Ecological dynamics

Abiotic Features:
This ecological site occurs in the highest elevations of the northern subalpine LRU, typically between 9,000 and 10,000 feet on mountain slopes. Soils are derived from granitic parent material, and are shallow to moderately deep over paralithic granitic bedrock, with a sandy skeletal particle size class. Cold temperatures and a short growing season restrict development of less frost resistant conifers. Coarse soils with very low water holding capacity support a minimal understory.

Ecology-Disturbance Factors:
Individual whitebark pine trees are very slow growing, and may be up to1500 years old (Millar 2014). Stands are composed of multiple age-class single and multiple stem trees because of ongoing seedling establishment. Caching of whitebark pine seeds by Clark’s nutcracker is the primary mode of seed dispersal, with birds often caching 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). In the absence of disturbance, ongoing recruitment from seed-caches occurs, leading to an increase in stand density over time.

Fire and avalanche are the primary natural drivers for succession. 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.

Whitebark pine forests are threatened by the non-native Cronartium ribicola, the cause of white pine blister rust (WPBR) and the native mountain pine beetle (Dendroctonus ponderosae) (Cox 2000, Tomback et al. 2001b, Howard 2002). Severe epidemics of WPBR in combination with MPB outbreaks 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 MPB outbreaks, WPBR infection, and climate. Mountain pine beetles prefer larger diameter trees (> 6 inch diameter at breast height), as these are necessary to complete their life cycle,, and attack at the warmer, lower elevation zone of whitebark pine. Mountain pine beetles preferentially attack trees infected by WPBR. White pine blister rust will infest all whitebark pines, regardless of age or elevation (Cluck 2014).

Mountain pine beetles are a native species in North American forests, but 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). Severe mountain pine beetle epidemics cause high mortality of overstory trees, while suppressed understory trees may be released (Meyer and Safford 2014). A flush of regeneration may occur due to the reduction in the overstory canopy providing new areas for establishment. However, the decline in seed production due to the loss of large overstory trees will leave fewer seeds to be consumed by Clark’s nutcracker and other animals which leaves fewer seeds available for regeneration, threatening stand sustainability.

The non-native WPBR was introduced into North America near Vancouver, British Columbia in approximately 1910, and has been slowly spreading across the western United States and Canada. It currently occurs throughout the Cascades, and north and central Sierra Nevada. So far, 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 in a high Sierra location on the western slope of the Sierra National Forest (Maloney 2011). A survey was conducted in 2009 to determine WPBR presence and effect on whitebark pine survivorship in the Lake Tahoe Basin. Mean incidence of WPBR among whitebark pine populations was 35 percent, with a range of 1 to 65 percent (Maloney et al. 2012).

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 WPBR entry (Maloney 2011). The basidiospores, which infect whitebark pine, are released in fall from the alternate host currants (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, and years with late summer or early fall precipitation are most likely when infection will occur. Whitebark pine may have early onset winter dormancy, so stomates are closed at the time WPBR basidiospores are released (Maloney 2011). The onset of winter dormancy is dependent upon the length of the growing season (temperature), precipitation and soil available water capacity (AWC).

There appears to be a relationship between soils with higher AWC and higher infection rates or intensity of stem girdling (Maloney et al. 2012). Higher soil moisture could increase WPBR mycelium growth rates and increase basidiospore production, while also allowing for whitebark pine stomates to remain open longer in the season, increasing the probability of infection (Maloney et al. 2012). This ecological site occurs on shallow to moderately deep sandy-skeletal soils, with lower AWC than the corresponding volcanic ecological site (R022AC200CA), and is likely less susceptible to WPBR infestation. A 2009 inventory of WPBR showed that the whitebark stands occurring on granitic soils had infestation rates ranging from 1 to 19%, while stands on volcanic soils ranged from -- to 65% (Maloney et al. 2012).

The main impact of WPBR on whitebark pine is reduction in stand cone production due to die-back of cone bearing branches from cankers girdling the branches. Mortality rates in older trees are low, and may take decades to occur. Younger trees may be killed quickly if main stem girdling causes disruption of water flow (Maloney et al. 2012). A few studies have been conducted on genetic resistance to WPBR, and results range from no resistance (Maloney, personal communication), to 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 Clark’s nutcracker, and thus the number and distribution of seed caches (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 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 broader 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 local 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.

High elevation areas with suitable soils and landforms for the upward migration of whitebark pine will be important for the sustainability of this community. However, in this region of the central Sierra Nevada, whitebark pine already occurs at the upper most elevations of the highest mountains in the area, so has little room to move upslope. The southern Sierra Nevada, with its higher mountain peaks, may prove to be an important refugium for this species.

The historic temperature range for this ecological site is between 34 to 37 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 become dominant in 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.