BG3.26 | Treeline ecotones under global change: linking spatial patterns to ecological processes
Treeline ecotones under global change: linking spatial patterns to ecological processes
Co-organized by ESSI4/GI2
Convener: Matteo Garbarino | Co-conveners: Maaike Bader, Alessandro VitaliECSECS, Johanna Toivonen, Nicolò AnselmettoECSECS
Orals
| Tue, 16 Apr, 14:00–15:45 (CEST)
 
Room 2.23
Posters on site
| Attendance Mon, 15 Apr, 16:15–18:00 (CEST) | Display Mon, 15 Apr, 14:00–18:00
 
Hall X1
Orals |
Tue, 14:00
Mon, 16:15
Treeline ecotones are transition zones between closed forest and climatically tree-less areas. Due to their climate sensitivity they are considered sentinels of global-change effects on terrestrial ecosystems. Vegetation patterns in treeline ecotones are constrained by multiple factors acting at different spatial and temporal scales. Climatic treeline positions are strongly influenced by global- and regional-scale climatic patterns, but other factors such as soil, meso-topography, and natural and anthropogenic disturbances dominate patterns at the landscape scale. Moreover, species competition/facilitation and micro-topographic heterogeneity are key factors for vegetation dynamics at finer scales. A current trend in vegetation dynamics both at latitudinal and altitudinal treelines is the accelerated encroachment of trees and shrubs, caused by interactions between climate and land-use changes. This encroachment can have far-reaching consequences for the biodiversity and functioning of mountain and subarctic ecosystems. Spatial vegetation patterns likely hold important information about the factors and processes (e.g. seed dispersal, safe-site characteristics, biotic interactions) that control this encroachment, yet few of treeline research deals with the spatial component of patterns and processes. For this reason, it is crucial to improve our understanding of spatial processes and the spatial signals of global change impacts in treeline ecotones and there is a need for a multiscale and multidisciplinary approach, to plan better adaptation strategies and monitor biodiversity trends in such sensitive ecosystems and to better link treeline metrics to ecological questions. Specifically, remote sensing can be combined with field data and modeling to capture the heterogeneity and variability of ecological conditions in treeline ecotones and couple observed spatial patterns to ecological processes. In this session, we invite contributions from all fields of research related to either the detection and description of treeline spatial and temporal patterns or the processes that may be relevant for these patterns.

SPECIAL ISSUE ON BIOGEOSCIENCES JOURNAL: A special issue based on the session topic is scheduled and will be focused on observational and modeling studies along latitudinal and elevational treelines of the globe.

Session assets

Orals: Tue, 16 Apr | Room 2.23

Chairpersons: Matteo Garbarino, Maaike Bader, Alessandro Vitali
14:00–14:05
14:05–14:15
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EGU24-1844
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ECS
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On-site presentation
Michael Maroschek, Rupert Seidl, Cornelius Senf, and Werner Rammer

Forest ecosystems are sensitive to global change; especially at the ecotones we expect high sensitivity to changes in climate, disturbance regimes or land use. For instance, the treeline ecotone is expected to move upward in elevation with global warming. The advent of machine learning, specifically computer vision, provides powerful tools for monitoring ecotones across large spatial scales using remote sensing data. In this study, we focused on the spatiotemporal development of ecotones bracketing the subalpine forest belt (i.e., the upper boundary, formed by the treeline, and the ecotone to montane forests as the lower boundary) in a protected forest landscape in the European Alps. Our objectives were threefold: First, we aimed to identify trees and shrubs on historic and recent orthophotos using deep learning, with special attention to integrating multiple sensor types into one computer vision framework. Second, based on the computer vision inference, we sought to map the a) treeline and b) montane-subalpine ecotone. Third, we aimed to describe the spatiotemporal changes occurring in both ecotones.

We based our analysis on historic and recent aerial images of Berchtesgaden National Park in the Northern Alps, covering roughly 210 km² in nine time steps from 1953 to 2020. The images were captured through both analog (panchromatic and color infrared) and digital (color infrared, RGB) sensors. To generate training data for deep learning, we manually interpreted randomly distributed 0.5 ha segments across all time steps, resulting in over 110,000 annotations of coniferous and broadleaved trees, shrubs, and standing dead trees. We tested different instance segmentation frameworks and selected the best performing model architecture to create wall-to-wall tree maps for each time step. Using structure and composition of the tree maps, we spatially delineated the ecotones and tracked their changes over time.

We did not find a spatially consistent pattern of ecotones shifting upwards, however we were able to identify areas of change and stability linked to climate, topography, disturbances and land use. We observed remarkable local upward shifts in ecotones, particularly of the montane-subalpine ecotone, which shifted up to five times faster than the treeline. In general, we found that subalpine forests, situated between the two ecotones, decreased in area because of an upward shift of its lower boundary, and exhibited an increase in crown cover over time.

Changes in these ecotones and related vegetation zones pose challenges to conservation, restoration and management. Our approach can help to address these challenges, e.g., in the combination with habitat modelling.

How to cite: Maroschek, M., Seidl, R., Senf, C., and Rammer, W.: Tracking shifts of mountain forest ecotones in aerial imagery with deep learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1844, https://doi.org/10.5194/egusphere-egu24-1844, 2024.

14:15–14:25
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EGU24-9480
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ECS
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On-site presentation
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Thiên-Anh Nguyen, Marc Rußwurm, Gaston Lenczner, and Devis Tuia

Over the last decades, the position of the upper treeline in the Swiss Alps has been highly affected by drivers such as climate change and land use change interacting at various spatial and temporal scales. To better understand these interactions, it is necessary to quantify treeline dynamics over large areas at high spatial resolution and over long time scales. This can be decomposed into three tasks: mapping forest cover, delineating the treeline, and comparing the treeline position through time.

We leverage archives of optical aerial imagery acquired over the Swiss Alps to map forest cover. These images constitute a large dataset of time series of 12 to 20 ortho-rectified aerial images at 1 m spatial resolution acquired throughout the time period 1946-2020. We have developed a deep learning-based method to automatically extract multi-temporal forest masks from these aerial images (under review).

We then explore how treeline dynamics can be characterized using these forest cover maps. More specifically, we look at designing a spatio-temporal processing pipeline that implements widely used definitions of the treeline and treeline displacement, while being robust to potential errors in our deep learning-generated maps, such as noise caused by differing sensors and imaging conditions. We find that through a series of pixel-based processing steps, based solely on the generated forest cover maps and a Digital Elevation Model, we manage to 1. delineate the treeline at a chosen spatial scale and 2. measure the elevational treeline shift between two dates. The flexible choice of the spatial scale enables multi-scale analysis and comparison with existing treeline shift measurements derived from different data sources and methods.

We hope that this automatic and flexible spatial analysis pipeline can link deep learning-based forest cover maps to ecologically relevant variables in a way that can foster the understanding of treeline dynamics.

How to cite: Nguyen, T.-A., Rußwurm, M., Lenczner, G., and Tuia, D.: Linking deep learning-based forest cover maps to treeline spatio-temporal patterns, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9480, https://doi.org/10.5194/egusphere-egu24-9480, 2024.

14:25–14:35
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EGU24-10296
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ECS
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On-site presentation
Jana-Sophie Kruse, Werner Rammer, Lisa Mandl, Rupert Seidl, and Michael Maroschek

Ecotones – transition zones between ecosystems – are sentinels of global change, as they are sensitive to changes in environmental conditions and land use. The alpine treeline ecotone - where the continuous, subalpine forest transitions into the treeless alpine zone – is a characteristic feature of many mountain ecosystems. The transition at the ecotone can be characterized by distinct treeline patterns. Treeline patterns can be simple, such as sharp transitions from forest to alpine vegetation, or complex, e.g., islands of trees and krummholz in a matrix of alpine vegetation. This variation can mediate the impact of global change at the alpine treeline ecotone. However, large-scale attribution, e.g., for an entire mountain range, and spatiotemporal quantification of treeline patterns remain challenging. Automated methods, such as deep learning-based computer vision systems, can help to overcome these challenges. Building on existing definitions of treeline patterns, we aim to characterize the alpine treeline ecotone for the entire mountain range of the European Alps. Our particular objectives are:

  • To characterize the patterns of a representative sample of the alpine treeline ecotone of the European Alps based on remote sensing information as training data for deep learning.
  • To quantify treeline patterns across the Alps and identify spatial differences in the prevalence of patterns.

In an alpine treeline ecotone, we considered the transition between three vegetation classes: trees (i.e., upright woody plants with a minimum height of 3m), krummholz (i.e., stunted trees and woody shrubs), and treeless alpine vegetation. Three spatial patterns were considered for trees and krummholz describing their state: closed, islands, or isolated individuals. The transitions between these states across elevation, a total of 24 combinations, were used to quantitatively characterize treeline patterns. We selected 1,000 randomly distributed elevational transects between 1,100 and 2,800 m.a.s.l. that include the alpine treeline ecotone across the European Alps. For each transect, we classified treeline patterns for areas of 90m×90m using satellite and orthophoto images. Based on this dataset, we quantified differences in treeline patterns and their distribution in elevation across the European Alps: While in the Prealps, the alpine treeline ecotone is located in lower elevations and treeline patterns tend to be more complex, the ecotone is higher in elevation and less complex in the Central Alps.

The quantification of treeline patterns and their distribution can serve as a basis for further investigations of the alpine treeline ecotone and its spatiotemporal development. We provide an outlook for a deep learning approach that uses the presented dataset combined with a time series of spectrally unmixed satellite data, i.e., fractional abundances of land cover per pixel, as training data. Utilizing satellite data of the past 35 years in annual resolution, we will be able to automatically classify and analyze treeline patterns and their changes across the entire European Alps.

How to cite: Kruse, J.-S., Rammer, W., Mandl, L., Seidl, R., and Maroschek, M.: Characterizing Spatial Patterns of the Alpine Treeline Ecotone Across the European Alps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10296, https://doi.org/10.5194/egusphere-egu24-10296, 2024.

14:35–14:45
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EGU24-4049
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ECS
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On-site presentation
Joanna Corimanya, Daniel Jimenez-Garcia, and A. Townsend Peterson

Previous research has shown that (1) treelines are shifting upward in elevation on high mountain peaks worldwide, and (2) the rate of the upward shift appears to have increased markedly in recent decades. Because treeline shift is a process manifested over broad scales of space and time, a particular challenge has been that of obtaining a broad-enough view of patterns of treeline shift to permit inferences about geographic and environmental patterns. What is more, intensive studies of treelines have been concentrated in North Temperate regions, such that little information is available about treeline shift patterns in the Tropics. We have attempted to broaden this viewpoint by means of analysis of long  time series of vegetation indices derived from Landsat imagery obtained and analyzed via Google Earth Engine for the 1980s to present. We sampled vegetation indices at points spaced every 100 m along 100 km transects radiating out from 120 high peaks across western North America (Canada to Central America); considerable data preparation was necessary, including ending transects <2 km into closed forest, identifying current treelines via reference to Google Earth imagery, and consideration only of up to <1 km above treeline. Patterns that emerged were—as is well known—that treelines are generally higher at lower latitudes, but also that magnitude of treeline shifts is nonrandomly distributed with respect to latitude, location with respect to coastlines, and size of the mountain mass within which the peak is located. Although analyses are continuing at the time of preparation of this abstract, this analysis offers a broadscale view of treeline shifts over a period of almost 40 years, and over a geographic span of more than 40° of latitude.

How to cite: Corimanya, J., Jimenez-Garcia, D., and Peterson, A. T.: Patterns of Treeline Rise with Climate Change Across Western North America from the 1980s to Present, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4049, https://doi.org/10.5194/egusphere-egu24-4049, 2024.

14:45–14:55
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EGU24-15620
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ECS
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On-site presentation
Lorena Baglioni, Donato Morresi, Enrico Tonelli, Emanuele Lingua, Raffaella Marzano, and Carlo Urbinati

Treelines are dynamic ecotones largely influenced by climate and land use changes. The increasing development of remote sensing techniques and the interest on the ecological effects of global warming on forest vegetation have raised the number of treeline studies.

The aims of this study were: i) to define an automatic approach for mapping the current position of the upper forestlines in the Italian Alps and Apennines and ii) to locate hotspots of long-term vegetation dynamics using Landsat-based spectral trend analysis. Hotspots will serve us to analyse the ecological drivers of vegetation change and to predict future vegetation dynamics.

We used the Tree Cover Density (TCD) dataset (Copernicus Land Monitoring Service) and a nationwide digital elevation model to define the polylines representing the forestlines for the reference year 2018. We used the main Italian mountain peaks, extracted from the Global Mountain Biodiversity Assessment (GMBA) dataset polygons, as reference points to detect only the upper forest ecotones based on the elevation difference between peaks and forest pixels. We defined our study areas by applying a positive and negative buffer around the forestlines and we calculated several spectral vegetation indices (e.g. NDVI, EVI, Tasseled Cap Angle) from Landsat timeseries of the last 40 years. In this way, we inferred inter-annual vegetation dynamics, discriminating two sub-areas of interest: the closed forest (below the current forestline) and the upper treeline ecotone (above the current forestline). It should be noted that on the Alps, treelines mainly host conifer species, whereas on the Apennines, broadleaf species (mostly European beach) prevail. We tested the significance of long-term spectral trends through a Mann-Kendall test for monotonicity that accounted for autocorrelation in space and time.

An important outcome of the study was to set up a replicable and unsupervised method to enhance the study of vegetation dynamics at treeline ecotones. This approach will allow the delimitation of the forestlines on a global scale and an ecologically sound comparison between different treeline ecotones. This study is the first step in a nationwide project and will provide the basis for future local-scale investigations of treeline ecotones.

How to cite: Baglioni, L., Morresi, D., Tonelli, E., Lingua, E., Marzano, R., and Urbinati, C.: Forestline detection and treeline ecotone dynamics in the Italian Alps and Apennines by satellite remote sensing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15620, https://doi.org/10.5194/egusphere-egu24-15620, 2024.

14:55–15:05
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EGU24-11942
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ECS
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Highlight
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On-site presentation
Erik Carrieri, Fabio Meloni, Carlo Urbinati, Emanuele Lingua, Raffaella Marzano, and Donato Morresi

Treeline ecotones spatial patterns and dynamics are influenced by factors acting at regional, landscape, and local scales. It is widely accepted that treelines change in complex ways depending on their diverse structural features and environmental conditions.
The high variability of environmental conditions and ecological drivers hampers the creation of a general pattern from case studies. A multi-scale approach applied at numerous locations is needed to discriminate between natural and anthropogenic factors that are driving treeline dynamics. Remote sensing techniques are today fundamental tools for a comprehensive assessment of the spatial heterogeneity of treeline patterns and their changes over space and time. Continuous improvements in remote sensing platforms, sensors, and methodologies have considerably increased the quality and reliability of spatial information, such as forest maps, which are essential for monitoring ecotonal dynamics.
In this study, we aimed to comprehensively map individual tree canopies at the treeline ecotone in 10  different sites distributed across the Italian Alps by integrating field and UAV-based data. We first mapped the position of the forestline using the 2018 pan-European Tree Cover Density layer provided by the Copernicus Land Monitoring service. In particular, we considered the pixel line where the tree canopy cover was less than 10% as the forestline. Field data consisted of position, height, and species of 100 trees taller than 50 cm scattered over a 9-hectare area. Each site was also flown over by a multirotor drone to produce an RGB orthomosaic, a digital surface model, and a canopy height model. A total of 1016 individual canopies of different coniferous species were manually classified on the orthomosaics with the aid of semi-automatic annotation software. These data were used to train a deep learning model based on the Mask R-CNN algorithm for object detection and segmentation. The classification masks were lastly combined with a canopy height model providing 3-dimensional information allowing to measure tree height. Preliminary results evidenced that remotely sensed data collected with low-cost equipment such as commercial drones with RGB cameras, coupled with the proposed canopy detection method can be used to produce highly accurate and reliable maps of treeline ecotones. These maps will serve as a starting point to study and monitor the spatio-temporal dynamics of treeline ecotones at the local scale and how they affect biodiversity in high-altitude environments.

How to cite: Carrieri, E., Meloni, F., Urbinati, C., Lingua, E., Marzano, R., and Morresi, D.: Treeline spatial patterns for biodiversity monitoring detected by spectral and 3D information from UAV‐based aerial imagery, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11942, https://doi.org/10.5194/egusphere-egu24-11942, 2024.

15:05–15:15
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EGU24-15287
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ECS
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Highlight
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On-site presentation
Noémie Delpouve, Cyrille Rathgeber, Laurent Bergès, Jean-Luc Dupouey, Sandrine Chauchard, and Nathalie Leroy

The forest line is a key feature of mountain landscapes around the world. Currently, most forest lines in the Northern Hemisphere are rising due to the combined effects of land-use and climate changes. This upward shift has led to major changes in the functions and services provided by the adjacent socio-ecosystems (e.g. carbon sequestration, biodiversity hosting, services to people…). However, it has not been elucidated how the recent forest-line upward shift fits into the longer context of land abandonment (occurring since the beginning of the 19th century in France), and how it is currently responding to the accelerating global warming. To answer this question, we assessed the elevation change of the forest line over the French northern Alps since the forest minimum (mid-19th century in France) using old and current land cover maps.

Three digitalised maps: the État-Major map, BD Forêt® v1.0 and BD Forêt® v2.0 were used to display forest cover at three dates: 1859, 1994 and 2007, respectively. These maps were standardized and combined with a digital elevation model to estimate the average elevation of the subalpine forest lines for 178 municipalities across the French departments of the Northern Alps: Haute-Savoie, Savoie and Isère. We compared forest-line elevations between dates and municipalities to explore temporal and spatial patterns.

The forest line in the French Northern Alps has risen by an average of 152 ± 18 m from its ancient position (1879 ± 21 m a.s.l. in 1859) to its current position (2032 ± 12 m in 1994).  However, no general upward shift was observed during the most recent period from 1994 to 2007, as the forest-line position was 2013 ± 13 m in 2007. In the Haute-Savoie department, a downward shift of 69 ± 12 m was even observed, while forest lines in Isère and Savoie were stable. Forest-lines upward shift in the French Northern Alps has been driven by agricultural abandonment, mountain land restoration and global warming since the period of the forest minimum (around 1860). However, it is noteworthy that forest line dynamics are no longer influenced by these factors nowadays and do not follow the acceleration of temperature increase. The current recession of the forest lines may be attributed to anthropogenic pressure related to the tourism activity. This new anthropogenic pressure corresponds to the development of alpine ski resorts and the increase in the human population in Haute-Savoie since 1925, and later in the other departments.

This large-scale spatial and temporal study shows how global and regional factors interact in the long-term to shape mountain landscapes, in particular the ecotone between subalpine forest and alpine grassland. Today, the dynamics of this ecotone is still linked to the contradictory tensions that divide our societies (conservation vs. exploitation). This is why we advocate the cautious management of alpine forest line ecotones, which could contribute to carbon sequestration and biodiversity conservation, provided they are not subjected to excessive human pressure (tourism and grazing).

How to cite: Delpouve, N., Rathgeber, C., Bergès, L., Dupouey, J.-L., Chauchard, S., and Leroy, N.: Is anthropogenic pressure limiting the climate-induced upward shift of the subalpine forest line in the French Northern Alps?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15287, https://doi.org/10.5194/egusphere-egu24-15287, 2024.

15:15–15:25
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EGU24-14114
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ECS
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Highlight
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On-site presentation
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Laurel Sindewald and Diana Tomback

Treeline elevation is expected to shift upward in response to climate warming. However, over half of alpine treelines worldwide appear to be lagging, possibly due to moisture limitations. Seedling niches tend to have narrower climate envelopes than those of mature, established trees, and regeneration requirements can vary substantially among species. We examined the density and species composition of recruitment at alpine treeline sites west and east of the Continental Divide, Central Rocky Mountains.  In the arid Colorado Front Range, the Divide results in a rain shadow on the east side due to orographic uplift. We stratified our sampling effort by proximity to subalpine limber pine (Pinus flexilis), a generalist, drought-tolerant conifer with a patchy metapopulation distribution in the Front Range. We expected to find higher abundance of limber pine regeneration than that of drought-averse Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) in two regeneration height categories (≤ 100 cm and ≤ 20 cm). Regeneration occurred at low densities on both sides of the Continental Divide and did not differ significantly between sites east and west of the Divide. Regeneration density also did not differ significantly between communities dominated by limber pine and communities dominated by Engelmann spruce and subalpine fir. However, the quadrats with highest regeneration densities were east of the Divide where limber pine was the dominant conifer. These sites were also in the rain shadow and associated with higher climate water deficit and lower growing season precipitation. Limber pine also comprised the majority of this regeneration. The site with the highest observed regeneration rates also had high rates of viable limber pine seed production at treeline. We observed a significantly higher proportional abundance of limber pine in the 100 cm regeneration class (relative to established trees) in quadrats east of the Divide, corresponding to establishment roughly in the last 30-70 years. The greater proportional abundance in limber pine regeneration at these treeline study sites occurred despite increasing temperatures, reduced growing season precipitation, and increased climate water deficit over the past 30 years. Drought-tolerant limber pine may therefore be the best-suited conifer in this region to persist and to migrate to higher elevations as temperatures continue to increase. Our findings underscore the importance of considering differences in seedling tolerances (niches) among different species in alpine treeline systems when aiming to predict landscape-scale treeline responses to climate warming.

How to cite: Sindewald, L. and Tomback, D.: Recruitment at treeline in the Central Rocky Mountains shifts in favor of a drought-tolerant species as climate water deficit increases, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14114, https://doi.org/10.5194/egusphere-egu24-14114, 2024.

15:25–15:35
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EGU24-10161
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On-site presentation
Eryuan Liang, Xiaoxia Li, J. Julio Camarero, Sergio Rossi, Jingtian Zhang, Haifeng Zhu, Yongshuo H. Fu, Jian Sun, Tao Wang, Shilong Piao, and Josep Peñuelas

 Despite the importance of species interaction in modulating the range shifts of plants, little is known about the responses of coexisting life forms to a warmer climate. Here, we combine long-term monitoring of cambial phenology in sympatric trees and shrubs at two treelines of the Tibetan Plateau, with a meta-analysis of ring-width series from 344 shrubs and 575 trees paired across 11 alpine treelines in the Northern Hemisphere. Under a spring warming of + 1°C, xylem resumption advances by 2–4 days in trees, but delays by 3–8 days in shrubs. The divergent phenological response to warming was due to shrubs being 3.2 times more sensitive than trees to chilling accumulation. Warmer winters increased the thermal requirement for cambial reactivation in shrubs, leading to a delayed response to warmer springs. Our meta-analysis confirmed such a mechanism across continental scales. The warming-induced phenological mismatch may give a competitive advantage to trees over shrubs, which would provide a new explanation for increasing alpine treeline shifts under the context of climate change.

How to cite: Liang, E., Li, X., Camarero, J. J., Rossi, S., Zhang, J., Zhu, H., Fu, Y. H., Sun, J., Wang, T., Piao, S., and Peñuelas, J.: Warming-induced phenological mismatch between trees and shrubs explains high-elevation forest expansion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10161, https://doi.org/10.5194/egusphere-egu24-10161, 2024.

15:35–15:45
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EGU24-20484
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Virtual presentation
Christian Körner

Since climatic treelines track the elevational position of isotherms across the globe, it is not the question if, but when and how they will arrive at a novel steady-state position. After briefly recalling the essential difference between the edge of the realized and fundamental niche of the life form tree (not to be confused with species’ range limits), I will present data on recent climatic trends in the Alps based on long term meteorological records. Two years of in-situ temperature records from Pinus cembra trees growing right at the current upper edge of tree size individuals in Eastern Tyrol (supplemented with data from the Swiss Engadin region), make it obvious that the current high elevation record positions around 2500 m elevation are lagging substantially behind the upslope shift of the isotherm. This explains, why these trees grew so exceptionally rapid over the past 10 years, partly growing a meter in height in only 6-8 years. The locations with rapid tree radiation are all under nutcracker control. These data permit projections on forthcoming treeline shifts. For the Austrian Alps, the current uppermost trees represent all-time elevation records, and will soon out-range the uppermost fossil elevation records of trees that date back to the warmest period of the Holocene. Suggested reading: Körner C, Hoch G (2023) Not every high-elevation or high-latitude forest edge is a treeline. J Biogeography, open access.

How to cite: Körner, C.: Treeline resposes to 2K warming in the Alps in half a century, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20484, https://doi.org/10.5194/egusphere-egu24-20484, 2024.

Posters on site: Mon, 15 Apr, 16:15–18:00 | Hall X1

Display time: Mon, 15 Apr 14:00–Mon, 15 Apr 18:00
Chairpersons: Johanna Toivonen, Nicolò Anselmetto, Matteo Garbarino
X1.1
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EGU24-21283
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ECS
Enrico Tonelli, Alessandro Vitali, Alma Piermattei, and Carlo Urbinati

In the context of ecological research, tree-ring analysis often deals with short time series (< 30 years). Their crossdating and averaging can be difficult but crucial to use such data for ecological modelling, multivariate statistics, and climate-growth analysis. Several studies were conducted in the Central Apennines (Italy) on recent encroachment of European black pine (Pinus nigra J.F. Arnold) on treeless areas above the current forestline. Growth of young trees is mainly controlled by endogenous or microclimatic factors making usual dendrochronology methods less applicable and crossdating very difficult or even impossible. The potential ecological information deriving from tree-ring growth in short series is therefore limited by this methodological bias. The aim of this study is to test suitable methods for optimizing the use of short ring series for further analytical use. A dataset of 734 tree-ring series of young European black pines (mean cambial age 15 years) growing at high altitude in 8 sites was used in this analysis. At each site tree-ring series were divided in two groups based on inter-series correlation: the crossdated or selected series (SEL), and non-crossdated or rejected ones (REJ). The following dendrochronological parameters were calculated for SEL and REJ series: mean tree-ring width, mean sensitivity, Gini coefficient, first order autocorrelation, inter-series correlation, and Gleichläufigkeit (GLK). Two methods of pointer years analysis were tested in order to detect years with synchronous growth: i) Normalization in a moving Window (NW) and ii) the RElative growth change method (RE). The two methods were applied to the raw series varying the standard thresholds, in order to detect synchronous growth-years in SEL and REJ group. A sensitivity analysis was included to assess how the threshold choice in the analysis could affect the results obtained. The term “common” was used to indicate years with similar tree growth response. Differences in the detected number of common years within SEL and REJ were obtained using different time windows with the RE and NW methods. The 47 % of all series were classified as SEL, showing more common years than the REJ series. However, a similar result occurred considering all the series together without SEL/REJ discrimination. In general, a significant occurrence of common years could be a tool to select series to be averaged for a site mean chronology. These are preliminary but encouraging results contributing to a more efficient use of the ecological information provided by short time series from young trees.

How to cite: Tonelli, E., Vitali, A., Piermattei, A., and Urbinati, C.: Young trees climate sensitivity above the forestline: the case study of Pinus nigra upward shift in Central Apennines (Italy) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21283, https://doi.org/10.5194/egusphere-egu24-21283, 2024.

X1.2
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EGU24-21595
Matteo Garbarino, Donato Morresi, Peter Weisberg, and Nicolò Anselmetto

Treelines are ecotones with a strong spatial nature. Remote sensing (RS) tools provide spatially explicit wall-to-wall maps in time. Nevertheless, despite the potential of RS to inform treeline ecologists on spatial patterns and underlying processes, its application is still scarce and heterogeneous. We performed a systematic review and meta-analysis of published literature with the aim to provide a question-oriented discussion of RS in treeline ecology. The main focus of the review was the role of RS as a tool for measuring spatial patterns and dynamics of treeline globally. We assessed the geographic distribution, scale of analysis, and relationships between RS techniques and ecological metrics through cooccurrence mapping and multivariate statistics. Only 10% of treeline studies applied RS. We observed four main types of applications; long-term aerial, long-term oblique, satellite timeseries, and high-resolution mapping. Long-term research and monitoring adopted coarser spatial resolution over long temporal extent, either with oblique or aerial photographs to measure treeline position and shift. Shorter temporal extents (i.e., up to 40 years) were investigated through satellite time-series, especially when dealing with coarse dynamics such as changes in climate. High-resolution imagery derived from UAV recently emerged as promising tools to measure tree height, canopy cover, and spatial patterns at a very fine spatial resolution (i.e., centimetres to metres). A multiscale and multi-sensor spatial approach was implemented in just 19% of papers. We advocate for an increasing interaction between classic treeline ecology based on field surveys and RS techniques. Also, the multi-dimensional structural complexity of treeline ecotones calls for a multiscale and multi-sensor approach, with high-resolution and low cost UAV acting as a powerful tool to fill the gap between local-scale ecological patterns and coarse-resolution satellite sensors.

How to cite: Garbarino, M., Morresi, D., Weisberg, P., and Anselmetto, N.: Linking ecological processes and spatial patterns: the promise of remote sensing in treeline ecology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21595, https://doi.org/10.5194/egusphere-egu24-21595, 2024.

X1.3
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EGU24-21653
Dynamics in human-shaped upper forest margins in southern European mountains
(withdrawn)
Carlo Urbinati, Enrico Tonelli, Matteo Garbarino, and Alessandro Vitali
X1.4
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EGU24-22195
Maaike Bader, Lukas Flinspach, Bradley Case, Julio Camarero, and Thorsten Wiegand

Spatial patterns in alpine-treeline ecotones reflect the ecological processes that have shaped and probably continue to shape these transition zones. Understanding these processes is essential for predicting treeline responses to global-change factors. To connect treeline-ecotone patterns and processes, we developed a spatially-explicit individual-based model. The first version of this Spatial Treeline Ecotone Model (STEM 1.0) represents the growth, mortality and dieback (biomass loss leading to stunted trees or krummholz) of all individual trees within a treeline transect, and uses variation in these demographic rates, imposed along elevation gradients or emerging as a result of neighbor interactions, to create treeline ecotones with different spatial patterns. The model could reproduce many of the expected treeline types, but some types required very particular parameter combinations. These results helped to identify missing elements in the model and thus to sharpen our conceptual model of treeline-forming processes. The next mayor development step for the model is to let demographic rates emerge from the interaction of environmental influences, modified by plant-plant interactions, rather than being imposed. However, this first version is a very important first step to formalizing and developing our conceptual model of pattern-process relationships in alpine-treeline ecotones.

How to cite: Bader, M., Flinspach, L., Case, B., Camarero, J., and Wiegand, T.: The Spatial Treeline-Ecotone Model (STEM) as a tool for understanding pattern-process relationships in alpine-treeline ecotones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22195, https://doi.org/10.5194/egusphere-egu24-22195, 2024.

X1.5
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EGU24-21746
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ECS
A standardized, globally applicable method for detecting spatial patterns at alpine treeline ecotones
(withdrawn)
Nishtha Prakash, Ribana Roscher, and Maaike Y. Bader
X1.6
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EGU24-21593
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ECS
Alessandro Vitali, Matteo Garbarino, J. Julio Camarero, Elvin Toromani, Velibor Spalevic, Milić Čurović, and Carlo Urbinati

In this study we compared the encroachment patterns of four pine species across anthropogenic forestlines in Southern Europe. Using a synchronic approach, we studied structure and recent spatio-temporal patterns of pine recruitment at upper forestline ecotones in Albania, Italy, Montenegro and Spain. Within altitudinal transects we mapped and sampled 964 living individuals of Pinus heldreichii, Pinus peuce, Pinus sylvestris and Pinus uncinata growing above the current forest line. We measured their basal diameter, total height, and counted the number of seed cones. We differentiated seedlings (height < 0.5 m) from saplings (0.5m≤height < 2 m) and trees (height≥2 m). From individuals with basal stem diameter>4 cm we extracted one increment core for cambial age determination and tree-ring width measurements. On smaller specimens, we estimated the age by counting annual internodes (terminal bud scars) along the whole stem. We compared the ground cover around each pine, applied point pattern analyses, modelled the probability of seed cone production and estimated the average distance of seed dispersal. The four pine species exhibited heterogeneous density values and the overall averaged means ranged 2–7 cm for basal diameter, 54–106 cm for total height and 9–20 years for cambial age, suggesting a recent encroachment process. None of these structural variables decreased with increasing relative altitude and distribution patterns exhibited a few higher density spots but not cohort spatial structure. Ground cover differed between species and more significantly between size classes. Grass was the most frequent type at all sites except for P. sylvestris where shrubs prevailed. Basal area increments increased from 1990 and stabilized in recent years at all species except for P. peuce. Height and basal diameter predicted cones production better than cambial age. P. heldreichii and P. peuce dispersed seeds at longer distances than P. uncinata and P. sylvestris, suggesting different potential for further encroaching. Pine recruitment above the forestlines is quite synchronic at all sites (last 30 years), but in some cases it appeared as a high altitude tree densification process, whereas in others as a starting forestline advance.

How to cite: Vitali, A., Garbarino, M., Camarero, J. J., Toromani, E., Spalevic, V., Čurović, M., and Urbinati, C.: Spatio-temporal dynamics of four pine species recolonization in Southern Europe human-disturbed forestlines, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21593, https://doi.org/10.5194/egusphere-egu24-21593, 2024.

X1.7
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EGU24-2339
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Walter Oberhuber, Andreas Gruber, and Gerhard Wieser

Green alder (Alnus alnobetula (Ehrh.) K. Koch = Alnus viridis (Chaix) DC), a tall multi-stemmed deciduous shrub, is widespread at high elevations in the Central European Alps especially within avalanche slide path, screes and steep, north-facing slopes with high water availability. The ascending growth of stems frequently leads to eccentric growth, discontinuous rings and elliptical shape of annual rings making development of representative ring-width time series, necessary to determine climate forcing of radial growth and long-term growth trends, a challenge. Therefore, the focus of this study was to assess growth variability among radii of one shoot (n=4 radii), among shoots belonging to one stock (n=20 shoots per stock) and among stocks exposed to different site conditions (n=3 sites). Stem discs were sampled within the treeline ecotone (c. 2150 m asl) on Mt. Patscherkofel (Tyrol, Austria), and annual increments were measured along 188 radii. Variability in inter-annual agreement among ring-width series was evaluated by applying dendrochronological techniques, i.e., the parameters (i) percentage of parallel variation (“Gleichläufigkeit”, Glk) and (ii) the correlation coefficient r, adjusted for the amount of overlap (tBP-score) were determined. Variation in intra-annual dynamics of radial growth among shoots belonging to different stocks was evaluated by mounting diameter dendrometers (n=6). Results revealed a high agreement in ring-width variation among radii of one shoot (Glk: P<0.001; tBP-score>5), among shoots of one stock (Glk: P<0.05; tBP-score>4) and among stocks from different sites (Glk: P<0.05; mean tBP-score=4.5). Dendrometer records gathered from shoots belonging to different stocks also revealed a high agreement in intra-annual radial growth dynamics, which started in 2023 at the end of June and already terminated in early August. In contrast to this, a high variability in both absolute growth rates and long-term growth trends was found at selected study sites. We attribute our findings to the pronounced limitation of radial stem growth in Alnus alnobetula by climate factors (mainly summer temperature and winter precipitation) leading to a high agreement among ring width series developed from different radii, shoots and individuals. On the other hand, differences in compressive and tensile forces and variation in microsite conditions determine absolute growth rates and long-term growth trends.

This research was funded by the Austrian Science Fund (FWF), P34706-B.

How to cite: Oberhuber, W., Gruber, A., and Wieser, G.: Climate factors control inter-annual variability of radial growth, while microsite conditions affect absolute growth and long-term growth trend in the multi-stemmed shrub Alnus alnobetula at the alpine treeline, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2339, https://doi.org/10.5194/egusphere-egu24-2339, 2024.

X1.8
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EGU24-21677
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ECS
Lirey A. Ramirez, Hannah Loranger, Lukas Flinspach, Nada Nikolic, Johanna Toivonen, Hanna Wenzel, Gerhard Zotz, and Maaike Y. Bader

Seedling establishment is a major bottleneck in plant community dynamics and is particularly critical for tree advance in the treeline ecotone. However, the characteristics and availability of safe sites for tree regeneration in alpine ecosystems remain unclear, while the criteria for safe sites may differ between tree species. Tree seedlings in the treeline ecotone are exposed to multiple environmental stressors that may differ from those affecting adult trees. Understanding the response of seedlings to different combinations of abiotic and biotic constraints is essential for predicting future treeline shifts. We therefore aimed to: 1) evaluate differences in microsite preferences of the conifers Larix decidua, Pinus uncinata, and P. cembra at treeline sites with two different types of bedrock chemistry, and 2) study the response of these species plus two further treeline-forming tree species, Picea abies and Sorbus aucuparia, to microclimatic manipulation. We evaluated microsite preferences at four sites in the upper treeline ecotone in the French Alps, two with calcareous and two with siliceous bedrock, and compared, at each site, the microsite characteristics of 50 tree species individuals with 50 random microsites, describing the substrate, ground cover, macro- and microtopography, and nearest shelter of each microsite. In a field experiment, also in the French Alps, seedlings were planted in 40 plots arranged in five blocks with the following treatments: Day warming, Day warming + watering, Night warming, Night warming + shade, Shade, Control, Watering, and Vegetation cover. We evaluated survival, growth, and biochemistry (chlorophyll fluorescence and nonstructural carbohydrates) of two seedling cohorts (planted in two consecutive years). We found that microsites were similar, and mostly sheltered, in both bedrock types, and the occupied microsites were a good representation of the available microsites in the respective areas, suggesting that safe-site availability does not limit the establishment of these species in the treeline ecotone. In the experiment, the two seedling cohorts responded differently to the treatments, but in general the vegetation treatment had the strongest effect on seedling performance in all the species studied. Our results imply that, contrary to our expectations, seed availability, rather than safe site availability, is a primary constraint for tree establishment in these alpine-treeline ecotones. Furthermore, in our experiment, the presence of vegetation affected seedling performance more than shading or warming, but given the differences between cohorts, this result must be carefully considered.

How to cite: Ramirez, L. A., Loranger, H., Flinspach, L., Nikolic, N., Toivonen, J., Wenzel, H., Zotz, G., and Bader, M. Y.: Exploring alpine seedling dynamics: microsite preferences and physiological performance in the French treeline ecotone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21677, https://doi.org/10.5194/egusphere-egu24-21677, 2024.

X1.9
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EGU24-5709
Svetlana Varsova, Veronika Lukasova, Milan Onderka, and Dusan Bilcik

Global warming affects the climatic conditions in the mountain environments. The climate of the alpine treeline ecotone (ATE) in the High Tatra Mts. is represented by unique conventional long-term climatological series from Skalnaté Pleso Observatory (49°11'21.9” N; 20°14'02.7” E). When considering the two last normal periods (1961-1990 and 1991-2020), the average air temperature in ATE increased by  1.1 °C. In this work, we analysed the altitudinal shift of the boundary 6°C isotherm, which represents the minimum temperature requirements for the growth and reproduction of tree vegetation. To determine the altitude of the cold treeline limit, i.e. upper limit of ATE, we used climate data from Skalnaté Pleso Observatory (1,778 m a.s.l.) and the near top meteorological station Lomnicky štít (2,634 m a.s.l.).  We found that over the analysed period 1951-2020, the limiting isotherm moved upwards from the level of 1,950 m a.s.l. to 2,200 m a.s.l.. Preliminary field monitoring and mapping indicated the colonisation of the dominant subalpine vascular species Pinus mugo Tura (mountain pine) into alpine summits. We identified young individuals or small groups of mountain pine at altitudes between 2,000-2,200 m a.s.l., which is consistent with the assumption of vertical extension of low tree vegetation due to positive changes in ambient thermal conditions. The warming of the alpine tree line ecotone may lead to a gradual reduction and eventual disappearance of montane species due to their strict ecological specialisation. The replacement of the populations of cold-adapted alpine species by those profiting from the warmer climate may cause a decline in the ecosystem's biodiversity. Therefore, further research will be focused on verification of the climate-related shift of the boundary line for the growth of mountain pine at the ATE zone in the highest mountain range of the central-eastern region of Europe.

How to cite: Varsova, S., Lukasova, V., Onderka, M., and Bilcik, D.: Dynamics of alpine treeline in the High Tatra Mts., Slovakia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5709, https://doi.org/10.5194/egusphere-egu24-5709, 2024.

X1.10
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EGU24-14685
Diana Tomback and Laurel Sindewald

Whitebark pine (Pinus albicaulis) and limber pine (Pinus flexilis), related five-needled white pines (Pinus subgenus Strobus, section Quinquefoliae, subsection Strobus), are distributed throughout the mountains of the western United States and Canada. Whitebark pine ranges from about  36° to 56 ° N latitude, and limber pine ranges from about 34° to 52° N latitude.  Both pines are tolerant of harsh sites, including poor soils and arid conditions, but whitebark pine inhabits colder sites and is restricted to high elevations, and limber pine occupies a broader elevational range and has more drought-resistance. The seeds of both pines are dispersed primarily by Clark’s nutcrackers (Nucifraga columbiana), which often cache seeds at treeline and in tundra.  In the Rocky Mountains, both pines are components of treeline communities but differ in growth form and foliage density and thus potential capacity to serve a facilitation function. Our previous studies identified different ecological functions or roles assumed by trees in Rocky Mountain treeline communities: isolated solitary tree, most windward tree of a tree island (potential tree island “initiator”), satellite tree (sheltered by a tree island), or tree island component (leeward of windward tree).  We examined whether whitebark and limber pine differ in ecological functions in treeline communities.  Whitebark and limber pine primarily co-occur with Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) at treeline, and both pines have higher abundance at treeline east of the Continental Divide. In treeline communities broadly sampled from 42° to 53° N latitude, whitebark pine was the majority solitary conifer in 9 out of 10 treeline study sites and had the highest representation within tree islands at 8 of 10 study sites.  Whitebark pine was the most frequently occurring windward conifer in tree islands at half of the study sites, and its proportional abundance as a solitary tree predicted its proportional abundance as a windward conifer.  Limber pine, in contrast, was rare at treeline at northern latitudes but more common in the arid southern Rocky Mountains.  We studied treeline communities in Rocky Mountain National Park, both east and west of the Continental Divide in 19 study sites.  Limber pine was found only east of the Divide and varied in prevalence from 0% to 97.6% of trees within a study site. It most frequently occurred as a satellite or solitary tree and less frequently as a windward tree than expected by its representation as a solitary tree.  We found a relationship between the proportion of limber pine at our treeline sites and the distance to a subalpine limber pine seed source, likely resulting from seed dispersal by nutcrackers against prevailing winds.  In sum, tolerance of harsh, windy conditions by both pines, coupled by avian seed dispersal, leads to their prevalence as solitary trees in treeline communities east of the Continental Divide.  Whitebark pine’s denser morphology likely facilitates establishment of conifers to its lee. Limber pine’s drought tolerance enables it to survive well on windswept ridges and slopes—which have earlier snowmelt dates—and may lead to increasing prevalence with climate change.

How to cite: Tomback, D. and Sindewald, L.: Differences in functional ecology of two western North American ‘five-needle’ white pines in treeline communities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14685, https://doi.org/10.5194/egusphere-egu24-14685, 2024.

X1.11
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EGU24-10168
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ECS
Linqing Zou, Gabriela Schaepman-Strub, Feng Tian, and Tianchen Liang

As the climate warms, vegetation within treeline ecotones is responding. The high-mountain ecotones, which are less affected by anthropogenic disturbances, present an optimal environment for investigating the effects of climate change on terrestrial ecosystems. Accurately delineating the trends of vegetation in high-mountain ecotones is pivotal for a comprehensive understanding how climate change affects these ecosystems. Remote sensing technology has a significant potential in detecting and quantifying vegetation variation. While previous studies have identified greening trends within certain mountainous regions, there remain a gap in global-scale analysis concerning vegetation dynamics in high-mountain ecotones.

In this study, we utilize long time-series Landsat imagery to monitor and analyze vegetation dynamics in high-mountain ecotones. Our approach includes assessing changes in the physiological properties of the vegetation and analyzing temporal patterns in spatial distribution changes. The results reveal a consistent trend of increased vegetation density and enhanced greening of vegetation in global high-mountain ecotones under the influence of climate change.

How to cite: Zou, L., Schaepman-Strub, G., Tian, F., and Liang, T.: Assessing vegetation dynamics in global high-mountain ecotones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10168, https://doi.org/10.5194/egusphere-egu24-10168, 2024.

X1.12
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EGU24-22109
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ECS
Plant facilitation and soil microbiome modulate treeline advancement across the Apennines
(withdrawn)
Giulio Tesei, Angelo Rita, Maurizio Zotti, Luigi Saulino, Marina Allegrezza, Antonio Saracino, Sergio Rossi, Emilia Allevato, Mohamed Idbella, and Giuliano Bonanomi