Data from: The importance of biotic filtering on boreal conifer recruitment at alpine treeline

Treeline, the ecotone where forest transitions to alpine or tundra
ecosystems, is considered the thermal limit to tree growth and survival.
Despite temperature increases across mountainous areas and high latitudes
globally, there has been no ubiquitous change in treeline position. The
process of range expansion must initially depend on increased recruitment
at, or beyond current range limits and recruitment limitations have been
hypothesized as a mechanism for the variable response of treeline position
to climate warming. We conducted a unique series of observational and
experimental studies to quantify early-life stage constraints, from seed
production to seedling establishment, on black spruce (Picea mariana) and
tamarack (Larix laricina) recruitment at a model alpine treeline in
Newfoundland, Canada. We found recruitment at treeline to be
simultaneously seed and establishment limited. The treeline population
produced fewer seeds than the forest population and black spruce seeds
produced at treeline were less viable. Tamarack was more seed limited than
black spruce where seed viability was low regardless of altitudinal
position. Post-dispersal seed predation greatly constrained recruitment
across the altitudinal gradient; however, black spruce seeds experienced
the lowest levels of invertebrate seed predation on the lichen mat at
treeline. If seeds were not consumed, individuals at treeline were
establishment limited where germination and seedling establishment was
both less abundant and delayed on lichen substrate. Our study highlights
the need for multiple factors to align temporally for significant
recruitment at treeline to occur.