Distribution and impact of Eastern dwarf mistletoe on black spruce in Bryant Bog.

Abstract

Eastern dwarf mistletoe, Arceuthobium pusillum Peck, is commonly found in northern Lower Michigan and the Upper Peninsula as a parasite of black spruce (Picea mariana) (Voss 1985). Within this area black spruce can be found in bogs and poor fens. Eastern dwarf mistletoe is host-specific, and is most often found on black spruce, but can also grow on white spruce (Picea glauca) and tamarack (Larix laricina) (Voss 1967). Dwarf mistletoe is a dioecious, leafless parasite with shoots up to two inches long. It relies primarily on insect pollination. Once pollination takes place in the spring, berries form, and ripen the following autumn. The seed is forcefully expelled up to fifty feet. According to Tinnin (1981), this is the major mode of dispersion, although animals can play a minor role. Each seed has a sticky coating, which allows it to adhere to host twigs. After reaching its host, precipitation can assist in moving the seed to the axil of a needle. The seed germinates the following spring, and establishes an endophytic root system in the branches of the host. This allows the mistletoe to sequester water and nutrients from the host tree. Two years after the initial infection, the first visible symptoms on the host appear. These generally include slight swelling of host branches. In the following two years, shoots and flowers of the parasite will appear (Tinnin 1981). In addition to swollen branches, dwarf mistletoe can also have a more dramatic effect on its host tree. It is common for the parasite to cause abnormal growth of the host twigs and branches, which increase in density, and alter in orientation. This phenomenon is known as a witches' broom. Eastern dwarf mistletoe forms a globose, dense systemic broom, as characterized by Tinnin et al. (1982). The presence of a broom is a direct benefit to the parasite, as it increases the amount of host tissue distal to the parasite relative to the trunk. This occurrence leads to increased photosynthesis, owing to a higher amount of surface area, and allows the endophytic system of the mistletoe to intercept the host tree's nutrients produced by photosynthesis. The high surface area of a broom allows reproduction to occur on the same tree without relying on long distance seed dispersal. WitchesƠbrooms lead to the decline in vigor, and ultimate death, of the host tree (Tinnin et al. 1982). Once dwarf mistletoe appears in a stand of black spruce, the infection expands outward in a circular pattern at a rate of one to two feet per year. Research has shown that tree mortality spreads at the same rate as the parasite (Baker and French 1991). Using this information, the origin of a stand? infection can be traced. In this study, we observed the distribution and attempted to discover the origin of Eastern dwarf mistletoe on black spruce at Bryant Bog, as well as the effects of the parasite on each tree. We also examined the intensity of infection and the relationship between tree height and infection.