Wind exposure and light exposure, more than elevation‐related temperature, limit tree line seedling abundance on three continents
Bibliographic record
Abstract
Summary 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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".