Distribution of western hemlock dwarf mistletoe (Arceuthobium tsugense [Rosendahl] GN Jones subsp tsugense) in mature and old-growth Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) forests

We investigated the landscape distribution and spatial patterns of western hemlock dwarf mistletoe in old-growth and mature Douglas-fir forests of the Wind River Experimental Forest. The study was conducted in two settings: the old-growth forest (500 year) of the 478 ha T.T. Munger Research Natural Area (T.T. Munger RNA) and the higher-elevation, predominantly 157-year old stand of the 1,400 ha Panther Creek Division of the Wind River Experimental Forest. Existing transects and tagged trees were used in the T.T. Munger RNA to survey for dwarf mistletoe infection. We surveyed for dwarf mistletoe-infected trees in the Panther Creek Division by hiking roads, trails, and drainages. Eighty-three percent of the transect segments within the T.T. Munger RNA had some level of dwarf mistletoe infection. Dwarf mistletoe was found only in legacy old-growth patches in the Panther Creek Division, and these comprised only 2.4 percent of the division area. The three legacy old-growth patches containing dwarf mistletoe were restricted to drainage bottoms and well-watered benches. None of these patches burned completely in the 1800's, as evidenced by large, old trees > 300 yrs. Spatial analysis of transect segments (using Moran's 1) in both T.T. Munger RNA and Panther Creek Divisions showed that within both old-growth and legacy patches in mature forest, western hemlock dwarf mistletoe infection is spatially aggregated. Dwarf mistletoe is maintained on this landscape by survival in refugia, most often in riparian areas, or, in the case of the T.T. Munger RNA, in low-density, low-productivity areas that do not burn completely during wildfire events. Dwarf mistletoe must persist in these refugia until host trees recover in burned areas, a process which may take many decades in intensely burned watersheds.