Effects of escaped settlement fires and logging on forest composition in the mixedwood boreal forest

The southern edge of the boreal forest in central Saskatchewan, Canada, has had its forest composition changed in the first decades of this century, primarily by logging and escaped fires from adjacent agricultural clearance. Three timber berths were established in 1884 within and immediately adjacent to the present southern half of Prince Albert National Park (established in 1927). These timber berths were selectively logged for saw timber between 1900 and 1918. Between 1907 and 1918, an average of 70 trees per hectare were removed by selective logging. Most of these trees were white spruce (Picea glauca (Moench) Voss). Since logging companies were required to remove all merchantable trees with a basal diameter greater than 25 cm, it is estimated that between 28 and 54% of the canopy trees were removed. Between 1883 and 1942, 81% of the timber berths were burned two or more times by crown fires that spread through the study area from adjacent agricultural clearances 30 km or more away. By 1945, agricultural clearance was largely complete and the clearance-caused fires stopped. The changes in tree composition were determined by transition probabilities between forest surveys taken in 1883 and 1994. Forests subjected to short-interval, clearance-caused fires but no logging were significantly reduced in their abundance of sexually reproducing trees such as white spruce, but increased in trees with either vegetative reproduction (i.e., underground stems, not just basal sprouts) or serotinous cones, such as aspen (Populus tremuloides Michx.) and jack pine (Pinus banksiana Lamb.), respectively. Transition probabilities for forests experiencing both short-interval, clearance-caused fires and logging reveal an even more marked compositional change in this direction.