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Both biotic and abiotic components, characterizing the mountain treeline ecotone, respond differently to climate variations. This study aims at reconstructing climate-driven changes by analyzing soil evolution in the late Holocene and by assessing the climatic trends for the last centuries and years in a key high-altitude climatic treeline (2515 m a.s.l.) on the SW slope of the Becca di Viou mountain (Aosta Valley Region, Italy). This approach is based on soil science and dendrochronological techniques, together with daily air/soil temperature monitoring of four recent growing seasons. Direct measurements show that the ongoing soil temperatures during the growing season, at the treeline and above, are higher than the predicted reference values for the Alpine treeline. Thus, they do not represent a limiting factor for tree establishment and growth, including at the highest altitudes of the potential treeline (2625 m a.s.l.). Dendrochronological evidences show a marked sensitivity of tree-ring growth to early-summer temperatures. During the recent 10-year period 2006–2015, trees at around 2300 m a.s.l. have grown at a rate that is approximately 1.9 times higher than during the 10-year period 1810–1819, one of the coolest periods of the Little Ice Age. On the other hand, soils show only an incipient response to the ongoing climate warming, likely because of its resilience regarding the changeable environmental conditions and the different factors influencing the soil development. The rising air temperature, and the consequent treeline upward shift, could be the cause of a shift from Regosol to soil with more marked Umbric characteristics, but only for soil profiles located on the N facing slopes. Overall, the results of this integrated approach permitted a quantification of the different responses in abiotic and biotic components through time, emphasizing the influence of local station conditions in responding to the past and ongoing climate change.

Tundra ecosystems are global belowground sinks for atmospheric CO2. Ongoing warming-induced encroachment by shrubs and trees risks turning this sink into a CO2 source, resulting in a positive feedback on climate warming. To advance mechanistic understanding of how shifts in mycorrhizal types affect long-term carbon (C) and nitrogen (N) stocks, we studied small-scale soil depth profiles of fungal communities and C–N dynamics across a subarctic-alpine forest-heath vegetation gradient. Belowground organic stocks decreased abruptly at the transition from heath to forest, linked to the presence of certain tree-associated ectomycorrhizal fungi that contribute to decomposition when mining N from organic matter. In contrast, ericoid mycorrhizal plants and fungi were associated with organic matter accumulation and slow decomposition. If climatic controls on arctic-alpine forest lines are relaxed, increased decomposition will likely outbalance increased plant productivity, decreasing the overall C sink capacity of displaced tundra.

This paper accounts for a case of substantial transformation of the treeline ecotone (Picea abies (L.) Karst.) in the Swedish Scandes. Duringthe past 100 years, coinciding with summer warming by 1.6 °C, the ecotonal landscape changed from predominance of stunted krummholzindividuals to a mosaic of tree groves and intervening alpine tundra.

Cudlín, Pavel et. al.- 16 páginas.- Ilustraciones.- Se acompaña suplemento de 3 páginas.- © The autors 2017. Open Access under Creative Commons by Attribution Licence. Use, distribution and reproduction are unrestricted. Authors and original publication must be credited., A growing body of evidence suggests that processes of upward treeline expansion and shifts in vegetation zones may occur in response to climate change. However, such shifts can be limited by a variety of non-climatic factors, such as nutrient availability, soil conditions, landscape fragmentation and some species-specific traits. Many changes in species distributions have been observed, although no evidence of complete community replacement has been registered yet. Climatic signals are often confounded with the effects of human activity, for example, forest encroachment at the treeline owing to the coupled effect of climate change and highland pasture abandonment. Data on the treeline ecotone, barriers to the expected treeline or dominant tree species shifts due to climate and land use change, and their possible impacts on biodiversity in 11 mountain areas of interest, from Italy to Norway and from Spain to Bulgaria, are reported. We investigated the role of environmental conditions on treeline ecotone features with a focus on treeline shift. The results showed that treeline altitude and the altitudinal width of the treeline ecotone, as well as the significance of climatic and soil parameters as barriers against tree species shift, significantly decreased with increasing latitude. However, the largest part of the commonly observed variability in mountain vegetation near the treeline in Europe seems to be caused by geomorphological, geological, pedological and microclimatic variability in combination with different land use history and present socio-economic relations., This paper is based firstly upon work from the COST Action ES 1203 SENSFOR, supported by COST (European Cooperation in Science and Technology; www.cost.eu). This international work was further supported by projects granted by the Ministry of Education Youth and Sports of the Czech Republic, grant NPU I LO1415 and LD 14039, by the agency APVV SR under projects APVV-14-0086 and APVV-15-0270. We acknowledge the E-OBS dataset from the EU-FP6 project ENSEMBLES (http://ensembles-eu.metoffice.com) and the data providers in the ECA&D project (www.ecad.eu)

Transition zones between mountain forests and treeless tundra, i.e. treeline ecotones, are characterized by great regional variety. In this paper, we discuss the biodiversity in various trophic levels in treeline ecotones throughout Europe, with particular focus on recent changes in land use and climate in northern and central mountains. In northernmost Europe, mountain birch prevails, while conifers (spruce, pine, larch) are the dominating species further south. While at continent-wide to global scales, the ecotone position is largely controlled by heat deficiency, it depends on a multitude of partly interacting abiotic and biotic factors other than climate at smaller scales. Climate change is a driving factor in treeline ecotone change, including physiognomic structure and biodiversity, although the effects of climate and other factors often overlap. Historical legacy plays an important role in this respect, and human impacts are particularly important. The recent decline in pastoral use of many European treeline areas often strongly influences plant diversity and re-growth of trees and other woody species. Climate change together with changing tree cover may influence snow cover, moisture regime, and nutrient conditions. Subsequently changed site conditions influence plant−plant interactions, favoring some species and disfavoring others, and plant−animal interactions. Native animals may cause widespread or local disturbances in treeline ecotone areas. Mass outbreaks of leaf-eating insects, for example, usually affect comparatively large forested areas whereas mammalian herbivores and birds have more local impact. However, high numbers of wild or domestic mammalian herbivores may challenge the carrying capacity of treeline ecotone areas at the same time as they preserve an open pasture character. This calls for cross-disciplinary study approaches, addressing the complexity of the ecotone regarding both causal background and biogeographic diversity. Treeline ecotone, ecotone change, land use change, climate change, anumal impact

The elevational and latitudinal diversity patterns of microbial taxa have attracted great attention in the past decade. Recently, the distribution of functional attributes has been in the spotlight. Here, we report a study profiling soil microbial communities along an elevation gradient (500-2200 m) on Changbai Mountain. Using a comprehensive functional gene microarray (GeoChip 5.0), we found that microbial functional gene richness exhibited a dramatic increase at the treeline ecotone, but the bacterial taxonomic and phylogenetic diversity based on 16S rRNA gene sequencing did not exhibit such a similar trend. However, the β-diversity (compositional dissimilarity among sites) pattern for both bacterial taxa and functional genes was similar, showing significant elevational distance-decay patterns which presented increased dissimilarity with elevation. The bacterial taxonomic diversity/structure was strongly influenced by soil pH, while the functional gene diversity/structure was significantly correlated with soil dissolved organic carbon (DOC). This finding highlights that soil DOC may be a good predictor in determining the elevational distribution of microbial functional genes. The finding of significant shifts in functional gene diversity at the treeline ecotone could also provide valuable information for predicting the responses of microbial functions to climate change.

Little is known about the relationships between treeline elevation and climate at regional and local scales. It is compelling to fill this research gap with data from the Tibetan Plateau where some of the highest alpine treelines in the world are found. This research question partially results from the lack of in situ temperature data at treeline sites. Herein, treeline variables (e.g., elevation, topography, tree species) and temperature data were collected from published investigations performed during this decade on the Tibetan Plateau. Temperature conditions near treeline sites were estimated using global databases and these estimates were corrected by using in situ air temperature measurements. Correlation analyses and generalized linear models were used to evaluate the effects of different variables on treeline elevation including thermal (growing-season air temperatures) and non-thermal (latitude, longitude, elevation, tree species, precipitation, radiation) factors. The commonality analysis model was applied to explore how several variables (July mean temperature, elevation of mountain peak, latitude) were related to treeline elevation. July mean temperature was the most significant predictor of treeline elevation, explaining 55% of the variance in treeline elevation across the Tibetan Plateau, whereas latitude, tree species, and mountain elevation (mass-elevation effect) explained 30% of the variance in treeline elevation. After considering the multicollinearity among predictors, July mean temperature (largely due to the influence of minimum temperature) still showed the strongest association with treeline elevation. We conclude that the coupling of treeline elevation and July temperature at a regional scale is modulated by non-thermal factors probably acting at local scales. Our results contribute towards explaining the decoupling between climate warming and treeline dynamics., This work was funded by the National Natural Science Foundation of China (41661144040, 41525001, and 41301207), the International Partnership Program of Chinese Academy of Sciences (131C11KYSB20160061), and the project supported by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

22 páginas, Background: The alpine treeline ecotone is regarded as a sensor of the effects of global changes on alpine plant communities. However, little is known about how treeline dynamics influence the diversity and composition of alpine plant communities. Such information is necessary to forecast how ascending montane forests may affect the composition of alpine flora. Aims: We analysed the temporal variations in tree cover, plant diversity and composition, and the effect of tree cover dynamics on field layer vegetation over a period of 11 years, in two alpine treeline ecotones in the central Pyrenees, Spain. Methods: Tree and field layer vegetation was sampled in permanent transects in 1998 and 2009, using the point-intercept method. Temporal changes in tree cover, plant species richness and abundance were characterised along the ecotone by using a randomisation approach, rarefaction curves, and a non-parametric multivariate test, respectively. Results: Tree cover increased significantly at one of the sites, whereas plant species richness only increased at the other site where tree cover had not changed. Vegetation composition changed significantly at both sites, but it was not spatially coupled with changes in tree cover along the ecotone. Conclusions: A change of tree cover does not necessarily trigger changes in the ground flora at the treeline over relatively short periods (decade scale). The results challenge our ability to infer short-term biodiversity impacts from upslope advance of forests. Integrated tree and field layer monitoring approaches are necessary to produce a better understanding of the impact of ongoing global change on treeline ecotones., We thank G. Sangüesa and J. Villellas for field assistance in 2009; J. Zabalza and P. Errea for technical assistance with Figure 1; L. Cayuela for comments on an earlier draft; J. Grace for suggestions and kindly improving the English; A. Hofgaard, L. Nagy and four anonymous referees for their useful comments and suggestions. We are also grateful to the staff of the Ordesa-Monte Perdido and Aigüestortes-Estany de Sant Maurici National Parks for permissions. Funding was provided by the Spanish Ministry of Science and Technology through a doctoral grant to IP (AP2008-01449), and projects (018/2008 of National Parks and CGL2010-21642 to MBG, and AMB95-0160 to EG). JJC thanks the support of the ARAID Foundation.

International audience; Within the alpine treeline ecotone tree growth is increasingly restricted by extreme climate conditions. Although intra-annual stem growth recorded by dendrometers can be linked to climate, stem diameter increments in slow-growing subalpine trees are masked by changes in tree water status. * We tested the hypothesis that intra-annual radial stem growth in Pinus cembra is influenced by different climate variables along the treeline ecotone in the Austrian Alps. Dendrometer traces were compared with dynamics of xylem cell development to date onset of cambial activity and radial stem growth in spring.* Daily fluctuations in stem radius reflected changes in tree water status throughout the treeline ecotone. Extracted daily radial increments were significantly correlated with air temperature at the timberline and treeline only, where budburst, cambial activity and enlargement of first tracheids also occurred quite similarly. A close relationship was detected between radial increment and number of enlarging tracheids throughout the treeline ecotone.* We conclude that (i) the relationship between climate and radial stem growth within the treeline ecotone is dependent on a close coupling to atmospheric climate conditions and (ii) initiation of cambial activity and radial growth in spring can be distinguished from stem re-hydration by histological analysis.; La croissance des arbres est fortement limitée par les conditions climatiques extrêmes de la limite altitudinale de la forêt. Les variations interannuelles de croissance radiale enregistrées par des dendromètres de tige peuvent être reliées au climat. Mais la croissance radiale peut être masquée par des changements de l'état hydrique des arbres.* Nous avons testé l'hypothèse selon laquelle les variations intra-annuelles de croissance radiale des troncs de P. cembra étaient contrôlées par différents facteurs climatiques le long de l'écotone de la limite altitudinale de la forêt dans les Alpes autrichiennes. Les enregistrements obtenus à partir de dendromètres ont été comparés à des dynamiques de développement des cellules cambiales, afin de détecter avec précision la date de départ de la croissance cambiale au printemps.* Les variations diurnes de diamètre du tronc reflétaient des variations de l'état hydrique des arbres le long de l'écotone. Les incréments quotidiens en diamètre extraits de ces signaux étaient corrélés à la température de l'air à la limite forestière, mais ne l'étaient plus à des altitudes supérieures, alors que le débourrement des bourgeons, la reprise d'activité cambiale et l'expansion des premiers trachéides se produisaient pratiquement en même temps le long de l'écotone. Une forte corrélation a été détectée tout au long de l'écotone, entre la croissance radiale et le nombre de trachéides en expansion.* Nous concluons que: (i) la croissance radiale le long de l'écotone de la limite altitudinale de la forêt est fortement couplée aux conditions climatiques dans l'atmosphère, et que : (ii) l'initiation de l'activité cambiale et de la croissance radiale au printemps peut être distinguée de la réhydratation des troncs en mobilisant des analyses histologiques.

Global warming is triggering some species to shift towards the poles or higher elevations, but spatial translocation is also influenced by land-use regime or intensity. The Himalayan climate is getting warmer and land use has changed, reducing in intensity in some areas. We estimated the upper species limit (USL) and tree limit of Abies spectabilis (D. Don) Spach and assessed whether these have changed over recent years. We hypothesise an upslope shift in response to enhanced temperature and changes in land-use intensity. Our four transects were located in treeline ecotones of two protected areas in Nepal, namely Manaslu Conservation Area (3 transects) and Gaurishankar Conservation Area (1 transect). Transects (20 m wide) ran from the USL of A. spectabilis down towards the treeline and beyond to the forest line. Length of each transect varied depending on local conditions. Co-ordinates, elevation, height and age of each A. spectabilis individual along the transects were recorded. We noted an upward shift of both the USL and the tree limit. The rate of shift was ca. 20 m per decade for the USL and 12 m per decade for the tree limit in the area of reduced land-use intensity and in the area with no change in land use, 5 m per decade for the USL, but almost nil for tree limit. The seedling density was higher below the treeline than above. Reduced intensity of land use was the dominant factor in upslope shift of A. spectabilis at both the USL and the tree limit. publishedVersion