Ecological dynamics

It is impossible to determine in any quantitative detail the Historic Climax Plant Community (HCPC) for this ecological site because of the lack of direct historical documentation preceding all human influence. In some areas, the earliest reports of dominant plants include the cadastral survey conducted by the General Land Office, which began in the late 19th century for this area. However, up to the 1870s the Shoshone Indians, prevalent in northern Utah and neighboring states, grazed horses and set fires to alter the vegetation for their needs. In the 1860s, Europeans brought cattle and horses to the area, grazing large numbers of them on unfenced parcels year-long. Itinerant and local sheep flocks followed, largely replacing cattle as the browse component increased.

Below is a State and Transition Model diagram that illustrates the “phases” (common plant communities), and “states” (aggregations of those plant communities) that can occur on the site. Differences between phases and states depend primarily upon observations of a range of disturbance histories in areas where this ESD is represented. These situations include grazing gradients to water sources, fence-line contrasts, patches with differing dates of fire, herbicide treatment, tillage, etc. Reference State 1 illustrates the common plant communities that probably existed just prior to European settlement.

The major successional pathways within states, (“community pathways”) are indicated by arrows between phases. “Transitions” are indicated by arrows between states. The drivers of these changes are indicated in codes decipherable by referring to the legend at the bottom of the page and by reading the detailed narratives that follow the diagram. The transition between Reference State 1 and State 2 is considered irreversible because of the naturalization of exotic species of both flora and fauna, possible extinction of native species, and climate change. There may have also been accelerated soil erosion.

When available, monitoring data (of various types) were employed to validate more subjective inferences made in this diagram. See the complete files in the office of the State Range Conservationist for more details.

The plant communities shown in this State and Transition Model may not represent every possibility, but are probably the most prevalent and recurring plant communities. As more monitoring data are collected, some phases or states may be revised, removed, and/or new ones may be added. None of these plant communities should necessarily be thought of as “Desired Plant Communities.” According to the USDA NRCS National Range & Pasture Handbook (USDA-NRCS 2003), Desired Plant Communities (DPC’s) will be determined by the decision-makers and will meet minimum quality criteria established by the NRCS. The main purpose for including descriptions of a plant community is to capture the current knowledge at the time of this revision.

Reference State
The Reference State is a description of this ecological site just prior to Euro-American settlement but long after the arrival of Native Americans. The description of the Reference State was determined by NRCS Soil Survey Type Site Location information and familiarity with rangeland relict areas where they exist. The least modified plant community would have been dominated by a low-statured curl-leaf mountain mahogany (Cercocarpus ledifolius), and a mixture of herbaceous species. Other common shrubs may have included antelope bitterbrush (Purshia tridentata) and mountain big sagebrush (Artemisia tridentata ssp. vaseyana). Dominant grasses would have included bluebunch wheatgrass (Pseudoroegneria spicata), needle and thread (Hesperostipa comata), spike fescue (Leucopoa kingii), and muttongrass (Poa fendleriana). Forbs would have included spiny phlox (Phlox hoodii), western mountain aster (Symphyotrichum spathulatum), and arrowleaf balsamroot (Balsamorhiza sagittata), among others (1.1). These sites would have had wildfire return intervals of approximately 20 to 50 years.

Community 1.1: scattered low-statured curl-leaf mountain mahogany and associated shrubs/ rich native perennial herbaceous understory
This plant community would have been characterized by a scattering of low-statured curl-leaf mountain mahogany. Antelope bitterbrush and mountain big sagebrush would have also been present with an understory composed of a variety of forbs (spiny phlox, arrowleaf balsamroot, etc.) and grasses (bluebunch wheatgrass, needle-and-thread, etc.).

Transition T1a: from State 1 to State 2 (Reference State to Curl-leaf Mountain Mahogany Non-native State)
The simultaneous introduction of exotic species, both plants and animals, possible extinctions of native flora and fauna, and climate change has caused State 1 to transition to State 2. Reversal of such historic changes (i.e. a return pathway) back to State 1 is not practical.

State 2 Curl-leaf Mountain Mahogany/ Introduced Non-natives State

State 2 is a description of the ecological site just following Euro-American settlement, which has been influenced by the introduction of several non-native plants and animals, possible extinctions of native species, and a different climate. The plant community will be very similar to State 1 with the exception that some introduced species are likely to be present. This plant community is considered the current potential, and is dominated by curl-leaf mountain mahogany with a scattering of antelope bitterbrush and mountain big sagebrush. The native perennial herbaceous understory is still intact, but a small component of non-native species will also be present. The resiliency of this state is maintained by the root-sprouting nature of the dominant shrubs. Reductions in livestock use during the growth season will maintain the stability of the state. Alternatively, continued season-long heavy livestock grazing will reduced the stability of this state.

Community Phase 2.1: scattered low-statured curl-leaf mountain mahogany and associated shrubs/ rich native perennial herbaceous understory
This plant community is characterized by a scattering of low-statured curl-leaf mountain mahogany, antelope bitterbrush, and mountain big sagebrush. The native perennial understory is composed of a variety of forbs (spiny phlox, western mountain aster, arrowleaf balsamroot, etc.) and grasses (bluebunch wheatgrass, spike fescue, needle and thread, etc.).

Transition T2a: from State 2 to State 3 (Curl-leaf Mountain Mahogany/ Introduced Non-natives State to Tall Dense Curl-leaf Mountain Mahogany State)
Heavy continuous livestock grazing took place on these sites between the 1850s and 1950s. The effects of this heavy livestock grazing were further worsened with fire exclusion policies. A rapid increase of deer numbers in the 1950s and 1960s caused high-lining of the trees. Curl-leaf mountain mahogany began to grow taller and larger in dbh (diameter at breast height), while the herbaceous understory species were reduced greatly. Key indicators of the approach to this transition are a loss of perennial grass understory, an increase in the shrub component relative to grasses, and an increase in height and dbh of mahogany, as well as high-lining and hedging of trees. This transition is triggered by sustained heavy grazing (over time), first by livestock, and later by excessive numbers of deer.

State 3 Tall Dense Curl-leaf Mountain Mahogany State
In the absence of fire, and continued heavy impacts from livestock, deer, and elk grazing, the native grasses will markedly decrease while the shrubs, mainly curl-leaf mountain mahogany, will grow taller and more dense as reproduction becomes rarer. The resiliency of this state is maintained root-sprouting nature of the dominant shrubs. Reductions in livestock use during the growth season will maintain the stability of the state. Alternatively, continued season-long use by both domestic and wild ungulates will reduced the stability of this state.

Community 3.1: abundant curl-leaf mountain mahogany/ depauperate understory
This plant community is characterized by a dramatic increase in curl-leaf mountain mahogany and substantial reduction in the perennial herbaceous component.

Transition T3a: from State 3 to State 4 (Tall Dense Curl-leaf Mountain Mahogany State to Tall Hedged Curl-leaf Mountain Mahogany State)
The region saw a marked increase in mule deer numbers during the 1940s to 1960s. As a preferred forage species for deer, curl-leaf mountain mahogany experienced heavy browsing pressure during that time. Heavy browsing combined with continued lack of fire caused these sites to transition into a near-monoculture of curl-leaf mountain mahogany. A key indicator of the approach to this transition is a loss of young, short mountain mahogany, and an increase in tall, large mountain mahogany trees. Excessive ungulate use will trigger this transition. A restoration pathway can be established by restoring fire to the ecosystem and reducing animal use on these sites.

Restoration Pathway R3a: from State 3 to State 2 (Tall Dense Curl-leaf Mountain Mahogany State to Curl-leaf Mountain Mahogany/ Introduced Non-natives State)
Brush management using prescribed fire, mechanical, or herbicide treatments, or wildfire, in combination with re-seeding efforts should be considered only on gentle to level slopes (these locations however are unusual). Curl-leaf mountain mahogany will not consistently re-sprout after these treatments, and invasive annuals and biennials are very prominent after disturbances. Re-seeding after wildfire must be done promptly or annuals and bi-annuals will takeover.

State 4
Tall Hedged Curl-leaf Mountain Mahogany State
Following a period of intense deer browsing, curl-leaf mountain mahogany will appear hedged. Without fire, mountain mahogany will grow taller and begin to form a monoculture. Introduced annuals and biennials are also quite common in the understory. In this state, the rockiness of the soil makes it self-armoring, thus reducing the effects of erosion. The steep, rocky locations of this site limit proactive management options.

Community 4.1: near monoculture curl-leaf mountain mahogany/ introduced annuals & biennials
This plant community is super-dominated by curl-leaf mountain mahogany with an understory composed mainly of introduced annuals and biennials. The site will remain in this state until wildfire occurs.