Abstract
ABSTRACT Cone and seed production at the forest-tundra ecotone, or treeline, depend on species-specific tolerances to limiting abiotic and biotic factors. As range expansion via seed dispersal is needed to keep pace with climate change, reproductive limitations act as a bottleneck for treeline advance. The treeline in the Mealy Mountains, central Labrador, was comprised of four codominant species: black spruce (Picea mariana [Mill.] B.S.P.), white spruce (Picea glauca [Moench] Voss), eastern larch (Larix laricina [Du Roi] K. Koch), and balsam fir (Abies balsamea [L.] Mill.). Conifer stem surveys from three treeline zones (forest, forest-tundra transition, krummholz) were used to assess patterns of altitudinal distributions, tree densities, and cone production to provide insight into overall reproductive potential. The altitudinal limit of the spruce species was 39 m a.s.l. higher than the altitudinal limit of black spruce cone production. Black spruce had the highest densities of cone-bearing trees across treeline with eastern larch values being comparable in the forest-tundra transition zone, although overall cone production was low and highly variable in all species. Compared to the other treeline species, black spruce has the greatest reproductive potential for upslope advance.
Highlights
As northern and alpine climates continue to warm (IPCC 2014), thermal constraints on cone and seed production may become less limiting (Brown et al 2018; Roland, Schmidt, and Johnstone 2014)
Altitudinal cone-production limits differed among species by more than 150 m: black spruce, 766 m a.s.l.; balsam fir, 721 m a.s.l.; white spruce, 642 m a.s.l.; and eastern larch 613 m a.s.l
Treeline varied substantially in species composition, density, height structure, and cone production across the treeline. Such variability has not to date been incorporated into models of treeline change, and may partially explain the wide variability encountered in treeline response to climate warming (e.g., Harsch et al 2009)
Summary
As northern and alpine climates continue to warm (IPCC 2014), thermal constraints on cone and seed production may become less limiting (Brown et al 2018; Roland, Schmidt, and Johnstone 2014). A northward and upslope advance of seed-mediated conifer regeneration is expected (Grace, Berginger, and Nagy 2002; Körner and Paulsen 2004) This seed-mediated range expansion depends on the production, dispersal, and establishment of viable seeds within and beyond the current distribution of trees (Gamache and Payette 2005; Lescop-Sinclair and Payette 1995; Szeicz and MacDonald 1995). Icy winds and late-season frosts at treeline can cause extensive bud loss and impede future production by damaging the terminal leader (Krasowski and Simpson 2001). Because of these and other factors, including cone and seed predation (Jameson, Trant, and Hermanutz 2015), sexual
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