Intermountain Mountain Mahogany Woodland

This ecological system occurs in hills and mountain ranges of the Intermountain West basins from the eastern foothills of the Sierra Nevada northeast to the foothills of the Bighorn Mountains. It typically occurs from 600 m to over 2650 m in elevation on rocky outcrops or escarpments and forms small- to large-patch stands in forested areas. Most stands occur as shrublands on ridges and steep rimrock slopes, but they may be composed of small trees in steppe areas. Scattered junipers or pines may also occur. This system includes both woodlands and shrublands dominated by Cercocarpus ledifolius. Artemisia tridentata ssp. vaseyana, Purshia tridentata, with species of Arctostaphylos, Ribes, or Symphoricarpos are often present. Undergrowth is often very sparse and dominated by bunchgrasses, usually Pseudoroegneria spicata and Festuca idahoensis. Cercocarpus ledifolius is a slow-growing, drought-tolerant species that generally does not resprout after burning and needs the protection from fire that rocky sites provide.

This system is includes both short and tall shrublands and short woodlands dominated by Cercocarpus ledifolius. Some stands occur as scattered shrub communities in steppe or on rocky outcrops or steep escarpments within forests and woodlands, especially on upper slopes and ridges. The woodlands occur mostly in the eastern Sierra Nevada and ranges in the Great Basin. Common shrub associates are Artemisia tridentata and Purshia tridentata, with species of Amelanchier, Arctostaphylos, Holodiscus, Prunus, Ribes, and Symphoricarpos commonly present. Scattered trees may also be present, including Pinus monophylla, Juniperus spp., Pinus ponderosa, Pinus flexilis, Pinus jeffreyi, Pseudotsuga menziesii, or Abies concolor. Undergrowth is often very sparse and dominated by bunchgrasses, usually Achnatherum hymenoides (= Oryzopsis hymenoides), Achnatherum occidentale (= Stipa occidentalis), Hesperostipa spp., Poa fendleriana, Poa secunda, Pseudoroegneria spicata, and Festuca idahoensis, and at higher elevations Calamagrostis rubescens and Festuca idahoensis.

Cercocarpus ledifolius woodlands and shrublands are poorly distinguished in the literature, as most authors describe the species as having either a tall-shrub or small-tree growth form within a single association. Some associations may have shrub-dominated stands in one area and also have a woodland physiognomy in another. The woodland physiognomy appears to be more typical, based on available literature. Near the northern edge of its range in Montana and Idaho, Cercocarpus ledifolius is described as occurring primarily in the shrub form (Mueggler and Stewart 1980, Tisdale 1986). These northern variants are the only described stands which appear to be clearly distinct from the woodland alliance.

The woodland stands may be dominated by different varieties of Cercocarpus ledifolius than shrubland stands. In Wyoming, the Natural Heritage Program is proposing to recognize two Cercocarpus ledifolius alliances, based upon varieties of Cercocarpus ledifolius. The most widespread proposed alliance (in Wyoming) is dominated by Cercocarpus ledifolius var. ledifolius, which grows up to about 1.5 m tall. The other proposed alliance, dominated by Cercocarpus ledifolius var. intercedens, is found only along the western border of the state, and the growth form is as small trees 4-5 m tall. The two taxa are obviously different in Wyoming, in stature and leaf characteristics, and are easily separated (Reid et al. 1998). The shorter variety, Cercocarpus ledifolius var. ledifolius, is not reported from Nevada or California (USDA NRCS 2011).

This ecological system is widespread in semi-arid hills and mountain ranges of the intermountain western U.S. from the eastern foothills of the Sierra Nevada and Cascade Range east into the Rocky Mountains including the foothills of the Bighorn Mountains. It also occurs south into the Mojave Desert and the Grand Canyon in northern Arizona. Stands mostly occur below montane conifer forests and above desert scrub from 1500 to 3200 m in elevation, extending down to 600 m in the north (Gucker 2006c). Higher-elevation stands typically occur on warmer and drier southerly slopes. Annual precipitation averages 25-45 cm, with a significant proportion falling as winter snow. Sites typically have shallow to deep, well-drained, often rocky, nutrient-poor, sandy loam soils frequently derived primarily from carbonate sediments (limestone or dolomite) or on sandstones rich in calcium carbonate (Reid et al. 1999). Other rock types include quartz, gneiss, and basalt.

Cercocarpus ledifolius is a slow-growing, drought-tolerant species which can inhabit very poor sites, such as cliffs, stony slopes, and outcrops. Stands are often small and clumped near ridgetops. These sites may also afford the species some protection from fire as the oldest individuals have been observed in these stands (Ross 1999). Succession in these stands is variable depending on site conditions and disturbance as Cercocarpus ledifolius is both a primary early-successional colonizer that rapidly invades bare mineral soils after disturbance and the dominant long-lived species in mid- and late-seral stands (Duncan 1975, Gruell et al. 1985). Shade tolerance is low so higher-elevation stands on sites where conifers can grow will eventually be overtopped by taller conifer trees forming woodlands with a Cercocarpus ledifolius subcanopy or shrub layer until replaced by more shade-tolerant shrubs such as Physocarpus malvaceus or Acer glabrum (Gruell et al. 1985, Steele and Geier-Hayes 1995).

Mature Cercocarpus ledifolius have thick bark and may survive "light" fires (Schultz 1987). However, more often they are killed by fire, and regeneration is by seedling establishment as sprouts following fire are rare and short-lived (Gruell et al. 1985, Gucker 2006c). Range expansion of this system in the last century has been attributed to decreased fire frequency (Gruell 1982, Gruell et al. 1994). From 1750 to the early 1900s, a mean fire-return interval was between 13 and 22 years, and stands were likely restricted to rocky sites where fuel levels were low. Since 1900 the fire-return interval has increased substantially because of fine fuel reductions with heavy livestock grazing, fire exclusion practices, and/or decreased human-caused fires (Arno and Wilson 1986). However, in the Petersen Mountains of western Nevada, the extent of curl-leaf mountain-mahogany has "decreased dramatically" from 1954 to 1997 as a result of increased fire incidence linked to increased cheatgrass dominance (Ross 1999).

Cercocarpus ledifolius is highly favored by native ungulates for winter range. Excessive browsing by deer and other wildlife has "high-lined" individual shrubs and reduced regeneration (West and Young 2000). Seeds are consumed by a variety of small mammals (Plummer et al. 1968). Mortality from bark damage (drilling) by red-breasted sapsuckers has been reported from Bald Mountain near the California-Nevada border (Ross 1999).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 1210620). These are summarized as:

A) Early Development 1 All Structures (10% of type in this stage): Curl-leaf mountain-mahogany rapidly invades bare mineral soils after fire. Litter and shading by woody plants inhibits establishment. Bunchgrasses and disturbance-tolerant forbs and resprouting shrubs, such as snowberry, may be present. Rabbitbrush and sagebrush seedlings are present. Vegetation composition will affect fire behavior, especially if chaparral species are present. Replacement fire (average FRI of 500 years), mixed-severity fire (average FRI of 100 years) and native herbivory of seedlings (2 out every 100) all affect this class. Replacement fire and native herbivory will reset the ecological clock to zero. Mixed-severity fire does not affect successional age. Succession to class C after 20 years.

B) Mid Development 1 Closed (10% of type in this stage): Young curl-leaf mountain-mahogany are common, although shrub diversity is very high. One out of every 1000 mountain-mahogany are taken by herbivores but this has no effect on model dynamics. Replacement fire (mean FRI of 150 years) causes a transition to class A. Mixed-severity fire can result in either maintenance (mean FRI of 80 years) in the class or a transition to class D (mean FRI of 200 years). Succession to class E after 90 years.

C) Mid Development 1 Open (15% of type in this stage): Curl-leaf mountain-mahogany may codominate with mature sagebrush, bitterbrush, snowberry and rabbitbrush. Few mountain-mahogany seedlings are present. Replacement fire (mean FRI is 150 years) will cause a transition to class A, whereas mixed-severity fire (mean FRI of 50 years) will thin this class but not cause a transition to another class. Native herbivory of seedlings and young saplings occurs at a rate of 1/100 seedlings but does not cause an ecological setback or transition. Succession to class B after 40 years.

D) Late Development 1 Open (20% of type in this stage): Moderate cover of mountain-mahogany. This class represents a combined Mid2-Open and Late1-Open cover and structure combination resulting from mixed-severity fire in class C (note: the combined class results in a slightly inflated representation in the landscape). Further, this class describes one of two late-successional endpoints for curl-leaf mountain-mahogany that is maintained by surface fire (mean FRI of 50 years). Evidence of infrequent fire scars on older trees and presence of open savanna-like woodlands with herbaceous-dominated understory are evidence for this condition. Other shrub species may be abundant, but decadent. In the absence of fire for 150 years (2-3 FRIs for mixed-severity and surface fires), the stand will become closed (transition to class E) and not support a herbaceous understory. Stand-replacement fire every 300 years on average will cause a transition to class A. Class D maintains itself with infrequent surface fire and trees reaching very old age.

E) Late Development 1 Closed (45% of type in this stage): High cover of large shrub or tree-like mountain-mahogany. Very few other shrubs are present and herb cover is low. Duff may be very deep. Scattered trees may occur in this class. This class describes one of two late-successional endpoints for curl-leaf mountain-mahogany. Replacement fire every 500 years on average is the only disturbance and causes a transition to class A. Class will become old-growth with trees reported to reach 1000+ years.

Curl-leaf mountain-mahogany is easily killed by fire and does not resprout (Marshall 1995b, Gucker 2006c). It is a primary early succession colonizer rapidly invading bare mineral soils after disturbance. Fires are not common in early-seral stages, when there is little fuel, except in chaparral. Replacement fires (mean FRI of 150-500 years) become more common in mid-seral stands, where herbs and smaller shrubs provide ladder fuels. By late succession, two classes and fire regimes are possible depending on the history of mixed-severity and surface fires. In the presence of surface fire (FRI of 50 years) and past mixed-severity fires in younger classes, the stand will adopt a savanna-like woodland structure with a grassy understory, spiny phlox and currant. Trees can become very old and will rarely show fire scars. In late, closed stands, the absence of herbs and small forbs makes replacement fires uncommon (FRI of 500 years), requiring extreme winds and drought. In such cases, thick duff provides fuel for more intense fires. Mixed-severity fires (mean FRI of 50-200 years) are present in all classes, except the late-closed one, and more frequent in the mid-development classes (LANDFIRE 2007a, BpS 1210620).

Ungulate herbivory: Heavy browsing by native medium-sized and large mammals reduces mountain-mahogany productivity and reproduction (Marshall 1995b, Gucker 2006c). This is an important disturbance in early- and mid-seral stages, when mountain-mahogany seedlings are becoming established. Browsing by small mammals has been documented (Marshall 1995b, Gucker 2006c), but is relatively unimportant and was incorporated as a minor component of native herbivory mortality.

Avian-caused mortality: In western Nevada, for ranges in close proximity to the Sierra Nevada, sapsucker's drilling of young curl-leaf mountain-mahogany has been observed to cause stand-replacement mortality (C. Ross, NV BLM, pers. comm. 2018). Windthrow and snow creep on steep slopes are also sources of mortality.

Cercocarpus ledifolius browse may have limited livestock use including domestic goats, sheep, or cattle in spring, fall, and/or winter but rarely in the summer (Gucker 2006c). Stands often occur on steep rocky slopes, but open shrubland or open woodland stands with grassy understory could provide significant livestock forage.

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.

Curl-leaf mountain-mahogany seedlings appear to be sensitive to drought, frost, and competition from exotic vegetation, especially Bromus tectorum (Plummer et al. 1968, Shaw et al. 2004, Gucker 2006c). High seedling mortality can also result from heavy browsing by wildlife and mature shrubs can be heavily pruned and suppressed as well (Gucker 2006c).

Fire suppression and exclusion have facilitated an increase in abundance of this system in the Intermountain West (Gruell et al. 1994, Gucker 2006c). However, increased fire frequency and severity from excessive fine-fuel buildup due to cheatgrass invasion may negatively impact some stands.

This system occurs in hills and mountain ranges of the Intermountain West basins from the eastern foothills of the Sierra Nevada northeast to the foothills of the Bighorn Mountains.