Transformation of the pre-settlement forest in the Gaspésie peninsula, eastern Canada: Anthropogenic disturbances or climate change?

Background/Question/Methods

Forest composition has changed since the pre-settlement period across North America. These changes are thought to have resulted from anthropogenic disturbances and climate change, but the relative contribution of these two factors is poorly understood. The Gaspésie peninsula is a mountainous region in eastern Canada, located at the transition between the boreal and the temperate ecozones, where several taxa attain both their latitudinal and altitudinal limits. Anthropogenic disturbances (settlement, fire and logging) have affected this region since the last 150 years but temperatures remained stable over the 20^th century, thus offering the opportunity of studying anthropogenic impacts in absence of warming trend. The post-settlement changes in forest composition were reconstructed for the boreal and temperate ecozones using early land survey records. In the 19^th and early 20^th centuries, surveyors divided the region into townships and recorded forest composition as line descriptions and witness trees data. In this study, surveyor's observations (n = 22 579) were georeferenced and compared to modern forest inventory plots (n = 27 521).

Results/Conclusions

Forest composition has remained stable in the boreal ecozone. These forests comprised mostly balsam fir (Abies balsamea (L.) Mill.) and spruces (Picea spp. Mill.), often in association with paper birch (Betula papyrifera Marsh.). An important increase in quaking aspen (Populus tremuloides Michx.) in burned sectors is the only noticeable changes that occurred in this ecozone since the pre-settlement period. In comparison, the temperate ecozone was more diversified comprising eastern white cedar (Thuja occidentalis L.), yellow birch (Betula alleghaniensis Britt.), pines (Pinus spp. L.) and maples (Acer spp. L.) in addition to the boreal taxa and has undergone more important compositional changes since the pre-settlement period. These changes have been similar to those observed elsewhere in the northern part of the North American temperate biome. Paper birch, quaking aspen and maples increased in dominance and prevalence, whereas eastern white cedar, pines and yellow birch decreased. These compositional changes are spatially associated with the 20^th century fire and logging disturbances, whereas the latitudinal and altitudinal limits remained stable for all taxa. We conclude that anthropogenic disturbances alone have been sufficient to cause large compositional changes in temperate forests of the Gaspésie peninsula and adjacent areas.