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Modelling impacts of dwarf mistletoe in coastal British Columbia western hemlock forests
Muir, John A.
Dwarf mistletoe, a parasitic plant, is endemic in coastal western hemlock forests. The mistletoe spreads by seeds dispersed from residual to nearby young trees. Over time, infestations significantly reduce growth of trees. Variable retention silviculture regimes create conditions that exacerbate spread, intensification and impacts of mistletoe, but data to evaluate these effects are lacking. We established new sampling protocols to measure effects of dwarf mistletoe on tree growth and wood quality characteristics, and sampled 18 trees at one site west of Lake Cowichan, Vancouver Island, that had been previously studied to determine dwarf mistletoe spread into young trees. Data are being analyzed. A website providing information on hemlock dwarf mistletoe and the research project was prepared, and a Silviculture Extension Decision Aid was published on-line by FORREX. A spatial model of mistletoe spread was incorporated in the growth model TASS and other models, and significant effects of mistletoe on tree growth will also be included. The new TASS/mistletoe model when completed will enable projections of effects on growth of trees and stands, and enable quantitative evaluations of sustainability, effectiveness of management regimes, and mistletoe impacts. Further work is needed to broaden the sample of sites and to determine effects of important factors on impacts such as site index and ecological subzone. John A. Muir.
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