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Free-living saprotrophic fungi and symbiotic mycorrhizal fungi affect organic matter dynamics differently because of contrasting ecological adaptations. We investigated how mass-loss, C:N-ratio and stable isotope dynamics of leaf litter and humus substrates depended on presence of living tree roots and associated fungal communities in a forest-to-tundra ecotone over three years. Litter mass-loss was stimulated by tree roots, contrary to a Gadgil effect. Increases in the litter nitrogen pool and δ15N suggested import of nitrogen from deeper soil by the dominating saprotrophic fungi. Over time, humus first lost, then gained, mass, and corresponding shifts in δ15N and δ13C suggested fluctuating pools of fine roots and fungal mycelium. Ectomycorrhizal tree roots consistently reduced longer-term humus mass-gain, counteracting positive effects of ericoid roots and associated fungi. Across all substrates, mass dynamics correlated with the balance between ectomycorrhizal and litter-saprotrophic fungi, both linked to mass-loss, and ericaceous shrubs and associated fungi, linked to mass-gain. [ABSTRACT FROM AUTHOR]

We present the longest high-elevation tree-ring width dataset in the Mediterranean reaching back to the 6th century CE. The network includes 101 living and 92 relict Pinus heldreichii Christ trees from four differently exposed sites in the 2100–2200 m a.s.l. elevation range of Mt. Smolikas in the Pindus Mountains in Greece. Though the sites were all sampled within a distance of <1 km, inter-site correlations are surprisingly low (r 1550–2014 = 0.65–0.87), indicating site exposure might affect tree-ring formation. We here explore the consequence of exposure differences on the climate signals in an eastern Mediterranean treeline ecotone. Temporally stable growth/climate relationships reveal similar seasonal patterns among the four sites, but differences in signal strength. P. heldreichii growth at Mt. Smolikas is significantly controlled by temperature in April (r 1951–2014 = 0.33–0.50) and precipitation in June-July (r 1951–2014 = 0.23–0.42), which emphasizes the overall importance of an early growth onset and subsequent moisture conditions. The association between stem growth and April climate is strongest in the South-facing stand, supporting the significance of higher insolation rates at this thermally privileged site. Strongest summer precipitation signals are found in the NE-facing stand, where trees seem to benefit least from an early growth onset and where reduced meltwater supply may enhance the dependency on early summer precipitation. The significance of spring temperature on tree growth in all four sites constrains the emergence of a distinct summer precipitation signal in the Mt. Smolikas high elevation ecotone. Exploration of the site-specific influences on a new millennium-long tree-ring width dataset is an important step towards an improved understanding of long-term climate variability in the Eastern Mediterranean. Site-related differences in climate sensitivity in the high-elevation tree-ring network at Mt. Smolikas indicate that both temperature and precipitation during different seasons could potentially be reconstructed if distinct site exposures (S versus NE) are considered. [ABSTRACT FROM AUTHOR]

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

Abstract: To evaluate the suitability of wood pastures as a managing tool in subalpine regions it is essential to know more about the influence of grazing on the ground vegetation. This study assessed native plant species selection by cattle at different stocking rates, feeding habits and site preferences of cattle. Based on the results, conclusions concerning the value of silvopastoral systems in the Alps were drawn. A field study on six different wood pasture areas, grazed by cattle at different stocking rates, was accompanied by an experiment on three adjoining areas of 0.51ha each, stocked with either three, six, or nine heifers. Plant species were recorded in plots of 20cm×20cm before and after grazing, and the intensity of grazing on each species was assessed. At low stocking rates, grasses and tall species were most intensely grazed, while at higher stocking rates the intake of forbs and small species increased. Since no relationship was found between nutritional value and species preference, other factors such as accessibility of a plant seem to be important for the feeding preferences of cattle. The preference for grasses at low and medium stocking rates suggests that an increased growth of forbs might lead to an increase in plant species diversity. [Copyright &y& Elsevier]

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• Nothofagus pumilio mortality is strongly related to elevation, followed by precipitation. • Dead patches are located in an ecotonal altitudinal strip: the treeline ecotone. • Dead patches are located in a longitudinal ecotone within the west-east rainfall gradient. • Decreasing rainfall and increasing temperatures were recorded. The world's forests are being affected by a changing climate. Recently, patches of dead trees have been found in forests of the Patagonian endemic species Nothofagus pumilio, which could not be related to insects or pathogenic fungi acting as primary agents. This study aimed to analyze environmental variables associated with N. pumilio mortality at a landscape scale. Dead patches were recorded from satellite image analysis and field trips throughout the distribution of N. pumilio forests in Chubut province (Argentina). The relation between forest mortality and environmental variables, including elevation, slope, aspect, recent volcanic ash deposits, precipitation, and temperature, was analyzed by contingency tables and chi-square analysis, and a risk model was developed using the Maximum Entropy (MaxEnt) algorithm. One hundred and seven dead patches were recorded only in the northern region of the study area (ca. −42.5°S to −44.5°S), where forests develop at higher elevations, slopes, precipitations, and temperatures than in the southern region. In the northern region, elevation, followed by precipitation, was the environmental variable most related to forest mortality. Nothofagus pumilio mortality was related to two ecotones: an altitudinal one, associated with middle-high mountainsides (1200 and 1400 m asl), and a longitudinal one, associated with intermediate values of mean annual precipitation within the west-east rainfall gradient. These results highlight the sensitivity of these transitional environments. According to the risk model based on abiotic features, more than 100,000 ha of forest (ca. 30%) are located under environmental variables favorable for decline and death. In the context of climate change and in the light of the results, abiotic factors seem to be the main predisposing factors of N. pumilio mortality. [ABSTRACT FROM AUTHOR]

• Rhododendron rupicola shrubs in the Hengduan mountains can modify abiotic microenvironmental conditions. • R. rupicola can increase diversity of soil associated fungi and modify their community composition. • R. rupicola facilitates growth and survival of treeline seedlings in a species-specific manner. • R. rupicola facilitates upward migration of the treeline and modifies its species composition. The alpine treeline is shifting upward due to climate warming. However, the treeline species composition and the pace of its upward migration can be mediated by ecological interactions. In particular, so-called ecosystem engineers, i.e. species that modulate the microscale environmental conditions, at the treeline may play a crucial role. We conducted a three-year seedling transplant experiment at the alpine treeline ecotone in southwest China to study how the shrub Rhododendron rupicola modifies the microscale physical and biotic environments and thus influences the establishment and performance of the two treeline species Larix potaninii and Picea likiangensis. Seedlings were transplanted to the current timberline and treeline, as well as above the current treeline in order to determine the responses of the two tree species to the shrub with respect to the current tree distribution. R. rupicola modified the microenvironment by increasing soil moisture and nutrient contents, buffering soil temperature fluctuations, and by increasing richness and changing the composition of root-associated fungi. As a result, tree seedlings planted under shrubs had significantly higher survival, growth rates and nutrient accumulations than those planted in open ground. Furthermore, seedlings planted at lower elevations performed better than those planted at higher elevations. Beyond the treeline, seedling survival was very low on open ground but strongly facilitated by the shrub. Finally, facilitation effects were species-specific, with Larix benefitting more from the shrub than Picea , while Picea had less mortality than Larix in the absence of the shrub. This study demonstrates that shrubs, through the amelioration of physical and biotic microenvironmental conditions, can act as stepping stones for the establishment of selective tree species beyond the current treeline. This suggests that biotic interactions can strongly modify the treeline species composition and push the treeline beyond its current climatic limits, thereby facilitating the upward shift with ongoing climate warming. [ABSTRACT FROM AUTHOR]

• No trends for advancing coniferous tree lines were detected in the satellite data. • Greening trends near the fell tops were detected by expanding shrub vegetation. • The NDVI indicates a densification of previously sparse vegetation cover. • Distinctive changes in the vegetation pattern arose from external disturbances. Global warming is predicted to affect ecosystems, particularly in high-latitude regions where polar amplification accelerates temperature rise and environmental changes. Here, where plants grow under adverse conditions, a warmer climate provides more favourable conditions for growth and regeneration. At the alpine and polar tree line in Finnish Lapland, rising temperatures are assumed to promote densification and expansion of conifers towards fell tops and treeless boreal heathlands beyond the recent tree-line position. In this study, we analysed vegetation changes in the pine treeline ecotone in six study sites in Finnish Lapland using multi-spectral satellite data during 1984–2017. All of the six sites were established in fell areas, covering the transition from closed forest stands of the lower elevations to the open fell tops beyond the treeline position. The southern sites were located in pine dominated-stands, where treelines were of alpine character. The northern sites were located in the polar treeline ecotone where mountain birch forests already dominate the landscape. We assessed shifts in the vegetation pattern of the fell sites using the normalized difference vegetation index (NDVI) and a RandomForest land-cover classification as indicators of potential change. We did not find clear trends for advancing coniferous tree lines towards open fell tops or treeless heath vegetation, neither by NDVI change detection nor by the land-cover classification. However, we found evidence for densification of open forest stands and sparse vegetation cover in lower elevations and the expansion of deciduous vegetation in higher elevations of previously vegetation-free or sparsely covered fell tops. Increasing stand density was detected mostly in the southern, pine-dominated sites, while the northern sites indicated increasing biomass near the fell tops. Prominent changes in vegetation patterns originated rather from human impact in the southern sites appearing as recent roads, clear-cuttings or infrastructure constructions in skiing areas. In the northern sites, distinctive changes arose from human impact or from biotic disturbance events such as moth outbreaks defoliating mountain birch stands at site Karigasniemi. [ABSTRACT FROM AUTHOR]