Ecological dynamics

This site occurs on infertile pumice soils that were laid down by the eruption of Mt. Mazama (Crater Lake) over 6,000 years ago. This site is within a very large flat pumice basin that is dominated by a single tree specie. Lodgepole pine (Pinus contorta ssp.murrayana ) is the seral/climax forest and the only tree specie that occurs on the site. The growing environment is harsh with long and cold winters. The topographic slope is low, ranging from 0.1 % to 2.0%. It is generally thought that due to the very flat slopes air drainage in the entire basin is slow. The cold air settles in, creating a harsh growing environment.

Under historic forest conditions there were 2 major disturbance factors in climax Lodgepole pine forests; Fire and Mountain pine beetle (Dendroctonus ponderosae) (Stuart et. el. 1989). Several research projects conducted in the lodgepole forests of the pumice zone concluded that fine fuels were not generally present. This type of fuel is needed to have frequent low intensity fires. Agee (1981) suggests a fire return interval of 60 years for a climax forest in Crater Lake National Park. Agee concludes that fire moved thru the Lodgepole stands in downed logs.

In a 130 year old Lodgepole pine stand (Geiszler, et. al. 1980) reported finding only 1 fire scar on the trees in his study area. Even though there had been 11 fires (1855-1903) the single fire scar per tree suggests that fire did not occur more than once in any location during the 130 years. In a study of the effect of a prescribed fire, in an open stand of Lodgepole pine, fire spread mostly in downed partially decayed logs that had been killed by mountain pine beetles many years prior (Agee 1981). Enough decayed downed wood needs to be of the forest floor accompanied by the right weather conditions for a fire to occur. Fire spreads slowly in the interior decayed material, igniting a surface fire when fine fuels (grass/pine litter) were present, damaging a tree bole by burning thru direct contact (burning log in direct contact with live tree) or killing tree roots thru heat transfer into the soil. In this ecological site, ground vegetation cover is limited, leading to the possible conclusion that this sites fire return interval was determined by the occurrence of downed logs. This is not to say that stand replacement fires do not occur, they do. A conceptual model of Lodgepole pine forest development (Gara, et al, 1985) proposes that stand replacement fires occur, leading to stand regeneration. Over time the stand grows, stagnates, snags fall and fuels accumulate. When enough fuels accumulate a ground fire will eventually occur. Damaged trees are invaded by beetles and killed or they become infected by fungi. The remaining trees continue to grow, but at a much slow pace due to fungal infestation. Over time the fungi kills the growing trees or weakens them to where a beetle outbreak occurs. The weak trees are attacked first and when the beetle populations build up they move into the older live trees, and then smaller trees if epidemic levels are reached. Trees die and fall over creating conditions for a possible stand replacement fire.

Historic regeneration patterns were determined by disturbance, either fire, mountain pine beetle outbreaks and fungi infections that occur in trees after injury or death. Successful regeneration was found to be limited by the lack of soil moisture (Geiszler et.al, 1980). So the killing of trees increased available water in the soil for seedling establishment. The type of disturbance determines that forest structure. A wildfire that killed most of the trees would encourage an even-aged forest. A light fire would maintain a predominance of two age classes (Agee 1981). Stands with periodic mountain pine beetle attacks could have 2 or 3 main age classes and stands with endemic beetle levels would be multi-aged (Stuart, 1989).

Mountain pine beetle have been found to attack older larger size Lodgepole pine trees first. Infestation maybe a function of lower tree resistance (older age) or size (availability of brooding sites) or fungal decay weakening the tree. Stands most likely to have a mountain pine beetle outbreak have a quadratic mean diameter of 20 cm (8 in) or more and have > 27 rings per cm (66 rings/inch) in the outermost centimeter (Stuart, et.al. 1989).


Fire frequency can also be influenced by a stand proximity to other forest types. In another study Gara et. al. (1985) surveyed 2 adjacent climax lodgepole stands and found one stand (A) had not had a fire for at least 350 years and the other had (B) a stand replacement fire in 1839 and a light ground fire in 1898. Stand B was immediately next to a Ponderosa pine stand that experienced frequent low intensity fires. Fire would move in to the climax Lodgepole forest when burning conditions were just right, burning and smoldering in downed logs. This created a two aged stand. Stand A, on the other hand, had a variety of age classes in clumps and no fire evidence, suggesting periodic pine beetle activity that would kill clumps of trees creating openings for regeneration.

Stand structure is influenced by the amount and timing of each occurrence, both singly and together along with the affect of fungi that invade damaged trees.