PARENT SESSION
Organized Oral Session 32: Forest disturbance regimes in the circumboreal forest zone: Natural variability and implications for forest management and biodiversity conservation
Organizer(s): S Gauthier, T Kuuluvainen, and D Kneeshaw
Wednesday, August 10, 8:00 AM - 11:30 AM, Meeting Room 516 C, Level 5, Palais des congrès de Montréal

Population dynamics of black spruce at its northern limit in the Brooks Range, Alaska.

Lloyd, Andrea^*,1, Wilson, Alexis¹, Fastie, Christopher¹, Landis, R. Matthew¹, ¹ Middlebury College, Middlebury, VT, USA

ABSTRACT- Black spruce is a common treeline species in eastern Canada, but is rare at treeline in Alaska, reaching its northern limit in the Brooks Range 30 km south of treeline. This research addressed two questions. First, what are the roles of fire, climate, and glacial history in controlling the northern limit of black spruce in Alaska? Second, how sensitive to future climate change is the northern limit of black spruce likely to be? We used tree rings to reconstruct stand history and documented current levels of recruitment and seed viability along a 45 km latitudinal transect in the Brooks Range. Black spruce abundance declined from approximately 75% in the southern sites to <10% of stems at the northernmost site. With the exception of the northernmost site, which has not burned for at least 200 years, our sites burned in a single fire in the early 1900s. High recruitment of black spruce occurred for <30 years post-fire at all of the burned sites, and at the 3 northern burned sites this post-fire recruitment represents the vast majority of non-clonal regeneration by black spruce. In recent decades, black spruce recruitment from seed has occurred primarily at the southern sites, and is confined to microsites with abundant sphagnum moss. In contrast, high rates of white spruce recruitment have occurred since the fire at all sites; the majority of seedlings, particularly in the northernmost stands, are now white spruce. Black spruce seed viability declined from >20% in the southernmost sites to 5-10% at the northernmost sites. The northern limit of black spruce appears to reflect an interaction between climate (which limits seed viability) and fire (which stimulates regeneration). Although northward expansion of white spruce populations has been widespread in recent decades, our data suggest that the northern limit of black spruce in Alaska is stable, and that future expansion may be dependent on an increase in fire frequency at its northern limit.

Key words: boreal forest, treeline, black spruce, fire history