Among-tree variability and feedback effects result in different growth responses to climate change at the upper treeline in the Swiss Alps

Fifty years of change in the lower tree line in an arid coniferous forest in the Qilian Mountains, northwestern China

Tree line areas exhibited significant changes in response to climate change, including upward migration. Lower tree line dynamics are rarely studied, but as unique features in arid and semi-arid areas, they may influence forest distribution. Here, eight lower tree line plots in a Picea crassifolia Kom. (Qinghai spruce) forest in the arid and semi-arid Qilian Mountains of northwestern China were used to determine changes in tree line location and relationships with meteorological factors during 1968-2018. The results showed that the lower tree line descended by an average of 9.82 m during 1968 to 2018, and exhibited almost no change after 2008. The change in the lower tree line was significantly correlated with the annual average temperature (°C) and annual precipitation (mm) and may be affected by human activities. In the past 50 years, the lower tree line in arid areas exhibited a downward trend. Our findings indicate that the movement of the lower tree line is also an important aspect of climatic changes in coniferous forest distribution in arid and semi-arid mountains.

Scientists' warning of the impacts of climate change on mountains

Mountains are highly diverse in areal extent, geological and climatic context, ecosystems and human activity. As such, mountain environments worldwide are particularly sensitive to the effects of anthropogenic climate change (global warming) as a result of their unique heat balance properties and the presence of climatically-sensitive snow, ice, permafrost and ecosystems. Consequently, mountain systems-in particular cryospheric ones-are currently undergoing unprecedented changes in the Anthropocene. This study identifies and discusses four of the major properties of mountains upon which anthropogenic climate change can impact, and indeed is already doing so. These properties are: the changing mountain cryosphere of glaciers and permafrost; mountain hazards and risk; mountain ecosystems and their services; and mountain communities and infrastructure. It is notable that changes in these different mountain properties do not follow a predictable trajectory of evolution in response to anthropogenic climate change. This demonstrates that different elements of mountain systems exhibit different sensitivities to forcing. The interconnections between these different properties highlight that mountains should be considered as integrated biophysical systems, of which human activity is part. Interrelationships between these mountain properties are discussed through a model of mountain socio-biophysical systems, which provides a framework for examining climate impacts and vulnerabilities. Managing the risks associated with ongoing climate change in mountains requires an integrated approach to climate change impacts monitoring and management.

Phenotypic plasticity of European larch radial growth and wood density along a-1,000 m elevational gradient

Phenotypic plasticity is a key mechanism for sedentary long-living species to adjust to changing environment. Here, we use mature Larix decidua tree-ring variables collected along an elevational transect in the French Alps to characterize the range of individual plastic responses to temperature. Stem cores from 821 mature Larix decidua trees have been collected from four plots distributed along a 1,000-m elevational gradient in a natural forest to build up individual linear reaction norms of tree-ring microdensity traits to temperature. The sign, magnitude and spread of variations of the slopes of the individual reaction norms were used to characterize variation of phenotypic plasticity among plots and traits. Results showed a large range of phenotypic plasticity (with positive and negative slopes) at each elevational plot and for each tree-ring variable. Overall, phenotypic plasticity tends to be larger but positive at higher elevation, negative at the warmer lower sites, and more variable in the center of the elevation distribution. Individual inter-ring reaction norm is a valuable tool to retrospectively characterize phenotypic plasticity of mature forest trees. This approach applied to Larix decidua tree-ring micro-density traits along an elevation gradient showed the existence of large inter-individual variations that could support local adaptation to a fast-changing climate.

Treeline Research—From the Roots of the Past to Present Time. A Review

Elevational and polar treelines have been studied for more than two centuries. The aim of the present article is to highlight in retrospect the scope of treeline research, scientific approaches and hypotheses on treeline causation, its spatial structures and temporal change. Systematic treeline research dates back to the end of the 19th century. The abundance of global, regional, and local studies has provided a complex picture of the great variety and heterogeneity of both altitudinal and polar treelines. Modern treeline research started in the 1930s, with experimental field and laboratory studies on the trees’ physiological response to the treeline environment. During the following decades, researchers’ interest increasingly focused on the altitudinal and polar treeline dynamics to climate warming since the Little Ice Age. Since the 1970s interest in treeline dynamics again increased and has considerably intensified from the 1990s to today. At the same time, remote sensing techniques and GIS application have essentially supported previous analyses of treeline spatial patterns and temporal variation. Simultaneously, the modelling of treeline has been rapidly increasing, often related to the current treeline shift and and its implications for biodiversity, and the ecosystem function and services of high-elevation forests. It appears, that many seemingly ‘new ideas’ already originated many decades ago and just confirm what has been known for a long time. Suggestions for further research are outlined.

Climate-growth relationships in a Larix decidua Mill. network in the French Alps

A better understanding of the respective role of key climatic variables on tree growth is crucial for an accurate assessment of how ongoing global changes may affect both dynamics and distribution of forest tree species in the future. The aim of this study was (i) to explore growth patterns of European larch (Larix decidua Mill.) through a network of tree-ring chronologies developed for the French Alps and (ii) to identify the main climatic drivers explaining radial growth. Climate-growth relationships were coupled with a hierarchical analysis. This relationship revealed significant variability expressed spatially by the existence of five clusters, initially discriminated by an elevational contrast related to (i) a negative correlation between summer temperatures and larch growth at lower elevations and (ii) a stronger response of low-elevation larch stands to winter precipitation. In the high-elevation clusters, tree growth depends on previous autumn and current summer temperatures and water supply in July. The differentiation, that portrays a strong geographical coherence, is mainly related to the latitudinal gradient; (i) the northwestern stands are mostly sensitive to high temperatures in summer; (ii) the growth of the southernmost clusters is equally driven by temperatures during autumn, winter, and summers; (iii) the populations of a cluster located in a transitional zone of the inner French Alps, subject to both Mediterranean and Continental influences, exhibit negative correlations to late winter and early spring precipitation. This significant spatial heterogeneity of climate-tree ring relationships in L. decidua clearly underlines the high plasticity of the species to adapt its growth to local climate conditions.

Among-tree variability and feedback effects result in different growth responses to climate change at the upper treeline in the Swiss Alps

Upper treeline ecotones are important life form boundaries and particularly sensitive to a warming climate. Changes in growth conditions at these ecotones have wide‐ranging implications for the provision of ecosystem services in densely populated mountain regions like the European Alps. We quantify climate effects on short‐ and long‐term tree growth responses, focusing on among‐tree variability and potential feedback effects. Although among‐tree variability is thought to be substantial, it has not been considered systematically yet in studies on growth–climate relationships. We compiled tree‐ring data including almost 600 trees of major treeline species (Larix decidua, Picea abies, Pinus cembra, and Pinus mugo) from three climate regions of the Swiss Alps. We further acquired tree size distribution data using unmanned aerial vehicles. To account for among‐tree variability, we employed information‐theoretic model selections based on linear mixed‐effects models (LMMs) with flexible choice of monthly temperature effects on growth. We isolated long‐term trends in ring‐width indices (RWI) in interaction with elevation. The LMMs revealed substantial amounts of previously unquantified among‐tree variability, indicating different strategies of single trees regarding when and to what extent to invest assimilates into growth. Furthermore, the LMMs indicated strongly positive temperature effects on growth during short summer periods across all species, and significant contributions of fall (L. decidua) and current year's spring (L. decidua, P. abies). In the longer term, all species showed consistently positive RWI trends at highest elevations, but different patterns with decreasing elevation. L. decidua exhibited even negative RWI trends compared to the highest treeline sites, whereas P. abies, P. cembra, and P. mugo showed steeper or flatter trends with decreasing elevation. This does not only reflect effects of ameliorated climate conditions on tree growth over time, but also reveals first signs of long‐suspected negative and positive feedback of climate change on stand dynamics at treeline.