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Several high-elevation plant species would experience an increased risk of regional extinction due to various climatic and anthropogenic factors. Information about the effects of climate change is urgently needed for modeling vegetation dynamics because it influences the various seed parameters like seed germination, seed maturation, seed mass, and seed bank in the soil. The present study was conducted at an elevation of 3145-3560 m in the treeline area of the western Himalayan region of India. The change in seed color is correlated with other seed parameters such as seed moisture content, seed germination, seed mass, and seed fall density. A decline in moisture content in maturing seeds is closely related to seed maturity (p < 0.05). Quercus semecarpifolia contains the highest seed mass followed by Abies spectabilis. Reportedly, the species with higher seed mass have an advantage in light-restricted environments for seed germination and seedling development. In addition, the fruit mass was observed to be the highest for Rhododendron campanulatum, while both Betula utilis and R. arboreum had similar fruit mass. The seed fall density varied between 1.55 and 7.85 seeds m-2 and the maximum mortality of up to 32% of seedlings was observed during post-monsoon season from November to February. The potential disruption in the timing of seed fall, soil seed bank, and seed germination due to climatic irregularities has broader implications for forest ecosystems. Generally, the soil in treeline areas gets frozen during winter, resulting in seedlings facing severe water stress and a high rate of transpiration. The present study addresses the issue regarding the survival and proliferation of important treeline species in the western Himalayan region of India. [ABSTRACT FROM AUTHOR]

There are growing evidences that indicate the Himalayan region is warming rapidly with more warming in high elevation areas. The elevation-dependent warming (EDW) accelerates the rate of change in mountain ecosystems, including cryosphere, hydrology, biodiversity and socio-economic systems. Here, we present temperature lapse rates (TLRs) based on primary data from 21 stations for three elevation transects leading to treeline (Western Himalaya: WH; Central Himalaya: CH; Eastern Himalaya: EH) representing different climate regimes along the Indian region of Himalayan Arc. TLRs were calculated using high temporal resolution data collected for 2 years (2017–2018) from complex Himalayan terrain. The annual mean TLR increased with decreasing moisture, being markedly higher for dry WH transect (− 0.66 °C/100 m) than at moderately moist CH (− 0.52 °C/100 m) and characteristically moist EH transect (− 0.50 °C/100 m). The One-Way Analysis of Variance (ANOVA) confirms that the TLR varied spatially, declining from West to East across the Himalayan Arc, and significantly differed seasonally. The lowest mean TLRs were found during the winter season (EH: − 0.46 °C/100 m; CH: − 0.40 °C/100 m; WH: − 0.31 °C/100 m). The monthly TLR for EH transect varied within a narrower range (− 0.32 °C/100 m to − 0.54 °C/100 m), than for CH transect (− 0.24 °C/100 m to − 0.68 °C/100 m), and WH transect (− 0.26 °C/100 m to − 0.90 °C/100 m). The lowest monthly TLR occurred in December (− 0.24 °C/100 m to − 0.32 °C/100 m) for all three transects. The relationship of TLR with rainfall and saturation vapor pressure was analyzed for CH transect to find out influence of these factors on seasonal variation in lapse rate. Moisture, snow albedo and reflectance are the factors which largely control the TLR along the elevation transects. The shallow TLR and higher growing season temperature values (9.2 ± 1.8 °C, 10.0 ± 1.4 °C, and 7.8 ± 1.7 °C), than normally found at treelines, may suggest that treeline environment in Himalaya is warming more rapidly than lowland areas. TLR was lowest in December due to reduced albedo and EDW, which influence treeline dynamics, snow and moisture regime, surface energy balance, species distribution, and growing season of alpine vegetation. The findings of this study provide useful insights to re-parameterize the climate models over the Himalayan region. This study facilitates in improving interpolation of air temperature for ecological studies in un-gauged and data-sparse regions, especially for the alpine region of Himalaya where observed data are extremely scarce. [ABSTRACT FROM AUTHOR]

One of the most noticeable environmental discontinuities in mountains is the transition that exists in vegetation below and above the treeline. In the North Patagonian Andean lakes (between 900 and 1950 m a.s.l.), we analyzed the bacterial community composition of lakes in relation to surrounding vegetation (erected trees, krummholz belt, and bare rocks), dissolved organic carbon (DOC), and total dissolved nutrients (nitrogen, TDN and phosphorus, TDP). We observed a decrease in DOC, TDP, and TDN concentrations with altitude, reflecting shifts in the source inputs entering the lakes by runoff. Cluster analysis based on bacterial community composition showed a segregation of the lakes below treeline, from those located above. This first cluster was characterized by the cyanobacteria Cyanobium PCC-6307, while in the krummholz belt and bare rocks, bacterial communities were dominated by Actinobacteria hgcl-clade and Proteobacteria (Sandarakinorhabdus and Rhodovarius), with the presence of pigments such as actinorhodopsin, carotenoids, and bacteriochlorophyll a. The net relatedness index (NRI), which considers the community phylogenetic dispersion, showed that lakes located on bare rocks were structured by environmental filtering, while communities of lakes below treeline were structured by species interactions such as competition. Beta-diversity was higher among lakes below than among lakes located above the treeline. The contribution of species turnover was more important than nestedness. Our study brings light on how bacterial communities may respond to changes in the surrounding vegetation, highlighting the importance of evaluating different aspects of community structure to understand metacommunity organization. [ABSTRACT FROM AUTHOR]

Biodiversity is a characteristic of ecosystems that plays a crucial role in the study of their evolution, and to estimate it, the species of all plants need to be determined. In this study, we used Unmanned Aerial Vehicles to gather RGB images of mid-to-high-altitude ecosystems in the Zao mountains (Japan). All the data-collection missions took place in autumn so the plants present distinctive seasonal coloration. Patches from single trees and bushes were manually extracted from the collected orthomosaics. Subsequently, Deep Learning image-classification networks were used to automatically determine the species of each tree or bush and estimate biodiversity. Both Convolutional Neural Networks (CNNs) and Transformer-based models were considered (ResNet, RegNet, ConvNeXt, and SwinTransformer). To measure and estimate biodiversity, we relied on the Gini–Simpson Index, the Shannon–Wiener Index, and Species Richness. We present two separate scenarios for evaluating the readiness of the technology for practical use: the first scenario uses a subset of the data with five species and a testing set that has a very similar percentage of each species to those present in the training set. The models studied reach very high performances with over 99 Accuracy and 98 F1 Score (the harmonic mean of Precision and Recall) for image classification and biodiversity estimates under 1% error. The second scenario uses the full dataset with nine species and large variations in class balance between the training and testing datasets, which is often the case in practical use situations. The results in this case remained fairly high for Accuracy at 90.64% but dropped to 51.77% for F1 Score. The relatively low F1 Score value is partly due to a small number of misclassifications having a disproportionate impact in the final measure, but still, the large difference between the Accuracy and F1 Score highlights the complexity of finely evaluating the classification results of Deep Learning Networks. Even in this very challenging scenario, the biodiversity estimation remained with relatively small (6–14%) errors for the most detailed indices, showcasing the readiness of the technology for practical use. [ABSTRACT FROM AUTHOR]

Treeline is a global ecological phenomenon in which tree populations decline, often abruptly, above a specific elevation or latitude. Temperature is thought to be a key determinant of treeline because it affects the rates at which trees establish, grow, produce seeds, and die. As climate change causes temperature increases, treelines have been observed to move in response—but there is considerable variability. In this study, we present a general mathematical model that provides possible explanations for both the general patterns observed in treelines and some of the variation. Avoiding system-specific details, our model assumes simply that all life processes are temperature-dependent. We incorporate the possibility of positive or negative feedback, in which the presence of trees either increases or decreases the temperature at their location. Our results indicate that this feedback and the relationship between temperature thresholds for growth, seed production, and seedling establishment are the key determinants of tree line form and movement. The model also shows that under many conditions bistability is predicted: treeline can equilibrate at two different elevations under the same conditions, depending on the system's history. General, flexible models like ours are essential for generating a unifying theory of treeline form and dynamics across multiple ecosystems. [ABSTRACT FROM AUTHOR]

In the European Alps, mean temperature has risen by 2.5 K since the end of the nineteenth century. A 2 K warming of the growing season has taken place in the last 4 decades only. The 2.5 K warming should rise the position of the climatic treeline by about 400 m. Actual shifts in uppermost tree positions reported here for the Austrian Defereggen Valley and the Swiss Lower Engadine region of the Eastern Alps reach only around 140 m of elevation above the limit of old trees that date back to the nineteenth century. Uppermost Pinus cembra trees of > 2 m height currently occur at c. 2500 m, representing elevation records for the Eastern Alps. In situ temperature records for 2022–2023 revealed seasonal mean temperatures for uppermost trees that are 1–3 K higher than the equilibrium treeline isotherm of c. 6 °C in both regions (corrected for temperature anomalies from long-term records). The 2 K span reflects microhabitat differences and two ways to define the season. Thus, tree advances lag behind the upslope shift of the treeline isotherm, on average, by more than 200 m. The uppermost trees currently grow under quite warm conditions with annual shoot length increments frequently reaching 20 cm. Even without additional future warming, the new steady-state climatic treeline will exceed the Holocene maximum elevation in the Eastern Alps substantially. [ABSTRACT FROM AUTHOR]

Invasive species represent a threat to many ecosystems but are nearly absent from arctic and subarctic Canada. The treeline town of Churchill, Manitoba, has an unusually high number of invasive species due to the presence of a grain port and railway, but few of these species have spread to the surrounding tundra; for instance, common dandelion (Taraxacum officinale) currently is restricted primarily to town, in contrast with its native congener, Taraxacum lacerum. To investigate whether the distribution of T. officinale is affected by the tendency of buildings to create warm, sheltered microenvironments, we surveyed the abundance of both dandelion species relative to existing structures in town. We established transects extending up to 5 m from each side of 23 buildings, and recorded the number of plants in 50 cm × 50 cm quadrats along each transect. We found that T. officinale performed better on the south sides of buildings, that flowering decreased away from buildings, and that these effects were stronger for T. officinale than the native T. lacerum. Soil temperatures were warmer on the south rather than on the north-facing sides, providing a possible explanation. Our results show that the distribution of T. officinale currently is more influenced by human-created microclimate compared to the abundance of its native relative. However, T. officinale may spread beyond these refuges as the climate continues to warm. [ABSTRACT FROM AUTHOR]

Abies spectabilis (East Himalayan Fir), a dominant timber-producing species in the Himalayas, plays a dynamic role in ecological balance and sustains various habitats across temperate to subalpine altitudes. Himalayan montane forests express signs of changes in the growth, regeneration, and population density of forest tree species due to ongoing climate change, including its upper distribution limit. The research was conducted at 30°11'02˝N and 79°39′36˝E, ranging from 3,100 to 3,500 m asl elevation in the treeline ecotone of western Himalaya. The study presents a comprehensive perspective on how water availability and its intricate interactions with climatic and ecological variables shape the dynamics of treeline ecosystems. The study's phytosociological analysis of treeline sites revealed the complex interplay between species composition, adaptability, and regeneration potential. The eco-physiological aspects of water balance were thoroughly explored, underscoring how plants employ various strategies to cope with changing water availability. The soil moisture content ranged from 32.17 to 73.50%. The dynamic nature of water potential (pre-dawn (ΨPD) and mid-day (ΨMD)) across seasons and years reflects and varies between -0.13 and - 1.25 MPa, the species' ability to adjust and manage water balance according to environmental shifts. Osmotic potential at full turgor varied from -0.72 to -1.77 MPa, these adjustments are crucial for key life cycle events of vegetative and reproductive phases of species. The strong correlation of water potential (Ψ) and phenophases emphasizes the importance of water status in regulating these vital processes. Furthermore, the study delved into the maturation and germination processes, highlighting the significance of Ψ and moisture content in seed development and germination success. The maximum germination of 46.33% was observed when the seed moisture content was 30.90 ± 3.11%. The observed correlations between cone characteristics and seed parameters indicate the existence of trade-offs that impact seed size, weight, and germination capacity, contributing to the overall fitness of the species. [ABSTRACT FROM AUTHOR]

Several high-elevation plant species would experience an increased risk of regional extinction due to various climatic and anthropogenic factors. Information about the effects of climate change is urgently needed for modeling vegetation dynamics because it influences the various seed parameters like seed germination, seed maturation, seed mass, and seed bank in the soil. The present study was conducted at an elevation of 3145–3560 m in the treeline area of the western Himalayan region of India. The change in seed color is correlated with other seed parameters such as seed moisture content, seed germination, seed mass, and seed fall density. A decline in moisture content in maturing seeds is closely related to seed maturity (p < 0.05). Quercus semecarpifolia contains the highest seed mass followed by Abies spectabilis. Reportedly, the species with higher seed mass have an advantage in light-restricted environments for seed germination and seedling development. In addition, the fruit mass was observed to be the highest for Rhododendron campanulatum, while both Betula utilis and R. arboreum had similar fruit mass. The seed fall density varied between 1.55 and 7.85 seeds m–2 and the maximum mortality of up to 32% of seedlings was observed during post-monsoon season from November to February. The potential disruption in the timing of seed fall, soil seed bank, and seed germination due to climatic irregularities has broader implications for forest ecosystems. Generally, the soil in treeline areas gets frozen during winter, resulting in seedlings facing severe water stress and a high rate of transpiration. The present study addresses the issue regarding the survival and proliferation of important treeline species in the western Himalayan region of India.

The present review article provides an overview about the timberlines of the Greek high mountains. It informs about the existing tree species and about the altitude, physiognomy and dynamics of the timber-line ecotones. Subsequently, it gives an interpretation of the human influence and the geo-ecological factors, which may have caused the altitudinal limit of tree growth in Greece. A latitudinal and local change in timberline-forming tree species creates high heterogeneity between the research areas and makes this their most remarkable feature. Moreover, in Greece there is no rise in timberline altitude from north to south. In some areas, fir-dieback causes a local decline in timberline altitude, which is the predominant dynamic process. An anthropogenic depression of the forest line is certainly common. Yet, a natural forest cover reaching up to the peak zones of the Greek high mountains is unthinkable. The identified timberline-forming factors and the prevailing site conditions speak against this. In accordance with the Mediterranean climate regime, it is obvious that not only the climatic conditions of winter (frost, snow) determine the limits of tree growth. The (macro- and micro) climatic impacts of summer (drought, high insolation) play a highly important role as well. Against this background, the climate-ecological hindrances to the establishment of trees, i.e. the germination and the survival of seedlings, are identified as key-factors for understanding the Hellenic timberlines (“regeneration hypothesis”). Against the background of global warming, suggestions for further research on the ecology and dynamics of the Greek high mountain timberlines are provided.

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Trees' growth and areal responses to changing climate are primarily expected within the edges of the species range. Here, we compared the responses of Siberian pine (Pinus sibirica Du Tour), a moisture-sensitive species, and drought-resistant larch (Larix sibirica Ledeb.) at the southern part of their ranges in the Siberian Mountains (the Tannu-Ola Ridge). We study the species' growth and proportion in the forests from forest-steppe to treeline ecotone along the elevation gradient. These studies are based on radial growth index (GI) analysis and GI dependence on the climate variables. We used satellite time series to detect the land cover changes (areas of larch and Siberian pine, as well as shrubs and birch). We compared trees' GI before and after warming "restart" in the late 1990s. Generally, GI dependence on the air temperature was negative at elevations below c. 1600 m a.s.l., whereas GI dependence on the moisture variables (precipitation, vapor pressure deficit, and soil moisture) was positive for both species. Above 1600 m, increasing air temperatures stimulated species growth, whereas the influence of moisture variables was negative (for larch) or neutral (for Siberian pine). After the warming restart, the GI of both conifers increased in moisture-sufficient high elevations and treeline ecotone, whereas within low elevations (<1300 m), the GI was stagnant or suppressed. Both species', especially Siberian pine, negative growth dependence on air temperature and positive dependence on the moisture variables strongly increased since the warming restart. We found a risen growth dependence of both species on the soil-stored water during the previous year (September–October), which smoothed moisture stress at the beginning of the growing season. Yet both species' growth also suffered as a result of early spring warms. We found that larch is migrating in both uphill and downhill directions, while Siberian pine is migrating uphill only. Forests loss occurred at low elevations (<1300 m), whereas forest and shrub gain occurred at high (>2000 m) ones. The upper boundary of the forests and shrubs is migrating uphill at rates of about 0.8 and 0.3 m/y, respectively. We observed a decrease in Siberian pine proportion in the forests, whereas areas of larch and birch strongly increased (by 150% and 100%, respectively), which indicates the retreat of Siberian pine from its southern habitat. We suggested afforestation of the areas of Siberian pine mortality by the drought-tolerant larch species. [ABSTRACT FROM AUTHOR]

Mountain habitats harbour significant breeding populations of alpine bird species, yet they can also be important from a biodiversity conservation point of view outside the breeding season. High-elevations are likely important fuelling habitats for birds during the post-breeding period due to seasonal peaks in arthropod abundance being later relative to lowlands. There is no detailed study of bird communities using high-elevations in the post-breeding migration period in Europe. In this study, we recorded birds on line transects from August to October in 2021 and 2022 across an elevational gradient in the Western Italian Alps to assess how the abundance and diversity of birds varied in time and space during the autumn migration period. We detected 104 species in total, representing 22% of the Italian list of terrestrial bird species, showing that a wide range of bird species exploit these habitats. The highest bird diversity and abundance were found during the early migration period at the end of August when long-distance migrants use high elevations. Treeline habitats and south-facing slopes held the highest abundance of migrants, probably due to the high structural diversity and relatively high solar radiation which might affect thermoregulation and prey activity. We suggest that future management of protected areas in the mountains should take these parameters into account in order to protect mountain bird diversity, not only during the breeding season, but also in other key periods of the annual cycle, such as post-breeding migration. It is also essential to counteract the effects of climate and land-use change in mountain habitats by promoting conservation strategies for Alpine biodiversity, including migrants, over the entire year. [ABSTRACT FROM AUTHOR]

This article discusses the concept of treeline dynamics in response to global warming. The authors argue that treeline ecotones in alpine and arctic settings are less sensitive to anthropogenic warming than commonly assumed. They propose the concepts of range-pinning and Allee effects as factors that may explain the reduced temperature sensitivity of treeline positions. The authors suggest that future treeline studies should consider eco-physiological and biochemical insights, empirical evidence for Allee effects, and interdisciplinary approaches to advance knowledge in biogeography and ecology. [Extracted from the article]

Abies spectabilis (East Himalayan Fir), a dominant timber-producing species in the Himalayas, plays a dynamic role in ecological balance and sustains various habitats across temperate to subalpine altitudes. Himalayan montane forests express signs of changes in the growth, regeneration, and population density of forest tree species due to ongoing climate change, including its upper distribution limit. The research was conducted at 30°11´02˝N and 79°39′36˝E, ranging from 3,100 to 3,500 m asl elevation in the treeline ecotone of western Himalaya. The study presents a comprehensive perspective on how water availability and its intricate interactions with climatic and ecological variables shape the dynamics of treeline ecosystems. The study’s phytosociological analysis of treeline sites revealed the complex interplay between species composition, adaptability, and regeneration potential. The eco-physiological aspects of water balance were thoroughly explored, underscoring how plants employ various strategies to cope with changing water availability. The soil moisture content ranged from 32.17 to 73.50%. The dynamic nature of water potential (pre-dawn (ΨPD) and mid-day (ΨMD)) across seasons and years reflects and varies between −0.13 and − 1.25 MPa, the species’ ability to adjust and manage water balance according to environmental shifts. Osmotic potential at full turgor varied from −0.72 to −1.77 MPa, these adjustments are crucial for key life cycle events of vegetative and reproductive phases of species. The strong correlation of water potential (Ψ) and phenophases emphasizes the importance of water status in regulating these vital processes. Furthermore, the study delved into the maturation and germination processes, highlighting the significance of Ψ and moisture content in seed development and germination success. The maximum germination of 46.33% was observed when the seed moisture content was 30.90 ± 3.11%. The observed correlations between cone characteristics and seed parameters indicate the existence of trade-offs that impact seed size, weight, and germination capacity, contributing to the overall fitness of the species.

Unravelling slow ecosystem migration patterns requires a fundamental understanding of the broad-scale climatic drivers, which are further modulated by fine-scale heterogeneities just outside established ecosystem boundaries. While modern Unoccupied Aerial Vehicle (UAV) remote sensing approaches enable us to monitor local scale ecotone dynamics in unprecedented detail, they are often underutilised as a temporal snapshot of the conditions on site. In this study in the Southern Alps of New Zealand, we demonstrate how the combination of multispectral and thermal data, as well as LiDAR data (2019), supplemented by three decades (1991–2021) of treeline transect data can add great value to field monitoring campaigns by putting seedling regeneration patterns at treeline into a spatially explicit context. Orthorectification and mosaicking of RGB and multispectral imagery produced spatially extensive maps of the subalpine area (~4 ha) with low spatial offset (Craigieburn: 6.14 ± 4.03 cm; Mt Faust: 5.11 ± 2.88 cm, mean ± standard error). The seven multispectral bands enabled a highly detailed delineation of six ground cover classes at treeline. Subalpine shrubs were detected with high accuracy (up to 90%), and a clear identification of the closed forest canopy (Fuscospora cliffortioides, >95%) was achieved. Two thermal imaging flights revealed the effect of existing vegetation classes on ground-level thermal conditions. UAV LiDAR data acquisition at the Craigieburn site allowed us to model vegetation height profiles for ~6000 previously classified objects and calculate annual fine-scale variation in the local solar radiation budget (20 cm resolution). At the heart of the proposed framework, an easy-to-use extrapolation procedure was used for the vegetation monitoring datasets with minimal georeferencing effort. The proposed method can satisfy the rapidly increasing demand for high spatiotemporal resolution mapping and shed further light on current treeline recruitment bottlenecks. This low-budget framework can readily be expanded to other ecotones, allowing us to gain further insights into slow ecotone dynamics in a drastically changing climate. [ABSTRACT FROM AUTHOR]

Birds of open alpine grasslands are threatened by the degradation and contraction of their breeding habitat. To assess potential impacts of environmental change in alpine environments, basic ecological knowledge, such as habitat requirements during pre-breeding and breeding periods, is needed. We conducted territory and habitat mapping of a population of the Northern Wheatear Oenanthe oenanthe breeding in alpine grasslands and lower elevation open habitats in the Western Italian Alps during two consecutive breeding seasons (2019-2020). For territory mapping, a study area of over 160ha was visited five times, covering an elevation gradient from 1,600 to 2,700m.a.s.l. Low elevation grasslands in the valley were exploited by birds as arrival habitat after spring migration, but were not selected as breeding territories. On territories, birds avoided areas with numerous small trees, selecting patchy habitats with high rock cover. Early arriving males first occupied the territories at lower elevations, and those with taller ground vegetation, but avoided overall higher vegetation cover. Birds were able to initiate the breeding season when snow cover was still high, as long as forbs and grass were present. The expected decrease in the area of open alpine grassland due to shrub encroachment will probably reduce the availability of suitable territory habitat and thus threatens the alpine population. Extensive seasonal grazing could help to maintain alpine grasslands above the treeline, and may provide suitable, but lower quality, habitat even at lower elevations by keeping the area open.—Alba, R., Chamberlain, D., Rosselli, D. & Sander, M.M. (2024). Habitat quality drives the spatio-temporal occupation of a migratory songbird, the Northern Wheatear Oenanthe oenanthe, in alpine grasslands. Ardeola, 71: 61-80. Las aves que se encuentran en pastizales alpinos abiertos están amenazadas por la degradación y contracción del hábitat de cría. Para evaluar los posibles impactos del cambio ambiental en entornos alpinos, se necesita conocimiento ecológico básico, como los requisitos de hábitat durante los períodos previos a la cría y la cría misma. Llevamos a cabo la cartografía de territorios y hábitats de una población de collalba gris Oenanthe oenanthe que se reproduce en pastizales alpinos y hábitats abiertos a menor altitud en los Alpes Occidentales Italianos durante dos temporadas de reproducción consecutivas (2019-2020). Durante la cartografía de territorios, se visitó una zona de estudio de más de 160 hectáreas en cinco ocasiones, cubriendo una gradiente de elevación desde 1.600 hasta 2.700 m s. n. m. Los pastizales de menor altitud en el valle fueron utilizados por las aves como hábitat de llegada después de la migración de primavera, pero no fueron seleccionados como territorios de cría. En los territorios, las aves evitaban áreas con gran cantidad de árboles pequeños, seleccionando hábitats con alta presencia de rocas de forma dispersa. Los machos que llegaron temprano ocuparon primero los territorios a menor altitud y aquellos con vegetación baja más alta, pero evitaron áreas con una cobertura general de vegetación más alta. Las aves pudieron iniciar la temporada de cría cuando todavía había una gran cobertura de nieve, siempre que hubiera hierbas y pasto presentes. La esperada disminución en el área de pastizales alpinos abiertos debido al avance de arbustos probablemente reducirá la disponibilidad de hábitat de territorio adecuado y, por lo tanto, amenaza a la población alpina. Actividades extensivas de pastoreo estacional podrían ayudar a mantener los pastizales alpinos por encima del límite arbóreo y podrían proporcionar hábitats adecuados, aunque de menor calidad, incluso a menor altitud al mantener el área abierta.—Alba, R., Chamberlain, D., Rosselli, D. y Sander, M.M. (2024). La calidad del hábitat determina la ocupación espacio-temporal de un ave cantora migratoria, la collalba gris Oenanthe oenanthe, en los pastizales alpinos. Ardeola, 71: 61-80. [ABSTRACT FROM AUTHOR]

The present review article provides an overview about the timberlines of the Greek high mountains. It informs about the existing tree species and about the altitude, physiognomy and dynamics of the timberline ecotones. Subsequently, it gives an interpretation of the human influence and the geo-ecological factors, which may have caused the altitudinal limit of tree growth in Greece. A latitudinal and local change in timberline-forming tree species creates high heterogeneity between the research areas and makes this their most remarkable feature. Moreover, in Greece there is no rise in timberline altitude from north to south. In some areas, fir-dieback causes a local decline in timberline altitude, which is the predominant dynamic process. An anthropogenic depression of the forest line is certainly common. Yet, a natural forest cover reaching up to the peak zones of the Greek high mountains is unthinkable. The identified timberline-forming factors and the prevailing site conditions speak against this. In accordance with the Mediterranean climate regime, it is obvious that not only the climatic conditions of winter (frost, snow) determine the limits of tree growth. The (macro- and micro) climatic impacts of summer (drought, high insolation) play a highly important role as well. Against this background, the climate-ecological hindrances to the establishment of trees, i.e. the germination and the survival of seedlings, are identified as key-factors for understanding the Hellenic timberlines ("regeneration hypothesis"). Against the background of global warming, suggestions for further research on the ecology and dynamics of the Greek high mountain timberlines are provided. [ABSTRACT FROM AUTHOR]

Species within tropical alpine treeline ecotones are predicted to be especially sensitive to climate variability because this zone represents tree species' altitudinal limits. Hawaiian volcanoes have distinct treeline ecotones driven by trade wind inversions. The local climate is changing, but little is known about how this influences treeline vegetation. To predict future impacts of climate variability on treelines, we must define the range of variation in treeline ecotone characteristics. Previous studies highlighted an abrupt transition between subalpine grasslands and wet forest on windward Haleakalā, but this site does not represent the diversity of treeline ecotones among volcanoes, lava substrates, and local climatic conditions. To capture this diversity, we used data from 225 plots spanning treelines (1500–2500 m) on Haleakalā and Mauna Loa to characterize ecotonal plant communities. Treeline indicator species differ by moisture and temperature, with common native species important for wet forest, subalpine woodland, and subalpine shrubland. The frequency or abundance of community indicator species may be better predictors of shifting local climates than the presence or absence of tree life forms per se. This study further supports the hypothesis that changes in available moisture, rather than temperature, will dictate the future trajectory of Hawaiian treeline ecotone communities. [ABSTRACT FROM AUTHOR]

Abstract Remote sensing applications have a long history in treeline research. Recent reviews have examined the topic mainly from a methodological point of view. Here, we propose a question‐oriented review of remote sensing in treeline ecology to relate remote sensing methodologies to key ecological metrics and identify knowledge gaps and promising areas for future research. We performed a meta‐analysis to assess the role of remote sensing as a tool for measuring spatial patterns and dynamics of alpine and Arctic treeline ecotone globally. We assessed the geographic distribution, scale of analysis, and relationships between remote sensing techniques and treeline ecological metrics through co‐occurrence mapping and multivariate statistics. Our analysis revealed that only 10% of treeline ecology studies applied remote sensing tools, often associated with the keyword ‘climate change’. Monitoring studies adopted coarser spatial resolutions over longer temporal extents in comparison with other treeline studies. A multiscale and multi‐sensor spatial approach was implemented in just 19% of papers. Long‐term research commonly relied on aerial and oblique photography to measure treeline shifts through photointerpretation within a multidisciplinary framework. More recent treeline dynamics were often quantified using greenness trends derived from the pixel‐based classification of satellite images. Many recent short‐term studies focused on delineating tree scale metrics derived from the object‐based classification of uncrewed aerial vehicle (UAV) images or LiDAR data. Over the past decade, high‐resolution and low‐cost UAV remote sensing has emerged as an interesting opportunity to fill the gap between local‐scale ecological patterns and coarse‐resolution satellite sensors. Additionally, treeline remote sensing applications would strongly benefit from multidisciplinary frameworks that integrate field studies in ecology and environmental science. The multi‐dimensional structural complexity of treelines typically responds to environmental drivers over multiple scales and thus is best described with multiscale and multi‐sensor approaches.

The Pinus cembra communities belong to the rarest and least studied woody habitats in Europe. This article reports 30 species of lignicolous Basidiomycota revealed in primeval spruce stands with the admixture of arolla pine in Gorgany Nature Reserve (the Ukrainian Carpathians). Five species, namely Aphanobasidium subnitens, Ceraceomyces eludens, Hyphoderma occidentale, Hypochnicium albostramineum and H. cremicolor, are firstly reported in Ukraine. The records of Cystostereum murrayi, Phellinus viticola and Pycnoporellus fulgens deserve particular attention due to their value as bioindicators of natural forest ecosystems. The species composition of Basidiomycota per individual fallen log turned out to be rather poor. The fungi forming thin resupinate corticioid basidiocarps on the substrate underside prevail among the finds, indicating a lack of moisture in the surveyed treeline stands. Detailed descriptions of the substrate and collection site are given for each find. An overview of previous research on the diversity of lignicolous Basidiomycota in arolla pine forests is provided.

Biodiversity is a characteristic of ecosystems that plays a crucial role in the study of their evolution, and to estimate it, the species of all plants need to be determined. In this study, we used Unmanned Aerial Vehicles to gather RGB images of mid-to-high-altitude ecosystems in the Zao mountains (Japan). All the data-collection missions took place in autumn so the plants present distinctive seasonal coloration. Patches from single trees and bushes were manually extracted from the collected orthomosaics. Subsequently, Deep Learning image-classification networks were used to automatically determine the species of each tree or bush and estimate biodiversity. Both Convolutional Neural Networks (CNNs) and Transformer-based models were considered (ResNet, RegNet, ConvNeXt, and SwinTransformer). To measure and estimate biodiversity, we relied on the Gini–Simpson Index, the Shannon–Wiener Index, and Species Richness. We present two separate scenarios for evaluating the readiness of the technology for practical use: the first scenario uses a subset of the data with five species and a testing set that has a very similar percentage of each species to those present in the training set. The models studied reach very high performances with over 99 Accuracy and 98 F1 Score (the harmonic mean of Precision and Recall) for image classification and biodiversity estimates under 1% error. The second scenario uses the full dataset with nine species and large variations in class balance between the training and testing datasets, which is often the case in practical use situations. The results in this case remained fairly high for Accuracy at 90.64% but dropped to 51.77% for F1 Score. The relatively low F1 Score value is partly due to a small number of misclassifications having a disproportionate impact in the final measure, but still, the large difference between the Accuracy and F1 Score highlights the complexity of finely evaluating the classification results of Deep Learning Networks. Even in this very challenging scenario, the biodiversity estimation remained with relatively small (6–14%) errors for the most detailed indices, showcasing the readiness of the technology for practical use.

Mountain treelines are thought to be sensitive to climate change. However, how climate impacts mountain treelines is not yet fully understood as treelines may also be affected by other human activities. Here, we focus on "closed‐loop" mountain treelines (CLMT) that completely encircle a mountain and are less likely to have been influenced by human land‐use change. We detect a total length of ~916,425 km of CLMT across 243 mountain ranges globally and reveal a bimodal latitudinal distribution of treeline elevations with higher treeline elevations occurring at greater distances from the coast. Spatially, we find that temperature is the main climatic driver of treeline elevation in boreal and tropical regions, whereas precipitation drives CLMT position in temperate zones. Temporally, we show that 70% of CLMT have moved upward, with a mean shift rate of 1.2 m/year over the first decade of the 21st century. CLMT are shifting fastest in the tropics (mean of 3.1 m/year), but with greater variability. Our work provides a new mountain treeline database that isolates climate impacts from other anthropogenic pressures, and has important implications for biodiversity, natural resources, and ecosystem adaptation in a changing climate. [ABSTRACT FROM AUTHOR]

Alpine treelines serve as vital indicators of the impacts of climate change on tree growth and forest distribution. They offer valuable insights into how shifting temperature and precipitation patterns affect ecosystems in treeline ecotones. Analyzing the age structure of tree stands at treelines provides a glimpse into how different generations of trees have responded to changing environmental conditions and aids in predicting future changes. Moreover, studying the spatiotemporal distribution of tree species at treelines helps us gain a comprehensive understanding of how forests adapt to climate variations. Tree rings at treelines can elucidate the climatic factors that limit tree growth and establishment patterns. Mountain environments, characterized by low temperatures at higher elevations, create constraints on tree growth. However, the intricate interplay between temperature and water availability, driven by precipitation gradients, means that predicting treeline shifts based solely on temperature changes is overly simplistic and may not fully reflect the complex reality. To assess the potential for such interactions, we contrasted the dendroecological performance of different tree species (Abies spectabilis, Betula utilis, Abies georgei, and Larix potaninii) in the Trans-Himalayan zone, Nepal, and Hengduan Mountains, China. We reconstructed the stand age structure by using dendrochronology. Statistical determination of climate-growth responses demonstrated that treeline is moisture-sensitive in the Himalaya and temperature- as well as moisture-sensitive in the Hengduan region. There was abundant seedling recruitment with consistent range shift of A. spectabilis and B. utilis treelines in Nepal, and lower seedling recruitment with lower shifting rates of treelines of A. georgei and L. potaninii in the Hengduan Mountains. We identify both moisture and temperature as critical environmental factors in determining tree radial growth and treeline response to climate. However, modifying factors such as microhabitat conditions and biotic interactions are also highly important to improve the accuracy of treeline dynamics. [ABSTRACT FROM AUTHOR]

Remote sensing applications have a long history in treeline research. Recent reviews have examined the topic mainly from a methodological point of view. Here, we propose a question‐oriented review of remote sensing in treeline ecology to relate remote sensing methodologies to key ecological metrics and identify knowledge gaps and promising areas for future research. We performed a meta‐analysis to assess the role of remote sensing as a tool for measuring spatial patterns and dynamics of alpine and Arctic treeline ecotone globally. We assessed the geographic distribution, scale of analysis, and relationships between remote sensing techniques and treeline ecological metrics through co‐occurrence mapping and multivariate statistics. Our analysis revealed that only 10% of treeline ecology studies applied remote sensing tools, often associated with the keyword 'climate change'. Monitoring studies adopted coarser spatial resolutions over longer temporal extents in comparison with other treeline studies. A multiscale and multi‐sensor spatial approach was implemented in just 19% of papers. Long‐term research commonly relied on aerial and oblique photography to measure treeline shifts through photointerpretation within a multidisciplinary framework. More recent treeline dynamics were often quantified using greenness trends derived from the pixel‐based classification of satellite images. Many recent short‐term studies focused on delineating tree scale metrics derived from the object‐based classification of uncrewed aerial vehicle (UAV) images or LiDAR data. Over the past decade, high‐resolution and low‐cost UAV remote sensing has emerged as an interesting opportunity to fill the gap between local‐scale ecological patterns and coarse‐resolution satellite sensors. Additionally, treeline remote sensing applications would strongly benefit from multidisciplinary frameworks that integrate field studies in ecology and environmental science. The multi‐dimensional structural complexity of treelines typically responds to environmental drivers over multiple scales and thus is best described with multiscale and multi‐sensor approaches. [ABSTRACT FROM AUTHOR]

Due to climate warming, forests are expanding to higher elevations and latitudes at the expense of tundra vegetation. While the subsequent increase in aboveground biomass is well‐documented, there is much speculation regarding the effects on soil organic carbon (SOC) stocks. To provide insight into the consequences of tree encroachment into treeless tundra, we sampled SOC stocks across 36 forest‐tundra ecotones along a 1100 km latitudinal gradient in Norway. Our results show that SOC stocks vary greatly within, as well as among treeline ecotones, and that SOC stocks do not correlate with tree biomass and tree species. SOC stocks do increase with temperature, and vary with slope steepness, slope aspect, and soil parent material. Applying a 'space‐for‐time substitution' perspective, our findings suggest that tree encroachment into tundra is unlikely to have immediate consequences for SOC stocks. [ABSTRACT FROM AUTHOR]

Climate change causes considerable shifts in the geographic distribution of species worldwide. Most data on range movements, however, derive from relatively short periods, within which it is difficult to distinguish directional shifts from random fluctuations. For detecting a shift, it is indispensable to delineate the range precisely. We propose a new method for the delineation based on percolation theory. We suggest marking the boundary between the connected and fragmented occurrence of the species (the hull). We demonstrate the advantages of this connectivity‐based method on simulated examples in which a metapopulation is advancing vs retreating along an environmental gradient with different velocities. The simulations show that the hull is a fractal and has the same dimension (7/4) even when the front is advancing or retreating relatively fast, compared to the generation time. It is particularly robust in the retreating (trailing) edge. Accordingly, we propose marking the range edge at the mean position of the hull, the 'connectivity limit' of the species. Theoretical considerations suggest that the position of the connectivity limit is statistically more reliable than those limits that are delineated according to the outermost occurrences, and the connectivity‐based method is broadly applicable to real‐life data. [ABSTRACT FROM AUTHOR]

A survey of Pinus cembra (Arolla pine) growing above 2800-m elevation was conducted in an ecoregion of the western Alps, where the highest treeline is at ~ 2650 m. The concepts of treeline and species line are discussed based on these field observations. The species line is the biological limit of a species, with inherited nested historical and current processes of high-mountain socio-ecosystems, while the treeline strictly concerns the limit of erect trees. Arolla pines were located and described (height, physiognomy) and their habitats recorded (elevation, slope, aspect). Individuals occurred up to 3200 m on east-facing steep and rocky slopes, with density decreasing exponentially with increasing elevation (halving every 40 m). East-facing habitats benefit from the main atmospheric moisture coming from the east (Italy). Individuals were mostly dwarf-shaped/prostrate (58%) or trees (39%; max height 3.8 m). They usually do not bear female cones, and needles are smaller by half compared to those of pines growing in lower elevation forests. Their morphology indicates harsh growing conditions due to elevation (aridity, frost) and disturbances (wind, snow, rockfall). Habitat preference primarily on grass-free steep rocky slopes and cliffs at high elevation is likely explained by the exclusion of livestock impacts (grazing, trampling). These pines constitute noteworthy "sky island" populations. Larix decidua and Pinus uncinata were also recorded above the treeline, but with lower maximum elevation and density than Pinus cembra. These observations highlight a complex pattern of species treeline in the western Alps, and probably beyond, with relevance to issues in conservation science. [ABSTRACT FROM AUTHOR]

As the climate has warmed, alpine treelines have moved to higher altitudes and the responses of tree growth to different climate factors have changed. We collected dendrochronologies of Larix gmelinii at different elevations on the Dabai Mountain, the highest peak in northeastern China's Greater Khingan range, to measure the sensitivity and stability of radial growth over time. We found that the treeline has moved upslope since 1970. From the mid-1980s, radial growth increased in the treeline ecotone but decreased in the subtimberline forest, an example of "growth divergence" under warming conditions: increases in the previous October's maximum temperatures promoted growth at higher altitudes but inhibited it at lower altitudes. The treeline ecotone appears to be more sensitive to climate change, with the effects on tree growth of different climate indicators varying by altitude in linear or U-shaped relationships. As warming continues, the management of boreal forests needs to consider the changing potential for tree growth and carbon sequestration capacity in relation to changing site conditions. [ABSTRACT FROM AUTHOR]

Alexander von Humboldt and Aimé Bonpland in their Essay on the Geography of Plants discuss what was known in 1807 about the elevational limits of vascular plants in the Andes, North America, and the European Alps and suggest what factors might influence these upper elevational limits. Here, in light of current knowledge and techniques, I consider which species are thought to be the highest vascular plants in twenty mountain areas and two polar regions on Earth. I review how one can try to compare elevational limits in different parts of Earth. I discuss recent advances in high-elevation plant ecology that would surely have fascinated von Humboldt such as the special snow-roots in some snow-bed plants and the coldest place on Earth where a vascular plant is growing. I briefly outline an ignored von Humboldt legacy, Mendelssohn’s Humboldt Cantata. In conclusion I summarise the foundations and legacies that von Humboldt created for global high-elevation ecology and biogeography.

Documenting the breeding habitat and distribution of migratory songbirds is essential for accurately assessing their conservation status. The “sagebrush” subspecies of the Brewer’s Sparrow ( Spizella breweri breweri ) breeds in greatest abundance in sagebrush-dominated ( Artemisia spp.) shrublands of western North America, and the “timberline” subspecies ( S. b. taverneri ) breeds in shrubs or krummholz at or above the tree line in mountain ranges from Alaska to Montana. Brewer’s Sparrows have also been reported at alpine sites in summer in mountain ranges across the western United States, but their taxonomic affiliation and breeding status are unknown. We reviewed monitoring, citizen science, and specimen data for Colorado and identified 186 historical summer observations of Brewer’s Sparrows at 59 alpine sites (3334–4288 m elevation). We surveyed 39 alpine sites in June–July 2021–2023, detected a total of 100 adults (mostly singing males) at 26 alpine sites (3395–3754 m elevation), and confirmed breeding at three sites. Males occupied mixed-species willow patches 0.9–1.8 m tall, often intermixed with sparse conifer krummholz. We recorded songs and captured, measured, photographed, and collected blood and feather samples from birds at a subset of alpine sites and nearby, lower-elevation sagebrush sites in May–July 2021. Vegetation associations and timing of breeding at alpine sites closely matched those of taverneri , but short songs, external morphology, coloration, and genetics of alpine birds overlapped with sagebrush birds and more closely matched those of range-wide breweri . Our results indicate that western Colorado supports a widely-distributed, but poorly-documented, population of alpine breweri breeding in willows with sparse conifer krummholz within a relatively narrow elevation band at or above the tree line. Whether alpine birds are itinerant breeders that first nested in sagebrush remains unknown. Our results complicate interpretation of differences in breeding habitat and breeding phenology as supporting criteria for subspecific identification and taxonomic delineation in this species.

The alpine treeline is highly sensitive to temperature change, and treelines worldwide have been rapidly responding to temperature warming by increasing in tree density. However, often inconsistent or even conflicting results, regarding the response of tree recruitment to temperature change across regions, still challenge our profound comprehension of warming‐induced treeline dynamics. Here, we quantified the temperature sensitivity of tree recruitment within alpine treeline ecotones (i.e. the strength of relationship between tree recruitment and temperature change), and tested whether and how the temperature sensitivity changed along latitudinal gradients by examining 124 alpine treelines across North America and Asia. Our analyses demonstrated that temperature changes could significantly influence tree recruitment at treeline ecotones and the sensitivity of tree recruitment to temperature change was significantly and positively associated with latitude, showing that tree recruitment was more sensitive to temperature change at high latitudes than at low latitudes. Our findings highlighted the heterogeneous responses of tree recruitment at alpine treelines to temperature changes across latitudinal gradients, which should be incorporated into assessment models to improve future projections of global warming impacts across alpine ecosystems. [ABSTRACT FROM AUTHOR]

Aim: The global relationship between treeline elevation and temperature (or latitude as a proxy) is well established. However, additional large‐scale and regional abiotic influences such as mass elevation effect (MEE), continentality and isolation are superimposed onto the latitude‐treeline relationship. To quantify these effects, we apply globally applicable measures and test the effects of MEE, an aspect of continental climate and isolation on treeline elevation. Location: Global treeline elevations (n = 629). Methods: We sampled treeline sites using earth observation. We calculated MEE as the distance to the nearest mountain chain limits. Continentality was assessed by the distance to the nearest coastline. Isolation was calculated by the nearest distance of a mountain chain to another mountain chain within a comparable elevational band. Results: The global latitudinal pattern showed a distinct bimodal latitude‐treeline elevation relationship. Treeline elevations increased substantially with increased MEE and distance to coastlines while isolation even decreased treeline elevations. Main Conclusions: Our study shows a globally consistent effect of MEE and distance to the coastline on treeline elevation, contributing to our basic understanding of large‐scale biogeographic processes governing treeline formation. MEE and continentality reduce cloudiness and increase solar radiation, resulting in higher treeline elevations. Isolation effects are not consistent and may be influenced by immigration and speciation. Understanding global treeline formation using comprehensive measures contributes to a better understanding of how environmental conditions determine vegetation boundaries at large spatial scales. [ABSTRACT FROM AUTHOR]

As plant species expand their upper limits of distribution under current warming, some retain both traditional climate space and biotic environment while others encounter novel conditions. The latter is the case for Rhododendron campanulatum, a woody shrub that grows both above and below treeline at our study site in the Eastern Himalayas where a very conspicuous, stable treeline was defined by a nearly contiguous canopy of tall Abies spectabilis trees, many of which are over a century old. Prior work showed that treeline had remained static in this region while R. campanulatum expanded its elevational range limit. We tested local adaptation of R. campanulatum by performing reciprocal transplants between the species' current elevational range limit (4023 m above sea level [asl]) and just above treeline (3876 m asl). Contrary to expectation, the coldest temperatures of late winter and early mid‐spring were experienced by plants at the lower elevation: R. campanulatum at species' limit (upper site) were covered by snow for a longer period (40 more days) and escaped the coldest temperatures suffered by conspecifics at treeline (lower site). The harsher spring conditions at treeline likely explain why leaves were smaller at treeline (15.3 cm2) than at species limit (21.3 cm2). Contrary to results from equivalent studies in other regions, survival was reduced more by downslope than by upslope movement, again potentially due to extreme cold temperatures observed at treeline in spring. Upslope transplantation had no effect on mortality, but mortality of species limit saplings transplanted downslope was three times higher than that of residents at both sites. A general expectation is that locals should survive better than foreign transplants, but survival of locals and immigrants at our species limit site was identical. However, those species limit saplings that survived the transplant to treeline grew faster than both locals at treeline and the transplants at species limit. Overall, we found asymmetric adaptation: Compared with treeline saplings, those at species limit (147 m above treeline) were more tolerant of extremes in the growing season but less tolerant of extremes in winter and early mid‐spring, displaying local adaptation in a more complex manner than simply home advantage, and complicating predictions about impacts of future regional climate change. [ABSTRACT FROM AUTHOR]

Most ecological studies are by necessity cast on rather short timescales, such that documenting change in phenomena that occur slowly (e.g., over decades to centuries) is quite difficult. Here, we explore variation in and covariation of two sources of information to address rates of treeline change on one mountain face in the central Rocky Mountains: repeat photography and remote sensing. The repeat photography work involved assembling and quantitatively comparing four photographs that span 148 years. The remote sensing analyses covered a shorter period (35 years) but provided quantitative measurements and fuller landscape coverage. The repeat photography results demonstrated pronounced elevational advance of erect forest, at a rate 1.8 m/year during 2004–2020. The remote sensing analyses reflected similar processes but also reflected considerable infilling of tree cover near treeline; consequent increases in Normalized Difference Vegetation Index (NDVI) values in the upper forested areas may thus indicate treeline advance or changes in forest characteristics below treeline. Overall, these results document (1) acceleration in rise of treeline at this site in recent decades and (2) elevational compression of vegetation zones above treeline, with important implications for conservation of nonforested alpine ecosystems. Integrating multiple streams of evidence offers complementary views and insights in analyzing this phenomenon.

It is important to develop a convenient method for predicting photosynthetic rates in cold regions such as high elevations. Dwarf pine Pinus pumila is a representative treeline species in Japan. In this study, we investigated how environmental factors affect diurnal and seasonal variations of the in situ photosynthetic rate of P. pumila. Five environmental factors (photosynthetic photon flux density [PPFD], air temperature [Ta], soil temperature [Ts], vapor pressure deficit [VPD], and soil water content [SWC]) were examined. Photosynthetic measurements were conducted for four needle ages at hourly intervals on fifteen days from July to October 2018. The net photosynthetic rate decreased with needle age and changed seasonally and diurnally for each needle age. The net photosynthetic rate showed unimodal patterns with Ta, Ts, and SWC. The seasonal and diurnal changes in the net photosynthetic rate were related to seasonal changes in the four environmental factors (PPFD, Ta, Ts, SWC); the net photosynthetic rate decreased in mid-summer when Ta, Ts, and PPFD were high due to a decrease in SWC. Seasonal and diurnal changes in net photosynthetic rates could be reproduced by the multiplicative model of the four environmental factors and needle age. Therefore, the photosynthetic rate of P. pumila can be explained and predicted using these environmental factors and needle age. This information will be useful for predicting the effects of climate change on the photosynthetic production of plants in cold regions.

Trees at their upper range limits are highly sensitive to climate change, and thus alpine treelines worldwide have changed their recruitment patterns in response to climate warming. However, previous studies focused only on daily mean temperature, neglecting the asymmetric influences of daytime and nighttime warming on recruitments in alpine treelines. Here, based on the compiled dataset of tree recruitment series from 172 alpine treelines across the Northern Hemisphere, we quantified and compared the different effects of daytime and nighttime warming on treeline recruitment using four indices of temperature sensitivity, and assessed the responses of treeline recruitment to warming‐induced drought stress. Our analyses demonstrated that even in different environmental regions, both daytime and nighttime warming could significantly promote treeline recruitment, and however, treeline recruitment was much more sensitive to nighttime warming than to daytime warming, which could be attributable to the presence of drought stress. The increasing drought stress primarily driven by daytime warming rather than by nighttime warming would likely constrain the responses of treeline recruitment to daytime warming. Our findings provided compelling evidence that nighttime warming rather than daytime warming could play a primary role in promoting the recruitment in alpine treelines, which was related to the daytime warming‐induced drought stress. Thus, daytime and nighttime warming should be considered separately to improve future projections of global change impacts across alpine ecosystems. [ABSTRACT FROM AUTHOR]

Inferences about the mechanisms of distributional change are often made from simple assessments of variation in the geographical positions of populations. However, direct assessments of species' responses to local habitat change may be necessary for proper understanding of the drivers of distributional dynamics.Amplified climate warming is inducing cascading impacts in boreal‐tundra regions including the expansion of conifers and deciduous shrubs (shrubs). In Denali National Park (Denali), Alaska, passerine birds are exhibiting rapid upslope shifts in distribution but the relative roles of conifer and shrub (woody vegetation) expansion in driving these shifts are unknown. Without directly assessing passerine‐vegetation dynamics, the assumption has been that the observed upslope shifts are indicative of shrub‐adapted passerines tracking the upslope expansion of shrubs. Here, we jointly investigate the processes of conifer and shrub expansion and their relationship to changes in passerine abundance in Denali.We used a remotely sensed vegetation cover timeseries (1985–2020) to assess the topographic and edaphic correlates of conifer and shrub expansion. We then assessed the impacts of changes in shrub and conifer cover on the relative abundance of 12 passerine species (1995–2020).Shrub and conifer colonization rates were highest at intermediate elevations near treeline. However, forest‐ and shrub‐adapted passerines differed in terms of the location in which their response was concentrated relative to treeline. The population growth rates of forest‐adapted passerines exhibited stronger effects of woody vegetation expansion at sites that were initially above treeline (IAT). In contrast, the population growth rates of shrub‐adapted passerines exhibited the negative effects of conifer expansion together with the positive effects of shrub expansion at initially below treeline sites. However, they showed a weak response to woody vegetation expansion at sites that were IAT.Below treeline conifer infilling appears to be pushing the elevational distributions of shrub‐adapted passerines upslope rather than these species following the pull of modest shrub expansion above treeline, as previously assumed.Overall, our findings illustrate the need for explicitly accommodating heterogeneity in habitat change at small spatial scales to properly view the distributional response, particularly when habitat change is concentrated at ecotones. [ABSTRACT FROM AUTHOR]

The eastern Andean treeline (EATL) is the world's longest altitudinal ecotone and plays an important role in biodiversity conservation in the context of land use/cover and climate change. The purpose of this study was to assess to what extent the position of the tropical EATL (9°N–18°S) is in near-equilibrium with the climate, which determines its potential to adapt to climate change. On a continental scale, we have used land cover maps (MODIS MCD12) and elevation data (SRTM) to make the first-order assessment of the EATL position and continuity. For the assessment on a local scale and to address the three-dimensional nature of environmental change in mountainous environments, a novel method of automated delineation and assessment of altitudinal transects was devised and applied to Landsat-based forest maps (GLAD) and fine-resolution climatology (CHELSA). The emergence of a consistent longitudinal gradient of the treeline elevation over half of the EATL extent, which increases towards the equator by ~30 m and ~60 m per geographic degree from the south and north, respectively, serves as a first-order validation of the approach, while the local transects reveal a more nuanced aspect-dependent pattern. We conclude that the applied dual-scale approach with automated mass transect sampling allows for an improved understanding of treeline dynamics. [ABSTRACT FROM AUTHOR]

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Unravelling slow ecosystem migration patterns requires a fundamental understanding of the broad-scale climatic drivers, which are further modulated by fine-scale heterogeneities just outside established ecosystem boundaries. While modern Unoccupied Aerial Vehicle (UAV) remote sensing approaches enable us to monitor local scale ecotone dynamics in unprecedented detail, they are often underutilised as a temporal snapshot of the conditions on site. In this study in the Southern Alps of New Zealand, we demonstrate how the combination of multispectral and thermal data, as well as LiDAR data (2019), supplemented by three decades (1991–2021) of treeline transect data can add great value to field monitoring campaigns by putting seedling regeneration patterns at treeline into a spatially explicit context. Orthorectification and mosaicking of RGB and multispectral imagery produced spatially extensive maps of the subalpine area (~4 ha) with low spatial offset (Craigieburn: 6.14 ± 4.03 cm; Mt Faust: 5.11 ± 2.88 cm, mean ± standard error). The seven multispectral bands enabled a highly detailed delineation of six ground cover classes at treeline. Subalpine shrubs were detected with high accuracy (up to 90%), and a clear identification of the closed forest canopy (Fuscospora cliffortioides, >95%) was achieved. Two thermal imaging flights revealed the effect of existing vegetation classes on ground-level thermal conditions. UAV LiDAR data acquisition at the Craigieburn site allowed us to model vegetation height profiles for ~6000 previously classified objects and calculate annual fine-scale variation in the local solar radiation budget (20 cm resolution). At the heart of the proposed framework, an easy-to-use extrapolation procedure was used for the vegetation monitoring datasets with minimal georeferencing effort. The proposed method can satisfy the rapidly increasing demand for high spatiotemporal resolution mapping and shed further light on current treeline recruitment bottlenecks. This low-budget framework can readily be expanded to other ecotones, allowing us to gain further insights into slow ecotone dynamics in a drastically changing climate.