Regional Mountain Conference

A better understanding of what conditions promote tree seedling recruitment in the alpine treeline ecotone is essential for predictions about where and how quickly the treeline will advance and which tree species will become dominant. Here, we conducted a multifactorial seed addition and seedbed disturbance (scarification) experiment at the current treeline and in the alpine zone in the Swiss Alps. We quantified the emergence, survival and height growth of seedlings of two high-elevation conifers, Larix decidua and Picea abies, over a 10-year period. Only two seedlings emerged naturally in non-seeded plots during the entire study (both Larix at treeline), pointing to strong recruitment constraints on the series of processes necessary for emergence. In seeded plots, initial emergence was greater at the alpine site (8.0% of viable seeds sown) than at treeline (5.8%), whereas the survival rate of emerged seedlings in subsequent years was higher at treeline (4.1% vs 0.6%). Winter was a survival bottleneck at the treeline and especially the alpine site, with 54% and 94%, respectively, of the initially emerged seedlings dying during the first winter after sowing and 76% of all subsequent mortality occurring outside the growing season. Survival rates were higher and seedlings were taller for Larix than for Picea and no Picea seedlings remained alive beyond 4 years at the alpine site, reflecting the higher distribution limit of Larix in the region. Seedbed scarification led to higher survival rates of both species, suggesting that a single disturbance event can enhance recruitment in this ecotone. Notably, the heights of the few surviving Larix seedlings at the alpine site matched those of the tallest seedlings of this species at the treeline site, highlighting that microsite conditions and ambient temperatures 300+ m above the current treeline support robust growth of this species once it establishes successfully.