Rocky Mountain Limber and Bristlecone Pine Woodland

This ecological system occurs throughout the Rocky Mountains, south of Montana, on dry, rocky ridges and slopes near upper treeline above the matrix spruce-fir forest. It extends down to the lower montane in the northeastern Great Basin mountains where dominated by Pinus flexilis. Sites are harsh, exposed to desiccating winds, with rocky substrates and a short growing season that limit plant growth. Higher-elevation occurrences are found well into the subalpine-alpine transition on wind-blasted, mostly west-facing slopes and exposed ridges. Calcareous substrates are important for Pinus flexilis-dominated communities in the northern Rocky Mountains and possibly elsewhere. The open tree canopy is often patchy and is strongly dominated by Pinus flexilis or Pinus aristata with the latter restricted to southern Colorado, northern New Mexico and the San Francisco Mountains in Arizona. In the Wyoming Rockies and northern Great Basin, Pinus albicaulis is found in some occurrences, but is a minor component. Other trees such as Juniperus spp., Pinus contorta, Pinus ponderosa, or Pseudotsuga menziesii are occasionally present. Arctostaphylos uva-ursi, Cercocarpus ledifolius, Juniperus communis, Mahonia repens, Purshia tridentata, Ribes montigenum, or Vaccinium spp. may form an open shrub layer in some stands. The herbaceous layer, if present, is generally sparse and composed of xeric graminoids, such as Calamagrostis purpurascens, Festuca arizonica, Festuca idahoensis, Festuca thurberi, or Pseudoroegneria spicata, or more alpine plants.

The open tree canopy is often patchy and is strongly dominated by Pinus flexilis or Pinus aristata with the latter restricted to southern Colorado, northern New Mexico and the San Francisco Mountains in Arizona. In the Wyoming Rockies and northern Great Basin, Pinus albicaulis is found in some occurrences, but is a minor component. Other trees such as Juniperus spp., Pinus contorta, Pinus ponderosa, or Pseudotsuga menziesii are occasionally present. Arctostaphylos uva-ursi, Cercocarpus ledifolius, Juniperus communis, Mahonia repens, Purshia tridentata, Ribes montigenum, or Vaccinium spp. may form an open shrub layer in some stands. The herbaceous layer, if present, is generally sparse and composed of xeric graminoids, such as Calamagrostis purpurascens, Festuca arizonica, Festuca idahoensis, Festuca thurberi, or Pseudoroegneria spicata, or more alpine plants.

This system is found throughout the Rocky Mountains, south of Montana, on dry, rocky ridges and slopes near upper treeline above the matrix spruce-fir forest. It extends down to the lower montane in the northeastern Great Basin mountains. Sites are harsh, exposed to desiccating winds with rocky substrates and a short growing season that limit plant growth. Higher elevation occurrences are found well into the subalpine - alpine transition on wind-blasted, mostly south- to west-facing slopes and exposed ridges. Pinus aristata forests are typically found on steep, south-facing slopes from 2700 to 3700 m (8850-12,140 feet) elevation. Pinus flexilis woodlands occupy similar habitats, but may occur at lower elevations than Pinus aristata. When found in the same landscape, stands dominated by Pinus aristata occur at higher elevation.

Both Pinus flexilis and Pinus aristata are short-statured, slow-growing, long-lived species in which individuals may live for 1000 or more years in fire-protected areas. They are adapted to cold, drought and extremely windy conditions with flexible branches that likely reduce wind damage. Fire is an important source of disturbance that facilitates stand regeneration in this system. Older woodlands are often broadly even-aged stands where seedlings are nearly absent, while areas that have recently burned may have abundant seedlings. Bristlecone pine is somewhat more tolerant of fire than is limber pine; however, both species appear to depend on fire for regeneration. Post-fire regeneration of bristlecone pine tends to be near burn edges and/or under surviving trees (Coop and Schoettle 2011). Regeneration of limber pine on burned areas is largely due to the germination of seeds cached primarily by Clark's nutcrackers (Nucifraga columbiana) and jays (i.e., corvid family), but also small mammals such as squirrels (Lanner and Vander Wall 1980, Tomback 2001, Lanner 2007, CNHP 2010b). Dispersal of the smaller winged seeds of bristlecone pine is primarily by wind, but seeds are likely to also be dispersed by birds (Coop and Schoettle 2011).

Fire occurrence in this ecosystem is low frequency and mixed severity. In the absence of wind, fires are likely limited in extent (two acres or less). Understories are often sparse, with little to carry fires across the surface (Landfire 2007a). Stand-replacement fires are usually wind-driven, especially in mid- and late-serial classes. Landfire (2007a) review estimated replacement fires occurring between 35-100+ years and 200+ years (Fire Regime Groups IV and V) with surface fires occurring every 1000 years. However, in northern New Mexico, some open stands transition into subalpine grasslands and have more frequent, less severe fires (Coop and Schoettle 2011).

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, some stands are converted to non-tree-dominated vegetation with loss of Pinus longaeva and Pinus flexilis from non-native, white pine blister rust (Cronartium ribicola) or epidemics of mountain pine beetle (Dendroctonus ponderosae).

Common stressors and threats include altered fire regime from fire suppression, fragmentation, extended drought which may make individuals more susceptible to mortality from non-native white pine blister rust or epidemics of native mountain pine beetle, and invasive non-native plant species (Schoettle and Sniezko 2007a, CNHP 2010b, Coop and Schoettle 2011). Threats to their seed dispersers, such as Clark's nutcracker (Nucifraga columbiana), are threats to the regeneration of limber pine and the ecosystem. Additionally, other plant species, such as Pseudotsuga menziesii and Ribes cereum (Baumeister and Callaway 2006), benefit from the presence of Pinus flexilis, which, if negatively influenced by threats or stress, could impact the entire ecosystem.

Potential climate change effects could include a change in the current extent of the ecosystem with higher tree mortality and lower recruitment if less moisture occurs with increasing mean temperature (Gibson et al. 2008, TNC 2013). Because Pinus aristata occurs within a narrow elevational gradient, its ability to adapt to changing climate might be limited (Gibson et al. 2008). McKinney et al. (2007) suggest Pinus flexilis shift will increase in areas with climate change. However, the influence of indirect stresses such as white pine blister rust and increased abundance of mountain pine beetle epidemics with warming climate (Schoettle and Sniezko 2007a, Schoettle et al. 2008) is unknown.

This system occurs throughout the Rocky Mountains south of Montana on dry, rocky ridges and slopes near upper treeline, including the Uinta and northern Wasatch mountains, and the Jarbridge Mountains in northeastern Nevada. It also occurs farther east, in the Bighorn Range of north-central Wyoming, although it is not common there.