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

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.