Vertical spread rate and intesification of dwarf mistletoe in western hemlock

The vertical rate of spread of dwarf mistletoe was studied in two actively growing, young hemlock stands. This was done by determining the height and age of successive oldest and highest female infections. The rate of spread was calculated by dividing the sum of the heights of advances by the total number of years lapse between successive advances. The mean vertical spread rate was 2.1 ± 0.1 ft./yr. in a relatively open stand and 1.0 ± 0.1 ft./yr. in a relatively dense stand. The mean rate of tree growth during the maximum growth phase in the open stand was 2.5 ft./yr. and for the dense stand 1.5 ft./yr. However, over the past 25 years, the growth rate of the trees in the open stand was 1.9 ft./yr. and for the dense stand 1.1 ft./yr. The number of new infections per year increased geometrically, doubling every four years in both the dense and open stands. However, the geometric increase levelled off six years ago in the open stand and five years ago in the dense stand. During the maximum growth phase of hemlock in an open and dense stand, the most photosynthetically active upper portion of the crown remains free of mistletoe infection. Until the senescent phase is reached, the trees can be expected to outgrow the mistletoe and intensification will be restricted to the lower portions of the crowns. It is tentatively concluded that provided there is no overstory seed source and no disruption of the natural stand, such as thinning, dwarf mistletoe on hemlock will not become serious until the rate of height growth of the trees falls below the rate of vertical spread, i.e., not until after the presently accepted rotation age.Forestry, Faculty ofGraduat