Data from: Competitor or facilitator? The ambiguous role of alpine grassland for the early establishment of tree seedlings at treeline

Alpine treelines are expected to move upslope with a warming climate. However, so far treelines have responded inconsistently and future shifts remain difficult to predict since many factors unrelated to temperature, such as biotic interactions, affect responses at the local scale. Especially during the earliest regeneration stages, trees can be strongly influenced by alpine vegetation via both competition and facilitation. We aimed to understand the relative importance of these two types of interaction in different vegetation structures for treeline regeneration dynamics. Effects of herbaceous alpine vegetation on seedling emergence and first-year performance were studied in a field experiment in the French Alps (2100 m a.s.l.) with five important European treeline tree species: Larix decidua, Picea abies, Pinus cembra, Pinus uncinata and Sorbus aucuparia. Total emergence and locally-germinated seedling survival were not affected, but for seedlings planted at two months of age, negative vegetation impacts dominated for all response parameters: first-year survival, growth and carbohydrate accumulation. However, in the winter half-year, evergreen tree seedlings increased carbohydrate reserves under the protection of senescent herbs. Also, responses of locally-germinated seedlings suggest facilitative vegetation effects in the first two months after emergence. Thus, the interaction switched between competition and facilitation according to ontogenetic stage and seasons. Still, the net outcome after one year was negative, but species differed in their susceptibilities. Because initial establishment is the first bottleneck determining whether treelines remain stable or move upslope, understanding establishment, including site-, life-stage and species-specific processes, is essential for understanding observed treeline spatial patterns and dynamics. When developing predictive models of treeline dynamics, all these ‘local’ aspects should be incorporated in addition to more global drivers like changes in temperature