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FS 3.103

Creating knowledge about alpine-treeline ecotones

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Full Title
FS 3.103: Creating knowledge about alpine-treeline ecotones in changing mountains
Scheduled
Talks:
2025-09-16, 10:00 - 12:00 (LT), MCI – Aula 302
Posters:
2025-09-16, 15:00 - 16:00 (LT), SOWI – Garden
Convener
Maaike Bader
Co-Conveners
Klaus Katzensteiner, Johanna Toivonen, Lirey Ramírez, Peter Bebi, Nishtha Prakash,
Assigned to Synthesis Workshop
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Thematic Focus
Conservation, Ecosystems, ES-Forests, Monitoring
Keywords
treeline, mountain forest, alpine vegetation, climate change, vegetation patterns

Description

Alpine-treeline ecotones mark the transition from mountain forest to alpine vegetation in mountains worldwide. As the physical realisation of the thermal limit to tree growth, these ecotones should respond strongly to climate warming, but additional limitations at various spatial scales lead to large variation in the observed responses. These include land management, natural disturbances, climatic and edaphic stress factors, tree population processes, and biotic interactions. To understand treeline dynamics, all of these limitations and the interactions among them need to be understood, which requires combining scientific disciplines and information gathered by many different methodological approaches. Important disciplines include plant ecology, ecophysiology, dendroecology, palaeoecology, human and physical geography, soil science, and (micro)climatology, while relevant approaches include field and lab experiments, field observations, remote sensing, and modelling. This session aims to bring together scientists studying alpine treeline ecotones from different scientific perspectives, with different methods and in different geographical contexts, in order to show the breadth of current treeline research. The session will combine regular talks and flash talks introducing posters. A related workshop then aims to synthesize and connect these different angles, methods and contexts. Also closely related is a pre-conference field trip for the 50th anniversary of the Stillberg treeline experiment near Davos, on September 12, before the #IMC25, offering an additional opportunity to discuss treeline research and visit this unique long-term treeline research experiment.

Registered Abstracts

ID: 3.10267

Tracking ecotones on aerial images with computer vision in a mountain forest landscape

Michael Maroschek

Seidl, Rupert; Rammer, Werner

Abstract/Description

Mountain forest ecosystems are sensitive to global change. Especially at the ecotones we often expect high sensitivity to changes in climate, disturbance regimes or land use. The advent of machine learning, specifically computer vision, provides powerful tools to investigate ecotones across extended spatiotemporal extents using remote sensing data. Here, we focused on the spatiotemporal development of the treeline and montane-subalpine forest ecotones in a protected area in the European Alps. First, we aimed to identify trees and shrubs on aerial images, with special attention to integrating multiple sensor types into one computer vision framework. Second, we mapped a) the montane forest zone, b) the subalpine forest zone, and c) the krummholz zone, as well as d) the ecotones in between. Third, we investigated the spatiotemporal changes occurring in the vegetation zones and the ecotones. We based our analysis on aerial images of Berchtesgaden National Park covering nine time steps from 1953 to 2020. The images were captured through analog (panchromatic, color infrared) and digital (color infrared, RGB) cameras. To generate training data, we manually interpreted randomly distributed 0.5 ha scenes across all time steps, resulting in >110,000 annotations of trees, shrubs, and standing dead trees. We tested a set of instance segmentation frameworks and compared individual models for each image type with models integrating all image types. We used the inference of the best performing model to generate wall-to-wall tree maps. Using structure and composition of the tree maps, we delineated zones and ecotones and tracked changes over time. We found that a combined computer vision model for all image types performed better than individual models for each image type. While the extent of montane and subalpine forest zones changed over time, krummholz was notably stable. Conversely, crown cover increased more strongly for krummholz and the subalpine zone than for the montane zone. Although we did not find a general pattern of ecotones shifting upward, we observed remarkable local upward shifts. On average, the upward shift of the montane-subalpine ecotone was roughly two times faster than the subalpine-alpine ecotone, decreasing the subalpine forest area by approximately 25% over 67 years.

ID: 3.11150

Biotic and abiotic constraints of Scots pine and mountain birch seedlings on the treeline ecotone of subarctic Finnish Lapland

Johanna Toivonen

Wallen, Henri; Stark, Sari; Kumpula, Jouko; Aakala, Tuomas

Abstract/Description

The range expansion and growth of tree seedlings on treeline ecotones are influenced by a combination of biotic and abiotic factors, including impacts of different land uses. Subarctic treeline ecotones are excellent manifestations of a stress gradient, where facilitation can be expected to increase and competition to decrease from favorable climatic conditions of closed forests to harsh climatic conditions of open tundra. We studied the growth and abundance of Scots pine (Pinus sylvestris) and mountain birch (Betula pubescens subsp. czerepanovii) seedlings on the treeline ecotone of the Finnish subarctic. We sampled 135 plots from an extensive reindeer pasture monitoring plot network, distributed in the main biomes of the ecotone: boreal coniferous forests, mountain birch forests and open tundra. In each plot, we measured seedling height, growth during the last three years (for Scots pine), described their microtopographic position, took soil samples and extracted climate data from the Finnish Meteorological Institute’s data. In this work, we will show the associations of seedling growth and density with climate and soil properties and intensity of reindeer pasture, aiming to shed light on the potential and requirements of these two species to maintain their dominance and to expand their ranges to open tundra, as the climate warms and environmental stress decreases in subarctic treelines.

ID: 3.11392

Forest-line dynamics in the French Northern Alps since 1860: a substantial upward shift, recently limited by human activities?

Noémie Delpouve

Rathgeber, Cyrille B. K.; Bergès, Laurent

Abstract/Description

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, leading to major changes in the adjacent alpine grasslands and subalpine forests functions. However, it has not been elucidated how the recent forest-line upward shift fits into the longer context of land abandonment, and how it is currently responding to the accelerating global warming. To answer this question, we assessed the forest-line elevation change over the French Northern Alps since the forest minimum using historical and current land cover maps, displaying forest cover at four dates: 1860, 1951, 1995 and 2007. These maps were standardized and combined with a digital elevation model to estimate the average forest-line elevation for 150 municipalities across three 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 was stable between 1860 and 1951, and then rose by 166 m, from 1865 m a.s.l. in 1951 to 2031 m a.s.l. in 1995. From 1995 to 2007, no general upward shift was observed and in the Haute-Savoie department, a downward shift of 5.3 m.yr-1 was even noticed, while forest lines in Isère and Savoie were stable. Forest-line upward shift in the French Northern Alps has been driven by pastoral abandonment and global warming. However, forest-line dynamics did not follow the recent acceleration of temperature increase. Its current limitation may be attributed to pastoral and tourism contemporary pressures. This regional study on long temporal scale shows how global and regional drivers interact in the long-term to shape mountain landscapes. Today, the forest-line dynamics is still linked to the contradictory tensions dividing our societies (conservation vs. exploitation). Thus, we advocate the cautious management of the adjacent alpine grasslands and subalpine forests, which could contribute to carbon sequestration and biodiversity conservation, provided they are not subjected to excessive human pressure.

ID: 3.11578

A standardized, globally applicable method for detecting spatial patterns at alpine treeline ecotones

Nishtha Prakash

Bader, Maaike

Abstract/Description

Treeline dynamics have been studied using a range of remote sensing and GIS methods depending on the scale of analysis (regional, landscape, hillslope and stand scale). At the landscape scale, alpine forest can be delineated from grassland using medium resolution imagery (10-30 m spatial resolution). At the hillslope scale, high resolution imagery (5 m spatial resolution) can help detect the general pattern of treeline ecotone and any vertical or lateral shifts, while very high-resolution imagery (<1 m spatial resolution) could help detect the pattern within the ecotone by telling individual trees or clusters of trees apart from the surrounding low-stature vegetation. However, there is a dearth of such very high-resolution data for treeline ecotone sites around the world, especially those in the global south. Therefore, there is the need for a treeline pattern detection method that works efficiently using limited data and that can be applied with high level of accuracy to diverse geographical regions. We are developing a deep learning method trained on the best available data; but that also works well for lower-quality data from new alpine treeline sites. Through this process, we are studying what level of detail and spatial accuracy is needed to characterise different aspect of treeline spatial pattern to answer different ecological questions. An effective method should be able to detect treeline-ecotone patterns in a consistent and comparable manner to allow a global comparison of patterns and their relation to driving factors and processes.

ID: 3.12506

Beneath the surface: How the declining health of Pinus mugo impairs soil carbon storage

Barbara Mateos Perez Bianco De Araujo

Maroschek, Michael; Göttlein, Axel; Rammer, Werner; Dollinger, Christina; Seidl, Rupert

Abstract/Description

Mountain pine (Pinus mugo) is a keystone species at the treeline ecotone of the European Alps. This pioneer shrub contributes to soil development, prevents erosion by gravitational processes, and contributes to carbon sequestration at high elevations. These ecosystem services are at risk as climate-related stressors and invasive species threaten mountain pine health. A widespread die-off could have serious consequences for alpine ecosystems and their role in the carbon cycle. We investigated 1. how declining mountain pine health influences topsoil carbon stocks; 2. whether changes in carbon concentration or soil volume drive these patterns; 3. when the carbon balance shifts from gains to losses upon mountain pine die-off; and 4. the potential impacts of a die-off on ecosystem-level soil carbon storage.
We collected soil samples from plots representing three conditions (healthy mountain pine, ailing mountain pine, no mountain pine) along an elevational gradient (1065 – 1810 m a.s.l.) in Berchtesgaden National Park, Germany. Organic carbon concentration was determined by elemental analysis. We analysed how mountain pine health and elevation influence topsoil carbon stocks using Linear Mixed Models (LMMs). Working with a simple model of soil carbon dynamics, we simulated how carbon stocks change over time upon mountain pine die-off and estimated the ecosystem-level impacts of a die-off within the national park.
Our LMM analyses showed that soil carbon stocks increased with elevation. In comparison to the healthy state, carbon stocks increased moderately under ailing mountain pines. However, carbon stocks in plots without mountain pines were considerably lower than those under healthy and ailing mountain pines. In our soil carbon simulations, the carbon gain from needle loss under ailing mountain pines lasted less than a decade before net losses from decomposition set in. This process slowed down as elevation increased. In Berchtesgaden National Park, our model estimated that loss of mountain pine would reduce soil carbon stocks above 1500 m a.s.l. by more than 15%.
Carbon stock decline upon mountain pine die-off is slower at higher elevations, yet the long-term trend shows a substantial net loss. Our results highlight the consequences of declining mountain pine health on the dynamics of alpine ecosystems through impaired soil carbon storage.

ID: 3.12598

Upper forestline dynamics in the Italian Alps and Apennines revealed by Landsat time-series

Lorena Baglioni

Morresi, Donato; Garbarino, Matteo; Urbinati, Carlo; Lingua, Emanuele; Marzano, Raffaella; Vitali, Alessandro

Abstract/Description

The interest on the ecological effects of global warming and land use changes on vegetation, combined with the increasing development of remote sensing techniques, have fostered the research about the successional dynamics at the upper forest ecotones . In this context, the aims of this study are: i) to define an automatic approach for mapping the current position of the most representative upper forestlines in the Alps and Apennines (Italy); ii) to detect and assess the long-term spectral changes at their ecotones using Landsat-based trend analysis; iii) to appraise the performance of greenness and wetness indices along a forestline buffer which includes the closed forest below and the ecotone above it. We used a regional scale approach to make the method replicable in different geographic areas. We calculated spectral greenness and wetness vegetation indices from Landsat timeseries for the period 1984 – 2023 and tested the significance of their long-term spectral trends with the Contextual Mann-Kendall test for monotonicity. Our results show an overall increasing trend, mainly close to the forestline ecotone and at lower elevations inside the buffer. Comparing the relative trends with the current canopy cover, we found in the Alps the highest values of greenness and wetness trends in the sparse and dense cover class respectively, as a result of encroachment and gap filling dynamics. We plan to analyse the detected trends integrating the Landsat data with others at higher-resolution to better assess the effect of structure and site-specific drivers.

ID: 3.13039

The role of biotic and abiotic drivers in shaping treeline spatial patterns in the Italian Alps

Erik Carrieri

Meloni, Fabio; Anselmetto, Nicoló; Morresi, Donato; Garbarino, Matteo

Abstract/Description

Treeline ecotones are extremely sensitive to multiple drivers and are generally considered as indicators of vegetation response to climate change. It is widely recognized that multiple factors in addition to climate drive the upward and poleward tree migration and that many other drivers act at finer spatial scales. Multi-scale approaches to better understand how the interaction of multiple drivers influences treeline position, its spatial patterns, and dynamics are needed. In a research conducted over 90 hectares of treelines divided in 10 sites and distributed across the Italian Alps, we have comprehensively stem-mapped the trees by integrating UAV and filed data collection. We here aim to provide valuable insights on the joint effect that several biotic and abiotic drivers have on the ecotone spatial patterns across the entire mountain range. The high-resolution ground cover classification obtained through a deep learning model was combined with fine-scale terrain features and topographic attributes, and with intra- and inter-specific relationships existing between coniferous and broadleaved species. The combination of the biotic and abiotic component was finally related to the spatial patterns of the treeline ecotones by using spatial and multivariate statistical tools (PPA, RDA, RF). Preliminary results highlight that tree establishment is majorly driven by ground cover, with grass and deadwood displaying a predominant role in driving the process. Apart for the heatload and the eastness, which displayed a direct, positive correlation with the seedlings presence, the other topographic variables did not show any particular influence. The presence of previously established trees in the close surrounding of a microsite seemed to have a negative effect on tree establishment, suggesting that competition prevails over facilitation in the studied ecotones. The applied setup aims at disentangling and giving a broader understanding of the role that different microsites and biotic interactions have on tree recruitment and survival at the upper tree limit. We expect our findings to explain the variability in treeline patterns and processes that macro- and mesoclimate factors alone cannot explain.

ID: 3.13300

Climatology of European treelines

Vaclav Treml

Kaczka, Ryszard; Kalita, Jakub; Romero, Eunice; Masek, Jiri; Tumajer, Jan; Lange, Jelena

Abstract/Description

Current uppermost tree positions represent cold boundaries of realized niche of trees which may or may not overlap with the boundaries of fundamental niche. Climatic characterization of uppermost tree locations allows an evaluation whether current treelines are in equilibrium with climate or they lag behind the pace of warming. Furthermore, climatic characteristics of treelines may indicate potential treelines where trees are absent due to various disturbances or long-term land-use pressures. Surprisingly, we lack comprehensive climatological characterization of European treelines. In this study, we systematically screened European treeline regions, identified uppermost tree positions and calculated treeline bioclimatic metrics derived from CHELSA dataset including growing season length, growing season temperature, degree days and growing season daily precipitation. We identified 5100 locations of uppermost trees in more than thousand 20 km gridcells that overlaid treeline areas. For each point we recorded treeline form, ground cover and topographic features. We hypothesized that treelines face varying climatic conditions with trade-offs between growing season heat sums and growing season moisture stress. Our results show that according to climatic metrics, European treelines can be divided into five distinct groups: (1) very warm and dry located predominantly in southern Europe, (2) moderately cold and dry located mainly in eastern Europe and in inland parts of Scandes and in very northern part of Europe, (3) cold found mainly in the Alps and Pyrenees, (4) moderately cold and wet with very short growing season located mainly in maritime parts of Scandes, and (5) moderately cold with very long growing season found in Scotland and Ireland. Treeline climatic groups differ mainly in their location, and to lesser extent also in terrain morphology and treeline form. Accordingly, the most influential predictors of treeline temperature metrics were geographical position, slope steepness, distance to summits and presence of broadleaves/conifers. Warmer treelines were drier suggesting trade-off between temperature and moisture-limited tree growth. Identified differences in temperature metrics between treeline groups indicate that there is a complex gradient of meteorologic conditions that influence occurrence and intensity of tree growth allowing woody plants to achieve tree stature. Our study brings a climatic context of European treelines.

ID: 3.13325

Integrating tree seedling establishment in Andean and Alpine treelines ecotones

Lirey Ramirez

Bader, Maaike

Abstract/Description

Understanding seedling responses to abiotic and biotic constraints is critical for predicting future treeline shifts. We investigate tree seedling establishment in treeline ecotones of the Venezuelan Andes and French Alps, integrating two field studies and one experiment, to explore how abiotic factors, microsite conditions, and plant-plant interactions influence seedling dynamics in tropical and alpine environments. In some inner valleys of the Venezuelan Andes, precipitation seasonality may critically impact seedling survival, however climatic and demographic data in this area is scarce. At a dry treeline ecotone, we analyzed climate, microclimate, seedling demography, and ecophysiological responses of two tree dominant species. We found a decrease in seedling density across the different environments -forest, forest-border and páramo- and positive associations of the seedlings with moisture-regulating elements under the forest and border suggesting that tree seedling depends on facilitative processes for recruitment, limiting rapid forest expansion. In the French Alps, we examined microsite preferences of three treeline-forming conifer species. Across four sites, we assessed key characteristics of microsites, including substrate, topography and shelter proximity. Species occupied similar microsites, typically with some shelter, while extreme microsites remained unoccupied. Results indicated that microsite availability is not a limiting factor suggesting that other factors, like seed availability could play a bigger role in new recruitment. We conducted a field experiment in the French Alps to examine how seedlings of five treeline-forming species respond to microclimatic modifications. Survival and biomass were negatively affected by vegetation cover and warming but benefited from shading and increased water availability. The results highlight the role of warming and competition in seedling survival. Our findings illustrate the multidimensional controls on seedling establishment. Although abiotic factors such as temperature and water availability influence survival and growth, positive and negative plant-plant interactions play a key role in establishment dynamics. As climate change accelerates treeline shifts, a better understanding of the factors that control seedling recruitment is an important step in treeline ecology. While adult trees at the treeline can survive for centuries, recruitment determines whether forests expand, remain stable or decline

ID: 3.14634

Fungi in pine forest regeneration at the upper tree line under the climate change

Jelena Lazarević

Topalović, Ana; Menkis, Audrius

Abstract/Description

Pinus heldreichii H. Christ and Pinus peuce Griseb are endemic and relict species to the Balkans, mostly remain as small, isolated stands on slopes and exposed terrains at altitudes between 1,200 and 2,100 m a.s.l. Fungi play key roles in alpine forest ecosystems and represent an essential part of biodiversity. They directly influence several physiological processes in trees and contribute to carbon, nutrient and water cycling. They can also significantly contribute to the successful regeneration, establishment and growth of pine trees, particularly in marginal habitats under harsh environmental conditions. To better understand the diversity, composition and possible roles, communities of fungi associated with forest trees and seedlings of P. heldreichii and P. peuce, as well as forest soils were studied in high-altitude forest sites in Montenegro. These sites are often subjected to forest fires, which represent a main ecological disturbance in forest ecosystems. In attempt to understand the mechanisms of ecosystem recovery and resilience following fire disturbances, we also studied the impact of forest fires on the composition and diversity of fungal communities in P. heldreichii forest soils at the upper forest line. Needles, rootlets and forest soil of P. heldreichii and P. peuce were sampled from their typical habitats, and associated fungi were studied using high-throughput sequencing. Soil physical and chemical properties were also determined through mechanical and chemical analyses. The results revealed a high fungal diversity associated with two pine species, as well as site-specific effects on the abundance and composition of fungal communities. In regard to forest fire, results showed that the fungal community composition differed markedly between the post-fire and unburned sites, and that there was a slow recovery of fungal communities in high-altitude environments. The new knowledge generated through these studies can contribute to the development of relevant strategies for management and conservation of P. heldreichii and P. peuce forests and their associated biodiversity. The need for active restoration strategies in fire-affected and other high-altitude regions is suggested.

ID: 3.18626

Climate, not land-use, drives a recent acceleration of larch expansion at the forest-grassland ecotone in the southern French alps

Baptiste Nicoud

Bayle, Arthur; Corona, Christophe; Francon, Loïc; Choler, Philippe

Abstract/Description

In recent decades significant forest expansion into treeless alpine zones has been observed across global mountain ranges, including the Alps, driven by a complex interplay of global warming and land-use changes. The upward shift of treelines has far-reaching implications for ecosystem functioning, biodiversity, and biogeochemical cycles. However, climate variables alone account for only a fraction of treeline dynamics, highlighting substantial research gaps concerning the influence of non-climatic factors. This study addresses these gaps by combining dendrochronological methods, high-resolution bioclimatic data, and historical land-use records to investigate treeline dynamics in the southern French Alps. Our results reveal a marked acceleration in tree establishment, starting in the early 2000s, attributable primarily to climate change rather than the pastoral abandonment of the 19th century. We demonstrate that historical land-use changes created predisposing conditions for tree establishment, while recent climate change has increasingly acted as an accelerator for this dynamic. While key climatic factors, such as thermal indicators and growing season length, are identified as significant contributors to treeline shifts, our study highlights the need for further research to disentangle the specific drivers of tree recruitment and survival in the context of ongoing climate change.

FS 3.233: Precipitation Changes in Mountainous Hydroclimates

FS 3.104: Treeline dynamics of pine and birch in response to climate change in the Karakoram

#IMC: September 14 - 18 2025

FS 3.103

Creating knowledge about alpine-treeline ecotones

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