United States Department of Agriculture

Forest Rocky Mountain General Technical Report January 2018

Service Research Station RMRS-GTR-372

Fire Patterns in Piñon and Juniper 

Land Cover Types in the Semiarid 

Western United States 

From 1984 Through 2013

David I. Board, Jeanne C. Chambers, 

Richard F. Miller, and Peter J. Weisberg
Board, David I.; Chambers, Jeanne C.; Miller, Richard F.; Weisberg, Peter J. 2018. 

Fire patterns in piñon and juniper land cover types in the Semiarid 

Western United States from 1984 through 2013. RMRS-GTR-372. Fort 

Collins, CO: U.S Department of Agriculture, Forest Service, Rocky Mountain 

Research Station. 57 p.

Abstract 

Increases in area burned and fire size have been reported across a wide range 

of forest and shrubland types in the Western United States in recent decades, but 

little is known about potential changes in fire regimes of piñon and juniper land 

cover types. We evaluated spatio-temporal patterns of fire in piñon and juniper land 

cover types from the National Gap Analysis Program using Monitoring Trends in 

Burn Severity (MTBS 2016) data (1984 through 2013) for Northern and Southern 

Intermountain and Central and Southern Rocky Mountain geographic regions. We 

examined differences in total area burned, fire rotation, fire size, fire number, and 

fire season among: (1) the four geographic regions; (2) the EPA level III ecoregions 

that occur within each geographic region; and (3) the piñon and juniper land cover 

types (woodlands, savannas, and shrublands) and other land cover types that occur 

within each geographic region and level III ecoregion. We found that area burned 

during the 30-year period, number of fires each year, and fire size followed a strong 

geographic pattern: Northern Intermountain > Southern Intermountain > Southern 

Rocky Mountain > Central Rocky Mountain. Area burned within piñon and juniper 

land cover types increased significantly during the 30-year period across the study 

area overall and for each geographic region, except the Southern Intermountain. 

Fire rotations were within reported historical ranges for sagebrush ecosystems 

and decreased over time. Also, fire number or fire size increased for the Southern 

Rocky Mountain and Southern Intermountain geographic regions. Across the study 

area, spatio-temporal patterns in fire regimes for piñon and juniper land cover types 

were similar to those for other land cover types. Careful monitoring of longer term 

trends in fire activity and the interacting effects of invasive annual grasses, bark 

beetles, and climate change is needed to access the dynamics of piñon and juniper 

land cover types and evaluate the efficacy of management treatments in piñon and 

juniper land cover types. 

Keywords: piñon, juniper, land cover types, fire size, fire number, fire season, 

total area burned, fire rotation, Intermountain geographic region, Rocky Mountain 

geographic region

Cover—A prescribed fire burning through Pinus monophylla and Juniperus osteosperma in 

the Great Basin Piñon-Juniper Woodland of the Southern Intermountain geographic region. 

(Photo by Jeanne Chambers, RMRS.)
Authors

David I. Board, Ecologist/Data Analyst, USDA Forest Service, Rocky Mountain 

Research Station, Reno, Nevada.

Jeanne C. Chambers, Research Ecologist, USDA Forest Service, Rocky Mountain 

Research Station, Reno, Nevada.

Richard F. Miller, Professor Emeritus, Oregon Agricultural Research Center, 

Oregon State University, Corvallis.

Peter J. Weisberg, Department of Natural Resources and Environmental Science, 

University of Nevada, Reno.

Acknowledgments

We thank Matthew L. Brooks, Douglas J. Shinneman, and Michele R. Crist for 

helpful review comments.

All Rocky Mountain Research Station publications are published by U.S. Forest Service 

employees and are in the public domain and available at no cost. Even though U.S. Forest 

Service publications are not copyrighted, they are formatted according to U.S. Department of 

Agriculture standards and research findings and formatting cannot be altered in reprints. Altering 

content or formatting, including the cover and title page, is strictly prohibited.
iv

Table of Contents

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

Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Data and Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Land Cover Type Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Fire Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Analysis of Fire Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Fire Patterns Across the Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Fire Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Fire Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Fire Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Fire Season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Fire Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Fire Patterns Within the Geographic Regions . . . . . . . . . . . . . . . . . . . . . 14

Fire Patterns in the Northern Intermountain Geographic Region . . . . . . . 17

Fire Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Fire Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Fire Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Fire Season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Fire Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Fire Patterns in the Southern Intermountain Geographic Region . . . . . . 24

Fire Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Fire Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Fire Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Fire Season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Fire Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Fire Patterns in the Central Rocky Mountain Geographic Region . . . . . . 29

Fire Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Fire Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Fire Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Fire Season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Fire Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Fire Patterns in the Southern Rocky Mountain Geographic Region . . . . . 34

Fire Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Fire Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Fire Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Fire Season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Fire Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Regional Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Changes in Fire Regimes in Piñon and Juniper Land Cover Types . . . . . 42

Differences Between Piñon and Juniper and Other Land Cover Types . . 44

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
USDA Forest Service RMRS-GTR-372. 2018. 1

Introduction

Increases in burned area and fire size have been reported for much of the semiarid Western United States in recent decades (Brooks et al. 2015; Dennison et al. 

2014; Littell et al. 2009; Westerling et al. 2006), but few studies have evaluated 

possible changes in fire regimes in piñon and juniper land cover types. In areas 

where increases in fire area or size have been reported, the increases have often 

been attributed to changes in plant composition and structure (Miller et al. 2011, 

2013). In many warmer and drier areas, invasive annual grasses are increasing the 

abundance and continuity of fine fuels and resulting in more frequent and larger 

fires (Balch et al. 2013; Brooks et al. 2004, 2015). In many cooler and moister areas 

piñon and juniper have infilled preexisting woodlands and expanded into shrubland 

communities resulting in increases in woody fuels and potentially higher fire severity (Barger et al. 2009; Biondi and Bradley 2013; Floyd et al. 2004; Miller et al. 

2008a). Climate warming has exacerbated the changes in fire regimes by increasing 

the length of the fire season and resulting in more extreme fire weather (McKenzie 

et al. 2004; Westerling et al. 2006, 2014). In addition, human population growth 

and increased land use and development have increased fire ignitions in many areas 

(Short et al. 2016). Here, we evaluated the spatio-temporal patterns of fire in piñon 

and juniper land cover types in the semi-arid Intermountain West for 1984–2013.

Piñon and juniper land cover types span a broad range of climatic regimes, are 

associated with numerous distinct vegetation types (West 1983, 1984), and exhibit 

different fire regimes (Romme et al. 2009). In general, fire regimes are influenced 

by the effects of topography and regional climate on composition, structure, 

and productivity of vegetation, seasonal patterns of fuel moisture, and ignitions 

(Abatzoglou and Kolden 2013; Littell et al. 2009; Liu and Wimberly 2015, 2016; 

Parks et al. 2012). In areas with piñon and juniper, local fire behavior and mortality 

of piñon and juniper trees are influenced by understory fuel characteristics, tree 

density, biomass and canopy cover, and fire weather (fig. 1; Arnold et al. 1964; 

Brown 1982; Dwyer and Pieper 1967; Martin 1978; Romme et al. 2009; Wright et 

al. 1979). In areas dominated by piñon and juniper trees with minimal understory 

cover, fire spread and mortality of trees greater than 2 m in height are typically low, 

but increase with tree density, biomass, canopy cover, and extreme fire weather 

(Blackburn and Bruner 1975; Wright et al. 1979). In areas where native grasses 

dominate the understory and shrubs are sparse, fire spread is moderate and mortality 

of trees greater than 2 m is low where tree densities are low to moderate. However, 

small trees (<1.3 m) are easily killed by fire (Arnold et al. 1964; Dwyer and Pieper 

1967; Jameson 1962) even under relatively mild to moderate weather conditions 

(Martin 1978). In areas where shrubs are dominant or codominant in the understory, 

fire intensities are typically moderate to high (Brown 1982) resulting in high mortality of trees (Martin 1978). Mortality of larger trees increases as a result of extreme 

fire weather, increasing shrub density providing ladder fuels, and increasing proportion of piñon to juniper.

Increases in tree density result in reduction of understory grasses and shrubs due 

to competition for resources, and high density stands are typically characterized by 

low understory biomass (Blackburn and Tueller 1970; Bristow et al. 2014; Bybee 

et al. 2016; Miller et al. 2000; Roundy et al. 2014; Tausch and West 1995; Young 

et al. 2015). Fires in late successional piñon and juniper vegetation with high bulk 
2 USDA Forest Service RMRS-GTR-372. 2018.

density of canopy fuels tend to be high severity and stand replacing (Bates et al. 

2014; Romme et al. 2009). There is little evidence of frequent, spreading, lowintensity fire in most piñon and juniper vegetation types such as occur in ponderosa 

pine types (Baker and Shinneman 2004). However, piñon and juniper savanna types 

with a high proportion of grasses are believed to have experienced low-severity fires 

that would have removed the small trees (Margolis 2014).

There are limited data on pre-settlement fire size or spatial distribution for the 

Western United States (Liu and Wimberly 2015, 2016) including for piñon and juniper vegetation types (Romme et al. 2009). Fire occurrence can be reported as fire 

rotation (the time required for the cumulative area burned to equal the size of the 

study area) (Sugihara et al. 2006). Although fire rotation does not account for the 

heterogeneity of fire frequency at local scales, it is useful in describing fire regimes 

at more regional scales. Existing data for pre-settlement piñon and juniper vegetation indicate that historical fire rotations ranged from about 400 to 600+ years for 

sites that included central Nevada (Bauer and Weisberg 2009), southern California 

Figure 1—Probable fire behavior following a single ignition event in piñon and juniper vegetation with respect to variablity in tree density (horizontal axis) and understory fuel characterization (veritcal axis). Split cells reflect variable fire 

behavior, spread dynamics, and tree mortality under modal (80th percentile) in the unshaded upper left versus extreme 

(96th percentile) fire weather conditions in the shaded lower right. (Used with permission from Romme et al. 2009.)
USDA Forest Service RMRS-GTR-372. 2018. 3

(Wangler and Minnich 1996), western Colorado (Shinneman and Baker 2009a), 

southern Utah (Floyd et al. 2008) and central Oregon (Waichler et al. 2001).

Fire history studies generally report that the large, stand-replacing fires that have 

become more frequent in recent decades, also occurred historically (Floyd et al. 

2004; Miller and Heyerdahl 2008; Romme et al. 2009). Increased frequency of 

high-severity fires has been attributed to tree expansion and increased tree density 

since the late 1800s (Romme et al. 2009), although some regions show minimal 

net change in density or extent of piñon and juniper vegetation since the 1930s 

(Manier et al. 2005). Increased tree densities in expansion areas (Miller et al. 2008b; 

Weisberg et al. 2007) have resulted in an increase in biomass (woody fuels) with the 

potential to increase high-intensity crown fires (Miller and Tausch 2001).

The primary objective of this report was to investigate the fire regimes of piñon 

and juniper land cover types in the Northern and Southern Intermountain and 

Central and Southern Rocky Mountain geographic regions of the semi-arid Western 

United States. We evaluated differences in total area burned, fire rotations, number 

of fires, length of fire season, and fire size among: (1) the four geographic regions; 

(2) level III ecoregions (USEPA 2013) that occur within each geographic region; 

and (3) piñon and juniper land cover types (USGS 2011) that occurred within each 

geographic region and level III ecoregion. We based our analyses on piñon and 

juniper and other land cover types from the National Gap Analysis Program (USGS 

2011) and on fire data from the Monitoring Trends in Burn Severity Program 

(MTBS 2016) for 1984 through 2013. We asked three questions: (1) how do fire 

regimes in the piñon and juniper land cover types vary geographically; (2) how have 

fire regimes changed in the regions with piñon and juniper land cover types in the 

last 30 years; and (3) are piñon and juniper land cover types more or less likely to 

burn than adjacent land cover types within the same region?

Study Area

The study area encompasses the distribution of two species of piñon, Pinus 

edulis Engelm. and P. monophylla Torr. & Frém. (fig. 2; Cole et al. 2008, 2013) 

and three species of juniper, Juniperus osteosperma (Torr.) Little, J. occidentalis

Hook., and J. deppeana Steud. (fig. 3; Little 1971; Adams 2014). The study area 

also includes portions of the distributions of J. scopulorum Sarg., J. grandis R.P. 

Adams, and J. monosperma (Engelm.) Sarg (fig. 3). Fifteen level III ecoregions 

(USEPA 2013) occur within the study area. For our analyses, we aggregated these 

ecoregions into four geographic regions based on climate, topography, and dominant vegetation types: (1) Northern Intermountain geographic region; (2) Southern 

Intermountain geographic region; (3) Central Rocky Mountain geographic region; 

and (4) Southern Rocky Mountain geographic region (fig. 4).

The ecoregions are characterized by distinct temperature and precipitation 

regimes (fig. 5) and are strongly influenced by the amount of precipitation received 

in winter versus summer (fig. 6). The Southern Rocky Mountains geographic region 

is influenced by monsoonal flows, with a bimodal precipitation pattern that has 

peaks in both the summer (June, July, and August) and winter (December, January, 

and February) months (fig. 6) with some areas receiving as much as 64 percent of 

the annual precipitation in summer. The Central Rocky Mountain geographic region 

receives relatively less precipitation in summer (27%); however, some areas receive 

as much as 37 percent of the annual precipitation in summer.
4 USDA Forest Service RMRS-GTR-372. 2018.

Figure 2—Distribution of the two piñon species of interest in the four geographic regions. Two varieties of Pinus monophylla are included (Cole et al. 

2008, 2013). 

In the Northern Intermountain and Southern Intermountain geographic regions 

most precipitation (>75%) arrives in winter and spring. Mean annual precipitation 

across the four geographic regions varies from as little as 46 mm in some southern 

areas to as much as 2,767 mm in some northern areas (30 year Normal Annual 

Precipitation, PRISM 2015). However, the majority of piñon and juniper land cover 

types receive between to 300 to 410 mm with extremes ranging between 200 and 

600 mm (Stringham et al. 2015; West 1999; West et al. 1975).

The primary study species and their corresponding land cover types (table 1) 

vary across the four geographic regions (figs. 2, 3, 4; Adams 2014; Cole et al. 

2008, 2013; Little 1971). Piñon species are restricted to a few populations of 

Pinus monophylla in northwestern Utah and southwestern Idaho in the Northern 

Intermountain geographic region. Pinus monophylla is most common in the 

Southern Intermountain geographic region, while P. edulis is most common in the 

Central Rocky Mountain and Southern Rocky Mountain geographic regions.

Northern

Intermountain

Region

Central Rocky

Mountain Region

μ

Southern Rocky

Mountain Region

0 80 160 320 480 640

Kilometers

Pinus edulis

Pinus monophylla(Fallax Type)

Pinus monophylla(Monophylla Type)

Southern

Intermountain

Region
USDA Forest Service RMRS-GTR-372. 2018. 5

Figure 3—Distribution of the three primary juniper species (Juniperus deppeana, 

J.occidentalis, and J. osteosprema) and three secondary juniper species (J. grandis, 

J. monosperma, and J. scopulorum) species in the four geographic regions. 

Juniperus occidentalis is the most common study species in the Northern 

Intermountain geographic region. Juniperus grandis occurs primarily in the Sierra 

Nevada ecoregion. Juniperus osteosperma occurs throughout the study areas, with 

the exception of the Northern Intermountain geographic region where it occurs 

only along the southern boundary. Juniperus scopulorum also occurs throughout 

the study area, but is more common in the two eastern Rocky Mountain geographic 

regions where its range extends northward along the Rockies into Canada and 

east into the plains. Juniperus deppeana occurs in the Southern Rocky Mountain 

geographic region, particularly in the Arizona and New Mexico Mountains and 

Madrean Archipelago ecoregions. Juniperus monosperma is found in the southeast 

area of the Southern Rocky Mountain geographic region and east into regions outside the study area.
6 USDA Forest Service RMRS-GTR-372. 2018.

Figure 4—Distribution of the 10 piñon-juniper land cover types (USGS 2011) within the 4 

geographic regions.

Data and Analyses

To describe wildfire patterns in the study area, we analyzed the area burned, fire 

rotation, fire number, length of the fire season, frequency of fire starts through the 

season, and fire size from 1984 through 2013 for all land cover types combined. 

Area burned and fire rotation were analyzed for piñon and juniper land cover types 

(described below) as well as for non-piñon and juniper land cover types. Data were 

summarized and compared for: (1) the four geographic regions; (2) the level III 

ecoregions (USEPA 2013) that occur within each geographical region; and (3) the 

piñon and juniper land cover types that occur within each geographic region and 

level III ecoregion. To evaluate if the fire regime characteristics had changed in 

Northern

Intermountain

Region

Southern

Intermountain

Region

Central Rocky

Mountain Region

Southern Rocky

Mountain Region

0 80 160 320 480 640

Miles

μ

Piñon-Juniper Land Cover Type

Madrean Pinyon-Juniper Woodland

Rocky Mountain Foothill Limber PineJuniper Woodland

Columbia Plateau Western Juniper

Woodland and Savanna

Great Basin Pinyon-Juniper Woodland

Inter-Mountain Basins Juniper Savanna

Colorado Plateau Pinyon-Juniper

Shrubland

Colorado Plateau Pinyon-Juniper

Woodland

Southern Rocky Mountain Juniper

Woodland and Savanna

Southern Rocky Mountain Pinyon-Juniper

Woodland

Madrean Juniper Savanna
USDA Forest Service RMRS-GTR-372. 2018. 7

Figure 5—The 30-year normal annual values for (A) precipitation and (B) temperature (PRISM 2016) in the four geographic regions.

the last 30 years for the study area as a whole and if burn area and fire rotation had 

changed in piñon and juniper land cover types, the Mann-Kendall statistic was used 

to evaluate trends. To evaluate if piñon-juniper land cover types were burning more 

or less than other land cover types, fire parameters for piñon and juniper land cover 

types were compared to those for non-piñon and juniper land cover types. To delineate burned areas within regions or land cover types, fires were split where they 

crossed geographic or land cover boundaries.

Land Cover Type Data

Land cover type data were derived from the National Gap Analysis Program, 

Land Cover Data, Version 2 (USGS 2011). The focus was on ten land cover types 

where piñon and/or juniper species are common (table 1). These ten land cover 

types comprised six piñon and juniper woodlands, three piñon and juniper savannas, 

and one piñon and juniper shrubland based on the land cover type descriptions in 
8 USDA Forest Service RMRS-GTR-372. 2018.

the Ecological System Comprehensive Report (NatureServe 2015). Shrublands were 

defined as areas characterized by natural or semi-natural woody vegetation with 

aerial stems generally less than 3 meters tall. Woodlands and savanna were characterized as having an overstory of trees, i.e., woody vegetation over 3 meters tall. 

Woodlands were described as typically having a shrub dominated understory while 

savannas were described as having a grass dominated understory. The Columbia 

Plateau Western Juniper Woodland and Savanna land cover type was characterized 

by open stands of J. occidentalis with an understory of open shrub-steppe; however, 

the land cover type includes areas where the trees intermingle or are expanding into 

the surrounding grassland and where conditions favor grasslands over shrublands. 

This land cover type was analyzed as part of the woodland land cover types. The 

Southern Rocky Mountain Juniper Woodland and Savanna land cover type was 

described as a savanna that has widely spaced trees but may have inclusions of 

Figure 6—The percent of summer precipitation (July, August, and September) based 

on 30-year normal annual values for the western United States (PRISM 2016).

Percent Summer Precipitation

65%

30%

1%

Northern

Intermountain

Region

Southern

Intermountain

Region

Central Rocky

Mountain

Region

Southern

Rocky

Mountain

μ

Region

0 80 160 320 480 640

Kilometers
USDA Forest Service RMRS-GTR-372. 2018. 9

denser juniper woodlands. This land cover type was analyzed as part of the savanna 

land cover types. All other land cover types included in the analyses were grouped 

into a non-piñon and juniper land cover types category.

Fire Data

Fire data were from the Monitoring Trends in Burn Severity (MTBS) program 

(MTBS 2016) for the years spanning 1984 through 2013, which was the last year 

that data were available at the beginning of the analysis. Fire perimeter polygons 

from MTBS were used to delineate burned areas and thus included unburned areas 

and low severity burn areas (fig. 7). The MTBS data included fires equal to or 
10 USDA Forest Service RMRS-GTR-372. 2018.

greater than 405 ha (1,000 ac). Fires smaller than 405 ha were not included in this 

study because they make up a small portion of the total area burned each year and 

were not always recorded accurately over the study period. Fires that were known 

to be prescribed fires were excluded from the data set as well as those that were 

not recorded with high confidence as reported in MTBS. Fires that burned across 

regional/land cover type boundaries were included in the calculations for both areas.

Figure 7—Fires larger than 450 ha occurring between 1984 and 2013 within the four 

geographic regions color coded by month of fire start. Number of fires are shown in parentheses for each month. Fires are overlaid on the percent of summer precipitation (July, 

August, and September) based on 30-year normal annual values for the western United 

States (MTBS 2016; PRISM 2016).

Fire Start Month

December (2)

January (10)

February (63)

March (123)

April (352)

May (794)

June (3,477)

July (6,675)

August (6,030)

September (1,199)

October (286)

November (33)

Percent Summer Precipitation

65%

30%

1%

Northern

Intermountain

Region

Southern

Intermountain

Region

Central Rocky

Mountain

Region

Southern

Rocky

Mountain

Region

0 75 150 300 450 600

Kilometers

μ
USDA Forest Service RMRS-GTR-372. 2018. 11

Analysis of Fire Parameters

Total area burned and area burned annually were calculated from the MTBS data. 

Trends in burn area were evaluated by summing the area burned annually within 

a geographic region and testing for a temporal trend for the period between 1983 

and 2013 using a Mann-Kendall statistic (Kendall 1975; Mann 1945). Where data 

were sufficient, the same test was conducted for piñon and juniper land cover types. 

Although the Mann-Kendall statistic is non-parametric and robust to skewed data 

distributions and nonlinear relationships, it can perform poorly if there is a cyclic 

signal within the data.

Fire rotation was calculated using data for the entire 30-year time span (1984 

through 2013) and the last 15 years (1999 through 2013). The last 15 years were 

compared to the 30-year time span to evaluate possible trends in changing fire 

regimes. However, 15 years is a limited time frame to extrapolate fire rotation, and 

estimates can be skewed by a single anomalous observation.

Fire number (number of fires >405 ha [1,000 ac]) was summarized on an annual 

basis, and temporal trends between 1984 and 2013 were tested using a MannKendall statistic. Fire number was evaluated further to describe the fire season.

Fire season was defined as the period from the start date of the first fire of the 

year greater than 405 ha to the start date of the last fire of the year greater than 

405 ha. Fire season was reported as the number and distribution of fire starts for 

each year between 1984 and 2013, including the first and last recorded fire, length 

of the fire season, and frequency of fire starts (shown as the number of fires per 

week during the fire season). To evaluate a potential change in fire season length 

and distribution of starts during the fire season, trends in the start date, end date, 

length of fire season, and the number of fires per week were tested for the period 

between 1984 and 2013 using the Mann-Kendall test.

Fire size was summarized using the median size of fires. Annual fire sizes 

(median size of all individual fires per year) were analyzed to determine if fire 

size was driving temporal trends in fire area. Temporal trends were tested using 

a Mann-Kendall statistic. To better understand the distribution of fire sizes and 

describe the distribution and potential change of fire size over time, the 25th, 50th 

(median), and 75th percentiles and maximum fire size for each year were evaluated 

for each geographic region (Liu and Wimberly 2015). It should be noted that the 

MTBS data was designed to evaluate large fires (>405 ha [1,000 ac]). This truncation of fire size limits inference about trends in small fires (i.e., the 25th percentile).

Results

Fire Patterns Across the Study Area

The four geographic regions that make up the study area extended across 165 million hectares of which 22.9 million ha (13.8%) were classified as piñon and juniper 

land cover types (table 1, fig. 4). Piñon and juniper woodlands accounted for 

20.7 million ha (12.6%) of the area, while piñon and juniper savanna and shrubland 

combined accounted for 2.2 million ha (1.3%).
12 USDA Forest Service RMRS-GTR-372. 2018.

Fire Area

From 1984 through 2013, a total of 13.9 million ha (8.4%) burned in the study 

area (table 2, fig. 7). Piñon and juniper land cover types accounted for almost 1.4 

million ha (10.0%) of the total area burned. The total number of hectares burned has 

been increasing since 1984 in both the study area overall (τ = 0.292, P = 0.0235) 

and the piñon and juniper land cover types (τ = 0.444, P = 0.0006) (table 3). The 

percentage of the area burned that was piñon and juniper land cover types also 

increased over time (τ = 0.366, P = 0.0046).

Total hectares burned on an annual basis in the study area ranged from 76,500 ha 

in 1991 to 1,630,000 ha in 2012 with a median of 293,000 ha. The area of piñon 

and juniper land cover types that burned annually ranged from 2,900 ha in 1991 to 

173,000 ha in 2012 (table 3, fig. 8).
USDA Forest Service RMRS-GTR-372. 2018. 13

Fire Rotation

Fire rotation for all land cover types in the study area was 356 years for 1984 

through 2013 (table 2). Piñon and juniper land cover types had longer return 

intervals (492 years) compared to non-piñon and juniper land cover types (341 

years). For all piñon and juniper land cover types, the fire rotation was less for 1999 

through 2013 than for 1984 through 2013.

Fire Number

Fire number for the study area overall ranged from 44 in 1991 to 321 in 2006 with 

a median of 133 (table 3, fig. 8). However, the number of fires did not appear to 

increase over the 30-year time period (τ = 0.206, P = 0.1121).

Fire Season

The length of the fire season did not increase over the 30-year period for the overall study area (τ = 0.242, P = 0.0690). However, length of fire season varied among 

years ranging from 165 days in 1993 to 343 days in 2006 with a median of 211 days 

for the study area (table 3, fig. 8). There was no temporal trend in fire season start 

date (τ = –0.203 0.1162) or end date (τ = 0.243, P = 0.0608). The median start date 

of the fire season was March 18 with start dates ranging from as early as January 

7, 2006 to as late as May 17, 2007. The median end date was October 24 with the 

earliest end date on September 16, 2013 and latest end date on December 16, 2006.
14 USDA Forest Service RMRS-GTR-372. 2018.

Fire Size

Median fire size for the overall study area increased between 1984 and 2013 (τ = 

0.287, P = 0.0257), and ranged from a minimum of 856 ha in 1988 to a maximum 

of 1,989 ha in 2012 (table 3, fig. 9). First quartile fire size ranged from 542 ha in 

1988 to 800 ha in 1996 and increased over the period (τ = 0.260, P = 0.0438). The 

third quartile fire size also increased (τ = 0.274, P = 0.0337) ranging from 1,294 

ha in 1991 to 5,097 ha in 2012. The maximum fire size increased dramatically (τ = 

0.434, P = 0.0007) ranging from 3,947 ha in 1991 to 229,622 ha in 2007.

Fire Patterns Within the Geographic Regions

The relative abundance of piñon and juniper land cover types differed among 

the four geographic regions (fig. 4). The two eastern regions had the largest percentages of piñon and juniper woodland land cover types (16.7% of the Central 

Rocky Mountain geographic region and 23.1% of the Southern Rocky Mountain 

geographic region). These two regions were also the only regions with piñon and 

juniper shrublands (3.6% of the Central Rocky Mountain geographic region and less 

than 0.1% of the Southern Rocky Mountain geographic region). Piñon and juniper 

savanna was most common in the Southern Rocky Mountain geographic region 

Figure 8—Characterization of the annual variation of fire within the study area overall and each geographic region. The 

first row of graphs shows the fire season. Each boxplot is a single year. The box shows the interquartile range with the 

median represented by a line within the box. The whiskers are the 10th and 90th percentile with earlier or later fires 

represented by dots. The dashed lines across the graphs represent the median earliest and latest fires. The solid line 

represents the median annual fire start. The second row of graphs illustrates the annual number of fires for the study area. 

The last row shows the total number of hectares burned in gray and the total number of hectares of piñon and juniper land 

cover types burned annually in black.
USDA Forest Service RMRS-GTR-372. 2018. 15

(1.9% of the region). However, the Columbia Plateau Woodland and Savanna land 

cover type (classified as a woodland type) occurred exclusively in the two western 

regions, contained savanna like vegetation, and was more abundant in the study area 

overall than any of the savanna piñon and juniper types.

The percentage of the total area that burned showed large differences among the 

four geographic regions. In the Northern Intermountain geographic region 15.2 

percent of the area burned from 1984 through 2013; this was 6.6 times greater 

than the Central Rocky Mountain geographic region where only 2.3 percent of the 

area burned (table 4). The percentage of piñon and juniper land cover types that 

burned followed the general trends of the geographic regions (table 4; fig. 8). Piñon 

and juniper land cover types were 4.4 times more likely to burn in the Northern 

Intermountain than the Central Rocky Mountain geographic region (table 4). In 

the Northern Intermountain and Southern Rocky Mountain geographic regions, a 

smaller percentage of piñon and juniper land cover types burned than in the regions 

as a whole, while the reverse was true for the Southern Intermountain and Central 

Rocky Mountain geographic regions.

Figure 9—Fire size for the 25th, 50th, 75th quartiles and the maximum for the study area.
16 USDA Forest Service RMRS-GTR-372. 2018.

The number and seasonal frequency of fires also differed among geographic 

regions. Annual fire number varied from 1.4 fires per million ha in the Northern 

Intermountain geographic region to 0.32 fires per million ha in the Central Rocky 

Mountain geographic region (table 5, fig. 8). The longest fire season occurred in 

the Southern Rocky Mountain geographic region with a median length of 188 days. 

The length of the season was countered by this region typically having only 1.1 fires 

per week (scaled to the area of the Central Rocky Mountain geographic region). 

The Northern Intermountain geographic region had a moderate fire season length, 

but had the highest number of fires per week, 2.5 (scaled to the area of the Central 

Rocky Mountain geographic region). The Southern Rocky Mountain geographic 

region had a distinct fire season with a propensity for spring and early summer 

burns, while the Northern and Southern Intermountain geographic regions had a 

propensity for late summer and fall burns.

Annual median fire size was 1,343 ha in the Northern Intermountain geographic 

region, which was 1.5 times greater than in the Central Rocky Mountain geographic 

region where annual fire size was 927 ha (table 5). However, the maximum annual 

fire size was 7 times greater in the Northern Intermountain geographic region 

(30,711 ha) than in the Central Rocky Mountain geographic region (4,297 ha) 

(fig. 10).
USDA Forest Service RMRS-GTR-372. 2018. 17

Fire Patterns in the Northern Intermountain Geographic Region

The Northern Intermountain geographic region encompassed 40.1 million ha, 

nearly a quarter of the area of interest and included five ecoregions and four major 

piñon-juniper land cover types (table 6, fig. 11). Piñon and juniper land cover types 

accounted for almost two million ha or about 5 percent of the region. The Columbia 

Plateau Western Juniper Woodlands and Savanna was the most common piñon and 

juniper land cover type in the region accounting for 3.7 percent of the total area and 

74.9 percent of the piñon and juniper land cover types. Piñon and juniper land cover 

types were most common in the Blue Mountains ecoregion and least common in the 

Snake River and Columbia Plateau ecoregions.

Figure 10—Fire size for the 25th, 50th, 75th quartiles and the maximum for the four geographic regions within the study 

area.
18 USDA Forest Service RMRS-GTR-372. 2018. Table 6—Aerial extent of land cover types within the Northern Intermountain geographic region (“All”) and within each ecoregion followed by percentage of the land cover type in the region and the percentage in the ecoregion(s), respectively, in parentheses. Dashes reflect areas where piñon-juniper land cover type did not occur. Land cover type All Eastern Cascades Slopes and Foothills Columbia Plateau Blue Mountains Snake River Plain Northern Basin and Range (ha) (ha) (ha) (ha) (ha) (ha) Piñon-juniper Piñon-juniper woodland Columbia Plateau Western Juniper Woodland and Savanna 1,495,449 (3.7%, 74.9%) 311,748 (5.9%, 86.7%) 61,496 (0.7%, 100.0%) 816,028 (11.5%, 100.0%) 228 (0.0%, 2.7%) 305,948 (2.2%, 40.7%) Great Basin Piñon-Juniper Woodland 378,839 (0.9%, 19.0%) 47,784 (0.9%, 13.3%) – – – 331,055 (2.4%, 44.0%) All piñon-juniper woodland types 1,874,770 (4.7%, 93.9%) 359,533 (6.8%, 100.0%) 61,496 (0.7%, 100.0%) 816,028 (11.5%, 100.0%) 665 (0.0%, 7.9%) 637,048 (2.4%, 84.7%) Juniper savanna Inter-Mountain Basins Juniper Savanna 122,644 (0.3%, 6.1%) 0 (0.0%, 0.0%) – – 7,716 (0.1%, 92.1%) 114,928 (0.8%, 15.3%) All piñon-juniper types 1,997,414 (5.0%, 100.0%) 359,533 (6.8%, 100.0%) 61,496 (0.7%, 100.0%) 816,028 (11.5%, 100.0%) 8,381 (0.2%, 100.0%) 751,976 (5.4%, 100.0%) All non- piñon-juniper types 38,113,820 (95%) 4,965,693 (93.2%) 8,251,266 (99.3%) 6,275,028 (88.5%) 5,354,030 (99.8%) 13,267,803 (94.6%) Total land area 40,111,235 (100%) 5,325,721 (100.0%) 8,023,904 (100.0%) 7,091,057 (100.0%) 5,362,411 (100.0%) 14,019,798 (100.0%)
USDA Forest Service RMRS-GTR-372. 2018. 19

Northern Basin

and Range

Columbia Plateau

Blue Mountains

Snake River Plain

Eastern

Cascades

Slopes

and Foothills

Piñon-Juniper Land Cover Type

Rocky Mountain Foothill Limber Pine-Juniper

Woodland

Columbia Plateau Western Juniper Woodland

and Savanna

Great Basin Pinyon-Juniper Woodland

Inter-Mountain Basins Juniper Savanna

Colorado Plateau Pinyon-Juniper Shrubland

Colorado Plateau Pinyon-Juniper Woodland

0 40 80 160 240 320

Kilometers

μ

Fire Area

From 1984 through 2013, 6.1 million ha or 15.2 percent of the Northern 

Intermountain geographic region burned (table 7). There was no temporal trend 

in the number of hectares burned (τ = 0.223, P = 0.8350) during the 30-year 

period. The number of acres burned annually ranged from 16,847 ha in 2009 to 

1,050,456 ha in 2012 with a median of 146,265 ha (table 8, fig. 12).

Piñon-juniper land cover types accounted for 269,259 ha of the burned area 

(4.4%) (table 7). There were significant increases in the annual area burned 

(τ = 0.457, P = 0.0004) and percentage of the area burned (τ = 0.494, P = 0.0001) 

for the piñon-juniper vegetation land cover types from 1984 through 2013 (table 8). 

Figure 11—Distribution of the piñon and juniper land cover types across the four ecoregions 

within the Northern Intermountain geographic region. Piñon and juniper land cover types account for almost two million ha or about 5 percent of the region.
20 USDA Forest Service RMRS-GTR-372. 2018.
USDA Forest Service RMRS-GTR-372. 2018. 21

The area of piñon-juniper land cover types that burned annually ranged from 626 ha 

in 1997 to 89,654 ha in 2012, while the percentage of the total burned area that was 

within piñon and juniper land cover types varied annually from 1.0 percent in 1985 

to 25.6 percent in 2004 (table 8, fig. 12, fig. A.1).

Total area burned varied among the five ecoregions with the Snake River Plain 

and Northern Basin and Range having the largest burned areas, as well as the largest percentages of the total ecoregion area burned (23.0% and 20.8%, respectively) 

(table 7, fig. 12, fig. A.1). The Northern Basin and Range had the largest area of 

piñon and juniper land cover types burned and the percentage of the area burned in 

these land cover types (18.8%) paralleled that for the ecoregion overall (20.8%). 

The burned areas of piñon and juniper land cover types in the other ecoregions 

ranged from 1,669 ha on the Snake River Plain to 77,396 ha in the Blue Mountains, 

and the percentage of the area burned within piñon and juniper land cover types 

ranged from a low of 8.3 percent in the Eastern Cascades Slopes and Foothills to a 

high of 31.2 percent in the Columbia Plateau.
22 USDA Forest Service RMRS-GTR-372. 2018. Figure 12—Start day, number of fires, and total area burned in the five ecoregions within the Northern Intermountain geographic region. The first row shows the fire season based on the ordinal day of ignition with each boxplot representing a single year. The box shows the interquartile range with the median represented by a line within the box. The whiskers are the 10th and 90th percentile with earlier or later fires represented by dots. The dashed lines across the graphs represent the median earliest and lat- est fires for the area of focus. The solid line represents the median of the annual median fire start for the area of focus. The second row represents the annual number of fires in the study area. The last row shows the total number of hectares burned in gray and the total number of hectares of piñon and juniper land cover types burned annu- ally in black. 
USDA Forest Service RMRS-GTR-372. 2018. 23

Fire Rotation

Mean fire rotation across piñon and juniper land cover types was 223 years in the 

Northern Intermountain geographic region (table 9). The longest rotation was 536 

years for the Columbia Plateau Western Juniper Woodland and Savanna land cover 

type in the Eastern Cascades Slopes and Foothills ecoregion, while the shortest rotation was 96 years for the Columbia Plateau Western Juniper Woodland and Savanna 

in the Columbia Plateau ecoregion.

Fire Number

The number of fires in the Northern Intermountain geographic region varied 

26-fold during the 30-year period, ranging from as few as 5 in 1993 to as many 

as 128 in 2007 with a median of 57 (table 8, fig. 12). The number of fires did not 

increase over the time period (τ = 0.145, P = 0.2609).

Fire Season

The median fire season was 125 days, ranging from 72 days in 2004 to 218 days 

in 2006 (table 8, fig. 12). Length of fire season did not show a trend between 1984 

through 2013 (τ = 0.074, P = 0.5677). The median start date for the season was 

June 5 and median end date for the season was October 6. Neither start nor end 

date showed a temporal change (τ = –0.134, P = 0.3004 and τ = 0.019, P = 0.8863, 

respectively).

Fire Size

Median fire size varied from 749 ha in 1997 to 2,906 ha in 2012 and did not 

increase over time (τ = 0.126, P = 0.3265; table 8, fig. 12). However, the size of 

large fires occurring in the maximum percentile did increase during the 30-year 

period (τ = 0.310, P = 0.0160).
24 USDA Forest Service RMRS-GTR-372. 2018.

Fire Patterns in the Southern Intermountain Geographic Region

The Southern Intermountain geographic region occupied 48.9 million ha or about 

30 percent of the study area and included three ecoregions with six piñon-juniper 

land cover types (table 10, fig. 13). Colorado Plateau Piñon-Juniper Shrubland and 

Madrean Piñon-Juniper Woodlands occupied less than 200 ha total and are included 

in the total for all piñon-juniper woodland types, but are not presented separately in 

table 10. Piñon and juniper land cover types accounted for 5.4 million ha or about 

11.0 percent of the region. The majority of the piñon-juniper land cover in the 

region was Great Basin Piñon-Juniper Woodland (94.0%). The Central Basin and 

Range was the largest ecoregion in this geographic region with 30.9 million ha and 

had the largest amount of piñon and juniper land cover (4.8 million ha).
USDA Forest Service RMRS-GTR-372. 2018. 25

Central Basin and Range

Mojave Basin and Range

Sierra Nevada

μ

Piñon-Juniper Land Cover Types

Colorado Plateau Pinyon-Juniper Woodland

Inter-Mountain Basins Juniper Savanna

Great Basin Pinyon-Juniper Woodland

Columbia Plateau Western Juniper Woodland

and Savanna

Rocky Mountain Foothill Limber Pine-Juniper

Woodland

Madrean Juniper Savanna

Southern Rocky Mountain Pinyon-Juniper

Woodland

Southern Rocky Mountain Juniper Woodland

and Savanna

Colorado Plateau Pinyon-Juniper Shrubland

Madrean Pinyon-Juniper Woodland

0 40 80 160 240 320

Kilometers

Fire Area

From 1984 through 2013, 4.26 million ha (8.7%) burned in the Southern 

Intermountain geographic region (table 11). There was no trend in total hectares 

burned (τ = 0.154, P = 0.232) during the 30-year period (table 12, fig. 14). The 

number of hectares burned in the study area ranged from 13,143 ha in 1991 to 

542,905 ha in 2006 with a median of 99,632 ha.

Piñon and juniper vegetation land cover types accounted for 528,738 ha (12.4%) 

of the total burned area (table 11). Great Basin Piñon-Juniper Woodlands accounted 

for the largest amount of the piñon and juniper land cover types burned (84.6%). 

Figure 13—Distribution of the piñon-juniper land cover types across the three ecoregions 

within the Southern Intermountain geographic region. Piñon and juniper land cover types account for 5.4 million ha or nearly 11.0 percent of the region.
26 USDA Forest Service RMRS-GTR-372. 2018.
USDA Forest Service RMRS-GTR-372. 2018. 27

The piñon and juniper land cover types showed no trend in total hectares (τ = 0.136,

P = 0.2925) or percentage of piñon and juniper land cover (τ = –0.030, P = 0.8166) 

burned during the 30-year period (table 12). The area of piñon and juniper land 

cover types burned ranged from 144 ha in 1991 to 39,987 ha in 2012, while the 

percentage of piñon and juniper land cover types burned varied from 1.1 percent in 

1991 to 20.2 percent in 2004 (table 12, fig. 14).

The three ecoregions were very different in the amount of area burned (table 11, 

fig. 14, fig. A.2). The Central Basin and Range was the largest ecoregion and had 

similar percentages burn in the ecoregion overall (9.6%) and the piñon and juniper 

land cover types (9.1%). In contrast, the Sierra Nevada and Mojave Basin and 

Range ecoregions had relatively low percentage area burn overall (2.3% and 4.6%, 

respectively), but high percentage area burn in piñon and juniper land cover types 

(19.6% and 10.7%, respectively).

Figure 14—Start day, number of fires, and total area burned in the five ecoregions within the Southern Intermountain 

Region. The first row shows the fire season based on the ordinal day of ignition with each boxplot representing a single 

year. The box shows the interquartile range with the median represented by a line within the box. The whiskers are the 

10th and 90th percentile with earlier or later fires represented by dots. The dashed lines across the graphs represent the 

median earliest and latest fires. The solid line represents the median of the annual median fire start. The second row represents the annual number of fires in the study area. The last row shows the total number of hectares burned in gray and 

the total number of hectares of piñon and juniper land cover types burned annually in black.
28 USDA Forest Service RMRS-GTR-372. 2018.

Fire Rotation

Fire rotation in piñon and juniper land cover types in the Southern Intermountain 

geographic region averaged 306 years (table 13). The longest rotation was 1,275 

years for the Inter-mountain Basins Juniper Savanna in the Mojave Basin and 

Range, while the shortest rotation was 42 years for the Great Basin Piñon-Juniper 

Woodland in the Central Basin and Range ecoregion.

Fire Number

The annual number of fires varied by 16-fold, ranging from 9 to 143 with a 

median of 36 (table 12, fig. 14). However, there did not appear to be an increase in 

the number of fires over time (τ = 0.127, P = 0.3265).

Fire Season

Length of the fire season did not show a significant increase during the time 

period (τ = 0.254, P = 0.0579) (table 12, fig. 14). The fire season ranged from 73 

days in 1991 to 256 days in 2012 with a median of 135 days. The median start date 

for the season was May 26 and the median end date for the season was October 2. 

By the end of the study period, the start date was significantly earlier (τ = 0.280, P = 

0.0362) and the end date was significantly later (τ = 0.302, P = 0.0243).

Fire Size

The median fire size ranged from 705 ha in 2010 to 1,921 ha in 1990 and did not 

show a significant trend (table 12, fig. 14, fig. A.2). Maximum fire size increased 

during the 30-year period (τ = 0.306, P = 0.0177), ranging from 2,567 ha in 1991 to 

229,622 ha in 2007.
USDA Forest Service RMRS-GTR-372. 2018. 29

Fire Patterns in the Central Rocky Mountain Geographic Region

The Central Rocky Mountain geographic region spanned 31.5 million ha, about 

20 percent of the study area, and included three ecoregions and six piñon and 

juniper land cover types. (The Southern Rocky Mountain Piñon-Juniper Woodland 

land cover type only occupied 401 ha and was not presented separately; table 14, 

fig. 15). Piñon and juniper land cover types occupied 5.3 million ha or 16.7 percent 

of the region. Colorado Plateau Piñon-Juniper Woodland was the most common 

piñon and juniper land cover type, occupying 11.3 percent of the region and 87.3 

percent of the piñon-juniper types. The piñon and juniper shrubland land cover type 

was found almost exclusively in the Colorado Plateaus ecoregion of the Central 

Rocky Mountain geographic region (table 14, fig. 15). The largest amount of piñon 

and juniper land cover types was in the Colorado Plateaus ecoregion (31.3%) with 

lesser amounts in the Wasatch and Uinta Mountains and Wyoming Basin ecoregions 

(12.2% and 3.2%, respectively).

Fire Area

From 1984 through 2013, 725,795 ha or 2.3 percent of Central Rocky Mountain 

geographic region burned (table 15). There was no detectable increase in annual 

area burned (τ = 0.254, P = 0.0579) (table 16). The area burned annually ranged 

from 890 ha in 2009 to 178,065 ha in 2012 with a median of 12,574 ha. Most of the 

area that burned was in the Colorado Plateau Piñon-Juniper Woodlands (101,090 ha) 

(fig. 16, fig. A.3).
30 USDA Forest Service RMRS-GTR-372. 2018.

Wyoming Basin

Colorado Plateaus

Wasatch and Uinta Mountains

Piñon-Juniper Land Cover Type

Southern Rocky Mountain Pinyon-Juniper

Woodland

Southern Rocky Mountain Juniper Woodland

and Savanna

Colorado Plateau Pinyon-Juniper Woodland

Colorado Plateau Pinyon-Juniper Shrubland

Rocky Mountain Foothill Limber Pine-Juniper

Woodland

Madrean Juniper Savanna

Inter-Mountain Basins Juniper Savanna

Great Basin Pinyon-Juniper Woodland

Columbia Plateau Western Juniper Woodland

and Savanna

Madrean Pinyon-Juniper Woodland

μ 0 40 80 160 240 320

Kilometers

Figure 15—Distribution of the piñon-juniper land cover types across the three ecoregions 

within the Central Rocky Mountain geographic region. Piñon and juniper land cover types occupy 5.3 million ha or 16.7 percent of the region.
USDA Forest Service RMRS-GTR-372. 2018. 31
32 USDA Forest Service RMRS-GTR-372. 2018.

Figure 16—Start day, number of fires, and total area burned in the five ecoregions within the Central Rocky Mountain 

geographic region. The first row shows the fire season based on the ordinal day of ignition with each boxplot representing a single year. The box shows the interquartile range with the median represented by a line within the box. The 

whiskers are the 10th and 90th percentile with earlier or later fires represented by dots. The dashed lines across the 

graphs represent the median earliest and latest fires. The solid line represents the median of the annual median fire 

start. The second row represents the annual number of fires in the study area. The last row shows the total number 

of hectares burned in gray and the total number of hectares of piñon and juniper land cover types burned annually in 

black.

The area burned in piñon and juniper land cover types increased significantly over 

the 30-year period (τ = 0.280, P = 0.0362) ranging from as little as 30 ha in 1998 to 

20,091 ha in 2012 (table 16, fig. 16). The percentage of burned area that was piñon 

and juniper land cover also increased significantly (τ = 0.302, P = 0.0243), ranging 

from 0.1 percent in 2010 to 27.8 percent in 2004.

The number of hectares burned was highly similar among the three ecoregions 

ranging from 215,004 ha in the Wasatch and Uinta Mountains to 291,920 ha in 

the Colorado Plateaus (table 15). However, the percentage of the total area that 

burned in piñon and juniper land cover types varied widely among ecoregions. The 

Colorado Plateaus had the largest percentage of piñon and juniper land cover types 

burn in the region (34.6%) followed by the Wasatch and Uinta Mountains (17.1%) 

and Wyoming Basin (10.3%).
USDA Forest Service RMRS-GTR-372. 2018. 33

Fire Rotation

Piñon and juniper land cover types had a fire rotation of 982 years in the Central 

Rocky Mountain geographic region (table 17). The longest rotation was 4,140 years 

for the Colorado Plateau Piñon-Juniper Shrubland in the Colorado Plateaus, while 

the shortest rotation was 113 years for the Great Basin Piñon-Juniper Woodland in 

the Wasatch and Uinta Mountains.

Fire Number

The median number of fires per year for the region was 10 (table 16), and the 

number of fires per year varied from 2 in 1997 to 35 in 2000 (table 16, fig. 16). The 

number of fires did not increase over time (τ = 0.108, P = 0.4275).

Fire Season

The median fire season was 91 days and ranged from 34 days in 1990 to 162 

days in 1988. The length of the fire season did not change over time (τ = 0.159, P = 

0.2359) (table 16, fig. 16). The median start date for the fire season was June 17 and 

the earliest start to the fire season was April 12, 2013. The median end date for the 

fire season was September 18 and the latest end to the season was October 25, 2003. 

There was no significant change in the start date (τ = –0.236, P = 0.0785) or end 

date (τ = –0.059, P = 0.6635) of the fire season.

Fire Size

The median size of fires was 927 ha and varied annually from 451 ha in 1998 to 

1,714 ha in 1996 (table 16, fig. 16). There was no significant change in fire size 

over time for any quantile.
34 USDA Forest Service RMRS-GTR-372. 2018.

Fire Patterns in the Southern Rocky Mountain Geographic 

Region

The Southern Rocky Mountain geographic region occupied 44.3 million ha and 

included four ecoregions and nine piñon and juniper land cover types (table 18, 

fig. 17). This geographic region had the largest cover of piñon and juniper land 

cover types of any region accounting for 10.2 million ha or 23.1 percent of the 

region. Although piñon and juniper woodlands were the dominant piñon and juniper 

land cover types, occupying 21.2 percent of the region and 91.8 percent of the types, 

juniper savanna was more abundant in the Southern Rocky Mountain geographic 

region than any other region, occupying 1.9 percent of the region and 8.1 percent 

of the piñon and juniper types. Each ecoregion had a unique composition of piñon 

and juniper land cover types. The most common piñon and juniper land cover 

type was the Colorado Plateau Piñon and Juniper Woodland, which accounted for 

57.5 percent of the piñon and juniper land cover types. The Arizona/New Mexico 
USDA Forest Service RMRS-GTR-372. 2018. 35

Southern Rockies

Arizona/New Mexico Plateaus

Arizona/New Mexico

Mountains

Madrean Archipelago

μ

Piñon-Juniper Land Cover Type

Madrean Juniper Savanna

Southern Rocky Mountain Pinyon-Juniper

Woodland

Southern Rocky Mountain Juniper Woodland

and Savanna

Colorado Plateau Pinyon-Juniper Woodland

Colorado Plateau Pinyon-Juniper Shrubland

Inter-Mountain Basins Juniper Savanna

Great Basin Pinyon-Juniper Woodland

Columbia Plateau Western Juniper Woodland

and Savanna

Rocky Mountain Foothill Limber Pine-Juniper

Woodland

Madrean Pinyon-Juniper Woodland

0 40 80 160 240 320

Kilometers

Figure 17—Distribution of the piñon-juniper land cover types across the four ecoregions 

within the Southern Rocky Mountain region. This geographic region has the largest cover of 

piñon and juniper land cover types of any region, accounting for 10.2 million ha or 23.1 percent of the region.
36 USDA Forest Service RMRS-GTR-372. 2018.

Plateaus and Arizona/New Mexico Mountains ecoregions had the largest amount of 

piñon and juniper land cover types with 4.4 million ha (30.2%) and 4.3 million ha 

(39.2%), respectively.

Fire Area

The Southern Rocky Mountain geographic region had 2.8 million ha or 6.3 

percent of the total area burn between 1984 and 2013 (table 19). There was a significant increase in the annual area burned from 1984 through 2013 (τ = 0.5035, P < 

0.0001) (table 20). The median annual area burned was 67,786 ha and ranged from 

2,530 ha in 1984 to 231,888 ha in 2002 (table 20, fig. 18).
USDA Forest Service RMRS-GTR-372. 2018. 37

Piñon and juniper land cover types accounted for 436,719 ha or 15.6 percent of 

the area burned (table 19). The median area of piñon and juniper land cover types 

burned annually was 10,405 ha and varied from only 46 ha in 1992 to 44,663 ha in 

2011 (table 20, fig. 18). The area of piñon and juniper types that burned increased 

over time (τ = 0.5310, P < 0.0001), but the percentage of the area burned that was 

piñon and juniper types did not change (τ = 0.2184, P = 0.0901). Madrean PiñonJuniper Woodlands were the most commonly burned type with 230,361 ha.

The total area burned in each ecoregion was very different in the Southern Rocky 

Mountain geographic region (table 19, fig. 18, fig. A.4). The Arizona/New Mexico 

Mountains had the largest area burned (1.5 million ha) and greatest percentage 

of the ecoregion burned (14.0%). This contrasted sharply with the Arizona/New 

Mexico Plateaus, where only 121,000 ha (0.8%) burned. The Madrean Archipelago 

ecoregion had the highest proportion of piñon and juniper land cover burn (27.8%).
38 USDA Forest Service RMRS-GTR-372. 2018. Figure 18—Start day, number of fires, and total area burned in the four ecoregions within the Southern Rocky Mountain geographic region. The first row shows the fire season based on the ordinal day of ignition with each boxplot representing a single year. The box shows the interquartile range with the median represented by a line within the box. The whiskers are the 10th and 90th percentile with earlier or later fires represented by dots. The dashed lines across the graphs represent the median earliest and latest fires. The solid line represents the median of the annual median fire start. The second row represents the an- nual number of fires in the study area. The last row shows the total number of hectares burned in gray and the total number of hectares of piñon and juniper land cover types burned annually in black. 
USDA Forest Service RMRS-GTR-372. 2018. 39

Fire Rotation

Fire rotation was highly variable in the Southern Rocky Mountain geographic 

region (table 21). Rotations ranged from 105 years for Madrean Piñon-Juniper 

Woodland in the Madrean Archipelago to 7,000 years for the Colorado Plateau 

Piñon Juniper Woodland in the Arizona/Mew Mexico Plateaus. Fire rotation for all 

piñon and juniper land cover types in the region was 702 years.

Fire Number

The median number of fires for the region was 30, and varied from 3 fires in 1984 

to 79 fires in 2011 (table 20, fig. 18). The number of fires increased significantly 

over the 30-year period (τ = 0.3210, P = 0.0131).
40 USDA Forest Service RMRS-GTR-372. 2018.

Fire Season

The Southern Rocky Mountain geographic region was the only region in which 

the length of the fire season increased significantly (τ = 0.2748, P = 0.0336; table 

20). The median length of the fire season was 188 days and ranged from 23 days in 

1992 to 343 in 2006 (table 20, fig. 18). The fire season typically started on March 

22 and ran until September 26. The earliest start date was January 7, 2006, and the 

latest end date was December 15, 2006, (which was a fire season length of almost 

the entire year).

Fire Size

The median fire size was 1,230 ha and ranged from 509 ha in 1984 to 2,363 ha 

in 2013. The Southern Rocky Mountain geographic region was the only region 

in which fire size increased across all fire sizes for the 30-year period (table 20, 

fig. 18).

Discussion

Regional Patterns

Fire characteristics across the study area and piñon and juniper land cover types 

differed among the geographic regions. Area burned during the 30-year period 

and on an annual basis, number of fires each year, and median or larger fire size 

all followed a strong geographic pattern: Northern Intermountain > Southern 

Intermountain > Southern Rocky Mountain > Central Rocky Mountain. These 

differences in fire patterns can be attributed to the influence of climate, vegetation composition and structure, and patterns of invasion by exotic grasses on fire 

regimes.

In the Southern Intermountain and especially the Northern Intermountain geographic regions, most precipitation arrives as winter snow and rain, while in the 

Central Rocky Mountain and especially the Southern Rocky Mountain geographic 

regions, as much as 30 to 60 percent of the annual precipitation arrives during the 

summer months of July, August, and September (fig. 6). Although topography 

strongly influences temperatures across the study area, there is a north-south gradient of increasing ambient temperature that is becoming more pronounced with 

climate warming (Kunkel et al. 2013 b,c) (fig. 5). These differences in summer 

precipitation and temperature correspond with differences in both fire season and 

length in the geographic regions (table 5, fig. 7).

The Southern Rocky Mountain geographic region has the longest fire season (188 

days), but most fires occur in relatively warm and dry portions of the region and 

burn early in the season (May and June) before the onset of the summer monsoons. 

In contrast, the Northern Intermountain geographic region has an intermediate fire 

season length (125 days), but most fires burn in July and August when surface fuel 

moisture is low and there can be abundant lightning with little to no precipitation 

and high summer temperatures. The Southern Intermountain geographic region also 

has an intermediate fire season (135 days) but summer precipitation is higher in the 

southern portion of the region. The Central Rocky Mountain geographic region has 

the shortest fire season (91 days) and is generally cool with an intermediate to high 

amount of summer precipitation.
USDA Forest Service RMRS-GTR-372. 2018. 41

The differences in climate among the geographic regions influence plant 

functional type dominance (Lauenroth et al. 2014; Sala et al. 1997) and fire characteristics (Brown et al. 1982; Romme et al. 2009). The amount of precipitation 

received during the period when temperature, and thus potential evapotranspiration 

are low, influences the amount of water stored in deep soil layers, which in turn 

affects the balance between woody and herbaceous species (Lauenroth et al. 2014; 

Sala et al. 1997). Areas that receive more winter and spring precipitation typically 

have more deep soil water storage and greater relative abundance of woody species, 

such as sagebrush, which are more effective at using deep soil water. In contrast, 

areas that receive predominantly summer precipitation typically have higher relative abundance of grasses. These differences in relative proportion of shrubs and 

grasses influence fire characteristics in the different regions (Brown 1982; Romme 

et al. 2009). In areas where grasses dominate, as in large areas of the Central and 

Southern Rocky Mountain geographic regions, fire spread is usually moderate. 

In areas where shrubs are dominant or codominant, such as in the Northern and 

Southern Intermountain geographic regions, fire intensities are typically moderate to 

high, and extreme fire weather can result in extensive spread. These differences in 

plant functional groups also influence fire characteristics in piñon and juniper vegetation (fig. 1; Romme et al. 2009). However, as tree densities increase and surface 

fuels become limited, fires tend to be high severity and stand replacing and occur 

during periods of extreme fire weather when fire can spread through these types of 

fuels (Bates et al. 2014; Blackburn and Bruner 1975; McCulloch 1969; Schroeder 

1966; Romme et al. 2009).

Ecosystem resistance to many invasive annual Bromus species, such as Bromus 

tectorum (cheatgrass), and the tendency for invasive annual grass/fire cycles to 

develop is also influenced by climate (Brooks et al. 2016, Chambers et al. 2016). 

In general, resistance to invasive annual grasses increases as summer precipitation 

and/or amount of precipitation increase due to higher perennial grass dominance 

and increased competition for resources (Bradford and Lauenroth 2006; Bradley 

2009; Chambers et al. 2016). Resistance to many invasive annual grasses, such as 

B. tectorum, also increases as soil temperature decreases along elevation gradients 

(Chambers et al. 2007, 2014a, 2016).

Annual grass/fire cycles are currently most problematic in piñon and juniper 

land cover types on warmer and drier soils in the western regions. They are also 

likely contributing to the large fire areas in the Northern Intermountain geographic 

region and northern portion of the Southern Intermountain geographic region, which 

receive little summer precipitation (fig. 7; Balch et al. 2013; Brooks et al. 2015; 

Downs et al. 2016). Cheatgrass and other invasive annuals now dominate at least 

6 percent (650,000 km2) of the central Great Basin (Balch et al. 2013). Invasive 

annual grasses increase the amount and continuity of fine fuels resulting in shorter 

fire return intervals and larger, more contiguous fires (Brooks et al. 2004, 2016; 

Pyke et al. 2016). In the central Great Basin, cheatgrass dominated lands have 

burned almost four times more frequently than any other vegetation type and are 

disproportionately represented in the largest fires (Balch et al. 2013).

In the Central and Southern Rocky Mountain geographic regions invasive annual 

grasses are a rapidly emerging problem (Baker 2011; Brooks et al. 2015; Mealor 

et al. 2012). In these regions as elsewhere, resistance to invasive annual grasses 

is reduced by anthropogenic disturbances that remove perennial native grass 

cover, especially where total precipitation and productivity are low (Bradford and 
42 USDA Forest Service RMRS-GTR-372. 2018.

Lauenroth 2006; Chambers et al. 2014a; Knight et al. 2014; Lauenroth et al. 2014). 

Invasive annual grasses are increasing with wildfire in both the Wyoming Basin 

(Knight et al. 2014) and eastern portion of the Colorado Plateau (Floyd et al. 2004; 

Shinneman and Baker 2009b).

Changes in Fire Regimes in Piñon and Juniper Land Cover Types

Various studies have found increasing trends in total area burned, fire size, or fire 

severity across the western United States (Dennison et al. 2014; Dillon et al. 2011; 

Miller and Safford 2012; Miller et al. 2012). We found that the total area of piñon 

and juniper land cover types that burned increased significantly from 1984 through 

2013 for the study area overall and for each geographic region, except the Southern 

Intermountain Mountain region. We found the same result for the percentage of the 

study area and each geographic region comprised of piñon and juniper land cover 

types that burned. Also, fire number increased for the Southern Rocky Mountain 

geographic region and fire size increased for both the Southern Rocky Mountain 

and Southern Intermountain geographic regions. These results are consistent with 

Dennison et al. (2014) who found that increases in fire size for the western United 

States were most significant for southern and mountainous ecoregions, and that the 

Arizona-New Mexico Mountains ecoregion exhibited significant positive trends in 

number of fires.

Fire rotations for 1984 through 2013 varied greatly among regions for piñon and 

juniper land cover types and were generally less in the Northern Intermountain 

geographic region (223 yrs) and Southern Intermountain geographic region (306 

yrs) than in the Central Rocky Mountain (982 yrs) and Southern Rocky Mountain 

geographic region (702 yrs). Fire rotations decreased by about 35 percent in piñon 

and juniper land cover types overall when 1999 through 2013 was compared to the 

30-year period. This decrease was generally reflective of all types except Madrean 

Juniper Savanna, which showed little change.

Our fire rotations for the Northern and Southern Intermountain geographic 

regions fell within ranges of historical fire rotations derived from historical 

General Land Office Survey (GLO) notes for Inter-Mountain Basins Big Sagebrush 

Shrubland and Steppe (based on Regional GAP Analysis Project data; Re-GAP) in 

Oregon (173–345), Idaho (131–262), and Nevada (251–502) (Bukowski and Baker 

2013). For the Central Rocky Mountain geographic region, our fire rotations were 

somewhat longer than reported from GLO data for Inter-Mountain Basins Montane 

Sagebrush Steppe in Wyoming (535–930 years; Bukowski and Baker 2013) and for 

specific areas such as the Uncompahgre Plateau (400–600+) (Shinneman and Baker 

2009a) and Mesa Verde (400+) (Floyd et al. 2004, 2008). Little data on fire rotations appear to be available for the Southern Rocky Mountain geographic regions, 

but our values were greater than those reported for P. edulis, J. osteosperma, and 

J. scopulorum woodland-ponderosa pine (Pinus ponderosa P. & C. Lawson) forest 

ecotones in Arizona (Tusayan–340 years ) and in New Mexico (Canjilon–290 years) 

(Huffman et al. 2008). However, a comparison of tree-ring and charcoal-based fire 

histories covering the past 7,000 years showed an unprecedented lack of fire in the 

Arizona/New Mexico Mountains ecoregion during the most recent century that was 

attributed to livestock grazing and fire suppression (Allen et al. 2008; Savage and 

Swetnam 1990). Limited fire rotation data from other studies make comparisons 

difficult, and estimates of the fire rotations for the two Rocky Mountain geographic 
USDA Forest Service RMRS-GTR-372. 2018. 43

regions were likely affected by a small number of fires and a relatively short time 

frame for comparison.

Several interacting factors are likely contributing to greater total area burned 

and reduced fire rotations for piñon and juniper land cover types over the 30-year 

period. Elevated levels of carbon dioxide have been shown to result in increased 

biomass of both woody species and invasive annual grasses during favorable 

periods for growth (Nowak et al. 2004; Polley et al. 2011) and thus higher fuel 

loads over large areas (Allen et al. 2015). Also, fire frequency is increasing due to 

human-caused fires that start in proximity to human settlements and road corridors 

(Narayanaraj and Wimberly, 2011, 2012; Syphard et al. 2007). However, invasive 

annual grasses and climate change are likely major contributors to changing fire 

regimes in semi-arid piñon and juniper land cover types.

Invasive annual grasses are likely influencing the increase in total and percentage area burned of piñon and juniper land cover types over time in the Northern 

Intermountain and Central Rocky Mountain geographic regions. In warmer and 

drier areas with piñon and juniper vegetation, cheatgrass is often a component 

of the understory that can increase significantly following wildfire (Condon et 

al. 2011), prescribed fire, or other management treatments if perennial native 

grasses and forbs have been depleted (Chambers et al. 2014b; Roundy et al. 2014; 

Urza et al. 2017).

In the Central Great Basin (Snake River Plain and Northern Basin and Range) 

fires that burn across multiple vegetation types are significantly more likely to have 

started in cheatgrass dominated vegetation (Balch et al. 2013). In the Colorado 

Plateaus, it has been suggested that shorter fire rotations caused by interactions with 

cheatgrass and other fire prone invasives are leading to a net decline of piñon and 

juniper land cover compared to their historical extent (Arendt and Baker 2013). For 

example, in southwestern Colorado in Mesa Verde National Park, a greater proportion of the piñon-juniper woodland burned in the decade between 1995 and 2005 

than had burned throughout the previous 200 years (Floyd et al. 2004). Those stands 

that had sparse understories prior to burning are now dominated largely by cheatgrass and other annual invaders (Floyd et al. 2004).

Increases in large wildfire activity including higher large-wildfire frequency, 

total burned area, longer wildfire durations, and longer wildfire seasons have been 

associated with increased spring and summer temperatures and earlier spring snowmelt due to climate change (Dennison et al. 2014; Westerling et al. 2006, 2014). 

Projected changes in precipitation due to climate change can have very different 

implications than changes in temperature in terms of the characteristics and spatial 

location of wildfire regime responses. Fuel availability effects are most important in 

arid, sparsely vegetated ecosystems, while flammability effects are most important 

in moist, densely vegetated ecosystems (Abatzoglou and Kolden 2013; Littell et al. 

2009; Westerling et al. 2014). In general cooler, moister areas with forested vegetation types have greater biomass, and wildfires tend to occur in dry years. Warmer 

and drier areas with grassland and shrubland vegetation types often have less 

biomass and wildfires tend to occur after one or more years with above-normal precipitation (Crimmins and Comrie 2004; Littell et al. 2009; Westerling et al. 2014). 

In these warmer and drier areas, long-term drought (>4 months) is not necessarily a 

prerequisite for extensive area burned, and seasonal climate can override the effect 

of antecedent climate (Abatzoglou and Kolden 2013). Piñon and juniper vegetation 

occurs over a range of precipitation and temperature regimes, is associated with a 
44 USDA Forest Service RMRS-GTR-372. 2018.

wide variety of vegetation types, and differs in amount of biomass both spatially 

and temporally.

There is general agreement that elevated temperatures and historical and current 

droughts have contributed to the recent increase in fire activity in the Southwest 

United States (Dennison et al. 2014; Westerling et al. 2014). We found that in the 

Southern Rocky Mountain geographic region the fire season was longer, while 

in the Southern Intermountain geographic region the fire season started earlier 

and ended later over the 30-year study period. The two southern regions also had 

increases in fire size, especially the Southern Rocky Mountain geographic region. 

The U.S. National Climate Assessment indicates that annual temperature in the 

Southwest United States has increased over the past 115 years with recent 10-year 

averages surpassing any previous decadal value in the southern portion of the area 

(Kunkel et al. 2013a). The frost-free season length has increased by about 2 weeks 

relative to the 1960s and 1970s and by a month relative to the early 1900s.

Since 2000, several fires in forested landscapes of the Southern Rocky Mountain 

geographic region exceeded the burned area of any other wildfire in at least the past 

100 years (e.g., most notably, the 189,651 ha [468,640 acre] Rodeo-Chediski Fire in 

central Arizona in 2002, the 217,741 ha [538,049 acre] Wallow Fire in east-central 

Arizona and west-central New Mexico in 2011, and the 63,000 ha [156,593 acre] 

Las Conchas Fire in New Mexico in 2011; Westerling et al. 2014).

In addition, large areas of piñon and juniper land cover types have experienced 

extensive tree mortality due to a combination of direct drought-induced physiological stress and mortality and effects of the piñon Ips bark beetle (Ips confusus) (Allen 

and Breshears 1998; Breshears et al. 2005). Williams et al. (2010) estimate that 

nearly 20 percent of forested areas in the region experienced high levels of tree mortality between 1984 and 2010 due to drought, fire, and bark beetles. Although both 

temperature and the frost-free length have increased in the more northern regions in 

this analysis (Kunkel et al. 2013b,c), the change may not yet be reflected in the fire 

season or other factors may currently have a greater influence on fire seasons in the 

more northern regions.

Differences Between Piñon and Juniper and Other Land Cover 

Types

The percentage of the study area that burned in piñon and juniper land cover types 

versus non-piñon and juniper land cover types differed among geographic regions. 

The relative influence of fire in piñon and juniper land cover types within the different regions was a function of the total area of the piñon and juniper land cover 

types, the area of piñon and juniper land cover types vs non-piñon and juniper land 

cover types that burned, and the rate of burning.

In the Northern Intermountain geographic region, the percentage of the area that 

burned within piñon and juniper land cover types was slightly less than the percentage of the area that burned within non-piñon and juniper land cover types (13.5 vs 

15.3%). Piñon and juniper land cover types were only 5 percent of total land cover 

and 4.4 percent of total area, but a relatively high percentage of piñon and juniper 

land cover burned annually. In contrast, the percentage of the area that burned 

within piñon and juniper land cover types in the Southern Intermountain geographic 

region was slightly more than the percentage of the area that burned within nonpiñon and juniper land cover types (9.8 vs 8.6%). Piñon and juniper land cover 

totaled 11 percent of total land cover and 12.4 percent of total area. A moderate 
USDA Forest Service RMRS-GTR-372. 2018. 45

to high percentage of piñon and juniper land cover burned annually. In both 

Intermountain geographic regions, similar factors appear to influence fire regimes 

in piñon and juniper and non-piñon and juniper land cover types. Invasive annual 

grasses (Downs et al. 2016) and higher burn probabilities (Short et al. 2016) have a 

relatively greater influence on fire regimes in the northern part of the Intermountain 

geographic region, and longer fire seasons have a stronger influence in the southern 

part.

In the Central Rocky Mountain geographic region, the percentage of the area that 

burned within piñon and juniper land cover types was more than the percentage of 

the area that burned within non-piñon and juniper land cover types (3.1 vs 2.3%). 

Piñon and juniper land cover types made up 16.7 percent of total land cover and 

22.1 percent of the total area burned, but a low percentage of piñon and juniper land 

cover burn annually. In the Southern Rocky Mountain geographic region the percentage of the area that burned within piñon and juniper land cover types was less 

than the percentage of the area that burned within non-piñon and juniper land cover 

types (4.3 vs 6.9%). Piñon and juniper land cover types were the greatest area of 

all the regions (23.1%). However, low to intermediate percentages of the piñon and 

juniper land cover types burned annually and only 15.6 percent of the total area that 

burned was piñon and juniper land cover. The higher percentage of burned area in 

piñon and juniper than non-piñon and juniper land cover types in the Central Rocky 

Mountain geographic region may be related to the increasing influence of climate 

warming, drought (Westerling et al. 2014), and invasive annual grasses (Arendt and 

Baker 2013; Floyd et al. 2004). A reverse trend in the Southern Rocky Mountain 

geographic region may be explained by the very large fires that burned in forested 

areas during the 30-year study period (Westerling et al. 2014).

Conclusions

Increases in the total area burned and decreases in fire rotations occurred in all 

four geographic regions for piñon and juniper land cover types during the 30-year 

period. Fire rotations for piñon and juniper land cover types were generally within 

historical ranges for the dominant shrubland or piñon and juniper land cover types 

within a region. Also, the percentage of the study area that burned in piñon and 

juniper land cover types was higher than in non-piñon and juniper land cover 

types in both the Southern Intermountain and Central Rocky Mountain geographic 

regions. These increases in fire coupled with other global change factors appear to 

be increasing the vulnerability of many piñon and juniper ecosystems.

In the Northern and Southern Intermountain geographic regions greater burn 

areas are linked to changes in climate and increases in invasive annual grasses that 

result in greater burn probabilities (Balch et al. 2013; Downs et al. 2016; Short et 

al. 2016). In the Colorado Plateau ecoregion, however, invasive annual grasses and 

associated increases in fire are counter-balancing the effects of tree expansion and 

resulting in net losses of piñon and juniper land cover types (Arendt and Baker 

2013). In the Southern Rocky Mountain geographic region, longer fire seasons 

coupled with hotter droughts and increases in bark beetles are resulting in significant tree mortality (Allen et al. 2015; Breshears et al. 2005). Similar trends have 

been observed in the Southern Intermountain geographic region (Greenwood and 

Weisberg 2008; Flake 2016).
46 USDA Forest Service RMRS-GTR-372. 2018.

The magnitude of the effects of increased fire activity versus other global change 

factors varied across the four geographic regions. Careful monitoring of longerterm trends in fire activity and the interacting effects of invasive annual grasses, 

bark beetles, and climate change is needed to better understand the dynamics of 

piñon and juniper land cover types and to develop predictive modeling capacity. 

Understanding the longer-term ecosystem trends and primary drivers for the different regions can help inform the desirability of management treatments to remove 

piñon and juniper.

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Appendix A—Summaries of Each Piñon and Juniper 

(PJ) Land Cover Type Burned Annually for Each 

Geographic Region and its Ecoregions.

Figure A.1— Percent area of each piñon and juniper (PJ) land cover type burned annually on a 100,000 hectare basis for 

the Northern geographic region overall and the ecoregions within the geographic region. 
USDA Forest Service RMRS-GTR-372. 2018. 55

Figure A.2—Percent area of each piñon and juniper (PJ) land cover type burned annually on a 

100,000 hectare basis for the Southern Intermountain geographic region overall and for the ecoregions within the geographic region. 
56 USDA Forest Service RMRS-GTR-372. 2018.

Figure A.3—Percent area of each piñon and juniper (PJ) land cover type burned annually on a 

100,000 hectare basis for the Central Rocky Mountain geographic region overall and the ecoregions within the geographic region. 
USDA Forest Service RMRS-GTR-372. 2018. 57

Figure A.4—Percent area of each piñon and juniper (PJ) land cover type burned annually on a 100,000 

hectare basis for the Southern Rocky Mountain geographic region overall and the ecoregions within the 

geographic region. 

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