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The ecology of Picea glauca (Moench) Voss at its range limits in northwest Canada McLeod, T. Katherine

Abstract

The northernmost conifers in North America are Picea glauca (Moench) Voss tree islands, located in the Tuktoyaktuk region, which encompasses Tuktoyaktuk Peninsula and the lower Anderson and Horton River valleys to the east. Using, ecological and dendroecological techniques, the role of these tree islands in the range of Picea glauca was examined. Three alternative hypotheses concerning Picea glauca tree islands were proposed: (i) the tree islands are of recent origin, (ii) the tree islands are a normal component in the range of Picea glauca, and (iii) the tree islands are relicts of past treeline advances. Specific objectives were (a) to locate and describe the northernmost Picea glauca, (b) ascertain the relative importance of vegetative and sexual reproduction, (c) examine current reproduction, and (d) determine the age structures of the tree island populations. Fifteen tree island sites were located in the Tuktoyaktuk region, with one to >300 tree islands at each site. Male cones were produced at all sites and female cones at most sites. There was a significant negative linear relation between seed production and distance from treeline. The germinability of seeds collected was less than 3% at tree island sites and reached a maximum of 33% within the Forest-Tundra. Seedbank soil samples yielded no germinable seed of Picea glauca. The low level of seed germinability at tree island sites was reflected in the lack of seedlings across the region. Survivorship o f transplanted seedlings was low over two growth seasons, varying from 20% to 45%, with no significant difference in survival inside versus outside the tree islands. In addition to low levels of seed germinability and limiting microclimatic conditions, other factors, such as competition from tundra vegetation, may contribute to low seedling recruitment levels. Tree islands maintained their populations primarily through vegetative reproduction by layering. There was a significant linear relation between the distance north of treeline and the proportion of the tree islands that established by layering or from seed. Reproductive origin and tree ring analysis revealed that large numbers of individuals established by layering in the 20th century. In contrast, only a few individuals established from seed and survived in most decades since 1700. The continuous nature of seedling establishment over the period of record and the lack of coincidence between establishment and published reconstructed climatic conditions, suggest the importance of site specific factors in successful sexual and vegetative regeneration of Picea glauca at its range limit. Light ring chronologies developed for six tree island sites showed a good correlation with other light ring chronologies from published sites in northwest Canada. Cross-dated dead stems and radiocarbon dates of wood remains extended the age of the tree islands back to the latter part of the Little Climatic Optimum (ca. AD 1000 to AD 1350). From these findings, it is suggested that tree islands in the Tuktoyaktuk region are relicts of more favourable climatic conditions in the past, when tree line and/or the species range limit advanced northward.

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