, the upper range limits of tree taxa that do not reach the alpine treeline are largely unexplained. In this study, we explored the causes of the exceptionally high elevation (4270m) occurrence of broad-leaved evergreen oaks (Quercus pannosa) in the south-eastern Himalayas. We assessed the course of freezing resistance of buds and leaves from winter to summer at the upper elevational limit of this oak species. Linked to leaf phenology, we analyzed freezing resistance and assessed minimum crown temperature for the past 65years. We also examined potential carbon limitation at the range limit of this species. Last season buds and leaves operated at a safety margin of 5.5 and 11K in mid-winter. Once fully dehardened early in July, last season foliage is damaged at -5.9 and new foliage at -4.6°C. Bud break is timed for late June to early July when low temperature extremes historically were never below -3.0°C. The monsoon regime ensures a long remaining season (149 days), thus compensating for the late onset of shoot growth. Compared with a site at 3450m, specific leaf area is reduced, foliar non-structural carbohydrate concentrations are similar and the δ13C signal is higher, jointly suggesting that carbon limitation is unlikely at the range limit of this species. We also show that these oaks enter the growing season with fully intact (not embolized) xylem. We conclude that the interaction between phenology and freezing tolerance results in safe flushing, while still facilitating shoot maturation before winter. These factors jointly determine the upper range limit of this oak species. Our study illuminates an exceptional case of broad-leaved evergreen tree performance near the treeline, and by exploring a suite of traits, we can underpin the central role of flushing phenology in such a stressful environment.

Regional variability in the response of alpine treelines to climate change

The distributions of many high-elevation tree species have shifted as a result of recent climate change; however, there is substantial variability in the movement of alpine treelines at local to regional scales. In this study, we derive records of tree growth and establishment from nine alpine treeline ecotones in the Canadian Rocky Mountains, characterise the influence of seasonal climate variables on four tree species (Abies lasiocarpa, Larix lyallii, Picea engelmannii, Pinus albicaulis) and estimate the degree to which treeline movement in the twentieth century has lagged or exceeded the rate predicted by recent temperature warming. The growth and establishment records revealed a widespread increase in radial growth, establishment frequency and stand density beginning in the mid-twentieth century. Coinciding with a period of warming summer temperatures and favourable moisture availability, these changes appear to have supported upslope treeline advance at all sites (range, 0.23–2.00 m/year; mean, 0.83 + 0.67 m/year). However, relationships with seasonal climate variables varied between species, and the rates of treeline movement lagged those of temperature warming in most cases. These results indicate that future climate change impacts on treelines in the region are likely to be moderated by species composition and to occur more slowly than anticipated based on temperature warming alone.

Effects of Litter Quality Diminish and Effects of Vegetation Type Develop During Litter Decomposition of Two Shrub Species in an Alpine Treeline Ecotone

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.

Repeat Oblique Photography Shows Terrain and Fire-Exposure Controls on Century-Scale Canopy Cover Change in the Alpine Treeline Ecotone

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.

Tree‐to‐tree interactions slow down Himalayan treeline shifts as inferred from tree spatial patterns

AbstractAimThe spatial patterns of tree populations reflect multiple ecological processes. However, little is known on whether these patterns mediate responses to climate in marginal tree populations such as those forming alpine tree lines. We examined the influence of tree‐to‐tree interactions on the responsiveness of tree lines to climate warming.LocationCentral Himalayas.TaxonBetula utilis; Abies spectabilis.MethodsWe analysed a network of 17 tree line sites located across the central Himalayas, encompassing a wide longitudinal gradient characterized by increasing precipitation eastwards. We quantified the changes in density and the spatial patterns of three 50‐year age classes of the two main tree species found at the tree line (Betula utilis and Abies spectabilis), and related them to reconstructed shifts in tree line elevation.ResultsYounger trees showed clustering near the tree line, while older trees tended to show random spatial distribution. Clustering decreased as climate conditions ameliorated, that is, in the wetter eastern sites. Lower rate of tree line elevation change was observed at the sites with higher clustering intensity.Main ConclusionsOur study indicates that tree aggregation weakens tree line responsiveness to climate warming, and thus warming‐induced drought stress tends to lower tree line shift rates by enhancing clustering. It also highlights the complexity and contingency of site‐dependent tree line responses to climate. Hence, to advance our understanding of tree line processes, we should consider both direct and indirect influences of relevant biotic (tree‐to‐tree interactions) and abiotic (climate) drivers of tree line dynamics.

Alpine treeline ecotone stasis in the face of recent climate change and disturbance by fire.

How species respond to climate change will depend on biological characteristics, species physiological limits, traits (such as dispersal), and interactions with disturbance. We examine multi-decadal shifts in the distribution of trees at the alpine treeline in response to regional warming and repeated disturbance by fire in the Victorian Alps, south-east Australia. Alpine treelines are composed of Eucalyptus pauciflora subsp. niphophila (Snow Gum, Myrtaceae) species. The location and basal girth of all trees and saplings were recorded across treelines at four mountains in 2002 and 2018. We quantify changes in treeline position (sapling recruitment above treeline) over time in relation to warming and disturbance by fire, and examine changes in stand structure below treeline (stand density, size class analyses). Short-distance advance of the treeline occurred between 2002 and 2018, but was largely restricted to areas that were unburned during this period. No saplings were seen above treeline after two fires, despite evidence that saplings were common pre-fire. Below treeline, subalpine woodland stands were largely resilient to fire; trees resprouted from lignotubers. However, small trees were reduced in number in woodlands when burned twice within a decade. Population dynamics at the alpine treeline were responsive to recent climate change, but other factors (e.g. disturbance) are crucial to understand recruitment trends. Establishment of saplings above treeline was largely restricted to unburned areas. These results indicate fire is a strong demographic filter on treeline dynamics; there is a clear need to frame alpine treeline establishment processes beyond just being a response to climate warming. Long lag periods in treeline change may be expected where recurrent disturbance is a feature of the landscape.

Some (do not) like it hot: shrub growth is hampered by heat and drought at the alpine treeline in recent decades.

Mountain ecosystems are particularly sensitive to climate change. However, only a very small number of studies exist so far using annually resolved records of alpine plant growth spanning the past century. Here we aimed to identify the effects of heat waves and drought, driven by global warming, on annual radial growth of Rhododendron ferrugineum. We constructed two century-long shrub ring-width chronologies from R. ferrugineum individuals on two adjacent, north- and west-facing slopes in the southern French Alps. We analyzed available meteorological data (temperature, precipitation and drought) over the period 1960-2016. Climate-growth relationships were evaluated using bootstrapped correlation functions and structural equation models to identify the effects of rising temperature on shrub growth. Analysis of meteorological variables during 1960-2016 revealed a shift in the late 1980s when heat waves and drought increased in intensity and frequency. In response to these extreme climate events, shrubs have experienced significant changes in their main limiting factors. Between 1960 and 1988, radial growth on both slopes was strongly controlled by the sum of growing degree days during the snow free period. Between 1989 and 2016, August temperature and drought have emerged as the most important. Increasing air temperatures have caused a shift in the growth response of shrubs to climate. The recently observed negative effect of high summer temperature and drought on shrub growth can, however, be buffered by topographic variability, supporting the macro- and microrefugia hypotheses.

Non-Structural Carbohydrate Storage Strategy Explains the Spatial Distribution of Treeline Species.

Environmental factors that drive carbon storage are often used as an explanation for alpine treeline formation. However, different tree species respond differently to environmental changes, which challenges our understanding of treeline formation and shifts. Therefore, we selected Picea jezoensis and Betula ermanii, the two treeline species naturally occurring in Changbai Mountain in China, and measured the concentration of non-structural carbohydrates (NSC), soluble sugars and starch in one-year-old leaves, shoots, stems and fine roots at different elevations. We found that compared with P. jezoensis, the NSC and soluble sugars concentrations of leaves and shoots of B. ermanii were higher than those of P. jezoensis, while the starch concentration of all the tissues were lower. Moreover, the concentration of NSC, soluble sugars and starch in the leaves of B. ermanii decreased with elevation. In addition, the starch concentration of B. ermanii shoots, stems and fine roots remained at a high level regardless of whether the soluble sugars concentration decreased. Whereas the concentrations of soluble sugars and starch in one-year-old leaves, shoots and stems of P. jezoensis responded similarly changes with elevation. These findings demonstrate that compared with P. jezoensis, B. ermanii has a higher soluble sugars/starch ratio, and its shoots, stems and fine roots actively store NSC to adapt to the harsh environment, which is one of the reasons that B. ermanii can be distributed at higher altitudes.

The importance of biotic filtering on boreal conifer recruitment at alpine treeline

Treeline, the ecotone where forest transitions to alpine or tundra ecosystems, is considered the thermal limit to tree growth and survival. Despite temperature increases across mountainous areas and high latitudes globally, there has been no ubiquitous change in treeline position. The process of range expansion must initially depend on increased recruitment at, or beyond current range limits and recruitment limitations have been hypothesized as a mechanism for the variable response of treeline position to climate warming. We conducted a unique series of observational and experimental studies to quantify early‐life stage constraints, from seed production to seedling establishment, on black spruce Picea mariana and tamarack Larix laricina recruitment at a model alpine treeline in Newfoundland, Canada. We found recruitment at treeline to be simultaneously seed and establishment limited. The treeline population produced fewer seeds than the forest population and black spruce seeds produced at treeline were less viable. Tamarack was more seed limited than black spruce where seed viability was low regardless of altitudinal position. Post‐dispersal seed predation greatly constrained recruitment across the altitudinal gradient; however, black spruce seeds experienced the lowest levels of invertebrate seed predation on the lichen mat at treeline. If seeds were not consumed, individuals at treeline were establishment limited where germination and seedling establishment was both less abundant and delayed on lichen substrate. Our study highlights the need for multiple factors to align temporally for significant recruitment at treeline to occur.

Observed data for publication "Sensitivity of recruitment and growth of alpine treeline birch to elevated temperature"

Observed data for publication "Sensitivity of recruitment and growth of alpine treeline birch to elevated temperature"

Spatial detection of alpine treeline ecotones in the Western United States

Human-mediated climate change over the past century has resulted in significant impacts to global ecosystems and biodiversity including accelerating redistribution of the geographic ranges of species. In mountainous regions, the transition zone from continuous closed-canopy montane forests to treeless alpine tundra areas at higher elevations is commonly referred to as the “alpine treeline ecotone” (ATE). Globally, warming climate is expected to drive ATEs upslope, which could lead to negative impacts on local biodiversity and changes in ecosystem function. However, existing studies rely primarily on field-based data which are difficult and time consuming to collect. In this study, we define an ATE-detection index (ATEI) to automatically identify the ATE positions from 2009 to 2011 in the western United States using geospatial tools and remotely sensed datasets provided by Google Earth Engine. A binomial logistic regression model was fitted between standardized ATEI components and a binary variable of pixel status of 141 sampled Landsat pixels manually classified with high-resolution imagery in Google Earth. The average model accuracy was around 0.713 (±0.111) and the average Kappa coefficient was approximately 0.426 (±0.221) based on a 100-time repeated 10-fold cross-validation. Furthermore, the ATEI-estimated elevation is highly correlated (Pearson's r = 0.98) with a published set of field-collected ATE elevations at 22 sampling sites across the region. The detection metric developed in this study facilitates monitoring the geographic location and potential shifts of ATEs as well as the general impact of climate change in mountainous areas during recent decades. We also expect this approach to be useful in monitoring other ecosystem boundaries.