Dwarf Mistletoe Research

Researchers need to climb into the canopy to study western dwarf mistletoe.

Western hemlock dwarf mistletoe (Arceuthobium tsugense subsp. tsugense) is a small, parasitic plant that infects the leaves and branches of its host plant, the western hemlock (Tsuga heterophylla) tree. Within a forest, like the HJ Andrews Experimental Forest, areas of mistletoe infection are patchy. Some areas of the forest have trees that are not infected, while other areas have trees that are heavily infected. Hemlock trees infected with dwarf mistletoe grow dense, multi-branched growths, called witches’ brooms. Researchers believe that mistletoe infections cause changes in the tree’s growth and water use. To understand the effect of mistletoe in the canopy of a tree, and in the broader area of a forest, graduate student Stephen Calkins and postdoc scholar Sky (Yung-Hsiang) Lan are climbing into the canopy of dozens of western hemlock trees to take a closer look. They measure the extent of the mistletoe infection by noting size and location of brooms in each tree crown. They also map each branch in the tree, recording its location and measuring the size. Each tree will also be cored to measure its sapwood. With these data, Stephen and Sky, together with their advisor, Dave Shaw, from Oregon State University, hope to learn more about how dwarf mistletoe may be affecting forest stands across the Pacific Northwest. See a little of the field work, high in the canopy, at https://andrewsforest.oregonstate.edu/gallery/dwarf-mistletoe-survey-2019