Unlike many of the cone-bearing, evergreen forests of the world, which are dominated by a single species of tree, the mixed-conifer montane forests that cloak the lower and middle slopes of the Sierra Nevada are remarkably diverse. Here ponderosa pine, incense-cedar, white fir, sugar pine, and scattered groves of giant sequoia intermix and coexist. These trees, many of which reach tremendous heights, form some of the most extensive stands of old-growth coniferous forest that remain in the world.
The iconic giant sequoias grow interspersed with other trees in a mixed-conifer forest. These include white fir, sugar pine, incense-cedar, red fir, and ponderosa pine. While giant sequoias dominate in terms of size and volume, they are outnumbered by other types of trees. Giant sequoias are the world's largest trees when measured by volume, and can live to be over 3,200 years old. They have a limited distribution, reflecting climatic patterns of the past several thousand years. Once more widespread, today they occur naturally only in the Sierra Nevada. In their current range, cold temperatures may have limited their expansion into higher elevations while drought has limited grove boundaries at low elevations. Soil characteristics also are important: Giant sequoias prefer deep, sandy loam soils that are wetter, less acidic, higher in calcium, and lower in nitrogen than soils associated with neighboring conifer forests.
Periodic fire plays an important role in giant sequoia reproduction. It stimulates seed release from cones, exposes mineral soil where the seeds can germinate, sterilizes the soil (killing seedling pathogens), and opens up the forest canopy to allow in enough sunlight for sequoia seedlings to establish and grow.
In the upper montane, the mixed coniferous forest is replaced by nearly pure stands of red fir and lodgepole pine. Characterized by deep snow accumulation during the winter months and a dense canopy that limits the amount of sunlight that reaches the forest floor, the red fir forests lack a diverse herbaceous component. Only the most shade tolerant herbs thrive beneath the towering trees. Lodgepole pines have an unusual distribution, growing in both moist lowlands and in drier sites on benches and ridges. In wetter sites, these forests can support a rich assortment of herbs and wildflowers in their understory.
Loss of historic fire frequency: Much of the montane forest zone is at risk of severe fire due to departure from its natural fire cycle. The giant sequoia forest type is furthest from its historic fire frequency.
Air pollution: Ozone causes injury to pines, particularly along the western edge of Sequoia National Park where exposure to ozone pollution is greatest. Nitrogen deposition along the western edge of both parks is a concern. Current nitrogen levels have potential to change the species composition, growth patterns, and nutrient cycling, altering the food sources for plants and animals that depend on forests.
Non-native pathogen: White pine blister rust has been a factor in increased sugar pine mortality especially in the Kaweah drainage of Sequoia National Park.
Climate change: Regional warming is implicated in the near doubling of annual tree-mortality rate measured in Sequoia National Park between 1983 and 2004. Recent years of severe drought in addition to higher than average temperatures, increased mountain pine beetle activity, and large wildfires have resulted in millions of pines dying across the Sierra Nevada.