Ecological dynamics

Abiotic Factors:
This ecological site occurs at the alpine treeline ecotone, typically between 10,000 and 11,000 feet, on mountain slopes. Wind exposure, cold temperatures, and a short 20 to 30 day growing season, limit the growth of whitebark pine to a shrubby krummholz stature. The whitebark pine krummholz canopy is typically less than less than 6 feet in tall, and is more often 2 to 3 feet tall. The soils associated with this ecological site are shallow with very gravelly coarse loamy sand surface textures, which have very low water holding capacity, and support a minimal but unique component of dwarf alpine plants. Whitebark pine roots can penetrate deeply into the soft decomposed granitic bedrock to access additional moisture.

Ecology-Disturbance Factors:

Individual whitebark pine trees are very slow growing, and may be 1500 years old (Millar 2014). The caching of whitebark pine seeds by Clark’s nutcracker is the primary mode of seed dispersal. The birds often cache seeds in open areas that are suitable for young seedlings. If all seeds are not consumed, they give rise to dense clusters of genetically similar whitebark pine. These clusters appear to be one tree with many stems, but are more often individual trees. (Burns et al. 1990, Tomback et al. 2001a). Whitebark pine has ongoing recruitment from Clark’s nutcracker in the absence of disturbance, thus over time stand density increases.

Fire and avalanche are the primary natural drivers for succession, but this ecological site is relatively stable. Fire ignition is frequent on these exposed ridges and mountain peaks, but there is minimal and discontinuous fuel to carry large or hot fires. Small fires may play a minor role in maintaining openings that favor the germination and survival of young whitebark pine seedlings (Burns et al. 1990, Tomback et al. 2001b, Howard 2002).

Avalanche is common among the alpine peaks and ridges, and can remove swaths of vegetation in avalanche prone chutes or below wind formed cornices. However, the removal of krummholz whitebark pine by avalanche in this ecological site is unlikely or infrequent due to the low stature of the trees, and the position of this ecological site on ridges and peaks.

Whitebark pine forests are threatened by the non-native white pine blister rust (Cronartium ribicola), and the native mountain pine beetle (Dendroctonus ponderosae) (Cox 2000, Tomback et al. 2001b, Howard 2002). Severe epidemics have killed large areas of forest in the Rocky Mountains, but the whitebark pine forests in the Sierra Nevada have not suffered as high mortality. There is a complex interaction between mountain pine beetle outbreaks, white pine blister rust (WPBR) infection rates, and climate. Mountain pine beetles prefer larger diameter trees, and attack at the warmer, lower elevation zone of whitebark pine. Thus, this krummholz whitebark pine ecological site is less susceptible to mountain pine beetle attacks. White pine blister rust will infest all whitebark pines, regardless of age or elevation (Cluck 2014). Younger trees are killed quickly, once the cancer girdles the tree. Older trees typically die slower as upper cone bearing limbs are slowly girdled by the cankers.

At this time, WPBR has varying rates of infection of whitebark pine in the Sierra Nevada, with several severe infestations in the Lake Tahoe region. The non-native WPBR was introduced into North America near Vancouver, British Columbia around 1910, and has been slowly spreading across the western United States and Canada. It was first detected in the southern Sierra Nevada Mountains in 1960’s. It has not been detected on whitebark pine in the southern extent of the Sierra Nevada, but has been found on a whitebark pine in Yosemite National Park and a high Sierra location on the western slopes of the Sierra National Forest (Maloney 2011). In order for WPBR to infect whitebark pine several synchronous phenological and environmental factors need to occur. For infection to occur in five-needled white pines, relative humidity has to be greater than 90 percent, temperatures have to be between 35.6 and 64.4 degrees F (2 to 18 degrees C), and stomates need to be open to allow the WPBR entry (Maloney 2011). The basidiospores, which infect whitebark pine, are released in fall from the alternate hosts gooseberry or currents (Ribes sp.), or less commonly, lousewort or Indian paintbrush (Pedicularis or Castilleja sp.). These spores do not travel far or last long in the environment. Whitebark pine may have early onset of winter dormancy, and the stomates may be closed at the time WPBR basidiospores are released (Maloney 2011). The onset of winter dormancy is dependent upon the length of growing season, precipitation and soil water holding capacity. This ecological site occurs on shallow, coarse textured soils on upper mountains slopes. The water drains rapidly out of the root zone, so the trees on this site may go into winter dormancy even earlier than whitebark pine on finer textures soils or cooler aspects, making them less susceptible to WPBR infestation.

WPBR affects the crown and cone producing limbs of mature trees, reducing cone production, and can kill younger trees within a year. The decrease in cone production and high mortality of young trees threatens the regenerative success of this species (Maloney et al. 2012). A few studies have been conducted on genetic resistance, and results vary from no resistance (Maloney, personal communication), and 26 to 47 percent in the Rocky Mountains and the Pacific Northwest (Keane et al. 2012).

Reduced seed production affects the presence and abundance of the Clark’s nutcracker, and in return the number and distribution of seed cache (Tomback and Resler 2007, Keane et al. 2012). This can lead to recruitment below the threshold required to sustain populations (McKinney et al. 2009).


Predictions about climate change due to global warming suggest that the whitebark pine communities in the Sierra Nevada Mountains may be threatened by rising temperatures and precipitation changes. Recent California based climate models predict a 9 degree F increase in temperature by 2100, and older models predict a 2 to 4 degree F increase in winter and 4 to 8 degree increase in summer (Safford et al. 2012). Models are more variable for precipitation, but recent models for the Sierra Nevada, predict similar to slightly less precipitation. Most models agree that summers will become drier, since more of the precipitation is predicted to come as rain, and snow melt-off will occur earlier in spring (Hayhoe et al. 2004, Safford et al. 2012). Presently a severe drought is occurring in the Sierra Nevada, with 10 to 30 percent of average precipitation and very little snow accumulation. Whether this is climate driven, and thus will become more of the future normal remains to be seen.

The alpine zone is defined as having mean annual summer temperatures of less than 43.5 Degrees F or (6.4 degrees Celcius), and a growing season of less than 94 days (Korner et al. 2011). The warming temperatures will affect the structure and distribution of the alpine ecotone. High elevation areas with suitable soils and landforms for the upward migration of whitebark pine will be important for the sustainability of this community. The long lived whitebark pine will most likely persist through the initial shift in climate, but the slow regeneration of this species in the limited expanses of suitable higher elevation habitat may be slow. The intermediate affect of warming temperatures may allow for more upright tree growth of whitebark pine and increased leader growth on compact krummholz forms. Indeed, upright tree growth forms have already significantly increased in alpine krummholz whitebark pine communities (Millar et al. 2004). However, a decrease in snow pack or earlier melt off may cause dieback of leaders and exposed krummholz, creating a more compact or less extensive krummholz community.

The dwarf alpine herbaceous community may also be impacted by warming temperatures. These
small forbs and grasses lie close to the surface to obtain the maximum heat possible at this elevation. They live in a much warmer microclimate than trees would be with their limbs exposed to air currents. The krummholz structure is also formed in part by the warmer temperatures near the soil surface. A few degrees of warming may make the temperatures too extreme for these alpine species. A modeling analysis of warming temperature on the distribution of alpine forbs in the White Mountains California, predicts local extinction of several species. Fewseed drab (Draba oligosperma), a common plant on this site, is predicted to be locally extinct in the White Mountains with 3 degrees of warming (Van de Ven et al. 2007). Because the peaks are not as high where this ecological site is found, these species have less room for upward migration.

Warmer temperatures have shifted the thermal zone for mountain pine beetles upslope, subjecting higher elevations of whitebark pine to beetle attacks (Craig 2010, Keane et al. 2012, Keane and al 2013). Growth rate of whitebark pine stems has nearly doubled since 1880, due to warmer temperatures (Millar et al. 2004). The new leaders may obtain sufficient size (greater than 6 inch dbh), to be targeted by mountain pine beetles.

The historic temperature range for this ecological site is between 20 to 35 degrees F. With a 2 to 6 degree warming, species such as Sierra lodgepole pine (Pinus contorta var. murrayana), or mountain hemlock (Tsuga mertensiana) may move into this zone. A 9 degree warming shift over the next 85 years could make conditions favorable for upper montane species to establish. Species such as Jeffrey pine (Pinus jeffreyi) and California red fir (Abies magnifica) could survive with the longer growing season and warmer temperatures for seedling germination and leader growth. If lower elevation conifers establish in the whitebark pine zone, whitebark pine may become a seral species, dependent upon fire for continued regeneration and elimination of competitors.

The reference state consists of the most successionally advanced community phase (numbered 1.1) as well as other community phases that 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. This phase is determined from the oldest modern day remnant forests and/or historic literature.

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.