Alpine treeline ecotones world wide: fine-scale spatial patterns and environmental controls.

Alpine treelines mark the physiological limit to tree growth along elevational gradients. Tree cover beneath this limit forms an ecotone with diverse spatial configurations. However, global-scale quantification of these ecotone patterns and their underlying environmental drivers remains lacking. We used very-high-resolution satellite imagery and landscape metrics to characterize fine-scale treeline-ecotone spatial patterns globally. The variations in tree-cover distributions within the ecotones were effectively described by the leading principal components from landscape metrics, which captured the level of tree-cover aggregation into patches and patch complexity. Highly patchy ecotones, where trees grow individually, are most common where snow, wind, topographic relief, and nontree vegetation cover are high, occurring in areas where the upper tree limits approach the thermal limits to tree growth. Upper tree limits located far below the thermal limits tend to exhibit discrete ecotones. Among the treeline tree genera, Pinus stood out as forming patchier and Nothofagus as forming more discrete ecotones. We show that alpine treeline ecotones follow predictable global spatial patterns, allowing the formulation of hypotheses about the processes controlling pattern formation and ecotone dynamics. With the presented method to describe fine-scale patterns globally, we gain a powerful indicator for predicting forest expansion under climate change.