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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., 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?, We tested five biological hypotheses to determine the relative strength of these tree line drivers on variable-size sampling plots of seedling abundance ( S) ( n = 1056). Specifically, we tested likely combinations of temperature limitation ( T), 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)., 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., 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. [ABSTRACT FROM AUTHOR]

Competition among conspecifics of the same cohort has been traditionally thought to be a main process driving population dynamics. In this classical view, however, the role of facilitation in stressful conditions has rarely been considered. Here, using a transplant experiment across a forest-prairie gradient, we test whether the stress gradient hypothesis (SGH) extends to individuals thought to be strongly competing. We transplanted 2-year-old seedlings of Nothofagus pumilio at two different densities (clusters of 10 and isolated) and at different distances from the forest edge (from 30 m inside the forest up to 50 m outside the forest in the prairie). We further stem-mapped all seedlings belonging to the clusters and computed a competition index (CI). After 3 years of growing, survival and increment growth in diameter and height were measured and analysed using mixed-effects models. We conducted a nearest-neighbour analysis using seedlings' CI and growth and computed model fit using the area under the curve (AUC) method. Seedlings planted in dense clusters had significantly higher survival than solitary seedlings at the stressful end of the gradient. This trend was reversed at the opposite end of the gradient, supporting the SGH at the intraspecific level. Pursuing this at the level of the individual, we found that higher CIs (more neighbours) in seedlings predicted higher probabilities of their survival (facilitation) in stressful conditions. Seedlings diameter and height increment growth were not affected by planting density and only diameter varied along the stress gradient; seedlings had higher diameter increments in growth outside the forest. Finally, when compared with conceptual models, our results mostly support predictions of a higher facilitation at intermediate position along the gradient. Synthesis. We showed that facilitation overrides competition among tree seedlings even at locations under moderate stress; the facilitation process occurs in resource-mediated interactions (niche overlapping). These results represent an important shift in our way to understand the density-dependent mortality process, and calls for a model reformulation including positive interactions even when competition is expected to be strongest (conspecifics of the same cohort). [ABSTRACT FROM AUTHOR]

An introduction to the journal is presented in which the editor discusses the use of globally distributed data from a large set of field experiments and observations.