Intraspecific competition drives recruit mortality in alpine treelines

Abstract Alpine treelines represent cold range limits of tree existence at high elevations. Climate is considered a major driver of treeline dynamics, but cannot well explain its lagged response to climate warming. In fact, predicting alpine treeline shifts remains a major challenge because of a poor understanding of how inherent processes, such as tree‐to‐tree interactions, drive treeline shifts. Both positive intraspecific facilitation and negative intraspecific competition are thought to play key roles in regulating seedling recruitment and mortality within treeline ecotones, yet empirical evidence remains scarce. Here, in 2024, we resurveyed 12 treeline plots established during 2011–2013 along an 1100‐km north–south transect on the eastern Tibetan Plateau to calculate annual per‐capita establishment, mortality and population growth rates. Point pattern analyses and generalised additive model were used to test how intraspecific facilitation or competition effects drive seedling establishment and mortality at the treeline. Nine out of twelve treeline plots exhibited positive population increase, while their treeline shift rate was low (average 0.56 m·year −1 with a range of 0.00–2.66 m·year −1 ). The average annual establishment and mortality rates were 2.74% year −1 (0.76%–7.92% year −1 ) and 1.95% year −1 (0.56%–5.73% year −1 ), respectively. Mortality rates were significantly lower near the treeline with sparse tree cover compared to the timberline with denser cover, showing a potential for treeline upward shift. Newly established seedlings did not show clustering around adult trees. Competition dominated tree‐to‐tree interactions, and smaller seedlings with higher competition intensity exhibited a greater risk of mortality. Synthesis . Our results do not support the hypothesis that facilitation constrains treeline advancement. Instead, competition explains the higher seedling mortality in treelines. This effect is alleviated near the sparsely populated upper treeline, providing strong upward migration potential with ongoing climate warming. Our findings elucidate how biotic factors mediate tree recruitment in alpine treeline ecotones, emphasising the need to incorporate negative density‐dependent interactions into future treeline models.