Alpine shrubs as facilitators: shrub structure, microsite exposure and site context shape seedling survival and growth dynamics

Abstract Shrub encroachment has become a common consequence of climate change in many alpine regions. However, the influence of existing shrubs on the growth of new shrub seedlings is not well understood. Facilitative interactions, such as microclimatic buffering, could play an increasingly important role in alpine seedling establishment as the climate changes. Here, we conducted a five-year field experiment in the Australian Alps to investigate whether seedling survival, leaf production and height growth of a common shrub ( Grevillea australis ) vary between leeward (SE-facing, sheltered) and windward (NW-facing, exposed) microsites associated with adult shrubs, and whether these responses are related to adult shrub structural traits. To do so, locally growing seedlings were transplanted to both sides of established shrubs of three dominant species and monitored annually. Overall, we found that seedling survival was higher on the leeward side of the adult shrubs, particularly near larger ones, but only at the site characterised by a dense and earlier melting snow and a drier growing season. In contrast, neither microsite position nor adult shrub traits affected seedling height or leaf production. Over the five-year period, survival and height growth trajectories differed between microsites;: leeward seedlings followed more stable, near-linear trajectories, whereas windward seedlings exhibited greater variability and more pronounced non-linear dynamics, particularly during periods of stress such as drought. These findings highlight the role of shrub-associated microsites and shrub structural traits in shaping seedling establishment, suggesting that local-scale environmental amelioration may become increasingly important for recruitment as snow cover decreases and climate extremes intensify.