Thermal control of treeline position is mediated by local environmental and ecological factors, making trends in treeline migration difficult to extrapolate geographically. We investigated the ecological dynamics of conifer establishment at treeline in the Mealy Mountains (Labrador, Canada) and the potential for its expansion with climate warming. Available seedbed and tree seedling emergence in the treeline ecotone were monitored, and seeds and seedlings of Picea mariana were planted along an elevational gradient from open-canopy forest through tree islands to alpine tundra. Experimental treatments included passive warming of daytime air, ground disturbance, and vertebrate herbivore exclosures. Responses in seed germination and seedling growth, damage, and mortality were monitored over two growing seasons, and re-surveyed after 5 years. While no tree seedlings were observed growing naturally above the treeline, planted seeds were able to germinate, develop and overwinter, and persist for 4 years in all habitats examined. Disturbance of the seedbed was important for seedling emergence in the forest and tree islands.While temperature enhancement alone had little impact on emergence, even moderate temperature increases had significantly disproportionate effects on emergence of seedlings in the alpine habitat when combined with soil disturbance, indicating that future climate warming could lead to treeline advance if viable seed and suitable substrate for recruitment are available. The positive effect of excluding herbivores suggests that herbivory may be an important filter modifying future species distribution. While seedbed conditions and herbivory would control the rate of individual species advance, the results indicate potential upslope migration of the treeline in the Mealy Mountains, with consequent loss of alpine ecosystems.

A preliminary population model was developed for the boreal felt lichen Erioderma pedicellatum (Hue) P. M. Jørg. in Newfoundland using life stage data collected in eastern and south-central Newfoundland, Canada. This Critically Endangered epiphytic lichen displayed a life history strategy with high adult survival and low recruitment. Deterministic models in 6 mo to 1 yr intervals were generated, yielding similar results to the overall mean values for the 4 yr of study in eastern Newfoundland. The populations of E. pedicellatum in Newfoundland are predicted by our models to be unsustainable because of adult mortality, and we attribute this problem to a decline in the forests of balsam fir Abies balsamea (Mill) that predominantly support this lichen. In eastern Newfoundland, thalli are located almost entirely on mature to over-mature balsam fir, and there is little regeneration because of heavy browsing by the introduced moose Alces alces population. The current and projected predictors indicate that habitat effects may be important in predicting future population size. An assessment of the stable stage distribution indicated that the current population has more juveniles and fewer apothecia-bearing thalli than projected, meaning the current population likely generated from a different set of survival and recruitment rates. The projected annual population growth rates calculated for 4 yr indicated that populations are declining (λ < 1.0, mean decline ± SD = -0.175 ± 0.079).The elasticity values support the fact that the population growth rates are most sensitive to changes in the survival of necrotic (apothecia-bearing) cohorts. We suggest that conservation is best focused on the inventory and protection of old-growth forests important to this species, the reduction of the introduced moose population and the use of herbivore exclosures in specific core population areas.

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