Conservation implications of browsing by Odocoileus virginianus in remnant upland Thuja occidentalis forests

Abstract

Regeneration success of canopy dominants is linked to multiple factors, including the ability of their seedlings to survive browsing and to utilize available resources in the understory. In remnant upland northern white cedar Thuja occidentalis L. forests, effects of browsing on recruitment of cedar seedlings were evaluated at 7 sites, including known deer yards, located on a portion of the Lake Superior Highlands in northeastern Minnesota. Experimental plantings and vegetation surveys were conducted inside and outside large deer exclosures constructed between 1937 and 1997. Objectives were to determine severity and intensity of browsing by Odocoileus virginianus Zimmerman, the ability of seedlings to survive browsing in environments beneath T. occidentalis and adjacent paper birch ( Betula papyrifera) Marshall canopies, and potential long-term sapling recruitment under different browsing scenarios. From 1994 to 1997, 76% of unexclosed planted T. occidentalis seedlings had been browsed at least once, compared with 0% of exclosed seedlings. Increased browsing intensity and decreased light availability increased T. occidentalis mortality rates. Simulation models and vegetation surveys demonstrated that the best recruitment rates occurred for seedlings planted under Betula canopy and subjected to low browsing severity, but no recruitment occurred under high browsing pressure under either canopy type. The current level of browsing in these forests has the potential to alter the future composition of canopy tree species through sustained prevention of T. occidentalis recruitment.