United States Department of Agriculture

Forest Service

Rocky Mountain Research Station

Research Paper Report RMRS-RP-69

January 2008

Age Structure and Expansion of 

Piñon-Juniper Woodlands: A Regional 

Perspective in the Intermountain West

Richard F. Miller

Robin J. Tausch

E. Durant McArthur

Dustin D. Johnson

Stewart C. Sanderson
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Miller, Richard F.; Tausch, Robin J.; McArthur, E. Durant; Johnson, Dustin D.; 

Sanderson, Stewart C. 2008. Age structure and expansion of piñon-juniper 

woodlands: a regional perspective in the Intermountain West. Res. Pap. 

RMRS-RP-69. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station. 15 p.

Numerous studies have documented the expansion of woodlands in the 

Intermountain West; however, few have compared the chronology of expansion for 

woodlands across different geographic regions or determined the mix and extent 

of presettlement stands. We evaluated tree age structure and establishment for 

six woodlands in four ecological provinces in the central and northern Great Basin. 

Since 1860, the area occupied by piñon and or juniper has increased 125 to 625 

percent. The increase of trees was a result of infill into shrub-steppe communities with relatively open low density stands of trees and expansion of piñon and 

juniper into sagebrush-steppe communities that previously did not support trees. 

Woodland expansion in Oregon, Utah, and Nevada were similar, but began two 

to three decades earlier in Idaho. The majority of woodlands are still in the early 

to mid phases of stand closure, which means they often support an understory of 

shrubs and herbaceous vegetation. This has implications for future changes that 

will occur within these woodlands in the next 30 to 50 years. In the absence of 

disturbance or management, the majority of these landscapes will become closed 

woodlands resulting in the loss of understory plant species and greater costs for 

restoration.

Key words: western juniper, Utah juniper, singleleaf piñon, succession, chronology, 

old-growth

Richard Miller is a professor with the Eastern Oregon Agricultural Research 

Station, Oregon State University in Corvallis, OR. He completed a B.S. in range 

management at Humboldt State University, an M.S. in range ecology and management at Oregon State University, and a Ph.D. in range ecology and plant physiology 

at New Mexico State University.

Robin Tausch is a research range ecologist with the U.S. Forest Service, Rocky 

Mountain Research Station Great Basin Ecology Laboratory in Reno, Nevada. He 

completed a B.S. in wildlife management at Humboldt State University, an M.S. in 

range management at the University of Nevada, Reno, and a Ph.D. in range ecology at Utah State University.

Durant McArthur is acting program manager, U.S. Forest Service, Rocky Mountain 

Research Station Grasslands, Shrublands, and Desert Ecosystem Research 

Program, Shrub Sciences Laboratory, Provo, Utah. His degrees (B.S., M.S., and 

Ph.D.) are from the University of Utah in biology, genetics, and botany.

Dustin Johnson is an assistant professor, Harney County Extension, Oregon 

State University. He completed a B.S. in range management at Eastern Oregon 

State University and M.S. in range ecology and management at Oregon State 

University.

Stewart Sanderson is a research geneticist, U.S. Forest Service, Rocky Mountain 

Research Station, Shrub Sciences Laboratory, Provo, Utah. He holds B.S. and 

M.S. degrees in botany and range science from Brigham Young University and a 

Ph.D. in plant genetics from the University of Texas, Austin.

Abstract

Authors
This study was funded by the U.S. Joint Fire Science Program and jointly supported by the Eastern Oregon Agricultural Research Center and the U.S. Forest 

Service, Rocky Mountain Research Station. The Eastern Oregon Agricultural 

Research Center is jointly operated by the USDA-ARS and the Oregon State 

University Agricultural Experiment Station. We wish to thank reviewers Jon Bates, 

Range Scientist for the USDA-ARS, and Richard Stevens, retired research leader 

for the Utah Division of Wildlife, for their thoughtful and constructive review of this 

manuscript.

Rocky Mountain Research Station

Natural Resources Research Center

2150 Centre Avenue, Building A

Fort Collins, CO 80526

Acknowledgment

Introduction............................................................................................... 1

Study Area................................................................................................ 1

Methods.................................................................................................... 4

Analysis .................................................................................................... 6

Results...................................................................................................... 6

Discussion ................................................................................................ 9

Management Implications....................................................................... 13

Literature Cited....................................................................................... 13

Contents

USDA Forest Service RMRS-RP-69. 2008. 1

Introduction

Piñon and juniper woodlands in the cold desert of 

the Intermountain West occupy over 18 million ha 

(44,600,000 acres) (Miller and Tausch 2001). These 

woodlands are commonly associated with sagebrush 

(Artemisia) communities forming a mosaic of shrubsteppe and woodlands across landscapes. Numerous 

studies have documented the expansion of these woodlands resulting in the replacement of shrub-steppe

communities (Adams 1975; Burkhardt and Tisdale 

1976; Cottam and Stewart 1940; Gedney and others 

1999; Miller and others 2005; Miller and Rose 1995, 

1999; Tausch and West 1988, 1995; Tausch and others 1981). However, few have documented woodland 

dynamics at the landscape level and across different 

ecological provinces. This has resulted in uncertainties as to the extent of piñon and juniper woodlands 

that have replaced or are encroaching sagebrushsteppe communities versus sites that have supported 

woodlands in the past.

As a result of continued expansion during the 20th

century, control of piñon and juniper has involved a 

major effort by both government agencies and private landowners since the early 1960s. The increase 

in piñon and juniper dominance within Intermountain 

plant communities can have significant impacts on 

soil resources, plant community structure and composition, forage quality and quantity, water and nutrient 

cycles, wildlife habitat, biodiversity, and fire severity 

and frequency (Miller and others 2005, Miller and 

Tausch 2001). However, in the 1970s various groups 

began challenging the removal of piñon and juniper 

woodlands in the Intermountain West (Belsky 1996). 

One area of concern was the limited scientific evidence documenting the expansion of these conifers 

at a broad scale (in other words, landscapes or entire 

woodlands) in the Intermountain Region. The fear of 

many groups is historic woodlands that occupied landscapes prior to Eurasian settlement in the late 1800s 

are being burned, cut, and chained. In the past, many 

treatments did not differentiate between post (historic) 

and presettlement woodlands (defined here as trees or 

woodlands <140 years old and >140 years old, respectively). If the primary emphasis of restoration is to 

restore and maintain potential natural vegetation, then 

understanding the mix of historic stands of trees with 

sagebrush-steppe communities and recently developed 

woodlands at the landscape scale becomes essential in 

the development of maintenance and restoration strategies for Intermountain plant communities.

The goal of our study was to document the temporal and spatial dynamics of tree establishment and 

expansion for piñon (Pinus monophylla) and two juniper species (Juniperus osteosperma and J. occidentalis 

ssp. occidentalis) across several different geographic 

regions in the Intermountain West. We also wanted to 

determine the ratio of post and presettlement trees for 

western juniper, Utah juniper, and singleleaf piñon 

alliances across these geographic locations. To accomplish this, we evaluated six woodlands from their 

lower to upper elevational boundaries in four different 

ecological provinces using both extensive and intensive sampling procedures. Questions we addressed in 

this study were:

1. What was the temporal pattern of woodlands 

expansion into sagebrush-steppe plant 

communities?

2. What was the chronological sequence of tree 

establishment?

3. What was the density and spatial extent of historic 

trees?

4. What is the current successional state of 

woodlands development?

The extent of woodland expansion is not always 

clear and has lead to debate and court appeals over proposed tree removal projects. The possible removal of 

old trees (>140 years old) or presettlement woodlands 

that are recovering from disturbance has resulted in 

considerable concern from a number of interest groups 

throughout the West. Addressing the above questions 

will help land managers develop tree removal projects 

that will more closely restore vegetation to presettlement conditions.

Study Area

The study was located in four ecological provinces 

in Idaho, Oregon, Nevada, and Utah (fig. 1), and included western juniper, Utah juniper, and singleleaf 

piñon.

Owyhee Mountains Idaho and Steens 

Mountain Oregon: Western Juniper

The study areas in the western juniper alliance were 

located on Steens Mountain in southeastern Oregon 

and in the Owyhee Mountains in southwestern Idaho 

(fig. 1). Climate, soils, and vegetation across the 

study areas are characteristic of those found throughout much of the range of western juniper in the High 

Desert, Klamath, and Humboldt ecological provinces. 

In Idaho, we sampled the woodlands on Juniper and 

South Mountains located in the Owyhee Mountains 
2 USDA Forest Service RMRS-RP-69. 2008.

in Owyhee County (42° north latitude, 116° west longitude) in the Humboldt Ecological Province. This 

area is managed by the Boise District of the Bureau 

of Land Management. The geomorphology of this area 

is characterized as an uplifted region with doming and 

fault blocking common. The Owyhee Mountains are 

predominantly comprised of granite; however, most of 

the uplands are rhyolites and welded tuffs with silicic 

volcanic flows, ash deposits, and wind-blown loess. 

Topographic characteristics of this area include mountains dissected by deep canyons, rocky tablelands, and 

rolling plains ranging in elevation from 1200 to 2375 m 

(3936 and 7790 ft). Woodlands described in Oregon 

were located on the north and south portions of Steens 

Mountain in the High Desert Ecological Province and 

managed by the Burns District of the Bureau of Land 

Management. Steens Mountain is an isolated volcanic 

fault-block that lies in the extreme northwest Basin and 

Range Province (Fenneman 1931) in Harney County, 

south-southeast of Burns (42° north latitude, 118° west 

longitude). The mountain is approximately 80 km 

(50 mi) long and oriented in a northeast direction (Orr 

and Orr 1999). The elevation of Steens Mountain ranges from 1268 to 2949 m (4160 to 9673 ft), with a steep 

east-facing escarpment and a gentle west-facing slope. 

Geomorphology of Steens Mountain is characterized 

as nearly level basins and valleys that are bordered by 

long, gently sloping alluvial fans. Pliocene volcanic 

and shallow intrusive igneous rocks occur, along with 

andesite, breccias, and basalt flows. Alluvial deposits, 

playas, marshes, and flat deposits occur in the valleys.

Climate across the Owyhee Mountains and Steens 

Mountain is characteristic of the northern Great 

Basin in that it is cool and semiarid. Mean annual 

North & South Steens Mt.

South Mt.

Juniper Mt. Owyhee Mts.

Bonneville

Humboldt

High

Desert

Shoshone Mts. Lahontan

East Tintic Mts.

Klamath

High Calcareous

High

Central

Figure 1. Map of the seven study locations (Ecological Provinces derived from Anderson and others 

1998 and Bailey 1994). Ecological provinces sampled are indicated by bold lettering.
USDA Forest Service RMRS-RP-69. 2008. 3

precipitation within the juniper belts varies between 

300 mm (11.8 in) at lower elevations increasing to 

>400 mm (15.7 in) at higher elevations. The majority of the annual precipitation is received as snow 

in November, December, and January and as rain 

March through June. Average temperatures vary from 

-6.6 °C (20.2 °F) in January to 34.5 °C (94.1 °F) in 

July. The growing season varies from 90 to 120 days. 

Soils range from shallow rock outcrops to moderately 

deep gravelly, sandy, or silt loams. Predominant soil 

taxa are Aridisols, Entisols, Alfisols, Inceptisols, and 

Mollisols that occur in combination with mesic and 

frigid soil temperature regimes and xeric and aridic 

soil moisture regimes. Cryic temperature regimes occur at higher elevations typically above the western 

juniper woodlands belt.

Western juniper woodlands form a discontinuous 

belt between 1450 and 2100 m (4760 to 6890 ft) in 

elevation on Steens Mountain and a near continuous belt between 1300 and 2100 m (4265 to 6890 ft 

on South and Juniper Mountains). Above 2100 m 

(6890 ft), extremes in temperatures limit western juniper establishment (Miller and Rose 1995). 

Limited western juniper distribution in the basins 

or below the toe-slopes of the mountains is likely 

an artifact of late spring frosts (Billings 1954) coupled with limiting moisture. The National Resources 

Conservation Service has described the areas potential natural vegetation as sagebrush-grassland. 

Current vegetation is predominantly of two types, 

sagebrush-grasslands and western juniper woodlands 

(Burkhardt and Tisdale 1976; Johnson 2005; Miller 

and others 2000). Predominant potential vegetation 

occupying the uplands are: 1) mountain big sagebrush 

(Artemisia tridentata ssp. vaseyana) (Cronquist and 

others 1972) associated with bluebunch wheatgrass 

(Psuedoroegneria spicata), Idaho fescue (Festuca 

idahoensis Elmer), western needlegrass (Acnatherum 

occidentalis), and Thurber needlegrass (Acnatherum 

thurberianum) on relatively deep, well-drained soils; 

and 2) low sagebrush (Artemisia arbuscula ssp. arbuscula) associated with bluebunch wheatgrass, Idaho 

fescue, or Sandberg bluegrass (Poa secunda) over restrictive layers of claypan or bedrock (Burkhardt and 

Tisdale 1976; Johnson 2005). Scientific nomenclature 

follows Cronguist and others (1972-2005).

Shoshone Mountains Nevada: 

Piñon Pine and Utah juniper

The study site in Nevada was located on the Shoshone 

Mountains in Nye and Lander counties of central 

Nevada, about 56.3 km (35 mi) southwest of Austin, 

Nevada, in the High Central Ecological Province 

(fig. 1). Sample locations are on land managed by both 

the Austin District of the Humboldt-Toiyabe National 

Forest and the Battle Mountain District of the Bureau 

of Land Management. The Shoshone Mountains are 

a typical north-south trending mountain range characteristic of the Basin and Range topography of the 

Great Basin, and the section where the study is located 

is approximately 80.5 km (50 mi) long. Elevations of 

the valley bottoms to the east and west range from 

1800 to 2000 m (5900 to 6560 ft), and elevations of the 

peaks range from 2550 to 3140 m (8366 to 10,302 ft). 

The geomorphology of the Shoshone Mountains includes gently sloping valleys on either side, alluvial 

fans at the mountain base, and relatively steep mountain slopes. The North Shoshone Peak area, where the 

study is located, represents the remnant of a caldera 

with a lithology that consists primarily of welded and 

non-welded silica ash flow tuffs.

Climate in the region is the characteristic cool and 

semiarid climate of the central Great Basin. Average 

annual precipitation in the woodlands belt ranges from 

230 mm (9.1 inches) at the base of the mountain to 

over 600 mm (23.6 inches) on the mountain peaks. 

The majority of the precipitation comes as winter 

snow and spring rains. Average annual temperatures 

in Austin, Nevada, range from -7.2 °C (19.0 °F) in 

January to 29.4 °C (84.9 °F) in July. Soils are generally extremely coarse grained gravelly loams that have a 

weak to moderate structure. Some sandy or silt loams 

are present but limited in distribution.

The woodlands across the Shoshone Mountains, 

form a continuous to sometimes discontinuous belt 

between about 2050 and 2700 m (6725 and 8860 ft) 

elevation with stands at the highest elevation on 

southerly aspects. Sagebrush-grasslands and pinyonjuniper woodlands are the predominant vegetation 

types. Characteristic species include Wyoming big 

sagebrush (Artemisia tridentata ssp. wyomingensis) at 

lower elevations and mountain big sagebrush at higher 

elevations. Shallow soils, particularly on broad ridge 

tops and adjacent slopes at higher elevations, are dominated by low sagebrush. Perennial grasses include 

Sandberg bluegrass, bottlebrush squirreltail (Elymus 

elymoides), Needle and thread (Hesprostipa comata), 

Thurber needlegrass, Idaho fescue, and bluebunch 

wheatgrass. Numerous forb species are also present. 

Cheatgrass (Bromus tectorum) is not a large component in the area. Woodlands are all single-leaf pinyon 

dominated with scattered presence of Utah juniper. At 

the upper elevational boundary of the pinyon-juniper 

woodlands are found pockets of curl-leaf mahogany 

(Cercocarpus ledifolius).
4 USDA Forest Service RMRS-RP-69. 2008.

East Tintic Mountains Utah: 

Piñon Pine and Utah juniper

The Utah study site was located in the East Tintic 

Mountains of the eastern Great Basin about 9.7 km 

(5 mi) northwest of Eureka and about 48.3 km (30 mi) 

southwest of Provo in the Bonneville Ecological 

Province. The portion of the woodlands sampled was from Broad Canyon over the East Tintic 

Mountain Range summit (latitude, 40°01’ N; longitude, 112°15’ W) to Black Rock Canyon in Utah and 

Tooele counties. The area is managed by the Salt Lake 

District of the Bureau of Land Management. The East 

Tintic Mountains are composed of two relatively parallel ranges of north-trending fault-block ridges near 

the eastern edge of the Great Basin (Stokes 1986). 

They are world famous for rich ore deposits that have 

been discovered and mined. The northern part of the 

range, including Black Rock and Broad canyons, is 

composed of Paleozoic limestones, dolomites, and 

shales. Tertiary intrusives are present in a few places 

in Broad Canyon and have been extensively mined 

further south in the Eureka area. Southern and eastern 

parts of the range consist of Tertiary basalt and volcanic ash. The East Tintic Mountains may be thought of 

as the eroded remnants of a large composite volcano 

whose eruptions buried a pre-existing structurally 

complex mountain range (Stokes 1986). Boulter Peak 

at 2532 m (8305 ft) is the highest elevation in the East 

Tintic Mountains. The mouth of Black Rock Canyon 

(running east from the south end of Rush Valley) is 

1920 m (6300 ft) in elevation and the mouth of Broad 

Canyon (running south from the south end of Cedar 

Valley) is 1630 m (5350 ft) in elevation.

Precipitation ranges from 203 to 254 mm (8 to 

10 inches) in the valleys to about 30.5 cm (12 inches) 

at the base of the mountains to as much as 660 mm 

(26 inches) at the highest elevations. Much of the precipitation is in the form of winter snow and spring rains. 

In Eureka, Utah, elevation 2000 m (6560 ft), 39°57’ N 

latitude, 112°07’ W longitude, the average maximum 

daily annual temperature is 14.5 °C (59.2 °F), ranging 

from 2.1 °C (36.4 °F) in January to 30.0 °C (86.0 °F) 

in July (Ashcroft and others 1992).

The vegetation is dominated by Wyoming big sagebrush and black sagebrush (Artemisia nova) at low 

elevations giving way to a piñon-juniper woodlands, 

then to mixed conifer (Abies concolor, Psuedotsuga 

menziesii) on north facing slopes at higher elevations 

(West 1989), with mountain mahogany on exposed 

ridges and bigtooth maple (Acer grandidentatum) in 

canyons. Scrub oak (Quercus gambellii), common on 

the nearby Wasatch Mountains, is not present. The 

woodlands and conifer stands are interspersed and underlain by black sagebrush or mountain big sagebrush, 

mountain brush species (Purshia sp., Amelanchier 

sp., Symphoricarpos oreophilus, Pachystima myrsinites), bluebunch wheatgrass and Indian ricegrass

(Acnatherum hymenoides), and mountain phlox (Phlox 

austromontana) and Mojave sandwort (Arenaria macradenia). More common grasses and forbs associated 

with Wyoming big sagebrush are western wheatgrass 

(Pascopyrum smithii), Sandberg bluegrass, and desert 

alyssum (Alyssum desertorum). At higher elevations, 

muttongrass (Poa fendleriana), needle and thread, 

and tall fescue (Festuca arundinacea) become more 

common.

Methods

To gain a landscape scale perspective of both spatial expansion and development of piñon and juniper 

woodlands in 2001, we established seven primary 

transects varying in length from 14 to 24 km (8.7 to 

15 mi) across six different woodlands. Each transect 

was located along an elevational gradient, which extended from the lower to upper boundaries of each 

woodland. We used both field reconnaissance and 

aerial photos to establish transects in areas where 

there was minimal evidence of woodlands disturbance 

and to ensure sampling represented a wide array of 

environmental variables typical of sites occupied by 

juniper and piñon.

Along each primary transect, circular plots for the 

extensive sample were placed approximately every 

500 m (1640 ft) along three parallel transects spaced 

roughly 500 m (1640 ft) apart (fig. 2). Total number 

of plots sampled/transect varied from 69 to 99. We 

Figure 2. Plot layout and design. Circular plots were positioned along three parallel transects that extended 

from the low to upper elevation boundaries of a woodland.
USDA Forest Service RMRS-RP-69. 2008. 5

adjusted plot locations to fit within a uniform stand 

of at least 0.5 ha (1.2 acres) in size and representing a 

single ecological site with uniform characteristics (for 

example, aspect, topography, soil, and vegetation). 

A variable plot radius was employed to improve 

sampling efficiency. At each location the plot radius 

was selected based on a cursory estimation of stand 

density: 15, 20, and 30 m (49.2, 65.6, and 98.4 ft) 

radiuses were employed on plots with greater than 600 

trees/ha (240/acres), between 200 and 600 trees/ha 

(80 to 240/acres), and with less than 200 trees/ha 

(80/acres), respectively. Densities of presettlement 

trees were always counted in a 30 m radius plot. We 

identified and recorded dominant understory plant 

species and remnants of stumps and logs to determine 

the potential plant alliance and association.

In each extensively sampled plot, we also identified 

the phase of woodlands development (in other words, 

I, II, III or early, mid, and late successional) following 

the guidelines developed by Miller and others (2000, 

2005) (table 1). Phases are described as:

• Phase I, trees are present but shrubs and herbs are 

the dominant vegetation that influence ecological 

processes on the site;

• Phase II, trees are co-dominant with shrubs and 

herbs and all three vegetation layers influence 

ecological processes on the site; and

• Phase III, trees are the dominant vegetation and 

the primary plant layer influencing ecological 

processes on the site.

Tree canopy cover during Phase I is typically less 

than 10 percent and is comprised primarily of juveniles 

(<1 m [3.3 ft]) and saplings (1 to 3 m [3.3 to 9.8 ft]). 

Phase II stands are generally composed of young trees 

interspersed with older dominant trees (typically 80 to 

140 years). Tree cover in Phase II commonly approaches 10 to 49 percent, depending on the ecological site. 

Characteristics employed for differentiating between 

Phases II and III include estimating terminal and lateral leader growth and evaluating the status of the shrub 

layer. In Phase III terminal and lateral leader growth of 

understory trees are suppressed by intraspecific competition as stands approach a full stocking of dominant 

trees. Lateral leader growth in the lower 75 percent 

of the canopy is the first suppressed, typically <6 cm 

(2.4 inches). During Phase II, a portion the shrub layer 

remains intact but with obvious mortality taking place. 

In Phase III, live shrubs are largely absent and dead 

shrub skeletons are common to abundant.

Density of pre- and postsettlement trees was measured in each extensively sampled plot. Presettlement 

juniper trees were identified in the field using morphological characteristics described by Waichler and 

others (2001) and Miller and others (2005). For piñon, 

we used similar characteristics including flat tops and 

bark. We determined if a stand replacement tree disturbance had occurred on the plot (in other words was the 

stand treeless prior to postsettlement establishment) by 

recording standing and down dead trees and charred 

tree remnants. We also used these data to estimate the 

Table 1. Stand characteristics differentiating the three transitional phases of woodland succession for several mountain big sagebrush 

associations, including Thurber needlegrass (maximum juniper cover = 25 to 41 percent, Idaho fescue (maximum juniper cover 

34 to 58 percent), and Columbia needlegrass (maximum cover = 60 to 75 percent) (derived from Miller and others 2000).

Characteristics Phase I Phase II Phase III

(post-settlement stands) (early) (mid) (late)

Tree canopy (percent of max) open, actively actively expanding expansion nearly stabilized >30

expanding ≤10 10 to 30

Leader growth (dominant trees) (cm/yr) terminal >10 terminal >10 terminal >10

lateral >10 lateral 5 to10 lateral <5

Crown lift1 (dominant trees) absent absent lower limbs dying or dead where 

tree canopy >40 percent

Potential berry production low moderate to high low to near absent

Tree recruitment active active limited primarily to beneath trees 

or absent

Leader growth (understory trees) (cm/yr) terminal >10 terminal >10 terminal <5

lateral >8 lateral 2 to >8 lateral <2

Shrub layer intact nearly intact ≥75 percent dead

significant thinning

1

 Crown lift is the mortality of lower tree limbs, usually due to shading by neighboring trees.
6 USDA Forest Service RMRS-RP-69. 2008.

density of piñon or juniper prior to a stand replacement 

disturbance. Plots occupied by pre- and postsettlement 

trees and/or large charred stumps or logs were named 

mixed aged stands and plots with no evidence of presettlement trees were named postsettlement.

To estimate the time when the first trees established 

in sagebrush-grassland communities across the six 

woodlands, we aged the three largest trees on all plots. 

An increment core was collected at 30 cm (11.8 inches) above ground level. Cores were air-dried, mounted, 

sanded until the cell structure was visible with a binocular microscope, and rings counted. Most cores 

(98 percent) intersected or were within 10 rings of the 

pith. For samples that did not include the pith, we used 

transparent overlays of concentric circles to estimate 

the number of rings to pith (Villaba and Veblen 1997). 

Thirty juvenile trees (15 to 45 cm [6 to 18 in]) growing in the tree interspace of open (Phase I) woodlands 

were sampled and aged to correct for core sample 

height (Johnson and Miller 2006).

To measure the chronology of woodlands development at the stand level, we intensively measured a 

sub-sample of approximately 25 percent of the extensively sampled stands in three of the six woodlands 

(Juniper Mountain, Idaho; East Tintic, Utah; and 

Shoshone Mountain, Nevada). On Juniper Mountain 

three 6 by 60 m (20 by 197 ft) belt transects were established for tree measurements in each intensive plot. 

On the East Tintic and Shoshone Mountains, a 20 by 

50 m (65.5 by 164 ft) (0.1 ha) plot was used for tree 

measurements on each intensive plot. Tree density was 

determined by counting each tree rooted in the three, 

6 by 60 m (20 by 197 ft) belt transects or all trees 

in the 20 by 50 m (65.5 by 164 ft) plot in one of the 

tree classes described in the procedures for the extensive plots. An increment core was collected from trees 

greater than 6 cm (2.4 inches) in basal diameter 30 cm 

(11.8 inches) above ground level. Cross-sections at 

ground level were collected from trees smaller than 

6 cm in basal diameter for aging. We aged a minimum 

of 40 trees in each plot within the 6 by 60 m (19.6 by 

197 ft) belt transects. If 40 trees did not occur within 

the three transects, we randomly selected additional 

trees immediately to the belt transects.

Samples from all sites were mounted and sanded 

and rings were counted. If coring missed the pith, the 

pith radius was estimated graphically (Villaba and 

Veblen 1997). On each of the three mountain ranges, a 

sample of 30 juveniles (15 to 45 cm [6 to 18 inches]) 

was collected from interspace areas and the mean age 

of a tree 30 cm (12 inches) tall was determined to correct age for core sample height.

Analysis

Differences in age structure among the six different 

woodlands were determined using the KolmogorovSmirnov nonparametric test to compare age class 

distributions and the Kruskal-Wallis test to compare 

average age at p = 0.05 (Zar 1984).

Results

Woodlands Chronology

Woodlands expansion and infill

Our extensive and intensive data suggests large increases in piñon and juniper started after 1850 across 

all six locations as a result of infill into open low density presettlement tree stands in sagebrush-steppe and 

expansion into sagebrush-steppe communities previously treeless. Trees establishing after 1860 accounted 

for 90 percent or more of the population measured 

in the extensively sampled plots (table 2). In Utah, 

Nevada, and Oregon, trees establishing prior to 1860 

accounted for only 2 percent or less of the total population of piñon and juniper. In Idaho, presettlement trees 

were more abundant across the landscape, accounting 

for 5 to 10 percent of the population. Prior to 1860, 

mean tree densities based on live trees, stumps, and 

Table 2. Proportion of pre- (>140 years) and postsettlement (<140 years) trees and total tree densities in the 

extensively sampled plots, based on tree morphology and aging the three largest trees.

Old-growth (percent) Post-settlement (percent) Total # Trees

piñon juniper piñon juniper trees/ha sampled

East Tintic, UT 0.8 1 99.2 99 885 4646

Shoshone, NV 0.6 1.5 99.4 98.5 344 3741

South Mt., ID - 5 - 95 197 4843

Juniper Mt., ID - 10 - 90 537 4332

South Steens, OR - 1 - 99 282 2816

North Steens, OR - 2 - 98 264 3345
USDA Forest Service RMRS-RP-69. 2008. 7

logs across South Mountain and Juniper Mountain, 

Idaho were 20 and 27/ha (8 and 11/acres), respectively. Frequency of occurrence of presettlement trees in 

the extensively measured plots in the six woodlands 

varied from 16 to 67 percent (table 3). In Oregon, 

Nevada, and Utah, presettlement trees occurred on 16 

to 30 percent of the extensive sampled stands in each 

state. However, in Idaho, presettlement trees occurred 

on nearly half or more of stands. Percent of historic sagebrush communities invaded

25

20

15

10

5

0

25

20

15

10

5

0

25

20

15

10

5

0

25

20

15

10

5

0

25

20

15

10

5

0

25

20

15

10

5

0

East Tintic, UT

South Steens Mt., OR South Mt., ID

North Steens Mt., OR Juniper Mt., ID

Shoshone Mt., NV

Decade

1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000

Figure 3. The proportion of decadal encroachment of piñon and/or juniper between 1860 and 2000 into historic sagebrushsteppe stands with no evidence of presettlement trees.

Table 3. Proportion of stands with one or more trees older than 140 years (mixed 

aged), and stands with all trees less than 140 years (postsettlement) in the 

extensively sampled plots.

Mixed aged Postsettlement Number of

Site plots (percent) plots (percent) plots

East Tintic, UT 20 80 75

Shoshone, NV 22 78 87

South Mt., ID 67 33 87

Juniper Mt., ID 48 52 69

South Steens, OR 16 84 78

North Steens, OR 30 70 99

Temporal pattern of woodlands expansion into 

shrub-steppe

The lack of large remnant wood or standing presettlement trees inferred nearly two-thirds of the 

landscapes measured were treeless prior to 1860. The 

period of most rapid expansion of piñon and juniper 

occurred between 1900 and 1920 in Utah, Nevada, 

and Oregon and 1880 to 1920 in Idaho (fig. 3). By 

1920, 50 to 73.5 percent of the sagebrush-steppe 

communities had been encroached by juniper and or 
8 USDA Forest Service RMRS-RP-69. 2008.

piñon. Although woodlands have continued to expand, rates of expansion have decreased since 1950 

compared to the first half of the 20th Century, possibly 

resulting in a decline in treeless plant communities 

open for first time expansion. In Idaho, expansion 

into sagebrush-steppe communities occurred at a faster rate during the late 1800s compared to the other 

regions. In the two Idaho woodlands, western juniper 

had encroached 75 and 78 percent of the mountain 

big sagebrush stands previously treeless by 1920. By 

the end of the 20th Century, trees remained absent on 

32 and 13 percent of the plots on the Shoshone and 

East Tintic Mountains, respectively, and absent on 

only 7 and 2 percent of the plots in Idaho and Oregon, 

respectively.

Temporal pattern of tree establishment

Tree establishment in both mixed age (infill) and 

shrub-steppe (expansion) began to increase after 1850 

and retained relatively high rates well into the 20th

Century, declining after 1950 (fig. 4). Rates of tree 

Decade

East Tintic, UT

Postsettlement stands

780 trees/ha

Percent established

40

30

20

10

0

15

10

5

0

15

10

5

0

Shoshone Mountain, NV

Postsettlement stands

299 trees/ha

Juniper Mountain, ID

Postsettlement stands

570 trees/ha

Mixed aged stands

952 trees/ha

Mixed aged stands

403 trees/ha

Mixed aged stands

502 trees/ha

Across all stands

885 trees/ha

Across all stands

344 trees/ha

Across all stands

537 trees/ha

40

30

20

10

0

15

10

5

0

15

10

5

0

15

10

5

0

15

10

5

0

40

30

20

10

0

1600 1650 1700 1750 1800 1850 1900 1950 2000 1600 1650 1700 1750 1800 1850 1900 1950 2000 1600 1650 1700 1750 1800 1850 1900 1950 2000

Figure 4. Current mean tree density and decadal establishment of piñon and juniper for postsettlement and mixed aged 

stands, and averaged across all stands for each location. Postsettlement stands are composed of trees establishing after 

1860 and contain no evidence of trees (stumps or snags) that established prior to 1860. Mixed aged stands are composed 

of trees that established prior to and after 1860. Across all stands represents decadal establishment for the entire woodland at each geographic location.

establishment between 1850 and 1900 appeared to be 

greater in mixed age stands than postsettlement stands. 

Although piñon and juniper followed the same general 

patterns of expansion and establishment across Utah, 

Nevada, and Idaho, the age structure of postsettlement 

stands was significantly different. The average age of 

trees in stands that developed after 1850 in Idaho were 

older (p <0.05) than in Utah and Nevada, and the average age of trees in Nevada were older (p <0.05) than in 

Utah. This was a result of an earlier beginning of tree 

establishment between 1860 and 1920 in Idaho compared to the other two locations (fig. 5). Total number 

of trees that established also differed across the three 

locations (fig. 4). Total density of Utah juniper trees in 

Utah was greater than woodlands in Oregon, Nevada, 

and Idaho.

Stage of woodlands development

The stage of development across all six woodlands 

varied from low density open stands of trees (Phase I) 

to dense closed stands (Phase III) (fig. 6). The majority 
USDA Forest Service RMRS-RP-69. 2008. 9

of stands along the Utah, Nevada, and north Steens, 

Oregon transects were still in the early to mid stages 

of woodlands development with less than 20 percent 

of the woodlands in Phase III. Stages of woodlands 

development in stands on South Mountain, Idaho and 

south Steens, Oregon, were nearly evenly split across 

all three Phases. Juniper Mountain, Idaho, was the only 

woodlands where over 50 percent of the stands were 

in Phase III. Initiation of tree establishment in stands 

currently in Phase I across Utah, Nevada, and Idaho 

occurred consistently later than stands in Phase II and 

III (fig. 7). In Phase I stands, tree establishment began slowly in the early 1900s and peak establishment 

occurred between 1960 and 1970. Declines in tree establishment have occurred across all three woodlands 

in Phase I during the last 2 to 3 decades of the 20th

Century. However, the majority of Phase I stands have 

a high enough tree density to fully occupy the sites.

Discussion

Woodlands Expansion and Infill

For over 200 years prior to settlement, woodlands 

across all sites sampled were relatively low density 

with limited rates of establishment. Our data supported other work suggesting substantial increases in 

piñon and juniper have occurred in the Intermountain 

Region since the late 1800s (Burkhardt and Tisdale 

1976; Cottam and Stewart 1940; Gedney and others 

1999; Knapp and Soulé 1998; Miller and Rose 1995, 

1999; Soulé and Knapp 2000; Tausch and others 

Frequency of tree establishment percent 100

80

60

40

20

0

Juniper Mt., ID

Shoshone Mts., NV

East Tintic, UT

Decade

1860 1880 1900 1920 1940 1960 1980 2000

Figure 5. Accumulative frequency of decadal tree 

establishment since 1860 for three woodland 

populations in the intensive plots.

Percent of total

Study site

60

50

40

30

20

10

0

NV UT IDsMT IDjMt ORn ORs

Phase I

Phase II

Phase III

Figure 6. The proportion of stands across extensive plots in each of the three different 

phases of woodland succession for the six 

study sites (Nevada = NV, Utah = UT, South 

Mountain Idaho = IDsMt, Juniper Mountain 

Idaho = IDjMt, North Steens Oregon = ORn, 

and South Steens Oregon = ORs). The average across all stands was 34 percent Phase I, 

40 percent Phase II, and 27 percent Phase III. 

Phase I—trees are present, but shrubs and 

herbs are the dominant vegetation that influence ecological processes on the site; Phase 

II—trees are co-dominant with shrubs and 

herbs and all three vegetation layers influence ecological processes on the site; and 

Phase III—trees are the dominant vegetation 

and the primary plant layer influencing ecological processes on the site.
10 USDA Forest Service RMRS-RP-69. 2008.

1981). Increases were the result of both infill in mixed 

age tree communities and expansion into shrub-steppe 

communities that appeared to not have supported trees 

over the past several centuries. Presettlement trees 

accounted for 5 and 10 percent of the population in 

Idaho and less than 2 percent on the remaining sites 

measured (table 2). This is similar to estimates reported for piñon and juniper woodlands in the Great 

Basin (Miller and others 1999; Miller and Tausch 

2001). However, the proportion of old-growth can 

vary across different ecological provinces, resulting 

from different disturbance regimes, soils, and climate. 

Old-growth western juniper in the pumice-sand region 

of the Mazama Ecological Province is estimated to occupy over 50 percent of the stands (Miller and others 

2005). Eisenhart (2004) documented that presettlement trees occupied the majority of stands measured 

on the Uncopahgre Plateau in western Colorado. And 

on Mesa Verde in Colorado, the majority of stands 

measured were composed of trees greater than 140 

years old (Floyd and others 2000). This suggests some 

sites are capable of supporting persistent woodlands, 

possibly a consequence of soils and climate resulting 

in infrequent stand replacement disturbance regimes. 

The shift from a relatively stable or limited rate of 

establishment to a substantial increase in conifer establishment in both space and time is generally attributed 

to the reduced role of fire, introduction of domestic 

livestock grazing, and shifts in climate (Burkhardt and 

Tisdale 1976; Heyerdahl and others 2006; Miller and 

Heyerdahl, in press; Miller and Rose 1999; Tausch 

1999). Under current climatic conditions, conifers are 

likely to continue expanding into shrub-steppe plant 

communities (Betancourt 1987; Miller and others 

2000; West and Van Pelt 1986).

Temporal Pattern of Woodlands 

Expansion into Shrub-Steppe

Since 1860, woodlands expansion across the six 

woodlands studied has resulted in the replacement 

of sagebrush-steppe plant communities. Evidence of 

presettlement trees (live, standing and down dead, and 

or remnants of large wood and charcoal) was found 

on 16 to 67 percent of the stands sampled, suggesting 

the current area occupied by trees has increased 140 

to 625 percent since 1860. Gedney and others (1999) 

compared two U.S. Forest Service surveys conducted 

Phase I

Phase II

Phase III

Trees/ha

Decade

50

40

30

20

10

0

140

120

100

80

60

40

20

0

140

120

100

80

60

40

20

0

Shoshonee Mountains, NV

East Tintic, UT

Juniper Mountain, ID

1800 1850 1900 1950 2000

Figure 7. Establishment of piñon and juniper (trees/ha by decade) for each 

successional phase (Phases I, II, and 

III) in three woodlands across the 

intensive plots. Phase I—trees are 

present, but shrubs and herbs are the 

dominant vegetation that influence 

ecological processes on the site; Phase 

II—trees are co-dominant with shrubs 

and herbs and all three vegetation layers influence ecological processes on 

the site; and Phase III—trees are the 

dominant vegetation and the primary 

plant layer influencing ecological processes on the site.
USDA Forest Service RMRS-RP-69. 2008. 11

in 1938 and 1988 across eastern Oregon and reported 

a 600 percent increase in area occupied by western 

juniper. Cottam and Stewart (1940) also reported a 

600 percent increase in area occupied by Utah juniper in southwest Utah between 1864 and 1940. In the 

Great Basin, old-growth trees have been reported by 

numerous authors to typically grow on rocky shallow 

or sandy soils that support little understory vegetation 

to carry a fire (Burkhardt and Tisdale 1976; Holmes 

and others 1986; Miller and Rose 1995, 1999; West and 

others 1998; Young and Evans 1981). In both the Idaho 

and Oregon woodlands, old-growth trees were associated with sites having a significantly greater cover of 

surface rock than where old-growth trees were absent 

(Johnson and Miller, in press). This was usually the 

case across all woodlands studied with the exception 

of the stands in Idaho. Within these woodlands, oldgrowth trees were found both on rocky, shallow soils 

and deep, well drained soils associated with mountain 

big sagebrush. In Idaho prior to the 1860s, trees occupied the landscape as widely scattered individuals 

or small stands of trees rarely exceeding a hectare, and 

accounted for 5 to 10 percent of the current population. This suggests a past fire regime characterized 

by frequent, lower intensity understory fires in open 

sagebrush/bunchgrass communities across much of 

the Owyhee Mountains. This fire regime resulted in a 

low density of widely scattered stands of trees, which 

is in contrast to the more commonly reported stand 

replacement fires that occur in woodlands (Baker and 

Shinneman 2004; Miller and others 2005; Miller and 

Tausch 2001) or fire return intervals and burn intensities that limited the development of mature trees in big 

sagebrush grassland communities (Miller and Rose 

1999; Miller and Tausch 2001; Miller and Heyerdahl, 

in press).

The most rapid period of expansion occurred between 1880 and 1920, which also coincided with the 

most rapid period of tree establishment. We were surprised by the consistent decline in expansion rates 

across all of the woodlands after 1950. The decline 

may be attributed to several factors. Climatic conditions were relatively milder and wetter during the 

early compared to latter part of the 20th Century. A 

shift in climate beginning in the 1960s is also evident 

for many regions of the world (Kerr 2007). Mild wet 

climatic conditions enhance juniper seedling establishment (Fritts 1974; Fritts and Wu 1986). A second 

factor may be the reduced levels of sagebrush-steppe 

plant communities still open to invasion by piñon or 

juniper within the six landscapes measured. This is in 

addition to the decline in sagebrush cover resulting in 

a reduction in safe-sites for tree establishment.

Tree Establishment

In the mid to late 1800s, the number of piñon and 

juniper trees establishing per decade began to increase compared to the previous several hundred 

years. Across the six woodlands, presettlement trees 

densities averaged 5 to 27/ha (2 to 11/acre). Current 

stand densities range from 197 to 885 trees/ha (80 to 

358/acre) (table 2) a 10- to 100-fold increase. The pattern of western juniper postsettlement establishment 

was similar to that of piñon and juniper in Utah and 

Nevada during this same period. Numerous other authors have reported increases in piñon, Utah juniper, 

and western juniper since the late 1800s (Adams 1975; 

Burkhardt and Tisdale 1976; Cottam and Stewart 1940; 

Gedney and others 1999; Miller and others 2005; 

Miller and Rose 1995, 1999; Tausch and West 1988, 

1995; Tausch and others 1981). In Utah and Nevada, 

increased establishment rates of piñon and Utah juniper 

began after 1860. This is similar to the period reported 

by the majority of others surveying woodlands in the 

Intermountain Region (Adams 1975; Blackburn and 

Tueller 1970; Burkhardt and Tisdale 1976; Eddleman 

1987; Gedney and others 1999; Gruell 1999; Miller 

and Rose 1995, 1999; Wall and others 2001). The timing of increased tree establishment after the 1860s in 

most areas is coincidental with the introduction of livestock and the reduction of fire occurrences (Swetnam 

1993; Miller and Rose 1999).

Western juniper establishment across the Idaho 

woodlands, however, began to increase around 1850, 

occurring just prior to the introduction of livestock and 

following the end of the Little Ice Age. Several other 

authors have reported increased establishment just prior to 1860 (Tausch and others 1981, Tausch and West 

1988). In Idaho, the early increase may be attributed 

to a shift to milder temperatures and wetter conditions 

following the end of the Little Ice Age (Antevs 1938; 

Graumlich 1987; LaMarche 1974; Wahl and Lawson 

1970) in addition to greater potential seed source, a 

result of the greater abundance of presettlement trees. 

Johnson and Miller (in press) reported stands occupied 

by at least one tree >140 years initiated woodlands 

expansion 24 years earlier than stands lacking presettlement trees.

The general pattern of tree establishment was a 

gradual increase during the latter half of the 1800s 

and then a large increase during the early to mid 

1900s followed by a decline in establishment during 

the second half of the 1900s. Other studies have reported tree establishment peaked during the first half 

of the 1900s (Burkhardt and Tisdale 1976; Gedney 

and others 1999; Miller and Rose 1999; Tausch and 
12 USDA Forest Service RMRS-RP-69. 2008.

others 1981; Tausch and West 1988; Wall 

and others 2001). The increasing rate of establishment during the late 1800s and early 

1900s may have been the consequence of 

optimal climatic conditions between 1850 to 

around 1920 and an increasing seed source 

resulting in an increasing population of seed 

disseminators. From 1850 to 1916 in much 

of the Great Basin, winters were generally 

milder and precipitation greater than the 

current long-term average (Antevs 1938; 

Graumlich 1987; LaMarche 1974; Wahl and 

Lawson 1970).

The rate of tree establishment occurred 

most rapidly in the two Idaho woodlands 

between 1860 and 1890, resulting in stands 

reaching closure (Phase III) more rapidly. Tree intraspecific competition in these 

stands started in the 1950s (Johnson and 

Miller 2006). The varying rates of tree establishment (figs. 4 and 5) are a function of the 

amount of seed input and success of seedling 

establishment on a site. Both woodlands in 

Idaho had the greatest density and distribution of presettlement trees, potentially 

resulting in a greater input of seed. Besides 

seed input, the timing and amounts of available water, temperature, and the abundance 

of safe sites (for example, beneath shrubs) 

are all factors that can influence the rate of 

tree establishment (Burkhardt and Tisdale 

1976; Chambers 2001; Eddleman 1987; 

Everett and Ward 1984; Miller and Rose 

1995).

The consistent decline in the number of trees establishing in the late 1900s across the three woodlands 

was not expected in Phase I stands. As trees increase 

in dominance and shrubs that provide safe sites for 

tree establishment decline (Miller and others 2000) we 

would expect a decline in establishment to occur during Phase II and be very limited in Phase III. However, 

we measured a general decline in establishment across 

all phases. This same trend has been reported on several other sites (Miller and Heyerdahl, in press). In 

addition to a decline in safe sites for tree establishment, this may be the result of climate, which in part 

would explain the decline in Phase I.

Stage of Woodlands Development

With the exception of the woodlands on Juniper 

Mountain, Idaho, almost 40 percent of the stands measured across the six woodlands were still in Phase I 

and another 40 percent in Phase II. This has implications for future changes that will occur within these 

woodlands (fig. 8). Assuming that past growth rates 

will continue into the future, tree dominated woodlands (Phase III) will increase from the current 20 to 

nearly 75 percent of the total woodland, within the next 

30 to 50 years (Johnson and Miller 2006). Tree establishment started considerably later in stands in Phase I 

compared to stands in Phases II and III. During the 

transition from shrub-steppe to woodland, understory 

species, particularly the shrubs decrease (Miller and 

others 2000). The majority of this loss from increasing tree dominance has yet to occur in Phase I and 

Figure 8. Woodlands closure over the 34-year period from 

1973 to 2007, resulting in a shift from Phase I and II to 

Phase II and III in the Shoshone Mountains, Nevada 

(photos by Robin Tausch).
USDA Forest Service RMRS-RP-69. 2008. 13

II woodlands. However, as stands approach Phase III, 

shifts in biomass over large areas from ground fuels 

to canopy fuels will significantly impact fire behavior. 

The more tree dominated piñon and juniper woodlands 

become, the less likely they are to burn under moderate 

conditions, resulting in infrequent high intensity fires. 

As Phase II approaches III, implementing prescribed 

fire, particularly in woodlands without a significant 

component of pinyon, becomes more difficult to apply 

in the absence some sort of mechanical pretreatment. 

This transition will occur on about half the current 

woodlands over the next 40 years. In addition, seed 

sources and other propagules of the understory species 

dramatically declines as the developing woodlands 

enter Phase III. The expansion and now increasing 

tree dominance in the resulting woodlands is putting 

remaining old-growth woodlands at increased risk of 

loss from high intensity fires (Tausch 1999). Scattered 

old-growth trees that are associated with sagebrush 

communities increases landscape heterogeneity, which 

can significantly increase the abundance and diversity 

of wildlife species (Reinkensmeyer and others 2007).

Management Implications

If management goals are to restore landscape function and processes to near presettlement conditions in 

the Great Basin, baseline information is needed that 

define the proportion of sagebrush-steppe and piñonjuniper woodlands on a landscape basis. Based on a 

broad examination of woodland age structure, our 

results indicate three general communities intermingled with one another at varying proportions across 

the landscape; (1) sagebrush-steppe with no trees, 

(2) sagebrush-steppe associated with a scattered lowdensity of trees, and (3) piñon and juniper woodlands. 

Their distribution is often determined by location 

specific interactions of slope, aspect, elevation, and 

topography. Prior to 1860 two-thirds of the landscape 

was treeless and occupied by sagebrush-steppe communities. Today, less than one-third of the landscape 

remains treeless and more than 90 percent of the trees 

have established since the 1860s. These data support 

active management in tree removal for over half of the 

landscape. In the absence of disturbance, woodlands 

will continue to expand, mature, and close, with the 

majority of these woodlands reaching Phase III over 

the next 40 to 50 years. To restore these landscapes 

managers should take the following steps:

• Inventory the distribution and density of 

presettlement trees, stumps or snags based on tree 

morphological characteristics defined by Miller 

and others (2006) to establish the proportion of 

shrub-steppe and woodland communities.

• Be sensitive to the presence of old-growth and 

develop their management plans accordingly to 

retain these trees and stands on the landscape.

• Emphasize treatments in woodlands in Phase I and 

II, particularly those on more productive sites with 

deeper soils, where treatment costs will be less and 

potential for success is greater than in Phase III 

woodlands.

The lack of active management will potentially 

result in the continued decline of historic sagebrush 

communities, structural diversity, understory species, herbaceous production, habitat for sagebrush 

obligates, and landscape heterogeneity. As a greater 

proportion of the landscape shifts towards Phase III 

the risk of larger, intensive wildfires and conversion to 

annual exotics will increase, as will the cost of treatment, and the potential for desirable outcomes will 

decrease. Infilling by younger trees also increases the 

risk for the loss of presettlement trees due to increased 

fire severity and size resulting from the increase in the 

abundance and landscape level continuity of fuels.

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