Causes for treeline stability under climate warming: Evidence from seed and seedling transplant experiments in southeast Tibet

Few data have examined the warming-related controls on seedling recruitment above the treeline, which is critical to understand causes for treeline stability under climate warming. We tested the hypothesis that compared to controls on seed dispersal and germination, the warming-induced decrease of seedling survival above the treeline is more likely to cause treeline stability. Long-term microclimate observations and 4-year seed and seedling transplant experiments were conducted in two contrasting treeline ecotones on the opposite slopes of a U-shaped valley at the peak of the Sergyemla Mountains. Seeds and young seedlings of Smith-fir (Abies georgei var. smithii) were transplanted to the treeline (4320 m) and Rhododendron shrub (4390 m) on the north-facing slope and to the juniper treeline (4425 m) on the south-facing slope. Transplanted seeds were collected from the Smith-fir treeline and its distribution center (3800 m). We also investigated the distribution of naturally-established seedlings and its correlation with shrub cover along elevation transects above the treeline. Smith-fir seedlings growing under shrubs were observed up to 40 m in elevation higher above the treeline, and there was either no correlation or a low correlation between seedling density and shrub cover. Harsh environments above the treeline did not limit the germination of seeds from different seed sources. However, transplanted seedlings cannot survival well above the treeline or at the juniper treeline due to high frequency of early-season freezing events and strong light intensity, while the absence of early-season freezing events in the Smith-fir treeline forest was more beneficial to seedling survival. During 2006-2015, the frequency of early-season freezing events increased with increasing annual mean air temperature above the treeline and at the juniper treeline but not in the Smith-fir treeline forest. The warming-induced increase of early-season freezing events, combined with high light intensity, mainly limits the survival of young seedlings above the treeline, which is more likely to cause treeline stability. Early-season freezing events should be an important index for evaluating the vulnerability of treeline forests to global warming.