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Treelines are sensitive to environmental changes, but few studies provide a mechanistic approach to understand treeline dynamics based on field experiments. The aim of this study was to determine how changes in the abiotic and/or biotic conditions associated with global change affect the performance of tree seedlings (later saplings) at the treeline in a 10-year experiment. A fully factorial experiment in the Central Pyrenees was initiated in autumn 2006; 192 Pinus uncinata seedlings were transplanted into microplots with contrasting environmental conditions of (1) increased vs. ambient temperature, (2) increased nutrient availability vs. no increase, and (3) presence vs. absence of the dominant shrub Rhododendron ferrugineum. We assessed the performance of young pines on several occasions over 10 years. The pines were removed at the end of the experiment in autumn 2016 to characterize their morphology and to conduct chemical and isotopic analyses on their needles. Both the warming and the fertilization treatments increased seedling growth soon after the start of the experiment. R. ferrugineum facilitated the survival and development of pine seedlings during the early years and affected the chemical composition of the needles. Toward the end of the experiment, the transplanted P. uncinata individuals, by then saplings, competed with R. ferrugineum for light and nutrients; the presence of the shrub probably altered the strategy of P. uncinata for acquiring nutrients and buffered the effects of warming and fertilization. The pines were highly sensitive to all factors and their interactions throughout the entire experimental period. These findings indicated that the interactive effects of several key abiotic and biotic drivers associated with global change should be investigated simultaneously for understanding the contribution of young trees to treeline dynamics. [ABSTRACT FROM AUTHOR]

Aim To assess the effects of climate change, past land uses and physiography on the current position of the tree line in the Catalan Pyrenees and its dynamics between 1956 and 2006. Location More than 1000 linear kilometres of sub-alpine tree line in the Catalan Pyrenees (north-east Spain) Methods Using aerial photographs and supervised classification, we reclassified the images into a binary raster with 'tree' and 'non-tree' values, and determined canopy cover in 1956 and 2006. We then determined the change in position of the tree line between 1956 and 2006 based on changes in forest cover. We used the distance from the position of the tree line in 1956 to the theoretical potential tree line - determined from interpretation of aerial photographs, identifying the highest old remnants of forest for homogeneous areas of the landscape in terms of bioclimatic conditions, bedrock, landform and exposure - as a surrogate of intensity of past land uses. Results Our analyses showed that the Pyrenean tree line has moved upwards on average almost 40 m (mean advance ± SE: 35.3 ± 0.5 m, P < 0.001), although in most cases it has remained unchanged (61.8%) or advanced moderately, i.e. between 25 and 100 m (23.7%); only 9.2% of the locations have advanced more than 100 m. Upward shifts of the tree line were significantly larger in locations heavily modified in the past by anthropogenic disturbance (mean advance 50.8 ± 1.1 m) compared with near natural tree line locations (19.7 ± 0.8 m, P < 0.001), where the mean displacement was much lower than expected and was not related to changes in temperature along the study period. Main conclusions Our results stress the impact of the cessation of human activity in driving forest dynamics at the tree line in the Catalan Pyrenees, and reveal a very low or even negligible signal of climate change in the study area. [ABSTRACT FROM AUTHOR]

Background:Climate, land-use and disturbance regimes are key drivers of treeline dynamics worldwide, but local and regional spatio-temporal patterns indicate that additional factors play an important role. Some studies suggest that shrub-tree interactions control tree seedling recruitment patterns across the treeline ecotone, but little is known about the generality of this interaction. Aims:We established an experiment in a treeline ecotone in the central Pyrenees to investigate the role of such interactions and other environmental factors on tree seedling growth and survival. It was based on a similar experiment we completed recently in the subarctic Scandes. By comparing local phenomena between both experiments we assessed the generality of the findings across regions with different biogeographic histories and species characteristics. Methods:We followed the survival and growth of transplantedPinus uncinataseedlings during three growing seasons in a multi-factorial design (forest vs. treeline, +/– shrub removal, +/– temperature increase and +/– nutrient addition). Results:There was better seedling growth at the treeline compared to the forest, and the presence of shrubs prevented seedling winter damage and herbivory. Both temperature and nutrient increase enhanced seedling performance. Conclusions:Although some particular details were exclusive to the Pyrenees or to the Scandes, the similarities indicated general patterns and help to interpret the underlying mechanisms of treeline dynamics in both regions. This convergence of responses points to the potential of the development of a robust mechanistically founded predictive framework for scaling up shrub impacts on treeline dynamics to other regions. [ABSTRACT FROM PUBLISHER]

Background: Studying how species richness varies with altitude has become an important part of investigations on mechanisms explaining regional-scale plant species diversity. Altitudinal diversity patterns are poorly known in the Pyrenees and elsewhere in the main mountain massifs in Catalonia (north-eastern Iberian Peninsula). Aim: We examine the altitudinal patterns of species richness in the Catalan Pyrenees and compare them with those in other mountains in Catalonia. Altitude patterns of species richness in the Pyrenees are compared for functional and taxonomic groups of vascular plants. Methods: Data on vascular plants were obtained from the Biodiversity Data Bank of Catalonia. We used rarefaction and interpolation to quantify the altitudinal diversity patterns. Results: The interpolated and rarefied richness generally peaked in the lower part of the altitudinal gradient, but with remarkable variation depending on the taxonomic or functional groups considered. Using all species, there was little variation between the different mountains, but the altitude at which richness started to decrease varied between them. Conclusions: The pronounced differences among different taxonomic groups and life-forms indicate a strong phylogenetic and adaptive signal on the altitudinal diversity pattern in the Catalan Pyrenees. The lack of regional differences within Catalonia is probably due to small differences in the intensity of climatic gradients across mountains. [ABSTRACT FROM PUBLISHER]