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

Abstract

Various types of open ecosystems across North America, including grasslands, savannas and barrens, have been shrinking in size over time as they are threatened by the succession of woody vegetation. The Frontenac Arch serves as a key linkage between ecosystems on the Canadian Shield to the north and the Adirondack and Appalachian Mountains to the south. Granite barrens are a unique ecosystem type on the Frontenac Arch that is of high conservation concern because they support many rare and threatened species, and a lack of periodic disturbances could be contributing to woody plant encroachment. The purpose of this research was to assess the extent to which these barrens have changed over the last 100 years and to investigate recent dynamics of woody vegetation on granite barrens in eastern Ontario. Using repeat aerial photography of 290 barrens, we found that the mean proportion of tree cover increased by 22.5% from 1925 to 2008 and the mean proportion of exposed granite or vegetated area decreased by 25.3% over the same period. Dendroecological analysis of the tree community in 10 granite barrens at Queen’s University Biological Station (QUBS) supports the aerial photography analysis. Very few trees were older than 100 years and there was a pulse of tree establishment in the 1960s. Species inventory at these same sites revealed that white ash, eastern red cedar, and common juniper are the most common woody species colonizing the barrens, despite evidence that eastern white pine was prominent prior to European settlement as indicated by the presence of charred pine tree stumps. These results have implications for the management and conservation of granite barrens that could be used to guide their restoration. Mechanical thinning or the removal of soil could be utilized to limit woody plant growth on granite barrens at QUBS. The potential use of prescribed fire as a management strategy is considered in other areas of the Frontenac Arch. These data, within a historical context, reveal the role that ecosystem disturbances can have in granite barrens and provide further insight than previous studies of land cover change and the infilling of woody plants in the Frontenac Arch.