Interactions between macroclimate, microclimate, and anthropogenic disturbance affect the distribution of aspen near its northern edge in Quebec: Implications for climate change related range expansions

• Aspen showed no systematic decline in abundance with increasing latitude in the James Bay area.
• Aspen abundance was more limited to south-facing slopes in the most northerly latitudinal zone.
• Aspen’s distribution and abundance correlate well with human activity.

Policy to address the shifting tree species distributions anticipated in coming decades requires a sound understanding of how forests respond to environmental change. Using a combination of remote sensing, geographic information systems (GIS) analysis, and ground-based techniques, we explored the environmental factors associated with the distribution and abundance of trembling aspen (Populus tremuloides Michx.) across regional gradients near its northern range limit in northwestern Quebec, Canada. Although not regionally abundant, aspen is the main deciduous tree species in this conifer-dominated landscape. Regionally, the ∼51,200-km2 study area has very few settlements or roads, and little industrial resource extraction. Most of the region is inaccessible to humans except by foot or water travel. We utilized Landsat Thematic Mapper images from 2010 and 2011, and a robust collection of ground reference data developed from aerial photography, supported by field verification (vegetation sampling) where access permitted, to construct a thematic map of 11 land cover classes. The map highlights the spatial distribution of aspen, which represents only 0.3% of the study area. Map validation indicated an overall mapping accuracy of 74%, and the aspen predicted class was determined to be over 77% accurate. The regional-scale distribution of aspen stands ⩾0.5 ha within the study area shows two patterns: (1) a shift toward greatest abundance on south-facing aspects with increasing latitude; and (2) a highly clustered pattern with a strong signal of concentration in areas of human activity. These patterns suggest that aspen range expansion due to climate warming will vary with topographic and other microclimatic factors (i.e. be a function of climate change interacting with landscapes) and that anthropogenic activities have the potential to influence future aspen abundance independently of climate. Forest management policies concerned with changing forest composition in these northern landscapes should recognize the potentially important role of human activity in driving the abundance of aspen.

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