CHANGES IN THE FOREST FLOOR OF CANADIAN SOUTHERN BOREAL FOREST AFTER DISTURBANCE

The concentrations and contents of organic matter and nutrients in org anic deposits on the forest floor were estimated along a 231-yr chrono sequence following fire at the southern limit of the boreal forest in eastern Canada. The sampling design was stratified to take into accoun t the variability related to the presence of the principal tree specie s as well as to the presence of large gaps created by a recent spruce budworm (Choristoneura fumiferana) outbreak. The forest floor showed a steady accumulation of organic matter and total nutrients with time-s ince-fire and a 50 % decrease in the concentrations of available P and K, but not N (as determined by aerobic incubation). The increase in f orest-floor weight was accompanied by an increased storage of availabl e N, Ca and Mg. The availability of N and Ca was more strongly affecte d by tree species and gaps than by time-since-fire. A high N-availabil ity was observed under Betula papyrifera and in gaps, while high a Ca- availability was found near Populus tremuloides and Thuja occidentalis . In old sites, the forest floor of gaps, created by a recent spruce b udworm outbreak, had a necromass similar to that of a young forest, bu t the low concentrations of available P and K of an old forest.