Fire-induced succession of lichen-spruce woodland in Central Labrador-Ungava, Canada
The well-drained soils of the Schefferville area in Central Labrador-Ungava are dominated by open lichen-spruce woodland. The dominant species within this plant community are the lichen Cladina stellaris (Opiz) Brodo and the spruce species Picea mariana (Mill.) BSP (black spruce) and Picea glauca (Moench) Voss (white spruce). The spatially heterogeneous nature of this lichen-spruce woodland and the juxtaposition of sites of several successional stages of different ages is due to the regular occurrence of fire. This study describes and analyses the succession of this community based on observations at 23 sites of varying postfire stages (1-315 years). The succession can be roughly divided into four stages: initial colonisation (1-4 years after fire), crustose lichens-Cladonia stage (4 ->30 years after fire), Cladina mitis stage (40-80 years after fire), and Cladina stellaris stage (> 80 years after fire). Tables of the plant community data show evidence of the high spatial variability within and between areas of equal or similar postfire age. This variability is due to different prefire conditions, the type of fire and its intensity. The interactions between the tree and the shrub layer on the one hand, and the ground cover vegetation on the other, also contribute to the large variation in community development. In addition, the relative percentage of each of the spruce species plays an important role in postfire succession. The first two successional stages are characterised by a well-defined spatial pattern of vegetation resulting from local variations in environmental conditions. This pattern, however, disappears, as a closed Cladina stellaris mat covers the surface. The Cladina stellaris stage can be divided into a mature stage at the age of 100-170 years and an older degenerating or disturbed stage. The latter is characterised by a number of disturbances (cryoturbation, windthrow) which create open spaces in the lichen cover and therefore new microhabitats to be colonised by tree seedlings. The successional sequence is then repeated (on a smaller spatial scale and mostly fragmentary) in these locations. This process ensures the regeneration of the ageing tree community as well as of the lichen mat itself. In accordance with the gap-theory the Cladina stellaris stage of the lichen-spruce woodland succession can therefore persist for more than 300 years in the absence of fire.