Alpine treelines and shrublines are highly sensitive to environmental change. In this Review, we summarize their global patterns and trends, underlying mechanisms and impacts. Continental alpine treeline and shrubline elevations are highest at mid-latitudes, declining towards the Equator and poles. Shrublines are typically 335 m higher than collocated treelines owing to morphological differences. The mass-elevation effect, whereby larger mountain masses retain more heat, largely governs this distribution. Indeed, temperature is a key factor determining ecotone elevation. For example, tree growth near the alpine treeline begins at 0.9 °C and continues as long as the average temperature during the growing season exceeds 6.4 °C for a minimum of 94 days. Water availability is also important, with 51% of treelines exposed to drought stress. Overall, between 1901 and 2021, alpine treelines and shrublines have shifted to higher elevations at an average rate of 0.40 and 0.49 m yr−1, respectively, with shift rates at high-latitude sites exceeding those at lower latitudes.
