The fundamental niche of many species is shifting with climate change, especially in sub-arctic ecosystems with pronounced recent warming. Ongoing warming in sub-arctic regions should lessen environmental constraints on tree growth and reproduction, leading to increased success of trees colonising tundra. Nevertheless, variable responses of treeline ecotones have been documented in association with warming temperatures. One explanation for time lags between increasingly favourable environmental conditions and treeline ecotone movement is reproductive limitations caused by low seed availability. Our objective was to assess the reproductive constraints of the dominant tree species at the treeline ecotone in the circumpolar north. We sampled reproductive structures of trees (cones and catkins) and stand attributes across circumarctic treeline ecotones. We used generalized linear mixed models to estimate the sensitivity of seed production and the availability of viable seed to regional climate, stand structure, and species-specific characteristics. Both seed production and viability of available seed were strongly driven by specific, sequential seasonal climatic conditions, but in different ways. Seed production was greatest when growing seasons with more growing degree days coincided with years with high precipitation. Two consecutive years with more growing degree days and low precipitation resulted in low seed production. Seasonal climate effects on the viability of available seed depended on the physical characteristics of the reproductive structures. Large-coned and -seeded species take more time to develop mature embryos and were therefore more sensitive to increases in growing degree days in the year of flowering and embryo development. Our findings suggest that both moisture stress and abbreviated growing seasons can have a notable negative influence on the production and viability of available seed at treeline. Our synthesis revealed that constraints on predispersal reproduction within the treeline ecotone might create a considerable time lag for range expansion of tree populations into tundra ecosystems.

ABSTRACT. While old trees have long been of interest, their significant role in responding to climate change at northern tree lines has been overlooked. Long-lived black spruces at the tree line in Labrador show a radial growth response that is synchronous with recent climate warming. The ability of individuals to persist with suppressed radial growth rates during adverse growing conditions may have significant implications for the rate at which these trees are able to respond when conditions become favourable.Key words: persistence, krummholz, dendrochronology, tree line, forest-tundra ecotone, Mealy Mountains, Labrador, climate change, black spruce, Picea mariana RÉSUMÉ. Même si les vieux arbres revêtent de l'intérêt depuis longtemps, on a négligé de considérer le rôle important qu'ils jouent en matière de réaction au changement climatique à la hauteur des limites forestières boréales. Au Labrador, les épinettes noires de longue date se trouvant à la limite forestière affichent une croissance radiale qui est synchrone avec le récent réchauffement climatique. L'aptitude des individus à persister malgré des taux de croissance radiale opprimés lorsque les conditions de croissance sont déficitaires peut avoir des incidences importantes sur le taux de réaction de ces arbres lorsque les conditions deviennent favorables.

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 conebearing 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.

ABSTRACT. As climate warms, abiotic factors, as well as plant community and biodiversity structure, may constrain or promote the movement of ecotone boundaries. Our study sought to examine how plant communities change across the tree-line ecotone of the Mealy Mountains in Labrador, Canada. We established eight transects (50-100 m in length) along an elevation gradient in three tree-line zones (forest, forest-tundra, and alpine-tundra) and recorded all species and cover of vegetation in contiguous 1 × 1 m quadrats. Companion abiotic parameters of nutrients and soil temperature were also measured. The absence of abrupt changes in important soil nutrients and growing season temperatures suggests that these factors do not limit tree species establishment beyond the current tree line. Vegetation cover and richness, however, were highly variable and in some cases changed non-linearly across the tree-line ecotone. Tree cover and species density generally decreased with elevation, while some field layer species (< 25 cm in height) increased; the latter change seems to be influenced by ground shrubs rather than herbaceous species. As expected, transects separated by the greatest difference in elevation were the least similar (higher beta diversity/species turnover); however, species turnover between the forest and forest-tundra transects was higher than it was between forest-tundra and alpine-tundra transects, even though the latter were separated by a greater elevation. Community structure and species turnover vary greatly across a tree line with the greatest differences between the forest and the forest-tundra, suggesting a biotic or abiotic barrier. While our ability to predict how the tree line will respond to continued climate change is complicated by these patterns in plant communities, the potential barriers investigated and others identified will be a useful focus for future studies.Key words: climate change; forest-tundra ecotone; Labrador; Mealy Mountains; soil temperature; species turnover RÉSUMÉ. Au fur et à mesure que le climat se réchauffe, les facteurs abiotiques de même que la structure des communautés végétales et de la biodiversité pourraient contraindre ou promouvoir le mouvement des délimitations des écotones. Notre étude cherche à examiner comment les communautés de plantes changent dans l'écotone de la limite forestière des montagnes Mealy au Labrador, Canada. Nous avons établi huit transects (de 50 à 100 m de longueur) le long d'un gradient d'altitude chevauchant trois zones de la limite forestière (forêt, toundra forestière et toundra alpine), puis répertorié toutes les espèces et la couverture végétale se trouvant dans des quadrats contigus d'un mètre sur un mètre. Nous avons également mesuré les paramètres abiotiques des éléments nutritifs et des températures du sol connexes. L'absence de changements abrupts en ce qui a trait aux importants éléments nutritifs du sol et aux températures de la période de croissance suggère que ces facteurs ne limitent pas l'établissement des espèces d'arbres au-de...