The ponderosa pine/fir historic plant association is in the northeastern and southeastern corners of the park (see fig. 5, page 101). Ponderosa pine is the historic climax tree species in areas frequently burned because it is more fire resistant than other associated tree species. In the northeastern corner of the park are other tree species, including Shasta red fir, white fir, and lodgepole pine. Research has shown that the average fire-free interval was 4.4 years during 1501 through 1850 (Mastrogiuseppe and Mastrogiuseppe, n.d.). The frequent fires resulted in open stands with few trees in the understory. In areas where fires occurred extremely frequently, the understory vegetation was mostly grasses and sedges. In areas where the fire interval was longest, a mixture of shrubs, forbs, grasses, and sedges was present. Use of fire suppression practices in the moister areas has allowed white fir to become a more common component in the overstory and understory.
The ecological sites in the part of the ponderosa pine/fir association in the northeastern corner of the park include ponderosa pine/greenleaf manzanita/longstolon sedge (Pinus ponderosa/Arctostaphylos nevadensis/Carex inops) and ponderosa pine/greenleaf manzanita-golden chinquapin (Pinus ponderosa/Arctostaphylos nevadensis-Chrysolepis chrysophylla).
In the southeastern corner of the park, ponderosa pine is the historic climax species at the lower elevations and ponderosa pine and Shasta red fir are the climax species at the higher elevations (4,800 to 5,600 feet). Other tree species include sugar pine, Douglas fir, and lodgepole pine. As a result of fire suppression practices, white fir and Shasta red fir successfully regenerate and make up a large component of the stand. If fire is suppressed for an extended period of time, white fir will become the dominant tree species in the overstory. As shade from the overstory increases, the understory composition changes. Shade-intolerant shrubs decrease in abundance, and shade-tolerant plants increase.
Large white fir trees have become established at the lower elevations (4,400 to 5,000 feet); thus, white fir is thought to be part of the historic climax plant community. If the average period between fires is 9 to 42 years, however, white fir trees would make up only a small to moderate component of the overstory (McNeil and Zobel, 1980). Because white fir seedlings, saplings, and poles have a thin bark and the resin on the surface of the bark blisters, these trees are highly susceptible to damage and death from fire (Laacke, 1990). Fires burn in a natural mosaic pattern, so white fir can survive and grow in the unburned areas. Other tree species at the lower elevations include sugar pine, Douglas fir, and lodgepole pine.
The ecological sites in the part of the ponderosa pine/fir association in the southeastern corner of the park are ponderosa pine-fir/trailing snowberry (Pinus ponderosa-Abies/Symphoricarpos hesperius), ponderosa pine/greenleaf manzanita/ longstolon sedge (Pinus ponderosa/Arctostaphylos nevadensis/Carex inops), and ponderosa pine/trailing snowberry/longstolon sedge (Pinus ponderosa/Symphoricarpos hesperius/Carex inops).