Recent dynamics of jack pine at its northern distribution limit in northern Quebec

Abstract

The northernmost jack pine (Pinus banksiana Lamb.) populations in northern Quebec are located at the boreal forest–forest tundra boundary, along the Grande rivière de la Baleine, where they colonize the sandy terraces affected by recurrent fires. The recent fire history in the study area, as deduced from fire scar and age structure data, spans a 216-year period from 1773 to 1988. Forest fires occurred on the sites at intervals averaging 40 to 80 years. The analysis of 19 coniferous stands (jack pine and black spruce (Picea mariana (Mill.) Bsp)) indicated that forest communities younger than 67 years old were open jack pine – Cladina mitis or jack pine – black spruce – C. mitis woodlands, while the oldest stands, more than 132 years old, were dominated by jack pine, black spruce, and Cladina stellaris. Stands less than 67-years-old had an age structure almost normally distributed and regeneration often occurred within less than 30 years after fire in both species, while most stands older than 132 years had a multiaged structure. In sites with a prolonged fire-free interval, jack pine was overgrown by black spruce. Spruce woodlands have developed on sites where the organic layer was relatively thick and continuous and they are the end result of the postfire successional process. However, at several sites both conifer species showed an ability to regenerate in prolonged absence of fire disturbance, particularly in open sites with exposed mineral substrates. At the regional scale, fire frequency during the last 200 years has been high enough to prevent pine exclusion at its range limit. The key requirement for the long-term maintenance of jack pine populations is that fires return at intervals shorter than the average life-span of individual trees. It is concluded that the northernmost jack pine populations are able to maintain and regenerate under present fire conditions. Key words: fire, subarctic, jack pine, postfire regeneration, boreal forest.