Abstract
Although there is a general consensus on the distribution and ecological features of terrestrial biomes, the allocation of alpine ecosystems in the global biogeographic system is still unclear. Here, we delineate a global map of alpine areas above the treeline by modelling regional treeline elevation at 30 m resolution, using global forest cover data and quantile regression. We then used global datasets to 1) assess the climatic characteristics of alpine ecosystems using principal component analysis, 2) define bioclimatic groups by an optimized cluster analysis and 3) evaluate patterns of primary productivity based on the normalized difference vegetation index. As defined here, alpine biomes cover 3.56 Mkm2 or 2.64% of land outside Antarctica. Despite temperature differences across latitude, these ecosystems converge below a sharp threshold of 5.9°C and towards the colder end of the global climatic space. Below that temperature threshold, alpine ecosystems are influenced by a latitudinal gradient of mean annual temperature and they are climatically differentiated by seasonality and continentality. This gradient delineates a climatic envelope of global alpine biomes around temperate, boreal and tundra biomes as defined in Whittaker's scheme. Although alpine biomes are similarly dominated by poorly vegetated areas, world ecoregions show strong differences in the productivity of their alpine belt irrespectively of major climate zones. These results suggest that vegetation structure and function of alpine ecosystems are driven by regional and local contingencies in addition to macroclimatic factors.
Highlights
The knowledge of the extent and climatic characteristics of terrestrial biomes, further linked to their functional aspects, is key to understanding ecological and biogeographical phenomena (Mucina 2019)
The mapped alpine ecosystems are grouped toward the colder end of the global climatic space (Fig. 2), with 99% of the grid cells situated below a mean annual temperature of 5.9°C and tropical alpine ecosystems lying on this threshold
Clustering the whole dataset of alpine regions using the clustering large applications (CLARA) algorithm with the first four weighted principal component analysis (PCA) axes highlighted the presence of four groups (Supplementary material Appendix 1 Fig. A3a), that were interpreted as 1) oceanic, 2) hemiboreal, 3) continental and 4) subtropical alpine ecosystems (Fig. 3, 4a)
Summary
The knowledge of the extent and climatic characteristics of terrestrial biomes, further linked to their functional aspects (e.g. productivity), is key to understanding ecological and biogeographical phenomena (Mucina 2019). Alpine ecosystems (i.e. high-elevation habitats above the climatic treeline) are the only biogeographic unit represented across all continents and latitudes (Körner 2003); they are characterized by a varied history of climatic changes and strong microhabitat differentiation (Körner 2003); they contain global biodiversity hotspots – e.g. the. In a different biogeographical context, Olson et al (2001) identified the ‘Montane Grasslands and Shrublands’ habitat type encompassing tropical and subtropical mountain ranges This classification is not based on vegetation patterns and left out temperate and boreal alpine regions of the northern hemisphere, that were either included in forest biomes or arctic tundra. Faber-Langendoen et al (2016) defined ‘Tropical Montane and High Montane Grasslands and Shrublands’ in a recent classification of world vegetation formations, separating these areas from the non-tropical alpine tundra
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