Recent White Spruce Dynamics at the Subarctic Alpine Treeline of North-Western Canada

Abstract

1 Dendroecological techniques are used to reconstruct the recent population dynamics of white spruce (Picea glauca) at the alpine treeline of north-western Canada in order to test the following hypotheses: (i) the recruitment/survival and mortality patterns of white spruce at treeline are episodic, controlled by climatic variations; and (ii) the response of treeline position to climate change has been limited by a degree of inertia inherent in marginal white spruce populations. 2 The oldest white spruce at all sites, including those at treeline, established at or before AD 1800. Rates of successful establishment were low in the early to mid-19th century, but have since increased substantially. Mathematical modelling of age distributions and comparison with proxy climate data suggest that summer temperatures both at time of germination and for up to 50 years after germination are important in determining recruitment and survival. 3 Dendrochronological dating of dead spruce remains indicates that a mass mortality of white spruce occurred at a hilltop site during the culmination of the Little Ice Age in the mid-19th century. Subsequent re-establishment at this site has not taken place despite a long-term warming trend. 4 The correlation of recruitment/survival records and mortality events with proxy climate data support the first hypothesis. The second hypothesis is supported by (i) the lack of any substantial increases in treeline altitude during the past 150 years; and (ii) the survival of white spruce throughout a long period of fluctuating climatic conditions at a site currently unsuitable for establishment of spruce. During the past 100-150 years the general trend within these forest-tundra stands has therefore been one of increasing population density, accompanied by only very minor changes in the upper limit of trees.