Winter conditions in eastern hemlock and mixed-hardwood deer wintering areas of Vermont

In regions experiencing harsh winter, eastern hemlock ( Tsuga canadensis (L.) Carrière) is thought to moderate winter conditions and provide cover for white-tailed deer ( Odocoileus virginianus Zimmerman, 1780). In Vermont, USA, eastern hemlock is the dominant tree in many white-tailed deer wintering areas. Hemlock woolly adelgid ( Adelges tsugae Annand; HWA), an insect introduced from Asia, is causing widespread mortality of eastern hemlock. HWA cold-hardiness research and cold-hardiness zone climate models predict that HWA is capable of winter survival in portions of southern Vermont. This study quantifies the effects of eastern hemlock cover on winter temperatures and snow depths and assesses the effect of hemlock dominance on the winter severity values for white-tailed deer and on cold hardiness zone designations. Within site-paired eastern hemlock and hardwood stands, temperatures were recorded at 30 min intervals and snow depths were recorded every 2 weeks in the winters of 2003–2004 and 2004–2005. Extreme temperatures and snow depths were significantly moderated within hemlock stands. In southern Vermont, eastern hemlock mortality caused by HWA will alter these pockets of moderated microclimate, adding additional stress to white-tailed deer. Furthermore, the magnitude of temperature buffering in hemlock stands suggests that HWA may be capable of winter survival farther north than previously estimated.