Woody plant encroachment in granite barrens on the Frontenac Arch, eastern Ontario, Canada

Prepared by Michelle Cohen and Ryan K. Danby

Granite barrens are sparsely treed bedrock outcrops that can be found across the Frontenac Arch ecoregion in eastern Ontario, Canada. They are typically located on hilltops and ridges and form a constellation of discrete openings within a forested matrix, ranging in size from several hundred to several thousand square metres.

There is concern that an absence of periodic disturbances, like fires, in recent years is resulting in an encroachment of woody species into the barrens or an extension of the adjacent tree canopy. This could lead to altered structure or size of granite barrens that would reduce their total area or possibly eliminate them from the landscape altogether. This presents particular conservation concern for the rare and endemic species which rely on these habitats and the agencies responsible for their management.

We determined the extent and timing of tree and shrub encroachment into granite barrens of the Frontenac Arch over the last century, and assessed implications for their ongoing management. We quantified the extent of change in woody vegetation in rock barrens using historical aerial photography from 1925, 1965, and 2008. We also conducted field work to assess the age and composition of woody plant communities in the barrens. Collectively, our results show that these ecosystems have changed substantially since the early 1900s – gradually being colonized by trees and shrubs and losing their distinctly open character.

The most common woody species to have colonized the barrens were common juniper, white ash, and eastern red cedar. The presence of charred stumps of eastern white pine at the field sites indicates a history of fire on this landscape. It also suggests that the vegetation of these granite barrens differed prior to European settlement, as living white pine was nearly absent from any of the areas we sampled. During the 1800s, large forest areas were logged in this region and subsequently burned to clear land for agriculture. Eastern white pine was one of the most economically important species and was heavily logged throughout Ontario. Land clearance and repeated burning are known to have led to immense soil erosion, which resulted in poor forest regeneration in many areas.

The rate of tree and shrub colonization we observed indicates that many rock barrens on the Frontenac Arch could disappear before the end of the century, meaning that population size and connectivity of regionally rare species would be seriously reduced. Active management, including prescribed fire and mechanical thinning, may be necessary if there is a desire to maintain these barrens and the rare species they support as components of the region’s biodiversity. All of the data we collected are available in open-access format online. We plan to continue research on these ecosystems to improve our understanding of their history and recent dynamics and to inform on management actions for maintaining regional biodiversity.

This is a plain language summary for the paper of Cohen and Danby published in Applied Vegetation Science (https://doi.org/10.1111/avsc.12733)