Ecological dynamics

This alpine forest is composed of whitebark pine (Pinus albicaulis) and mountain hemlock (Tsuga mertensiana). Total canopy cover is about 25 percent. In some areas trees grow as single upright stems and reach approximately 45 feet in height, while in other areas they are multi-stemmed and shrub-like (Krummholz). Although the cover of bluntlobe lupine (Lupinus obtusilobus) is often very high, it is absent in other areas. Bare soil and gravels cover most of the surface, with 1 to 3 percent vegetative ground cover other than lupine. Common plants in addition to lupine are western needlegrass (Achnatherum occidentale), pioneer rockcress (Arabis platysperma), squirreltail (Elymus elymoides), marumleaf buckwheat (Eriogonum marifolium), oceanspray (Holodiscus discolor), and Davis' knotweed (Polygonum davisiae).

The whitebark pine-mountain hemlock forest is found near treeline in Lassen Volcanic National Park, but may be found in other similar elevations in the Southern Cascade Mountains. The trees at this elevation are very slow growing. Older trees may be 500 years old while younger trees appear to be 75 to 200 years old. On the steep slopes, this forest develops on bedrock controlled ridges. The depth to bedrock is 40 to 60 inches. The bedrock provides a solid anchor for the tree roots. The surrounding colluvial soils have a low cover of forbs and grasses, with very few trees. Trees may be inhibited in the nearby areas because of cold air that drains down the mountain to lower positions, where it pools in basins. These areas are more prone to summer frost, which can kill young mountain hemlock and whitebark pine seedlings. On steep southern slopes, whitebark pine may be inhibited by excessively warm temperatures.

The high elevations are buried with deep snow from November to June and remain cool for most of the year. Several physiological adaptations allow mountain hemlock and white bark pine to survive in this cold environment. They have maximum photosynthetic rates at colder temperatures than lower elevation trees, and close stomata to reduce water loss during dormant periods. The tips of mountain hemlock are very flexible, an attribute that reduces snow build-up and stem breakage. Snow burial can be helpful in protecting trees from strong winter winds, desiccation from warm winter winds and sunny winter days, extreme cold, and repeated freezing and thawing (Arno and Hammerly, 1984). Snow burial can, however, be detrimental as well. In some areas, those portions of the trees exposed above the snow can die back, leaving short multi-stemmed trees. Snow creep can create pistol-butted trees, and avalanches can destroy swaths of forest.

Timberline trees are able to withstand extremely cold winter conditions when they are dormant but need at least a 2 to 3-month frost free growing period in the summer. During this short growing season, usually in July and August, new mountain hemlock and whitebark pine growth is susceptible to frost. The new shoots are soft and succulent and need time to "ripen" (Arno and Hammerly, 1984). The duration of the growing season is crucial for seedling establishment. As elevations increase, temperatures drop and the growing season is shortened. Growing season length is one of the limiting factors to determine treeline. Another is wind. Wind induced treelines can be caused by drought conditions, due to increased evapotranspiration (Tomback, et al. 2001).

Whitebark pine is a long-lived timberline tree species that grows 40 to 60 feet tall in favorable conditions. At upper treeline limits and on exposed ridges it is reduced to its Krummholz or low shrub form. In its upright form it develops multiple branches along the upper stem and creates a broad canopy, rather than the tapered canopy of many conifers. Needles are formed in bundles of 5 that vary in length from 1.5 to 7 inches. The female cones are 1.6 to 3 inches in length. The cones are indehiscent, meaning they do not open at maturity. They heavily rely on the Clark’s Nutcracker (Howard, 2002) as these birds often cache the seeds in open areas that are suitable for young seedlings. If the seeds are not completely consumed, they give rise to dense clusters of genetically similar whitebark pine. These clusters appear to be one tree with many stems but are actually individual trees (Arno and Raymond 1990, Tomback et al. 2001).

White bark pine germination and seedling survival is best in canopy openings, such as those created by small fires. This is especially important in areas where whitebark pine develops dense canopies, as in the northern Cascades and the Rocky Mountains (Arno and Raymond, 1990, Howard, Janet L. 2002, Tomback et al. 2001). In Lassen Volcanic National Park, whitebark pine is usually found on exposed ridges and mountain slopes near timberline. It grows naturally into an open canopy with low levels of litter and woody debris accumulations. The understory is primarily bare soil, composed of loose single-grain gravels or coarse sand. Lightning is the primary cause of natural fires in this area, but the discontinuity of forest fuels will usually not allow it to spread far or burn very hot (Arno and Raymond, 1990, Howard, Janet L. 2002).

Whitebark pine forests are diminishing rapidly across the western United States. This is caused by fire exclusion and climate change, as well as impacts by the introduction of white pine blister rust (Cronartium ribicola) and from the native mountain pine beetle (Dendroctonus ponderosae) (Cox, 2000, Howard, Janet L. 2002, Tomback et al. 2001). In 2005, the National Park Service surveyed for white pine blister rust infestation in Lassen Volcanic National Park. There was a 2 percent infection rate in 1 of the 2 plots within the whitebark pine forest (personal communication, LVNP). The sites have not been resurveyed since. Conditions for the white pine blister rust require sufficient moisture during early summer to allow an alternate host to be infected, usually local currents and gooseberries (Ribes ssp.), and continuing moisture throughout summer to maintain the leaf moisture (Arno and Raymond, 1990). Alternate hosts are often found in lower elevation forests and wind can carry the fragile spores short distances up slope.

Predictions about climate change due to global warming suggest that the whitebark pine communities may be threatened by rising temperatures and precipitation changes. These changes may cause lower elevation tree species to extend their elevation range and encroach into the whitebark pine community (Cox, 2000). These invading trees, which may include California red fir (Abies magnifica), mountain hemlock (Tsuga mertensiana) and Sierra lodgepole pine (Pinus contorta var. murrayana) could over-top the whitebark pine and replace it successionally (Cox, 2000).

The fire return intervals for whitebark pine and mountain hemlock forests in this area are poorly documented. Fire occurrence for mountain hemlock may range from 400 to 800 years (Tesky, 1992); for white bark pine, the range is from 50 to over 300 years (Tomback et al. 2001). There were 9 fires documented in the mountain hemlock zone of Lassen Volcanic National Park between 1933 and 1977, resulting in a single tree being burned (Taylor, 1995). Lightning strikes are very common in this area, but the fuel loads and their capacity to carry fire is low. Even if fire started to spread, these forests are often dissected by unvegetated slopes, wind exposed ridges and rock outcrops.

Mountain hemlock is a slow-growing native conifer. On this site it is generally less than 45 feet tall with branches covering the entire stem. Low-lying branches may root by layering. Trees produce single needles that tightly overlap all surface area of the twigs. The needles generally curve upward. The species exhibit shallow wide-spreading root systems. It is shade tolerant and will reproduce in the understory (Tesky, 1992). Reestablishment of mountain hemlock after a fire or other disturbance is often slow, and in some areas growth never regains its tree-like stature (Arno and Hammerly, 1984).

Mountain hemlock is not generally as susceptible to forest pathogens as the lower elevation conifers, but trees over 80 years old are very susceptible to laminated root rot (Phellinus weirii). Laminated root rot can rapidly spread by root contact and kill acres of forests (Tesky, 1992). Other common fungal and parasitic pests of mountain hemlock include several heart rots, of which Indian paint fungus (Echinodontium tinctorum) is the most common and damaging, various needle diseases, snow mold (Herpotrichia nigra), and dwarf-mistletoe (Arceuthobium tsugense) (Tesky, 1992).

Other pests that affect whitebark pine include aphids (Essigella gillettei), mealybugs (Puto cupressi and P. pricei), lodgepole needletier (Argyrotaenia tabulana), Monterey pine Ips (Ips mexicanus), other bark beetles (Pityogenes carinulatus and P. fossifrons), and ponderosa pine cone beetle (Conophthorus ponderosae). Other diseases that infect whitebark pine include stem infections from (Atropellis pinicola), (A. piniphila), (Lachnellula pini), (Dasyscypha pini) and (Gremmeniella abietina), all of which form cankers. Wood rots are caused by (Phellinus pini), (Heterobasidion annosum), (Phaeolus schweinitzii), and (Poria subacida). Needle cast fungi include (Lophodermium nitens), (L. pinastri), (Bifusella linearis), and (B. saccata). The snow mold (Neopeckia coulteri) occasionally forms on leaves when they are buried by snow for long periods. Several species of dwarf mistletoes (Arceuthobium spp.) can infest whitebark pine and cause localized mortality. The limber pine dwarf mistletoe (A. cyanocarpum), lodgepole pine dwarf mistletoe (A. americanum), and hemlock dwarf mistletoe (A. tsugense) can damage whitebark pine.

The reference state consists of the most successionally advanced community phase (numbered 1.1) as well as other community phases which result from natural and human disturbances. Community phase 1.1 is deemed the phase representative of the most successionally advanced pre-European plant/animal community including periodic natural surface fires that influenced its composition and production. Because this phase is determined from the oldest modern day remnant forests and/or historic literature, some speculation is necessarily involved in describing it.

All tabular data listed for a specific community phase within this ecological site description represent a summary of one or more field data collection plots taken in communities within the community phase. Although such data are valuable in understanding the phase (kinds and amounts of ground and surface materials, canopy characteristics, community phase overstory and understory species, production and composition, and growth), it typically does not represent the absolute range of characteristics nor an exhaustive listing of species for all the dynamic communities within each specific community phase.