IV. Parasitic Plants Affecting Forest Structure

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Dwarf mistletoe (Arceuthobium americanum) is a higher plant which grows as a parasite in the stem and branches of lodgepole pine, from which it receives the water and most of the carbohydrates it needs. It causes swelling of branches and, as it grows, causes abnormal branching above the infection, forming "witches brooms." Tree height and density of the upper crown are reduced in heavily infected trees at Crater Lake, and diameter and root growth may be likewise affected. One hypothesis states that thickness and food content of the phloem tissue may be reduced by heavy mistletoe infection, rendering those trees less susceptible to mortality from bark beetles. Trees with heavy dwarf mistletoe infection often have dead tops. Heavy infection in a young tree may prevent its development to mature size and form. Infection is often inconspicuous when the parasite does not produce aerial shoots or does not cause "witches broom" formation.

These mistletoe effects on growth and form of individual trees seem to result in a more open canopy in heavily infected stands. On severe sites, the largest, oldest trees are almost all heavily infected with dwarf mistletoe. Perhaps they reach their size chiefly as a result of their lower ability to support bark beetle attack.

This species of mistletoe, of which the primary host is lodgepole pine, disappears from a site when the host is totally destroyed or replaced by fir or hemlock. Thus it must be reintroduced to a new population of pine. It moves into an uninfected stand slowly, about 0.7 m yr^-1, primarily by short-range mechanical seed dispersal, although long-distance transport by birds occasionally occurs. Thus, areas from which lodgepole is periodically absent tend to have less infection than those where the tree can reproduce without catastrophic destruction. In these all-aged forests with large mistletoe populations few new trees reach the overstory without considerable mistletoe infection. These forests which are open enough to allow continuous reproduction of lodgepole pine have very low and discontinuous surface fuels. Most fires would have been confined to local pockets of continuous fuel, small enough to have their new trees immediately reinfected by seeds from plants on adjacent infected trees. Dwarf mistletoe is not responsible for the sparse nature of the stands where primeval lodgepole pine continuously reproduced. We believe heavy infections have always been present; management to reduce mistletoe on these sites is not necessary.

Western gall rust (Peredermium harknessii) infects many lodgepole pine stands. Trees with a stem infection often snap off at the canker. In some spots this may cause small openings in the forest canopy and speed fuel buildup on the forest floor, perhaps allowing lodgepole reproduction, or releasing small trees of shade tolerant species, such as fir and hemlock.