Lodgepole Pine at Crater Lake: History and Management of the Forest Structure
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.