Alpine Treeline Research Articles

Assessing Changes in Mountain Treeline Ecotones over 30 Years Using CNNs and Historical Aerial Images

Historical black-and-white (B&W) aerial images have been recognized as an important source of information for assessing vegetation dynamics. However, the use of these images is limited by the lack of multispectral information, as well as by their varying quality. It is therefore important to study and develop methods that are capable of automatic and accurate classification of these B&W images while reducing the need for tedious manual work. The goal of this study was to assess changes over 30 years in woody vegetation cover along alpine treeline ecotones using B&W aerial images from two time points. A convolutional neural networks model was firstly set up based on three structure classes calculated from Airborne Laser Scanning data using the B&W aerial images from 2010. Then, the model was improved by active addition of training samples of those that were wrongly predicted from historical B&W aerial images from 1980. A comparison with visual image interpretation revealed generally high agreement for the class “dense forest” and lower agreement for the class “group of trees”. The study illustrates that vegetation changes at the treeline ecotone can be detected in order to assess areawide long-term vegetation dynamics at a fine spatial resolution.

Dynamics of the Alpine Treeline Ecotone under Global Warming: A Review

The alpine treeline ecotone is defined as a forest-grassland or forest-tundra transition boundary either between subalpine forest and treeless grassland, or between subalpine forest and treeless tundra. The alpine treeline ecotone serves irreplaceable ecological functions and provides various ecosystem services. There are three lines associated with the alpine treeline ecotone, the tree species line (i.e., the highest elevational limit of individual tree establishment and growth), the treeline (i.e., the transition line between tree islands and isolated individual trees) and the timber line (i.e., the upper boundary of the closed subalpine forest). The alpine treeline ecotone is the belt region between the tree species line and the timber line of the closed forest. The treeline is very sensitive to climate change and is often used as an indicator for the response of vegetation to global warming. However, there is currently no comprehensive review in the field of alpine treeline advance under global warming. Therefore, this review summarizes the literature and discusses the theoretical bases and challenges in the study of alpine treeline dynamics from the following four aspects: (1) Ecological functions and issues of treeline dynamics; (2) Methodology for monitoring treeline dynamics; (3) Treeline shifts in different climate zones; (4) Driving factors for treeline upward shifting.

Alpine Treeline Dynamics and the Special Exposure Effect in the Hengduan Mountains.

Alpine treeline is highly sensitive to climate change, but there remains a lack of research on the spatiotemporal heterogeneity of treeline and their relationships with climate change at the landscape scale. We extracted positions of alpine treeline from high-resolution Google Earth images from three periods (2000, 2010, and 2020) and analyzed the elevation patterns and dynamics of treeline positions in the Hengduan Mountains. Based on the treeline positions in 2020, a buffer zone of 300 m is established as the treeline transition zone, and the changing trend of the fraction vegetation cover (FVC) from 2000 to 2020 and its relationship with climate are also analyzed. Due to the special geographical and climatic environment, the treeline in the Hengduan Mountains area is high in the middle but lower in the surrounding areas. We found that over the past 20 years, the treeline position did not change significantly but that the FVC increased in 80.3% of the treeline areas. The increase in FVC was related to the decrease in precipitation in the growing season. The results also revealed a special exposure effect on the alpine treeline in the Hengduan Mountains. Because of the lower treeline, isotherm position caused by the monsoon climate, the treeline position on south-facing slopes is lower than that on slopes with other exposures. Our results confirmed that the pattern and dynamics of the alpine treeline are driven by the regional monsoon climate regime.

Transient Effects of Snow Cover Duration on Primary Growth and Leaf Traits in a Tundra Shrub.

With the recent climate warming, tundra ecotones are facing a progressive acceleration of spring snowpack melting and extension of the growing season, with evident consequences to vegetation. Along with summer temperature, winter precipitation has been recently recognised as a crucial factor for tundra shrub growth and physiology. However, gaps of knowledge still exist on long-living plant responses to different snowpack duration, especially on how intra-specific and year-to-year variability together with multiple functional trait adjustments could influence the long-term responses. To fill this gap, we conducted a 3 years snow manipulation experiment above the Alpine treeline on the typical tundra species Juniperus communis, the conifer with the widest distributional range in the north emisphere. We tested shoot elongation, leaf area, stomatal density, leaf dry weight and leaf non-structural carbohydrate content of plants subjected to anticipated, natural and postponed snowpack duration. Anticipated snowpack melting enhanced new shoot elongation and increased stomatal density. However, plants under prolonged snow cover seemed to compensate for the shorter growing period, likely increasing carbon allocation to growth. In fact, these latter showed larger needles and low starch content at the beginning of the growing season. Variability between treatments slightly decreased over time, suggesting a progressive acclimation of juniper to new conditions. In the context of future warming scenarios, our results support the hypothesis of shrub biomass increase within the tundra biome. Yet, the picture is still far from being complete and further research should focus on transient and fading effects of changing conditions in the long term.

Treeline Ecotone Progression and Stability: Time Series Analysis of Individual Photographic Data 1973-2021 in the Swedish Scandes

Positional and population change of the alpine treeline ecotone were studied at a site (isolated low-fell) in the Swedish Scandes. Methods included early-20th century records and subsequent re-visitations, including repeat photography, up to 2021. Substantial temperature rise (summer and winter) during the past 100 years, has left the elevational forest border virtually unchanged. However, the treelines, i.e. scattered solitary trees >2 m tall, of mountain birch (Betula pubescens ssp. czerepanovii) and Scots pine (Pinus sylvestris) have risen by maximum 70 and 65 m, respectively since the early 20th century, in contrast to inertia of Norway spruce (Picea abies). Hitherto, pine appears as the most successful species, particularly in the light of the relatively large numbers of new trees established around the elevated treeline and the proliferous regeneration in the treeline ecotone during the past few decades. Presumably, a subalpine pine belt may eventually replace the sparse birch belt in the snow-poor SE-facing slope, in accordance with regional tendencies. Climate warming has reduced the snow cover duration on the NE-facing slope, to the benefit of mountain birch. Possibly, closed birch stands will evolve here. The study mountain displays a spectrum of different local habitat types, with contrasting quantitative and qualitative responses to climate change, representative of prevailing regional treeline ecotone dynamics. Important and complex constraints to projections of treeline growth and rise, in response to alleged future climate warming, include geomorphology, herbivory and snow pack.

Fine-Spatial Boreal–Alpine Single-Tree Albedo Measured by UAV: Experiences and Challenges

The boreal–alpine treeline is a fine-spatial heterogeneous ecotone with small single trees, tree clusters and open snow surfaces during wintertime. Due to climate change and decreased grazing of domestic animals, the treelines expand both upwards into the mountains and northwards into the tundra. To quantify and understand the biophysical radiative climatic feedback effect due to this expansion, it is necessary to establish measurement strategies of fine-spatial albedo by which relationships with the tree structure and snow-masking effect can be quantified. In this study, we measured single-tree Norway spruce albedo for small trees using an unmanned aerial vehicle (UAV). The platform allows the measurement of proximal remotely sensed albedo, enabling the provision of fine-spatial reflectance distributed over larger geographical areas. The albedo measurements varied from 0.39 to 0.99. The interaction between the diurnal course of the sun and sloping terrain constituted the most important driving factor on the albedo. Surprisingly, all tree structural variables revealed smaller correlations with the albedo than typically found for boreal and boreal–alpine summertime albedo. The snow-masking effect of the trees on the albedo was statistically significant and was found to be stronger than the effects of tree structural variables. Only the canopy density had a statistically significant effect on the albedo among the tree structural variables. This was likely explained by the imprecise heading of the hoovering positions of the UAV and insufficient spatial resolution of the reflected radiation measurements. For further development of fine-spatial UAV-measured albedo, we recommend the use of UAVs with high-precision navigation systems and field-stop devices to limit the spatial size of the measured reflected radiation.

Land cover classification of treeline ecotones along a 1100 km latitudinal transect using spectral‐ and three‐dimensional information from UAV‐based aerial imagery

AbstractThe alpine treeline ecotone is expected to move upwards in elevation with global warming. Thus, mapping treeline ecotones is crucial in monitoring potential changes. Previous remote sensing studies have focused on the usage of satellites and aircrafts for mapping the treeline ecotone. However, treeline ecotones can be highly heterogenous, and thus the use of imagery with higher spatial resolution should be investigated. We evaluate the potential of using unmanned aerial vehicles (UAVs) for the collection of ultra‐high spatial resolution imagery for mapping treeline ecotone land covers. We acquired imagery and field reference data from 32 treeline ecotone sites along a 1100 km latitudinal gradient in Norway (60–69°N). Before classification, we performed a superpixel segmentation of the UAV‐derived orthomosaics and assigned land cover classes to segments: rock, water, snow, shadow, wetland, tree‐covered area and five classes within the ridge‐snowbed gradient. We calculated features providing spectral, textural, three‐dimensional vegetation structure, topographical and shape information for the classification. To evaluate the influence of acquisition time during the growing season and geographical variations, we performed four sets of classifications: global, seasonal‐based, geographical regional‐based and seasonal‐regional‐based. We found no differences in overall accuracy (OA) between the different classifications, and the global model with observations irrespective of data acquisition timing and geographical region had an OA of 73%. When accounting for similarities between closely related classes along the ridge‐snowbed gradient, the accuracy increased to 92.6%. We found spectral features related to visible, red‐edge and near‐infrared bands to be the most important to predict treeline ecotone land cover classes. Our results show that the use of UAVs is efficient in mapping treeline ecotones, and that data can be acquired irrespective of timing within a growing season and geographical region to get accurate land cover maps. This can overcome constraints of a short field‐season or low‐resolution remote sensing data.

Causes for the increase of early-season freezing events under a warmer climate at alpine Treelines in southeast Tibet

Few data have demonstrated why early-season freezing events may increase under a warmer climate at alpine treelines, which is critical to understand the key limiting factors determining treeline dynamics under future climate warming. Here we test the hypothesis that the increase of early-season freezing events under a warmer climate is mainly associated with advanced onset of growing season and enhanced radiative cooling effect in the pre-monsoon season. We conducted 11-year observations of microclimate factors across north-facing and south-facing treelines in the Sergyemla Mountains, southeast Tibet. We found that the frequency, intensity, and duration of early-season freezing events were generally higher under a warmer climate on the south-facing slope or in the warmer years within a slope, in which the frequency of early-season freezing events significantly increased with increasing annual mean air-temperature. During 2006–2016, the frequency, intensity and duration of early-season freezing events typically showed a negative correlation with the onset date of growing season, while their frequency was positively correlated with the early-season global radiation. In each of both slopes, global radiation was significantly higher and long-wave radiation balance was much more negative on days with daily minimum air-temperature (Tmin) < 0 °C than with Tmin > 0 °C, indicating the generality of radiative cooling effect on early-season freezing events at high elevations. Our data support the hypothesis, revealing physical mechanisms for the increase of early-season freezing events with climate warming. The physical mechanisms should provide a general explanation for the topography-dependent pattern of tree species distribution and alpine treeline stability under climate warming in high mountainous regions like the Himalayas.

Both specific plant functional type loss and vegetation change influence litter metallic element release in an alpine treeline ecotone

Both specific plant functional type loss and vegetation change influence litter metallic element release in an alpine treeline ecotone

Minimum Leaf Conductance (g min) Is Higher in the Treeline of Pinus uncinata Ram. in the Pyrenees: Michaelis' Hypothesis Revisited.

The search for a universal explanation of the altitudinal limit determined by the alpine treeline has given rise to different hypotheses. In this study, we revisited Michaelis’ hypothesis which proposed that an inadequate “ripening” of the cuticle caused a greater transpiration rate during winter in the treeline. However, few studies with different explanations have investigated the role of passive mechanisms of needles for protecting against water loss during winter in conifers at the treeline. To shed light on this, the cuticular transpiration barrier was studied in the transition from subalpine Pinus uncinata forests to alpine tundra at the upper limit of the species in the Pyrenees. This upper limit of P. uncinata was selected here as an example of the ecotones formed by conifers in the temperate mountains of the northern hemisphere. Our study showed that minimum leaf conductance in needles from upper limit specimens was higher than those measured in specimens living in the lower levels of the sub-alpine forest and also displayed lower cuticle thickness values, which should reinforce the seminal hypothesis by Michaelis. Our study showed clear evidence that supports the inadequate development of needle cuticles as one of the factors that lead to increased transpirational water losses during winter and, consequently, a higher risk of suffering frost drought.

Upward Treeline Shifts in Two Regions of Subarctic Russia Are Governed by Summer Thermal and Winter Snow Conditions

Climate warming impacts on alpine treeline dynamics. However, we still lack robust assessments of the long-term impacts of climate on tree recruitment at the treeline, particularly in remote areas such as the subarctic regions of Russia subjected to different climate influences. We expected that the treelines in two regions may have different features and dynamics patterns. We analyzed climate variables and assessed treeline dynamics by quantifying recruitment using the tree rings of ca. 7000 trees of four species (Betula pubescens Ehrh. ssp. tortuosa, Pinus sylvestris L., Picea abies Ledeb. ssp. obovata, Larix gmelinii Rupr.) along 14 altitudinal transects (series of study plots). We compared the Khibiny Massif (Kola Peninsula) and the western Putorana Plateau, subjected to oceanic and continental influences, respectively. In both regions, summers became warmer, and winters became snowier during the past century. At the low part of the treeline ecotone, tree recruitment has slowly increased since the mid-18th century at the Putorana Plateau and the mid-19th century at the Khibiny but accelerated in the early 20th century at both regions and reached a maximum peak in the second half of the past century. Treeline encroachment intensified in the 1930s at the Khibiny and the 1950s at the Putorana Plateau. Trees encroached in the tundra leading to upward treeline shifts in the late 20th century. The slope exposure affected the rates of treeline shift with higher upward advances on southern-oriented slopes. Tree recruitment and early-winter precipitation were positively correlated. The differences in species composition, treeline altitude and influences of slope orientation on treeline dynamics can be explained primarily by differences in the degree of continentality. The abundance of saplings in both regions allows the future encroachment of trees into tundra and further treeline upward shifts to be forecast.

Intraspecific trait variation, growth, and altered soil conditions at tree species distribution limits: From the alpine treeline to the rear edge

Alpine treelines are expected to shift upward due to climate warming, whereas warmer conditions can have negative impacts on forests located near the xeric, equatorward limit of the distribution of tree species (rear edge). We compare tree populations forming the distribution limits of mountain pine (Pinus uncinata) in north-eastern Spain: two cold-limited alpine treeline populations, and a rear-edge drought-prone stand. In the treelines, trees were sampled in three altitudinal belts to evaluate within-site variations considering the forest, the transitional ecotone and the treeline.Tree growth was markedly higher in one of the treeline sites (Tesso), particularly in the forest belt, as compared to the other treeline (Las Cutas), and to the warmest rear-edge stand. Tree growth in the rear-edge population depended on June precipitation. Trees presented also comparatively smaller leaves with higher Specific Leaf Area (SLA) there than in the two treelines. Regarding the differences between altitudinal belts within the treelines, treeline soils showed lower N concentrations and a lower content of sand than in forests. Soil microbiota was dominated by bacteria in the treeline and by fungi in the forest, although with differences between sites. Soil characteristics and microbial composition were strongly related, whereas its relationship with tree growth and functional traits was less clear.The marked differences in leaf traits and growth response to climate found when comparing treeline and rear-edge sites highlights the biogeographical uniqueness of sites forming the equatorward distribution limit. The greater tree growth in the Tesso treeline as compared to Las Cutas indicate that interactions between climate and physical and chemical properties of the soil influence tree growth and its interactions with soil microbial communities in the treeline. A better understanding of plant-soil feedbacks may help to understand the future dynamics of tree populations forming the species’ climatic and geographic limits of distribution.

Soil extracellular enzyme activities and their stoichiometric ratio in the alpine treeline ecotones in Gongga Mountain, China

Soil extracellular enzyme activities and their stoichiometric ratio in the alpine treeline ecotones in Gongga Mountain, China

Automatic alpine treeline extraction using high-resolution forest cover imagery

Automatic alpine treeline extraction using high-resolution forest cover imagery

Tree line shifts, changing vegetation assemblages and permafrost dynamics on Galdhøpiggen (Jotunheimen, Norway) over the past ~4400 years

An environmental reconstruction based on palynological evidence preserved in peat was carried out to examine late-Holocene alpine tree line dynamics in the context of past climatic changes on Galdhøpiggen (Jotunheimen, southern Norway). We analysed a peat core taken from a mire at the present-day tree line (1000 m a.s.l.), c. 450 m downslope from the lower limit of sporadic permafrost. We adopted a combination of commonly used indicators of species’ local presence to reconstruct past vegetation assemblages, such as the relative pollen abundance (%), pollen accumulation rate (PAR), and presence of indicator species. Additionally, fossil pollen from the peat sequence was compared to modern pollen from a surface moss polster to establish a modern analogue. The results were compared with studies covering the late-Holocene climatic changes in the area. The reconstruction demonstrates that a pine-dominated woodland reached above the present-day tree line at c. 4300 cal. yr BP, suggesting a warmer climate suitable for Scots pine ( Pinus sylvestris) growth at this altitude. Scots pine retreated to lower altitudes between c. 3400 and 1700 cal. yr BP, accompanied by the descent of the low-alpine shrub-dominated belt, in response to cooling climatic conditions. The colder period covered c. 1700–170 cal. yr BP, and an open downy birch ( Betula pubescens) woodland became widespread at 1000 m a.s.l., whilst pine remained sparse at this altitude. From c. 170 cal. yr BP onwards, warming allowed pine to re-establish its local presence alongside downy birch at 1000 m a.s.l.

Has tree density increased at alpine treelines on the eastern Tibetan Plateau?

Treeline densification, along with increased growth rates, is considered a primary manifestation of climate warming at alpine treelines. However, treeline densification has typically been inferred from comparisons of present-day tree density with estimates of former densities inferred from current age structure; the densification has not been verified with long-term monitoring data and thus empirical testing is needed. In this study, a series of plots was established along an altitudinal gradient in a treeline ecotone on the eastern Tibetan Plateau; the plots were then surveyed repeatedly for ten years to analyse spatiotemporal variation in tree regeneration. The densities of Abies fargesii var. faxoniana seedlings and saplings increased from low altitude sites to high altitude sites, before dropping to zero beyond the treeline. The density of fir seedlings at the treeline in 2018 was significantly lower than in 2008. There were no significant differences in the density of saplings, small trees, medium trees, or old-growth trees between 2018 and 2008. As compared to regeneration patterns from 65 years ago, treeline densification represents a spatial phenomenon related to altitude, but not a temporal pattern on the eastern Tibetan Plateau. A more comprehensive understanding of the effects of climate warming on treeline regeneration will require further long-term monitoring and research.

Land Use Changes in the Alpine Tree Line Ecotone in the Hrubý Jeseník Mountains (Czech Republic)

Abstract Currently, we expected a altitudinal shift of forest vegetation upward under predicted climate changes impacts to European mountains ecosystems. In this context can be very interesting, that changes in alpine tree line ecotone in high European mountains were induced by human activities relatively often also in history. Probably one of the most important driver in land-use changes in high mountains was grazing, which significantly have influenced both mountain spruce forests and open alpine grasslands in central-Europe. This paper deals with historical changes of land-use in Hruby Jesenik Mountains (Czech Republic) during 19th and 20th centuries. Results revealed consequences of former grazing in mountain grasslands above alpine tree line ecotone in the frame of land-use analyses based on historical maps and other sources. These results, based on historical data, support our better understanding to current dynamic changes in European mountain landscapes.

Tree Regeneration Patterns on Contrasting Slopes at Treeline Ecotones in Eastern Tibet

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.

Branch water uptake and redistribution in two conifers at the alpine treeline

During winter, conifers at the alpine treeline suffer dramatic losses of hydraulic conductivity, which are successfully recovered during late winter. Previous studies indicated branch water uptake to support hydraulic recovery. We analyzed water absorption and redistribution in Picea abies and Larix decidua growing at the treeline by in situ exposure of branches to δ2H-labelled water. Both species suffered high winter embolism rates (> 40–60% loss of conductivity) and recovered in late winter (< 20%). Isotopic analysis showed water to be absorbed over branches and redistributed within the crown during late winter. Labelled water was redistributed over 425 ± 5 cm within the axes system and shifted to the trunk, lower and higher branches (tree height 330 ± 40 cm). This demonstrated relevant branch water uptake and re-distribution in treeline conifers. The extent of water absorption and re-distribution was species-specific, with L. decidua showing higher rates. In natura, melting snow might be the prime source for absorbed and redistributed water, enabling embolism repair and restoration of water reservoirs prior to the vegetation period. Pronounced water uptake in the deciduous L. decidua indicated bark to participate in the process of water absorption.

Opposite Tree-Tree Interactions Jointly Drive the Natural Fir Treeline Population on the Southeastern Tibetan Plateau

The long-term stability of alpine treeline positions and increased stem density are frequently reported by recent studies; however, whether a denser treeline forest is relevant to competitive tree–tree interactions remain unclear. Herein, we mapped and surveyed individual trees in two undisturbed Smith fir (Abies georgei var. smithii) treeline plots (with a size: 30 m × 200 m; plot NE1: 4477 m, NE2: 4451 m) near Ranwu Lake (RW) on the southeastern Tibetan Plateau. The surface pattern method and spatial point pattern analysis were used to detect the spatial distribution patterns of three size classes (seedlings, juveniles, adults) and spatial associations between the pairwise size classes. We also compared our results to the spatial patterns of the five other treeline forests (Deqin, Linzhi, Changdu, Yushu, Aba) reported from the Tibetan Plateau. Young trees dominated the two fir treeline plots. Both positive and negative spatial autocorrelations for all of the trees were detected in two study plots. Intraspecific facilitation and competition coexisted at the fir treelines in three forest regions (RW, Linzhi, Aba) characterized by a mild moist climate, whereas intraspecific facilitation dominated the other three forest regions (Changdu, Deqin, Yushu), which featured seasonal climatic stress or high disturbance pressure. Thus, increased stem density at alpine treeline can be linked to competitive interactions in relatively favorable environmental conditions. Overall, the spatial patterns of the treeline population are mainly shaped by the combination of thermal and moisture conditions and are also modulated by non-climatic variables (e.g., disturbance history and microtopography).