Types of Forest History - Lodgepole Pine at Crater Lake

The lodgepole forests in Crater Lake National Park have several apparent types of stand history: Type 1) Those seral lodgepole forests which are rapidly replaced by fir and hemlock need to be considered as a part of the larger complex of fir-hemlock forests. At any one time, part of this complex is in mature fir-hemlock forest, part in seral lodgepole stands, and part in transition (Fig. 1).

Proposed model of lodgepole pine stand development, Crater Lake National Park

Figure 1. A proposed model of stand development in areas supporting seral P. contorta forest on pumice soils. Heavy arcs indicate phases in which intense fires are unlikely. Solid fine curves indicate phases in which intense fires are more probable. Broken lines signify fires. Fire types a and c are intense enough to initiate a new P. contorta stand. Fire type b may initiate a new age class or only burn the understory and tree reproduction. (from Zeigler, 1978).

Intense fires, which kill the fir and hemlock, create new lodgepole forests, which may then develop into fir-hemlock with time. The lodgepole reproduces poorly in these dense seral forests and beetle kill hastens its demise. Litter is heavy; a reburn would probably kill most of the lodgepole but would also produce a new lodgepole forest. In most stands of this type, the major age class originated after white man arrived. From historical records it seems that a larger proportion of the area which can potentially support fir-hemlock is now in lodgepole than there was in primeval conditions. This resulted from the many fires in 1855 to 1900, some of which burned in mature forest.

Type 2) Some areas which can support fir-hemlock are invaded by trees only slowly after forest destruction by fire, probably due to a relatively dense herbaceous cover. Gradually the lodgepole pine increases in number, with most reproduction being near older trees, forming islands of forest in which fir and hemlock become established. These tree islands gradually spread into the meadow between, and, probably after two or more generations of lodgepole without a major disturbance, a closed forest may form. In the meantime, however, some individual tree islands were probably destroyed or thinned by local fires and by bark beetles, delaying forest closure. Tree growth is very rapid once trees finally become established, and they reach beetle-susceptible size at a relatively young age.

Type 3) In some lower elevation areas, contiguous to the ponderosa pine forest, periodic ground fires probably maintained a mixed forest of lodgepole, white pine, fir and hemlock. Fuel loads are low, and the burns were probably small or patchy, and of low intensity. Large trees, even lodgepoles, were scarred without dying, but most of the reproduction in the burns would have been killed, and lodgepole reproduction increased in the openings. One fire, which increased the fuel load as dead trees fell, probably led to a greater chance of a later reburn there. If a long enough time passed without fire, an intense fire, killing most trees, could probably have been supported by the accumulated fuel. Lodgepole re-invasion could have led to another forest maintained by periodic ground fires. The burning interval between the only two fires recorded on scars was 30 years, but now, 80 years later, fuel loads still appear too low to allow other than patchy fires.

Type 4) Some areas appear to be lodgepole pine climax, where fir and hemlock rarely establish. In the better sites, an open or patchy lodgepole forest gradually may become a quite dense thicket, stopping lodgepole reproduction at least in spots. In the patchy phase fires were probably small or of low intensity due to discontinuous or light cover of litter. These small fires and beetle kills delayed the development of closed forest. After the forest closed, and most likely following heavy beetle kill, intense fires occurred, killing all trees and beginning the cycle again. After 75 years of fire protection, some of these forests have developed densities and fuel loads which are very conducive to intense fire and probably equal or exceed the maximum present under primeval conditions.

Type 5) Some climax lodgepole forests are very sparse in all layers, and grow in habitats which appear incapable of supporting denser forest. Fire would be confined to very small patches of continuous fuel. It seems unlikely that extensive fires of any type could have been supported. Reproduction occurs more or less continuously. Beetle kill or local fires remove the older trees; however, some with heavy mistletoe survive longer than any trees in other communities. In these forests, fire effects appear minor, the stands are in more or less a steady state, and further stand development will be a process of primary succession, occurring only over many generations of trees.