IV. Parasitic Plants Affecting Forest Structure

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.

V. Primary Causes of Death of Lodgepole Pine

Very few lodgepole pines reach the age and size of which they are capable; most probably die at a relatively young age following either fire or infestation by mountain pine beetle (Dendroctanus ponderosae).

A. Mountain Pine Beetle

Mountain pine beetles often attack lodgepole pines. The female bores through the outer bark and lays her eggs in the inner bark; after hatching, the larvae feed on the phloem tissue. A heavy attack quickly results in death.

After an initial attack the females may abandon a tree if conditions are unsuitable. Suitability is apparently associated with phloem thickness; phloem thickness increases with tree diameter; thus beetles preferentially attack larger trees, which suffer the greatest mortality. Trees with thin phloem, due to their small size (or, in cases, perhaps due to heavy mistletoe infection), are relatively immune. The usual diameter of susceptibility is 25-35 cm in the Rocky Mountains and seems similar here.

At the elevations encountered in the Park mountain pine beetle populations are food-limited. Under endemic conditions beetle populations are low, selectively removing only a few large individuals from a susceptible stand each year. The populations may be kept at endemic levels for several reasons: there may not be enough large trees to support increasing numbers of beetles; the trees may be vigorous enough to successfully resist attack; environmental conditions may be too severe (e.g. low temperature) to permit large scale brood survival. At Crater Lake conditions restricting beetle population buildup may be encountered in a multi-aged lodgepole stand where there are only a few trees of susceptible size at any given time. There are apparently no stands at Crater Lake that are either vigorous enough to perpetually resist attack or at high enough elevations so that environmental extremes always restrict beetle activity.

Epidemic conditions arise when the available food supply is large and environmental conditions (both physical and biotic) permit large-scale brood survival. Populations increase as the beetles successfully attack most of the large trees, each of which produces large numbers of adults. Thus, epidemics are more likely to occur, and impact is most severe, in single-aged stands where most individuals reach a susceptible size at about the same time. As most of the large trees are killed the beetles are forced to attack trees as small as 10 cm dbh. These trees with thin phloem are incapable of supporting large numbers of brood. As the brood starve to death in the smaller trees, and disease and predators increase, the beetle population declines.

Following an epidemic, activity may remain low for years until surviving trees reach the most susceptible size class. In a lodgepole climax stand, openings from beetle-caused mortality permit increased lodgepole reproduction. As this age class reaches susceptible size and conditions permit, another bark beetle epidemic is likely. In seral stands the shade tolerant species are released and replace the pine unless fire recycles the stand to lodgepole. In both cases epidemics greatly increase the amount of fuel on the forest floor.

No known control method for mountain pine beetle is effective over large areas. The last attempts at control at Crater Lake were abandoned several years ago. Beetle activity, since it is affected by the number of susceptible trees, will probably continue to be high as the lodgepole stands which originated in 1850-1900 reach susceptible size. Then the level will probably wane somewhat as some seral stands are replaced by fir and hemlock.

B. Fire

Lodgepole pine is easily killed by fire, as it has thin bark even when old. Trees affected by fire but not killed directly succumbed in 10-12 months to bark beetles (Ips pini and Dendroctanus ponderosae) in the 1976 Panhandle control burn. Fire decreases the seed availability on the site, because cones are not serotinous. However, removal of overstory shade and litter enhances seedling survival. Major tree competitors, western white pine, the firs and mountain hemlock, are all very susceptible to fire when young, but develop thicker bark with age, and become more resistant than lodgepole. Many understory plants such as grasses and sedges may recover rapidly after fire and some may increase with repeated fires (see Appendix D). Others may be reduced in importance or eliminated at least temporarily. Thus, response of tree regeneration to fire may vary with the ground cover present, as a result of its interference with seedling establishment.

Although fire will reduce the litter on the forest floor, the dead lodgepole needles and twigs will rapidly replenish the fine litter and, as the snags fall, heavy fuels may become very dense. (In the Rocky Mountains, half the snags fall in about 15 years). The usual increase in fuels following fire in lodgepole is in sharp contrast to the fuel reduction which occurred after fire in the primeval ponderosa pine forests, where most of the overstory survived. Fire scar and age class data indicate that some areas which burned in the primeval forest were reburned within twenty to thirty years.

Evidence for the fire history of lodgepole forests comes from several sources: (1) Fire scars are rare. The few are mostly in one community. Those on other species in lodgepole forests are also rare, with the most common, on western white pine, having a record of only two fires. (2) Charcoal is present in variable amounts in the forests. Surface charcoal collected in many stands was identified as lodgepole pine, or white pine, or non-pine species. This can separate stands where fir and hemlock were previously present from those which were only lodgepole pine. (3) Presence of very common age classes may indicate an origin after fire; they may also indicate disturbance by bark beetles or wind effects, or simply the coincidence of heavy seed years with very favorable conditions for seedling establishment, in some communities. (4) Reports by the early qualified observers (e.g. Leiberg 1900), histories of Indian activity, and park records of lightning fires provide much pertinent information.

Lightning fires are common (7 per year recently) in the Crater Lake area and were almost certainly the major ignition source in primeval lodgepole pine forests. Although some low elevation stands were probably burned by Indian-caused fires moving up slope, there was little Indian activity at high elevations where most lodgepole forests are. This situation changed drastically with the arrival of white man in the area about 1855. Fires were used in roadbuilding and caused by visitors and hunters. Grazing on the west slope was accompanied by extensive burning. Considerable fir and hemlock forest was converted to lodgepole pine by this burning, which certainly also burned some of the lodge pole already present. Our age data confirm the historical reports, with many lodgepole stands originating between 1855 and 1900, and many older ones having large age classes established then. These are particularly evident in the areas of greatest activity by white man, the west slope, Pinnacles Valley, and the general route of the Union Creek - Fort Klamath road. With fire suppression, man-caused fires and the size of lightning fires were greatly reduced. These activities of white man have thus resulted in differences from the amounts of lodgepole forest one would have expected with primeval conditions; there is more area of 75 to 120 year old stands and less of younger stands than there would have been.

C. Fire - Bark Beetle Interactions

The effects of fire and bark beetles are not independent of each other. Lodgepole trees which survive fire seem very susceptible to bark beetles, perhaps capable of triggering an epidemic. Trees killed by bark beetles quickly become fuel to support more intense fires Fire allows another generation of lodgepole pine, which can eventually support more beetles. In contrast, beetle kill of lodgepole in seral forests opens the canopy and thus accelerates growth of the fir and hemlock and the transition to the more fire-proof fir-hemlock forest. The long term effect of beetles thus may be to decrease chance of fire earlier than otherwise, if the stage of high fuel loads passes without fire.

In many northern Rocky Mountain forests, fire suppression led to abnormally large areas of old lodgepole pine with resulting massive beetle kills, much larger than would have occurred in the primeval condition. At Crater Lake, some seral forests have recently reached the size of susceptibility to beetles; recycling them to new, beetle-proof stands with controlled fire might seem a logical thing to do. However, this appears NOT to be appropriate. Many of these stands were converted from fir-hemlock to lodgepole by fires caused by white man and a return to primeval conditions requires some area of lodgepole forest be allowed to revert to fir-hemlock. Furthermore, controlled burning in lodgepole reduces fuel loads only temporarily, since the overstory is usually killed, producing extremely high ground fuels as the debris falls, and probably requiring a reburn for safety. In the resulting lodgepole stand, fire danger and beetle susceptibility eventually will be high again. Thus, a general program of controlled burning in seral stands is ruled out by (1) the policy to return to primeval forest, which requires conversion of some lodgepole forest to fir-hemlock, and (2) long-range safety considerations, i.e., allowing stands to develop naturally to fir-hemlock and thus reducing the fire danger permanently at no management cost. A present period of widespread beetle kills and the resultant high fire danger appear to be the price of a return toward primeval conditions in several of the seral communities.