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.