Dwarf Mistletoe And Its Effect Upon The Growth Of Larch And Douglas Fir In Western Montana

Abstract

Dwarf mistletoes are intolerant, dioecius seed plants belonging to the Loranthaceae family. Five species, Arceuthobium pussillum, A. americanum, A, douglasii, A. vaginatum, and A. campylopodum have been recognized in North America. All of them are restricted to coniferous hosts.

The aerial portions of the plant are small, not exceeding 2 1/2 inches in length, and constrain very little chlorophyll. They persist for only a short time, varying with the species, and their primary purpose seems to be reproduction. Pollination is by insects and the small dense seeds are dispersed by mechanical propulsion.

The greater portion of this pest lies within the host tree, forming the entophytic system. Infection usually originated on the younger branches of the host and persists as long as that part of the tree remains alive.

Stands infected with this parasite do not begin to produce the volume or quality of products of which they are capable. This is the result of a reduction in the growth rate, of poorer form, distortion of grain, large knots, and higher mortality in the infected trees. In addition there is reduction in the size of the viable seed crop and infected seedlings are usually unable to reach merchantable size.

Merchantable size larch and Douglas fir with less than 33 per cent of their crown infected with mistletoe will have about 14 per cent slower basal area growth than a healthy tree. Trees of the same species with over 60 per cent of their crown infected with the parasite can have their basal area growth reduced as much as 68 per cent.

Because of the magnitude of the growth losses incurred from this parasite it must be controlled before the production of timber in the future can become an economic possibility for those stands now infected.