The Influence of Island and Mainland Lakeshore Landscapes on Boreal Forest Fire Regimes

Abstract

In order to characterize the fire regime of the southern boreal forest and to understand the way in which landscape and fire regime interact, a detailed study of fire history was undertaken in two adjacent contrasting landscapes in northwestern Quebec. The fire history for the islands of Lake Duparquet was reconstructed and compared to that of the surrounding lakeshore. Fire occurrence was determined by archival search and by collection of information from the fire scars. Dendroecological techniques were used to determine years of stand initiation on the lakeshore and for a subsample of the islands. Stand initiation data were used to estimate fire frequency, fire cycle, and fire intensity (lethal and non—lethal fires). Tree composition and surficial deposits were sampled in order to assess the possible effect of topography and forest cover on fire frequency. Dated fire scars (n = 273) revealed 82 fire years, with the earliest dated to 1593. Islands experienced more fire years (56) than the lakeshore (37), and fire years were uncorrelated. Most 72%) of the lakeshore stands were initiated by a few large fires (eight), whereas the fire size on islands was variable, ranging from <10% to 100% of the total surface of the island. Despite these differences in fire regime, fire occurrence and fire frequency decreased concurrently on the islands and the lakeshore starting °120 yr ago. The lakeshore fire cycle was 63 yr before 1870 and 99 yr since 1870, whereas the fire cycle on the islands was slightly longer (74 yr and 112 yr) during the same intervals. The long—term decrease in fire frequency of both the lakeshore and the islands was possibly driven by climatic change. Fire frequency was generally similar for all surficial deposits and fuel types on the lakeshore, whereas fires were more abundant in pine woodland growing on bedrock on the islands. Lakeshore fires were generally of lethal intensity, whereas both a long cycle of lethal fires and a shorter cycle of non—lethal fires were observed on 50% of the islands. The morphology and location of islands with both types of fires suggest that a higher frequency of lightning strikes resulted in a regime characterized by more frequent fires and fires of variable intensity. The specific fire regime observed in the island landscape may be the cause of the abundance of pines, whose presence may contribute, in turn, to the continuation of this regime of less intense and more frequent fires. We suggest that fire regime in the boreal forest is controlled by long—term climate change at the regional scale, and by a strong interaction with landscape at the local scale, both components having a great impact on the distribution and the dynamics of boreal vegetation.