Historical and physiographical determinants of tree species distribution in human-dominated boreal landscapes

Background/Question/Methods

Historical land-use practices have changed natural disturbance dynamics, causing widespread modifications in species composition of the earth’s forests. Over the last two centuries, at the interface of boreal and temperate forests of north America, escaped settlement fires and industrial logging have altered the frequencies and spatial footprint of stand-replacing disturbances. In turn, these have modified successional pathways, favoring the expansion of two of the most widely-distributed tree species in North America, trembling aspen (Populus tremuloides Michx.) and paper birch (Betula papyrifera Marsh.). In this study, we reconstructed a spatially-explicit, multi-century history of fire and logging (1820-2005) using historical forestry maps (1925, 1949, 1959, 1969, 1981, 1990), terrestrial forest inventory data (1924-2011) and historical syntheses. We evaluated how the present-day (2005) distribution of tree species could be explained by the interacting effects of fire and logging disturbances over the last two centuries in a southern boreal forest landscape (6 050 km²) of eastern Canada. We also evaluated how physiography (elevation, slope, soil drainage and surficial deposit) have influenced the currently observed landscape composition.

Results/Conclusions

Strong interactions between disturbances, elevation and physiography are found to explain the present-day distribution of boreal tree species. Stand-replacing disturbances were more prevalent at low-elevation sites accessible for timber harvesting and which were subject to escaped fires during the colonization era. Trembling aspen was restricted to low-elevation sites (< 600 m) where fires have occurred at least once since 1820. In areas that have experienced successive fires, balsam fir has been excluded to the profit of trembling aspen and paper birch. We conclude that human-induced fires during colonization activities of the 19^th and 20^th centuries, as well as industrial logging of the 20^th century, have promoted early-successional species in the southern boreal forest. In the near future, the expected increase of wildfire frequencies in North America’s eastern boreal forest may foster the expansion of pioneer tree species. This study emphasizes the fact that a good knowledge of disturbance history is key to better understanding the current and future composition of forest landscapes.