Great Basin Subalpine Bristlecone Pine Woodland

This ecological system extends from the Mojave Desert and Sierra Nevada across the central Great Basin to the central Wasatch and western Uinta mountains. These open woodlands are typically found on high-elevation ridges and rocky slopes above subalpine forests and woodlands. Site are harsh, exposed to desiccating winds with rocky substrates and a short growing season that limit plant growth. Parent materials include dolomitic, limestone or granitic rocks. Occurrences can be found on all aspects but are more common on southwestern exposures on steep convex slopes and ridges between 2530 and 3600 m (8300-12,000 feet). Stands are strongly dominated by Pinus flexilis and/or Pinus longaeva. Pinus monophylla may be present in lower-elevation stands. If present, shrub and herbaceous layers are generally sparse and composed of xeric shrubs, graminoids and cushion plants. Associated species may include Antennaria rosea, Arenaria kingii, Artemisia tridentata, Cercocarpus intricatus, Chamaebatiaria millefolium, Cymopterus cinerarius, Elymus elymoides, Erigeron pygmaeus, Eriogonum ovalifolium, Festuca brachyphylla, Koeleria macrantha, Linanthus pungens, Ribes cereum, or Ribes montigenum.

Vegetation is characterized by a typically open tree canopy (<25% cover) with heights ranging from 1-2 m (krummholz) to over 10 m. Pinus flexilis and/or Pinus longaeva dominate the tree canopy, alone or in combination. Pinus longaeva stands tend to occur at higher elevation with less mixed canopies. Other trees present to codominant include Picea engelmannii, Pseudotsuga menziesii, Populus tremuloides, or Abies concolor. In the Sierra Nevada stands, Pinus albicaulis, Pinus balfouriana, and/or Pinus contorta var. murrayana may be present. Understory layers, if present, are sparse to moderately dense and composed of xeric shrubs, graminoids and cushion plants. Characteristic shrubs include Arctostaphylos patula, Artemisia arbuscula, Artemisia tridentata ssp. vaseyana, Ericameria discoidea, Juniperus communis, Mahonia repens, Ribes cereum, and Ribes montigenum. Cercocarpus intricatus, Cercocarpus ledifolius, or Chrysolepis sempervirens frequently occur in stands in the Sierra Nevada. The herbaceous layer is typically sparse. Associated herbaceous species are diverse given the wide elevational range, with alpine species occurring near the upper treeline and montane and subalpine species below. Associated species may include Antennaria rosea, Aquilegia scopulorum, Arabis drummondii, Arenaria congesta, Arenaria kingii, Astragalus kentrophyta, Astragalus platytropis, Calamagrostis rubescens, Carex rossii, Cirsium eatonii, Cymopterus cinerarius, Cymopterus nivalis, Elymus elymoides, Eriogonum gracilipes, Eriogonum holmgrenii, Eriogonum ovalifolium, Erigeron pygmaeus, Erigeron tener, Festuca brachyphylla, Koeleria macrantha, Linanthus pungens (= Leptodactylon pungens), Packera werneriifolia, Penstemon leiophyllus, Poa fendleriana, Phlox pulvinata, Trifolium gymnocarpon, and Trisetum spicatum. Selaginella watsonii is common in some high-elevation stands. The vegetation description is based on several references, including Graybosch and Buchanan (1983), Lanner (1983), Holland (1986b), Holland and Keil (1995), Nachlinger and Reese (1996), Reid et al. (1999), Fryer (2004), Thorne et al. (2007), and Sawyer et al. (2009).

This ecological system extends from the Mojave Desert and eastern Sierra Nevada across the central Great Basin to the central Wasatch and western Uinta mountains. These open woodlands are typically found on high-elevation ridges and rocky slopes above subalpine forests and woodlands, sometimes extending down into the montane zone. Sites are harsh, exposed to desiccating winds, with rocky substrates and a short growing season that limit plant growth. Occurrences can be found on all aspects but are more common on southwestern exposures on steep convex slopes and ridges between 2530 and 3600 m (8300-12,000 feet) elevation. Most sites are droughty, with gravel in the shallow subsurface horizons. Surface textures vary depending upon substrate, which are best represented on colluvium derived from limestone and dolomite or Tertiary and Cretaceous sandstone parent materials. Steep slopes, high-intensity summer convection storms, and only partial ground cover for interception often result in severe sheet erosion of fine particles. This usually leads to the development of gravel pavements. Additional erosion can be expected from wind action. High insolation and wind during the winter usually result in reduced snowpack accumulations. However, soils can be expected to freeze. The sparsity of shrubs, forbs, grasses, and litter, in addition to the widely spaced trees, usually means that fire does not carry easily. Individual trees may be ignited from lightning, but seldom is an entire occurrence burned. The environmental description is based on several other references, including Graybosch and Buchanan (1983), Lanner (1983), Holland (1986b), Holland and Keil (1995), Nachlinger and Reese (1996), Reid et al. (1999), Fryer (2004), Thorne et al. (2007), and Sawyer et al. (2009).

Both Pinus longaeva and Pinus flexilis are slow-growing, long-lived trees that are intolerant of shade. Pinus longaeva may attain nearly 4900 years in age and 12 m in height, whereas Pinus flexilis may live 1000 years and attain 18 m in height. Bristlecone pine branches retain needles for as long as 30 years, whereas limber pine needles are lost after only several years. Bristlecone pine trees produce dense, resinous wood that is resistant to rot and disease. Mature trees have massive, contorted trunks with mostly dead and gnarled wood (Sawyer et al. 2009). Tree-ring data over the last 4000 years indicate that droughts of 200 years or more have occurred.

Natural regeneration of both species appears to be closely associated with caching of the large wingless seeds, primarily by Clark's nutcracker (Nucifraga columbiana) (Lanner and Vander Wall 1980). Germination of cached seeds often results in the multi-stemmed clumps characteristic of these sites, although the species may produce multiple stems from boles damaged near the ground. Germination and rooting will sometimes be restricted to crevices in rock. Pinus longaeva has smaller winged seeds and should be wind-disseminated. However, caching by nutcrackers does take place, especially when other Pinus species are also available (Dr. R. Lanner pers. comm.). The longevity of individuals enables stands to persist for centuries between times of favorable seedling establishment (Keeley and Zedler 1998). Stands are subject to long, intense droughts.

These pines have relatively thin bark adapted to survive only low-severity surface fires. However, fires seldom destroy stands due to the sparse nature of the canopy cover of trees and abundant bare ground. When fire occurs on high-elevation sites, they are usually small, low-severity surface fires (Bradley et al. 1992).

Pinus longaeva and Pinus flexilis are both experiencing mountain pine beetle (Dendroctonus ponderosae) infestations throughout much of their ranges (Lanner 1983). Logan and Powell (2001) provide information on the ecology and management of mountain pine beetles in high-elevation ecosystems. Gibson et al. (2008) reported recently detected mortality of Pinus longaeva in the Great Basin, including 100 acres in 2005, 60 acres in 2006, and 300 acres in 2007, all within the Snake Range in east-central Nevada (aerial detection surveys). Western dwarf mistletoe (Arceuthobium campylopodum) infests Great Basin bristlecone pines in southern Nevada and Utah (Mathiasen and Hawksworth 1990).

Both pine species are five-needle white pines and are also susceptible to the exotic fungus white pine blister rust (Cronartium ribicola). The arid climate and isolated stands appear to have protected most Pinus longaeva and Pinus flexilis from infection in the Great Basin, although an incidental level of the infection was found in the Wasatch Mountains of Utah (Fryer 2004). There is potential for blister rust to spread into arid zones, especially during wet years. This system occurs in the White and Inyo mountains, which lie close to moderately high infestation centers in the Sierra Nevada, and may be at greatest risk for blister rust infestation and spread (Smith and Hoffman 2000).

Pinus longaeva populations are sensitive to fluctuations in climate. Low seedling establishments were documented in eastern Nevada populations during cool, dry periods approximately 900 and 2500-3000 BP (Hiebert and Hamrick 1984b). Effects of current climatic conditions on Great Basin bristlecone pine regeneration are uncertain. Regeneration is generally sparse, and there is concern that climate warming is hindering Great Basin bristlecone pine regeneration on sites in the interior Great Basin (Lanner 1983).

In most high-elevation five-needle pine stands throughout the West, populations of mountain bark beetles have increased dramatically since the late 1990s, and it is anticipated that populations will remain high as long as weather conditions are conducive to beetle survival and/or until most mature host trees have been killed (Gibson et al. 2008). The bark beetles are the most serious short-term threat, but the most serious long-term threat is white pine blister rust, which affects all aspects of the forest regeneration process and will impair ecosystem recovery long after the current beetle epidemic is over (Schoettle and Sniezko 2007a). Logan and Powell (2005) report range expansion into high-elevation, five-needle pines stands and loss of biodiversity.

Conversion of this type has commonly come from mining activities and other very localized removal of stands for various kinds of development, but conversion is not a major factor for this system. However, with loss of Pinus longaeva and Pinus flexilis trees from non-native white pine blister rust (Cronartium ribicola) or epidemics of mountain pine beetle (Dendroctonus ponderosae), some stands may be converted to non-tree-dominated vegetation or stands dominated by other tree species. Where infected, white pine blister rust will likely prevent successful stand regeneration.

Common stressors and threats include altered fire regime from fire suppression, fragmentation, and extended drought which may make individuals more susceptible to mortality from non-native white pine blister rust or epidemics of native mountain pine beetle (Dendroctonus ponderosae), and invasive non-native plant species. Pinus longaeva and Pinus flexilis are both experiencing mountain pine beetle infestations throughout much of their ranges (Lanner 1983). Pinus flexilis and possibly Pinus longaeva are dependent on animals for longer distance dispersal. Threats to these dispersers such as Clark's nutcracker are threats to the regeneration of these pines and the ecosystem in areas with high tree mortality.

Potential climate change effects could include a change in the current extent of the ecosystem with higher tree mortality and lower recruitment if climate change has the predicted effect of less effective moisture with increasing mean temperature (TNC 2013). McKinney et al. (2007) suggest limber pine will increase in area with climate change, but do not consider indirect stresses such as white pine blister rust and increased abundance of mountain pine beetle epidemics with warming climate (Schoettle et al. 2008).

Human development has impacted many locations throughout the ecoregion. High- and low-density urban and industrial developments also have large impacts. For example, residential development has significantly impacted locations within commuting distance to urban areas. Impacts may be direct as vegetation is removed for building sites or more indirectly through natural fire regime alteration, and/or the introduction of invasive species. Mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species.

This system extends from the Mojave Desert and Sierra Nevada across the Great Basin to the central Wasatch and extreme western Uinta mountains.