Wind exposure and light exposure, more than elevation-related temperature, limit tree line seedling abundance on three continents

1. The transition from seedlings into trees at alpine tree lines is a temperature-limited process that ultimately sets the tree line elevation at a global scale. As such, tree lines may be key bioassays of global warming effects on species distributions. For global warming to promote upward tree line migration, as predicted, seedlings must be available. We examined, for the first time at a global scale, elevational patterns and drivers of seedling availability at tree lines. 2. Working at 10 sites across five mountain regions (dry Andes, humid Andes, Patagonian Andes, Swiss Alps and US Rocky Mountains) with different tree line forms (abrupt and diffuse) and dominated by different tree species (broadleaves and conifers), we answered the following question: How is seedling abundance affected by elevation (as a coarse grain surrogate of temperature), light exposure (openness immediately above plots) or wind exposure (an index for openness in the horizontal direction), or combinations thereof and what is the relative importance of each factor? 3. We tested five biological hypotheses to determine the relative strength of these tree line drivers on variable-size sampling plots of seedling abundance (S) in = 1056). Specifically, we tested likely combinations of temperature limitation (7), light as a resource (light, L) and as a radiation stress (via high light at low temperature, R), wind exposure as a tree line stressor (W) and tree line form (a coarse scale test: abrupt vs. diffuse, D). 4. We found strong, moderate and weak negative effects of our estimates of wind exposure, radiation stress and elevation-related temperature on seedling abundance, respectively. We also found a positive effect, at tree line, for site-level tree line diffuseness. Two distinct facilitation mechanisms likely improved seedling abundance at tree line elevation: wind blockage by neighbourhood trees (the sheltering effect) and partial shading by overhead trees. 5. Synthesis. Seedling abundance at alpine tree lines is limited by multiple simultaneous factors with the temperature decrease with elevation playing a relatively minor role. We therefore note that if the temperature threshold limiting the conversion from seedlings to adult trees is relaxed because of global warming, upward tree line migration will depend on the availability of shelter sites for seedlings.