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Dryad

Disentangling seed availability and establishment filters at alpine treelines through a decade-long field manipulation

Lin, Wentao1; Liang, Eryuan1; Camarero, J. Julio2

Published Nov 07, 2025 on Dryad. https://doi.org/10.5061/dryad.jh9w0vtq1

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Abstract

Climate warming is expected to increase upslope shifts of alpine treelines globally. However, the ecological filters controlling tree recruitment in the alpine belt remain poorly quantified, limiting accurate predictions of treeline responses to climate change. To fill this gap, we conducted a decade‑long, factorial field experiment at abrupt (Ordesa) and diffuse (Tessó) Pinus uncinata treelines located in the Spanish Pyrenees. Our experimental treatments manipulated seed addition, herbivory exclusion, shrub competition, and soil scarification. We monitored seedling presence and abundance annually and analyzed their interactions with climate variables, specifically growing-season temperature and snow depth. Seedling recruitment at alpine treelines was strongly filter-limited and varied between sites. Seed addition enhanced emergence at both treeline types, with a steeper, density-dependent response at the diffuse treeline. Herbivore exclusion (1-mm mesh) consistently increased densities across cohorts, underscoring herbivory as a critical biotic filter. Climatic and biotic factors interacted to shape establishment: at the abrupt treeline, warmer growing-season maxima and dense shrub cover suppressed recruitment, while snow depth exerted contrasting effects across cohorts, from protective to limiting. At the diffuse treeline, year-one seedlings peaked at intermediate growing-season maximum temperatures under low shrub cover, whereas older cohorts showed more variable responses, occasionally persisting under dense shrubs when cooler growing season and deeper snow provided facilitative microclimatic conditions. These patterns highlight that both propagule supply and the interplay of climatic stress and biotic interactions jointly determine early recruitment above treeline. Treeline advance under warming occurs only when multiple filters align—adequate seed supply, favourable microclimatic windows, moderated herbivory, and shrub effects that remain facilitative rather than competitive. Because these filters are context- and life-stage dependent, forecasts must move beyond climate envelopes to integrate fine-scale propagule dynamics, episodic heat/snow extremes, and density-dependent biotic interactions. Models coupling these processes with long-term observations will better predict forest–tundra change.