Loss of growth resilience towards the alpine shrubline

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The loss of resilience and prolonged recovery times after extreme climate events can be used as early warning signals of impending tipping points or abrupt irreversible changes. However, evidence of such critical slowing down in growth series is still lacking in terrestrial ecosystems under harsh environmental conditions such as alpine shrublands. Alpine shrublines, at which shrublands shift into grasslands under climate thresholds, constitute unique settings to look for critical slowing down phenomena. On the northeastern Tibetan Plateau, we sampled Salix oritrepha Schneid. (willow shrubs) in three elevational sites along each of two transects (QL, Qilian; HSX, Huashixia) running from alpine treeline position to shrubline. We then quantified the radial growth resilience and recovery indices of alpine willow shrubs to cold spells along elevational gradients by using dendrochronology. Willow growth was primarily constrained by low winter temperatures in QL and low summer temperatures in HSX. Cold spell events were detected in 2008 and 2011 in HSX and QL sites, respectively. Because temperature decreases with elevation, both growth resilience and recovery indices differed within each transect, showing a decreasing trend with elevation. Growth in 78% and 56% willow shrubs recovered back to the level before cold spell within two years in QL and HSX sites, respectively. Moreover, the average growth recovery period was longer at the shrubline (ca. 2–4 years) than near the treeline position (ca. 1–2 years). Critical slowing down theory could thus be applied to assess growth resilience towards the alpine shrubline, a thermal threshold for shrub growth. Our results and framework highlight the priority to directly measure the resilience metrics and also offer a promising perspective that the critical slowing down phenomenon may be universal in various ecosystems.