Are sub-alpine species' seedling emergence and establishment in the alpine limited by climate or biotic interactions?

One of the ways in which plants are responding to climate change is by shifting their ranges to higher elevations. Early life-history stages are major bottlenecks for species' range shifts, and variation in seedling emergence and establishment success can therefore be important determinants of species' ability to establish at higher elevations. Previous studies have found that warming per se tends to not only increase seedling establishment in alpine climates but it also increases plant productivity, which could limit establishment success through increased competition for light. Here we disentangle the relative importance of several climate-related abiotic and biotic factors on sub-alpine species' seedling emergence and survival in the alpine. Specifically, we test how temperature, precipitation and competition from neighbouring vegetation impacts establishment, and also whether species' functional traits, or strategies impact their ability to colonise alpine locations. We found that our six sub-alpine study species were all able to recruit from seed in alpine locations under the extant alpine climate, but their emergence was limited by competition from neighbouring vegetation. This indicates that biotic interactions can hinder the range shifts expected as a result of climate warming. Species with a resource conservative strategy had higher emergence in the extant alpine climate than species with a resource acquisitive strategy, and they were largely unaffected by changes in temperature. The resource acquisitive species, in contrast, had faster emergence under warming, especially when they were released from competition from neighbouring vegetation. Our results indicate that competition from the established vegetation is limiting the spread of lowland species into the alpine, and as the climate continues to warm, species with resource acquisitive traits might gain an advantage.