Postfire stand dynamics in a southern boreal forest (Quebec): A dendroecological approach

Arboreal succession in the southern boreal forest of Quebec was documented through a dendroecological analysis of a mid-successional stand originating from fire 75 years ago. The studied stand was located in the forest surrounding Lake Duparquet, south of Lake Abitibi in northwestern Quebec. Tree composition and ecological characteristics were assessed in quadrats distributed systematically within the burnt area. One site was selected for a detailed dendroecological analysis as it was representative of the average composition found on moderately well-drained clay deposits. In a 20 m x 20 m quadrat, all living and dead tree stems (> 1 cm d.b.h.) were mapped and cut down. Cross sections collected at the root collar and at every metre were analyzed using standard dendrochronological techniques. While the prefire composition was characterized by an old (> 210 years) forest of Thuja occidentalis L. and Abies balsamea (L.) Mill., Populus tremuloides Michx and Betula papyrifera Marsh dominated the composition of the burnt area. These two species, originating most likely from suckering and sprouting, were recruited immediately after the fire while most of the Picea glauca (Moench) Voss and Abies regeneration was delayed for five years. Most Populus rapidly attained dominance while, with few exceptions, the other species were suppressed and occupied lower positions in the canopy. Differences in growth between Populus and Betula were accentuated by an important growth decrease in the mid-1950s, from which only Populus had fully recovered. Defoliation by the forest tent caterpillar caused hardwood growth decrease in the 1950s, as well as previous ones, while defoliation by the spruce budworm caused growth decreases and mortality in conifers around 1940 and from 1972 to 1987. Picea and Abies were regularly recruited since the fire and they formed an abundant understory cover. The recent recruitment of a few Thuja seedlings may be incidental or related to an increase in mosses with time since fire. Our results suggest that all species, except Thuja, can be abundant in the postfire cohort and that differences in diameter and height among species were explained by their respective growth and not by successive species invasion. This relatively regular stand development was, however, significatively influenced by episodes of insect defoliation. Mechanisms involved in subsequent stands development towards the mixed and coniferous compositions observed in older stands cannot be easily extrapolated from the present stand structure. Similar studies are therefore needed to explain these mechanisms in older stands.