Dynamics of Forest-Grassland Boundaries in Cypress Hills Interprovincial Park
Robinov, Larissa Raquel
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My research evaluated the rate and nature of forest expansion within the Cypress Hills Interprovincial Park in western Canada, where it is currently a topic of management interest. I analyzed historical aerial photographs to estimate the rate and pattern of forest expansion over two time intervals (1970s-1990s and 1990s-2018). Using aerial laser scanning (ALS), I assessed canopy height and canopy cover in newly established forested areas. From field transects, I measured the density of tree regeneration outwards from the forest edge and assessed its relationship to grazing activity. The estimated rate of forest expansion was slower in the second time interval (1.9% every 20 years) than the first (2.8% every 20 years). Overall, the rate of woody plant expansion (0.12% per year) was on the low end of expansion rates calculated for North America (<0.1-2.3% per year) and elsewhere. Mean canopy height and cover of 20- year-old and 40-year-old forests were similar, but were 5.5 m shorter and had 10% less canopy cover than older forests. Grasslands within 50 m of the forest edge had appreciable tree cover, which suggests that forest expansion in the study area is likely the result of sporadic tree establishment in grasslands followed by a slow infilling process that produces a closed forest canopy over time. Most forest expansion in the last 40 years occurred within 25-50 m of established forests, and 68% of all tree regeneration was recorded within 15 m of the forest edge. Continuous grazing, at the intensity in Cypress Hills, did not have an effect on tree regeneration along the ecotone. The observed rates of forest expansion between the 1970s and 2018 indicate that slow land cover transitions are decreasing grassland area by 0.15% per year. It is important to quantify land cover change where there is potential risk of losing biodiversity or ecosystems at risk, such as grasslands. Quantifying the rate of land cover change and the structural development of newly established vegetation is important for management and restoration practices, and contributes to a better understanding of global trends.