A Guide for Identifying and Managing Old-Growth Pinyon and Juniper Woodlands

Large old trees growing with grass and sagebrush

Authors: Jeanne C. Chambers, Richard F. Miller, Eva K. Strand, and Lisa M. Ellsworth

Introduction

In recent years there has been increased focus on old-growth (persistent) pinyon and juniper woodlands, but information on their identification and management is limited. Management objectives for pinyon and juniper are often twofold: (1) decreasing threats to old-growth woodlands to conserve their ecological and cultural values; and (2) reducing pinyon and juniper encroachment into shrublands to maintain their ecological resilience. It has been estimated that old-growth pinyon and juniper woodlands comprise only 2% of Forest Service (FS) and Bureau of Land Management (BLM) lands combined (USDA FS 2023). Old-growth woodlands can take centuries to recover if damaged by wildfire or managed inappropriately (Waichler et al. 2001). This guide is intended to provide information to help managers identify old-growth woodlands and pinyon and juniper encroached shrublands and develop effective management strategies for woodlands. The primary focus is on old-growth woodlands characterized by western juniper (Juniperus occidentalis), Utah juniper (J. osteosperma), and (Pinus monophylla). The guide has the following components:

Figure 1. Old-growth Utah juniper tree.

1. Purpose and approach
2. Definitions
3. Ecological site characteristics of old-growth woodlands
4. Old-growth tree morphological characteristics
5. Fire history and stand development of old-growth woodlands
6. Tree infill in old-growth woodlands and tree encroachment in shrublands
7. Ecological and cultural importance of old-growth woodlands
8. Threats to old-growth woodlands
9. Use of ecological site descriptions to identify old-growth woodlands and encroached shrublands
10. Management strategies for old-growth woodlands
11. Acknowledgements
12. References

1 - purpose and approach

This field guide provides the information needed to identify old-growth (persistent) woodlands by evaluating ecological site characteristics, tree morphology, and stand characteristics. The approach in this field guide is based on the understanding that old-growth woodland ecological types can exist in different ecological states and successional phases after disturbances, such as wildfires or tree harvesting, and thus may have multiple tree ages. Old-growth stands in the later stages of woodland development are defined as having many trees that established prior to European and Anglo-American settlement and that are potentially many hundreds to over a thousand years old (Miller and Rose 1999; Miller et al. 2005). The approach used here for defining old-growth woodlands differs from that of a collaborative, national report by the USDA FS and USDOI BLM (USDA FS 2023). That report provides working definitions of old-growth forests, including old-growth pinyon and juniper woodlands, based on minimum criteria for live tree age, tree diameter, or number of trees per acre (see Appendix in this guide). While minimum criteria like those provided in the USDA FS (2023) report may be useful for planning, the ecological site characteristics and current ecological states of old-growth woodlands vary across landscapes. This field guide describes how to assess site characteristics, tree morphology, and stand characteristics to identify old-growth woodlands within planning areas.

To help identify old-growth woodlands and inform management strategies, this guide provides information on fire history, stand development, effects of land-use history, and climate on tree infill in old-growth woodlands and tree encroachment in shrublands. The ecological and cultural importance of old-growth woodlands are described, and the threats to old-growth woodlands are discussed.

To help determine if a planning area has the potential to support a shrubland or an old-growth woodland, ecological site concepts are used. State-and-transition models for generalized sagebrush shrubland and pinyon and juniper old-growth woodland ecological site types are provided to illustrate the various vegetation states (persistent plant communities) and phases (successional stages) that exist for the two different types. Transitions to alternative states and restoration pathways are shown.

Management strategies based on knowledge of ecological site type characteristics, current ecological state or plant community, and fuel structure, as well as effects of potential treatments are provided for old-growth and mature woodlands. Treatment location, type, and configuration all affect landscape appearance, cultural values, use by wildlife, and public perception, and the guide discusses considerations for each of these factors when developing management strategies.

2 - definitions

Encroached Shrubland ─ Areas where pinyon and juniper trees are establishing into sites historically characterized by shrubland, grassland, or other ecological types and altering their structure and function (Romme et al. 2009; Stringham et al. 2015a, b). Most trees established after European and Anglo-American settlement in the mid-to-late 1800s.

In some earlier guides and papers, encroached shrubland has been referred to as expansion woodland. Specifically, shrubland ecological types have been termed woodlands if pinyon and juniper trees were present, and the term expansion has been used instead of encroachment to provide a more positive connotation. In this guide, the term encroached shrubland is used to (1) explicitly differentiate shrubland and old-growth (persistent) pinyon and juniper ecological types, and (2) more accurately describe of pinyon and juniper trees into shrubland ecological types. The term expansion is used in paleobotany and defined in this guide as the regional or geographic increase in the range of a pinyon or juniper resulting from long-term climate change.

Three phases (successional stages) of tree stand development apply to pinyon and juniper encroachment into shrublands (Miller et al. 2005, 2019) that are characterized by which of the three vegetation layers (trees, shrubs, or perennial herbaceous vegetation) dominate (Fig. 2):

Phase I ─ trees are present, but shrubs and herbs are the dominant vegetation influencing ecological processes.
Phase II ─ trees are codominant with shrubs and herbs, and all three vegetation layers influence ecological processes.
Phase III ─ trees are the dominant vegetation on the site and the primary plant layer influencing ecological processes.

Phase I more shrubs, less trees, which changes as move to phase III

Figure 2. The tree dominance index = tree cover/tree + shrub + perennial grass and forb cover (modified from Roundy et al. 2014).

Infill ─ The increase in tree density within either old-growth woodlands or pinyon and juniper encroached shrublands that previously supported lower tree density.

Mature Woodland ─ Stage of old-growth (persistent) woodland development immediately before old-growth trees dominate the site (USDA FS 2023). Stands exhibit structural characteristics lacking in earlier stages of development and may contain some but not all the structural attributes of old-growth woodlands (USDA FS 2023). Mature trees typically characterize the stand. These trees are 80 to 100 years old and near maximum size depending on site history and site characteristics. Some trees may have established prior to European and Anglo-American settlement (Miller et al. 2019). Note that mature trees can also occur in Phase III encroached shrublands.

Old-Growth (Persistent) Woodland ─ Ecological sites that are typically characterized by shallow (< 19.7 inches [0.5-m ]deep), coarse, and/or rocky soils, and occur on certain landscape positions, such as ridges and steep slopes or other non-depositional sites (Barney and Frischknecht 1974; Romme et al. 2009; Stringham et al. 2015a; Duniway et al. 2022). Potential understory vegetation varies with climate, soils, tree canopy characteristics, and past disturbance (Stringham et al. 2015 a, b). Later stages differ from earlier stages of old-growth woodland development in a variety of characteristics, including old-growth tree size and morphological characteristics, accumulation of large dead woody material, and ecosystem function (Hamilton 1993; Waichler et al. 2001; USDA FS 2023). Many trees established prior to European and Anglo-American settlement in the mid to late1800s and may be many hundreds to over a thousand years old in relatively undisturbed sites.

Pinyon or Juniper Contraction ─ The regional or geographic decrease in the range of a pinyon or juniper species resulting from long-term climate change (e.g., Wigand and Rhode 2002; Cole et al. 2013). Term used in paleobotany.

Pinyon or Juniper Expansion ─ The regional or geographic increase in the range of a pinyon or juniper species resulting from long-term climate change (e.g., Wigand and Rhode 2002; Cole et al. 2013). Term used in paleobotany.

3 - ecological site characteristics of old-growth woodlands

Topography ─ Typically ridge tops or relatively steep side slopes.

Soils ─ Usually shallow (< 0.5-m deep), coarse, or rocky; may be highly calcareous or have a high clay or sand content (Knutson and Pyke 2008; Romme et al. 2009; Stringham et al. 2015a, b). Soil water availability is typically low compared to adjacent sites due to the soil characteristics. In contrast, pinyon and juniper encroached big sagebrush shrublands (basin, Artemisia tridentata ssp. tridentata; Wyoming, A. t. ssp. wyomingensis; and mountain, A. t. ssp. vaseyana) are often associated with deeper, less coarse, or less rocky soils.

Climate ─ Pinyon and juniper species and associated vegetation change across regional environmental gradients as temperatures increase from north to south, and the timing of precipitation changes from winter- to summer-dominated from west to east. Western juniper typically occurs in areas with cool to cold, wet winters and relatively dry summers. Utah juniper and singleleaf pinyon both occur in areas with cool to cold winters. However, singleleaf pinyon occurs in climates with relatively dry summers while the distribution of Utah juniper spans dry to wet summers. Twoneedle pinyon (P. edulis) occurs in areas with cool winters and warm, wet summers.

Figure 3. Old-growth twoneedle pinyon pine woodland.

Elevation ─ Many elevations support pinyon and juniper species across the Intermountain West. The elevations of old-growth woodlands vary with climate, tree species, aspect, and soil characteristics (Tausch et al. 1981; Johnson and Miller 2008; Weisberg et al. 2008; Loehman et al. 2023).

Fire-safe sites ─ Old growth woodlands typically occur on relatively fire-safe sites due to a combination of site characteristics, including topography, relatively low tree cover, and limited understory fuels (Miller et al. 2019).

4 - old-growth tree morphological characteristics

Size and shape ─ Diameter at the root crown (DRC) and height are useful indicators of old-growth status but become less predictive as trees age (Gascho Landis and Bailey 2006; Weisberg and Ko 2012).

Reduced crown ─ Large bole (trunk) and lower branch dimensions, but small crown area
Squat ─ Often large diameter relative to height
Other ─ Dead wood in canopy and exposed large roots (Jacobs 2008)

Examples of Old-Growth Tree Morphological Characteristics

Western Juniper (Juniperus occidentalis)

Western juniper trees with sparse crowns and shreddy bark

Figure 4a. Old-growth western juniper with a diminished crown.

Figure 4b. Old-growth western juniper with a lot of dead branches in the crown.

Utah Juniper (Juniperus osteosperma)

Flattened or rounded crowns that may have dead tops
Limited terminal and lateral growth on the branch tips
Dead wood in the canopy
Large, twisted trunks

Figure 6a. Lichen on juniper on old-growth juniper bark

Lichen on juniper on old-growth juniper bark

Crustose lichen (Teuvoa junipericola) causes a black stain that can be mistaken for charred wood on old-growth juniper tree (Sohrabi et al. 2013).

Fruticose lichen (Letharia spp.) is often present on old western juniper trees and on old Utah juniper trees in cooler and moister environments (Bunting et al. 2007).

Singleleaf Pinyon (Pinus monophylla)

Twoneedle Pinyon (Pinus edulis)

Singleleaf Pinyon (Pinus monophylla)

Twoneedle pinyon (Pinus edulis)

Pinus monophylla and P. edulis trees with large rounded crowns and exposed roots

Flattened or rounded crowns that may have dead tops
Dead branches in the canopy
Large trunks
Exposed roots

Bark on Western Juniper

Bark on Utah Juniper

Typically flaky and thin at < 130 to 150 years old.

Thicker bark and very fibrous at > 200 to 250 years. Heartwood may be exposed on old-growth trees.

Bark on Singleleaf Pinyon

Relatively thin and flaky with weak vertical furrows on trees < 130 to 150 years old.

Thicker and more plate-like than furrowed on trees > 150 to 180 years old.

Characteristic	Younger Trees (< 130-150 years)	Old-Growth Trees (> 150-200 years)
─ *Juniper* ─
Crown shape	Conical with pointed tip	Flattened, rounded, or uneven top
Branch structure	Branches progressively smaller from bottom to top of tree	In open stands, large branches near the base
Bark	Flaky, relatively thin with limited or shallow vertical furrows	Thick and fibrous with well-developed vertical furrows
Leader growth	Leader growth at tips of main branches in the upper 1/4 of the tree usually > 2 inches (5.08 cm). In open stands, leader growth > 2 inches per year	Leader growth at tips of main branches in the upper 1/4 of the tree usually < 1 inch (2.54 cm)
─ *Pinyon* ─
Crown shape	Conical with pointed to slightly rounded tip	Flattened, rounded, or uneven top
Branch structure	Branches smaller from bottom to top of tree - general orientation is vertical	In open stands, branches large near base and relatively large into the crown, but more randomly oriented
Bark	Relatively thin, flaky, with weak vertical furrows	Thicker, more plate-like than furrowed structure
Leader growth	Leader growth at tips of main branches in pinyon similar to juniper but not directly visible; look at bud scale scars to determine amount of growth	Leader growth at tips of main branches in upper 1/4 of the tree usually < 2 inches (5.08 cm)
─ *Juniper and Pinyon* ─
Tree canopy lichen	Little or no foliose lichen	Juniper often covered by bright green foliose lichen; pinyon is not
Dead wood in living tree	Little dead wood in the main stem of the tree	Dead branches and bark missing, black stain or lichen on main stem
Downed dead wood across the site	Large diameter logs and stumps absent	Large diameter logs and stumps often charred and scattered across the site

Change in singleleaf pinyon morphology over time (Weisberg and Ko 2012).

A. 61 yrs ─ Symmetrical and full green crown, small diameter

B. 140 yrs ─ Forked stem, thickened lower branches, large diameter

C. 286 yrs ─ Crown dieback, diminished canopy volume, thick lower branches, asymmetric crown, sinuous branching

D. 307 yrs ─ Extensive crown dieback and diminished canopy, thick lower branches, and short, stubby growth form

5 - FIRE HISTORY AND STAND DEVELOPMENT OF OLD GROWTH

Woodland heterogeneity supports small patchy fires, and small patchy fires beget woodland heterogeneity.

The historical fire regime of old-growth woodlands has been characterized by a frequency of 200+ years for fires of stand-replacement severity (Schmidt et al. 2002).
In the past, fuels in old-growth woodlands were often discontinuous due to the ecological site conditions and the patchiness of the vegetation. Fires ignited by lightning burned individual trees or groups of trees, however under drought conditions high winds could cause larger fires to burn (Waichler et al. 2001).
Disturbances resulting in tree mortality included not only fire, but also drought, insect infestation, and disease (Miller et al. 2019).
Slow regeneration of the trees following these disturbances favored the persistence of shrub, grass, and forb patches (Barney and Frischknecht 1974; Miller and Heyerdahl 2008).
Old-growth woodlands typically experienced long fire-free intervals, and fires that did occur were usually small and patchy due to the variability of the vegetation and lack of surface fuels (Miller et al. 2019).
In combination, these conditions generated patchiness at both stand and landscape scales and uneven-aged stand structures within the old-growth woodland patches.

6 - TREE INFILL IN OLD-GROWTH WOODLANDS AND TREE ENCROACHMENT IN SHRUBLANDS

The extent and cover of pinyon and juniper trees have increased in recent decades across the Intermountain West (Filippelli et al. 2020; Morford et al. 2022). Infill in old-growth woodlands and encroachment of pinyon and juniper trees into adjacent vegetation types have been observed since European and Anglo-American settlement of the western U.S. Both land-use history and climate help explain the increase in pinyon and juniper as indicated by literature reviews, field assessments, and demographic models (Wright et al. 1979; West 1999; Heyerdahl et al. 2001; Miller et al. 2008; Marlon 2012; Miller et al. 2019; Noel et al. 2023).

Grazing. Excessive livestock grazing in the late 1800s through the early 1900s caused a significant decline in perennial grasses, altering the structure and function of shrubland ecosystems across much of the western U.S. (Stewart 1936; Stoddart et al. 1975). Decreased competition for available resources from perennial grasses altered plant community composition and likely created more favorable conditions for tree seedling establishment. For example, shrubs typically increase following removal of perennial grasses (Chambers et al. 2017; Pierce et al. 2019), and shrubs often serve as nurse plants and facilitate pinyon and juniper establishment (Chambers et al. 1999, Chambers 2001).

Climate. A favorable climatic period during the first two decades of the 1900s helped create the conditions needed for seedling establishment and a rapid increase in pinyon and juniper populations (Miller et al. 2008; Shriver et al. 2025). The increase in seed trees during this period likely created multiplicative or compounding tree population growth that persisted throughout the 20th and into the 21^st century (Miller et al. 2008; Shriver et al. 2025).

Fire. A sharp decline in fire occurred across the Intermountain West in the late 1800s and early 1900s and coincided with a large increase in both pinyon and juniper trees (West 1983; Baisan and Swetnam 1990; Miller et al. 2019). The decline in fire preceded fire exclusion policies and was likely the result of excessive livestock grazing and depletion of grasses and forbs (fine fuels) necessary for fire spread (Heyerdahl et al. 2001; Marlon et al. 2012). The decrease in fires exacerbated tree infill in old-growth woodlands and encroachment in shrublands.

Recent Trends. The rate of tree seedling establishment has declined since the early 1900s. However, despite warmer temperatures and increasing aridity in recent decades (Bradford et al. 2020), tree infilling in old-growth woodlands and encroachment into shrublands continue in many areas (Miller et al. 2008; Filippelli et al. 2020; Morford et al. 2022). Pinyon and juniper tree species are well adapted to dryland environmental conditions but experience mortality during severe drought and are most vulnerable in the warmer and drier portions of their ranges and at higher stocking rates (Shriver et al. ; Noel et al. 2023.

Research indicates that elevated atmospheric CO2 increases water-use efficiency and allows continued growth in western juniper despite increasing aridity (Soulé and Knapp 2019).
Utah juniper can tolerate extremely dry soils, maintaining modest photosynthetic rates during drought and responding rapidly to summer rains (Leffler et al. 2002).
Demographic models of the expected effects of climate change on singleleaf pinyon and Utah juniper indicate that both species have relatively low vulnerability to climate warming (Noel et al. 2023).
Demographic models of the expected effects of climate change on twoneedle pinyon and oneseed juniper (J. monosperma) indicate that increasing aridity in the warmer and drier Southwest is resulting in population declines due to rising mortality and decreasing recruitment rates (Noel et al. 2023).

The current degree of infilling and encroachment as well as mortality differs across the landscape and is influenced by:

Differences in climate and their influence on both tree establishment and wildfire (Miller et al. 2008; Noel et al. 2023),
Site productivity and its effects on tree regeneration and growth (Johnson and Miller 2006),
Type and extent of previous fuel treatments and other management activities (Reinhardt et al. 2020),
Invasion of annual forbs and grasses and its influence on fire spread and fire return intervals (Board et al. 2018; Williams et al. 2023), and
Effects of climate warming and increasing drought severity on both tree mortality and tree recruitment (Noel et al. 2023).

7 - ecological and cultural importance of old-growth woodlands

Ecological Value

Partial canopy mortality and thinning crowns provide a mosaic of sunlit and shaded microsites promoting a higher diversity of understory plants (Waichler et al. 2001).
Old-growth trees provide greater habitat heterogeneity benefiting many bird, small mammal, and reptile species (Floyd et al. 2003).
Old-growth trees provide habitat for tree-dwelling animal taxa and epiphytic plants (Floyd et al. 2003).
Old-growth trees provide breeding habitat for cavity-nesting bird species (Reinkensmyer et al. 2007).
Deep, furrowed bark is beneficial to insects and arthropods and provides habitat for bats (Kunz et al. 2003).

Juniper titmouse at hole in old-growth juniper

Figure 12. Photo of juniper titmouse (Baeolophus ridgwayi) from Cornell Laboratory of Ornithology.

Cultural Value

Pinyon and juniper are of deep cultural and spiritual significance to Native Americans (D’Azevedo 1986; Miller 1997).

Pinyon nuts are a prized food that has been a diet staple for thousands of years.
Juniper berries and needles are used in traditional medicine to treat many ailments including influenza and indigestion. Pinyon pitch is used to make salves for skin issues and wounds.
Trees are used for fuel for cooking and heating, as well as wood for construction. Juniper bark is used for materials like rope.
Many old-growth woodlands are considered sacred cultural sites that represent a deep, long-standing connection to the land.

Figure 13. Burned old-growth, where?

Larger and more severe wildfires across the western U.S. mean that more wildfires are affecting old-growth woodlands and that fires are less patchy than they were historically (Abatzoglou and Williams 2016; Board et al. 2018; Strand and Bunting 2023; Strand et al. 2025).

Fire regimes have been substantially altered and in some areas multiple fire intervals have been missed. This has resulted in increases in fire size, intensity, and severity and elevated the risk of losing key ecosystem components following wildfires.
Infilling of old-growth woodlands is associated with increased continuity of canopy fuels. Infilling of old-growth woodlands also leads to increased drought stress, tree mortality, and dead, standing fuels.
Increasingly hotter and drier climatic conditions lengthen fire seasons and increase rates of tree mortality.

Biotic disturbances (insects and diseases) also increase rates of tree mortality and numbers of standing dead trees.
Woodland invasion by non-native annual grasses increases fine fuel loading and can result in grass-fire cycles.
Wood cutting or woody fuel treatments can remove old-growth trees and provide access to an area by ATV/UTVs, increasing the risk of invasion by non-native annual grasses and forbs.

9 - using ecological site concepts to identify old-growth woodlands and encroached shrublands

Managing for old-growth woodlands begins with determining if the planning area has the potential to support a shrubland, an old-growth woodland, or both based on environmental characteristics.

The types of management actions are based on the current ecological state or condition of the shrubland or old-growth woodland.
Ecological site descriptions (ESDs) and state-and-transition models (STMs) provide information needed to distinguish old-growth from shrubland ecological types and make informed management decisions.
ESDs and their associated STMs have been developed for much of the western U.S. and are available in the USDA ESD Catalog. STMs for Nevada, which include old-growth woodlands, are available at the University of Nevada Rangeland Ecology Lab.

The information provided by ESDs and STMs, and illustrations of different shrubland and woodland ESDs, follow.

Ecological Site Descriptions (ESDs)

Site Characteristics of Ecological Site Types – physiographic, climate, soil, and water features
Plant Communities – plant species, vegetation states, and ecological dynamics
Site Interpretations – management alternatives
Supporting Information – relevant literature and information

State and Transition Models (STMs)

Identify the different vegetation states (persistent plant communities)
Show the phases (successional stages) within the vegetation states
Describe the disturbances that drive transitions between vegetation states
Define the restoration activities needed to restore plant communities

Examples of Ecological Site Types

Sagebrush shrubs growing with grasses and forbs, healthy

Generalized Big Sagebrush State and Transition Model (STM)

Soils are loamy and precipitation is 10 to 14 inches per year. These ecological types have the potential for encroachment of pinyon and juniper (PJ) trees and invasion by non-native annual grasses and forbs.

Explanation of Big Sagebrush STM States and Phases within States

Reference state – Shows transitions among phases (1-5) expected with periodic fire. Phase II encroached shrubland is a phase at risk of transitioning to a less desirable ecological state.

In the reference state, perennial grasses and forbs are sufficient to promote recovery following disturbances like wildfire that result in mortality of the dominant shrubs and encroaching trees.

Encroached state – Phase III encroached shrubland. Primarily younger trees but may include some old-growth trees.

Eroded state – Phase III encroached shrubland experiencing active soil erosion.

Invaded state – State invaded by non-native annual grasses and forbs. This state can occur with or without pinyon and juniper (PJ) encroachment. Risk of invasion by non-native annual grasses is higher on warmer and drier ecological sites than on cooler and moister sites due to lower resistance to invasion.

Seeded state – State resulting from seeding treatments. Seeding can result in permanent conversion to introduced species depending on the species seeded.

Differences in Species Composition in Great Basin Big Sagebrush ESDs

Lower elevation, warmer and drier (10-12 inches annual precipitation) sites - typically characterized by Wyoming or basin big sagebrush (Artemisia tridentata wyomingensis or tridentata), bluebunch wheatgrass (Pseudoroegneria spicata), Thurber’s needlegrass (Achnatherum thurberianum), Indian ricegrass (Achnatherum hymenoides), and Sandberg bluegrass (Poa secunda).
Higher elevation, cooler and moister (12-14 inches annual precipitation) sites - typically characterized by mountain big sagebrush ( t. ssp. vaseyana), antelope bitterbrush (Purshia tridentata), bluebunch wheatgrass, and Idaho fescue (Festuca idahoensis).

Example of a State and Transition Model (STM) for an Old-Growth Pinyon and/or Juniper Ecological Site Type (Adapted from Stringham et al. 2015a, b, 2019)

The diagram below is typical of an STM and includes the following elements.

Potential ecological states (persistent plant communities) are shown in the large boxes.
Phases (successional stages) within the states are shown in the smaller boxes.
“T” indicates a transition to an alternative state.

“R” indicates transition back to a previous state due to restoration or other management intervention.

Generalized Old-Growth Pinyon and/or Juniper STM

Sites are typically located on ridge tops or steeper slopes, soils are coarse or rocky and may have a high percentage of clay, sand, or calcium carbonate. Precipitation ranges from 10 to 20 inches per year depending on ecoregion and species.

Large, downed wood and stumps of old-growth trees can help identify old-growth sites after disturbances such as wildfire or tree harvesting, which was common in the mining era.

Explanation of STM States and Phases within States

Reference state – Transitions occur due to succession following disturbances that remove trees. Phases are (1) perennial grass and forb dominance, (2) immature tree dominance, and (3) old-growth tree dominance. Infilling of old-growth dominated areas can result in a phase at risk (5). Disturbances that remove trees cause areas with immature trees or multiple age classes to transition to perennial grass and forb dominance (4, 6).

Infilled tree state – Time and lack of disturbance allow trees to dominate; inappropriate grazing management can favor shrub and tree dominance.

Eroded state – Soil erosion occurs over time due to mortality of understory vegetation. This state is most likely in Utah juniper and singleleaf pinyon ecological sites; freezing and thawing, soil crusting (hardening), and soil impermeability are causal factors.

Invaded state – Crown fire can result in invasion. Invasions are more likely following fires where tree cover and fire severity were high and on warmer and drier ecological sites due to lower resistance to invasion.

Differences in Species Composition in Old-Growth Pinyon and Juniper ESDs

Lower elevation, warmer and drier (10-12 inches annual precipitation) sites - Utah juniper dominant, associated with Wyoming big sagebrush and black sagebrush (Artemisia nova), bluebunch wheatgrass, Thurber’s needlegrass, Indian ricegrass, and Sandberg bluegrass.
Higher elevation, cooler and moister (14-20 inches annual precipitation) sites - western juniper or singleleaf pinyon dominant depending on ecoregion. Associated species include ow sagebrush, mountain big sagebrush, mountain mahogany (Cercocarpus ledifolius), bluebunch wheatgrass, and Idaho fescue.