Document: LUC-3-68-29

Dynamics and distribution of tree species across a fire frequency gradient in the James Bay sector of Quebec.

PARISIEN, M.* and L.SIROIS

Université du Québec à Rimouski, Rimouski, Quebec, Canada ¹

Abstract:
It is well known that fire frequency is largely responsible for the distribution of tree species in boreal ecosystems. Indeed, forest structure and composition differ greatly under different fire regimes; furthermore, as a fire regime changes in space or in time, the vegetation will shift accordingly. This study's aim is to document the distribution and dynamics of tree species across a gradient of fire frequencies in the James Bay sector of Quebec. This territory of more than 400 000 sq km encompasses the northern boreal forest from the James Bay coast to longitude 68W. Our central hypothesis is that changes in vegetation are mainly associated with changes in the fire regimes. First, the longitudinal distribution of trees was described across the gradient. Vegetation sampling was carried out in twenty-three stations, distributed as a function of forest heterogeneity, extending from the James Bay coast to 115 km hinterland. Almost pure white spruce (Picea glauca (Moench) Voss) stands form a narrow strip along the coast, where fire occurrence is most probably the lowest. At 0.2 to 4 km from the coast, white spruce is gradually replaced by black spruce (Picea mariana (Miller) BSP). Jack pine (Pinus banksiana Lambert), a species highly dependent of fire for its propagation, first appears at approximately 20 km west of the bay. Hinterland, with a fire rotation period estimated at 109 years, this species is extremely prolific, but it is totally excluded from the coastal region. With spatio-temporal data of all fires of 14 ha and over from 1930 to 1999, the role of fire will be quantified with GIS technology and used to document tree species distribution and dynamics under different fire regimes. In the James Bay sector, jack pine seems to be very highly correlated with a high fire frequency. As for white and black spruce however, this relationship does not seem to be as straightforward: other factors might prevail in determining species presence-absence. In northern forests, tree species are at their ecological limits, therefore, where they are likely the most change-sensitive. The results of this study should increase our knowledge of how northern forests respond to changing fire regimes and exemplify the importance of maintaining many natural fire regimes to conserve diversity at a very large spatial scale.

Keywords: fire regime, species distribution, northern forests, GIS, Black Spruce, White Spruce, Jack Pine

This abstract is being presented at: 4:00 PM in session:
Oral Session #35: Fire Ecology.