Influence of eastern spruce dwarf mistletoe on stand structure and composition in northern Minnesota

Black spruce (Picea mariana (Mill.) Britton, Sterns & Poggenb.) is one of the most common tree species in the boreal forest. However, there is limited information on one of the major disturbance agents, eastern spruce dwarf mistletoe (Arceuthobium pusillum Peck; EDM) a native, parasitic plant that influences stand development especially at the southwestern range limit of black spruce in Minnesota. The goal of our study was to quantify overstory and understory (sapling and seedling) structure and composition in productive black spruce stands across three EDM severity categories: uninfected (0% EDM), low ( 50% of stand infected with EDM) across northern Minnesota during the summer of 2017. The presence of EDM infection in stands shifted species composition and structure compared to uninfected stands. Uninfected stands were dominated by black spruce in both overstory and understory and on average had a uni-modal diameter distribution. Both low and high EDM severity stands had significantly greater overstory species richness, Shannon’s diversity index, and Shannon’s Evenness compared to uninfected stands. However, stands with high EDM infections had the most black spruce infected with EDM in both the overstory and sapling layer, which will likely kill the infected trees within seventeen years and may result in a shifting in forest composition or a transition to non-forested conditions. The presence and severity of EDM influences stand structure and composition and subsequently stand development. Depending on the goals and objectives and ultimate desired future conditions both at the stand and the broader landscape level, natural resource managers may choose more or less intensive management of EDM.