Abstract ID: 3.13325 | Accepted as Talk | Talk/Oral | TBA | TBA

Understanding seedling responses to abiotic and biotic constraints is critical for predicting future treeline shifts. We investigate tree seedling establishment in treeline ecotones of the Venezuelan Andes and French Alps, integrating two field studies and one experiment, to explore how abiotic factors, microsite conditions, and plant-plant interactions influence seedling dynamics in tropical and alpine environments. In some inner valleys of the Venezuelan Andes, precipitation seasonality may critically impact seedling survival, however climatic and demographic data in this area is scarce. At a dry treeline ecotone, we analyzed climate, microclimate, seedling demography, and ecophysiological responses of two tree dominant species. We found a decrease in seedling density across the different environments -forest, forest-border and páramo- and positive associations of the seedlings with moisture-regulating elements under the forest and border suggesting that tree seedling depends on facilitative processes for recruitment, limiting rapid forest expansion. In the French Alps, we examined microsite preferences of three treeline-forming conifer species. Across four sites, we assessed key characteristics of microsites, including substrate, topography and shelter proximity. Species occupied similar microsites, typically with some shelter, while extreme microsites remained unoccupied. Results indicated that microsite availability is not a limiting factor suggesting that other factors, like seed availability could play a bigger role in new recruitment. We conducted a field experiment in the French Alps to examine how seedlings of five treeline-forming species respond to microclimatic modifications. Survival and biomass were negatively affected by vegetation cover and warming but benefited from shading and increased water availability. The results highlight the role of warming and competition in seedling survival. Our findings illustrate the multidimensional controls on seedling establishment. Although abiotic factors such as temperature and water availability influence survival and growth, positive and negative plant-plant interactions play a key role in establishment dynamics. As climate change accelerates treeline shifts, a better understanding of the factors that control seedling recruitment is an important step in treeline ecology. While adult trees at the treeline can survive for centuries, recruitment determines whether forests expand, remain stable or decline