United States Department of Agriculture

Forest Rocky Mountain Resource Bulletin

Service Research Station RMRS-RB-25 June 2018

Arizona’s Forest Resources, 2001–2014

John D. Shaw, Jim Menlove, Chris Witt, Todd A. Morgan, 

Michael C. Amacher, Sara A. Goeking, Charles E. Werstak, Jr.
All Rocky Mountain Research Station publications are produced 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.

Shaw, John D.; Menlove, Jim; Witt, Chris; Morgan, Todd A.; Amacher, Michael C.; Goeking, 

Sara A.; Werstak, Charles E., Jr. 2018. Arizona’s forest resources, 2001–2014.

Resour. Bull. RMRS-RB-25. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station. 126 p.

Abstract

This report presents a summary of the most recent inventory of Arizona’s forests based 

on field data collected between 2001 and 2014. The report includes descriptive highlights 

and tables of forest and timberland area, numbers of trees, biomass, volume, growth, 

mortality, and removals. Most sections and tables are organized by forest type or foresttype group, species group, diameter class, or owner group. The report also describes the 

inventory’s design, inventory terminology, and data reliability. Results show that Arizona’s 

forest land covers 18.6 million acres. Forty-one percent (7.7 million acres) of this forest 

land is administered by the USDA Forest Service, and another 39 percent (7.3 million 

acres) is privately owned. The State’s most abundant forest type is pinyon/juniper woodland, which covers more than 7.3 million acres. Pinyon/juniper woodlands, combined with 

juniper woodland, cover over 11 million acres, or almost 60 percent of Arizona’s forest land 

area. Common pinyon is the most abundant tree species by number of trees, and ponderosa pine is the most abundant by volume and biomass. Arizona’s forests contain 14.5 

billion cubic feet of net volume in trees 5.0 inches diameter and larger. Gross growth of all 

live trees 5.0 inches diameter and larger averaged 184 million cubic feet per year. Average 

annual mortality totaled 238 million cubic feet per year, and net growth was –53.6 million 

cubic feet per year, or about a 0.37 percent reduction of the State’s total wood volume.

Keywords: Arizona, forest inventory, field data, trees, biomass, volume, growth, mortality, 

removals

Front cover photos by John D Shaw..
Authors

John D. Shaw is a Biological Scientist and Analysis Team Leader with the Interior West 

Forest Inventory and Analysis Program at the USFS Rocky Mountain Research Station in 

Ogden, Utah. He holds B.S. and M.S. degrees in Natural Resources Management from the 

University of Alaska Fairbanks and a Ph.D. in Forest Ecology from Utah State University.

Jim Menlove is an Ecologist and a member of the Analysis Team with the Interior West 

Forest Inventory and Analysis Program at the USFS Rocky Mountain Research Station in 

Ogden, Utah. He holds a B.S. degree in Biology from the University of Utah and an M.S. 

degree in Zoology and Physiology from the University of Wyoming, both with an emphasis 

in ecology.

Chris Witt is an Ecologist and a member of the Analysis Team with the Interior West Forest 

Inventory and Analysis Program at the USFS Rocky Mountain Research Station in Ogden, 

Utah. He holds B.S. and M.S. degrees in Ecology from Idaho State University.

Todd A. Morgan is the Director of the Forest Industry Research Program at The University 

of Montana’s Bureau of Business and Economic Research in Missoula, Montana. He received a B.A. degree in Philosophy and a B.S. degree in Forest Science from Pennsylvania 

State University and an M.S. degree in Forestry from the University of Montana.

Michael C. Amacher is a National Program Manager, Urban & Community Forestry at the 

USFS Rocky Mountain Research Station in Logan, Utah. He also serves as the Western 

Soils Indicator Advisor in the Indicators of Forest Health program. He has B.S. and M.S. 

degrees in Chemistry and a Ph.D. in Soil Chemistry, all from Pennsylvania State University.

Sara A. Goeking is a Biological Scientist and a member of the Analysis Team with the Interior West Forest Inventory and Analysis Program at the USFS Rocky Mountain Research 

Station in Ogden, Utah. She holds a B.S. degree in Environmental Studies and an M.S. 

degree in Forest Ecology, both from Utah State University.

Charles E. Werstak, Jr. is a Biological Scientist and a member of the Analysis Team with 

the Interior West Forest Inventory and Analysis Program at the USFS Rocky Mountain Research Station in Ogden, Utah. He holds a B.S. degree in Geography from Western Illinois 

University and an M.S. degree in Geography from Utah State University.
ii

Acknowledgments

The Rocky Mountain Research Station gratefully acknowledges the cooperation and 

assistance of the USFS Southwestern Region; the Arizona State Forester; the Bureaus 

of Land Management and Indian Affairs; and the U.S. Department of the Interior National 

Park Service. The authors extend a special note of thanks to private landowners who provided information and access to field sample plots, and to the field staff who collected the 

inventory data.

Report Highlights

Forest Area

• There are an estimated 18.6 million acres of forestland in Arizona, about 25 percent of 

the State’s total land area. Almost 91 percent (16.9 million acres) of the forest land is 

unreserved, meaning that it is available to be managed for production of various forest 

resources. Almost 18 percent (3.0 million acres) of the unreserved forest land is classified as timberland, meaning that the land has the capability of producing greater than 20 

cubic feet per acre per year of wood volume. 

• Arizona’s most common forest-type group is pinyon/juniper, which covers 11.1 million 

acres or almost 60 percent of the State’s forest land. The woodlands hardwoods group 

is the second most abundant, making up 3.6 million acres (19.5 percent) of the State’s 

forest land. 

• Arizona’s largest owner of forest land is the USDA Forest Service’s National Forest System, which manages about 7.7 million acres (41.3 percent) of forest land. Collectively, 

privately owned forest land totals 7.3 million acres (39.3 percent) of Arizona’s total forest 

land area, second only to National Forest System lands.

Numbers of Trees Volume, and Biomass

• Arizona has an estimated 4.0 billion live trees 1 inch in diameter or larger and over 75 

million dead trees 5 inches in diameter or larger. Over 68 percent of live trees are softwood species, and 35 percent of live softwoods are under 5 inches in diameter. 

• The net volume of all live trees on Arizona’s forest land totals 14.6 billion cubic feet, 7.7 

billion cubic feet of which (53 percent) is located on lands managed by the U.S. Forest 

Service. Privately owned forests contain 36.4 percent of the State’s total live volume. 

The western woodland softwoods forest-type group contains almost 42 percent of the 

State’s live tree volume, followed by the ponderosa pine group, with over 30 percent.

Forest Growth and Mortality

• Gross growth of wood volume is over 181 million cubic feet per year, but annual mortality is nearly 238 million, resulting in a negative net growth (reduction in live tree volume) 

of nearly 54 million cubic feet per year on forest land in Arizona. As a percentage of total 

live volume —0.37 percent per year— this results in a relatively small change in the 

live-tree inventory.
iii

Contents

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

Arizona’s Annual Forest Inventory................................................................1

Previous Inventories of Arizona’s Forests......................................................3

Accessing Arizona’s Forest Inventory Data ...................................................4

Overview of Standard and Supplemental Tables...........................................4

Inventory Methods..............................................................................................5

Plot Configuration........................................................................................5

Sample Design ............................................................................................6

Three-phase Inventory .................................................................................6

Sources of Error .........................................................................................11

Quality Assurance .....................................................................................12

Overview of Arizona’s Forests...........................................................................16

Ecoregion Provinces of Arizona .................................................................16

Forest Land Classification ..........................................................................19

Forest Land Ownership..............................................................................20

Forest Types and Forest-Type Groups .........................................................22

Stand Age ..................................................................................................27

Number of Trees........................................................................................28

Tree Volume and Biomass..........................................................................30

Forest Change Components: Growth, Mortality, and Removals..................33

Stand Density Index (SDI) ..........................................................................38

Arizona’s Forest Resources................................................................................40

FIA Data as a Habitat Monitoring Tool: The Mexican Spotted Owl 

as a Case Study......................................................................................40

Understory Vegetation ...............................................................................42

Forest Soils ................................................................................................44

Down Woody Material ..............................................................................46

Arizona’s Primary Wood Products Industry ................................................49

Current Issues in Arizona’s Forests....................................................................54

Drought-Related Effects on Pinyon/Juniper Woodlands..............................54

Expansion of Pinyon/Juniper Woodlands....................................................58

Aspen Status and Trends ............................................................................61

Damage to Live Trees.................................................................................63

Recent Fires...............................................................................................66

Invasive and Noxious Weeds.....................................................................71

Cheatgrass.................................................................................................73

Comparisons Between Arizona’s Periodic and Annual Forest Inventories....75

Conclusions and Future Analyses......................................................................79

References ........................................................................................................80
iv

Appendix A: Standard Forest Inventory and Analysis Terminology......................86

Appendix B: Standard Forest Resource Tables....................................................94

Appendix C: Arizona Forest-Type Groups and Forest Types, With 

Descriptions and Timber (T) or Woodland (W) Designations........................118

Appendix D: Tree Species Groups and Tree Species Measured in 

Arizona’s Annual Inventory, With Common Name, Scientific 

Name, and Timber (T) or Woodland (W) Designation ..................................124

Appendix E: Volume and Site Index Equation Sources.....................................126
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 1

Introduction______________________________________________

This report contains highlights of the status of Arizona’s forest resources, 

with discussions of pertinent issues based on the first 14 years of inventory under 

the Forest Inventory and Analysis (FIA) annual system (Gillespie 1999). In 1998, 

the Agricultural Research Extension and Education Reform Act (also known as 

the Farm Bill) mandated that inventories would be conducted throughout United 

States’ forests on an annual basis. This annual system integrates FIA and Forest 

Health Monitoring (FHM) sampling designs resulting in the mapped-plot design, 

which includes a nationally consistent plot configuration with four fixed-radius 

subplots; a systematic national sampling design consisting of one plot in each 

approximately 6,000-acre hexagon; annual measurement of a proportion of permanent plots; data or data summaries within 6 months after yearly sampling is 

completed; and a State summary report after 5 years. The inventory strategy for 

the western United States involves measurement of 10 systematic samples, or subpanels, where one subpanel is completed each year and all subpanels are measured 

over a 10-year period. Each subpanel is pre-assigned to be surveyed during a specific calendar year, which is referred to as the inventory year (see Appendix A for 

standard FIA terminology). The year in which each plot was actually surveyed is 

recorded as its measurement year. Annual inventory summaries are updated each 

spring to include the most recent subpanels of data. These data are usually available to the public within 6 months of the close-out of the previous field season. 

Data may be downloaded in table form or queried using a variety of online tools 

(http://fia.fs.fed.us/tools-data/default.asp).

Arizona’s Annual Forest Inventory

Interior West Forest Inventory and Analysis (IWFIA) implemented the new 

annual inventory strategy in Arizona in 2001, shortly after conclusion of the 1999 

periodic inventory (see next section). At the time annual inventory was started, 

the IWFIA program used criteria of 5 percent projected canopy cover or 40 trees 

per acre (including seedlings and saplings) as part of the definition of forest land. 

About the time that the first report on Arizona’s forest resources, based on annual inventory, was scheduled to begin (mid-2006), the Forest Service Washington 

Office Director for Science Policy Planning, Inventory, and Information directed 

a change in the FIA definition of forest for inventory purposes. There were two 

parts to the justification of this change: (1) crown cover was more consistent with 

international definitions of forest land; most international definitions use some 

minimum measure of crown or canopy cover in the forest land definition; and (2) 

FIA was expanding the national inventory to include areas not traditionally viewed 

as forest land. Several pilot projects underway at the time highlighted the need 

to define and categorize areas not traditionally viewed as forest land, and crown 

cover was considered superior as a measure of tree stocking in making these classifications. The new forest definition used a minimum projected canopy cover of 
2 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

10 percent, which includes standing dead trees and trees recently removed from 

the stand.

The directive also included a requirement that the new definition be applied 

retroactively. Plots measured previously that didn’t meet the new forest definition were to be excluded from the inventory of forest land and re-classified as 

nonforest. However, there was no set of variables in the FIA database that could 

be used to definitively assess whether a plot that was measured using a minimum 

of 5 percent cover would still meet the new forest definition of 10 percent cover. 

Moreover, there was no nationally consistent approach to qualifying land as forest; although the IWFIA program used a cover-based definition, the three other 

regional FIA programs used different approaches—for example, based on local 

stocking equations. As a result, the directive only started the process of moving 

toward a national, cover-based definition with a 10 percent minimum.

In response to this situation, the IWFIA program implemented two actions. 

First, for the upcoming (2007) field season, three temporary variables were added 

to the inventory—live tree canopy cover percent, live and dead/missing canopy 

cover percent, and number of seedlings per acre. These variables were assessed 

not on the plot footprint, but on the 1-acre area that encompassed the plot. Second, 

each of the plots measured in Arizona from 2001–2006 was assessed, using a variety of methods, as to whether they met the 10 percent cover definition. This approach was called “plot filtering” and is described in detail in the 2000–2005 report 

on Utah’s forests (DeBlander et al. 2010, Appendix A). That report was nearly 

completed using the 5 percent cover definition and was modified in accordance 

with the revised definition, whereas the Arizona report was delayed until the new 

definition and related procedures were fully implemented.

The new, nationally consistent forest definition was implemented in 2011, 

with version 5.0 of the IWFIA field manual (http://www.fs.fed.us/rm/ogden/datacollection/pdf/iwfia_p2_50.pdf). As a result of the filtering approach and the addition of temporary variables, the first 10 years of annual data (2001–2010) for 

Arizona included a combination of plots that were classified as forest based on 

filtering, and plots that were classified as forest using variables that eventually 

were added to version 5.0 of the field manual. Because Arizona has a large area of 

marginally forested land, this meant that many forest/nonforest calls from 2001–

2006 were considered tentative, pending remeasurement visits to those plots during 2011–2016. However, using a combination of the temporary canopy cover 

variables and the application of the new definition and procedures since 2011, the 

majority of previously tentative forest/nonforest determinations have now been 

verified and the results are reflected in this report.

The inventory years covered in this report are 2001 through 2014. Because 

this span of subpanels includes 4 years of remeasurement, only the most recent 10 

subpanels (2005–2014) are used to produce current inventory estimates, such as 

are presented in the Appendix B tables. Data from the first 4 inventory years are 

used for other analyses, such as trends in certain variables since the beginning of 

annual inventory.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 3

Previous Inventories of Arizona’s Forests

Prior to implementation of the annual forest inventory, Arizona’s forest attributes were estimated from periodic inventories. Periodic inventories in most 

States generally used data collected over a span of 2 to 5 years, which were then 

grouped into a single “inventory year.” This process was done periodically (hence 

the name), with an interval between inventories of 10 to 15 years. There were three 

periodic inventories in Arizona, dated 1966 (Spencer 1966); 1985 (Conner et al. 

1990) and 1999 (O’Brien 2002).

The 1966 periodic inventory report (Spencer 1966) was the most comprehensive assessment of Arizona’s forests at the time. Some statistics on Arizona’s 

forests were published earlier within national reports (USDA FS 1958, 1965), but 

the underlying data were not comprehensive for the State. The 1966 inventory was 

based on a combination of airphoto analysis and ground verification. The 1985 periodic inventory was conducted with support from several cooperators, including 

the Forestry Division of the Arizona State Land Department, the Bureau of Land 

Management, and the Bureau of Indian Affairs (Conner et al. 1990). Two national 

forests collected data for the inventory, and the U.S. Forest Service Southwestern 

Region supplied data for other national forests. The Bureau of Indian Affairs supplied data for part of the Fort Apache Indian Reservation. Data collection in the 

remaining areas of the State was done by the FIA staff (then known as Forest 

Survey) with the assistance of State, Federal, and Tribal cooperators. The 1999 periodic inventory report was based on data collected on 2,763 plots (O’Brien 2002), 

some of which were carry-over plots from the 1985 inventory. Most plots used a 

mapped-plot design similar to that used in the current annual inventory.

Data from new inventories are often compared with data from earlier inventories to quantify forest trends. However, for the comparisons to be valid, the 

procedures used in the inventories must be compatible. Procedures for past inventories of Arizona are different enough from present procedures that direct comparisons of summary results are not recommended. However, it is possible to compare 

individual plots that were measured during both inventories. The plot design used 

during the 1999 periodic inventory was very similar to the annual inventory’s plot 

design. Therefore, plots on forest land that were sampled during both inventories 

can be compared to evaluate changes in attributes such as per-acre estimates of 

live volume, mortality, growth, and biomass. Details on this approach, as well 

as comparison of selected characteristics between Arizona’s 1999 periodic inventory and annual inventory, have been published (Goeking 2015). A more detailed 

description of the differences between the periodic and annual forest inventories 

of Arizona, as well as results of plot-to-plot comparisons of periodic and annual 

inventory data, can be found in this report in the “Comparisons Between Arizona’s 

Periodic and Annual Forest Inventories” chapter.
4 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Accessing Arizona’s Forest Inventory Data

FIA data are publicly available from the national FIA website at http://fia.

fs.fed.us. This site includes data downloads; online tools that allow users to perform 

custom queries; and documentation of FIA’s field inventory protocols, database 

structure, and publications. Plot data may be downloaded in table form or summarized using a variety of online tools (http://fia.fs.fed.us/tools-data/default.asp).

The national FIA database contains data from the 1985 periodic inventory 

(Conner et al. 1990) and the 1999 periodic inventory (O’Brien 2002), as well as 

annual forest inventory data, which are updated each year as additional measurements are collected. Data collected as part of the annual inventory are assigned an 

inventory year that corresponds to the year in which the plot was scheduled for 

measurement within a 10-year remeasurement cycle, although measurement year 

can differ from inventory. This can occur, for example, when a plot is inaccessible 

during its scheduled year because of active fires in the vicinity. For assistance with 

finding information on this site or with performing custom analyses, data users 

are encouraged to contact one of the members of the Analysis Team of the Interior 

West FIA Program who are listed as authors at the beginning of this report.

Overview of Standard and Supplemental Tables

Forest Inventory and Analysis produces a set of standard tables that incorporates most of the core FIA program, using both Phase 2 and Phase 3 data. Appendix 

B presents tables B1–B37, which summarize annual forest inventory data collected in Arizona between 2005 and 2014 in terms of traditional FIA attributes. 

These tables encompass statistics for land area, numbers of trees, wood volume, 

biomass (oven-dry weight), growth, mortality, and sampling errors. Table B1 is 

the only table that includes all land cover types, and it summarizes the proportions 

of sample plots that were recorded as forest, nonforest, and nonsampled (e.g., due 

to inaccessibility). All other tables exclude nonforest land and therefore include 

only accessible forest land or timberland (see Appendix A for definitions). Table 

B37 shows sampling errors for area, volume, net growth, and mortality at the 67 

percent confidence level.

This report also contains supplemental tables within the body of the report. 

To avoid confusion between supplemental tables found in individual report chapters and the standard FIA tables found in Appendix B, supplemental tables in the 

body of this report are labeled consecutively as they appear, beginning with table 

1. The standard tables located in Appendix B will be referred to with the appendix 

letter followed by the table number (e.g., table B1).
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 5

Inventory Methods_________________________________________

This chapter briefly describes five key aspects of the FIA program. The first 

four sections describe configuration of field plots, the national sample design, the 

three-phase inventory system, and sources of error, which are consistent among all 

States. The last section describes FIA’s quality assurance program and presents the 

results of quality assessments for the current forest inventory of Arizona.

Plot Configuration

The national FIA plot design consists of four 24-foot radius subplots configured as a central subplot and three peripheral subplots (USDA FS 2011; see 

fig. 1). Centers of the peripheral subplots are located at distances of 120 feet and at 

azimuths of 0 degrees, 120 degrees, and 240 degrees from the center of the central 

subplot. A circle passing through the centers of the peripheral subplots encompasses about one acre. Each standing tree with a diameter at breast height (d.b.h.) 

for timber trees, or a diameter at root collar (d.r.c.) for woodland trees, of 5 inches 

or larger is measured on these subplots. Each subplot contains a 6.8-foot radius 

microplot with its center located 12 feet east of the subplot center on which each 

tree with a d.b.h./d.r.c. from 1 inch to 4.9-inches is also measured.

To enable division of the forest into various domains of interest for analysis, 

it is important that the tree data recorded on these plots are properly associated 

with stand-level data. In addition to the tree data recorded on FIA plots, data are 

also gathered about the condition class in which the trees are located. A condition 

Figure 1—Plot configuration used by the Forest 

Inventory and Analysis program. Each plot consists of four subplots with a 24-foot radius. The 

three outer subplots are located 120 feet from the 

central subplot’s center at azimuths of 0, 120, and 

240 degrees. Microplots with radii of 6.8 feet are 

located on each subplot, with centers located 12 

feet from the subplot center at an azimuth of 90 

degrees.
6 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

class (or condition) is the combination of discrete landscape and forest attributes 

that define and describe the area associated with a plot. The six variables that define distinct condition classes are forest type, stand origin, stand size, ownership 

group, reserved status, and stand density (Bechtold and Patterson 2005). In some 

cases, the plot footprint spans two or more conditions if there is a distinct change 

in any of these six variables. For example, the four subplots on a plot may intersect 

both forest and nonforest areas, the plot may include distinct stands differentiated 

by forest type and/or stand size, or the plot may straddle an ownership boundary. 

All three of these examples would result in more than one condition per plot. Field 

crews assign numbers to condition classes in the order they are encountered on a 

plot. Each tree is assigned the number of the condition class in which it stands to 

enable partitioning of tree data into meaningful categories for analysis.

Sample Design

Based on historic national standards, a sampling intensity of approximately 

one plot per 6,000 acres is necessary to satisfy national FIA precision guidelines 

for area and volume. Therefore, FIA divided the area of the United States into nonoverlapping, 5,937-acre hexagons and established one plot in each hexagon using 

procedures designed to preserve existing plot locations from previous inventories. 

These sample plots, designated as the Federal base sample, were divided into five 

spatially interpenetrating panels and 10 subpanels, where each panel consists of 

two subpanels. In the eastern United States, two subpanels are measured each year 

such that the inventory cycle is on a 5-year rotation, while in the western United 

States, including Arizona, one subpanel is measured each year and inventory cycles 

are completed on a 10-year rotation (Gillespie 1999). For estimation purposes, the 

measurement of each subpanel of plots can be considered an independent, equal 

probability sample of all lands in a State, or all plots can be combined to represent 

the State.

Three-phase Inventory

FIA conducts inventories in three phases. In Phase 1, remote sensing data 

are digitally analyzed to stratify each State into homogeneous groups such as forest and nonforest areas. In Phase 2, a permanent network of ground plots is first 

examined using digital imagery, and then forested plots are visited in the field and 

traditional inventory variables such as forest type and tree diameter are measured. 

In Phase 3, additional variables associated with forest and ecosystem health are 

measured on a subset of Phase 2 plots.

Phase 1

Phase 1 uses remote sensing data to delineate homogeneous areas, or strata, 

throughout the entire State. The purpose of this delineation is to reduce the variance of FIA estimates through post-sampling stratification of field data. The initial 

Phase 1 strata map consisted of forest, nonforest, and census water strata (see 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 7

Appendix A for definitions), which were delineated at a spatial resolution of 250 

meters using a combination of 2004 MODIS satellite imagery, other geospatial 

datasets, and plot-based calibration data (Blackard et al. 2008).

In most Interior West States, post-sampling stratification is based solely on 

forest and nonforest strata under the assumption that any nonresponse plots occur 

randomly across the plot grid. Non-response plots are defined as plot locations that 

cannot be sampled by a field crew. These situations typically occur when landowners or managers do not grant permission for field crews to access plot locations 

on their lands, although some plots are not sampled due to hazardous conditions 

that may be permanent in nature, such as sheer cliffs, or temporary hazards, such 

as current wildfires or active logging operations. Only 4.8 percent of the plots in 

Arizona were nonresponse (table B1), including 3.5 percent that were classed as 

access denied.

FIA produces estimates at the scale of individual States, which can then be 

aggregated into regional estimates, as well as at smaller scales within each State. 

Within-State population estimates are constructed at two scales: survey units that 

are comprised of groups of counties, and smaller estimation units that represent 

individual counties. Arizona consists of two survey units and 15 estimation units 

denoted as g, each containing ng ground plots. The area of each estimation unit is 

divided into strata of known size using the State’s stratification map (fig. 2), which 

divides the total area of the estimation unit into 250-meter pixels and assigns each 

pixel to one of H strata. Each stratum, h, within an estimation unit, g, then contains 

nhg ground plots where the Phase 2 attributes of interest are observed.

To illustrate, the area estimator for forest land within an estimation unit in 

Arizona is defined as:

Âg = ATg Ʃ Whg

_____ H

h = 1

Ʃ yihg

i = 1

nhg

nhg

where:

Âg = total forest area (acres) for estimation unit g

ATg = total land area (acres) in estimation unit g

H = number of strata

Whg = proportion of Phase 1 pixels in estimation unit g that occur in stratum h

Yihg = forest land condition proportion on Phase 2 plot i in stratum h in estimation 

 unit g

nhg = total number of Phase 2 plots in stratum h in estimation unit g

Phase 2

Phase 2 pertains to FIA’s network of permanent plot locations, where each 

plot is assigned spatial coordinates and represents approximately 6,000 acres. To 
8 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

minimize inventory costs, plots that are obviously and entirely nonforest are not 

designated for field sampling, and these plots are recorded as nonforest. A human 

interpreter examines each plot location using digital imagery from the National 

Agriculture Imagery Program and distinguishes plots that potentially contain forest or wooded land from those that do not intersect any forest or wooded land. This 

process is known as prefield interpretation, and it was historically considered part 

of Phase 1 because both prefield interpretation and Phase 1 relied on remote sensing data. However, Phase 1 delineation of forest and nonforest strata occurs independently of current prefield interpretation of the Phase 2 grid. Therefore, prefield 

data collection is considered part of Phase 2 and not part of Phase 1.

Figure 2—The strata used for poststratification of Arizona’s forest 

inventory, 2005–2014; background 

shows shaded relief and county 

boundaries.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 9

The status of each plot in the Phase 2 grid is eventually assigned as accessible forest land, nonforest land, or non-repsonse (fig. 3). Plots that were not 

designated for field sampling by prefield interpreters are automatically recorded 

as nonforest plots. For plots that are designated for field sampling, field crews 

record the plot status as accessible forest land if (a) they can physically visit 

the plot location, and (b) the plot satisfies FIA’s definition of forest land (see 

Appendix A). Some field plots are recorded as nonforest because the field crew 

determines that they do not meet FIA’s definition of forest land. A field plot may 

be recorded as non-response if a field crew cannot safely measure the plot or if 

they cannot obtain permission to access the plot location. Before visiting privately owned plot locations, FIA crews identify each plot’s ownership status by consulting county land records and then seek permission from private landowners to 

Figure 3—Plot status of the 12,274 

Phase 2 plots in Arizona’s annual forest inventory, 2005–2014. 

(Note: Plot locations are approximate; some plots on private land 

were randomly swapped.)
10 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

measure plots on their lands. Information about individual landowners and the 

existence of FIA plots on their property is considered confidential and is never 

shared outside the FIA program, regardless of whether permission to access the 

plot location is granted. Table B1 shows the total percentage of Phase 2 plot 

areas that represent forest, nonforest, and non-response conditions. Note that 

figure 3 and table B1 are the only portions of this report that include summaries 

of non-response plots; all other summaries of forest and nonforest are based on 

sampled plots, and estimates have been adjusted to account for missing observations at non-response plots as described by Goeking and Patterson (2013).

Field crews record a wide variety of data on plot locations that contain 

accessible forest land. Crews locate the geographic center of the plot using geographic positioning system (GPS) receivers and then establish markers to facilitate relocation of the plot for future remeasurement. They record condition-level 

variables that include land use, forest type, stand origin, stand-size class, stand 

age, site productivity class, forest disturbance history, silvicultural treatment, 

slope, aspect, and physiographic class. Some of these area attributes are measured or observed (e.g., regeneration status), some are assigned by definition 

(e.g., ownership group), and some are computed from tree data (e.g., percent 

stocking). For each tree on the plot, field crews record a variety of attributes 

including species, live/dead status, diameter, height, crown ratio, crown class, 

damage, and decay status. The field procedures used in Arizona’s forest inventory are described in detail in the FIA field guide (USDA FS 2011). Data analysts apply statistical models using field measurements to calculate additional 

variables such as volume and biomass for individual trees, as well as volume, 

biomass, growth, mortality, and number of trees per unit area.

Phase 3

The third phase of the enhanced FIA program focuses on forest and ecosystem health. The Phase 3 sample consists of a 1/16 subset of the Phase 2 plots, 

which equates to one Phase 3 plot for approximately every 96 thousand acres. 

All the measurements collected on Phase 2 plots are collected on Phase 3 plots, 

plus an extended suite of ecological data (forest health indicators) pertaining to 

soil samples, down woody materials, lichen communities, tree crowns, and understory vegetation structure. Phase 3 measurements are obtained by field crews 

during the growing season. The original Phase 3 approach is being superseded 

in the IWFIA States by a shift toward expanding some protocols to all Phase 2 

plots. Down wood materials, vegetation structure, and tree crown data are all 

collected on Phase 2 plots at the same intensity as all other data but with slightly 

different sampling designs than the Phase 3 versions. Soil data collection has 

been expanded beyond the original Phase 3 plots and will eventually cover the 

entire set of Phase 2 plots. The remaining indicators are being implemented only 

as special studies at this time.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 11

Sources of Error

Sampling error

The process of sampling (selecting a random subset of a population and calculating estimates from this subset) causes estimates to contain error they would 

not have if every member of the population had been observed and included in the 

estimate. The 2005–2014 FIA inventory of Arizona is based on a sample of 12,274 

plots systematically located across the State. The total area of Arizona is 73.1 million acres, so the sampling rate is approximately one plot for every 5,953 acres.

The statistical estimation procedures used to provide the estimates of the 

population totals presented in this report are described in detail in Bechtold and 

Patterson (2005). Along with every estimate is an associated sampling error that 

is typically expressed as a percentage of the estimated value, but it can also be 

expressed in the same units as the estimate or as a confidence interval (the estimated value plus or minus the sampling error). This sampling error is the primary 

measure of the reliability of an estimate. An approximate 67 percent confidence 

interval constructed from the sampling error can be interpreted to mean that under 

hypothetical repeated sampling, approximately 67 percent of the confidence intervals calculated from the individual repeat samples would include the true population parameter if it were computed from a 100-percent inventory. The sampling 

errors for State-level estimates are presented in table B37.

Because sampling error increases as the sample size decreases, users should 

aggregate data categories as much as possible. Sampling errors obtained from data 

aggregation are only approximations of reliability because homogeneity of variances is assumed. Users may compute statistical confidence for subdivisions of the 

reported data using the formula below:

� SEs

 = SEt

�_____ Xt

Xs

SEs = sampling error for subdivision of State total.

SEt

 = sampling error for State total.

Xs

 = sum of values for the variable of interest (area, volume, biomass, etc.) for 

 subdivision of State total.

Xt = sum of values (area, volume, biomass, etc.) for State total.

Measurement error

Measurement errors are associated with the methods and instruments used 

to observe and record the sample attributes. On FIA plots, attributes such as the 

diameter and height of a tree are measured with specialized instruments; other attributes, such as species and crown class, are observed without the aid of an instrument. On a typical FIA plot, 30 to 70 trees are observed with 15 to 20 attributes 

recorded on each tree. In addition, many attributes that describe the plot and conditions on the plot are observed. Errors in any of these observations affect the quality 
12 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

of the estimates. If a measurement is biased—such as tree diameters consistently 

taken at a height other than 4.5 feet from the ground—then the estimates that use 

this observation (e.g., calculated volume) will reflect this bias. Even if measurements are unbiased, high levels of random error in the measurements will add to 

the total random error of the estimation process. To ensure that FIA observations 

meet the highest standards possible, a quality assurance program, described below, 

is integrated throughout all FIA data collection efforts.

Prediction error

Prediction errors are associated with using mathematical models (such as volume models) to provide information about attributes of interest based on sample 

attributes. Area, number of trees, volume, biomass, growth, removals, and mortality are the primary attributes of interest presented in this report. FIA estimates of 

area and number of trees are based on direct observations and do not involve the 

use of prediction models; however, estimates of volume, biomass, growth, and 

mortality use model-based predictions in the estimation process and are thus subject to prediction errors.

Quality Assurance

FIA employs a Quality Assurance (QA) program to ensure the quality of all 

collected data. The QA program provides a framework to assure the production of 

complete, accurate, and unbiased forest information of known quality. There are 

two primary facets of FIA’s QA program: quality control and quality assessment.

FIA’s quality control process is carried out using data quality inspectors, 

who assess individual field crews and then provide timely feedback to improve the 

crews’ performance. This is accomplished by means of hot checks and cold checks. 

During a hot check, an inspector accompanies a field crew to a plot and provides 

immediate feedback on the quality of their measurements. Cold checks occur when 

an inspector visits a recently completed plot, typically in possession of the original 

crew’s data but without the crew present, and then verifies each measurement and 

provides the crew an overall score as well as feedback on measurements that did 

not meet FIA specifications. On average, hot checks are done on 2 percent of all 

field-sampled plots and cold checks are done on 5 percent of field-sampled plots.

Quality assessment is the second facet of FIA’s QA program, and this process 

quantifies the overall precision of field measurements by comparing two independent measurements of the same plot. The independent measurements are collected 

by means of blind checks, where a regular field crew collects measurements and 

then a second crew collects a second set of measurements, without knowledge of or 

access to the first crew’s measurements (Pollard et al. 2006). Thus, these paired observations provide a means of assessing repeatability of FIA’s field measurements.

Quality control and quality assessment both require a data quality standard 

that defines the target level of precision for field measurements. FIA has specific 

Measurement Quality Objectives (MQOs) that enumerate data quality standards for 

individual field-measured variables. These data quality objectives were developed 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 13

from knowledge of measurement processes in forestry and forest ecology as well 

as the requirements of the FIA program. MQOs for each variable consist of a measurement tolerance and a compliance standard. Measurement tolerances define 

the acceptable range of variability between two independent observations, and 

compliance standards define the target percentage of observations that should be 

within the measurement tolerance when recorded by two independent observers. 

The practicality of these MQOs, as well as the measurement uncertainty associated 

with a given field measurement, can be tested by comparing the results of quality 

assessments using blind check data.

Quality assessment data for Arizona’s current inventory were collected between 2010 and 2014. The results of the QA analysis for this period are presented 

in tables 1 and 2. Table 1 describes tolerances for condition-level variables, and table 2 describes tree-level variables. Each variable and its associated measurement 

tolerance are followed by the percentage of total paired records that fall within one, 

two, three, and four times the tolerance. The last four columns show the number 

of observations that fell outside the tolerance. For example, table 1 shows that 

there were 62 paired records, representing 62 conditions that were measured independently by two field crews, for the variable “Disturbance 1.” About 89 percent 

of those records fell within the tolerance of having no errors. The percentage of 

observations that fall within the 1X tolerance level is referred to as the observed 

compliance rate, which can be compared to the compliance standard for each variable’s MQO to determine that variable’s performance. Compliance standards and 

measurement tolerances for FIA’s field measurements are listed within the field 

manual (USDA FS 2011).

The information in tables 1 and 2 shows variables with varying degrees of 

repeatability. For example, one condition-level regional variable that appears to be 

fairly repeatable is “percent crown cover.” At the 1X tolerance level, its observed 

compliance rate was about 98 percent. This represents that 98 percent of 56 paired 

observations were within plus or minus 10 percent of each other. In contrast, the 

compliance rate for “Habitat Type 1,” which has no tolerance variability, was only 

55 percent. This low compliance rate warrants further investigation into the potential repeatability issues associated with evaluating habitat type, which can provide 

insight into successional status when combined with existing vegetation (such as 

tree numbers, size class, and species by habitat types or series). Habitat types are 

represented as a categorical value, and it is likely that the compliance rate for habitat types would be higher if we could consider habitat type groups (or groups of 

types that are very similar) in our quality assurance analysis.

The tree measurements that have the biggest influence on estimates of forest 

volume are tree species, tree diameter, and tree height. As shown in table 2, the 

compliance rate for the variable “Species” was 87 percent. The diameter variables 

“d.b.h.” and “d.r.c.” represent the respective diameters of timber and woodland 

tree species (see appendices C and D). Whereas timber species are measured at 

breast height (4.5 feet above ground level), woodland species are measured near 

ground level at root collar. The 1X compliance rate was 91 percent for d.b.h., 
14 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 Table 1—Results of quality assessment for condition-level variables from 71 conditions in Arizona, 2010–2014. Variables that did not have any non-null values recorded on any QA plots are not shown; these include regeneration species as well as secondary and tertiary disturbances and treatments. Variable Tolerance Percentage of data within tolerance Number of times data exceeded tolerance Number of records @1x @2x @3x @4x @1x @2x @3x @4x National core variables Condition status No tolerance 98.6 1 71 Reserve status No tolerance 98.6 1 71 Owner group No tolerance 100.0 0 71 Forest type (type) No tolerance 82.3 11 62 Stand size No tolerance 83.9 10 62 Regeneration status No tolerance 100.0 0 62 Tree density No tolerance 100.0 0 62 Owner industrial status No tolerance 100.0 0 19 Disturbance 1 No tolerance 88.7 7 62 Disturbance Year 1 ±1 yr 66.7 66.7 66.7 100.0 2 2 2 0 6 Treatment 1 No tolerance 100.0 0 62 Treatment year 1 ±1 yr 100.0 0 1 Physiographic class No tolerance 48.4 32 62 Regional variables Percent crown cover ±10 % 98.2 100.0 1 0 56 Percent bare ground ±10 % 74.2 88.7 91.9 95.2 16 7 5 3 62 Habitat type 1 No tolerance 54.8 28 62 Habitat type 2 No tolerance 53.2 29 62
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 15 Table 2—Results of quality assessment for tree-level variables from 1,066 trees in Arizona, 2010–2014. Variable Tolerance Percentage of data within tolerance Number of times data exceeded tolerance Number of records @1x @2x @3x @4x @1x @2x @3x @4x National core variables Diameter (d.b.h.) ±0.1 /20 in. 91.2 97.2 97.6 98.0 22 7 6 5 249 Diameter (d.r.c. using IW MQO) ±0.2 in*#stems 88.2 95.3 97.7 98.9 96 38 19 9 815 Azimuth ±10 o 98.8 99.2 99.3 99.4 13 8 7 6 1066 Horizontal distance ±0.2 /1.0 ft 94.4 97.1 98.3 98.9 60 31 18 12 1066 Species No tolerance 86.5 144 1066 Tree status No tolerance 99.8 2 1066 Rotten/missing cull ±10 % 94.9 98.5 99.4 99.8 51 15 6 2 993 Total length ±10 % 83.6 96.9 98.6 99.4 175 33 15 6 1066 Actual length ±10 % 84.2 97.0 98.4 99.6 111 21 11 3 703 Compacted crown ratio ±10 % 100.0 0 968 Uncompacted crown ratio (P3) ±10 % 82.5 94.4 97.7 98.6 157 50 21 13 897 Decay class ±1 class 100.0 0 96 Cause of Death No tolerance 54.4 26 57 Mortality year ±2 yr 71.9 86.0 98.2 100.0 16 8 1 0 57 Condition class No tolerance 100.0 0 1066 Regional Variables Mistletoe ±1 class 97.5 98.6 99.2 99.3 24 14 8 7 968 Number of Stems ±1 stem 96.1 98.8 99.4 99.5 32 10 5 4 815 Percent missing top ±10 % 97.3 97.3 97.3 97.3 27 27 27 27 993 Sound dead ±10 % 41.5 41.5 41.5 41.5 581 581 581 581 993 Form defect ±10 % 59.8 59.8 59.8 59.8 49 49 49 49 122 Current tree class No tolerance 98.5 16 1066 Tree age ±5 % 80.2 83.0 86.8 89.6 42 36 28 22 212 Horiz dist-timberland ±0.2 /1.0 ft 98.0 98.4 98.8 98.8 5 4 3 3 250 Horiz dist-woodland ±0.2 /1.0 ft 93.3 96.7 98.2 98.9 55 27 15 9 816
16 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

which has a 0.1-inch tolerance, and 88 percent for d.r.c., which has a 0.2-inch per 

stem tolerance to allow for larger tolerances on multi-stemmed trees. Tree height 

is represented by the variables “total length” and “actual length.” Both variables 

have a tolerance level of ±10 percent of the observed length, and compliance rates 

at the 1X level were about 84 percent for both variables. “Sound dead,” which 

indicates the percentage of the tree’s volume that is estimated as cull due to sound 

dead wood, was the least repeatable tree-level variable, with a 1X compliance rate 

of only 42 percent.

When a variable’s compliance rate falls below the compliance standard, it 

indicates that one of three things should occur to address the shortfall. First, more 

intensive crew training may be required. Second, a variable’s MQO may need to 

be adjusted accordingly to better reflect the realistic expectation of quality for that 

variable. Third, the variable could be eliminated from FIA’s field protocols. As a 

result, MQOs are used not only to assess the reliability of FIA measurements and 

their ability to meet current standards, but also to identify areas of improvement of 

data collection protocols and training. This ongoing process improves repeatability 

or may even lead to elimination of variables that prove to be unrepeatable.

Overview of Arizona’s Forests_______________________________

This chapter summarizes the current status of Arizona’s forests in terms of 

traditional forest attributes such as forest ownership, forest type, stand age, numbers of trees, volume, biomass, growth, mortality, removals, and stand density index. Nearly all attributes are based directly on FIA measurements, except where 

noted in individual sections.

Ecoregion Provinces of Arizona

Although Arizona is well-known as a “desert” State because of its iconic saguaro cactus stands and spectacularly exposed geology, the State contains a wide 

variety of forests and woodlands. About 25.5 percent of the surface area of the 

State is covered by forest. The distribution of forest types is strongly controlled by 

an interaction of climate and soils (Pearson 1931). In fact, the obvious zonation of 

vegetation types in the San Francisco Peaks and vicinity led to the development of 

the life zone concept by Merriam in 1889 (Merriam 1898; Merriam and Steineger 

1890; see Colorado Plateau Semi-Desert Province below). Precipitation ranges 

from about 4 to over 28 inches annually across the State (Greening 1941), but it 

is the interaction of precipitation and evaporation—ultimately controlling available soil moisture—that truly determines potential vegetation (Pearson 1931; Zon 

1941). For example, 15 inches of precipitation is sufficient to support ponderosa 

pine forests in Montana, but 20 or more inches are necessary to offset the higher evaporation potential in Arizona and New Mexico (Pearson 1931; Zon 1941). 

Some forests and woodlands actually experience negative soil moisture balances 

at times (Pearson 1931).
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 17

Because of these limitations, Arizona’s forests are largely confined to the 

Mogollon Rim portion of the Colorado Plateau, the Kaibab Plateau, and the 

Chuksa Mountains. Isolated pockets of forest land are also found in the Madrean 

Archipelago, or “Sky Islands,” that are found in the southeastern section of the 

State. In all cases, the high-elevation forest types are surrounded by wide bands 

of woodland types and other wooded land. These bands are, in turn, surrounded 

by large expanses of nonforest. The abundance of these transition zones makes it 

possible to observe some or all of Merriam’s life zones (Merriam and Steineger 

1890) by driving short distances at many different locations throughout the State. 

This feature of Arizona’s forests makes their diversity as important as their extent. 

Many aspects of this diversity are discussed in this report.

The multitude of factors that influence forest conditions often occur across 

political and ownership boundaries. Forest scientists and land managers must assess and manage for these issues regardless of such boundaries. Ecological units 

provide an alternative spatial framework for assessing and managing forest resources because they characterize areas of similar vegetation, climate, soils, hydrologic processes, disturbance regimes, topography, geology, and other processes 

such as nutrient cycling and plant community succession (Cleland et al. 1997). 

Each ecological unit is therefore similar with regard to natural processes and probable responses to management activities (Bailey 1983). Ecoregions in the United 

States are hierarchically subdivided, in descending order of size, into domains, 

divisions, provinces, sections, and subsections. Provinces are defined largely by 

vegetation patterns and are therefore the most relevant units for describing forest 

lands.

FIA uses the modifications to Bailey (1995) of Cleland et al. (2007) to assign plots to ecological provinces, sections, and subsections. Arizona spans four 

ecological provinces (fig. 4): the Chihuahuan Semi-Desert (321); American SemiDesert and Desert Province (322); the Colorado Plateau Semi-Desert (313); and 

the Arizona-New Mexico Mountains Semi-Desert–Open Woodland–Coniferous 

Forest–Alpine Meadow (M313). All four provinces contain some amount of forest 

land, although the composition and extent vary widely.

The Chihuahuan Semi-Desert Province (321) covers southeastern Arizona 

and extends through southern New Mexico and west Texas. The province is primarily desert, with long, hot summers and short winters that have brief periods below 

freezing. Spring and summer are very dry, but local, sometimes heavy rains start in 

July and extend through October. The province covers 13.2 percent of Arizona (9.7 

million acres) and is 28.3 percent forested (2.7 million acres). Mesquite woodland 

accounts for about 45 percent of forested land, followed by evergreen oak woodland (27.2 percent) and pinyon/juniper woodland (10.4 percent).

The American Semi-Desert and Desert Province (322) covers southwestern Arizona, southeastern California, and southern Nevada. Like the Chihuahuan 

province, summers are long and hot, and winters are moderate. Precipitation is not 

regular anywhere in the province; rain falling in winter is usually widespread and 

gentle, whereas summer rain comes in the form of localized thunderstorms. The 
18 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

province covers 37.9 percent of Arizona (27.7 million acres), but forest cover is 

only 6.4 percent (1.8 million acres). Pinyon/juniper woodland is most common, 

accounting for 41.2 percent of the forested land, followed by juniper woodland 

(37.7 percent) and mesquite woodland (13.9 percent).

The Colorado Plateau Semi-Desert Province (313) covers the Colorado River 

Plateau regions of Arizona, Colorado, New Mexico, and Utah. The province is 

intersected in some areas by the Arizona-New Mexico Mountains Semi-Desert–

Open Woodland–Coniferous Forest–Alpine Meadow (M313; see below), which 

represents the highest portions of the plateau, including several mountain ranges. 

Figure 4—The four ecological 

provinces in Arizona; background 

shows shaded relief and county 

boundaries.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 19

Plateau areas are relatively high elevation (5,000–7,000 ft), so winters are relatively cold, but temperatures in canyon bottoms can be more moderate. Summers 

are characterized by hot days and cool nights, with wide daily temperature fluctuations. Precipitation varies with elevation, with lower areas averaging less than 10 

inches annually. By comparison, the highest elevations of the San Francisco Peaks, 

near Flagstaff, average 35 to 40 inches annually. The wide range of climate found 

in the province, such as the variation found from the bottom of the Grand Canyon 

to the top of the San Francisco Peaks (which is actually in the adjacent, related 

province; see below), led to development of the life zone concept (Merriam and 

Steineger 1890). The province covers 40 percent of Arizona (29.2 million acres), 

and is 31 percent forested (9.1 million acres). There is relatively high forest diversity, with 18 forest types represented. However, of the forested land, 55.5 percent 

is the pinyon/juniper type, 22.6 percent is juniper woodland, and 5.4 percent is evergreen woodland oak. Ponderosa pine is the non-woodland type with the highest 

portion of area, with 6.6 percent.

The Arizona-New Mexico Mountains Semi-Desert–Open Woodland–Coniferous 

Forest–Alpine Meadow (M313) covers parts of Arizona and New Mexico. As implied by the lengthy name, the province is very diverse and characterized by high 

relief. In general, the province encompasses the highest plateaus and mountains (elevation >7,000 ft) within the Colorado Plateau Semi-Desert Province (313), although 

some of the province includes elevations as low as 4,500 feet. Climate varies with 

elevation; precipitation ranges between 10 and 35 inches annually and arrives mostly 

as snow in the high mountains. Late spring is relatively dry, with moisture deficits 

relieved by summer thunderstorms. The province covers 8.8 percent of Arizona (6.4 

million acres), but is 78.0 percent forested (5.0 million acres). The province contains 

the largest variety of forests in Arizona, with 19 forest types represented. This is the 

only province in which a timber forest type (ponderosa pine) is the most common, 

covering 35.4 percent (1.8 million acres) of the forested area. However, the total area 

of woodland types (52.7 percent, or 2.7 million acres) is still greater than the total 

acres of timber forest types (2.2 million acres, or 43.1 percent). As is the case elsewhere in the State, the most common woodland types are pinyon/juniper woodland, 

with 26.1 of the forested area, and juniper woodland, with 15.3 percent.

Forest Land Classification

FIA uses a nationally consistent standard for defining different categories 

of forest land based on reserved status and productivity. These categories were 

originally developed for the purpose of separating forest land deemed suitable for 

timber production from forest land that was either not suitable or unavailable for 

timber harvesting activity, which includes woodland forest types. The first division of forest land is unreserved forest land and reserved forest land. Unreserved 

forest land is considered available for harvesting activity where wood volume can 

be removed for wood products. Reserved forest land is considered unavailable for 

any type of wood utilization management practice through administrative proclamation or legislation.
20 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Both unreserved and reserved forest lands are further divided based on productivity. Unreserved forest land is subdivided into timberland and unproductive 

forests. Timberland is defined as unreserved forest land capable of producing 

20 cubic feet of wood per acre per year of trees designated as a timber species. 

Unproductive forests, because of a combination of species’ characteristics and site 

conditions, are not capable of producing 20 cubic feet of wood per acre per year of 

trees designated as a timber species (see Appendix A). Reserved forest land is also 

divided into productive and nonproductive forests. Some characteristics that contribute to productivity can be visibly obvious, such as the presence or absence of 

non-commercial species, rocky substrates, steep slopes, and high elevation. While 

these distinctions may be important for understanding reserved area management 

concerns (e.g., their effect on visitor experience), wood production capability on 

reserved forest land is useful only as a potential indicator of non-timber values. For 

example, higher-productivity reserved lands will tend to produce higher fuel loads 

than lower-productivity lands.

The State of Arizona encompasses 73.1 million acres of land area, of which 

18.6 million acres (25.5 percent) are estimated to be forest land by FIA. Unreserved 

forest land accounts for 90.9 percent of the forest land in Arizona and totals 16.9 

million acres (table B2). Timberland constitutes almost 18 percent (3.0 million 

acres) of Arizona’s unreserved forest land, and the remaining 82 percent (13.9 million acres) is classified as unproductive forest land. Reserved forests account for 

over 9 percent (1.7 million acres) of total forest land, with most of that area being 

unproductive forests.

Forest Land Ownership

Federal agencies manage more forest land in Arizona than any other land 

ownership or management group (table B2). Arizona’s largest manager of forest 

land is the USDA Forest Service’s National Forest System (NFS), which manages 

about 7.7 million acres of forest land. This represents over 10 percent of Arizona’s 

total land area and 41.3 percent of its forest land area. NFS lands in Arizona consist of six different national forests. More than 91 percent, or 7.0 million acres, of 

the forest land managed by NFS is classified as unreserved forest land. About 28 

percent, or 2.2 million acres, of unreserved forest land managed by NFS is further 

classified as unreserved timberland, while the remaining 72 percent is classified as 

unproductive (table B2). The net volume of live trees (table B12), as well as the 

average annual tree mortality (table B25), is higher on NFS lands than any other 

owner class.

Other public agencies managing large portions of Arizona’s forest land include the State of Arizona, with 8.7 percent of forest land (1.6 million acres), and 

the Bureau of Land Management (BLM), with 7.6 percent (1.4 million acres). 

The Departments of Defense and Energy (DOD/DOE), the National Park Service 

(NPS), the U.S. Fish and Wildlife Service (FWS), and other Federal ownerships 

account for only 2.8 percent of forest land, or just over 524 thousand acres. All of 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 21

Arizona’s State-managed forest land is classified as unreserved, but nearly all of it 

is classified as unproductive forest.

Privately owned forest land totals 7.3 million acres, or 39.3 percent of the 

State’s total forest land area, second only to national forest land (fig. 5). Arizona’s 

diverse array of private landowners consists of private individuals/families, corporations, tribes, and non-governmental organizations such as private associations 

or conservation groups. Although conservation easements can cover substantial portions of private land, all private forest land is categorized as unreserved. 

About 766 thousand acres, or 10.5 percent of all private forest land, are classified 

Figure 5—Distribution and percentage of inventory plots on forest 

land by owner class, Arizona, 

2005–2014. (Note: Plot locations 

are approximate; some plots 

on private land were randomly 

swapped.)
22 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

as timberland while 6.5 million acres are classified as unproductive forest land. 

Average annual net growth is higher overall on private lands than any other owner 

class (table B21).

Forest Types and Forest-Type Groups

Forest type is a classification of forest land based on the species forming 

a plurality of living trees growing in a particular forest. Forest type names may 

be based on a single species or groups of species. Forest types are an important 

measure of diversity, structure, and successional stage. The distribution of forest 

types across the landscape is determined by factors such as climate, soil, elevation, 

aspect, and disturbance history. The loss or gain of a particular forest type over 

time can help assess the impact of major disturbances related to fire, weather, climate, insects, disease, and human-caused disturbances such as timber harvesting 

or ecosystem restoration.

Forest types are aggregated into forest-type groups to simplify interpretation of large-scale forest trends. Arizona’s forests are classified into 10 foresttype groups that are further divided into 21 distinct forest types, all of which are 

described in Appendix C. Some forest-type groups contain only one forest type, 

while other forest-type groups include several individual forest types. An example 

of a forest-type group with multiple forest types is the pinyon/juniper forest-type 

group, which consists of the Rocky Mountain juniper forest type, the pinyon/juniper forest type, and the juniper woodland forest type. The distribution of forest 

types as well as individual tree species may vary among ecological provinces. 

Figure 6 shows the area occupied by each forest-type group in Arizona. Figures 

7–10 illustrate the spatial distribution of inventory plots in the most common forest-type groups and the forest types within those groups.

	-				 	5,000,000		 	10,000,000		 	15,000,000		

Other	hardwoods	group	

Other	western	so5woods	group	

Elm	/	ash	/	co;onwood	group	

Aspen	/	birch	group	

Fir	/	spruce	/	mountain	hemlock	group	

Douglas-fir	group	

Nonstocked	

Ponderosa	pine	group	

Woodland	hardwoods	group	

Pinyon	/	juniper	group	

Area	(acres)	

Figure 6—Area of land by forest-type group, Arizona, 2005–2014. See Appendix C for forest types and tree species 

included in each group.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 23

Figure 7—Distribution of inventory plots in the pinyon/juniper 

forest-type group, by forest type 

and basal area class, Arizona, 

2005–2014. (Note: Plot locations 

are approximate; some plots 

on private land were randomly 

swapped.)

Arizona’s most abundant forest-type group is the pinyon/juniper group, 

which covers more than 11.1 million acres and accounts for 59.6 percent of forest 

land in the State (table B3). Within this forest-type group, the pinyon/juniper forest 

type is most abundant (7.3 million acres), followed by the juniper woodlands forest 

type (3.6 million acres). Arizona’s second most abundant forest-type group is the 

woodland hardwoods group, which comprises 3.6 million acres and 19.5 percent 

of the State’s forest land. The most abundant forest types in this group includes the 

mesquite forest type (1.6 million acres), the evergreen oak woodland forest type 

(1.5 million acres), and the deciduous oak woodland forest type (0.5 million acres). 
24 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Figure 8—Distribution of inventory 

plots in the woodland hardwoods 

forest-type group, by forest type 

and basal area class, Arizona, 

2005–2014. (Note: Plot locations 

are approximate; some plots 

on private land were randomly 

swapped.)

The ponderosa pine forest-type group is Arizona’s third most abundant group, covering 2.4 million acres and 13 percent of the State’s forest land. The remaining 

forest-type groups covering at least 1 percent of forest land are nonstocked forests 

(768 thousand acres), the Douglas-fir forest-type group (229 thousand acres), and 

the fir/spruce/mountain hemlock group (221 thousand acres). Note that although 

mountain hemlock is part of the group name, the species does not naturally occur 

in Arizona (see Appendices C and D). Although the aspen/birch group, which is 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 25

Figure 9—Distribution of inventory 

plots in the aspen/birch, Douglasfir, and Ponderosa pine forest-type 

groups, by forest type and basal 

area class, Arizona, 2005–2014. 

Each of these forest-type groups 

contains only one forest type. 

(Note: Plot locations are approximate; some plots on private land 

were randomly swapped.)

entirely the aspen forest type, occupies only 0.8 percent of forest land (153 thousand acres), aspen is a prominent feature on many more acres, where it occurs as 

a minority stocking in other forest types (see section on Aspen Status and Trends).
26 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Figure 10—Distribution of inventory 

plots in the fir/spruce/mountain 

hemlock forest-type group, by 

forest type and basal area class, 

Arizona, 2005–2014. (Note: Plot 

locations are approximate; some 

plots on private land were randomly swapped.)
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 27

Stand Age

The age structure of forest land provides insight into prospective shifts in 

stand structure and composition over time. On every FIA plot that samples forest 

land and includes suitable trees for increment core extraction, stand age is estimated based on the average age of only those trees that fall within the calculated 

stand-size category. For example, suppose an FIA plot sampled a softwood forest 

type where about 30 percent of the live trees were in the large diameter stand-size 

class (trees at least 9.0 inches d.b.h. and larger) and 70 percent were in the medium 

diameter size class (trees between 5.0 and 9.0 inches d.b.h.). The stand would be 

classified as a medium diameter stand-size class, and therefore only the medium 

size trees would be used in determining stand age.

There are limitations to collecting data for stand age computation. Repeatable 

measurements of increment cores are difficult to collect from certain tree species, 

particularly woodland species or those that may be very long-lived. Stand age may 

not accurately depict the age structure of uneven-aged stands, which encompasses 

multiple age classes. Stand ages are difficult to accurately determine for stands that 

are predominated by small diameter tree species such as Gambel oak trees. Stand 

ages are not assigned to nonstocked conditions, which are stands that contain less 

than 10 percent stocking of live trees because of disturbance.

Table B6 shows the area of forest land, by age class and forest-type group, 

with 20-year intervals representing stand-age classes. Nearly half of Arizona’s forest land, or 9.0 million acres, is between 60 and 140 years of age, and nearly twothirds are between 60 and 180 years old. There are approximately equal areas of 

forest in the 81–100 year and the 101–120 year class, which are the two largest 20-

year classes with about 2.7 million acres each. Forests younger than 60 years cover 

about 2.8 million acres (15 percent), while forests older than 180 years cover 3.5 

million acres (18.5 percent). Only 8.8 percent of Arizona’s forest land, or 1.6 million acres, is in stands less than 20 years of age; this includes nonstocked stands, 

for which age is not computed, and is set to zero. The high abundance of middleaged forests (80–120 years) in comparison to young forests (0–20 years) suggests 

that disturbance is lagging behind the rate needed to rejuvenate the population and 

early seral habitats may be lacking.

There is a considerable difference in stand age distribution among the major 

forest-type groups in the State (fig. 11). Half of the six most abundant forest-type 

groups—Douglas-fir, fir/Spruce/Mountain Hemlock, and ponderosa pine groups—

have more forest land area in the 81–120 year age classes than any other class. The 

pinyon/juniper forest-type group has the most even distribution among age classes, 

with seven of the 20-year classes accounting for about 73 percent of area. The 

201+ year age class has the largest area of any class, but this is expected given the 

longevity of pinyons and junipers, coupled with the fact that the age class is openended. The Douglas-fir, fir/spruce/mountain hemlock, and ponderosa pine foresttype groups all have a very small proportion of forest land area that is younger than 

60 years (0 percent, 0.4 percent, and 5.8 percent, respectively). Compared to these 
28 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

coniferous forest-type groups, aspen forests have a much higher percentage of area 

that is younger than 60 years (64 percent). More than 55 percent of aspen forest 

type area is less than 20 years old. A very small percentage of aspen stands are 

older than 120 years (7.8 percent). After the aspen type, the woodland hardwoods 

group has the next greatest proportion of its area in young stands, with 15.8 percent 

younger than 20 years; the majority of acreage is irregularly distributed among all 

of the other age classes.

Number of Trees

Estimates of the numbers of trees are beneficial to a variety of silvicultural, 

forest health, and habitat management applications. These estimates are typically 

combined with information about the size and species of the trees to provide meaningful summaries of forest dynamics and stand structure. Younger forest stands 

usually consist of large numbers of small-diameter trees, whereas older forest 

stands contain small numbers of large-diameter trees. FIA classifies individual tree 

species into species groups and categorizes each species and species group as either softwood or hardwood (Appendix D).

Arizona contains an estimated 4.0 billion live trees 1 inch in diameter or 

larger (table B10) and over 75 million dead trees 5 inches in diameter or larger. 

Softwood species total 2.8 billion trees or 68.5 percent of the State’s live trees. 

Almost 35 percent of live softwood trees are under 5.0 inches in diameter and 5.8 

percent are 15.0 inches and larger in diameter. The western woodland softwoods 

species group accounts for 69.3 percent (1.9 billion live trees) of the softwood trees 

(fig. 12). Oneseed juniper and common pinyon are the most abundant tree species in this group, which also includes Pinchot juniper, redberry juniper, alligator 

0%	

10%	

20%	

30%	

40%	

50%	

60%	

1-20	 21-40	 41-60	 61-80	 81-100	 101-120	121-140	141-160	161-180	181-200	 201+	

Percent	of	Area	in	Group	

Stand	Age	Class	

Aspen	/	birch	group	

Douglas-fir	group	

Fir	/	spruce	/	mountain	hemlock	group	

Pinyon	/	juniper	group	

Ponderosa	pine	group	

Woodland	hardwoods	group	

Figure 11—Distribution of 

forest land by stand age 

class for major foresttype groups, Arizona, 

2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 29

juniper, Utah juniper, Rocky Mountain juniper, Mexican pinyon, and Arizona pinyon. The second most abundant softwood group is the ponderosa and Jeffrey pine 

group with 605 million live trees (22 percent of softwood trees), all of which are 

ponderosa pines. Each of the other four forest-type groups account for less than 4 

percent of live trees.

Hardwood species account for 1.3 billion trees, or 31.5 percent of Arizona’s 

live trees. The western woodland hardwoods group includes the vast majority (90.4 

percent) of the live hardwood trees occurring in Arizona, followed by the cottonwood 

and aspen species group (8.5 percent percent). The hardwood species group also includes Gambel oak, bigtooth maple, honey mesquite, velvet mesquite, Arizona white 

oak, Emory oak, Mexican blue oak, silverleaf oak, gray oak, and netleaf oak.

Figure 13 shows the number of live trees by diameter class for seven species 

groups in Arizona. The pattern of many smaller trees compared to larger ones is 

expected for most species, but it also illustrates the different life histories of various species groups. Most species in the woodland hardwoods species group, which 

includes several species of oak and mesquite, do not attain large size. A large fraction of trees (69.8 percent) occur in the small diameter classes (less than 5 inches 

diameter), with just over 6 percent of trees 11.0 inches and greater. In contrast, 

most timber species groups have more balanced size distributions. Less than half 

of ponderosa pine and Douglas-fir trees are under 5.0 inches, and approximately 

20 percent of trees of each species are 11.0 inches or greater. Spruces, true firs, and 

poplars (cottonwoods and aspen) have distributions that more closely resemble the 

woodland species, possibly because of recent disturbance and regeneration. The 

size distribution of western woodland softwoods, which include pinyons and junipers, more closely resembles that of some timber types. This situation is likely due 

to the fact that pinyons and junipers are long-lived, reproduce slowly, and self-thin 

relatively little during stand development.

	-				 	500,000		 	1,000,000		 	1,500,000		 	2,000,000		

Other	western	softwoods	

Engelmann	and	other	spruces	

True	fir	

Douglas-fir	

Ponderosa	and	Jeffrey	pine	

Western	woodland	softwoods	

Other	western	hardwoods	

Cottonwood	and	aspen	

Western	woodland	hardwoods	

Softwoods	 Hardwoods	

Number	of	live	trees	(thousands)	

Species	Group	

Figure 12—Number of live trees 1.0 inch diameter and larger on forest land, by species group, Arizona, 2005–2014. 
30 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Tree Volume and Biomass

The amount of cubic-foot volume of wood in a forest is important for determining the sustainability of current and future wood utilization. The forest products industry and forest managers are interested in knowing the tree species composition and size distribution, as well as the geographic location and ownership 

status, of available wood volume. Estimates of gross and net volume include only 

the merchantable portion or sawlog portion (e.g., cubic-foot or board-foot) of live 

trees 1.0 inch in diameter and larger. Net volume is computed by deducting rotten, 

missing, or form defects from gross volume. Net volume is reported below as net 

volume of all live trees, net volume of growing-stock trees, net volume of sawtimber, and net volume of sawlogs. All of these terms are defined below as well as 

in Appendix A. Tree biomass estimates are based on gross volumes and describe 

aboveground tree weight (oven-dry) by various components (merchantable bole 

and bark, tops and limbs, saplings). This method of estimating tree biomass is 

referred to as the component ratio method and is described by Woudenberg et al. 

(2010, Appendix J). Note that FIA’s biomass estimates are produced in units of 

oven-dry weight; estimates of bone-dry weight can be calculated using the following conversion: one bone-dry unit equals 2,400 pounds of oven-dry wood (Morgan 

et al. 2006).

Tables B12 through B16 show the net volume of all live trees 5.0 inches diameter and larger on Arizona’s forest land, by various categories. The net volume 

of all live trees on Arizona’s forest land totals 14.6 billion cubic feet (table B12). 

	-				

	100,000		

	200,000		

	300,000		

	400,000		

	500,000		

	600,000		

	700,000		

1.0-2.9	 3.0-4.9	 5.0-6.9	 7.0-8.9	 9.0-10.9	 11.0-12.9	13.0-14.9	15.0-16.9	17.0-18.9	19.0-20.9	21.0-24.9	 25+	

Number	of	trees	(thousands)	

Diameter	class	(in.)	

Western	woodland	softwoods	

Western	woodland	hardwoods	

Ponderosa	and	Jeffrey	pine	

Cottonwood	and	aspen	

True	fir	

Douglas-fir	

Engelmann	and	other	spruces	

Figure 13—Number of live trees on forest land, by species group and diameter class, Arizona, 2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 31

Almost 53 percent of the live volume, or 7.7 billion cubic feet, is located on lands 

managed by the NFS. Only 6.4 percent of the NFS-managed volume exists on 

reserved lands and is unavailable for harvest. Privately owned forests contain 36.4 

percent of the State’s total live volume, or 5.3 billion cubic feet. Lands managed 

by Federal agencies other than the NFS include 7.4 percent of net volume, or less 

than 1.1 billion cubic feet. The remainder, about 0.5 billion cubic feet, is on lands 

managed by State and local government. For all owner classes, unreserved forests 

have somewhat lower volumes on timberland than on unproductive forest land (6.2 

billion vs. 7.1 billion cubic feet), although on NFS lands the volume on unreserved 

timberland is about 60 percent of the unreserved volume. The total live volume on 

unreserved timberland is 6.2 billion cubic feet.

Live tree volume can also be reported by forest-type group and tree species 

group. The pinyon/juniper forest-type group contains more live tree volume than 

any other forest-type group (table B13). Similarly, the woodland softwoods species 

group, which includes all of Arizona’s pinyon and juniper species, contains more 

live tree volume than any other species group (tables B14 and B15). The western 

woodland softwoods include 41.6 percent of the State’s live tree volume and 32 

percent of the standing dead volume (fig. 14). The ponderosa forest-type group, 

with 34.3 percent of live volume, and the ponderosa pine species group, with 36 

percent of live volume, account for the majority of the remaining volume among 

forest type- and species-groups. Pinyon and juniper species are not considered 

to be timber species, so they are not included in the estimates of growing-stock 

volume and sawtimber volume that are presented below and in tables B17–B20. 

When the volumes of individual tree species are compared, ponderosa pine has 

more volume than any other species with 5.3 billion cubic feet Statewide.

The availability of timber volume for harvest is affected by three primary 

factors: reserved status, productivity, and merchantability. Timberland is defined 

as unreserved forest land capable of producing in excess of 20 cubic feet per acre 

	-				 	1,500		 	3,000		 	4,500		 	6,000		 	7,500		

Other	western	hardwoods	

Cottonwood	and	aspen	

Western	woodland	hardwoods	

Other	western	softwoods	

True	fir	

Engelmann	and	other	spruces	

Douglas-fir	

Ponderosa	and	Jeffrey	pine	

Western	woodland	softwoods	

Hardwoods	 Softwoods	

Cubic	feet	(millions)	

Species	Group	

Live	

Growing	Stock	

Standing	Dead	

Figure 14—Net 

cubic-foot volume of trees 5.0 

inches diameter or larger on 

forest land, by 

species group, 

Arizona, 2005–

2014. (Note 

that the volume 

of trees in the 

“Other western 

hardwoods” species group is too 

small to appear 

on this graph.)
32 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

per year of wood at culmination of mean annual increment. Merchantability refers 

to growing-stock trees, which are at least 5 inches in diameter and contain, or 

have the potential to produce, an 8-foot sawlog that is reasonably free of defects. 

Therefore, growing-stock volume on timberland represents the amount of timber 

that is potentially available for harvest. The net volume of growing-stock trees on 

timberland in Arizona totals 5.9 billion cubic feet (table B17), or 40.5 percent of 

the total live volume on forest land.

The distribution of growing-stock volume varies by species or species group 

and also by owner class (table B18). Across all owner classes, nearly 85 percent 

of the State’s growing-stock volume is composed of two species: ponderosa pine 

and Douglas-fir. Ponderosa pine constitutes over 75 percent of Arizona’s growing-stock volume, or 4.4 billion cubic feet, and Douglas-fir contains 9 percent, or 

528 million cubic feet (table B18). The remainder of growing stock is somewhat 

evenly divided among the species groups true fir (4.1 percent or 241 million cubic 

feet), Engelmann and other spruces (4.4 percent or 262 million cubic feet), and 

cottonwood and aspen (5.8 percent or 339 million cubic feet). NFS lands include 

4.1 billion cubic feet, or almost 70 percent of the State’s growing stock. Almost 

1.7 billion cubic feet, or 29 percent of the total growing stock, occur on privately 

owned lands. Live volume is also reported for sawtimber trees, which are defined 

as softwood trees 9.0 inches in diameter or larger, or hardwood trees 11.0 inches in 

diameter or larger (International 1⁄4-inch rule). The net volume of sawtimber trees 

on timberland totals 28.5 billion board feet (table B19), over 76 percent of which 

is ponderosa pine.

The total weight of oven-dry aboveground biomass on Arizona’s forest land 

is 269 million tons, 60 percent (162 million tons) of which exists on public lands 

(table B29). Although biomass is typically sold by green weight, the water content 

of wood is highly variable geographically, seasonally, and even across portions 

of a single tree. Therefore, live-tree inventory estimates of green biomass may be 

unreliable or even misleading. In contrast, oven-dry weight does not change due to 

fluctuations in tree water content.

Volume and biomass can also be expressed in terms of the amount per acre. 

Table 3 shows live tree volume (in cubic feet per acre) and biomass (in tons per 

acre) by forest type. The estimates for each forest type include all of the different 

species that occur within that forest type. Because estimates for forest types with 

small samples may not be representative, only forest types sampled on at least 20 

plots are included in this discussion. The Douglas-fir forest type has the highest 

per-acre net volume of live trees 5.0 inches diameter and larger, with 2,842 cubic 

feet per acre, and also has the highest biomass of live trees 1.0 inch diameter 

and larger with 55.3 dry tons per acre. Not surprisingly, the forest types with the 

six largest average net volumes and biomasses are all timber types (Engelmann 

spruce and Engelmann spruce / subalpine fir are combined, because separately 

they occupy relatively small area). The woodland forest type with the highest peracre net volume is the Rocky Mountain juniper forest type, with 957 cubic feet. 

Deciduous oak woodlands contain more biomass per acre (18.4 dry tons per acre) 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 33

than other woodland forest types, although Rocky Mountain juniper has higher 

average volume.

Forest Change Components: Growth, Mortality, and Removals

Forest vigor, sustainability, and timber supply are often assessed by what are 

referred to as forest change components: growth, mortality, and removals. The relationship among these three change components quantifies the change in tree volume over time. Growth is typically expressed as net annual growth and is defined 

as the gross, or total, average annual growth in tree volume minus the volume lost 

through mortality. Mortality is the average annual net volume of trees dying over a 

given time period due to natural causes and excludes the volume removed through 

harvesting. Tree mortality often occurs at low and predictable rates due to insects 

and disease, suppression by overstory trees, or advanced tree age. Occasionally, 

highly concentrated and localized losses occur due to insect and disease epidemics, 

wildfire, or severe weather events. Removals represent the net volume of growingstock trees removed from the inventory by harvesting or other cultural operations 

(such as timber-stand improvement), by land clearing, or by changes in land use 

(such as designation as Wilderness or other reserved status).

The three components of forest change—growth, mortality, and removals—

are typically analyzed using measurements of the same plot at two points in time. 

It is possible, however, to also estimate growth and mortality rates based on a 

single inventory, as described below. In contrast, removals cannot be reliably estimated without having two measurements of the same set of plots, and the Arizona 

inventory did not begin remeasurement until 2011. Because fewer than half of 

plots are available for removal estimation, recent removals are still estimated using 

Table 3—Net volume (cubic feet per acre) of live trees 5.0 inches diameter and larger, and biomass (tons per acre) of live 

trees 1.0 inches diameter and larger, averaged by common forest types, Arizona, 2005–2014.

Forest type

Forest type area 

(acres)

Net volume (ft3 per 

acre)

Biomass (dry tons per 

acre)

Douglas-fir 228,661 2,842 55.3

White fir 70,663 2,308 42.0

Ponderosa pine 2,411,236 2,066 37.7

Aspen 153,237 1,868 35.0

Engelmann spruce and Engelmann spruce / 

subalpine fir

96,188 1,888 31.1

Deciduous oak woodland 539,169 846 18.4

Rocky Mountain juniper 94,293 957 16.6

Pinyon / juniper woodland 7,354,124 649 11.6

Evergreen oak woodland 1,456,812 579 11.5

Juniper woodland 3,635,485 408 7.1

Mesquite woodland 1,630,770 165 4.6

Nonstocked 768,390 25 0.5
34 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

information about the amount of wood cut and processed by the forest products 

industry. Due to this difference in analysis methods, growth and mortality are analyzed and discussed separately from removals.

At this stage of annual inventory in Arizona, the procedures used to estimate 

tree growth and mortality depended on the remeasurement status of the plot. A 

remeasured or paired plot refers to a plot that was established during the previous 

inventory cycle (time 1), and the field crews were able to relocate the plot during 

the current inventory (time 2) and account for all trees previously measured. In 

the current inventory (2005–2014), over 38 percent of all plots that sample forest 

land in Arizona were remeasured, so the same trees were measured at two points in 

time. For trees that were alive at time 1 and time 2, growth is calculated based on 

the change in volume over the time interval between plot visits. The time interval 

between remeasured plot visits in Arizona varied between 4 and 11 years with 

an average interval of about 10 years. Mortality volume is based on the volume 

of any tree that qualifies as a mortality tree over the time interval between plot 

visits. A tree is classified as mortality if it was alive at time 1 but dead at time 2. 

A new plot is a plot established for the first time where there was no previous colocated plot to be remeasured. On new plots, annual growth is estimated from a 

sample of increment core measurements based on the previous 10 years of radial 

growth. Mortality is estimated from trees that died in the 5 years prior to the year 

of measurement.

The annual estimate of gross growth of all live trees 5.0 inches diameter and 

greater on forest land in Arizona totaled nearly 184.1 million cubic feet. This is the 

sum of growth on all survivor and ingrowth trees. Survivor trees are live trees 5.0 

inches and larger in diameter at time 1 and still alive at time 2 on remeasured plots, 

and live trees determined to be 5.0 inches and larger in diameter 10 years prior to 

the current measurement on new plots. Ingrowth trees are live trees 5.0 inches and 

larger in diameter that grew over the 5.0-inch threshold between time 1 and time 

2 on remeasured plots or during the previous 10 years on new plots. The average annual mortality of trees 5.0 inches and larger in diameter was 237.8 million 

cubic feet (table B25). The difference between the live tree growth and mortality 

indicates a net annual growth estimate of –53.6 million cubic feet on forest land in 

Arizona (see tables B21–B24).

The negative net annual growth of 53.6 million cubic feet of in Arizona signifies an inventory of live trees that is decreasing annually in the absence of trees 

removed from human-caused activities. The annual decrease appears to be a large 

number, but in relative terms is small; net annual growth as a percentage of net volume of all live trees 5.0 inches and larger in diameter averages only –0.37 percent 

per year. However, negative net growth means that high levels of tree mortality are 

exceeding gains from live tree growth. In figure 15, the map of net annual growth 

at individual plots shows that plots with large values of net growth, whether positive or negative, tend to be concentrated in particular areas of the State. Areas 

where plots with high negative net annual growth are clustered represent areas 

affected by major disturbances such as insect outbreaks and fires.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 35

Net growth varies considerably by major owner group. Figure 16 illustrates 

the relationship between net growth and mortality by owner group in Arizona. 

Mortality of all trees on forest lands managed by the NFS totaled 161.9 million 

cubic feet (table B25) compared to –60.7 million cubic feet of net annual growth 

(table B21). In contrast, net annual growth exceeded mortality on privately owned 

forests; net growth totaled 5.4 million cubic feet compared to 61.5 million cubic 

feet of mortality.

Figure 17 illustrates the relationship between net growth and mortality for 

the 13 major inventory species—those with the greatest total volume—in Arizona. 

With the exception of ponderosa pine, Utah and oneseed junipers, and velvet 

Figure 15—Net annual growth at 

inventory plots on forest land, 

Arizona, 2005–2014. Negative 

values indicate plots where mortality exceeded gross growth. (Note: 

Plot locations are approximate; 

some plots on private land were 

randomly swapped.)
36 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

mesquite, annual mortality exceeded growth for all other major species. Douglasfir, Engelmann spruce, and quaking aspen recorded negative net growth. As a 

proportion of live volume, velvet mesquite was the highest with 1.1 percent net 

growth, followed by Utah juniper (0.44 percent), oneseed juniper (0.39 percent), 

and ponderosa pine (0.19 percent). This is in reverse order of what would be expected in normal periods, because timber species generally grow at higher rates 

than woodland species. Of the species with negative net growth, the worst loss as a 

proportion of volume occurred in white fir (–6.2 percent), Engelmann spruce (–3.7 

percent), common pinyon (–1.6 percent), and Gambel oak (–1.7 percent).

Since high mortality is the driving force behind the large differences between 

gross and net growth, further examination of this change component by other resource attributes can help explain the factors behind the high level of tree volume 

-100	

-50	

0	

50	

100	

150	

200	

National	Forest	 Other	Federal	 State	and	Local	

Government	

Undifferentiated	

private	

Million	cubic	feet	

Ownership	Group	

Net	Annual	Growth	 Average	Annual	Mortality	

Figure 16—Net annual 

growth and average annual 

mortality on forest land by 

ownership group, Arizona, 

2005–2014. 

-30	 -20	 -10	 0	 10	 20	 30	 40	 50	 60	 70	

Ponderosa	pine	

Utah	juniper	

Common	pinyon	

Oneseed	juniper	

Alligator	juniper	

Douglas-fir	

Arizona	white	oak	

Gambel	oak	

Quaking	aspen	

Velvet	mesquite	

White	fir	

Engelmann	spruce	

Arizona	pinyon	pine	

Million	Cubic	Feet	

Species	

Mortality	 Net	Growth	

Figure 17—Net annual 

growth and average 

annual mortality on 

forest land for 13 major species, Arizona, 

2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 37

estimated to have died in the previous 10 years. Substantial differences were observed in per-acre estimates of mortality between major ownership groups and 

reserved statuses. Converting the State-level estimates of mortality into per-acre 

estimates removes the effect of differences in the amount of forest land controlled 

by different ownership groups. The per-acre estimate of annual mortality volume 

averages 12.8 cubic feet per year on forest land across all ownerships. Mortality on 

reserved forest land was appreciably higher than unreserved land, averaging 21.2 

cubic feet per acre, compared to 11.9 cubic feet per acre on unreserved forest land. 

Figure 18 illustrates per-acre estimates of mortality by two major owner categories 

and reserved status. Reserved lands managed by the NFS recorded the highest 

average level of per-acre mortality at 37.6 cubic feet, whereas unreserved land 

controlled by private landowners, other Federal agencies, and State agencies had 

the lowest at 6.6 cubic feet. Many factors account for this difference, such as the 

varying ability to manage lands of different reserved status and the different forest 

type distributions among different combinations of ownership and reserved status.

All trees classified as mortality trees are assigned a cause of death in the field. 

Drawing conclusions from mortality estimates by cause of death should be done 

with caution because the actual agent that caused a tree’s death may be difficult, if 

not impossible, to determine. The “other” cause of death category includes trees 

that have died due to reasons the field crews are unable to determine. Interactions 

between insects and diseases are complex and make identification of causal agents 

difficult. Figure 19 illustrates per-acre estimates of mortality by reserved status 

and cause of death. Substantial differences were noted between reserved and unreserved forest land for mortality caused by insects and fire. Mortality due to fire 

accounted for the majority (41.5 percent) of total mortality. Insects were the second leading contributor to mortality, accounting for 32.3 percent of total mortality, 

followed by disease with 11.7 percent.

The leading causes of mortality are typically the same in most Interior West 

States, although the order of importance of fire and insects changes over time 

and space. In the neighboring State of New Mexico, insects were found to be the 

0	

5	

10	

15	

20	

25	

30	

35	

40	

Forest	Service	 Other	federal,	state,	or	private	

Annual	Mortality	(cubic	feet	per	

acre)	

Reserved	 Unreserved	

Figure 18—Average annual peracre mortality on forest land 

by two major owner categories 

and reserved status, Arizona, 

2005–2014.
38 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

leading cause of mortality in the most recent inventory (Goeking et al. 2014). The 

reasons behind the differences in levels of tree mortality by owner class and reserved status deserve further investigation. These differences have been observed 

in other State inventories (Menlove et al. 2012; Witt et al. 2012), which may suggest that reserved NFS lands have a larger share of aging forest stands that are 

more susceptible to insect and disease. These differences factors will be investigated with additional analysis of stand age, structure, density, species composition, 

and management regimes.

Stand Density Index (SDI)

Stand density index (Reineke 1933) is a relative measure of stand density, 

based on quadratic mean diameter of the stand and the number of trees per acre. 

In the western States, silviculturists often use SDI as one measure of stand structure to meet diverse objectives such as ecological restoration and wildlife habitat 

(e.g., Lilieholm et al. 1994; Long and Shaw 2005; Shaw and Long 2007; Smith 

and Long 1987). Originally developed for even-aged stands, SDI can also be applied to uneven-aged stands (Long and Daniel 1990; Shaw 2000). Stand structure 

can influence the computation of SDI, so the definition of maximum SDI must be 

compatible with the computation method. SDI was computed for each condition 

that sampled forest land using the summation method (Shaw 2000), and the SDI 

percentage was calculated using the maximum SDI for the forest type found on the 

condition.

Maximum SDI is rarely, if ever, observed in nature at the stand scale because the onset of competition-induced (self-thinning) mortality occurs at about 

60 percent of the maximum SDI. Within-stand variability of density results in the 

average stand density being well below that of the densest patches. A site is considered to be fully occupied at 35 percent of maximum SDI. Below about 25 percent 

0	

2	

4	

6	

8	

10	

12	

14	

Insect	 Disease	 Fire	 Animal	 Weather	 Vegetation	 Unknown	/	

other	

Human	

Annual	Mortality	(cubic	feet	per	acre)	

Cause	of	Death	

Reserved	 Unreserved	

Figure 19—Average 

annual per-acre mortality on forest land by 

reserved status and 

cause of death, Arizona, 

2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 39

of maximum SDI, individual trees are considered “free to grow.” At these lower 

densities, individual tree growth is maximized but stand growth is below potential, while at higher densities, individual tree growth is below potential, but stand 

growth is maximized (Long 1985). There are several reasons why stands may have 

low SDI. Stands typically have low SDI following major disturbances, such as 

fire, insect attack, or harvesting. These stands remain in a low-density condition 

until regeneration fills available growing space. Stands that are over-mature can 

also have low SDI, because growing space may not be re-occupied as fast as it is 

released by the mortality of large, old trees. Finally, stands that occur on very thin 

soils or rocky sites may remain at low density indefinitely, because limitations on 

physical growing space do not permit full site occupancy.

Because of the length of time that Arizona has been under annual inventory, 

it is possible to look at changes in stand density over time. For this analysis, conditions measured during the first half of the inventory period (2001–2007) are compared to conditions measured during the second half of the inventory (2008–2014). 

Each condition-level SDI value was binned at 20-unit intervals, and then the number of conditions in each bin was normalized to account for slightly differing numbers of observations during each 7-year span. After normalization, values from 

the two periods can be plotted together in each bin for direct comparison (fig. 20).

The general distribution of SDI values is somewhat left-skewed, which is 

consistent with the fact that Arizona is dominated by dryland forests and woodlands. These forests tend to have a lower capacity for stand density than, for example, the moister forests of the Pacific Northwest or the west side of the Rocky 

Mountains in northern Idaho and western Montana. Comparison of SDI means 

and distributions suggests that there had been an overall lowering of stand density 

since the early 2000s. Mean SDI was 131.8 for conditions measured 2001–2007 

and 124.4 for conditions measured 2008–2014, which is a small but statistically 

significant difference, based on a t-test (P<0.01). This difference does not appear 

to have been caused by the densest of stands being heavily disturbed and changed 

0.00	

0.02	

0.04	

0.06	

0.08	

0.10	

0.12	

0.14	

0	

20	40	60	80	

100	120	140	160	180	200	220	240	260	280	300	320	340	360	380	400	420	440	460	480	500	520	540	560	580	600	

More	

Proportion	of	Conditions	

SDI	Class	

2001-2007			Average	131.8	

2008-2014			Average	124.4	

Figure 20—Distribution 

and average Stand 

Density Index for the 

first half and second 

half of annual inventory in Arizona.
40 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

to a much lower-density state; there are very few conditions with SDI = 0. Rather, 

it appears that conditions previously distributed among the upper two-thirds of the 

range of SDI have shifted into the lower third.

The overall reduction of stand density is consistent with other results of the 

inventory. Many species have experienced a period of negative net growth (tables 

B21–B24), which means that mortality is freeing up growing space faster that the 

surviving trees on a site can re-occupy that space. It is also consistent with the 

impacts of fire. Across the Interior West, inventory data has shown that mortality 

is complete on only about 25 percent of burned conditions, and that less than 10 

percent of burned conditions have no regeneration tallied at the time of plot visit 

(Shaw et al. 2017). The conclusion drawn from this analysis is that while disturbances such as drought, fire, and insects have slightly reduced the overall density 

of Arizona’s forests, the impacts have not been catastrophic at the population scale.

Arizona’s Forest Resources_________________________________

FIA Data as a Habitat Monitoring Tool: The Mexican Spotted Owl as a Case 

Study

As additional forest attributes such as understory vegetation and down woody 

material have been added to the Phase 2 protocols, FIA data has become increasingly useful for estimating and monitoring wildlife habitat and tracking changes in 

its quality and quantity over time. These data can be especially useful in monitoring the changes in habitat of organisms listed under the Endangered Species Act 

(ESA). Species listed under ESA are afforded certain legal protections, one being 

a Recovery Plan (Plan) developed by the FWS. The Plan usually outlines a monitoring protocol for assessing the effectiveness of the recovery actions over time. 

Monitoring is often expensive and logistically cumbersome and might not provide 

useful information to FWS biologists in a timely fashion. The FIA program can 

assist with wildlife habitat monitoring, particularly in cases of forest-dwelling species. Inventory data can be useful for estimating habitat over large areas and comparing trends over time with little or no additional costs to the agencies managing 

the habitat in question. If FIA currently collects data on forest attributes found 

important to a listed species, estimates of these attributes can be produced quickly. 

By comparing past and present estimates, FWS staff can develop trends in habitat 

for a species over large geographic areas in perpetuity.

The Mexican spotted owl (Strix occidentalis lucida; “owl”) is a resident of the 

coniferous forests and canyon country of the Southwest, including a large area of 

Arizona. The owl prefers heavily stocked mixed-confer and pine-oak forests with 

large diameter trees and a complex understory for nesting and roosting (USFWS 

2012). Citing the alteration of its habitat due to timber management practices and 

the threat of stand-replacing wildlife in its remaining habitat, the owl was listed 

as threatened under the ESA in 1993, with a Recovery Plan being implemented 

in 1995. The Plan describes minimum thresholds for forest stand characteristics 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 41

known to be important for owl nesting and roosting in mixed-conifer and pineoak forests. These include the percentage of live basal area (BA) comprised of 

medium and large diameter trees, the density of large trees, total BA of a stand, 

and canopy cover. In the following application, we use FIA condition and tree data 

to identify plots meeting the Plan’s definition of the mixed-conifer forest type. In 

Arizona, mixed-conifer is more common than pine-oak and used by nesting owls 

more often (USFWS 2012). FIA forest type, habitat type, and the contribution of 

certain tree species to live basal area were the most important components used to 

redefine forest types. We then produce estimates of current suitable nesting/roosting habitat in the mixed-conifer forest type of Arizona’s Federally administered 

lands. Area of suitable habitat in mixed-conifer forest on Federal lands from past 

periodic inventory data (1995–2000) is compared to 2006–2015 estimates to illustrate trend analysis capabilities of FIA data. Area estimates for individual habitat 

characteristics are presented as well as those for Federal lands that satisfy all habitat characteristics concurrently.

Nearly 275 thousand acres of mixed-conifer forest reside on Federally administered lands in Arizona for the 2015 time series compared to an estimated 278 

thousand acres in 2000 (fig. 21). Just over 4 percent of these acres are estimated to 

meet all of the minimum nesting/roosting requirements outlined in the Plan in both 

inventories. This indicates that mixed-conifer forest nesting habitat has been maintained through the life of the Recovery Plan. In 2000, 61 percent (170,422 acres) of 

all mixed-conifer forests met the total basal area requirement, compared to nearly 

57 percent (274,860 acres) in 2015. This is the most frequently satisfied habitat 

component of the owl in mixed-conifer forests in either time period. Basal area 

from large trees (>18 in. d.b.h.) was the rarest habitat feature found in the 2015 

inventory. Nesting habitat in mixed-conifer forests appears to be relatively static 

over the years between inventories, the largest change being the large increase in 

stands meeting medium-sized tree basal area contributions. It also appears that 

forests meeting all criteria concurrently are still a very small percentage of the 

potential habitat.

0	 50,000	 100,000	 150,000	 200,000	 250,000	 300,000	

Meet	all	criteria	

BA:	trees	12-18	in.	dbh	

BA:	trees	≥	18	in.	dbh	

BA:	all	trees	

Trees	per	acre	≥	18	in.	dbh	

Mixed-confier	forest	

Acres	

Nesting	habitat	criteria	

2000	 2014	

Figure 21—Comparison 

of mixed-conifer nesting habitat of Mexican 

spotted owls on federal 

lands in Arizona between the 2000 and 

2015 inventories.
42 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

This exercise illustrates the utility of FIA data for future monitoring applications where methodology and temporal distribution of sampling will remain consistent. These estimates can be compared to data collected in the future to assess 

continuing trends of habitat quality in each forest type. This can help assess the 

effectiveness of management actions taken to either maintain or increase habitat 

for the owl over a given period of time. The Interior West FIA plot remeasurement 

schedule closely approximates the monitoring timeline described in the Plan and 

allows FIA data to be easily used as a habitat monitoring tool with little additional 

investment to data collection and monitoring efforts.

Understory Vegetation

The structure and composition of understory vegetation contributes to the 

diversity, productivity, and habitat structure of forest ecosystems. FIA collects 

understory vegetation data using two distinct protocols that characterize overall 

vegetation structure as well as species composition. Under the vegetation structure 

protocol, field crews record the height class and percent cover that is occupied by 

each of four plant growth habits: forbs, graminoids, shrubs, and understory trees, 

which are defined as trees less than 5 inches d.b.h. Under the species composition 

protocol, height class, growth habit, and percent cover are recorded for plant species that individually occupy at least 3 percent of the ground area. If more than 

four species occupy more than 3 percent cover, then only the most abundant four 

species per life form are recorded.

Figure 22 depicts the average percent cover of each plant growth habit for 

Arizona’s 10 most abundant forest types, and nonstocked land. Understory trees 

cover more area than the other three growth habits on all forest types except for 

juniper woodlands and nonstocked land. Juniper woodlands have more graminoid 

0.0	 2.0	 4.0	 6.0	 8.0	 10.0	 12.0	

Nonstocked	

Mesquite	woodland	

Evergreen	oak	woodland	

Deciduous	oak	woodland	

Aspen	

White	fir	

Ponderosa	pine	

Douglas-fir	

Pinyon	/	juniper	woodland	

Juniper	woodland	

Rocky	Mountain	juniper	

Percent	Cover	

Forest	Type	

Understory	Trees	 Shrubs	 Graminoids	 Forb	

Figure 22—The average percent cover of the four understory vegetation growth habits, by forest type, Arizona, 2005–2014. The 10 

most common forest types and nonstocked area are shown.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 43

cover than understory tree cover, whereas on nonstocked land both graminoids and 

shrubs have greater cover. Graminoids and shrubs have about equal cover in most 

forest types, but shrubs have relatively low cover as compared to graminoids in the 

ponderosa pine forest type. There were 670 individual plant species recorded on 

Arizona’s forest inventory plots. The five most frequently recorded species within 

each growth habit are listed in table 4. Blue grama (Bouteloua gracilis) was the 

most frequently encountered species by a large margin and was recorded on nearly 

a third of all forested plots. On average, blue grama covered 11 percent of all plots 

where it occurred. Forbs, in general, had relatively low frequencies of occurrence; 

western brackenfern (Pteridium aquilinum) was the most common, but was found 

on less than 2 percent of plots. It is possible, and perhaps likely, that some forbs 

are recorded at lower than actual frequencies because the timing of plot visits may 

not coincide with their emergence. Shrubs, in contrast, are more easily detectable 

because their woody components are generally visible year-round. Still, the five 

most common shrubs were only found on 6 to 10 percent of plots. Four of the five 

most common understory trees were also tally tree species. Only Sonoran scrub 

oak (Quercus turbinella), which is usually found in shrubby form or as small trees, 

did not represent regeneration of overstory tree species.

Table 4—Five most frequently recorded plant species in each understory vegetation growth habit, and average cover 

where they occur, Arizona, 2005–2014.

Growth habit Scientific name Common name

Number of 

plots

Average cover 

(percent)

Forb Pteridium aquilinum western brackenfern 57 7.3

Erodium cicutarium redstem stork’s bill 25 7.0

Chenopodium graveolens fetid goosefoot 20 3.9

Cordylanthus wrightii Wright’s bird’s beak 18 3.9

Amaranthus palmeri carelessweed 17 6.6

Gaminoid Bouteloua gracilis blue grama 863 11.0

Bouteloua curtipendula sideoats grama 480 6.1

Muhlenbergia emersleyi bullgrass 136 4.6

Bromus rubens red brome 104 5.5

Poa fendleriana muttongrass 99 4.1

Shrub Gutierrezia sarothrae broom snakeweed 341 3.2

Artemisia tridentata big sagebrush 288 7.9

Arctostaphylos pungens pointleaf manzanita 256 9.2

Cercocarpus montanus alderleaf mountain mahogany 227 5.0

Garrya wrightii Wright’s silktassel 184 4.6

Understory tree Pinus edulis common pinyon 426 3.0

Quercus gambelii Gambel oak 316 7.6

Quercus turbinella Sonoran scrub oak 280 9.0

Pinus ponderosa ponderosa pine 271 5.0

Prosopis velutina velvet mesquite 171 6.4
44 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Forest Soils

Soils on the landscape are the product of five interacting soil forming factors. These are parent material, climate, landscape position (topography), organisms (vegetation, microbes, other soil organisms), and time (Jenny 1994). Many 

external forces can have a profound influence on forest soil condition and hence 

forest health. These include agents of change or disturbances to apparent steadystate conditions such as shifts in climate, fire, insect and disease activities, land use 

activities, and land management actions.

The Soil Indicator of forest health was developed to assess the status and 

trend of forest soil resources in the United States across all ecoregions, forest types, 

and land ownership categories. For this report, data were analyzed and are being 

reported by forest-type groups. This forest type stratification not only reflects the 

influence of forest vegetation on soil properties, but also the interaction of parent 

material, climate, landscape position, and time with forest vegetation and soil organisms. Some of the key soil properties in Arizona were graphed by forest-type 

group, and to place these results in a regional context, these graphs are placed side 

by side with graphs of regional results. Because the western hardwoods, pinyon/

juniper, and ponderosa pine groups occupy extensive areas in Arizona, there are 

many soil samples from these forest-type groups (35, 96, and 26 plots with soil 

samples, respectively). The Douglas-fir, aspen, and spruce/fir groups occupy far 

less area in Arizona so the number of plots sampled for soil are very limited (3, 2, 

and 2 plots, respectively).

Soil C and N percentages generally increase from drier to wetter forest environments (fig. 23). Generally, soil moisture increases with elevation and latitude (cooler temperatures) and forest types reflect this climatic gradient. When 

expressed in terms of megagrams of C or N per hectare of forest area, C stocks also 

generally increase with elevation and latitude (fig. 24).

Aspen forests store more N in the mineral soil than any other forest group 

in the Interior West (fig. 24, right side). Aspen forests store significantly more N 

than spruce/fir forests, which often intermingle with aspen. High N levels in aspen 

forest floor and soils leads to lower C/N ratios than those found in forest floor and 

soils under spruce/fir. Since low C/N is a good indicator of relative organic matter decomposition rate, nutrient-rich aspen leaves decompose quickly and easily 

compared to spruce/fir needles.

Soil pH generally decreases with increasing elevation, latitude, and precipitation (fig. 25). The more acidic soils are found in the wetter high-elevation forest 

types. This is also reflected in higher levels of exchangeable Al in wetter highelevation forest soils (fig. 25, right side), although this effect is not seen in the 

Arizona data because of high variability and a very limited sample population. In 

the Interior West as a whole, much higher levels of Al are found in spruce/fir than 

aspen soils. Aspen are intolerant of high levels of exchangeable Al. In the Interior 

West as a whole and to some extent in Arizona, aspen soils store more K than 

other forest-type groups (fig. 25). High levels of exchangeable Ca are found in the 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 45

calcareous, high-pH soils under western hardwoods including oaks and pinyon/

juniper group woodlands (fig. 25).

Western aspen are in decline in Arizona (see Aspen Status and Trends section 

below) and other Interior West States. Causes include fire suppression, overbrowsing by native ungulates and domestic livestock, and forest succession in which 

aspen are replaced by invading conifers (Bartos and Campbell 1998). Loss of aspen on the landscape can lead to decline or loss of ecosystem benefits provided by 

aspen. Since aspen soils store large amounts of N and K, and maintain moderate 

soil pH levels, conifer replacement of aspen can lead to nutrient loses and soil 

Figure 23—Forest floor and 0–20 cm mineral soil % C, % N, and C/N arranged by forest-type groups in Arizona (left 

side) and in five Interior West States (Arizona, Colorado, Idaho, Montana, and Utah) (right side). The forest-type 

groups are arranged left to right in order of increasing latitude, elevation, and precipitation with some overlap 

among forest types. The western hardwoods group in Arizona includes mesquite woodland, deciduous oak 

woodland, and evergreen oak woodland. For the Interior West as a whole, the western oak group (deciduous 

and evergreen oak woodlands) was separated from the other western hardwoods (mesquite and Cercocarpus 

woodlands). The pinyon/juniper group in Arizona includes Juniper woodland and pinyon/juniper woodland. For the 

Interior West, this group also includes Rocky Mountain and western juniper types. The spruce/fir group in Arizona 

includes Engelmann spruce and blue spruce. For the Interior West, this group also includes white fir, grand fir, subalpine fir, and mixed Engelmann spruce/subalpine fir. The western softwoods group in the Interior West includes 

western white pine, foxtail/bristlecone pines, limber pine, and whitebark pine.
46 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

acidification. If these soil changes are permanent, aspen may be unable to recolonize areas where they formerly thrived.

Down Woody Material

Down woody material (DWM) is an important component of forests that 

impacts fire behavior, wildlife habitat, and carbon storage and sources. Some examples of DWM are fallen trees, branches, and leaf litter commonly found within 

forests in various stages of decay. The main components of DWM include fine 

woody debris (FWD), coarse woody debris (CWD), litter, and duff. FWD comprises the small diameter (up to 2.9-inch) fire-related fuel classes (1-hr, 10-hr, 100-hr 

moisture equilibration), and CWD comprises the large diameter (3-inch+) 1,000-hr 

fuels. In 2006, due to the increasing need for more intensive DWM information, 

IWFIA initiated a DWM inventory in all its annual States. This DWM analysis 

used data from regional Phase 2 (P2) protocols (USDA FS 2006, 2011) for plots 

measured between 2006 and 2012.

Figure 24—Soil organic carbon (top) and total nitrogen (bottom) stocks (Mg/ha) in the forest floor and 0–10 and 

10–20 cm soil layers arranged by forest-type groups in Arizona (left side) and in five Interior West States (Arizona, 

Colorado, Idaho, Montana, and Utah) (right side).
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 47

Figure 26 shows DWM biomass value classes in the State of Arizona, in tons 

per acre, based on the sum of all six DWM components (CWD, 3 classes of FWD, 

litter, and duff).

The highest biomass values tend to be found in the higher elevation areas 

of the State, such as the Kaibab Plateau, the San Francisco Peaks, and White 

Mountain. This is because higher levels of DWM are generally associated with 

higher-productivity forest types, and the higher-productivity forest types are generally found where there is higher precipitation (Garbarino et al. 2015). Given that 

elevation and precipitation are highly correlated in the mountain areas of the West, 

the pattern emerges clearly.

This can also be shown by plotting DWM biomass averages by forest type 

and elevation (fig. 27). Note that the woodland types, such as mesquite, juniper, and 

pinyon/juniper, which occur on dry sites at low elevations, group together with relatively low total DWM biomass values. Aspen and the different spruce-dominated 

Figure 25—Soil pH and exchangeable potassium, calcium, and aluminum in the 0–20 cm soil layers arranged by foresttype groups in Arizona (left side) and in five Interior West states (Arizona, Colorado, Idaho, Montana, and Utah) (right 

side).
48 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

types occur at the opposite end of the scale, both in biomass and elevation. There 

are a few exceptions to the pattern, such as the cottonwood, cottonwood/willow, 

and limber pine forest types. Although cottonwood is capable of high growth rates, 

stands with a large cottonwood component are generally located in riparian areas. 

While these forest types may produce DWM in proportion to their productivity 

rates, material on the ground is likely to decay quickly because of the availability 

of moisture, or even be washed from the stand during periods of high water. The 

limber pine type is an exception for different reasons. Although limber pine as a 

species is found in many forest types, sites where limber pine forms its own forest 

type tend to be rocky or otherwise harsh. Therefore, although moisture is likely 

to be high because of the elevations at which these stands are found, other limiting factors, like low temperatures and rocky soil, reduce overall productivity and 

therefore the average amount of DWM found in these stands.

Figure 26—Total biomass (tons per 

acre) of downed woody material (coarse, fine, litter, and duff 

components), Arizona, 2006–2014. 

(Note: Plot locations are approximate; some plots on private land 

were randomly swapped).
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 49

Remeasurement of DWM started during the 2016 field season, using a nationally consistent approach for Phase 2 plots. As additional data are collected on 

DWM components, it should be possible to produce trend information for down 

woody volume, biomass, and carbon, just as for the standing live and dead components of the forest.

Arizona’s Primary Wood Products Industry

The Bureau of Business and Economic Research at the University of Montana 

conducted censuses of Arizona’s primary wood products industry for calendar 

years 1998, 2002, 2007, and 2012 documenting timber use and the condition of the 

industry (Hayes et al. 2012; Keegan et al. 2001; Morgan et al. 2006; Sorenson et 

al. 2016). Primary wood products manufacturers are firms that process timber into 

products such as lumber, house logs, and biomass energy. The secondary industry 

further processes outputs from the primary industry into other value-added wood 

products, such as cabinets, doors, or furniture.

Arizona’s primary wood products industry consisted of 25 active facilities in 

2012 (table 5; fig. 28). The sawmill sector, manufacturing lumber and other sawn 

products, was the largest sector, operating 14 mills during 2012—six more than 

operated in 2007. Only two log home and viga producers operated during 2012, 

while nine other products manufacturers were active.

Figure 27—Average down woody material biomass by forest type and elevation.
50 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Table 5—Active Arizona primary wood production facilities, selected years (sources: Hayes et al. 2012; Keegan et al. 

2001; McLain 1988; Morgan et al. 2006).

County Lumber

House logs and 

vigas

Other 

productsa

Pulp and 

paper Total

2012 total 14 2 9 0 25

2007 total 8 5 4 0 17

2002 total 11 5 7 0 23

1998 total 6 4 2 1 13

1990 total 14 3 1 1 19

1984 total 20 0 2 1 23

a Other products include industrial fuelwood, fuel pellets, biomass energy, posts and poles.

Figure 28—Locations and type 

of wood processing facilities in 

Arizona, 2012.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 51

Timber processors in Arizona received 69.6 million board feet (MMBF) 

Scribner of timber in 2012. The vast majority (96 percent) of timber came from 

national forests. Tribal lands provided 3 percent, and less than 1 percent came from 

private and State lands combined (table 6). During previous study years, private 

and Tribal timberland accounted for 60 to 75 percent of the timber received by 

Arizona mills.

Despite having six more sawmills operating during 2012 than in 2007, total 

lumber production in Arizona was 49.3 MMBF in 2012 versus 54.9 MMBF in 

2007. Average lumber production declined to just 3.5 MMBF per mill in 2012, the 

lowest level identified since the 1960s (Hayes et al. 2012).

Sales of finished products (excluding mill residue) from Arizona’s primary 

wood products industry increased substantially during 2012 and totaled $83.8 

million (table 7). Sawmill sales accounted for 39 percent ($32.4 million) of total sales, while biomass energy, log homes, and other products accounted for 61 

percent ($51.4 million). Arizona was the leading overall destination for primary 

wood products, with in-State sales accounting for almost 71 percent of total sales. 

The leading destination for lumber was Mexico, and within State was the second 

leading market area for lumber from Arizona mills. Arizona was also the leading 

destination for biomass energy, log homes, and other primary products, with nearly 

90 percent of sales within State

Arizona timber processors produced almost 68 thousand bone-dry units 

(BDU) of mill residue in 2012, with 99.9 percent utilized, versus 98.5 percent of 

Table 6—Ownership of timber received by Arizona’s primary wood products industry, selected years (sources: Hayes et al. 2012; Keegan et al. 

2001; Morgan et al. 2006).

1998 2002 2007 2012

Ownership class

MBF 

Scribner

Percentage 

of total

MBF 

Scribner

Percentage 

of total

MBF 

Scribner

Percentage 

of total

MBF 

Scribner

Percentage 

of total

Private and tribal 

timberland 48,102 71.1 58,108 76.3 31,706 60.8 2,623 3.8

Tribal 45,964 68.0 56,150 73.8 4,400 8.4 2,220 3.2

Private 2,138 3.2 1,958 2.6 27,306 52.4 403 0.6

National forests 19,510 28.9 18,006 23.7 20,427 39.2 66,858 96.0

State lands — — — — — — 130 0.2

All owners 67,612 100 76,114 100 52,133 100 69,611 100

Table 7—Sales value of finished products from Arizona’s primary wood products industry, selected years (sources: Hayes and 

others 2012; Keegan and others 2001; Morgan and others 2006; WWPA various years).

Sector 1984 1990 1998 2002 2007 2012

---------------------------------------Thousands of 2012 dollars---------------------------------------------

Sawmills 193,169 158,069 33,452 30,217 22,335 32,403

Log home and other sectorsa 271 622 2,613 7,853 17,551 51,397

Totalb 193,440 158,692 36,064 38,070 39,886 83,800

a Other sectors include producers of industrial fuelwood, fuel pellets, biomass energy, posts, poles, and vigas.

b All sales are reported F.O.B. the manufacturer’s plant. Sales of mill residue, mulch, and paper are not included for comparison to previous 

years.
52 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

residue utilized in 2007. Sawmills produced the majority of mill residue, generating 1.07 BDU per MBF of lumber in 2012, compared to 1.22 BDU per MBF of 

lumber in 2007.

The classification of forest industries used here follows the North American 

Industry Classification System (NAICS) available online via the U.S. Department 

of Commerce Census Bureau (USDC CB 2014). The forest industry can be found 

in four categories: NAICS 113—forestry and logging; NAICS 1153—forestry 

support activities; NAICS 321—wood product manufacturing; and NAICS 322—

paper manufacturing. These categories include employees that work in both the 

primary and secondary wood products sectors, as defined above.

Based on the four NAICS sectors of the forest industry (113, 1153, 321, and 

322), about 6,700 workers were directly employed in the primary and secondary 

forest products industry in Arizona during 2012 (USDC BEA 2014). This marked 

a 42 percent decline from 2007 employment in the industry, with most of the loss 

coming from wood products manufacturing, which declined from over 8,400 jobs 

in 2007 to about 3,900 in 2012. Just fewer than 800 workers were employed in 

harvesting and processing timber or in private sector land management (i.e., the 

primary sector) in 2012, roughly the same level of primary sector employment as 

in 2007.

Volume removed from forest inventory during the harvesting of timber is 

referred to as removals. Removals are an important indicator of the sustainability 

of timber harvest levels. Removals exceeding net growth over an extended period could indicate over-harvesting and decreasing forest inventory. Conversely, 

growth or mortality greatly exceeding removals could signal a need for increased 

vegetation management to decrease risks of tree mortality, insect outbreaks, or 

wildfire.

Removals can come from two sources: growing-stock (portions of live, commercial tree species meeting specified quality or vigor standards) or dead trees and 

other non-growing stock sources (e.g., tree limbs and tops). The two general types 

of removals are timber products and logging residue (i.e., volume cut or killed 

but not utilized). Removals, as reported here, are based on 2002, 2007, and 2012 

surveys of Arizona’s primary forest products industry (Hayes et al. 2012; Morgan 

et al. 2006; Sorenson et al. 2016) and the U.S. Energy Information Administration 

data for residential fuelwood consumption (EIA 2016). More detailed timber 

products and logging residue data for Arizona and other States are available from 

FIA’s Timber Products Output (TPO) website: https://www.fs.usda.gov/srsfia/php/

tpo_2009/tpo_rpa_int1.php.

Total removals in Arizona during 2012 were almost 31.1 million cubic feet 

(MMCF). Total removals included 28.4 MMCF of timber products, including industrial and residential fuelwood, and 2.6 MMCF of logging residue left in the 

forest as slash (table 8).

Growing-stock sources accounted for approximately 7.8 MMCF of total removals. Very little (0.1 MMCF) growing-stock volume was unutilized and left in the 

forest as logging residue, with almost 99 percent of growing-stock removals used 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 53

to produce wood products. Ponderosa pine accounted for 86 percent (6.7 MMCF) 

of growing-stock removals and Douglas-fir accounted for 7 percent (0.5 MMCF). 

Almost all of the growing-stock removals in Arizona came from public lands, with 

about 96 percent (7.5 MMCF) originating on national forests, while only 4 percent 

of the volume came from non-industrial private and Tribal timberlands.

In 2012, removals for all timber products (including industrial fuelwood and 

residential firewood) totaled 28.4 MMCF accounting for 92 percent of the total 

removals for the year (table 8). Of the 18.9 MMCF of removals for industrial 

products, approximately 50 percent came from industrial fuelwood, making it the 

leading timber product in 2012. Sawlogs accounted for 7.9 MMCF (almost 41 

percent) of industrial product removals. Logs for miscellaneous wood products accounted for approximately 7 percent, with logs for posts and poles accounting for 

the remaining 2 percent of industrial product removals. Approximately 99 percent 

(18.7 MMCF) of removals for industrial wood products consisted of softwood species. The largest volume of both softwood and hardwood removals was used for 

industrial fuelwood.

Across Arizona’s nearly 3 million acres of timberland, approximately 71 million board feet (MMBF) Scribner, about 18.9 MMCF, of industrial timber products were harvested in 2012. The 2012 industrial harvest was 17.2 MMBF (33 

percent) higher than 2007, but only 56 percent of the State’s 2002 harvest levels. 

Dead trees, resulting from wildfires and insect outbreaks, have a dynamic role in 

Arizona’s annual timber harvest level. While dead trees accounted for only 8 percent (4 MMBF) of the harvest in 2007, harvest of dead timber accounted for 70 

percent (90 MMBF) of the 2002 harvest and about 56 percent (40 MMBF) of the 

2012 harvest (Hayes et al. 2012; Morgan et al. 2006; Sorenson et al. 2016).

Total removals for timber products and logging residue decreased about 42 

percent over the past 10 years (table 8), from 53.5 MMCF in 2002 to about 31.1 

MMCF in 2012. Total removals decreased approximately 12 percent between 2002 

and 2007, and about 34 percent from 2007 to 2012. The EIA reported a dramatic 

Table 8—Arizona total removals for products and logging residues, selected years.

Product 2002 2007 2012 Annual average

--------- million cubic feet ---------

Sawlogs 21.648 9.112 7.876 12.9

Composite products 0.421 0.421 — 0.3

Industrial fuelwood 4.143 1.524 9.410 5.0

Post & poles 0.152 — 0.306 0.2

Miscellaneous 0.307 0.242 1.302 0.6

Industrial products 26.671 11.299 18.894 19.0

Residential fuelwooda 23.079 31.882 9.520 21.5

Total products 49.750 43.181 28.414 40.4

Logging residue 3.707 3.800 2.638 3.4

Total removals 53.457 46.980 31.052 43.8

a Residential fuelwood is reported by the U.S. Energy Information Administration.
54 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

decrease of 70 percent in residential fuelwood between 2007 and 2012. Removals 

for industrial products also decreased over the decade, with 2012 approximately 

7.7 MMCF (29 percent) lower than in 2002. However, removals for industrial 

products actually increased between 2007 and 2012 by about 67 percent (7.6 

MMCF) after a dramatic decrease from 2002 to 2007.

Removals for sawlogs decreased nearly 64 percent from 2002 to 2012, with 

the steepest (58 percent) decline occurring from 2002 to 2007. Conversely, industrial fuelwood more than doubled from 2002 to 2012, with an increase of about 5.3 

MMCF. Removals for post and poles doubled and other miscellaneous products 

increased more than four-fold from 2002 to 2012. Logging residue removals increased by about 3 percent from 2002 to 2007, but fell approximately 31 percent 

from 2007 to 2012. Less volume is being left unused after product harvest activities due to technological advancements in harvesting and milling operations and to 

notable growth in removals for industrial firewood.

Long-term sustainability of Arizona’s timberlands depends upon several interrelated factors: active management of lands available for timber production, the 

presence of a forest products industry capable of processing harvested timber, and 

harvest levels that meet societal demands while fostering continual site productivity. To ensure sustainable forests and communities, careful consideration should 

be given not only to growth, removals, and mortality across Arizona’s available 

timberlands, but also to the forest industry and employees who conduct management activities and utilize timber in Arizona.

Current Issues in Arizona’s Forests__________________________

Drought-Related Effects on Pinyon/Juniper Woodlands

Collectively, pinyon/juniper and juniper woodlands make up the most common forest type in the American Southwest, covering over 36 million acres in 

10 States and extending into Mexico. In Arizona, these types account for 11.1 

million acres, or nearly 60 percent of the State’s forest land. Within the pinyon/

juniper forest-type group, FIA distinguishes three main forest types: pinyon/juniper woodlands, juniper woodlands, and Rocky Mountain juniper woodlands (see 

Appendix C). The pinyon/juniper forest type is defined by the presence of one or 

more pinyon species—usually common or singleleaf pinyon—and one or more 

juniper species; pure stands of pinyon are not considered a separate type by the 

FIA program. Juniper woodland types are dominated by various juniper species, 

but other species—exclusive of pinyons—may be present as a minor component. 

To most laypersons and many managers, the term pinyon/juniper woodland (or 

P-J, for short) includes all lands dominated by pinyons, junipers, or both. For convenience, in this section the term “pinyon/juniper woodland” refers to all lands 

covered by this common use of the term and thus includes the juniper and Rocky 

Mountain juniper forest types as well as the pinyon/juniper forest type.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 55

The IWFIA program operates in Arizona, Colorado, Idaho, Montana, Nevada, 

New Mexico, Utah, and Wyoming; these States include most of the range of pinyon/juniper woodlands in the United States. A drought began across much of the 

Southwest, including Arizona, about the time that an annual forest inventory was 

started in the Interior West States. As a result, forest managers and researchers 

began to notice an increase in the incidence of insects and disease in several forest 

types, including pinyon/juniper. As the drought progressed, tree mortality appeared 

to be increasing and there was increasing interest in using FIA data to quantify the 

effects of drought, insects, and disease on pinyon/juniper woodlands (Shaw et al. 

2005). The drought-related mortality episode has provided an opportunity to test 

the utility of the FIA annual inventory system for quantifying rapid change in pinyon/juniper woodlands over a large geographic area (Shaw 2006).

Since the annual inventory system was implemented in Arizona in 2001, the 

progression of drought-related mortality was captured as it occurred in the early 

2000s. During the first 10 years of annual inventory, all plots visits were treated as 

“first-time” visits, even though there may have been a previous visit during one or 

both of the preceding periodic inventories. During first-time visits, trees judged to 

have died during the 5 years prior to plot visit were considered as mortality trees, 

and trees judged to have died earlier considered “old dead.” Assignment of mortality year for visits up to 5 years post-mortality is known to be relatively reliable 

because of rapid changes in tree condition (Kearns et al. 2005), but correct assignment becomes increasingly difficult with time. Most mortality trees removed 

by firewood cutting, blown down, or no longer “on the stump” for other reasons 

would still be missed by the current inventory, but these situations cover only a 

small percentage of tally trees. In the case of remeasurement of annual plots, each 

of the trees from the previous annual visit is accounted for. In this case the assignment of mortality year is less critical, since the status of each tree was known from 

the previous visit. Because annual inventory was implemented in Arizona before 

the early 2000s increase in mortality, and the inventory data currently include 40 

percent of plots with remeasurements, we consider our estimate of drought-related 

mortality in Arizona to be a good approximation.

Pinyon mortality from all causes in the Interior West began to increase in 

2001 and appears to have peaked between 2005 and 2007 (fig. 29a). Since then, 

mortality rates have decreased and some states appear to be returning to background mortality rates. However, drought persists in the Southwest, including in 

Arizona, and fire continues to impact pinyon/juniper woodlands in many areas. 

Mortality of pinyon in Arizona in the 2000s tended to be somewhat higher than 

mortality rate of pinyons in all Interior West states combined—peaking at about 

15 percent of the total basal area. Among the Four Corners States, Arizona ranks 

highest in the level of pinyon mortality.

Juniper species have shown to be much more resistant to drought-related 

mortality than pinyon species. From 2000 to 2003, the mortality rate of juniper 

species in the Interior West states, from all causes, rose from a very small fraction to about 2 percent and has remained relatively steady since then (fig. 29b). 
56 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

0.0%	

2.0%	

4.0%	

6.0%	

8.0%	

10.0%	

12.0%	

14.0%	

16.0%	

18.0%	

1998	 2000	 2002	 2004	 2006	 2008	 2010	 2012	 2014	

Mortality	percentage	of	total	basal	area	

Year	

Arizona	

Colorado	

New	Mexico	

Utah	

All	IW	States	

A	

0.0%	

2.0%	

4.0%	

6.0%	

8.0%	

10.0%	

12.0%	

14.0%	

16.0%	

18.0%	

1998	 2000	 2002	 2004	 2006	 2008	 2010	 2012	 2014	

Mortality	percentage	of	total	basal	area	

Year	

Arizona	

Colorado	

New	Mexico	

Utah	

All	IW	States	

B	

Figure 29—Annual 

mortality, by 

measurement year, 

for pinyon (A) and 

juniper (B) species 

in the Interior West 

States.

The apparent spike in juniper mortality seen in 2007 is largely a result of a large 

number of “catch-up” plots in Arizona that were located in burned areas. As with 

the pinyon species, the juniper mortality rate in Arizona appears somewhat higher 

than the mortality rate for the Interior West as a whole.

The dramatic visual effect of drought-related mortality of pinyon species—

dying trees with reddened foliage covering entire landscapes—brought public and 

media attention to the event. Because there were typically localized hot spots of 

mortality that were surrounded by large areas of relatively low mortality, it was 

difficult to obtain unbiased, quantitative estimates of the true extent of mortality. In some cases, mortality estimates were extrapolated from local sites to entire 

States. For example, one account reported that 90 percent of the pinyon trees in the 

State of Arizona had been killed (Society of American Foresters 2004). However, 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 57

a preliminary analysis of the available data in Arizona, Colorado, and Utah (Shaw 

et al. 2005) showed that there was clearly an upward trend in mortality but population-level mortality was not nearly as high as initially feared.

Unlike in other states, Arizona pinyon/juniper woodlands have not maintained 

positive net growth during the past decade (see table B22); recent mortality has 

exceeded recent growth. However, even though the numeric amount of negative 

net growth (–8.2 million cubic feet per year) is comparable to the numeric amounts 

of other species (e.g., Douglas-fir –9.6 million cubic feet, aspen/birch –9.3 million 

cubic feet), the net change in volume in pinyon/juniper woodlands is very small 

compared to the live inventory. As a proportion, pinyon/juniper woodlands changed 

at an average annual rate of only –0.13 percent, as compared, for example, to the 

Douglas-fir forest-type group (–0.83 percent) and the aspen/birch group (–3.25 

percent). Practically speaking, the total live volume of trees in the pinyon/juniper 

group has remained nearly constant during the past decade in Arizona.

One persistent question about the current episode of drought-related mortality is: “How does the current episode compare with previous drought-related dieoffs?” The climatic record shows that similar droughts occurred in the Southwest 

during the early 1900s and mid-1950s (National Climatic Data Center 1994). 

Breshears et al. (2005) characterized the recent mortality event as a response to 

“global-change-type drought” and suggested that recent conditions have been hotter than in the 1950s. Some of the conclusions about the relative magnitude of 

recent mortality and the mortality of the 1950s are based on the lack of evidence, 

in the form of remaining dead woody material, from the 1950s. However, despite 

the perceived long-term persistence of woody material in the arid Southwest, pinyons may decay or physically break down relatively quickly. Although Kearns et 

al. (2005) found that pinyon snags could persist as long as 25 years, they found 

that “extremely fragmented” trees were dead for an average of 16.2 years. Because 

the impacts of the 1950s drought were not well-studied and there is a great deal of 

uncertainty surrounding the possible surviving evidence of pinyon mortality, the 

relative magnitude of the two mortality episodes remains uncertain.

The recent drought has undoubtedly impacted the pinyon/juniper resource in 

Arizona and around the Four Corners States, but the magnitude of impact varies 

widely between the pinyon and juniper components. Differential mortality among 

species on the same site has been shown by Mueller et al. (2005), who found 

mortality of common pinyon to be 6.5 times higher than oneseed juniper mortality 

during two drought events in northern Arizona. Future mortality rates will likely 

depend on temperature and precipitation trends. The mortality event of the early 

2000s corresponded with a shift of the Palmer Drought Severity Index (PDSI) 

from positive (wetter) to negative (drier) values, while the decrease in mortality 

rate corresponded with a temporary shift back to positive values. However, in recent years PDSI has once again become negative. Whether there is a resurgence in 

mortality or not depends on a number of factors, including what effects the earlier 

drought-induced thinning of dense stands will have on competition and water relations. The dynamics of this forest type have important implications for carbon 
58 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

storage, because dead trees have released growing space to the survivors and new 

regeneration. Although there has been a short-term loss in living biomass, there 

may be a long-term increase in carbon storage while dead wood persists and new 

growth accumulates. It will be possible to determine the actual trends as FIA continues to monitor these woodlands into the future.

Expansion of Pinyon/Juniper Woodlands

The pinyon/juniper forest-type group in Arizona consists of three forest 

types, pinyon/juniper woodland, juniper woodland, and Rocky Mountain juniper, 

and is the most common group in Arizona, covering 11.1 million acres, or nearly 

60 percent of the forest land (Appendix B, tables B3–B7). In addition, 52 percent 

of the nonstocked area was identified by field crews as formerly and/or potentially 

stocked by pinyon/juniper group forest types. In total, over 12.5 million acres are 

either now, were recently, or will soon be pinyon/juniper forest-type groups, or 62 

percent of the forest. The dominant species are Utah juniper, oneseed juniper, alligator juniper, redberry juniper, Rocky Mountain juniper, California juniper, common or two-needle pinyon, singleleaf pinyon, Arizona pinyon pine, border pinyon, 

and Mexican pinyon pine.

There has been concern that since Euro-American settlement of the West, 

pinyons and junipers have been expanding their ranges dramatically, encroaching 

on and degrading grasslands and shrublands. This expansion has been well documented in many parts of the pinyon/juniper range (Burkhardt and Tisdale 1976; 

Tausch and Hood 2007). Generally, expansion has been attributed to direct or indirect alterations of pre-settlement fire regimes. These fire regimes are theorized to 

be of three general types (Baker and Shinneman 2004; Romme et al. 2009):

1. Low intensity, frequent fires that tend to “thin from below.” These would occur in pinyon/juniper stands where the understory is dominated by grasses and 

forbs, and the tree density should be comparatively low. This type is most common in the extreme Southwest United States. and northern Mexico. It would be 

expected in parts of Arizona.

2. Less frequent, high intensity stand-replacing fires. This regime occurs in pinyon/

juniper stands where the understory is dominated by shrubs, notably sagebrush. 

This type is common in the Great Basin, but it only occurs in northern Arizona. 

Elsewhere in Arizona, the shrub component is commonly shrub oaks, Arctostaphylos spp. (manzanita) or Garrya spp. (silktassel).

3. Very rare, mostly localized fires that occur under only the most extreme conditions and that may only burn small areas. These are pinyon/juniper stands with 

rocky substrates or cryptobiotic crusts that support little, if any, understory. The 

topography is often rugged with features such as cliffs and bare bedrock, which 

prevent or inhibit the spread of fires. This type is documented on the Colorado 

Plateau (Romme et al. 2003), so would be expected in northern Arizona.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 59

Inventory data were used to evaluate the age, structure, and potential status 

of pinyon/juniper stands. The age chosen to represent pre-settlement stands was 

150 years. The first estimate was stand age, but since this is based on the age of the 

trees in the dominant size class rather than the oldest trees in the stand, the maximum ages were also evaluated, along with the proportion of trees over 150 years 

old. Since tree and stand ages are determined from a few live trees on a plot, the 

presence of very large dead trees (14.5 inches diameter) and dead basal branches 

(12.4 inches diameter) were evaluated as evidence that the stand was in existence 

at least 150 years ago. More recently disturbed stands, with few or no live trees, 

were classified as disturbed. Understory cover and layering, along with tree cover, 

were evaluated to characterize fire regimes (Scott and Burgan 2005) and estimate 

fine fuel loading (Caratti 2006).

About 1.1 percent of the pinyon- and juniper-dominated land in Arizona 

has been recently disturbed. Of the remaining, about 54 percent, or 6.6 million 

acres, was classified as older than 150 years. The remaining 46 percent, 5.8 million 

acres, was characterized as having established within the last 150 years. Pinyon 

and juniper occur in the northern and eastern parts of Arizona, in foothills, and in 

high plateaus, mostly at elevations between 4,000 and 8,000 feet. Younger stands 

were more common at lower elevations, and older stands were more common at 

higher elevations (fig. 30). Also, younger stands tended to be more prevalent in the 

0.0	

0.5	

1.0	

1.5	

2.0	

2.5	

3.0	

3.5	

<	3000	

3000	-	3999	 4000	-	4999	 5000	-	5999	 6000	-	6999	 7000	-	7999	

8000	+	

Million	Acres	

Elevation	class	(feet)	

Old	(>150	years)	

Young	(<150	years)	

Figure 30—Pinyon/juniper woodlands by age and elevation class.
60 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

southwestern parts of the range, while the older stands occurred more frequently 

in the northern part of the State (fig. 31).

In terms of fire regime and fuel loading, a larger proportion of the older stands 

were found to have very low fuel loadings, indicating a low probability of severe 

fire. Fifty-three percent of older stands had very low fuel loadings, while 44 percent of younger stands did. A higher percentage of younger stands had understories 

dominated by shrubs, grass, or shrub-grass mixtures (31 percent) than did older 

stands (17 percent). Fourteen percent of both younger and older stands were not 

assigned a fire classification, primarily due to the uncertain effects of having over 

10 percent of their live tree basal area in species other than pinyons and junipers.

Figure 31—Pinyon area and 

stand age classification by 

ecoregion.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 61

The spatial distribution of Arizona’s pinyon- and juniper-dominated stands 

can be addressed using ecoregions (Bailey 1995), using a combination of ecoregion provinces and sections (fig. 31). The highest proportions of stands older than 

150 years occur in the northern part of the State, in the Navajo Canyonlands and 

Grand Canyon sections of the Colorado Plateau province. This distribution, combined with the finding that a greater proportion of older stands than younger stands 

have very sparse understories, supports the expectation that the third fire regime, 

characterized by very rare, localized fires, is operating in this area. This situation 

results in most pinyon/juniper stands having existed in similar densities, structures, 

and age classes as they are currently found for hundreds, if not thousands, of years 

(Romme et al. 2003).

Pinyon- and juniper-dominated stands younger than 150 years are more prevalent toward the Southwest, in the Tonto Transition section of the Colorado Plateau 

province, and the Chihuahuan Semi-Desert and American Semi-Desert and Desert 

provinces. The common occurrence of younger stands, along with younger stands 

having more shrub-dominated understories and lower elevations than older stands, 

tends to support the expectation that this part of the State is dominated by the 

second fire regime. In this situation, pinyon and juniper populations are maintained by relatively small areas with fire-resistant properties, similar to those in the 

third fire regime (Weisberg et al. 2008). In many cases, these areas would tend to 

be at higher elevations. When conditions favor tree growth, pinyons and junipers 

expand from these refugia into lower elevation, mostly shrub-dominated areas, 

where eventually they experience high intensity stand-replacing fires, which halt 

tree expansion. If the fire frequency is artificially lengthened, trees can increase 

uninterrupted, displacing shrublands.

Over 10 percent of pinyon and juniper stands were characterized by understories dominated by grasses, or grasses and shrubs. These are stands where the 

first fire regime would be expected to operate. However, the extent to which these 

stands actually experience low intensity, “thin from below” fires is not well understood (Romme et al. 2009). Stands younger than 150 years outnumbered older 

stands two to one in these types.

Some younger pinyon/juniper stands probably represent recovery from severe human-caused disturbances, such as early chaining or harvest for charcoal to 

support the mining and railroad industries (Romme et al. 2009).

Aspen Status and Trends

Aspen is the widest-ranging species in North America. It is present in all 

States in the Interior West and occupies a wide elevational range—from 2,000 ft 

in northern Idaho to 11,700 ft in Colorado. It is also found on a wide range of sites 

and occurs in 26 of the forest types that occur in the Interior West. The species is 

intolerant of shade and relatively short-lived, which makes it prone to replacement by conifers through successional change. In the Interior West, it also reproduces infrequently by seeding, relying mostly on root sprouting for reproduction. 

However, aspen responds well to fire and cutting, and it is able to dominate heavily 
62 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

disturbed sites for many years following severe disturbance. In addition, there is 

some evidence that aspen is able to persist in conifer-dominated forests by exploiting gaps in the conifer canopy that are caused by insects, disease, windthrow, and 

other smaller-scale disturbances.

In recent years there has been concern about the future of aspen on the landscape, primarily due to the characteristics of aspen and how they relate to changes 

in disturbance regimes. The earliest concerns were related to successional change 

in the Interior West, where fire suppression has decreased disturbance rates and, as 

a result, aspen regeneration rates. In addition, it has been shown that large populations of herbivores can inhibit aspen regeneration where it occurs spontaneously 

or after disturbance (e.g., Hessl and Graumlich 2002). The lack of disturbance allows conifers to gain dominance where they are present, and in pure aspen stands, 

consumption of regeneration by ungulates could lead to loss of senescing overstory trees without replacement. More recent concerns are related to a period of 

drought that has an impact on aspen and other forest types (e.g., Shaw et al. 2005; 

Thompson 2009). Drought appears to have contributed to mortality in many lowelevation stands (Worrall et al. 2008), and in some of these regeneration is either 

lacking or suppressed by herbivores.

Johnson (1994) suggested that the acreage of aspen-dominated stands had 

declined as much as 46 percent in Arizona and New Mexico between the 1960s 

and late 1980s, with most of these acres becoming dominated by mixed-conifer 

forest types. Bartos (2001) suggested that similar changes—aspen acres dropping 

by 96 percent—had occurred in Arizona as compared with “historical” extent, although the time scale over which this change is believed to have occurred was not 

specified. These assessments of “lost” aspen acres were based on the assumption 

that forested acres with a minority aspen component were, at one time in the recent 

past, dominated by aspen in pure or nearly pure stands. This assumption may not 

be reasonable because there are many situations where aspen may persist normally 

as a minor stand component.

The current inventory of Arizona shows that there are over 153 thousand 

acres of the aspen forest type in Arizona, as compared to just over 147 thousand 

acres found during the 1999 periodic inventory. When considering all acres where 

aspen is present, the current inventory shows that aspen is present on over 602 

thousand acres, as compared to over 603 thousand acres during the 1999 inventory. 

This result suggests no net change in the extent of aspen over the past 15 years. 

However, there appears to have been a decline in aspen live volume since the late 

1990s. Using the 1999 values of 603 thousand acres with aspen present and 383 

million cubic feet of live aspen volume gives an average volume of 634.4 cubic 

feet of live aspen per acre. The equivalent computation in the current inventory 

gives 539.8 live cubic feet per acre, or just over 85 percent of the 1999 average. On 

an absolute basis, the result is similar; the current volume of 325 million cubic feet 

is about 85 percent of the live aspen volume found in 1999 (fig. 32).

This is consistent with the recent effects of drought and fire on the forests of 

Arizona. Younger stands of aspen tend to be less susceptible to drought, but the 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 63

effect of drought on aspen stand regeneration has not yet been well-assessed. The 

response to fire, on the other hand, is well-known; fire is essential to the regeneration of aspen in most of its western range. Recent fires may be responsible for a 

trend that appears to be emerging from inventory results. In the 1999 periodic 

inventory, stands where aspen was dominant (aspen forest type) made up over 24 

percent of the total area with aspen present. This proportion appeared to decline 

slightly (19 to 22 percent) during the mid-2000s, but the proportion is again over 

25 percent in the current inventory. Whether this fluctuation is due to sampling 

error or represents a real trend will require more years of monitoring. However, 

even if the apparent decline in volume persists it would take another 50 years for 

total aspen volume to reach half its present level. It is unlikely that aspen will not 

regenerate during that period. In fact, since aspens that have regenerated from recent disturbances will not have an impact on total volume until they reach larger 

size classes, there may be a surge of aspen volume in the coming years regardless 

of future regeneration. In any case, Johnson’s (1994) projection that “aspen will 

cease to exist as a distinct cover type” in Arizona and New Mexico before the year 

2020 is highly unlikely.

Damage to Live Trees

As field crews measure live trees on subplots (trees 5 inches diameter and 

greater), they carefully examine those trees for the presence of damaging agents. 

They record up to three damaging agents if the damages meet one or more of the 

following three criteria:

1. The damage will prevent the tree from living to maturity if immature, or living 

10 more years if mature.

2. It will prevent the tree from producing marketable products.

0.0%	

1.0%	

2.0%	

3.0%	

4.0%	

5.0%	

6.0%	

7.0%	

8.0%	

9.0%	

10.0%	

	-		

	40		

	80		

	120		

	160		

	200		

	240		

	280		

	320		

	360		

	400		

1998	 2000	 2002	 2004	 2006	 2008	 2010	 2012	 2014	 2016	

Annual	mortality	(basal	area	basis)	

Net	volume	(million	cubic	feet)	

Measurement	/	Inventory	Year	

Live	Aspen	Volume	

Annualized	Aspen	Mortality	

1999	aspen	volume	estimate	

Figure 32—Aspen trends in Arizona by 

total volume estimate (blue) and annualized mortality rate (red).
64 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

3. It will seriously reduce (or has reduced) the quality of the tree’s marketable 

products.

Since the last two criteria are less applicable to woodlands species, which 

produce few marketable products, results from damage data can be quite different 

between woodland type species and timber type species.

Damaging agents are grouped into several general classes: insects, diseases, 

fire, animals, humans, and miscellaneous. The miscellaneous group includes suppression, some symptomatic groups whose ultimate causation can be difficult to 

determine in the field (unhealthy foliage and heartwood scars), unknown causes, 

and timber form defects that affect commercial products (forking, excessive lean, 

broken tops, and excessive crook, sweep, or taper). Crews also estimate cull volume percentages (missing top, dead wood, rotten and missing wood, and timber 

form). When these percentages exceed specific thresholds, the damaging agent 

must be recorded, although these thresholds can vary by timber or woodland species type.

Of the estimated 1.7 billion live trees 5.0 inches diameter and greater in 

Arizona, about 405 million, or 23.6 percent, are estimated to have at least one 

damage meeting one or more of the three damage evaluation criteria above. Of 

these, 81.7 million (4.7 percent) have a second damaging agent and 8.2 million 

(0.48 percent) have three. Timber species had damage agents recorded only slightly more frequently than did woodland species (26.4 percent versus 22.5 percent), 

mostly due to loss of marketable products, although there were other differences 

between damages to timber and woodland species. For the five most common timber species, the frequency of at least one recorded damage was over 20 percent: 

quaking aspen (47.3 percent), white fir (26 percent), ponderosa pine (25 percent), 

Douglas-fir (24.3 percent), and Engelmann spruce (22.8 percent). Many of the less 

common timber species had higher rates of damage than aspen, but these estimates 

were based on small samples and their proportions of the total number of damaged 

trees was very small. The three common woodland species with the highest rates 

of damage were all oaks: Gambel oak (47.3 percent), Arizona white oak (36.9 percent), and Emory oak (36.4 percent). The remainder of common woodland species 

had damage rates of less than 25 percent: velvet mesquite (24.1 percent), oneseed 

juniper (22.9 percent), Utah juniper (21.2 percent), common pinyon (15 percent), 

and alligator juniper (12.9 percent).

Because the damage criteria above can be divided into pathogenic or lifethreatening (#1) and non-life-threatening (#2 and #3) categories, the proportions 

of damaging agents will be described separately. Of the pathogenic damages, stem 

decay affected the largest proportion of trees (fig. 33). Arizona white oak accounted for the largest portion of the trees with stem decay (30.9 percent), more than 

the next two species combined (velvet mesquite, 12.8 percent; Utah juniper, 11.8 

percent). The remainder of trees with stem decay were generally well-distributed 

among 31 other species. Parasitic plants, which include true and dwarf mistletoes, were the next most common damage agent. Of the trees found with parasitic 

plants, common pinyon and ponderosa pine accounted for the majority, accounting 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 65

for 39.8 and 25.6 percent, respectively, of infected trees. All of the records of parasitic plants on these species were dwarf mistletoes, and the Hawksworth rating 

(Hawksworth 1977) was 3 or greater.

Two damaging agents that were found relatively infrequently but merit mention are insects and fire, because they are the most common causes of tree mortality. Insect damage, particularly bark beetles, can be difficult to see in live trees 

but can rapidly lead to tree death. Thus, insect damage would be more commonly 

recorded as a mortality agent than a damage agent. Bark beetles were the most 

common insects recorded in timber species, but overall, defoliators were the most 

common insect damaging agent. Similarly, fires in Arizona kill most trees and 

leave relatively few survivors that meet damage thresholds with fire damage.

Of the damage agents that primarily affect potential wood products and that 

are typically non-lethal, dead tops were recorded on more than half of the trees 

with these kinds of damages (fig. 34). Although dead tops may affect tree vigor, 

depending on the proportion of live crown remaining, there is not likely to be a 

substantial effect on the quality of the lower stem. The same is true for broken 

tops, which only represent 5 percent of the form damages. However, it is possible to quantify the amount of missing stem by taking the difference in actual 

(as measured) and estimated (as if the top were present) heights. Based on this 

method, the average length of the upper stem missing from trees with broken tops 

is about 25 percent for timber species, and 30 percent for all trees. The next three 

most common form damages, which together account for 41 percent of this class 

of damages, can have substantial effects on the merchantability of the lower stem. 

Depending on the location of the fork, forked trees can reduce the merchantable 

volume by limiting log length or requiring removal of a long section from the butt 

log. Crook and sweep affect the ability to saw logs efficiently, although modern 

General	Insects	

1%	

Bark	Beetles	

2%	

Defoliators	

1%	

Root/Butt	Diseases	

1%	 Cankers	

2%	

Stem	Decay	

36%	

Parasitic	Plants	

15%	

Broom	Rusts	

1%	

Fire	

10%	

Wild	Animals	

3%	

Abiotic	Damage	

9%	 Competition	

2%	 Human	(non-harvest)	

2%	

Harvest	

3%	

Misc.	Minor	Types	

2%	

Unknown	Type	

12%	

Figure 33—Frequency of damaging agents on an estimated 

203 million trees in Arizona, 

2005–2014. This represents 

about half of the trees with 

recordable damage in the 

inventory and excludes formrelated damage types (see fig. 

34). Trees with these damages generally meet criterion 

1 above.
66 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

sawing methods tend to maximize sawn volume. The impact of open wounds vary 

considerably, because factors such as the time between the wound and harvest, or 

the degree to which a wound has allowed decay to the stem, have an effect on the 

degree of merchantability.

Recent Fires

Fire is an important disturbance that influences the structure and dynamics 

of Arizona’s forests. In some forest types, such as ponderosa pine, fire can maintain open stands and stimulate the growth of grasses and forbs in the understory. 

Throughout the Interior West, a century of fire suppression has led to a buildup 

of fuels and stand densification, which may lead to uncharacteristically intense 

fires in some areas (Reinhardt et al. 2008). Areas that burn intensely may experience slow regeneration, but others may recover relatively quickly. For example, 

the area inside the boundary of the large 1910 fires in Idaho and Montana (Cohen 

and Miller 1978; Egan 2009; Pyne 2008) now carries about the same amount of 

live tree volume per acre as areas outside the fires, although the mean stand age 

is somewhat lower and the volume is generally distributed among smaller trees 

(Wilson et al. 2010).

There were many fires and fire complexes in Arizona during the period covered by this report. Some FIA plots within fire boundaries were measured before 

and some were measured after the fires occurred. As a result, some fire perimeters 

contain both prefire and postfire plots, while others may contain only prefire or 

only postfire plots (fig. 35). Prefire plots represent the original conditions in areas 

that later burned, while only postfire plots provide insight into the short-term effects of fire. This means that normal data compilation methods cannot be used 

without introducing some element of temporal bias. These limitations on analysis 

Broken	Top	

5%	

Dead	Top	

52%	

Forked	Below	

Merchantable	Top	

11%	

Crook	or	Sweep	

16%	

Foliage	Discoloration	

1%	

Dieback	

1%	

Open	Wound	

14%	

Figure 34—Frequency 

of form-related damaging agents on an 

estimated 202 million 

trees in Arizona, 

2005–2014. This 

represents about 

half of the trees with 

recordable damage in the inventory 

and excludes most 

pathogenic damage 

types. Trees with 

these damages generally meet criteria 2 

and 3 above.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 67

will be reduced as the current inventory cycle is completed and remeasurement 

data are acquired during the next cycle. However, there are some general analyses 

that can be conducted with the current data.

We used data from the Monitoring Trends in Burn Severity (MTBS) project, which is an interagency effort being conducted and maintained by the USDA 

Forest Service Remote Sensing Applications Center and the U.S. Geological 

Survey National Center for Earth Resources Observation and Science. The purpose of the MTBS project is to map the perimeters and severities of large wildland 

fires (including wildfire, wildland fire use, and prescribed fire) across all lands 

of the United States. In western States, the project includes all fires larger than 

1,000 acres (Eidenshenk et al. 2007) but usually captures some that are below that 

threshold. The analysis presented here is based on fire perimeters identified by the 

MTBS program between 1984 and 2014 and FIA plot data collected between 2005 

and 2014 in Arizona.

Figure 35—Distribution of FIA plots 

in Arizona that occur within MTBS 

fire perimeters for fires occurring 

between 1984 and 2014, by plot 

status (forest = triangle; nonforest 

= circle) and timing of the most 

recent plot visit (prefire = green; 

postfire = red ). Mapped MTBS fire 

areas, including those not sampled 

by FIA plots, shown in orange.
68 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

The MTBS program mapped 823 fires covering a total of 5.7 million acres 

in Arizona between 1984 and 2014. These fires ranged from 265 acres to more 

than 563 thousand acres, with an average size of 7.1 thousand acres. FIA plot 

locations fell within the perimeters of 391 of the 823 mapped fires. Forested conditions measured between 2005 and 2014 fell within the boundaries of 187 fires 

perimeters, while the remaining 134 fire perimeters encompassed only nonforest 

or nonsampled plots. Of the 10 largest fires that occurred during the 31-year period 

covered by MTBS, three occurred in 2011 and three occurred in 2005. Together, 

the 10 largest fires account for 1.9 million burned acres.

As noted in the early sections of this report, the State of Arizona encompasses 73.1 million acres of land area, of which 18.6 million acres (25.5 percent) 

are estimated to be forest land by FIA. The unique area burned over the 31-year 

period is about 4.8 million acres, or only 6.6 percent of the State’s area. Totals of 

area burned do not tell the full story of fire in forest ecosystems. Although MTBS 

products include severity and cover type data, they cannot be used to differentiate 

between forest and nonforest as defined by the FIA program. However, we can use 

the status of FIA conditions as a sample of burned area to estimate the proportions 

of forest and nonforest area burned, as well as the area of different forest types 

within burned forest. According to this kind of analysis, 58.3 percent of the burned 

area in Arizona was on forest land and the remaining 41.7 percent was nonforest. 

Given that Arizona is only 25 percent forested, this would suggest that far more 

forest land burned than would be expected given its proportion of the landscape. 

However, much of the area of Arizona is in “unburnable” land cover types, such 

as the rocky areas of the Grand Canyon and certain desert vegetation types, so this 

proportion is difficult to analyze. As the FIA program collects data on nonforest 

plots, it should be possible to analyze these proportions further.

Plot data from within fire boundaries can also be used to characterize some 

fires individually. Although most of the small fires were not sampled by FIA plots, 

several of the largest fires were sampled by enough plots to provide some analysis 

capability. The largest fire recorded in Arizona, the 563 thousand-acre Wallow fire 

of 2011, encompassed 94 plots, some of which sampled both forest and nonforest 

conditions. Based on the number of forest conditions sampled, 82.3 percent of the 

area of the Wallow fire was forest land. The Rodeo fire of 2002, which until the occurrence of the Wallow fire was the State’s largest at over 461 thousand acres, was 

95.6 percent forest land. Most of the remaining top 10 fires burned mostly forest 

land. The Cave Creek complex, in which 61 percent of the 245,013 acres burned 

were nonforest, and the Goldwater fire, in which the entire burned area of 65,148 

acres were nonforest, were the only fires in the top 10 in which forest land was a 

minority.

In addition to estimating forest vs. nonforest proportions, it is also possible 

to analyze proportions among forest types. Ninety-three percent of the burned area 

that was classified as forest fell into seven forest types: juniper woodland, pinyon/

juniper woodland, Douglas-fir, ponderosa pine, deciduous oak woodland, evergreen oak woodland, and mesquite woodland. Forest area calculations are based 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 69

on field-determined forest type instead of calculated forest type, because high rates 

of mortality on recently burned land tends to result in a large proportions to be 

classified as nonstocked. Juniper woodland, pinyon/juniper woodland, and mesquite woodland burned in lower proportions than their occurrences on the landscape; the remainder burned in higher proportion, and in some cases, much higher 

proportion (table 9). The ponderosa pine forest type burned almost double the area 

in proportion to its forest type, but the actual proportion is likely to be higher 

because the ponderosa pine type is known to be converted to oak woodland types 

after fire (table 10).

In most cases, forest type change is related to fire severity. Low-severity fires 

may leave most overstory trees intact and have a minimal effect on composition. 

High-severity fires, which by definition kill a major fraction of the overstory, have 

Table 9—Proportions of all forest area and proportions of burned area for seven forest types representing most burned area between 1984 and 2014.

Forest type Percent of forest area Percent of burned area

Juniper woodland 14.1 8.5

Pinyon/juniper woodland 45.2 13.9

Douglas-fir 1.6 5.4

Ponderosa pine 16.7 31.3

Deciduous oak woodland 1.8 6.3

Evergreen oak woodland 7.9 21.8

Mesquite woodland 10.0 5.8

Table 10—Forest type group change matrix for 193 plots in Arizona with pre- and postfire measurements. Shaded cells 

indicate cases where burned plots did not change type. 

Forest-type group (postfire)

Forest-type group (prefire)

Aspen/birchDouglas-firElm/ash/cottonwoodFir/spruce/mountainOther western softwoodsPinyon/juniperPonderosa pineWoodland hardwoodsTotal

Aspen/birch 1 2 3

Douglas-fir 1 3 3 3 10

Elm/ash/cottonwood

Fir/spruce/mountain 2 1 1 4

Other western softwoods 1 3 4

Pinyon/juniper 1 36 2 21 60

Ponderosa pine 1 2 12 38 18 71

Woodland hardwoods 3 38 41

Total 5 5 1 1 52 46 83 193
70 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

varying effects by forest type. For example, in stands composed of a combination 

of seeding and sprouting species, such as ponderosa pine (seeding) with associated 

aspen or Gambel oak (sprouting), a high-severity fire can kill most trees. When 

all or most of the ponderosa pine overstory is killed by fire, aspen and oak species 

usually sprout in abundance and can become the dominant species in the stand. 

This is an example of successional change, and it is expected that ponderosa will 

re-establish and regain dominance in the future. Lower-severity fires may kill firesensitive species like aspen or consume the crowns of low-stature species like 

Gambel oak but not affect the fire-resistant ponderosa pine component. Therefore, 

low-severity fire can actually increase and maintain the relative dominance of ponderosa pine.

Our limited data on pre- and postfire conditions, based on 193 conditions 

with pre- and postfire measurements, show a gradient of mortality within fire 

boundaries in Arizona (fig. 36). About 16 percent of remeasured conditions had 

more live basal area at the time of remeasurement than at the time of first measurement, meaning that a combination of the growth of residual trees and regeneration 

has offset whatever reductions in basal area might have resulted from fire. Only 

about 12 percent of remeasured plots had no live component with a diameter of 1.0 

inches or greater at the time of remeasurement. This is partly because the average 

time between the fire and time of remeasurement was only 4.1 years, so regeneration might not yet be dense enough to be captured on the 1/300th-acre microplots 

that are used for seedlings and saplings. However, examination of seedling data 

shows that seedlings have been recorded on one-third of plots with no trees at 

least 1.0 inch in diameter, so while seedlings are being captured by the sample, on 

these plots they have not yet transitioned to sapling size. Seedlings are primarily 

ponderosa pine and aspen, but Douglas-fir, Gambel oak, and some juniper species 

Figure 36—Distribution of basal 

area change from prefire 

to postfire measurements. 

Positive differences indicate 

more basal area at the time 

of remeasurement than at 

initial measurement. Values of 

negative 100 percent indicate 

conditions with no remaining live 

basal area at the time of remeasurement. Graph is truncated 

at 100 percent positive change 

(2x original basal area), so the 

amount of change exceeding 

a doubling of basal area is not 

shown for some conditions.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 71

have also been recorded. Many of these plots are coming up for remeasurement in 

the coming 5 years, so it will be possible to monitor the progress of regeneration 

of these stands in the near term.

The analysis of fire and fire effects in this section should be considered only 

a first approximation of fire effects on Arizona’s forests. Although the results are 

generally consistent with expectations, the magnitude of fire-related mortality cannot be stated with precision at this point in the inventory. Nonetheless, the data 

confirm that there has been only partial mortality within fire boundaries. Additional 

data and analysis will be required to determine whether, for example, mortality is 

more or less evenly distributed among plots within the burned areas or mortality 

tends to be all-or-none at the plot scale. Remeasurement data will be necessary to 

confirm the portions of standing live and dead trees that are consumed by fire and 

converted to the DWM pool. The Accelerated Remeasurement and Evaluation of 

Burned Areas (AREBA) project may also shed light on the actual rates of mortality and conversion of standing dead trees to DWM. The objective of AREBA is 

to remeasure FIA plots that fall within MTBS perimeters within 1 year of the fire 

(Megown et al. 2011). Future remeasurements of FIA plots, whether on regularly 

scheduled inventory cycles or immediately following fire under the AREBA project, will not only enable analysis of fire’s effects on specific forest types such as 

aspen, but they will also provide important information on the amount and rate of 

recovery in all burned areas over time.

Invasive and Noxious Weeds

Noxious plant species can have many negative effects on forest communities. 

Noxious species can displace native flora, alter fire regimes, reduce diversity in the 

plant and pollinator communities, and generally reduce the diversity and resiliency 

of forest ecosystems. FIA field crews record any instance where a noxious species 

is found on a plot that contains a forested condition. This allows for the spatial and 

temporal extent of these species to be documented as plots are revisited. Although 

cheatgrass (Bromus tectorum) is not listed as noxious in Arizona, it is a nonnative 

annual grass that is quickly invading many areas of the State. There is considerable 

interest in the occurrence of cheatgrass on Arizona’s forests, but since it is not considered noxious by the State of Arizona, cheatgrass data are collected in a different 

manner and is discussed in the next section.

A total of 2,105 samples were used to assess the occurrence of noxious plant 

species in Arizona. These samples represent plots or portions of plots that had 

a forested condition recorded somewhere within the boundaries of the four subplots. Twelve different species were documented on forested plots in Arizona, with 

one or more found on 37 (1.8 percent) of the sampled plots. Yellow starthistle 

(Centaurea solstitialis) and Canada thistle (Cirsium arvence) were the most common noxious species found in the forested condition. These two species accounted 

for 46 percent of the weed occurrences (fig. 37).

The pinyon/juniper forest-type group has the highest number of locations 

with at least one noxious species detection (fig. 38). However, when viewed as a 
72 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

percentage of total plots within a forest-type group, the pinyon/juniper group is one 

of the least-invaded groups found in Arizona (fig. 39). The elm/ash/cottonwood 

group appears to have the highest percentage of its plots occupied (17 percent), but 

with such a low sample size in this group, the result should be viewed cautiously. 

The nonstocked forest type had the next highest occurrence of noxious species 

with just over 3 percent of the sampled plots found with detections. Many of these 

stands have recently been disturbed by fire, harvest, disease, or other perturbation 

that has removed most or all of the trees from it. These plots are more susceptible 

to invasion because noxious species can compete with native pioneer species for 

resources that were unavailable when the stand was stocked with mature trees and 

an established understory component.

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

Yellow 

starthistle 

Canada 

thistle 

Russian 

knapweed 

Common 

purslane 

Scotch 

thistle 

Plumeless 

thistle 

Leafy 

spurge 

Purple 

starthistle 

Halogeton Dalmation 

toadflax 

Burclover Perennial 

sowthistle 

Number of detections 

Figure 37—Weed species found on FIA plots and number of occurrences.

0 

2 

4 

6 

8 

10 

12 

14 

16 

Pinyon-Juniper 

Non-stocked 

Ponderosa Pine 

Western Oaks 

Other Western Hardwoods 

Elm/Ash/Cottonwood 

Douglas-fir 

Spruce/Fir/Mtn. Hemlock 

Aspen/Birch 

Other Western Softwoods 

Number of infested plots 

Forest type group 

Figure 38—Number of plots within forest-type groups that had occurrences of noxious weeds.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 73

The paucity of noxious species found in Arizona’s forests suggests that they 

are resistant to invasion and/or persistence of listed species. However, these species often occur along linear features such as roads and streams, and FIA sampling 

may not adequately sample such features for noxious plants. In addition, many 

noxious species are associated with grasslands, agricultural lands, and developed 

lands because of the persistent disturbance that occurs in such areas. Arizona certainly has these features but a much larger portion of the State is rural, undeveloped, or remote. The xeric conditions associated with Arizona and its forests may 

also limit the opportunities for infestation and establishment of noxious species.

Cheatgrass

Cheatgrass (Bromus tectorum) is a nonnative annual grass that has invaded 

and displaced native vegetation throughout the West. Cheatgrass germinates earlier than most native species, thus gaining a competitive advantage for the limited 

resources in the arid environments of the States of the Southwest. The fine fuels 

created by cheatgrass alter fire frequency in the areas where it is found in abundance (e.g., Balch et al. 2013). These fuels can perpetuate the spread of the species 

by creating new areas to invade after a fire disturbance. Both public and private 

land managers are interested in understanding the spatial and temporal patterns of 

cheatgrass and any other information that can be used to curb infestation. Because 

cheatgrass is not listed as noxious by the State of Arizona, FIA field crews document it as understory vegetation. A threshold of 3 percent or greater ground cover 

must be reached before an understory plant is recorded on a plot. In addition, only 

those areas with a forested condition present on a plot have understory vegetation 

recorded. Therefore, these data do not reflect trace amounts of cheatgrass (<3 percent cover) or cheatgrass on nonforested portions of plots.

In Arizona, cheatgrass was documented on 62 occasions in six different forest types at elevations ranging from 2,900 feet to 8,900 feet. Cheatgrass was found 

in stands that ranged from 0 (nonstocked) to 369 years old. The pinyon/juniper 

0.0% 

2.0% 

4.0% 

6.0% 

8.0% 

10.0% 

12.0% 

14.0% 

16.0% 

18.0% 

Elm/Ash/Cottonwood 

Non-stocked 

Other Western 

Western Oaks 

Grand Total 

Ponderosa Pine 

Pinyon-Juniper 

Douglas-fir 

Spruce/Fir/Mtn. Hemlock 

Other Western 

Aspen/Birch 

Percentage of plots 

 infested within group 

Forest type group 

Figure 39—Proportion of plots with noxious weed detections by forest-type groups. 
74 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

forest-type group had by far the most records of cheatgrass (41), with about 66 

percent of the total number of cheatgrass occurrences sampled on Arizona forest 

land (fig. 40). The woodland hardwoods and nonstocked groups had the next highest occurrences with nine records each (15 percent), followed by four records in 

the ponderosa pine group (3 percent) and one record in the other hardwoods group. 

Most of the cheatgrass sampled were on plots between 6,000 and 7,000 feet in elevation, with an equal distribution of occurrences in the higher and lower elevation 

classes (fig. 41). This suggests that adequate growing conditions for cheatgrass 

exist in forested areas at these elevations in Arizona.

In contrast to species officially listed as noxious in Arizona, cheatgrass is 

not found in abundance in any timber forest types. With the exception of two occurrences in ponderosa pine and nine in nonstocked forests types, all cheatgrass 

occurrences that reached 3 percent ground cover were found in woodland forest 

types—the vast majority in the pinyon/juniper group. These forest types often occur in areas that have lower soil moisture and understory species diversity than 

0.0%	

10.0%	

20.0%	

30.0%	

40.0%	

50.0%	

60.0%	

70.0%	

0	

5	

10	

15	

20	

25	

30	

35	

40	

45	

Pinyon/	juniper		 Woodland	

hardwoods	

Non-stocked	 Ponderosa	pine		 Other	

hardwoods	

Percent	of	total	

Number	of	detections	

Forest	type	groups	

Number	of	detections	 Percent	of	total	

Figure 40—Cheatgrass detections by forest-type group in 

Arizona, 2005–2014.

0	

5	

10	

15	

20	

25	

30	

≤	5000	 5001-6000	 6001-7000	 7001-8000	 >	8000	

Number	of	detections	

Elevation	class	(feet)	

Figure 41—Cheatgrass detections by elevation class in 

Arizona, 2005–2014
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 75

higher elevation sites. These may be factors that affect an area’s susceptibility to 

invasion. Other possible factors include the type, level, and frequency of disturbance (natural and human-induced) in these areas. The pattern of cheatgrass across 

the elevation gradient may be driven by the corresponding moisture gradient. Plots 

located at higher elevations typically have more soil moisture, lower temperatures 

and a shorter growing season. These factors may impede the introduction and/or 

establishment of cheatgrass in these areas.

Cheatgrass presents a threat to western ecosystems and a challenge to those 

that manage the lands where it has established a population. FIA data can be used 

to identify areas infested and/or most susceptible to infestation. This information 

can also be used to test cheatgrass models currently being developed or refine 

those already being implemented.

Comparisons Between Arizona’s Periodic and Annual Forest Inventories

One purpose of Arizona’s annual forest inventory is to provide information 

about changes in forest attributes over time. Prior to the implementation of the 

annual inventory, two plot-based periodic inventories were conducted in Arizona. 

One Statewide periodic inventory was conducted between 1981 and 1986 and is 

referred to as the 1985 inventory (Conner et al. 1990). Another was conducted primarily between 1995 and 1999 and is referred to as the 1999 inventory (O’Brien 

2002). If the definitions and methods used during the periodic inventories were 

compatible with those used during the annual inventory, we could quantify trends 

over the past 30 years. However, the sampling and field procedures used during 

the periodic inventories were different enough from those of the annual inventory 

to preclude reliable trend analysis. Therefore, direct comparisons of periodic and 

annual inventories in their entireties are not recommended and may even produce 

misleading results (Goeking 2015). This section describes the primary differences 

between the periodic and annual inventories; presents an appropriate method for 

comparing periodic and annual inventory data at plots that were measured during 

both inventories, or co-located plots; and summarizes some changes in forest attributes that have occurred at co-located plots.

The primary differences between Arizona’s periodic and annual forest inventories pertain to the plot design, sample design, and operational definitions 

used during field data collection. The periodic inventories of the 1980s and 1990s 

used a variable-radius plot design with varying numbers of subplots. In contrast, 

the plot design of the annual inventory consists of four fixed-radius subplots, as 

described in the Plot Configuration section of this report’s “Inventory Methods” 

chapter. Sample designs also changed appreciably, from different sample intensities across ownership groups (O’Brien 2002) to a spatially representative plot grid 

with consistent sample intensity across all forest types and management categories 

(Bechtold and Patterson 2005). Arizona’s periodic inventories also used an operational definition of “tree” that differentiated between tree-form and shrub-form 

trees. For example, pinyon pines that were less than 6 feet tall and were not expected to eventually produce a straight, 8-foot trunk section were not considered to be 
76 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

trees and were not measured, so they were not included in volume-based estimates 

such as biomass, growth, and mortality. In contrast, the annual inventory identifies 

trees strictly by their species, regardless of growth form. Therefore, trees on many 

woodland plots in the current annual inventory would not have been measured 

under previous definitions.

Due to these differences in forest inventories over time, users of FIA data 

should be aware of appropriate methods of evaluating trends and avoid inappropriate methods. Examples of inappropriate comparisons between periodic and annual inventories range from comparing the tree volume on a specific forest type to 

directly comparing the total area of forest land. Instead, an appropriate method of 

quantifying trends is to first identify forest plots that were measured during both 

periodic and annual inventories, and then to assess trends at only those plots. FIA 

refers to such plot locations as co-located plots. Although different plot designs 

were used during the periodic and annual measurements of co-located plots, each 

plot design allows estimation of volume, growth, and mortality per acre as well 

as stand-level variables such as forest type. Therefore, comparisons of multiple 

measurements at co-located plots are useful for quantifying trends in attributes on 

a per-acre basis, such as volume, mortality, growth, biomass, and number of trees 

per acre.

This section presents the results of two analyses of co-located plot data collected during periodic versus annual inventories. The first analysis compares data 

collected at co-located plots that were measured once between 1990 and 1999 and 

again during a full cycle of the annual inventory. The second analysis compares 

co-located plot measurements from 1981–1986 to the annual inventory measurements. Figure 42 shows the distribution of all plots in the 1985 inventory, all plots 

in the 1999 inventory, all plots in the annual inventory, and the plots that were colocated between the annual inventory and each of the periodic inventories.

The analysis of change between the 1990s and the annual inventory consisted 

of 1,900 co-located plots that were measured during both periods. Average annual 

tree mortality at co-located plots increased by 471 percent, from 3.5 to 19.8 cubic 

feet per acre per year, and net growth decreased by 135 percent, from 16.3 to –5.7 

cubic feet per acre per year. Over this same time period, mean net volume of dead 

trees per acre more than doubled from 78 to 162 cubic feet per acre. Live net volume per acre decreased by 11 percent, from 1,307 to 1,070 cubic feet per acre. The 

changes in net volume, net growth rates, and mortality rates are similar to those 

observed for Arizona by Goeking (2015) in a comparison of plots measured during 

the 1990s and measured again between 2003 and 2012.

The second analysis consisted of 629 co-located plots that were measured 

during both the 1981–1986 inventory and the annual inventory. The results showed 

the same patterns as those from the first comparison: large increases in dead net 

volume and mean annual mortality, and decreases in net growth and live net volume. In both analyses, the mean annual net growth at co-located plots was negative, indicating that mortality exceeds gross growth. One difference between the 

two analyses is the temporal difference in total net volume, which includes both 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 77

live and dead trees. Average total net volume at co-located plots measured during 

the 1990s and again in the annual inventory decreased by about 11 percent, from 

1,384 to 1,232 cubic feet per acre. Co-located plots measured during the 1980s and 

again during the annual inventory showed a slight increase in total net volume, 

from 669 to 642 cubic feet per acre. Note that the average net volume at co-located 

plots measured during the 1980s inventory was much smaller than the average net 

volume at co-located plots from the 1990s inventory. That is likely because the 

1990s inventory of Arizona over-represented the ponderosa pine forest type and 

under-represented pinyon/juniper forest types and, to a lesser extent, the woodland 

hardwoods forest-type group (Goeking 2015). In contrast, the 1980s inventory was 

more representative of woodland forest types and therefore shows lower per-acre 

volumes, on average.

To investigate changes for individual tree species, we quantified live basal 

area, dead basal area, mean annual growth, and mean annual mortality, as measured at co-located plots in the 1990s inventory and again in 2005–2014, for the 

six tree species that make up most of Arizona’s tree volume. These are, in decreasing order of total volume, ponderosa pine, Utah juniper, common pinyon, 

oneseed juniper, alligator juniper, and Douglas-fir. Figure 43 shows that all species 

are exhibiting lower mean annual net growth, and higher mean annual mortality, 

during the annual inventory period (t2) than during the 1990s (t1). Recent annual 

net growth rates are positive for three of the species (ponderosa pine, common 

pinyon, and oneseed juniper), while annual net growth is negative for the other 

three species (Utah juniper, alligator juniper, and Douglas-fir). Negative annual net 

growth indicates that mortality rates are greater than gross growth rates on a basis 

Figure 42—Approximate locations of periodic inventory plots measured in the 1980s and 1990s (A), annual inventory plots measured 

in 2005–2014 (B), and co-located plots measured during one of the periodic inventories and again during the annual inventory (C).
78 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

of cubic-foot volume per year. The total amount of gross growth can be inferred 

from the combination of net growth and mortality (fig. 43). Gross growth during 

2005-2014 was greater than gross growth during the 1990s for three species: Utah 

juniper, common pinyon, and alligator juniper. However, the negative net growth 

of Utah juniper and alligator juniper, as well as the small yet positive net growth of 

common pinyon, show that high mortality rates are approximately offsetting gains 

due to relatively high growth rates.

In 2005–2014, all six major species except Utah juniper had less live basal 

area per acre, and all species except for oneseed juniper had more dead basal area 

per acre, than in the 1990s (fig. 44). Given the recent negative net growth of some 

of these species (fig. 43), we could expect that the decline in live basal area will 

continue into the next few years of measurement in Arizona’s forest inventory. 

Total basal area declined for most of these species; the exceptions were alligator 

juniper and Douglas-fir, which actually have slightly more total basal area in the 

annual inventory than in the 1990s.

Figure 44—Mean basal 

area for major tree 

species of Arizona, 

as measured at colocated plots that were 

measured during the 

1990s inventory (t1) 

and again between 

2005 and 2014 (t2).

-10	

-5	

0	

5	

10	

15	

20	

25	

30	

t1	 t2	 t1	 t2	 t1	 t2	 t1	 t2	 t1	 t2	 t1	 t2	

ponderosa	pine	 Utah	juniper	 common	

pinyon	

oneseed	

juniper	

alligator	juniper	 Douglas-fir	

Mortality	and	net	growth	(feet3/acre/year)	

Mortality	 Net	growth	

Figure 43—Mean annual 

mortality and mean 

annual net growth for 

major tree species of 

Arizona, as measured 

at co-located plots 

that were measured 

during the 1990s inventory (t1) and again 

between 2005 and 

2014 (t2).

0	

10	

20	

30	

40	

50	

60	

70	

80	

t1	 t2	 t1	 t2	 t1	 t2	 t1	 t2	 t1	 t2	 t1	 t2	

ponderosa	pine	 Utah	juniper	 common	pinyon	 oneseed	juniper	 alligator	juniper	 Douglas-fir	

Basal	area	(ft2/acre)	

Live	trees	 Dead	trees	
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 79

The caveat of the co-located plot analysis presented here is that results cannot 

be scaled to the entire State and cannot overcome the limitations of the periodic 

sample design. For example, the periodic inventory under-sampled some woodland forest types, and any analysis of co-located plots will still under-represent that 

forest type and will instead exhibit trends that occurred on forest types that were 

sampled more intensively when all plots are considered together. Nonetheless, it 

provides an indication of the direction of change in Arizona’s forests. As Arizona’s 

forest inventory continues into its second cycle and plots are remeasured at a consistent 10-year interval, FIA’s ability to quantify trends in forest attributes will 

expand from analyses of co-located periodic plots to robust Statewide estimates of 

change based on the spatially representative annual plot grid.

Conclusions and Future Analyses____________________________

There has been considerable change in Arizona’s forests since annual inventory was started in the State in 2001. Several of the large fires that occurred in the 

early 2000s, such as the Rodeo fire, were only partially sampled before the occurrence of fire, so most of the plot visits have occurred afterward. Later large fires, 

such as the Wallow fire, occurred after a complete first cycle of annual inventory. 

As a result, there is both a good baseline inventory and several years of remeasurement, and every new year of annual inventory brings in new remeasurement data.

The situation is similar with drought and its effects on several species in the 

State. Shortly after annual inventory began in Arizona, the cumulative effects of 

several years of drought began to affect several species. Of particular note were 

localized outbreaks of bark beetles in ponderosa pine and widespread die-off of 

aspen. The most notable drought-related event was the pinyon ips beetle outbreak 

that caused high levels of mortality in common pinyon nearly range-wide, and 

most severely along the Mogollon Rim in Arizona. The outbreak accelerated and 

peaked during the period of 2003–2006, but small pockets of mortality have occurred during the intervening years.

Because of the timing of these events, FIA annual inventory was able to capture widespread change with annual resolution—something that was impossible to 

do with the periodic inventory system. Periodic inventories effectively took only 

“snapshots” of forest conditions at intervals of a decade or more, providing very 

little information on how the forest changed between inventories. As a result of 

the timing of disturbance events and implementation of annual inventory, the most 

recent 10 years of annual inventory, on which all of the standard tables (Appendix 

B) and many analyses are based, mostly describe a recovering forest.

However, recovery from past events occurs even as new changes affect 

Arizona’s forests. It is impossible for forests to grow indefinitely, so one goal of 

FIA inventory is to monitor the near-term trends in accumulation of live and dead 

forest biomass, including transitions between the major pools of woody material. 

In the process, FIA data provide information on forest composition, structure, and 

age-class distribution, all of which can provide some insight to changes that might 
80 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

be coming in the future. Younger and more diversified forests tend to be more 

resilient forests, so a potential positive effect of the recent, relatively high rates 

of disturbance includes a forest that is not as susceptible to the same kinds of disturbances that created it. As the forest continues to change, FIA will monitor the 

change and compare the outcome to expectations that are formed by past research.

References_______________________________________________

Bailey, R.G. 1983. Delineation of ecosystem regions. Environmental Management. 7(4): 

365–373.

Bailey, R.G. 1995. Descriptions of the ecoregions of the United States. (2nd ed. rev. and 

expanded). Misc. Publ. No. 1391. Washington, DC: U.S. Department of Agriculture, 

Forest Service. http://www. fs.fed.us/land/ecosysmgmt/ [Accessed June 1, 2013].

Baker, W.L.; Shinneman, D.J. 2004. Fire and restoration of pinon-juniper woodlands in the 

western United States: A review. Forest Ecology and Management. 189(1): 1–21.

Balch, J.K.; Bradley, B.A.; D’Antonio, C.M.; [et al.]. 2013. Introduced annual grass increases regional fire activity across the arid western USA (1980–2009). Global Change 

Biology. 19: 173–183.

Bartos, D.L. 2001. Landscape dynamics of aspen and conifer forests. In: Shepperd, 

W.D.; Binkley, D.; Bartos, D.L.; [et al.], comps. Sustaining aspen in western landscapes: Symposium proceedings; 2000 13–15 June; Grand Junction, CO. Proceedings 

RMRS-P-18. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station: 5–14.

Bartos, D.L.; Campbell, R.B., Jr. 1998. Decline of quaking aspen in the Interior West—

Examples from Utah. Rangelands. 20(1): 17–24.

Bechtold, W.A.; Patterson, P.L., eds. 2005. The enhanced Forest Inventory and Analysis 

program—National sampling design and estimation procedures. Gen. Tech. Rep. SRS80. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research 

Station. 85 p.

Blackard, J.A.; Finco, M.V.; Helmer, E.H.; [et al.]. 2008. Mapping U.S. forest biomass 

using nationwide forest inventory data and moderate resolution information. Remote 

Sensing of Environment. 112: 1658–1677.

Breshears, D.D.; Cobb, N.S.; Rich, P.M.; [et al.]. 2005. Regional vegetation die-off in 

response to global-change-type drought. Proceedings of the National Academy of 

Sciences. 102: 15144–15148.

Brickell, J.E. 1968. A method for constructing site index curves from measurements of tree 

age and height—Its application to inland Douglas-fir. Res. Pap. INT-47. Ogden, UT: 

U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 23 p.

Brickell, J.E. 1970. Equations and computer subroutines for estimating site quality of 

eight Rocky Mountain species. Res. Paper INT-75. Ogden, UT: U.S. Department of 

Agriculture, Forest Service, Intermountain Research Station. 22 p.

Burkhardt, J.W.; Tisdale, E.W. 1976. Causes of juniper invasion in southwestern Idaho. 

Ecology. 57(3): 472–484.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 81

Caratti, J.F. 2006. Line Intercept (LI). In: Lutes, D.C.; Keane, R.E.; Caratti, J.F.; [et al.]. 

FIREMON: Fire effects monitoring and inventory system. Gen. Tech. Rep. RMRSGTR-164-CD. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station: LI-1-13.

Chojnacky, D.C. 1988. Juniper, pinyon, oak and mesquite volume equations for Arizona. 

Res. Note INT-391. Ogden, UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Research Station. 11 p.

Chojnacky, D.C. 1994. Volume equations for New Mexico’s pinyon-juniper dryland forests. Res. Note INT-471. Ogden, UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Research Station. 9 p.

Cleland, D.T.; Avers, P.E.; McNab, W.H.; [et al.]. 1997. National Hierarchical Framework 

of Ecological Units. In: Boyce, M.S.; Haney, A., eds. Ecosystem management applications for sustainable forest and wildlife resources. New Haven, CT: Yale University 

Press: 181–200.

Cleland, D.T.; Freeouf, J.A.; Keys, J.E., Jr.; [et al.]. 2007. Ecological subregions: Sections 

and subsections for the conterminous United States. Gen. Tech. Rep. GTR-WO-76. 

Washington, DC: U.S. Department of Agriculture, Forest Service.

Clendenen, G.W. 1977. Base-age conversion and site-index equations for Engelmann 

spruce stands in central and southern Rocky Mountains. Res. Note INT-223. Ogden, 

UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 

6 p.

Cohen, S.; Miller, D. 1978. The Big Burn: The Northwest’s fire of 1910. Missoula, MT: 

Pictorial Histories Publishing Co. 96 p.

Conner, R.C.; Born, J.D.; Green, A.W.; [et al.]. 1990. Forest resources of Arizona. Resour. 

Bull. INT-69. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain 

Research Station. 92 p.

DeBlander, L.T.; Shaw, J.D.; Witt, C.; [et al.]. 2010. Utah’s forest resources, 2000–2005. 

Resour. Bull. RMRS-RB-10. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station. 144 p.

Edminster, C.B.; Mowrer, H.T.; Sheppard, W.D. 1985. Site index curves for aspen in the 

Central Rocky Mountains. Res. Note RM-453. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 4 p.

Egan, T. 2009. The Big Burn: Teddy Roosevelt and the fire that saved America. Boston: 

Houghton Mifflin Harcourt. 324 p.

Energy Information Association (EIA). 2016. U.S. States: State profiles and 

energy estimates. State Energy Data System (SEDS): 1960–2013. [Data files-Consumption-in physical units]. http://www.eia.gov/state/seds/seds-data-complete.

cfm?sid=US#CompleteDataFile.

Eidenshenk, J.; Schwind, B.; Brewer, K.; [et al.]. 2007. A project for monitoring trends in 

burn severity. Fire Ecology Special Issue. 3(1): 3–21.

Garbarino, M.; Marzano, R.; Shaw, J.D.; [et al.]. 2015. Environmental drivers of deadwood 

dynamics in woodlands and forests. Ecosphere. 6(3): Article 30.

Gillespie, A.J.R. 1999. Rationale for a national annual forest inventory program. Journal 

of Forestry. 97(12): 16–20.
82 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Goeking, S.A. 2015. Disentangling forest change from forest inventory change: A case 

study from the US Interior West. Journal of Forestry. 113(5): 475–483.

Goeking, S.A.; Patterson, P.L. 2013. Stratifying to reduce bias caused by high nonresponse 

rates: A case study from New Mexico’s forest inventory. Res. Note RMRS-RN-59. 

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

Research Station. 22 p.

Goeking, S.A.; Shaw, J.D.; Witt, C.; [et al.]. 2014. New Mexico’s forest resources, 2008–

2012. Resour. Bull. RMRS-RB-18. Fort Collins, CO: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station, 144 p.

Greening, G.K. 1941. Climate of Arizona. In: Climate and man: Yearbook of Agriculture 

1941. Washington, DC: U.S. Government Printing Office: 761–772.

Hann, D.W.; Bare, B.B. 1978. Comprehensive tree volume equations for major species of 

New Mexico and Arizona: I. Results and methodology. Res. Note INT-209. Ogden, UT: 

U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 43 p.

Hawksworth, F.G. 1977. The 6-class dwarf mistletoe rating system. Gen. Tech. Rep. RM48. U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range 

Experiment Station: 7 p.

Hayes, S.W.; Morgan, T.A.; Berg, E.C.; [et al.]. 2012. The Four Corners timber harvest 

and forest products industry, 2007. Resour. Bull. RMRS-RB-13. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station. 61 p.

Hessl, A.E.; Graumlich, L.J. 2002. Interactive effects of human activities, herbivory and 

fire on quaking aspen (Populus tremuloides) age structures in western Wyoming. Journal 

of Biogeography. 29: 889–902.

Jenny, H. 1994. Factors of soil formation—A system of quantitative pedology. Dover edition. New York: Dover Publications. 191 p.

Johnson, M. 1994. Changes in Southwestern forests: Stewardship implications. Journal of 

Forestry. 92(12): 16–19.

Kearns, H.S.J.; Jacobi, W.R.; Johnson, D.W. 2005. Persistence of pinyon pine snags and 

logs in southwestern Colorado. Western Journal of Applied Forestry. 20(4): 247–252.

Keegan, C.E.; Chase, A.L.; Morgan, T.A.; [et al.]. 2001. Arizona’s forest products industry: 

A descriptive analysis 1998. Missoula, MT: University of Montana, Bureau of Business 

and Economic Research. 20 p.

Kemp, P.D. 1956. Region 1 volume tables for ADP cruise computations. Timber Cruising 

Handbook R1-2430-31. Missoula, MT: U.S. Department of Agriculture, Forest Service, 

Northern Region.

Lilieholm, R.J.; Long, J.N.; Patla S. 1994. Assessing goshawk nest stand habitat using stand 

density index. Cooper Ornithological Society. Studies in Avian Biology. 16: 18–24.

Long, J.N. 1985. A practical approach to density management. Forest Chronicle. 61: 23–37.

Long, J.N.; Daniel, T.W. 1990. Assessment of growing stock in uneven-aged stands. 

Western Journal of Applied Forestry 5: 93–96.

Long, J.N.; Shaw, J.D. 2005. A density management diagram for even-aged ponderosa pine 

stands. Western Journal of Applied Forestry. 20: 205–215.

McArdle, R.E.; Meyer, W.H.; Bruce, D. 1961. The yield of Douglas-fir in the Pacific 

Northwest. Tech Bull. 201. Second revision. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 601 p.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 83

Megown, K.; Finco, M.; Brewer, K.; [et al.]. 2011. Accelerated remeasurement and evaluation of burned areas. Fire Management Today. 71(2): 9–11.

Menlove, J.; Shaw, J.D.; Thompson, M.T.; [et al.]. 2012. Montana’s forest resources, 2003–

2009. Resour. Bull. RMRS-RB-15. Fort Collins, CO: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station. 140 p.

Merriam, C.H. 1898. Life zones and crop zones of the United States. Bulletin 10. 

Washington, DC: U.S. Department of Agriculture, Division of Biological Survey.

Merriam, C.H.; Steineger, L. 1890. Results of a biological survey of the San Francisco 

Mountain Region and the Desert of the Little Colorado, Arizona. North American Fauna 

Report No. 3. Washington, D.C.: U.S. Government Printing Office.

Morgan, T.A.; Dillon, T.; Keegan, C.E., III; [et al.]. 2006. Four Corners timber harvest 

and forest products industry, 2002. Resour. Bull. RMRS-RB-7. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station. 64 p.

Mueller, R.C.; Scudder, C.M.; Porter, M.E.; [et al.]. 2005. Differential tree mortality in response to severe drought: Evidence for long-term vegetation shifts. Journal of Ecology. 

93: 1085–1093.

National Climatic Data Center. 1994. Time bias corrected divisional temperature-precipitation-drought index. Documentation for dataset TD-9640. 12 p. Washington, DC: U.S. 

Department of Commerce, National Climatic Data Center. https://www7.ncdc.noaa.

gov/CDO/divisionaldesc.html [Last accessed September 18, 2017].

 O’Brien, R.A. 2002. Arizona’s forest Resources, 1999. Resour. Bull. RMRS-RB-2. Ogden, 

UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 

116 p.

Pearson, GA. 1931. Forest types in the southwest as determined by climate and soil. Tech. 

Bull. 247. Washington, DC: U.S. Department of Agriculture. 143 p.

Pollard, J.E.; Westfall, J.A.; Patterson, P.L.; [et al.]. 2006. Forest inventory and analysis national data quality assessment report 2000 to 2003. Gen. Tech. Rep. RMRS-GTR-181. 

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

Research Station. 43 p.

Pyne, S.J. 2008. Year of the fires: The story of the Great Fires of 1910. Revised ed. Missoula, 

MT: Mountain Press Publishing Company. 320 p.

Reineke, L.H. 1933. Perfecting a stand density index for even-aged forests. Journal of 

Agricultural Research. 46(7): 627–638.

Reinhardt, E.D.; Keane, R.E.; Calkin, D.E.; [et al.]. 2008. Objectives and considerations 

for wildland fuel treatment in forested ecosystems of the interior western United States. 

Forest Ecology and Management. 256: 1997–2006.

Romme, W.H., Allen, C.D., Bailey, J.D., [et al.]. 2009. Historical and modern disturbance 

regimes, stand structures, and landscape dynamics in pinon-juniper vegetation of the 

western United States. Rangeland Ecology and Management. 62(3): 203–222.

Romme, W.H.; Floyd-Hanna, L.; Hanna, D. 2003. Ancient pinyon-juniper forests of Mesa 

Verde and the West: A cautionary note for forest restoration programs. In: Omi, P.N.; 

Joyce, L.A., tech. eds. Fire, fuel treatments, and ecological restoration: Conference proceedings; 2002 16–18 April; Fort Collins, CO. Proceedings RMRS-P-29. Fort Collins, 

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

335–350.
84 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Scott, J.H.; Burgan, R.E. 2005. Standard fire behavior fuel models: A comprehensive set 

for use with Rothermel’s surface fire spread model. Gen. Tech. Rep. RMRS-GTR-153. 

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

Research Station. 72 p.

Shaw, J.D. 2000. Application of stand density index to irregularly structured stands. 

Western Journal of Applied Forestry. 15: 40–42.

Shaw, J.D. 2006. Drought-related mortality in pinyon/juniper woodlands: A test case for 

the FIA annual inventory system. In: McRoberts, R.E.; Reams, G.A.; Van Deusen, P.C.; 

[et al.], eds. Proceedings of the sixth annual forest inventory and analysis symposium; 

2004 September 20–24; Denver, CO. Gen. Tech. Report WO-70. Washington, DC: U.S. 

Department of Agriculture, Forest Service. 126 p.

Shaw, J.D.; Long, J.N. 2007. A density management diagram for longleaf pine stands with 

application to red-cockaded woodpecker habitat. Southern Journal of Applied Forestry. 

31(1): 28–38.

Shaw, J.D., Goeking, S.A.; Menlove, J.; [et al.]. 2017. Assessment of fire effects based 

on forest inventory and analysis data and a long-term fire mapping data set. Journal of 

Forestry. 115(4): 258–269.

Shaw, J.D.; Steed, B.E.; DeBlander, L.T. 2005. Forest Inventory and Analysis (FIA) annual inventory answers the question: What is happening to pinyon-juniper woodlands? 

Journal of Forestry. 103(6): 280–285.

Smith, F.W.; Long, J.N. 1987. Elk hiding and thermal cover guidelines in the context of 

lodgepole pine stand density. Western Journal of Applied Forestry. 2: 6–10.

Society of American Foresters. 2004. Western US drought outlook improves. The Forestry 

Source. 9(12): 1,5. Bethesda, MD: Society of American Foresters.

Sorenson, C.B.; Hayes S.W.; Morgan, T.A.; [et al.]. 2016. The Four Corners timber harvest 

and forest products industry, 2012. Resour. Bull. RMRS-RB-21. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station. 62 p. 

https://www.fs.usda.gov/treesearch/pubs/50961.

Spencer, J.S., Jr. 1966. Arizona’s forests. Resour. Bull. RMRS-RB-2. Ogden, UT: U.S. 

Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment 

Station. 57 p.

Tausch, R.J.; Hood, S. 2007. Pinyon/juniper woodlands. In: Hood, S.M.; Miller, M., eds. 

Fire ecology and management of the major ecosystems of southern Utah. Gen. Tech. 

Rep. RMRS-GTR-202. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station: 57–71

Thompson, M.T. 2009. Analysis of conifer mortality in Colorado using Forest Inventory 

and Analysis’s annual forest inventory. Western Journal of Applied Forestry. 24(4): 

193–197.

USDA Forest Service. 1958. Timber resources for America’s future. Forest Resour. Rep. 

No. 14. Washington, DC: U.S. Department of Agriculture, Forest Service. 713 p.

USDA Forest Service. 1965. Timber trends in the United States. Forest Resour. Rep. No. 

17. Washington, DC. U.S. Department of Agriculture, Forest Service. 235 p.

USDA Forest Service. 2006. Interior West Forest Inventory and Analysis P2 field procedures, Ver. 4.0. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station. 369 p. http://www.fs.fed.us/rm/ogden/data-collection/pdf/

p2_midas_40_final_reduced.pdf; [Accessed June 2013].
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 85

USDA Forest Service. 2011. Interior West Forest Inventory and Analysis P2 field procedures, Ver. 5.0. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station. 398 p. http://www.fs.fed.us/rm/ogden/data-collection/pdf/

iwfia_p2_50. pdf; [Accessed June 2013].

USDC Bureau of Economic Analysis (BEA). 2014. Regional accounts data. Table SA25N. 

Washington, DC: U.S. Department of Commerce, Bureau of Economic Analysis. http://

www.bea.gov/regional/. [Accessed March 9, 2015].

USDC Census Bureau (USDC CB). 2014. North American Industry Classification System: 

Introduction to NAICS. Washington, DC: U.S. Department of Commerce, Census 

Bureau. http://www.census.gov/eos/www/naics/. [Accessed March 9, 2015].

USDOI Fish and Wildlife Service [USFWS]. 1995. Recovery plan for the Mexican spotted owl: Vol. I. Albuquerque, NM: U.S. Department of the Interior, Fish and Wildlife 

Service.

USDOI Fish & Wildlife Service [USFWS]. 2012. Final recovery plan for the Mexican spotted owl (Strix occidentalis lucida). First Revision. Albuquerque, NM: U.S. Department 

of the Interior, Fish and Wildlife Service. 413 p.

Weisberg, P.J.; Ko, D.; Py, C.; [et al.]. 2008. Modeling fire and landform influences on 

the distribution of old-growth pinyon-juniper woodland. Landscape Ecology. 23(8): 

931–943.

Wilson, M.J.; Deblander, L.T.; Halverson, K.A. 2010. Resource impacts of the 1910 fires: 

A Forest Inventory and Analysis (FIA) perspective. Powerpoint presentation given at: 

1910 Fires: A Century Later. Wallace, Idaho, May 20-22, 2010. Unpublished document 

on file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station, Forestry Sciences Laboratory, Ogden, UT.

Witt, C.; Shaw, J.D.; Thompson, M.T.; [et al.]. 2012. Idaho’s forest resources, 2004–2009. 

Resour. Bull. RMRS-RB-14. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station. 134 p.

Worrall, J.J.; Egeland, L.; Eager, T.; [et al.]. 2008. Rapid mortality of Populus tremuloides

in southwestern Colorado, USA. Forest Ecology and Management. 255(3-4): 686–696.

Woudenberg, S.W.; Conkling, B.L.; O’Connell, B.M.; [et al.]. 2010. The Forest Inventory 

and Analysis database: Database description and users manual version 4.0 for Phase 2. 

Gen. Tech. Rep. RMRS-GTR- 245. Fort Collins, CO: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station. 339 p.
86 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Appendix A: Standard Forest Inventory and Analysis Terminology

Note: For the FIA national glossary please go to https://www.nrs.

fs.fed.us/fia/data-tools/state-reports/glossary/default.asp.

Average annual mortality—The average annual volume of trees 5.0 inches 

d.b.h./d.r.c. and larger that died from natural causes.

Average annual net growth—Average annual net change in volume of trees 5.0 

inches d.b.h./d.r.c. and larger in the absence of cutting (average annual gross 

growth minus average annual mortality).

Basal area (BA)—The cross-sectional area of a tree stem/bole (trunk) at the point 

where diameter is measured, inclusive of bark. BA is calculated for trees 1.0 

inch and larger in diameter, and is expressed in square feet. For timber species, 

the calculation is based on diameter at breast height (d.b.h.); for woodland species, it is based on diameter at root collar (d.r.c.).

Biomass—The quantity of wood fiber, for trees 1.0 inch d.b.h./d.r.c. and larger, expressed in terms of oven-dry weight. It includes aboveground portions of trees: 

bole/stem (trunk), bark, and branches. Biomass estimates can be computed for 

live and/or dead trees.

Board-foot volume—A unit of measure indicating the amount of wood contained 

in an unfinished board 1 foot wide, 1 foot long, and 1 inch thick. Board-foot 

volume is computed for the sawlog portion of a sawtimber-size tree; the sawlog 

portion includes the part of the bole on sawtimber-size tree from a 1-foot stump 

to a minimum sawlog top of 7 inches diameter outside bark (d.o.b.) for softwoods, or 9 inches d.o.b. for hardwoods. Net board-foot volume is calculated 

as the gross board-foot volume in the sawlog portion of a sawtimber-size tree, 

less deductions for cull (note: board-foot cull deductions are limited to rotten/

missing material and form defect—referred to as the merchantability factor—

board-foot). Board-foot volume estimates are computed in both Scribner and 

International 1⁄4-inch rule, and can be calculated for live and/or dead (standing 

or down) trees.

Census water—Streams, sloughs, estuaries, canals, and other moving bodies of 

water 200 feet wide and greater, and lakes, reservoirs, ponds, and other permanent bodies of water 4.5 acres in area and greater.

Coarse woody debris—Down pieces of wood leaning more than 45 degrees from 

vertical with a diameter of at least 3.0 inches and a length of at least 3.0 feet.

Condition class—The combination of discrete landscape and forest attributers 

that identify, define, and stratify the area associated with a plot. Such attributes 

include reserved status, owner group, forest type, stand-size class, stand origin, 

and tree density.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 87

Crown class—A classification of trees based on dominance in relation to adjacent 

trees in the stand as indicated by crown development and amount of sunlight 

received from above and the sides.

Crown cover (Canopy cover)—The percentage of the ground surface area covered by a vertical projection of plant crowns. Tree crown cover for a sample site 

includes the combined cover of timber and woodland trees 1.0 inch d.b.h./d.r.c. 

and larger. Maximum crown cover for a site is 100 percent; overlapping cover 

is not double counted.

Cubic-foot volume (merchantable)—A unit of measure indicating the amount 

of wood contained in a cube 1-by-1-by-1 foot. Cubic-foot volume is computed 

for the merchantable portion of timber and woodland species; the merchantable 

portion for timber species includes that part of a bole from a 1-foot stump to a 

minimum 4-inch top d.o.b, or above the place(s) of diameter measurement for 

any woodland tree with a single 5.0-inch stem or larger or a cumulative (calculated) d.r.c. of at least 5.0 inches to the 1.5-inch ends of all branches. Net cubicfoot volume is calculated as the gross cubic-foot volume in the merchantable 

portion of a tree, less deductions for cull.

Diameter at breast height (d.b.h.)—The diameter of a tree bole/stem (trunk) 

measured at breast height (4.5 feet above ground), measured outside the bark. 

The point of diameter measurement may vary for abnormally formed trees.

Diameter at root collar (d.r.c.)—The diameter of a tree stem(s) measured at root 

collar or at the point nearest the ground line (whichever is higher) that represents 

the basal area of the tree, measured outside the bark. For multi-stemmed trees, 

d.r.c. is calculated from an equation that incorporates the individual stem diameter measurements. The point of diameter measurement may vary for woodland 

trees with stems that are abnormally formed. With the exception of seedlings, 

woodland stems qualifying for measurement must be at least 1.0 inch in diameter or larger and at least 1.0 foot in length.

Diameter class—A grouping of tree diameters (d.b.h. or d.r.c.) into classes of a 

specified range. For some diameter classes, the number referenced (e.g., 4”, 6”, 

8”) is designated as the midpoint of an individual class range. For example, if 

2-inch classes are specified (the range for an individual class) and even numbers 

are referenced, the 6-inch class would include trees 5.0 to 6.9 inches in diameter.

Diameter outside bark (d.o.b.)—Tree diameter measurement inclusive of the 

outside perimeter of the tree bark. The d.o.b. measurement may be taken at 

various points on a tree (e.g., breast height, tree top) or log, and is sometimes 

estimated.

Field plot/field location—A reference to the sample site or plot; an area containing the field location center and all sample points. A field location consists of 

four subplots and four microplots.

• Subplot—A 1/24-acre fixed-radius area (24-foot horizontal radius) used to 

sample trees 5.0 inches d.b.h./d.r.c. and larger and understory vegetation.
88 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

• Microplot—A 1/300-acre fixed-radius plot (6.8-foot radius), located 12 feet 

from the center of each subplot at an azimuth of 90 degrees, used to inventory 

seedlings and saplings.

Fixed-radius plot—A circular sample plot of a specified horizontal radius: 1/300 

acre = 6.8-foot radius (microplot); 1/24 acre = 24.0-foot radius (subplot).

Forest land—Land that has at least 10 percent cover of live tally tree species of 

any size, or land formerly having such tree cover, and not currently developed 

for a nonforest use. The minimum area for classification as forest land is 1 acre. 

Roadside, stream-side, and shelterbelt strips of trees must be at least 120 feet 

wide to qualify as forest land. Unimproved roads and trails, streams and other 

bodies of water, or natural clearings in forested areas are classified as forest if 

less than 120 feet in width or 1 acre in size. Grazed woodlands, reverting fields, 

and pastures that are not actively maintained are included if the above qualifications are satisfied. (Note that the canopy cover threshold for forest land was 

formerly 5 percent rather than 10 percent, and field crews in New Mexico from 

2008 to 2012 used the 5 percent threshold. However, sampled conditions with 

5-9 percent cover were treated as nonforest for the purposes of this report, and 

forest attributes are therefore based on the new 10-percent threshold.)

Forest type—A classification of forest land based on the species forming a plurality of live-tree stocking.

Gross growth—The annual increase in volume of trees 5.0 inches d.b.h. and larger 

in absence of cutting and mortality. Gross growth includes survivor growth, 

ingrowth, growth on ingrowth, growth on removals before removal, and growth 

on mortality prior to death.

Growing-stock trees—A live timber species, 5.0 inches d.b.h. or larger, with less 

than 2/3 (67 percent) of the merchantable volume cull, and containing at least 

one solid 8-foot section, now or prospectively, reasonably free of form defect, 

on the merchantable portion of the tree.

Growing-stock volume—The cubic-foot volume of sound wood in growing-stock 

trees at least 5.0 inches d.b.h. from a 1-foot stump to a minimum 4-inch top 

d.o.b. to the central stem.

Hardwood trees—Dicotyledonous trees, usually broadleaf and deciduous.

Inventory year—The year in which a plot was scheduled to be completed. Within 

each subpanel, all plots have the same inventory year. Inventory year may differ 

from measurement year.

Land use—The classification of a land condition by use or type.

Litter—The uppermost layer of organic debris on a forest floor; that is, essentially 

the freshly fallen, or only slightly decomposed material, mainly foliage, but also 

bark fragments, twigs, flowers, fruits, and so forth. Humus is the organic layer, 

unrecognizable as to origin, immediately beneath the litter layer from which it 

is derived. Litter and humus together are often termed duff.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 89

Logging residue/products—

• Bolt—A short piece of pulpwood; a short log.

• Industrial wood—All commercial roundwood products, excluding fuelwood.

• Logging residue—The unused sections within the merchantable portions of 

sound (growing-stock) trees cut or killed during logging operations.

• Mill or plant residue—Wood material from mills or other primary manufacturing plants that is not used for the mill’s or plant’s primary products. Mill 

or plant residue includes bark, slabs, edgings, trimmings, miscuts, sawdust, 

and shavings. Much of the mill and plant residue is used as fuel and as the 

raw material for such products as pulp, palletized fuel, fiberwood, mulch, 

and animal bedding. Mill or plant residue includes bark and the following 

components:

• Coarse residue—Wood material suitable for chipping, such as slabs, edgings, 

and trim.

• Fine residue—Wood material unsuitable for chipping, such as sawdust and 

shavings.

• Pulpwood—Roundwood, whole-tree chips, or wood residues that are used for 

the production of wood pulp.

• Roundwood—Logs, bolts, or other round sections cut from trees.

Mapped-plot design—A sampling technique that identifies (delineates or maps) 

and separately classifies distinct “conditions” on the field location sample area. 

Each condition must meet minimum size requirements. At the most basic level, 

condition class delineations include forest land, nonforest land, and water. Forest land conditions can be further subdivided into separate condition classes if 

there are distinct variations in reserved status, owner group, forest type, standsize class, stand origin, and stand density, given that each distinct area meets 

minimum size requirements.

Measurement year—The year in which a plot was completed. Measurement year 

may differ from inventory year.

Merchantable portion—For trees measured at d.b.h. and 5.0 inches d.b.h. and 

larger, the merchantable portion (or “merchantable bole”) includes the part of 

the tree bole from a 1-foot stump to a 4.0-inch top (d.o.b.). For trees measured 

at d.r.c., the merchantable portion includes all qualifying segments above the 

place(s) of diameter measurement for any tree with a single 5.0-inch stem or 

larger or a cumulative (calculated) d.r.c. of at least 5.0 inches to the 1.5-inch 

ends of all branches; sections below the place(s) of diameter measurement are 

not included. Qualifying segments are stems or branches that are a minimum of 

1 foot in length and at least 1.0 inch in diameter; portions of stems or branches 

smaller than 1.0 inch in diameter, such as branch tips, are not included in the 

merchantable portion of the tree.
90 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Mortality tree—All standing or down dead trees 5.0 inches d.b.h./d.r.c. and larger 

that were alive within the previous 5 years (in most States); for the 2008–2012 

New Mexico inventory, this includes trees that were alive within the previous 

10 years.

National Forest System (NFS) lands—Public lands administered by the Forest 

Service, U.S. Department of Agriculture, such as National Forests, National 

Grasslands, and some National Recreation Areas.

National Park lands—Public lands administered by the Park Service, U.S. Department of the Interior, such as National Parks, National Monuments, National 

Historic Sites (such as National Memorials and National Battlefields), and some 

National Recreation Areas.

Noncensus water—Portions of rivers, streams, sloughs, estuaries, and canals that 

are 30 to 200 feet wide and at least 1 acre in size; and lakes, reservoirs, and 

ponds 1 to 4.5 acres in size. Portions of rivers and streams not meeting the criteria for census water, but at least 30 feet wide and 1 acre in size, are considered 

noncensus water. Portions of braided streams not meeting the criteria for census 

water, but at least 30 feet in width and 1 acre in size, and more than 50 percent 

water at normal high-water level are also considered noncensus water.

Nonforest land—Land that does not support, or has never supported, forests, and 

lands formerly forested where tree regeneration is precluded by development 

for other uses. Includes areas used for crops, improved pasture, residential areas, city parks, improved roads of any width and adjoining rights-of-way, power 

line clearings of any width, and noncensus water. If intermingled in forest areas, 

unimproved roads and nonforest strips must be more than 120 feet wide, and 

clearings, etc., more than 1 acre in size, to qualify as nonforest land.

Nonstocked stand—A formerly stocked stand that currently has less than 10 percent stocking, but has the potential to again become 10 percent stocked. For 

example, recently harvested, burned, or windthrow-damaged areas.

Other Federal lands—Public lands administered by Federal agencies other than 

the Forest Service, U.S. Department of Agriculture, or the Bureau of Land Management, U.S. Department of the Interior.

Other public lands—Public lands administered by agencies other than the Forest 

Service, U.S. Department of Agriculture. Includes lands administered by other 

Federal, State, county, and local government agencies, including lands leased 

by these agencies for more than 50 years.

Poletimber-size trees—For trees measured at d.b.h, softwoods 5.0 to 8.9 inches 

d.b.h. and hardwoods 5.0 to 10.9 inches d.b.h. For trees measured at d.r.c., all 

live trees 5.0 to 8.9 inches d.r.c.

Primary wood processing plants—An industrial plant that processes roundwood 

products, such as sawlogs, pulpwood bolts, or veneer logs.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 91

Private lands—All lands not owned or managed by a Federal, State, or other public entity, including lands owned by corporations, trusts, or individuals, as well 

as Tribal lands.

Productive forest land—Forest land capable of producing 20 cubic feet per acre 

per year of wood from trees classified as a timber species (see Appendix D) on 

forest land classified as a timber forest type (see Appendix C).

Productivity—The potential yield capability of a stand calculated as a function 

of site index (expressed in terms of cubic-foot growth per acre per year at age 

of culmination of mean annual increment). Productivity values for forest land 

provide an indication of biological potential. Timberland stands are classified 

by the potential net annual growth attainable in fully stocked natural stands. For 

FIA reporting, Productivity Class is a variable that groups stand productivity 

values into categories of a specified range. Productivity is sometimes referred to 

as “yield” or “mean annual increment.”

Removals—The net volume of sound (growing-stock) trees removed from the 

inventory by harvesting or other cultural operations (such as timber-stand improvement), by land clearing, or by changes in land use (such as a Wilderness 

designation).

Reserved land—Land withdrawn from management for production of wood products through statute or administrative designation; examples include Wilderness 

areas and National Parks and Monuments.

Sampling error—A statistical term used to describe the accuracy of the inventory estimates. Expressed on a percentage basis in order to enable comparisons 

between the precision of different estimates, sampling errors are computed by 

dividing the estimate into the square root of its variance.

Sapling—A live tree 1.0–4.9 inches d.b.h./d.r.c.

Sawlog portion—The part of the bole of sawtimber-size trees between a 1-foot 

stump and the sawlog top.

Sawlog top—The point on the bole of sawtimber-size trees above which a sawlog 

cannot be produced. The minimum sawlog top is 7 inches d.o.b. for softwoods, 

and 9 inches d.o.b. for hardwoods.

Sawtimber-size trees—Softwoods 9.0 inches d.b.h. and larger and hardwoods 

11.0 inches and larger.

Sawtimber volume—The growing-stock volume in the sawlog portion of sawtimber-size trees in board feet.

Seedlings—Live trees less than 1.0 inch d.b.h./d.r.c.

Site index—A measure of forest productivity for a timberland tree/stand. Expressed in terms of the expected height (in feet) of trees on the site at an index 

age of 50 (or 80 years for aspen and cottonwood). Calculated from height-toage equations.
92 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Site tree—A tree used to provide an index of site quality. Timber species selected 

for site index calculations must meet specified criteria with regards to age, diameter, crown class, and damage.

Snag—A standing dead tree.

Softwood trees—Coniferous trees, usually evergreen, having needle- or scale-like 

leaves.

Stand—A community of trees that can be distinguished from adjacent communities due to similarities and uniformity in tree and site characteristics, such as 

age-class distribution, species composition, spatial arrangement, structure, etc.

Stand density—A relative measure that quantifies the relationship between trees 

per acre, stand basal area, average stand diameter, and stocking of a forested 

stand.

Stand density index (SDI)—A widely used measure developed by Reineke 

(1933), and is an index that expresses relative stand density based on a comparison of measured stand values with some standard condition; relative stand density is the ratio, proportion, or percent of absolute stand density to a reference 

level defined by some standard level of competition. For FIA reporting, the SDI 

for a site is usually presented as a percentage of the maximum SDI for the forest type. Site SDI values are sometimes grouped into SDI classes of a specified 

percentage range. Maximum SDI values vary by species and region.

Standing dead tree—To qualify as a standing dead tally tree, dead trees must be 

at least 5.0 inches in diameter, have a bole that has an unbroken actual length of 

at least 4.5 feet, and lean less than 45 degrees from vertical as measured from 

the base of the tree to 4.5 feet. Portions of boles on dead trees that are separated 

greater than 50 percent (either above or below 4.5 feet), are considered severed and are included in Down Woody Material (DWM) if they otherwise meet 

DWM tally criteria. For western woodland species with multiple stems, a tree is 

considered down if more than 2/3 of the volume is no longer attached or upright; 

do not consider cut and removed volume. For western woodland species with 

single stems to qualify as a standing dead tally tree, dead trees must be at least 

5.0 inches in diameter, be at least 1.0 foot in unbroken actual length, and lean 

less than 45 degrees from vertical.

Stand-size class—A classification of forest land based on the predominant diameter size of live trees presently forming the plurality of live-tree stocking. 

Classes are defined as follows:

• Sawtimber stand (Large-tree stand)—A stand at least 10 percent stocked 

with live trees, in which half or more of the total stocking is from live trees 

5.0 inches or larger in diameter, and with sawtimber (large tree) stocking 

equal to or greater than poletimber (medium tree) stocking.

• Poletimber stand (Medium-tree stand)—A stand at least 10 percent stocked 

with live trees, in which half or more of the total stocking is from live trees 
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 93

5.0 inches or larger in diameter, and with poletimber (medium tree) stocking 

exceeding sawtimber (large tree) stocking.

• Sapling/seedling stand—A stand at least 10 percent stocked with live trees, in 

which half or more of the total stocking is from live trees less than 5.0 inches 

in diameter.

• Nonstocked stand—A formerly stocked stand that currently has less than 10 

percent stocking, but has the potential to again become 10 percent stocked. 

For example, recently harvested, burned, or windthrow-damaged areas.

Stocking—An expression of the extent to which growing space is effectively utilized by live trees.

Timber species—Tally tree species traditionally used for industrial wood products. These include all species of conifers, except pinyon and juniper. Diameters 

for timber species are measured at breast height (d.b.h.).

Timber-stand improvement—A term comprising all intermediate cuttings or 

treatments, such as thinning, pruning, release cutting, girdling, weeding, or poisoning, made to improve the composition, health, and growth of the remaining 

trees in the stand.

Timberland—Unreserved forest land capable of producing 20 cubic feet per acre 

per year of wood from trees classified as a timber species (see Appendix D) on 

forest land designated as a timber forest type (see Appendix C).

Unproductive forest land—Forest land not capable of producing 20 cubic feet 

per acre per year of wood from trees classified as a timber species (see Appendix 

D) on forest land designated as a timber forest type and all forest lands designated as a woodland forest type (see Appendix C).

Unreserved forest land—Forest land not withdrawn from management for production of wood products through statute or administrative designation.

Wilderness area—An area of undeveloped land currently included in the Wilderness System, managed to preserve its natural conditions and retain its primeval 

character and influence.

Woodland species—Tally tree species that are not usually converted into industrial wood products. Common uses of woodland trees are fuelwood, fenceposts, 

and Christmas trees. These species include pinyon, juniper, mesquite, locust, 

mountain-mahogany (Cercocarpus spp.), Rocky Mountain maple, bigtooth maple, desert ironwood, and most oaks (note: bur oak and chinkapin oak are classified as timber species). Because most woodland trees are extremely variable 

in form, diameter is measured at root collar (d.r.c.).
94 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Appendix B: Standard Forest Resource Tables

Percentage of sample

Accessible forest land

Unreserved forest land

Timberland 3.9

Unproductive 18.1

Total unreserved forest land 22.0

Reserved forest land

Productive 0.3

Unproductive 1.9

Total reserved forest land 2.2

Total accessible forest land 24.2

Nonforest and other areas

Nonforest land 70.7

Water 0.3

Census 0.2

Non-Census 0.1

Total nonforest and other areas 71.0

Non-response

Access denied 3.5

Hazardous conditions 0.7

Other 0.7

Total non-response 4.8

All land 100.0

Forest 3186 26.0%

Nonforest 8513 69.4%

Nonsampled 575 4.7%

12274 100.0%

Land status

Table B1—Percentage of plot area by land status, Arizona, 2005–2014.

All table cells without observations in the inventory sample are indicated by --. 

Table value of 0.0 indicates the percentage rounds to less 

than 0.1 percent. Columns and rows may not add to their totals due to rounding.

Owner class Timberland Unproductive Total Productive Unproductive Total

Forest Service

National Forest 2,182.1 4,838.8 7,020.8 111.9 554.4 666.3 7,687.1

Other Federal

National Park Service - - - - - - 84.2 286.8 371.0 371.0

Bureau of Land Management 5.7 828.5 834.2 12.2 563.0 575.3 1,409.5

U.S. Fish and Wildlife Service - - - - - - - - 86.2 86.2 86.2

Departments of Defense and Energy 26.8 34.3 61.1 - - - - - - 61.1

Other Federal - - 6.2 6.2 - - - - - - 6.2

State and local government

State 7.5 1,616.3 1,623.8 - - - - - - 1,623.8

County and municipal 0.6 48.5 49.1 - - - - - - 49.1

Private

Undifferentiated private 766.0 6,546.4 7,312.4 - - - - - - 7,312.4

All owners 2,988.6 13,919.0 16,907.6 208.4 1,490.3 1,698.7 18,606.4

Unreserved forests Reserved forests

All forest land

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less than 0.1 thousand acres. 

Columns and rows may not add to their totals due to rounding.

Table B2—Area of accessible forest land, in thousand acres, by owner class and forest land status, Arizona, 2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 95

Other Other Other Other All

Timber- forest Timber- forest Timber- forest Timber- forest forest

Forest-type group land land land land land land land land land

Pinyon / juniper group - - 3,670.0 - - 1,337.8 - - 831.0 - - 5,245.3 11,084.1 

Douglas-fir group 135.2 26.1 - - 9.0 - - - - 58.4 - - 228.7 

Ponderosa pine group 1,716.4 38.0 25.3 60.6 6.6 - - 557.9 2,411.3 6.5

Fir / spruce / mountain hemlock group 115.2 12.1 - - 6.0 - - - - 87.7 - - 221.0 

Other western softwoods group 11.9 18.1 1.5 - - - - - - - - 6.5 38.1 

Elm / ash / cottonwood group 9.1 6.1 5.7 - - - - 4.6 15.6 - - 41.1 

Aspen / birch group 90.7 17.0 - - 20.9 - - - - 24.7 - - 153.2 

Other	hardwoods	group 2.8 4.9 - - 6.9 1.5 4.8 - - 6.4 27.3 

Woodland hardwoods group - - 1,484.2 - - 366.0 - - 727.0 - - 1,056.1 3,633.3 

Nonstocked 100.7 228.5 - - 94.3 - - 97.5 21.7 225.6 768.4 

All forest-type groups 2,182.1 5,505.1 32.5 1,901.5 8.1 1,664.8 766.0 6,546.4 18,606.4 

Table B4—Area of accessible forest land, in thousand acres, by forest type group, ownership group, and land status, Arizona, 2005–2014.

Forest Service Other Federal

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less than 0.1 thousand acres. 

Columns and rows may not add to their totals due to rounding.

State and local

government

Undifferentiated

private

Forest-type group 0-19 20-49 50-84 85-119 120-164 165-224 225+

Pinyon / juniper group 11,084.1 - - - - - - - - - - - - 11,084.1 

Douglas-fir group - - 51.7 170.8 6.2 - - - - - - 228.7 

Ponderosa pine group 1,870.8 6.5 528.0 6.0 - - - - - - 2,411.3 

Fir / spruce / mountain hemlock group - - 32.0 136.2 45.0 7.7 - - - - 221.0 

Other western softwoods group 11.7 20.3 6.0 - - - - - - - - 38.1 

Elm / ash / cottonwood group 10.7 16.4 9.1 4.9 - - - - - - 41.1 

Aspen / birch group 4.6 85.3 60.4 3.0 - - - - - - 153.2 

Other	hardwoods	group 23.0 4.3 - - - - - - - - - - 27.3 

Woodland hardwoods group 3,633.3 - - - - - - - - - - - - 3,633.3 

Nonstocked 635.5 100.9 32.1 - - - - - - - 768.4 

All forest-type groups 15,409.4 2,181.7 942.6 65.0 7.7 - - - - 18,606.4 

Site-productivity class (cubic feet/acre/year) Total all 

classes

Table B3—Area of accessible forest land, in thousand acres, by forest type group and productivity class, Arizona, 2005–2014.

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less 

than 0.1 thousand acres. Columns and rows may not add to their totals due to rounding.

Large Medium Small All size

Forest-type group diameter diameter diameter Nonstocked classes

Pinyon / juniper group 9,976.3 727.0 380.8 - - 11,084.1 

Douglas-fir group 228.7 - - - - - - 228.7

Ponderosa pine group 2,289.6 50.0 71.6 - - 2,411.3

Fir / spruce / mountain hemlock group 220.1 - - 0.9 - - 221.0

Other western softwoods group 38.1 - - - - - - 38.1

Elm / ash / cottonwood group 36.4 1.6 3.0 - - 41.1

Aspen / birch group 54.8 7.7 90.8 - - 153.2

Other hardwoods group 13.2 11.3 2.8 - - 27.3

Woodland hardwoods group 2,137.4 633.8 862.0 - - 3,633.3

Nonstocked - - - - - - 768.4 768.4

All forest-type groups 14,994.5 1,431.5 1,412.0 768.4 18,606.4

Stand-size class

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less 

than 0.1 thousand acres. Columns and rows may not add to their totals due to rounding.

Table B5—Area of accessible forest land, in thousand acres, by forest type group and stand-size class, Arizona, 2005–2014.
96 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 Non- All Forest-type group stocked 1-20 21-40 41-60 61-80 81-100 101-120 121-140 141-160 161-180 181-200 201+ classes Pinyon / juniper group - - 155.1 210.9 416.3 879.2 1,250.5 1,408.8 1,371.7 1,277.9 1,093.9 786.8 2,233.0 11,084.1 Douglas-fir group - - - - - - - - 48.9 44.3 73.0 42.4 12.3 6.2 - - 1.6 228.7 Ponderosa pine group - - 48.8 34.7 55.1 346.3 838.5 651.1 186.3 103.8 73.5 24.1 49.1 2,411.3 Fir / spruce / mountain hemlock group - - 0.9 - - - - 30.9 76.3 43.1 31.8 7.5 24.3 6.2 - - 221.0 Other western softwoods group - - - - - - - - - - 6.0 6.0 8.4 - - 17.7 - - - - 38.1 Elm / ash / cottonwood group - - 3.0 - - 6.5 15.3 6.1 - - 5.7 4.5 - - - - - - 41.1 Aspen / birch group - - 84.6 6.2 7.7 6.2 12.1 24.5 11.9 - - - - - - - - 153.2 Other hardwoods group - - 2.8 6.4 - - - - 4.9 1.5 - - - - - - - - 11.7 27.3 Woodland hardwoods group - - 574.3 247.4 168.7 332.1 481.6 441.8 331.3 424.8 293.5 102.7 235.1 3,633.3 Nonstocked 768.4 - - - - - - - - - - - - - - - - - - - - - - 768.4 All forest-type groups 768.4 869.6 505.6 654.3 1,658.8 2,720.3 2,649.8 1,989.6 1,830.6 1,509.0 919.7 2,530.5 18,606.4 Stand-age class (years) Table B6— All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less than 0.1 thousand acres. Columns and rows may not add to their totals due to rounding. Area of accessible forest land, in thousand acres, by forest type group and stand-age class, Arizona, 2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 97

Natural Artificial All forest

Forest-type group stands regeneration land

Pinyon / juniper group 11,084.1 - - 11,084.1

Douglas-fir group 228.7 - - 228.7

Ponderosa pine group 2,411.3 - - 2,411.3

Fir / spruce / mountain hemlock group 221.0 - - 221.0

Other western softwoods group 38.1 - - 38.1

Elm / ash / cottonwood group 41.1 - - 41.1

Aspen / birch group 153.2 - - 153.2

Other hardwoods group 27.3 - - 27.3

Woodland hardwoods group 3,633.3 - - 3,633.3

Nonstocked 768.4 - - 768.4

All forest-type groups 18,606.4 - - 18,606.4

Stand origin

All table cells without observations in the inventory sample are indicated by --. 

Table value of 0.0 indicates the acres round to less than 0.1 thousand acres. 

Columns and rows may not add to their totals due to rounding.

Table B7—Area of accessible forest land, in thousand acres, by forest type group and stand origin, 

Arizona, 2005–2014.

Wild Domestic All forest

Forest-type group None Insects Disease Fire animals animals Weather Vegetation Other Human Geological land

Pinyon / juniper group 9,702.8 727.9 199.5 179.2 - - 30.0 190.2 - - - - 54.4 - - 11,084.1

Douglas-fir group 154.1 12.7 20.9 41.0 - - - - - - - - - - - - - - 228.7

Ponderosa pine group 1,909.9 79.0 54.6 332.9 - - - - 24.0 - - - - 10.9 - - 2,411.3

Fir / spruce / mountain hemlock group 164.7 19.7 36.6 - - - - - - - - - - - - - - - - 221.0

Other western softwoods group 38.1 - - - - - - - - - - - - - - - - - - - - 38.1

Elm / ash / cottonwood group 25.8 - - - - 9.1 - - - - 6.2 - - - - - - - - 41.1

Aspen / birch group 94.8 - - 6.2 46.2 - - - - 6.0 - - - - - - - - 153.2

Other hardwoods group 24.5 - - - - - - - - - - 2.8 - - - - - - - - 27.3

Woodland hardwoods group 2,958.0 63.9 28.9 386.1 - - 62.2 134.3 - - - - - - - - 3,633.3

Nonstocked 503.3 3.1 6.0 223.0 - - - - 18.1 - - 6.0 8.9 - - 768.4

All forest-type groups 15,575.8 906.3 352.7 1,217.6 - - 92.2 381.6 - - 6.0 74.2 - - 18,606.4

Disturbance class

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less than 0.1 thousand acres. 

Columns and rows may not add to their totals due to rounding.

Table B8—Area of forest land. In thousand acres, by forest type group and primary disturbance class, Arizona, 2005–2014.

Large Medium Small All size

Forest-type group diameter diameter diameter Nonstocked classes

Douglas-fir group 193.6 - - - - - - 193.6

Ponderosa pine group 2,191.0 50.0 65.1 - - 2,306.2

Fir / spruce / mountain hemlock group 201.9 - - 0.9 - - 202.9

Other western softwoods group 13.4 - - - - - - 13.4

Elm / ash / cottonwood group 27.4 - - 3.0 - - 30.4

Aspen / birch group 48.8 7.7 58.9 - - 115.4

Other hardwoods group 1.5 - - 2.8 - - 4.3

Nonstocked - - - - - - 122.5 122.5

All forest-type groups 2,677.7 57.7 130.8 122.5 2,988.6

Stand-size class

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less 

than 0.1 thousand acres. Columns and rows may not add to their totals due to rounding.

Table B9—Area of timberland, in thousand acres, by forest type group and stand-size class, Arizona, 2005–2014.
98 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 1.0- 3.0- 5.0- 7.0- 9.0- 11.0- 13.0- 15.0- 17.0- 19.0- 21.0- 25.0- 29.0- 33.0- All Species group 2.9 4.9 6.9 8.9 10.9 12.9 14.9 16.9 18.9 20.9 24.9 28.9 32.9 36.9 37.0+ classes Douglas-fir 30,249 12,396 11,809 9,419 6,721 5,108 3,757 2,662 1,962 840 1,103 412 110 74 37 86,659 Ponderosa and Jeffrey pine 155,865 119,223 75,444 69,746 55,591 44,301 30,219 18,716 13,323 8,431 9,877 3,042 985 144 182 605,089 True fir 34,050 14,125 6,308 5,136 3,490 2,128 1,728 1,097 740 588 549 36 113 - - - - 70,088 Engelmann and other spruces 20,047 11,408 4,300 3,264 3,086 1,947 1,360 1,170 657 404 550 36 - - - - - - 48,229 Other western softwoods 14,777 7,732 3,552 2,995 2,144 1,656 824 713 177 219 259 36 36 - - - - 35,119 Western woodland softwoods 644,485 331,675 247,269 196,016 142,327 108,111 74,480 52,383 35,500 26,041 30,167 12,035 6,059 1,442 1,265 1,909,257 899,474 496,559 348,682 286,577 213,359 163,252 112,367 76,740 52,358 36,523 42,506 15,597 7,303 1,660 1,484 2,754,441 Cottonwood and aspen 65,820 18,123 6,589 3,784 3,831 3,311 2,574 1,579 703 149 329 108 - 36 36 106,971 Other western hardwoods 8,751 3,480 1,114 187 146 335 254 73 114 72 - - - - - - - - - - 14,526 Western woodland hardwoods 581,231 216,344 132,566 90,744 52,407 29,679 17,413 9,690 5,663 2,631 3,140 1,096 458 37 - - 1,143,100 655,802 237,947 140,270 94,714 56,384 33,325 20,241 11,342 6,480 2,852 3,468 1,204 458 73 36 1,264,597 1,555,275 734,506 488,951 381,291 269,743 196,576 132,609 88,082 58,838 39,375 45,974 16,802 7,761 1,732 1,520 4,019,038 Western hardwood species groups Other Table B10— All softwoods Hardwood species groups All hardwoods All species groups All table cells without observations in the inventory sample are indicated by --. Table value of 0 indicates the number of trees rounds to less than 1 thousand trees. Columns and rows may not add to their totals due to rounding. Number of live trees (at least 1 inch d.b.h./d.r.c.), in thousand trees, on forest land by species group and diameter class, Arizona, 2005–2014. Diameter class (inches) Softwood species groups Western softwood species groups Other 5.0- 7.0- 9.0- 11.0- 13.0- 15.0- 17.0- 19.0- 21.0- 25.0- 29.0- 33.0- All Species group 6.9 8.9 10.9 12.9 14.9 16.9 18.9 20.9 24.9 28.9 32.9 36.9 37.0+ classes Douglas-fir 9,370 7,540 4,866 3,667 2,624 2,370 1,593 768 919 299 37 74 37 34,163 Ponderosa and Jeffrey pine 57,287 56,177 45,737 37,062 25,732 15,572 11,369 6,922 8,118 2,607 836 144 146 267,708 True fir 5,445 4,478 3,055 1,766 1,547 953 702 336 370 36 113 - - - - 18,801 Engelmann and other spruces 3,973 2,829 2,723 1,512 1,141 953 549 367 514 36 - - - - - - 14,596 Other western softwoods 1,528 1,529 1,056 908 365 400 109 147 183 36 36 - - - - 6,297 77,602 72,552 57,437 44,915 31,409 20,248 14,322 8,539 10,104 3,014 1,022 218 183 341,565 Cottonwood and aspen 5,334 3,314 3,246 2,983 2,283 1,253 588 149 184 36 - - 36 36 19,442 Other western hardwoods 112 76 110 - - 37 - - - - - - - - - - - - - - - - 334 5,446 3,390 3,356 2,983 2,319 1,253 588 149 184 36 - - 36 36 19,776 83,047 75,942 60,793 47,899 33,728 21,501 14,911 8,688 10,288 3,050 1,022 254 219 361,341 Table B11— All hardwoods All species groups All table cells without observations in the inventory sample are indicated by --. Table value of 0 indicates the number of trees rounds to less than 1 thousand trees. Columns and rows may not add to their totals due to rounding. Number of growing stock trees (at least 5 inches d.b.h.), in thousand trees, on timberland by species group and diameter class, Arizona, 2005–2014. Diameter class (inches) Softwood species groups Western softwood species groups All softwoods Hardwood species groups Western hardwood species groups
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 99

All

Owner class Timberland Unproductive Total Productive Unproductive Total forest land

Forest Service

National Forest 4,332.4 2,859.6 7,192.1 195.5 298.0 493.4 7,685.5

Other Federal

National Park Service - - - - - - 200.4 161.9 362.4 362.4

Bureau of Land Management 26.7 260.9 287.6 16.7 321.2 338.0 625.6

U.S.	Fish	and	Wildlife	Service - - - - - - - - 20.3 20.3 20.3

Departments	of	Defense	and	Energy 51.5 11.4 62.9 - - - - - - 62.9

Other Federal - - 0.1 0.1 - - - - - - 0.1

State and local government

State 8.1 481.7 489.8 - - - - - - 489.8

County and municipal 3.4 4.7 8.1 - - - - - - 8.1

Private

Undifferentiated private 1,777.2 3,522.3 5,299.5 - - - - - - 5,299.5

All owners 6,199.3 7,140.8 13,340.0 412.6 801.5 1,214.1 14,554.1

Unreserved forests Reserved forests

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million cubic 

feet. Columns and rows may not add to their totals due to rounding.

Table B12—Net volume of all live trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, by owner class and forest land status, Arizona, 2005–2014.

Large Medium Small All size

Forest-type group diameter diameter diameter Nonstocked classes

Pinyon / juniper group 6,103.5 217.8 22.6 - - 6,343.9

Douglas-fir group 649.8 - - - - - - 649.8

Ponderosa pine group 4,913.5 47.8 19.4 - - 4,980.7

Fir / spruce / mountain hemlock group 565.6 - - 0.0 - - 565.7

Other western softwoods group 67.7 - - - - - - 67.7

Elm / ash / cottonwood group 69.4 0.2 0.1 - - 69.7

Aspen / birch group 234.9 26.1 25.2 - - 286.2

Other hardwoods group 1.8 1.7 - - - - 3.5

Woodland hardwoods group 1,305.8 218.2 44.1 - - 1,568.1

Nonstocked - - - - - - 19.0 19.0

All forest-type groups 13,912.0 511.8 111.3 19.0 14,554.1

Stand-size class

All table cells without observations in the inventory sample are indicated by --. 

Table value of 0.0 indicates the volume rounds to less than 0.1 million cubic feet. 

Columns and rows may not add to their totals due to rounding.

Table B13—Net volume of all live trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, on forest land by forest type 

group and stand-size class, Arizona, 2005–2014.
100 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Forest Other State and local Undifferentiated All

Species group Service Federal government private owners

Douglas-fir 444.5 13.9 - - 200.3 658.7

Ponderosa and Jeffrey pine 3,676.9 203.9 11.5 1,353.5 5,245.8

True fir 192.6 19.9 - - 75.2 287.8

Engelmann and other spruces 145.3 28.2 - - 130.0 303.5

Other western softwoods 106.8 9.8 - - 18.1 134.6

Western woodland softwoods 2,141.6 670.1 374.7 2,866.7 6,053.0

6,707.7 945.6 386.2 4,643.8 12,683.4

Cottonwood and aspen 144.0 36.7 0.7 201.4 382.7

Other western hardwoods 11.7 0.1 0.8 4.4 17.0

Western woodland hardwoods 822.2 88.8 110.2 449.9 1,471.1

977.8 125.6 111.7 655.7 1,870.8

7,685.5 1,071.2 497.9 5,299.5 14,554.1

Western hardwood species groups

Other

All hardwoods

All species groups 

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million 

cubic feet. Columns and rows may not add to their totals due to rounding.

All softwoods

Hardwood species groups

Table B14—Net volume of all live trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, on forest land by species group and ownership group, 

Arizona, 2005–2014.

Ownership group

Softwood species groups

Western softwood species groups

Other

5.0- 7.0- 9.0- 11.0- 13.0- 15.0- 17.0- 19.0- 21.0- 25.0- 29.0- 33.0- All

Species group 6.9 8.9 10.9 12.9 14.9 16.9 18.9 20.9 24.9 28.9 32.9 36.9 37.0+ classes

Douglas-fir 23 41 58 71 79 82 80 46 86 49 16 14 13 659

Ponderosa and Jeffrey pine 123 298 454 605 629 554 549 469 892 406 170 33 64 5,246

True fir 10 22 28 30 38 36 32 31 43 3 15 - - - - 288

Engelmann and other spruces 8 17 33 33 37 47 35 31 58 4 - - - - - - 303

Other western softwoods 5 13 16 20 17 18 5 10 19 5 8 - - - - 135

Western woodland softwoods 297 504 649 750 704 641 531 463 686 384 258 74 112 6,053

466 896 1,239 1,509 1,503 1,377 1,233 1,049 1,783 852 467 121 188 12,683

Cottonwood and aspen 14 26 48 67 75 63 36 9 21 8 - - 8 8 383

Other western hardwoods 1 1 2 4 3 1 3 2 - - - - - - - - - - 17

Western woodland hardwoods 160 222 222 205 175 137 108 59 101 52 26 3 - - 1,471

176 249 272 276 254 202 147 70 122 60 26 10 8 1,871

All species groups 642 1,144 1,511 1,784 1,757 1,579 1,380 1,119 1,905 912 493 131 196 14,554

Western hardwood species groups

Other

All hardwoods

All table cells without observations in the inventory sample are indicated by --. Table value of 0 indicates the volume rounds to less than 1 million cubic 

feet. Columns and rows may not add to their totals due to rounding.

Table B15—Net volume of all live trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, on forest land by species group and diameter class, 

Arizona, 2005–2014.

Diameter class (inches)

Softwood species groups

Western softwood species groups

Other

All softwoods

Hardwood species groups
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 101

Natural Artificial All forest

Forest-type group stands regeneration land

Pinyon / juniper group 6,343.9 - - 6,343.9

Douglas-fir group 649.8 - - 649.8

Ponderosa pine group 4,980.7 - - 4,980.7

Fir / spruce / mountain hemlock group 565.7 - - 565.7

Other western softwoods group 67.7 - - 67.7

Elm / ash / cottonwood group 69.7 - - 69.7

Aspen / birch group 286.2 - - 286.2

Other	hardwoods	group 3.462662 - - 3.462662

Woodland hardwoods group 1,568.1 - - 1,568.1

Nonstocked 19.0 - - 19.0

All forest-type groups 14,554.1 - - 14,554.1

Stand origin

All table cells without observations in the inventory sample are indicated by --. 

Table value of 0.0 indicates the volume rounds to less than 0.1 million cubic feet. 

Columns and rows may not add to their totals due to rounding.

Table B16—Net volume of all live trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, on 

forest land by forest type group and stand origin, Arizona, 2005–2014.
102 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 5.0- 7.0- 9.0- 11.0- 13.0- 15.0- 17.0- 19.0- 21.0- 25.0- 29.0- 33.0- All Species group 6.9 8.9 10.9 12.9 14.9 16.9 18.9 20.9 24.9 28.9 32.9 36.9 37.0+ classes Douglas-fir 18 33 44 51 57 75 68 42 72 36 7 14 13 528 Ponderosa and Jeffrey pine 97 246 379 513 542 467 475 388 742 351 148 33 52 4,434 True fir 9 20 25 25 35 32 31 19 28 3 15 - - - - 241 Engelmann and other spruces 7 15 29 26 30 39 30 27 55 4 - - - - - - 262 Other western softwoods 3 7 9 11 8 11 4 7 13 5 8 - - - - 86 134 320 486 626 671 623 607 483 910 398 178 47 65 5,551 Cottonwood and aspen 12 24 45 62 71 51 32 9 13 4 - - 8 8 339 Other western hardwoods 0 0 1 - - 0 - - - - - - - - - - - - - - - - 2 12 24 46 62 71 51 32 9 13 4 - - 8 8 341 147 344 533 688 742 675 639 492 924 402 178 55 74 5,891 Table B17— All hardwoods All species groups All table cells without observations in the inventory sample are indicated by --. Table value of 0 indicates the volume rounds to less than 1 million cubic feet. Columns and rows may not add to their totals due to rounding. Net volume of growing stock trees (at least 5 inches d.b.h.), in million cubic feet, on timberland by species group and diameter class, Arizona, 2005–2014. Diameter class (inches) Softwood species groups Western softwood species groups All softwoods Hardwood species groups Western hardwood species groups
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 103

9.0- 11.0- 13.0- 15.0- 17.0- 19.0- 21.0- 25.0- 29.0- 33.0- All

Species group 10.9 12.9 14.9 16.9 18.9 20.9 24.9 28.9 32.9 36.9 37.0+ classes

Douglas-fir 155 222 281 397 375 242 413 210 44 90 81 2,511

Ponderosa and Jeffrey pine 1,295 2,231 2,680 2,487 2,666 2,266 4,552 2,174 949 213 312 21,825

True fir 87 108 166 161 161 98 152 11 81 - - - - 1,024

Engelmann and other spruces 125 128 165 217 170 158 318 23 - - - - - - 1,304

Other western softwoods 30 44 36 55 23 39 78 33 42 - - - - 381

1,691 2,733 3,327 3,318 3,396 2,804 5,514 2,450 1,116 302 393 27,045

Cottonwood and aspen - - 325 405 307 186 47 82 21 - - 45 51 1,468

Other western hardwoods - - - 1 - - - - - - - - - - - - - - - - 1

- - 325 406 307 186 47 82 21 - - 45 51 1,469

1,691 3,058 3,733 3,625 3,582 2,851 5,596 2,471 1,116 347 444 28,514

All hardwoods

All species groups 

All table cells without observations in the inventory sample are indicated by --. Table value of 0 indicates the volume rounds to less than 1 million board 

feet. Columns and rows may not add to their totals due to rounding.

Table B19—Net volume of sawtimber trees, in million board feet (International 1/4 inch rule), on timberland by species group and diameter class, Arizona, 

2005–2014.

Diameter class (inches)

Softwood species groups

Western softwood species groups

All softwoods

Hardwood species groups

Western hardwood species groups

Forest Other State and local Undifferentiated All

Species group Service Federal government private owners

Douglas-fir 356.6 - - - - 171.2 527.8

Ponderosa and Jeffrey pine 3,263.3 45.3 11.5 1,114.3 4,434.4

True fir 167.1 - - - - 73.6 240.7

Engelmann and other spruces 131.5 - - - - 130.0 261.6

Other western softwoods 69.0 4.4 - - 12.7 86.1

3,987.5 49.6 11.5 1,501.9 5,550.5

Cottonwood and aspen 121.2 26.0 - - 191.6 338.9

Other western hardwoods 1.9 - - - - 0.2 2.1

123.1 26.0 - - 191.8 340.9

4,110.6 75.7 11.5 1,693.7 5,891.4

69.8% 1.3% 0.2% 28.7% 100.0%

All hardwoods

All species groups 

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million 

cubic feet. Columns and rows may not add to their totals due to rounding.

Table B18—Net volume of growing stock trees (at least 5 inches d.b.h.), in million cubic feet, on timberland by species group and ownership group, 

Arizona, 2005–2014.

Ownership group

Softwood species groups

Western softwood species groups

All softwoods

Hardwood species groups

Western hardwood species groups
104 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Forest Other State and local Undifferentiated All

Species group Service Federal government private owners

Douglas-fir 311.6 - - - - 148.8 460.4

Ponderosa and Jeffrey pine 2,913.5 41.6 11.0 1,016.3 3,982.4

True fir 142.2 - - - - 61.4 203.6

Engelmann and other spruces 116.2 - - - - 113.0 229.2

Other western softwoods 59.0 4.3 - - 10.4 73.8

3,542.6 45.9 11.0 1,349.9 4,949.3

Cottonwood and aspen 76.4 22.7 - - 126.6 225.6

Other western hardwoods 0.3 - - - - - - 0.3

76.7 22.7 - - 126.6 226.0

3,619.3 68.6 11.0 1,476.4 5,175.3

All hardwoods

All species groups 

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million 

cubic feet. Columns and rows may not add to their totals due to rounding.

Table B20—Net volume of sawlog portion of sawtimber trees, in million cubic feet, on timberland by species group and ownership group, Arizona, 

2005–2014.

Ownership group

Softwood species groups

Western softwood species groups

All softwoods

Hardwood species groups

Western hardwood species groups

All

Owner class Timberland Unproductive Total Productive Unproductive Total forest land

Forest Service

National Forest -7.1 -34.1 -41.2 -12.4 -6.8 -19.2 -60.4

Other Federal

National Park Service - - - - - - -3.9 -1.3 -5.2 -5.2

Bureau of Land Management 0.7 1.3 2.0 0.2 1.0 1.1 3.1

U.S. Fish and Wildlife Service - - - - - - - - 0.3 0.3 0.3

Departments of Defense and Energy 0.4 0.1 0.5 - - - - - - 0.5

Other Federal - - 0.0 0.0 - - - - - - 0.0

State and local government

State 0.2 2.3 2.6 - - - - - - 2.6

County and municipal 0.1 0.1 0.1 - - - - - - 0.1

Private

Undifferentiated private 14.1 -8.7 5.4 - - - - - - 5.4

All owners 8.4 -39.0 -30.7 -16.1 -6.9 -23.0 -53.6

Unreserved forests Reserved forests

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million cubic 

feet. Columns and rows may not add to their totals due to rounding.

Table B21—Average annual net growth of all live trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, by owner class and forest land status, Arizona, 2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 105

Large Medium Small All size

Forest-type group diameter diameter diameter Nonstocked classes

Pinyon / juniper group -5.4 0.7 -3.5 - - -8.2

Douglas-fir group -5.4 - - - - - - -5.4

Ponderosa pine group 38.6 0.1 -1.9 - - 36.8

Fir / spruce / mountain hemlock group -9.6 - - 0.0 - - -9.6

Other western softwoods group 0.7 - - - - - - 0.7

Elm / ash / cottonwood group 0.9 0.0 -0.1 - - 0.8

Aspen / birch group 3.3 0.6 -13.2 - - -9.3

Other hardwoods group 0.1 -0.1 - - - - 0.1

Woodland hardwoods group -6.3 1.1 -24.0 - - -29.1

Nonstocked - - - - - - -30.4 -30.4

All forest-type groups 16.9 2.5 -42.6 -30.4 -53.6

Stand-size class

All table cells without observations in the inventory sample are indicated by --. 

Table value of 0.0 indicates the volume rounds to less than 0.1 million cubic feet. 

Columns and rows may not add to their totals due to rounding.

Table B22—Average annual net growth of all live trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, on forest land by 

forest type group and stand-size class, Arizona, 2005–2014.

Forest Other State and local Undifferentiated All

Species group Service Federal government private owners

Douglas-fir -9.5 -0.2 - - 0.2 -9.4

Ponderosa and Jeffrey pine -4.4 0.8 0.3 12.7 9.4

True fir -14.3 -1.1 - - 0.3 -15.1

Engelmann and other spruces -4.7 -0.9 - - -4.3 -9.9

Other western softwoods 0.0 -0.1 - - 0.2 0.2

Western woodland softwoods -10.7 1.3 1.2 -3.5 -11.7

-43.6 -0.2 1.5 5.7 -36.5

Cottonwood and aspen -2.5 -1.5 0.0 1.1 -2.8

Other western hardwoods 0.2 0.0 0.1 -0.1 0.1

Western woodland hardwoods -14.5 0.4 1.1 -1.3 -14.4

-16.8 -1.1 1.1 -0.3 -17.1

-60.4 -1.3 2.7 5.4 -53.6

Western hardwood species groups

Other

All hardwoods

All species groups 

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million 

cubic feet. Columns and rows may not add to their totals due to rounding.

All softwoods

Hardwood species groups

Table B23—Average annual net growth of all live trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, on forest land by species group and 

ownership group, Arizona, 2005–2014.

Ownership group

Softwood species groups

Western softwood species groups

Other
106 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Forest Other State and local Undifferentiated All

Species group Service Federal government private owners

Douglas-fir -6.2 - - - - 2.3 -3.8

Ponderosa and Jeffrey pine 18.8 0.3 0.3 13.8 33.1

True fir -10.6 - - - - 1.6 -9.0

Engelmann and other spruces -2.1 - - - - -4.3 -6.4

Other western softwoods 0.3 0.1 - - 0.4 0.8

0.1 0.4 0.3 13.9 14.6

Cottonwood and aspen -1.1 0.6 - - 1.1 0.6

Other western hardwoods -0.1 - - - - 0.0 0.0

-1.2 0.6 - - 1.1 0.6

-1.1 1.0 0.3 15.0 15.2

All hardwoods

All species groups 

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million 

cubic feet. Columns and rows may not add to their totals due to rounding.

Table B24—Average annual net growth of growing stock trees (at least 5 inches d.b.h.), in million cubic feet, on timberland by species group and 

ownership group, Arizona, 2005–2014.

Ownership group

Softwood species groups

Western softwood species groups

All softwoods

Hardwood species groups

Western hardwood species groups
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 107 All Owner class Timberland Unproductive Total Productive Unproductive Total forest land Forest Service National Forest 73.8 62.9 136.8 15.2 9.9 25.1 161.9 Other Federal National Park Service - - - - - - 6.7 2.6 9.3 9.3 Bureau of Land Management - - 1.0 1.0 - - 1.6 1.6 2.7 U.S. Fish and Wildlife Service - - - - - - - - 0.1 0.1 0.1 Departments of Defense and Energy 0.4 0.0 0.4 - - - - - - 0.4 State and local government State - - 2.0 2.0 - - - - - - 2.0 County and municipal - - 0.0 0.0 - - - - - - 0.0 PrivateUndifferentiated private 18.7 42.8 61.5 - - - - - - 61.5 All owners 92.9 108.9 201.8 21.9 14.1 36.0 237.8 Table B25—Average annual mortality of trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, on forest land by owner class and forest land status, Arizona, 2005–2014. Unreserved forests Reserved forests All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million cubic feet. Columns and rows may not add to their totals due to rounding.
108 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Large Medium Small All size

Forest-type group diameter diameter diameter Nonstocked classes

Pinyon / juniper group 58.1 3.2 4.0 - - 65.3

Douglas-fir group 15.9 - - - - - - 15.9

Ponderosa pine group 36.1 1.1 2.4 - - 39.6

Fir / spruce / mountain hemlock group 20.7 - - - - - - 20.7

Other western softwoods group 0.4 - - - - - - 0.4

Elm / ash / cottonwood group 1.0 - - 0.1 - - 1.1

Aspen / birch group 1.5 0.0 13.7 - - 15.2

Other hardwoods group - - 0.1 - - - - 0.1

Woodland hardwoods group 20.6 2.9 25.0 - - 48.5

Nonstocked - - - - - - 30.9 30.9

All forest-type groups 154.4 7.3 45.1 30.9 237.8

Stand-size class

All table cells without observations in the inventory sample are indicated by --. 

Table value of 0.0 indicates the volume rounds to less than 0.1 million cubic feet. 

Columns and rows may not add to their totals due to rounding.

Table B26—Average annual mortality of trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, on forest land by forest type 

group and stand-size class, Arizona, 2005–2014.

Forest Other State and local Undifferentiated All

Species group Service Federal government private owners

Douglas-fir 17.3 0.5 - - 4.1 21.9

Ponderosa and Jeffrey pine 58.2 1.6 - - 9.8 69.5

True fir 18.3 1.6 - - 2.2 22.0

Engelmann and other spruces 7.2 1.3 - - 6.9 15.3

Other western softwoods 1.7 0.2 - - 0.3 2.2

Western woodland softwoods 30.6 4.1 1.8 28.6 65.2

133.3 9.3 1.8 51.8 196.2

Cottonwood and aspen 5.6 2.3 - - 3.0 11.0

Other western hardwoods 0.1 - - - - 0.2 0.3

Western woodland hardwoods 22.9 0.7 0.2 6.5 30.3

28.6 3.1 0.2 9.7 41.5

161.9 12.4 2.0 61.5 237.8

Western hardwood species groups

Other

All hardwoods

All species groups 

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million 

cubic feet. Columns and rows may not add to their totals due to rounding.

All softwoods

Hardwood species groups

Table B27—Average annual mortality of trees (at least 5 inches d.b.h./d.r.c.), in million cubic feet, on forest land by species group and ownership group, 

Arizona, 2005–2014.

Ownership group

Softwood species groups

Western softwood species groups

Other
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 109

Forest Other State and local Undifferentiated All

Species group Service Federal government private owners

Douglas-fir 12.5 - - - - 1.2 13.7

Ponderosa and Jeffrey pine 29.3 0.4 - - 4.7 34.3

True fir 14.1 - - - - 0.9 14.9

Engelmann and other spruces 4.4 - - - - 6.9 11.2

Other western softwoods 0.9 - - - - 0.0 0.9

61.1 0.4 - - 13.7 75.1

Cottonwood and aspen 3.9 - - - - 2.7 6.5

Other western hardwoods 0.1 - - - - - - 0.1

4.0 - - - - 2.7 6.6

65.0 0.4 - - 16.3 81.7

All hardwoods

All species groups 

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million 

cubic feet. Columns and rows may not add to their totals due to rounding.

Table B28—Average annual mortality of growing stock trees (at least 5 inches d.b.h.), in million cubic feet, on timberland by species group and 

ownership group, Arizona, 2005–2014.

Ownership group

Softwood species groups

Western softwood species groups

All softwoods

Hardwood species groups

Western hardwood species groups
110 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 All Owner class Timberland Unproductive Total Productive Unproductive Total forest land Forest Service National Forest 79,426 52,202 131,627 3,713 5,670 9,382 141,010 Other Federal National Park Service - - - - - - 3,630 3,183 6,812 6,812 Bureau of Land Management 504 5,055 5,559 307 6,164 6,471 12,030 U.S. Fish and Wildlife Service - - - - - - - - 565 565 565 Departments of Defense and Energy 934 234 1,168 - - - - - - 1,168 Other Federal - - 2 2 - - - - - - 2 State and local government State County and Municipal 61 203 264 - - - - - - 264 PrivateUndifferentiated private 32,334 64,609 96,943 - - - - - - 96,943 All owners 113,408 131,903 245,312 7,650 15,581 23,230 268,542 Unreserved forests Reserved forests Table B29— All table cells without observations in the inventory sample are indicated by --. Table value of 0 indicates the aboveground tree biomass rounds to less than 1 thousand dry tons. Columns and rows may not add to their totals due to rounding. Aboveground dry weight of all live trees (at least 1 inch d.b.h./d.r.c.), in thousand dry short tons, by owner class and forest land status, Arizona, 2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 111 1.0- 3.0- 5.0- 7.0- 9.0- 11.0- 13.0- 15.0- 17.0- 19.0- 21.0- 23.0- 25.0- 27.0- 29.0+ All Species group 2.9 4.9 6.9 8.9 10.9 12.9 14.9 16.9 18.9 20.9 22.9 24.9 26.9 28.9 classes Douglas-fir 99 212 476 847 1,186 1,435 1,585 1,630 1,594 909 984 701 472 487 840 13,457 Ponderosa and Jeffrey pine 283 1,126 2,314 5,482 8,240 10,866 11,217 9,829 9,705 8,261 8,171 7,501 4,308 2,807 4,652 94,763 True fir 84 186 174 369 472 491 620 588 523 505 328 367 43 - - 247 4,997 Engelmann and other spruces 43 171 128 264 501 506 553 708 522 453 583 271 56 - - - - 4,759 Other western softwoods 25 83 102 244 294 356 304 320 100 179 284 58 - - 101 139 2,588 Western woodland softwoods 1,590 2,906 4,680 7,993 10,366 12,212 11,570 10,810 9,072 7,974 6,303 5,584 3,827 2,827 7,486 105,201 2,124 4,683 7,873 15,199 21,059 25,864 25,849 23,886 21,516 18,281 16,654 14,483 8,706 6,223 13,364 225,765 Cottonwood and aspen 216 350 295 491 877 1,175 1,292 1,056 602 155 295 70 69 72 298 7,315 Other western hardwoods 48 76 36 11 30 80 71 27 64 41 - - - - - - - - - - 484 Western woodland hardwoods 2,497 3,650 3,141 4,364 4,383 4,059 3,420 2,676 2,116 1,170 1,163 784 647 341 568 34,978 2,761 4,076 3,472 4,867 5,290 5,315 4,783 3,759 2,782 1,366 1,458 854 716 413 866 42,777 All species groups 4,885 8,759 11,345 20,065 26,350 31,179 30,632 27,645 24,298 19,648 18,112 15,337 9,422 6,636 14,230 268,542 Western hardwood species groups Other Table B30— All hardwoods All table cells without observations in the inventory sample are indicated by --. Table value of 0 indicates the aboveground tree biomass rounds to less than 1 thousand dry tons. Columns and rows may not add to their totals due to rounding. Aboveground dry weight of all live trees (at least 1 inch d.b.h./d.r.c.), in thousand dry short tons, on forest land by species group and diameter class, Arizona, 2005–2014. Diameter class (inches) Softwood species groups Western softwood species groups Other All softwoods Hardwood species groups
112 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 All Timberland Unproductive Total Productive Unproductive Total forest land Southern Cochise 7.2 853.6 860.8 18.0 111.2 129.2 990.0 Graham 91.9 765.5 857.4 7.6 80.3 87.9 945.3 Greenlee 141.0 499.1 640.0 12.0 6.0 17.9 658.0 La Paz - - 4.6 4.6 - - - - - - 4.6 Maricopa 1.3 63.2 64.5 - - 17.0 17.0 81.5 Pima 13.1 618.0 631.1 6.6 187.6 194.1 825.3 Pinal 4.7 187.5 192.2 6.9 13.7 20.6 212.8 Santa Cruz - - 359.6 359.6 - - 24.0 24.0 383.7 Yuma - - 16.1 16.1 - - - - - - 16.1 Total 259.1 3,367.3 3,626.4 51.0 439.8 490.8 4,117.2 Northern Apache 876.6 2,015.6 2,892.2 18.5 - - 18.5 2,910.7 Coconino 1,435.7 2,687.3 4,123.0 107.5 267.8 375.4 4,498.3 Gila 115.9 1,515.1 1,630.9 6.1 123.2 129.3 1,760.3 Mohave - - 902.9 902.9 12.2 562.7 575.0 1,477.8 Navajo 252.3 1,895.4 2,147.6 - - - - - - 2,147.6 Yavapai 49.0 1,535.6 1,584.7 13.0 96.7 109.7 1,694.4 Total 2,729.5 10,551.7 13,281.3 157.4 1,050.5 1,207.9 14,489.2 All counties 2,988.6 13,919.0 16,907.6 208.4 1,490.3 1,698.7 18,606.4 Unreserved forests Reserved forests Table B31— Inventory unit and county All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less than 0.1 thousand acres. Columns and rows may not add to their totals due to rounding. Area of accessible forest land, in thousand acres, by Forest Survey Unit, county and forest land status, Arizona, 2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 113

Other Other Other Other All

Inventory unit Timber- forest Timber- forest Timber- forest Timber- forest forest

land land land land land land land land land

Southern

Cochise - - 354.7 7.2 130.6 - - 247.6 - - 249.9 990.0

Graham 35.7 250.2 - - 121.2 - - 77.1 56.2 405.0 945.3

Greenlee 136.5 446.2 - - 37.5 - - 18.3 4.5 14.9 658.0

La Paz - - - - - - 4.6 - - - - - - - - 4.6

Maricopa 1.3 54.5 - - 3.0 - - 3.0 - - 19.6 81.5

Pima 13.1 206.9 - - 172.2 - - 355.4 - - 77.7 825.3

Pinal - - 49.6 - - 26.2 - - 78.7 4.7 53.6 212.8

Santa Cruz - - 298.8 - - - - - 31.1 - - 53.7 383.7

Yuma - - - - - - - - - 5.8 - - 10.2 16.1

Total 186.6 1,661.0 7.2 495.4 - - 817.0 65.3 884.7 4,117.2

Northern

Apache 345.7 87.9 - - 23.4 - - 165.5 531.0 1,757.4 2,910.7

Coconino 1,367.4 1,494.5 25.3 316.4 6.6 210.8 36.5 1,041.0 4,498.3

Gila 105.7 1,017.4 - - - - - - 7.6 10.1 619.3 1,760.3

Mohave - - - - - - 1,026.9 - - 153.0 - 297.9 1,477.8

Navajo 135.7 268.8 - - 11.4 - - 45.0 116.6 1,570.2 2,147.6

Yavapai 41.0 975.4 - - 28.0 1.5 266.0 6.5 375.9 1,694.4

Total 1,995.5 3,844.1 25.3 1,406.0 8.1 847.8 700.7 5,661.7 14,489.2

All counties 2,182.1 5,505.1 32.5 1,901.5 8.1 1,664.8 766.0 6,546.4 18,606.4

Table B32—Area of accessible forest land, in thousand acres, by Forest Survey Unit, county, ownership group and forest land status, 

Arizona, 2005–2014.

and county

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less 

than 0.1 thousand acres. Columns and rows may not add to their totals due to rounding.

Forest Service Other Federal State and local Undifferentiated

government private
114 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Large Medium Small All size

diameter diameter diameter Nonstocked classes

Southern

Cochise 7.2 - - - - - - 7.2

Graham 78.2 6.1 6.1 1.5 91.9

Greenlee 129.0 6.0 6.0 - - 141.0

Maricopa - - - - 1.3 - - 1.3

Pima 1.6 - - 11.5 - - 13.1

Pinal 4.7 - - - - - - 4.7

Total 220.7 12.1 24.8 1.5 259.1

Northern

Apache 780.1 24.7 31.7 40.1 876.6

Coconino 1,330.6 1.5 46.9 56.7 1,435.7

Gila 109.8 - - 3.0 3.0 115.9

Navajo 189.0 19.5 22.7 21.1 252.3

Yavapai 47.5 - - 1.5 - - 49.0

Total 2,457.0 45.7 105.9 121.0 2,729.5

All counties 2,677.7 57.7 130.8 122.5 2,988.6

Stand-size class

Inventory unit and county

Table B33—Area of timberland, in thousand acres, by Forest Survey Unit, county and stand-size class, Arizona, 

2005–2014.

All table cells without observations in the inventory sample are indicated by --. 

Table value of 0.0 indicates the acres round to less than 0.1 thousand acres. 

Columns and rows may not add to their totals due to rounding.

Poorly Moderately Fully OverNonstocked stocked stocked stocked stocked All classes

Southern

Cochise - - - - - - 7.2 - - 7.2

Graham 1.5 24.3 45.6 14.4 6.1 91.9

Greenlee 1.5 35.9 85.7 17.9 - - 141.0

Maricopa - - - - - - - - 1.3 1.3

Pima - - - - 1.6 11.5 - - 13.1

Pinal - - - - 4.7 - - - - 4.7

Total 3.0 60.2 137.6 51.0 7.4 259.1

Northern

Apache 53.4 297.9 289.2 211.4 24.7 876.6

Coconino 68.7 428.3 584.9 341.7 12.2 1,435.7

Gila 3.0 49.7 44.0 19.1 - - 115.9

Navajo 27.6 108.8 82.0 32.3 1.6 252.3

Yavapai - - 13.0 26.1 9.9 - - 49.0

Total 152.7 897.6 1,026.2 614.4 38.6 2,729.5

All counties 155.7 957.8 1,163.8 665.4 45.9 2,988.6

Stocking class of growing-stock trees

Inventory unit and county

All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the acres round to less 

than 0.1 thousand acres. Columns and rows may not add to their totals due to rounding.

Table B34—Area of timberland, in thousand acres, by Forest Survey Unit, county and stocking class, Arizona, 2005–2014.
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 115 Inventory unit Other Soft Hard All Other Soft Hard All Pine softwoods hardwoods hardwoods species Pine softwoods hardwoods hardwoods species (In million cubic feet) (In million board feet) SouthernCochise 4.4 - - - - 26.0 30.4 24.3 - - - - 151.7 176.0 Graham 145.5 67.3 4.8 - - 217.7 737.5 352.5 15.2 - - 1,105.2 Greenlee 231.0 66.0 9.0 - - 306.0 1,126.7 312.9 55.0 - - 1,494.6 Pima - - 11.7 - - - - 11.7 - - 63.4 - - - - 63.4 Pinal - - - - - - 5.5 5.5 - - - - - - 15.9 15.9 Total 381.0 145.0 13.8 31.6 571.3 1,888.5 728.8 70.3 167.6 2,855.1 NorthernApache 1,123.1 568.9 225.9 - - 1,918.0 5,553.6 2,662.5 970.0 - - 9,186.1 Coconino 2,453.7 228.4 56.0 - - 2,738.0 12,122.1 1,034.1 216.8 - - 13,373.1 Gila 178.1 28.9 4.4 7.4 218.9 868.3 139.7 16.7 23.4 1,048.1 Navajo 319.6 17.8 0.2 1.5 339.1 1,459.8 73.3 - - 4.6 1,537.7 Yavapai 65.0 41.1 - - - - 106.1 314.0 200.2 - - - - 514.3 Total 4,139.5 885.1 286.6 9.0 5,320.1 20,317.8 4,109.9 1,203.6 28.0 25,659.2 All counties 4,520.5 1,030.1 300.4 40.6 5,891.4 22,206.3 4,838.7 1,273.8 195.5 28,514.3 Table B35— and county All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million cubic or board feet. Columns and rows may not add to their totals due to rounding. Net volume of growing stock trees (at least 5 inches d.b.h.), in million cubic feet, and sawtimber trees, in million board feet (International 1/4 inch rule), on timberland by Forest Survey Unit, county, and major species group, Arizona, 2005–2014. Growing stock Sawtimber Major species group Major species group
116 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 Inventory unit Other Soft Hard All Other Soft Hard All Pine softwoods hardwoods hardwoods species Pine softwoods hardwoods hardwoods species (In million cubic feet) (In million board feet) SouthernCochise 0.1 - - - - 0.6 0.7 0.4 - - - - 3.8 4.2 Graham 1.6 -4.1 0.1 - - -2.4 12.6 -23.2 0.4 - - -10.2 Greenlee 0.5 -2.6 0.0 - - -2.1 5.1 -10.8 0.3 0.0 -5.4 Pima - - -0.6 - - - - -0.6 - - -2.6 - - - - -2.6 Pinal - - - - - - 0.1 0.1 - - - - - - 0.2 0.2 Total 2.2 -7.4 0.1 0.7 -4.4 18.1 -36.6 0.7 4.0 -13.8 NorthernApache 11.1 -6.0 1.1 - - 6.2 80.6 -7.6 25.5 - - 98.5 Coconino 21.9 -5.5 -1.4 - - 15.0 148.4 -19.5 -3.3 - - 125.6 Gila -0.3 0.1 0.0 0.2 0.0 1.7 2.4 0.0 6.8 10.9 Navajo -0.2 -0.4 0.0 0.0 -0.6 12.5 0.2 - - -0.1 12.7 Yavapai -0.7 -0.2 - - - - -0.9 -1.4 0.2 - - - - -1.2 Total 31.7 -11.9 -0.4 0.2 19.6 241.8 -24.3 22.3 6.7 246.5 All counties 33.9 -19.3 -0.3 0.9 15.2 259.9 -60.9 22.9 10.7 232.6 Table B36— and county All table cells without observations in the inventory sample are indicated by --. Table value of 0.0 indicates the volume rounds to less than 0.1 million cubic or board feet. Columns and rows may not add to their totals due to rounding. Average annual net growth of growing stock trees (at least 5 inches d.b.h.), in million cubic feet, and sawtimber trees, in million board feet (International 1/4 inch rule), on timberland by Forest Survey Unit, county, and major species group, Arizona, 2005–2014. Growing stock Sawtimber Major species group Major species group
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 117 Inventory unit Forest Timberland area area Volume Growth Removals Mortality Volume Growth Removals Mortality Southern Cochise 6.0 84.8 90.0 95.0 - - - - 90.5 94.6 - - - - Graham 5.2 24.2 29.7 100.0 - - 48.9 30.6 100.0 - - 51.8 Greenlee 4.4 18.4 22.5 100.0 - - 43.2 24.1 100.0 - - 45.2 La Paz 100.0 - - - - - - - - - - - - - - - - - - Maricopa 23.9 100.0 - - - - - - - - - - - - - - - - Pima 7.9 77.6 100.0 100.0 - - 100.0 100.0 100.0 - - 100.0 Pinal 14.9 100.0 100.0 100.0 - - 100.0 100.0 100.0 - - - - Santa Cruz 8.0 - - - - - - - - - - - - - - - - - - Yuma 55.2 - - - - - - - - - - - - - - - - - - Total 2.8 14.1 17.4 84.7 - - 31.0 18.4 100.0 - - 33.2 Northern Apache 2.2 6.8 9.4 84.1 - - 17.1 9.8 27.2 - - 17.9 Coconino 1.6 5.4 6.7 35.2 - - 17.7 7.1 22.6 - - 20.4 Gila 3.2 21.6 25.2 100.0 - - 44.4 26.2 97.5 - - 51.6 Mohave 4.3 - - - - - - - - - - - - - - - - - - Navajo 3.0 14.4 18.7 100.0 - - 31.0 20.2 73.1 - - 34.0 Yavapai 4.3 34.6 40.4 100.0 - - 61.3 40.4 100.0 - - 63.1 Total 1.1 4.0 5.2 40.3 - - 11.0 5.4 16.9 - - 12.2 All counties 1.1 3.8 5.0 57.5 - - 10.4 5.2 19.9 - - 11.6 All table cells without observations in the inventory sample are indicated by --. Sampling errors that exceed 100% are reported as 100%. Growing stock (on timberland) Sawtimber (on timberland) Table B37 and county—Sampling errors by Forest Survey Unit and county for area of timberland, volume, average annual net growth, average annual removals, and average annual mortality on timberland, Arizona, 2005–2014.
118 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Appendix C: Arizona Forest-Type Groups and Forest Types, With Descriptions and Timber (T) or Woodland (W) Designations

Forest-type groups and forest types are usually named for the predominant 

species (or group of species) on the condition. In order to determine the forest type, 

the stocking (site occupancy) of trees is estimated by softwoods and hardwoods. 

If softwoods predominate, then the forest type will be one of the softwood types 

and if hardwoods predominate, then the forest type will be one of the hardwood 

types. Some other special stocking rules apply to individual forest types and are 

described below.

Associate species are defined as those that regularly form the majority of 

the non-predominant species stocking of mixed-species conditions. These descriptions are applicable to the current inventory; species importance, including predominance in some cases, will vary for other States or inventory years. When 

species are listed, they are in decreasing order of overall forest type stocking.

Aspen/Birch Group (T)

Aspen

Predominant species: quaking aspen

Associate species: ponderosa pine, Douglas-fir, Engelmann spruce,

Other species: blue spruce, white fir, corkbark fir, southwestern white pine, limber 

pine, bigtooth maple, subalpine fir

Douglas-Fir Group (T)

Douglas-fir

Predominant species: Douglas-fir

Associate species: ponderosa pine, quaking aspen, white fir, Gambel oak, southwestern white pine

Other species: Engelmann spruce, corkbark fir, netleaf oak, Arizona white oak, 

limber pine, border pinyon, Mexican pinyon pine, common or two-needle pinyon, blue spruce, alligator juniper, silverleaf oak, Rocky Mountain juniper, bigtooth maple, Arizona walnut

ELM/Ash/Cottonwood Group (T)

Cottonwood

Predominant species: Fremont cottonwood, narrowleaf cottonwood

Associate species: none identified
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 119

Other species: oneseed juniper, Emory oak, Utah juniper, Arizona white oak, honey mesquite, Arizona walnut, boxelder, velvet mesquite

Special rules: Stocking of cottonwoods must be at least 50 percent of total stocking.

Cottonwood/willow

Predominant species: Fremont cottonwood, narrowleaf cottonwood

Associate species: none identified

Other species: ponderosa pine, Arizona walnut, Gambel oak, Emory oak, Rocky 

Mountain juniper, Arizona alder, alligator juniper, Arizona white oak, Arizona 

pinyon pine

Special rules: Stocking of cottonwoods is less than 50 percent, but predominant. In 

order to meet 50 percent hardwood stocking, other hardwoods must be present.

Fir/Spruce/Mountain Hemlock Group (T)

Blue spruce

Predominant species: blue spruce

Associate species: quaking aspen, ponderosa pine, Douglas-fir, white fir

Other species: subalpine fir, southwestern white pine, Engelmann spruce

Engelmann spruce

Predominant species: Engelmann spruce

Associate species: Douglas-fir, quaking aspen, corkbark fir, white fir, subalpine fir

Other species: ponderosa pine, southwestern white pine, blue spruce

Special rules: In order to use Engelmann spruce stocking predominance, subalpine 

fir and/or corkbark fir stocking must be less than 5 percent of the total. If subalpine fir and/or corkbark fir stocking is 5 percent or more, Engelmann spruce 

stocking must be at least 75 percent of the total.

Engelmann spruce/subalpine fir

Predominant species: Engelmann spruce, corkbark fir, subalpine fir

Associate species: quaking aspen, Douglas-fir, white fir

Other species: ponderosa pine, southwestern white pine, Gambel oak

Special rules: The combined stocking of Engelmann spruce with subalpine fir and/

or corkbark fir is predominant. Stocking of both Engelmann spruce and subalpine fir/corkbark fir must each be between 5 and 74 percent of the total.

White fir

Predominant species: white fir
120 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Associate species: quaking aspen, Douglas-fir, ponderosa pine, southwestern white 

pine

Other species: Engelmann spruce, bigtooth maple, Gambel oak, corkbark fir, velvet ash

Nonstocked

Nonstocked

Predominant species: various, most commonly velvet mesquite, but many nonstocked conditions have no live-tree stocking.

Associate species: various, frequently common or two-needle pinyon

Other species: seldom more than two species on a condition. Complete species list: 

velvet mesquite, Utah juniper, oneseed juniper, common or two-needle pinyon, 

ponderosa pine, Arizona white oak, Emory oak, redberry juniper, alligator juniper, honey mesquite, singleleaf pinyon, Mexican blue oak, Mexican pinyon 

pine, Engelmann spruce, California juniper

Special rules: Used when all live stocking is less than 10 percent. Implies disturbance, but may be used for sparse stands with no disturbance, especially with 

woodland species.

Other Hardwoods Group (T)

Other hardwoods

Predominant species: velvet ash, Arizona walnut, Arizona sycamore

Associate species: none identified

Other species: Gambel oak, oneseed juniper, Utah juniper, velvet mesquite, alligator juniper, Fremont cottonwood, Arizona pinyon pine

Special rules: A “catch-all” type, typically for hardwood species with a limited 

geographical range. Arizona alder and Arizona madrone are also evaluated for 

this forest type, but did not determine or appear in the forest type in this inventory.

Other Western Sofwoods Group (T)

Limber pine

Predominant species: limber pine

Associate species: too few occurrences to evaluate

Other species: quaking aspen, corkbark fir, Engelmann spruce

Miscellaneous western softwoods

Predominant species: Arizona cypress, Chihuahuan pine, Apache pine
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 121

Associate species: none identified

Other species: Arizona pinyon pine, ponderosa pine, silverleaf oak, netleaf oak, 

alligator juniper, Douglas-fir, Emory oak, Mexican pinyon pine, Gambel oak, 

Arizona white oak, Arizona madrone, velvet ash

Special rules: A “catch-all” type, typically for softwood species with a limited 

geographical range.

Southwestern white pine

Predominant species: southwestern white pine

Associate species: none identified

Other species: Apache pine, silverleaf oak, quaking aspen, white fir, Douglas-fir, 

Arizona white oak, ponderosa pine, bigtooth maple

Pinyon/Juniper Group (W)

Juniper woodland

Predominant species: Utah juniper, oneseed juniper, alligator juniper, redberry juniper, California juniper

Associate species: Arizona white oak, ponderosa pine, velvet mesquite

Other species: Gambel oak, Emory oak, Douglas-fir, honey mesquite, Rocky 

Mountain juniper, Arizona walnut, Chihuahuan pine, silverleaf oak

Special rules: Predominance of any combination of junipers other than Rocky 

Mountain juniper, and live pinyons are NOT present.

Pinyon/juniper woodland

Predominant species: Utah juniper, common or two-needle pinyon, oneseed juniper, alligator juniper, Arizona pinyon pine, singleleaf pinyon, redberry juniper, 

border pinyon, Mexican pinyon pine

Associate species: Arizona white oak, ponderosa pine

Other species: Gambel oak, Emory oak, Rocky Mountain juniper, silverleaf oak, 

netleaf oak, gray oak, Arizona cypress, Douglas-fir, Arizona sycamore, white 

fir, velvet mesquite, Chihuahuan pine, Mexican blue oak, velvet ash, honey 

mesquite, Arizona walnut

Special rules: Any combination of pinyons and junipers other than Rocky Mountain juniper predominate. Pinyons must be present.

Rocky Mountain juniper

Predominant species: Rocky Mountain juniper

Associate species: ponderosa pine, Gambel oak, alligator juniper, Arizona white 

oak, Douglas-fir, common or two-needle pinyon
122 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Other species: Utah juniper, Arizona sycamore, Emory oak, oneseed juniper, Arizona alder, Arizona pinyon pine

Ponderosa Pine Group (T)

Ponderosa pine

Predominant species: ponderosa pine, Arizona pine

Associate species: Gambel oak, alligator juniper, Douglas-fir, Arizona white oak, 

quaking aspen, Rocky Mountain juniper

Other species: white fir, common or two-needle pinyon, southwestern white pine, 

Utah juniper, Emory oak, Engelmann spruce, netleaf oak, blue spruce, bigtooth 

maple, oneseed juniper, silverleaf oak, limber pine, border pinyon, subalpine fir, 

gray oak, Mexican pinyon pine, Arizona walnut, corkbark fir, Chihuahuan pine, 

Arizona madrone, singleleaf pinyon

Woodland Hardwoods Group (W)

Deciduous oak woodland

Predominant species: Gambel oak

Associate species: ponderosa pine, alligator juniper, Arizona white oak, common 

or two-needle pinyon, Douglas-fir, Emory oak

Other species: bigtooth maple, Rocky Mountain juniper, white fir, boxelder, southwestern white pine, quaking aspen, oneseed juniper, Utah juniper, netleaf oak, 

Arizona cypress, velvet ash, Arizona walnut, Chihuahuan pine, silverleaf oak

Special rules: Gambel oak is the only species evaluated for this type.

Evergreen oak woodland

Predominant species: Arizona white oak, Emory oak, silverleaf oak, Mexican blue 

oak, netleaf oak, gray oak

Associate species: alligator juniper, ponderosa pine, Mexican pinyon pine, common or two-needle pinyon, Gambel oak, Utah juniper, border pinyon, Arizona 

pinyon pine

Other species: redberry juniper, velvet mesquite, Rocky Mountain juniper, oneseed juniper, Arizona madrone, Douglas-fir, Chihuahuan pine, Arizona alder, 

Apache pine, Arizona sycamore, southwestern white pine, Arizona pine, Arizona cypress, honey mesquite, Arizona walnut, velvet ash

Special rules: Any combination of southwestern evergreen oaks. The only Arizona 

oak not included is Gambel oak.

Intermountain maple woodland

Predominant species: bigtooth maple
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 123

Associate species: too few occurrences to evaluate

Other species: Gambel oak

Special rules: Currently, bigtooth maple is the only species evaluated for this type. 

In the previous periodic inventory, Rocky Mountain maple was included.

Mesquite woodland

Predominant species: honey mesquite, velvet mesquite, screwbean mesquite

Associate species: none identified

Other species: redberry juniper, Mexican blue oak, Fremont cottonwood, sugarberry, Emory oak, oneseed juniper, Utah juniper, velvet ash, Arizona walnut
124 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Appendix D: Tree Species Groups and Tree Species Measured in 

Arizona’s Annual Inventory, With Common Name, Scientific Name, and 

Timber (T) or Woodland (W) Designation

Hardwoods

Cottonwood and aspen group (T)

Fremont cottonwood (Populus fremontii)

Narrowleaf cottonwood (Populus angustifolia)

Quaking aspen (Populus tremuloides)

Other western hardwoods group (T)

Arizona alder (Alnus oblongifolia)

Arizona madrone (Arbutus arizonica)

Arizona sycamore (Platinus wrightii)

Arizona walnut (Juglans major)

Boxelder (Acer negundo)

Sugarberry (Celtis laevigata)

Velvet ash (Fraxinus velutina)

Woodland hardwoods group (W)

Arizona white oak (Quercus arizonica)

Bigtooth maple (Acer grandidentatum)

Emory oak (Quercus emoryi)

Gambel oak (Quercus gambelii)

Gray oak (Quercus grisea)

Honey mesquite (Prosopis glandulosa)

Mexican blue oak (Quercus oblongifolia)

Netleaf oak (Quercus rugosa)

Screwbean mesquite (Prosopis pubescens)

Silverleaf oak (Quercus hypoleucoides)

Velvet mesquite (Prosopis velutina)

Softwoods

Douglas-fir group (T)

Douglas-fir (Pseudotsuga menziesii)

Engelmann and other spruces group (T)

Blue spruce (Picea pungens)

Engelmann spruce (Picea engelmannii)
USDA Forest Service Resour. Bull. RMRS-RB-25. 2018 125

Other western softwoods group (T)

Apache pine (Pinus engelmannii)

Arizona cypress (Cupressus arizonica)

Chihuahua pine (Pinus leiophylla)

Limber pine (Pinus flexilis)

Southwestern white pine (Pinus strobiformis)

Ponderosa and Jeffrey pines group (T)

Arizona pine (Pinus arizonica)

Ponderosa pine (Pinus ponderosa)

True fir group (T)

Corkbark fir (Abies lasiocarpa var. arizonica)

Subalpine fir (Abies lasiocarpa)

White fir (Abies concolor)

Woodland softwoods group (W)

Alligator juniper (Juniperus deppeana)

Arizona pinyon pine (Pinus monophylla var. fallax)

Border pinyon (Pinus dicolor)

California juniper (Juniperus califonica)

Common or two-needle pinyon (Pinus edulis)

Mexican pinyon pine (Pinus cembroides)

Oneseed juniper (Juniperus monosperma)

Redberry juniper (Juniperus coahuilensis)

Rocky Mountain juniper (Juniperus scopulorum)

Singleleaf pinyon (Pinus monophylla)

Utah juniper (Juniperus osteosperma)
126 USDA Forest Service Resour. Bull. RMRS-RB-25. 2018

Appendix E: Volume and Site Index Equation Sources

Volume

Chojnacky 1988 was used for bigtooth maple, honey mesquite, velvet mesquite, 

Arizona white oak, Emory oak, Gambel oak, Mexican blue oak, silverleaf oak, 

gray oak, and netleaf oak volume estimation.

Chojnacky 1994 was used for Pinchot juniper, redberry juniper, alligator juniper, Utah juniper, Rocky Mountain juniper, oneseed juniper, common pinyon, 

Mexican pinyon, and Arizona pinyon volume estimation.

Hann and Bare 1978 was used for white fir, corkbark fir, Engelmann spruce, blue 

spruce, bristlecone pine, limber pine, southwestern white pine, Chihuahuan 

pine, ponderosa pine, Douglas-fir, and aspen volume estimation.

Kemp 1956 was used for narrowleaf cottonwood and Fremont cottonwood volume 

estimation.

Volume equations provided by the USDA Forest Service’s Northern Research 

Station were used for boxelder, velvet ash, Arizona walnut, chinkapin oak, and 

Siberian elm volume estimation. [Documentation on file at Rocky Mountain 

Research Station, Ogden, UT.]

Site Index

Brickell 1968 was used for Douglas-fir site index estimation.

Brickell 1970 was used for bristlecone pine, limber pine, Chihuahua pine, ponderosa pine, and southwestern white pine site index estimation.

Clendenen 1977 was used for subalpine fir, corkbark fir, Engelmann spruce, and 

blue spruce site index estimation.

Edminster et al. 1985 was used for boxelder, velvet ash, narrowleaf cottonwood, 

Fremont cottonwood, aspen, gray oak, chinkapin oak, netleaf oak, and Siberian 

elm site index estimation.

McArdle et al. 1961 was used for white fir site index estimation

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