Testing the response of northern white-cedar to simulated browsing: Evidence of apparent compensatory growth

The impacts of ungulate browsing on forested ecosystems largely stem from events happening at the plant scale, and plant-level responses to browsing vary according to factors including light environment and browsing intensity, frequency, and timing. Northern white-cedar (Thuja occidentalis), a slow-growing, shade tolerant, and highly browse-selected species, is experiencing regeneration problems in large part due to heavy browsing pressure by white-tailed deer (Odocoileus virginianus). Although survival and recovery following browsing are essential for cedar recruitment, we know little about the reactions of individual cedar plants to browsing. Our objective was to evaluate the impact of browsing, as emulated by clipping, on cedar height, diameter, and branch growth. We applied combinations of shading (full sun or 50% sun) and clipping timing (early growth or dormancy), intensity (apex or 50% previous year's growth), and frequency (one or two years) to planted cedar seedlings. Partially shaded trees had systematically faster height, diameter and branch growth than trees in full sun. Clipping timing had no effect on height or branch growth but growing season clipping negatively affected diameter growth. Clipping frequency had no detectable effect on cedar growth. We observed aboveground growth compensation except under high clipping intensity. We confirm the expected browse tolerance of cedar and its high recovery potential following low to moderate browsing pressure. We underline that higher clipping intensities or longer-term browsing pressure than those we tested, which can be expected in areas with heavy, sustained browsing pressure, could negatively impact cedar's recovery potential.