Ecological dynamics

Abiotic Factors:
This ecological site occurs on glacially scoured valley walls and basins. The soils are typically shallow to moderately deep in linear bedrock fissures that support whitebark pine. Soils are typically non-skeletal with sandy loam textures and low organic matter accumulation. However, some fissures are filled with a deep layer of poorly decomposed pine needles. Whitebark pine grows as a multi-stemmed tree or in dense clusters of individual trees. Canopy height ranges from 6 to 25 feet tall. At upper elevations and in exposed positions trees develop a compact krummholz form. Mountain pride, Ross' sedge (Carex rossii), and Parry's rush are common in these fissures and small soil pockets. There is a diversity of microhabitats in these bedrock pockets, due to water accumulation or run off, degree of shading due to bedrock forms, and soil characteristics. A wide variety of alpine and subalpine forbs and grasses can be found among the bedrock outcrops.

Ecology-Disturbance Factors:

In this ecological site whitebark pine grows in linear bedrock fissures on glacially scoured granite bedrock outcrops. These trees are typically multi-stemmed, and canopy cover is limited due to the high percentage of bedrock. In areas exposed to wind, cold air drainage, or avalanche, whitebark pine is maintained in a krummholz form by wind and ice pruning of branches exposed above the snow pack. Krummholz trees may be greater than 1,500 years old (Miller 2014), but are typically less than 800 years, and continuously develop seedlings from seed caches. They may attain these older ages by layering of branches, which allows for new stem growth while older branches die back. Caching of whitebark pine seeds by Clark’s nutcracker (Nucifraga columbiana) 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 caches in the absence of disturbance, resulting in an increase in stand density over time.

Fire and avalanche are the primary natural drivers for succession in this ecological site, which is relatively stable. Fire ignition is frequent on these exposed high elevation bedrock outcrops, 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). Fire studies specific for the Sierra Nevada in whitebark pine forests are lacking. However, studies in whitebark pine forest across the US, show a mean fire return interval that ranges from 29 to 300 years, while moderate severity fires range from 25 to 75 years, and stand replacing fires have greater than 140 year return interval (Fryer 2002).

Avalanche is common among the alpine peaks and ridges, and can remove swaths of whitebark pine in avalanche prone chutes or below wind formed cornices. Areas prone to frequent avalanche are not stable long enough to support whitebark pine, but areas prone to less frequent avalanches or rare large avalanches will develop whitebark pine over time.

Whitebark pine is 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 epidemics have not been as severe or as extensive in the Sierra Nevada. 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 Sierra Nevada Mountains in 1960’s. White pine blister rust is present in the northern Sierra Nevada and has been found at a few locations in the central Sierra Nevada, including Yosemite National Park and at a high Sierra location on the western slopes of the Sierra National Forest (Maloney 2011). It has not been detected on whitebark pine in the most southern Sierra Nevada including Sequoia and Kings Canyon National Park.

White pine blister rust could potentially infect whitebark pine in the southern Sierras, but synchronous phenological and environmental factors need to occur for WPBR to infect whitebark pine. Within the dry and high southern Sierra Nevada, this may be less common than at lower elevations and more northern latitudes (Maloney 2011). 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 typically be closed at the time WPBR basidiospores are released (Maloney 2011). The onset of winter dormancy for whitebark pine is dependent upon the length of growing season, precipitation and soil water holding capacity. This ecological site occurs on medium textured soils on shallow to moderately deep bedrock fissures in southern latitudes with lower precipitation than the northern latitudes. Due to the drier climate, these whitebark stands may be less prone to white pine blister rust.

A state altered by WPBR has been included in this state-and-transition model because of the documented incidences of WPBR in the central Sierra Nevada, and the slight potential that it may infect the southern Sierra Nevada. In other regions of whitebark pine forest, there is a potential to transition to a third state, where WPBR has cause a decline in cone production of less than 1000 cones/ Ha and basal area is < .5 m2/ acre. Below this threshold there may be insufficient seeds for Clark’s nutcracker and the regeneration of whitebark pine (McKinney et al. 2009).

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, Vogler et al. 2012). A few studies have been conducted on genetic resistance, and results vary from no resistance (Maloney, personal discussion), and 26 to 47 percent in the Rocky Mountains and the Pacific Northwest (Keane, Tomback et al. 2012).

Mountain pine beetles (MPB) preferentially attacks whitebark pine trees that are heavily infected with WPBR. They also appear to prefer whitebark pine over lodgepole pine, and trees with diameters over 6” dbh. Mountain pine beetles targets larger diameter trees at lower elevations but WPBR infected trees at all elevations and ages (Cluck 2014).

Reduced seed production affects the presence and abundance of Clark’s nutcracker, and in turn 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 due to global warming suggest that the whitebark pine communities in the Sierra Nevada Mountains may be threatened by rising temperatures and precipitation changes, primarily due to secondary impacts as pathogens and lower elevation conifer species extend their ranges to higher elevations. 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 Celsius), 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 effect 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, with their limbs exposed to air currents. 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 ecological site, is predicted to be locally extinct in the White Mountains with 3 degrees of warming (Van de Ven et al. 2007).

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 rates 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) 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.

In the state-and-transition model below, the reference community within 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.

The potential for crossing the threshold for State 2 and State 3 is low to very low, but this ecological site may extend into the central Sierra Nevada where whitebark pine bister rust has been documented, and therefore may potentially affect this ecosystem. Potential shifts due to warming temperatures are not included in this state-and-transition model.

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.