Diameter Caps and 

Forest Restoration

Evaluation of a 16-inch Cut Limit on 

Achieving Desired Conditions 

F. Jack Triepke, Bruce J. Higgins, Reuben N. 

Weisz, James A. Youtz, and Tessa Nicolet

United States

Department of

Agriculture

Forest Service

Southwestern

Region

FR-R3-16-3

November 2011
Citation

Triepke, F. J., B. J. Higgins, R. N. Weisz, J. A. Youtz, and T. Nicolet. 2011. Diameter caps and forest restoration — 

Evaluation of a 16-inch cut limit on achieving desired conditions. USDA Forest Service Forestry Report FR-R3-16-3. 

Southwestern Region, Regional Office, Albuquerque, NM. 31 pp.

Summary 

With forest plan revision in the U.S. Forest Service, the Southwestern Region is evaluating means for achieving desired 

conditions of the region. Some forests within the region have previously applied a diameter cap cutting limit as a means 

of responding to public concerns over the preservation of large diameter trees in forested ecosystems. We analyzed the 

effects of a 16-inch cap on forest structure in the coming decades, and the ability of managers to achieve and maintain 

desired conditions within two major forest ecosystems—ponderosa pine and dry mixed conifer. The effects of a cap on 

forest composition and fire behavior were also considered. The results suggest that a blanket policy of diameter-limit cutting 

impairs the ability of resource managers to achieve or maintain desired conditions, and is not sustainable in the mid to long 

term.

Keywords

Uneven-aged management, forest restoration, diameter cap, diameter cutting limit, desired conditions, ponderosa pine, dry 

mixed conifer.

The Authors 

F. Jack Triepke is regional ecologist for the U.S. Forest 

Service Southwestern Region in Albuquerque, New Mexico.

Bruce Higgins is a retired planner and forester from 

the Kaibab National Forest. He currently works as a 

planning and forestry consultant on local and regional land 

management projects.

Reuben Weisz is a retired regional analyst from the U.S. 

Forest Service Southwestern Region in Albuquerque, New 

Mexico.

Jim Youtz is regional silviculturist for the U.S. Forest 

Service Southwestern Region in Albuquerque, New Mexico.

Tessa Nicolet is regional fire ecologist for the U.S. Forest 

Service Southwestern Region stationed in Payson, Arizona.

Acknowledgements

The authors gratefully acknowledge Bob Davis, director 

of Ecosystem Assessment and Planning for the U.S. Forest 

Service Southwestern Region, and personnel from the 

Kaibab and Apache-Sitgreaves National Forests, including 

Jared Scott, for their key support in this effort. We also 

thank Don Vandendriesche of the U.S. Forest Service Forest 

Management Service Center for his help with calibration of 

the Forest Vegetation Simulator Model. Our thanks also go 

to Kristen Waring at Northern Arizona University, and to 

several staff of the Southwestern Region for their considered 

review of this paper.

Cover: Photo taken by David A. Conklin, of the U.S. Forest Service Southwestern Region forestry staff, showing a ponderosa 

pine stand on the Mescalero Indian Reservation in southeastern New Mexico. The photo represents a managed uneven-aged 

forest stand structure and desired conditions for ponderosa pine forests of the Southwestern Region.
1

Introduction

Each national forest and grassland of the U.S. Forest 

Service (USFS) develops and periodically revises a land 

and resource management plan known as a “forest plan” 

as its chief policy document. The forest plan addresses the 

management of its resources with a framework of goals, 

standards, and guidelines. As outlined in the provisions 

of the 1982 Planning Rule (USDA Forest Service 1999; 

36 CFR § 219), desired conditions are descriptions of 

goals to be achieved at some time in the future. Desired 

conditions, by their contributions to social, economic, 

and ecological sustainability, are the vision that drives the 

forest plan revision and implementation process. While 

the planning rule is currently under revision, the concept 

of desired condition is likely to persist. The proposed 

rule issued in February 2011 states that “A desired 

condition is a description of specific social, economic, 

and/or ecological characteristics of the plan area, or a 

portion of the plan area, toward which management of 

the land and resources should be directed” (draft new rule 

section 219.7(d)(10(i))). The 1982 rule (USDA Forest 

Service 1999) states that “The forest plan shall contain 

the following...multiple-use goals and objectives that 

include a description of the desired future condition of the 

forest or grassland...” (Section 219.11) with the following 

purpose and principles from Section 219.1:

(a)(1) “The resulting plans shall provide for 

multiple use and sustained yield of goods and 

services from the National Forest System in a 

way that maximizes long-term net public benefits 

in an environmentally sound manner;

(b)(1) “Establishment of goals and objectives for 

multiple-use and sustained-yield management of 

renewable resources without impairment of the 

productivity of the land;

(b)(2) “Consideration of the relative values of all 

renewable resources, including the relationship 

of nonrenewable resources, such as minerals, to 

renewable resources;

(b)(3) “Recognition that the national forests are 

ecosystems and their management for goods and 

services requires an awareness and consideration 

of the interrelationships among plants, animals, 

soil, water, air, and other environmental factors 

within such ecosystems; and

(b)(4) “Protection and, where appropriate, 

improvement of the quality of renewable 

resources....”

Desired conditions are an expression of ecological, 

economic, and social sustainability toward which 

management of the land and resources is to be directed 

over the life of the forest plan. Ecologically speaking, 

they describe species composition, structure, and 

landscape pattern. Desired conditions also incorporate 

the natural processes such as nutrient cycling, trophic 

interactions, fire, insects, and diseases. For forests and 

woodlands, desired conditions include age class diversity, 

tree density, overstory and understory composition, and 

the size and density of snags and woody debris. Desired 

conditions are aspirations and are not commitments 

or final decisions approving projects and activities. 

Ecosystem-based management may require long planning 

horizons and must, therefore, incorporate dynamics—

spatial and temporal changes resulting from vegetation 

growth and succession and the periodic resetting of 

these by natural and human-caused disturbances such 

as fire, wind, insects, and tree harvest. For some plant 

communities, desired conditions may be achieved in a 

single management entry, followed by continued natural 

or human maintenance. For others, desired conditions 

may not even be achieved except over a long time period, 

involving multiple successive entries.

The Southwestern Region has established desired 

conditions for the management of vegetation across all 

major ecosystems of the region (USDA Forest Service 

2010). Ecosystems are analyzed using a framework of 

potential natural vegetation types (PNVTs) (TNC 2006), 

a coarse map unit scheme building on similarities in site 

potential and historic fire regime. Desired conditions are 

described in terms of the variability of vegetation patterns 

across spatial and temporal scales within the respective 

PNVTs. Vegetation is quantified and categorized in terms 

of vegetation structure, composition, and dynamics to 

help characterize the desired vegetation conditions.
2

The Southwestern Region’s desired conditions were 

developed by an interdisciplinary team that utilized 

historic range of variation concepts as a beginning 

point. Historic conditions (prior to approximately 1880) 

provide a strong inference of ecological sustainability 

(reference condition). Descriptions of historic conditions 

were modified for the desired conditions based upon 

socioeconomics, operational feasibility, Agency 

policy and direction, and legal requirements such as 

the Endangered Species Act. Other social, political, 

and economic factors are much different today than a 

century ago and there are valid considerations for leaving 

uncharacteristically high densities in some areas to meet 

management needs (USDA Forest Service 1992). Desired 

conditions are broad in theme and may require further 

modification based on the best available science or on 

specific management concerns and issues.

Desired conditions, particularly for dry forest systems, 

reflect a careful balance among ecological, economic, 

and social sustainability considerations. Identifying this 

balance is especially challenging in ponderosa pine and 

dry mixed conifer forests (AKA mixed conifer—frequent 

fire), where much of the region’s social and economic 

desires for preservation and commodity production come 

to bear, and where much of the region’s management 

activities are concentrated. The resulting desired 

conditions form standard regional direction for these 

systems, each reflecting overall sustainability, including 

the economic realities of the limited management options 

to restore and maintain desired composition and structure. 

In this study we focus on the ponderosa pine and dry 

mixed conifer PNVTs where diameter caps are the most 

relevant in regards to desired conditions and forest 

restoration.

Ecosystem Description

Ponderosa Pine Forest

The ponderosa pine forest ecosystem is widespread in 

the Southwest, occurring at elevations typically ranging 

from 6,000 to 7,500 feet (1,800 to 2,300 m) on igneous, 

metamorphic, and sedimentary parent soils with good 

aeration and drainage, and across elevational and moisture 

gradients (Fitzhugh et al. 1987, Romme et al. 2009). As 

currently described, this PNVT is comprised of both the 

“ponderosa pine/bunchgrass” (PPG) and “ponderosa pine/

Gambel oak” (PPO) subclasses (Muldavin et al. 1996), 

collectively referred to as ponderosa pine forest. The 

dominant species in this system is ponderosa pine (Pinus 

ponderosa C. Lawson var. scopulorum Engelm.). Other 

trees, such as Gambel oak (Quercus gambelii Nutt.), 

pinyon pine (Pinus edulis Engelm.), evergreen oak trees 

(Quercus L.), and juniper species (Juniperus L.) may be 

present. There may be a shrubby understory mixed with 

grasses and forbs, although this type sometimes occurs as 

savannah with extensive grasslands interspersed between 

widely spaced clumps or individual trees. This system is 

adapted to drought during the growing season, and has 

evolved several mechanisms to tolerate frequent surface 

fires (Ffolliott et al. 2008). Historically, plant communities 

had over 10 percent tree canopy cover by definition of 

the PNVT. On contemporary landscapes, much of this 

type is dominated by closed forest structure (>30 percent 

tree cover) as a result of fire suppression and other past 

management practices. 

Dry Mixed Conifer

The dry mixed conifer PNVT (AKA mixed conifer—

frequent fire) spans a variety of semimesic environments 

in the Rocky Mountain, Colorado Plateau, and Madrean 

biomes of the Southwest. Mixed conifer forests are 

typically found at elevations between 5,000 and 10,000 

feet (1,500 to 3,050 m), often situated between ponderosa 

pine forests below and wet mixed conifer (AKA “mixed 

conifer with aspen”) or spruce-fir forests above (Romme 

et al. 2009, Sesnie et al. 2009). For the most part, this 

frequent fire type occupies warmer and drier sites of 

the mixed conifer life zone, and are characterized by an 

historic fire regime of frequent (9 to 22 years; Baisan 

and Swetnam 1990; Dietrich 1983; Grissino-Mayer et 

al. 1995; Heinlein et al. 2005), low severity surface fires 

and infrequently mixed severity fires. Typically these 

types were dominated by ponderosa pine in an open 

forest structure (<30 percent tree cover) with Douglasfir (Pseudotsuga menziesii (Mirb.) Franco var. glauca

(Beissn.) Franco) and minor occurrence of quaking aspen 

(Populus tremuloides Michx.) and white pines (Pinus 

strobiformis Engelm. and P. flexilis James). Aspen in the 

dry mixed conifer (DMC) occurs within inclusions of 
3

dissimilar site potential and not as a seral cover type as it 

does within the wet mixed conifer type (Moir and Ludwig 

1979). More shade tolerant conifers, such as Douglas-fir, 

white fir, and blue spruce (Picea pungens Engelm.), tend 

to increase in cover in late succession, and would not 

typically achieve dominance under the characteristic fire 

regime (Brown et al. 2001, Heinlein et al. 2005). These 

species could achieve dominance in localized settings 

where aspect, soils, and other factors limited the spread of 

surface fire. Historically plant communities had over 10 

percent tree canopy cover by definition of the PNVT. On 

contemporary landscapes, much of this type is dominated 

by closed structure (>30 percent tree cover) and climax 

conifer species as a result of fire suppression and other 

past management practices. 

Historically ponderosa pine and dry mixed confer PNVTs 

were composed of mosaics of tree groups of one to 

many trees dominated by ponderosa pine in a generally 

uneven-aged structure, with occasional groups of evenaged forest structure present (Fule et al. 1997, Moore et 

al. 1999, TNC 2006). An understory of grasses, forbs, 

and sometimes shrubs was present and carried frequent 

surface fire. In the DMC PNVT, Douglas-fir frequently 

codominated plant communities with ponderosa pine. The 

abundance of Douglas-fir and other more shade tolerant 

tree species was limited by the high frequency of fire 

disturbance that favored the dominance of ponderosa 

pine due to its fire resistant characteristics. Historical 

inventories of ponderosa pine forests in the Southwest 

provide insight into structure conditions that are 

characteristic of these systems. From Woolsey (1911):

“The typical western yellow pine forest of the 

Southwest is a pure park-like stand made up of 

scattered groups of from 2 to 20 trees, usually 

connected by scattering individuals. Openings 

are frequent, and vary greatly in size. Within the 

type are open parks of large extent.... In pure 

stand of yellow pine the ground cover is usually 

pine grass, bunch grass, and in openings grama 

of various species. The grasses are distributed 

in clumps or patches, interspersed with layers of 

pine leaves of various depths, according to the 

density of the sand [sic]. Underbrush is rare... 

Varying age classes give pure western yellow 

pine a variety of aspects. In places it is made 

up of thrifty pole stands of blackjack, with an 

occasional mature yellow pine fast declining 

in vigor. In others there may be an old mature 

stand of veterans, with complete reproduction 

beneath.”

Three separate areas on the Kaibab National Forest in 

northern Arizona, ranging from 128 to 6,000 acres (52 

to 2,430 ha), that were inventoried by Woolsey (1911) 

averaged about 30 ft2

/ac (7 m2/ha) tree basal area. One 

sample area of about 500 acres (200 ha) on the northern 

rim of the Grand Canyon, had a basal area of about 55 ft2/

ac (13 m2/ha)(Lang and Stewart 1910). This description 

is dated from about 25 years after the onset of heavy 

livestock grazing and the disruption of the characteristic 

fire regime to suggest that forest conditions may have 

been even more open than what are described in this 

excerpt. 

Desired Conditions – Ponderosa 

Pine Forest and Dry Mixed Conifer

The following description of desired conditions was 

abstracted from guidelines written for the USFS 

Southwestern Region (USDA Forest Service 2010). 

The description describes forest structure, composition, 

process, and spatial pattern for the ponderosa pine and dry 

mixed conifer systems.

Uneven-aged forest structures comprise a distribution 

of age classes within stands. Overall, the landscape is 

generally dominated by ponderosa pine, the tree species 

most resilient to wildfire effects. Juniper, pinyon, oak, and 

other hardwood species are collectively well represented 

depending on the plant association, and are regenerating 

successfully where local forest biophysical conditions are 

appropriate for development of these species. Aspen can 

occur in inclusions of dissimilar site potential. Overall, 

forests are vigorous with characteristic levels of native 

insect and disease occurrences. In the DMC Douglas-fir 

and white pine can occur as codominants. White pine is 

present through much of this PNVT, with a wide range of 

genetic diversity, and regenerating in suitable locations. 
4

In both the ponderosa pine and dry mixed conifer systems, 

a variety of forest density, spatial arrangement, and age 

and structure conditions exists across the landscape 

similar to historic conditions. Conifer forest types are 

composed of a distribution of age classes that comprise 

a sustainable mix of structural stages (early, middle, and 

late succession). Forest canopy gaps and openings occur 

on 20 to 40 percent of each stand area, with the exception 

of designated Mexican spotted owl habitats, and goshawk 

nesting areas where canopy cover is more continuous, but 

gaps can still be present. These canopy gaps and forest 

openings mimic historic spatial patterns and provide for 

the regeneration of ponderosa pine and the development 

of herb and shrub species, and facilitate reintroduction 

and maintenance of frequent surface fire as an ecological 

process. Forest canopy gaps and openings are dynamic 

over time, shaped by small-scale disturbances including 

fire and subsequent vegetation development, with some 

areas regenerating new tree groups of one to many trees, 

while other areas remain as openings that contribute to 

ecosystem diversity by supporting tree group rooting 

zones and grass-forb and shrub habitat. Managed unevenaged stands range from 15 to 45 percent of maximum 

stand density index (see appendix B). In areas outside of 

MSO nest/roost and replacement nest/roost habitats, basal 

areas average less than 80 ft2

/acre (18 m2/ha), and bark 

beetle hazard is low. In most cases, ponderosa pine and 

mixed conifer forest stands exhibit open and uneven-aged 

characteristics (multistoried), except for stands managed 

for goshawk nest and MSO nest/roost and replacement 

nest/roost habitats (MSO habitat may or may not be 

multistoried).

Dwarf mistletoe is an element of the forest landscape. 

There is a varied level of mistletoe across the landscape, 

comparable with historic conditions (Conklin and 

Fairweather 2010) such that it does not impede achieving 

and sustaining desired uneven-aged forest conditions. 

Desired stand dwarf mistletoe infection levels do not 

exceed 20 percent infection of the host species (trees per 

acre basis) or 25 percent of the area infected for any given 

tree species. Dwarf mistletoe infections are irregularly 

distributed among tree groups, such that effects are 

limited to the forest group scale.

Detailed descriptions of desired conditions are in 

appendix A.

Diameter Cap

An issue confronting the implementation of forest 

desired conditions is the concept of a diameter cut limit 

imposed with harvest prescriptions. In the past, diameter 

caps have been used as a means to preserve large trees, 

often those over 16 inches (23 cm) in diameter, leading 

to a so-called “16-inch cap.” The approach originated 

in the environmental community over concerns for the 

preservation of large trees including:

 Impression that there are relatively few large trees 

remaining on the landscape (Abella et al. 2006);

 Loss of large trees through commercial logging;

 Harvesting trees is aesthetically unappealing; and

 The removal of large trees is a return to 

commercially-focused logging (Coughlan 2003, 

Larson and Mirth 2001).

The 16-inch threshold is an arbitrary value given its 

relation to forest restoration and desired conditions. There 

may be instances where the removal of small diameter 

trees (that happen to all be less than 16 inches in size) 

favor the desired tree age distribution and the restoration 

of an uneven-aged system. We differentiate the intent 

of such a prescription from an indiscriminate, broadly 

applied diameter cap, when considering disparities 

between existing and desired conditions.

Previous compromises were negotiated among local 

stakeholders resulting in project-level agreements to 

implement diameter caps. Diameter caps have since 

become a common practice on some national forests 

(Abella et al. 2006, Coughlan 2003, Fule et al. 2006, 

Larson and Mirth 2001). The few related empirics that 

have been generated to date suggest that such a policy is 

unsustainable (see “Related Studies”). The Southwestern 

Region has concerns that such constraints will limit 

achievement and maintenance of desired conditions for 

long-term forest structure, composition, and dynamics, 

particularly in frequent fire types characterized by open 

tree canopies and multistoried conditions. 
5

Related Studies

Abella and others (2006) recently assessed the 

effects of diameter cap constraints in ponderosa pine 

forest restoration focused treatments and considered 

arguments for and against the approach. Treatment 

objective outcomes were similar to Forest Service 

desired conditions: the treatment guidelines focused on 

reduction of tree densities to within historic ranges, and 

reestablishment of presettlement tree patterns of one 

to several trees clumped among forest openings, based 

on historical evidence (Covington et al. 1997). In the 

diameter cap scenario, guidelines were implemented only 

as the 16-inch cap would allow. Results are summarized 

here:

 Final basal area was 87 and 39 ft2/acre (20 and 

9 m2/ha) for the cap and no-cap treatments, 

respectively, a difference of approximately 50 ft2/

acre (11 m2/ha). 

 Where there were relatively few trees greater than 

16 inches in diameter in the current condition:

 The effect of a diameter cap was negligible.

 Tree groups and openings were effectively 

created in the diameter cap treatment.

 The difference in foliar biomass (a metric 

positively linked to crown fire behavior) 

between the treatment types was negligible.

 Where the current conditions was comprised of 

a relatively high number of trees greater than 16 

inches in diameter:

 The diameter cap treatment retained an 

additional 28 trees/acre (69 trees/ha) compared 

to the no-cap treatment.

 It was problematic to establish canopy gaps 

and retain tree groups necessary for an unevenaged forest management prescription.

 Foliar biomass was significantly higher in the 

cap-constrained treatment versus the no-cap 

treatment (51 percent vs. 21 percent). 

Abella et al. (2006) also caution about assessing the 

effects of a diameter cap based on conditions immediately 

after treatment. The cap may respond to certain shortterm management objectives of fuel reduction or large 

tree retention, but the cap essentially sets a minimum tree 

density, basal area, and canopy biomass, and limits the 

developmental dynamics typical of this forest ecosystem. 

The residual trees will continue growing and increasing 

biomass, canopy closure, and fire risk.

An unpublished study by Higgins (2011) has also 

challenged the notion that large trees have become rare 

in the Southwest. In an assessment of historic inventory 

data from the Kaibab National Forest and other forests 

in northern Arizona, Higgins compared contemporary 

and historic conditions in ponderosa pine forests. He 

determined that all ranger districts on the Kaibab have 

a greater number of trees 16 inches in diameter and 

larger than they did historically, based upon inventories 

reported in Woolsey (1911) and Lang and Stewart 

(1910). It’s only at larger tree diameters that current and 

historic tree densities are approximately equivalent—20 

inches on the Tusayan district, 22 inches on the Williams 

district, and over 24 inches on the North Kaibab Ranger 

District. Higgins’ study included a similar assessment of 

conditions across broader scales in northern and central 

Arizona that are indicative of the region (figure 1). In this 

assessment, the current average and maximum density 

of trees 16 inches and over was compared with historic 

conditions as interpreted from Woolsey and Lang and 

Stewart.

Figure 1 suggests that the density of large trees, 16 inches 

and greater, has increased in recent decades compared 

to historic conditions. Though the density of large trees 

has increased, the overall proportion of large trees today 

has decreased with fire suppression and the increasing 

abundance of small trees. The historic stands of maximum 

density (“Max”) reflect the efforts of early inventory 

crews charged with deliberately locating and measuring 

those stands with the highest densities of large trees. 

Today’s large tree density in northern Arizona is greater 

than historic in all but the densest stands located in the 
6

early 1900s. This assessment does not account for spatial 

variation, and it’s likely that the spatial distribution of 

large trees is more homogenous than it was historically. 

Under a diameter cap scenario, the proportion of large 

trees continues to increase with each thinning treatment 

that targets only small trees. Figures 2 and 3 display 

similar trends but on a regional basis for the Southwest.

Study Objectives

The current study seeks to supplement the work of Abella 

and others, including Fiedler et al. (2002), who have 

explored diameter cap strategies and related issues. With 

this study, we expand the scope of the evaluation of a 

diameter cap to include extended model simulations and 

additional empirics and model calibration for the analysis 

of structure.

A 16-inch diameter cap has been suggested, either 

specifically or in practical effect, as a programmatic 

approach to restoring forest systems that are at moderate 

to high risk of stand-replacing fire, as a management 

theme in forest plan revision processes currently 

underway in the region. This study represents an effort 

to compile, develop, and report available science on the 

effects of a diameter cap to achieving and maintaining 

desired conditions within uneven-aged forest ecosystems. 

We’ve documented the results of Vegetation Dynamics 

Development Tool (VDDT) (ESSA 2006) and Forest 

Vegetation Simulator (FVS) modeling (Dixon 2002), 

testing the effects of a programmatic application of 

a 16-inch diameter cap on management for desired 

conditions in ponderosa pine and dry mixed conifer types. 

This manuscript is based in large part on methodology 

recently developed to calibrate VDDT models using FVS 

(Weisz et al. 2010). We did not examine site-specific 

circumstances where application of a prescription 

incorporating a diameter cap may meet project objectives. 

Rather the study is focused on the effects of a diameter 

cap on achieving or maintaining desired conditions at the 

programmatic level, at the scale of national forests.

For the purposes of this analysis and discussion, three 

primary elements are considered to determine whether 

management strategies move conditions toward or away 

from desired conditions: (1) forest structure (age/size 

class distribution, density, spatial arrangement), (2) forest 

composition (species), and (3) ecological function (fire 

behavior).
7

Figure 2. Tree density by 

size class for ponderosa 

pine forest types on 

nonreserve forest lands of 

national forests in Arizona 

and New Mexico, by 

inventory period. 

Figure 3. Density of trees 

15 inches in diameter and 

greater, for the ponderosa 

pine types on nonreserve 

forest lands of national forests 

in Arizona and New Mexico, 

by inventory period.

Figure 1. Current average and 

maximum density of trees ≥ 16 

inches compared with historic 

conditions as interpreted from 

Woolsey (W) (1911) and Lang 

and Stewart (L&S) (1910). The 

proportion of all trees that is made 

up of those 16 inches and greater is 

also included. Historic data sources 

represent the Prescott NF (PNF), 

Tusayan NF (TNF) (now part of 

the Kaibab NF), and Coconino 

NF (CNF). “Max” reflects the 

densest stands located during the 

early 1900s inventories. Current 

conditions are represented by FIA 

(FIA-All) across the same region of 

north-central Arizona. 
8

Study Area

Our analysis was focused on the dry forest systems 

of the USFS Southwestern Region, as they occur on 

Forest Service lands of Arizona and New Mexico. All 

Forest Service lands have been mapped by PNVT, and 

the analysis specifically considered the ponderosa pine/

grassland, ponderosa pine/Gambel oak, and dry mixed 

conifer PNVTs as represented by a probability sample of 

vegetation across these systems (see “Data”). Descriptions 

for these systems are included in the introduction. The 

analysis focused on actively managed lands. 

Data

Data for the modeling analysis were derived from the 

Forest Inventory Analysis (FIA) Program. The FIA 

established a systematic vegetation inventory where data 

are periodically gathered on permanent plots from all 

forested lands across the United States (U.S. Code 1998; 

16 USC § 1642). Plots are randomly located 

on a sample design with approximately 5,000 

meters between plots, and the exact plot 

locations are kept secret to prevent bias. Plots 

that occur on national forests in Arizona and 

New Mexico in PPG, PPO, and DMC were 

used as statistical samples for modeling with 

the Forest Vegetation Simulator (FVS) (see 

“FVS Modeling”). All plot data were used 

to calibrate natural growth in the absence of 

disturbance, while finer subsets were used to 

calibrate the effects of specific management 

activities (Moeur 2011). The FVS model is 

a well established stand simulator program 

(Dixon 2002) used to project stand growth over 

time under the influences of site productivity, 

natural disturbance, and silviculture. A 

detailed process of sample data compilation 

and synthesis, over several steps (Weisz et al. 

2011), was required to stratify and prepare FIA 

tree list data sufficient to enable FVS model 

projections and the analysis of a diameter cap 

scenario.

Methods

Stratification within Ecosystems

Both the ponderosa pine and dry mixed conifer systems 

have been stratified by ecosystem states of specific 

combinations of size, cover, canopy layering, and 

dominance type (Triepke et al. 2005). State and transition 

models have been built for each major ecosystem of the 

Southwest using the Vegetation Dynamics Development 

Tool (VDDT), a landscape analysis software developed 

to project vegetation conditions over time (Weisz et al. 

2009). Each VDDT state represents an important phase 

in the ecosystem dynamics of a PNVT, either in terms 

of ecological processes or management themes. Table 1 

summarizes the ecosystem states and their differentiating 

criteria for the ponderosa pine and dry mixed conifer 

systems. For each of the three PNVTs, FIA samples were 

respectively assigned to the appropriate model state 

(table 1), and then grouped with similar strata (table 2). 

Grouping by similar strata ensured the larger sample 

Table 1. Stratification of ponderosa pine and dry mixed 

conifer states A through M, according to key attributes 

of canopy layering, canopy cover, and predominant tree 

diameter

Canopy 

Layering

Canopy 

Cover

GFBSHR1

Tree Diameter2

0-5" 5-10" 10-20" 20"+

Single Open3 A, N4 B C D E

Single Closed3 F G H I

Multi Open J5 K5

Multi Closed L M

1 GFB-SHR – Plant communities with ≤10 percent tree canopy cover, 

dominated by grass, forb, or shrub species.

2 Tree diameter classes determined by the size class (0–5", 5–10", 

10–20", 20"+) with a plurality of basal area.

3 Tree canopy cover classes include “open” (10–29.9 percent) or “closed” 

(≥30 percent).

4 There are two grass-forb-shrub states differentiated by successional 

sequence, where state A represents the historic trajectory of reforestation 

following localized disturbance, and state N represents a static, 

uncharacteristic state with little or no tree regeneration following standreplacing fire on contemporary landscapes. 

5 Desired condition, open multilayered (representing young, mid, and old 

trees), average diameter varying by site productivity
9

Table 2. FIA sample plot numbers and proportion for each model stratum 

of the PPG, PPO, and DMC PNVTs

Model 

Stratum

PPG PPO DMC Total

n Percent n Percent n Percent n Percent

JKLM 84 30 48 33 91 48 223 33

DEHI 97 35 38 26 29 15 223 33

CG 57 21 46 32 42 22 145 22

ABF 40 14 14 10 28 15 82 12

Total 278 100 146 100 190 100 673 100

sizes necessary for reliable model projections, while still 

allowing for some evaluation of variance in the outputs.

FVS Modeling

Simulations were conducted using FVS with the objective 

of modeling a 16-inch cap. Eighty-year simulations were 

used with successive prescribed cutting treatments to 

achieve and maintain 60 ft2/ac of tree basal area, but with 

an overriding retention of all trees 16 inches and above 

in diameter. A value of 60 ft2/ac is near the middle of the 

desired range of 30–80 ft2/ac (7–18 m2/ha) for ponderosa 

pine, which may be retained in this range for most of a 

30-year period between scheduled treatments. For DMC, 

60–80 ft2/ac (14–18 m2/ha) is considered a suitable range 

of mid-points given the greater target basal area range 

of 40–100 ft2/ac (9–23 m2/ha) for mixed conifer, but 

modeling results are based on 60 ft2/ac. Accordingly, the 

FVS model was carefully parameterized to reflect cutting 

treatments designed to achieve or maintain stand densities 

at 60 ft2

/ac, while imposing strict retention of all trees 

over 16 inches. Where current stand conditions conflict 

with attainment of both objectives, retention of all trees 

greater than 16 inches in diameter was given preference 

over the density target for modeling the effects of this 

constraint. Two subsequent cuttings with the identical 

treatment prescriptions were scheduled in the model at 30 

and 60 years following the initial treatment. Unlike other 

variants of FVS, the Southern Rockies variant used with 

this analysis does not include default imputation for tree 

regeneration. For parameterization of tree regeneration 

we followed Vandendriesche (2010), and assumptions 

involving stand density index are 

discussed in the following section, 

“Species Composition in Dry 

Mixed Conifer.” 

The FVS model is capable of 

reporting several stand variables 

to support the evaluation of 

silvicultural treatments and effects 

of stand dynamics. Averages and 

variance of the variables were 

reported for each stratum, along 

with values for individual plots, all 

computed for each decade of the simulation. The stratum 

averages include:

 Basal area (BA)

 Quadratic mean diameter (QMD)

 Stand density index (SDI)

 Trees per acre (TPA) by species and diameter class

The variables computed for each plot were given by the 

diameter classes of 0–5" (0–13 cm), 5–10" (13–25 cm), 

10–20" (25-51 cm), and 20"+ (51 cm+) and include:

 BA

 TPA

 SDI

For ecological sustainability analysis in the Southwestern 

Region, a canopy cover threshold of 30 percent was 

established to differentiate open and closed forest 

canopies. Ecological syntheses by TNC (2006) and others 

have shown key differences in the fire regime and the 

resulting forest structure and wildlife habitat below and 

above this approximate value for ponderosa pine and dry 

mixed conifer systems. For purposes of this analysis, a 

basal area threshold of about 60 ft2/ac (14 m2/ha) is used 

to represent an equivalent level of forest structure to 

differentiate open and closed conditions by the basal area 

metric (Mitchell and Popovich 1996). Open states are 

those with basal area values less than about 60 ft2/ac—

states A, B, C, D, E, J, and K (table 1). Closed states—F, 

G, H, I, L, and M—are states with at least 60 ft2/ac of 

basal area. Note that the PNVTs evaluated with this study 
10

share the same naming convention for their respective 

model states.

The FVS also provides both age diversity and canopy 

layering, or storiedness, as a means to evaluate the 

uneven-agedness of a stand. Given the positive 

relationships of tree size and diameter, storiedness gives 

a useful inference of age diversity that is apparent. 

Storiedness is also one of the primary differentia of 

ecosystem states (table 1), and a key characteristic of 

these ecosystems, where uneven-aged and multistoried 

conditions reflect the desired condition (states J and K). 

Storiedness was reported, along with the variables listed 

above, immediately preceding the initial cutting treatment 

and then for every subsequent decade over the next 70 

years of the simulation. The storiedness algorithm used in 

our analysis was used as an inference of tree age diversity, 

and is described in the following decision tree:

Is the basal area of trees that are ≥20" (51 cm) in 

diameter at least 20 percent greater than the basal area 

of trees that are ≥10" (25 cm) and <20"?

 NO – Single storied

 YES – Is the basal area of trees that are ≥20" in 

diameter at least 20 percent greater than the basal 

area of trees that are ≥0" and <10"?

 NO – Single storied

 YES – Multistoried

Results of FVS modeling for PPG, PPO, and DMC are 

reported by each decade of the 80-year simulation (table 

3). The basal area and storiedness values calculated 

for each stratum and each decade were weighted by 

the number of FIA plots in each stratum, and then 

summarized into a percent attainment of desired 

conditions. Individual states within each stratum were 

identified by computations made for each decade 

according to differentiating values of basal area, 

storiedness, and dominant tree size range. The most 

frequent state was also identified for each decade and 

for the modeling period. These same variables were also 

averaged across each stratum to assess the attainment 

of desired conditions in each reporting year across the 

simulation.
11

The FVS modeling results are summarized by the proportion 

of stands that meet desired conditions with a 16-inch 

cap strategy (table 3). Conditions listed for 2010 reflect 

overall stand conditions just prior to the first treatment. As 

mentioned, prescribed cutting treatments were repeated two 

more times during the simulations in years 2030 and 2060. 

The table includes critical attributes of basal area (BA) and 

multistoried (MS) conditions, where storiedness is used as 

an inference of age diversity. In this study, whether overall 

desired conditions are met is determined by whether both 

attributes of basal area and storiedness are met.

In an effort to provide an unbiased accounting of the 

effects of a diameter cap policy, model simulations were 

made on the assumptions that regulatory framework and 

the availability of resources did not limit the access and 

treatment to all stands. In our modeling, each suitable stand 

was subject to the regular cutting treatments prescribed in 

FVS. And in this context for a stand to be suitable it must 

have had merchantable trees less than 16 inches in diameter 

in a stand that did not meet desired conditions.

The results in table 3 show that nearly all stands were 

considered multistoried at the beginning of the modeling 

period. In all three PNVTs, the percentage of stands that 

meet multistoried conditions continued to decline as the 

proportion of trees larger than 16 inches increased across 

the simulation and the positive effects of treatments were 

negated. The loss of multistoried conditions appears to 

accelerate in the first half of the modeling period. By the 

time the third treatment is implemented, conditions in 

both storiedness and basal area are more or less static, 

unless natural disturbance creates new canopy gaps (see 

“Discussion”). This analysis shows that within 3 decades, 

all simulation stands would be converted to conditions 

that are functionally even-aged, given that younger 

cohorts do not take on diameter growth typical of unevenaged systems due to the poor physiological conditions 

of trees growing under high-density forest canopies, and 

given the lack of tree regeneration. 

Results also show with all three PNVTs that basal area 

targets are achieved one decade after the initial treatment 

and, in the case of ponderosa pine, the percentages trend 

downward with time as fewer trees under 16 inches 

remain to present opportunities to modify stand conditions 

toward desired conditions. On the other hand, dry mixed 

conifer affords the ability to achieve basal area targets 

at least as reflected in the beginning 80 years of the 

management scenario, but with an increasing plurality of 

shade tolerant conifer species (see “Discussion”). In part 

this is due to differences in site types in mixed conifer 

that favor greater tree density and regeneration of shade 

tolerant species (see “Species Composition in Dry Mixed 

Conifer”). However, the cutting constraints can also 

force tradeoffs between basal area and desired species 

composition targets in dry mixed conifer forests (see 

“Discussion”).

In nearly all stages of the simulations and across all three 

PNVTs, the inability to develop or maintain uneven-aged 

forest structure is the most limiting factor in achieving 

desired conditions. The ability to progress toward 

desired structure conditions is achievable through the 

first treatment in some stands, with overall percentages 

declining significantly approximately 40 years into the 

simulation to 14 percent, 10 percent, and 37 percent for 

PPG, PPO, and DMC, respectively. That multistoried 

conditions are achieved in DMC is enabled, in large part, 

by the regeneration of shade tolerant conifers, counter 

to the desired species composition for the PNVT (see 

“Discussion”).

Results

Table 3. The proportion of forest ecosystems 

meeting desired conditions under a diameter cap 

policy are listed according to target ranges for basal 

area (BA) and multistoried (MS) conditions; results 

based on FVS modeling over an 80-year simulation

Year

PPO PPG DMC

BA MS BA MS BA MS

2010A 14% 100% 10% 100% 15% 100%

2020 100% 35% 100% 41% 100% 100%

2030 14% 35% 10% 41% 100% 100%

2040 70% 35% 68% 41% 63% 52%

2050 35% 14% 10% 10% 100% 37%

2060 0% 14% 0% 10% 100% 37%

2070 14% 14% 10% 10% 100% 37%

2080 14% 14% 10% 10% 100% 37%

AConditions prior to initial cutting treatment.
12

Our analysis indicates that maintaining desired conditions 

for the PPG, PPO, and DMC systems is problematic 

with a diameter cap policy, and that achieving desired 

conditions where they do not exist on contemporary 

landscapes is even more difficult. Tree density, as 

determined with basal area, and age diversity, as inferred 

from forest canopy storiedness, were the two principle 

attributes used to assess consistency with the desired 

conditions that have been formally defined for these 

three systems for the USFS Southwestern Region (USDA 

Forest Service 2010). In large measure, basal area and 

storiedness alone can be used for evaluation. In DMC, 

the composition of tree species is another primary 

consideration (Arno and Fiedler 2005).

Analysis by FVS suggests that desired conditions for 

basal area in ponderosa pine, 30–80 ft2

/ac, cannot be 

maintained over time and even in the short term can only 

be met on a minor percentage of the area. Examination of 

individual FIA samples tells us that in some cases where 

desired conditions have been met, too few large trees exist 

in the first place so that a diameter cap is irrelevant, and 

the “diameter cap” label does not fit the true silvicultural 

method. That only the mid-point of the target range was 

considered for modeling suggests that the likelihood of 

achieving lower values of the range, even more indicative 

of historic conditions, would be that much more difficult 

on a programmatic scale. And even when desired 

conditions are met for basal area, a much smaller minority 

of stands meet age diversity targets in most instances. 

Meeting composition targets in DMC is also unlikely (see 

“Species Composition in Dry Mixed Conifer”).

At the beginning of the simulation nearly all stands were 

considered multistoried, suggesting that the algorithm 

used is very liberal in making the assignment—i.e., the 

stand is more likely to meet desired conditions by this 

algorithm. The current algorithm represents only a 2-class 

system, versus a 3-plus story system indicative of true 

uneven-aged conditions. The storiedness algorithm is 

being refined for the Southwestern Region to reflect a 

3-class system to reflect forest plant communities that are 

truly uneven-aged.

Following implementation of a 16-inch cap, the 

percentage of stands that meet uneven-aged conditions 

declined precipitously due to the preservation of all large 

trees representing the upper cohorts, treatment tradeoffs 

between stand structural diversity and basal area targets, 

and the simultaneous reduction in opportunities for tree 

regeneration necessary to sustain the recruitment essential 

for an uneven-aged/diameter distribution within the plant 

community. Imposing diameter caps in managed density 

stands most often results in the elimination of smalland mid-sized diameter trees, due to the tradeoff forced 

between achievement of stand structure and density 

targets.

As expected, the manager can ostensibly progress toward 

desired conditions with diameter constraint following 

the initial entry on a minor percentage of the landscape; 

but, the effects of the harvest are short term so that 

the percentage moving toward desired conditions is 

reduced to less than half of the peak overall percentages 

only 40 years into the projection (table 3). Despite the 

immediate pulse in stands progressing toward desired 

density conditions, even a temporary diameter cap policy 

has a long-term liability on opportunities to manage for 

(implement or maintain) uneven-aged forest structure. 

All else equal, maintaining a diameter cap policy for even 

a short term reduces the number of stands suitable for 

treatment to achieve uneven-aged conditions, given the 

lost opportunities for ensuring continual tree regeneration 

and age diversity that an increasingly dense single-storied 

component of 16" plus trees forestalls. Growth rate is 

likewise affected. Forty years of sample data from the 

Taylor Woods study, at Fort Valley Experimental Forest 

in northern Arizona, show that, for example, a 12-inch 

tree (30 cm) will attain a 16 to 18 inch diameter in about 

a decade at a rate of disturbance commensurate with preEuropean settlement disturbance regimes (Bailey 2008). 

Trees in lower density conditions grew significantly faster 

in diameter, crown length, and crown width; that is, the 

smaller size of younger cohorts is temporary as long as 

competition is sufficiently controlled by silvicultural 

treatments or other disturbance. 

As mentioned, the FVS simulations in this study were 

conducted under the assumption that forest managers 

would have resources to consider treatment in all stands, 

without regulatory hindrance, to reduce complicating 

Discussion
13

factors in the analysis. In reality, resources, regulation, 

and policy will substantially limit the manager’s ability 

to restore and maintain desired conditions on a broad 

scale. Results by this study suggest that the effects of a 

diameter cap policy would be further exacerbated by these 

limitations, given that managers will tend to focus their 

energy on the limited opportunities where the economics 

support harvest activities or where stand conditions 

favor successful outcomes. In these cases the likelihood 

of achieving desired conditions over the long term are 

reversed by a policy that incidentally favors dense singlestoried forest conditions. The remainder of untreated 

stands will tend to average higher densities with time (see 

table 3, 2010 conditions).

Collateral effects of a diameter cap policy in PPG, 

PPO, and DMC involve changes in fire risk and habitat 

diversity associated with changing fire regimes and, 

in DMC, significant shifts toward shade tolerant, fire 

intolerant species of conifers, which impact long-term 

sustainability. The composition shifts also reduce 

biodiversity and key wildlife habitat components such as 

shade intolerant hardwood species, including aspen and 

oak.

Effects of Stand Density

Stand level characteristics for desired conditions of PPG, 

PPO, and DMC have also been qualitatively described 

(see appendix A), and include spatial arrangement 

for trees within the plant community subject to local 

circumstances. Desired conditions for all three PNVTs 

specify an arrangement of trees into clusters or groups of 

one to many trees, some with high density, surrounded 

by openings of sparse tree cover that are dominated 

by grasses or shrubs (depending on the particular 

plant association). Under a diameter-cap policy, the 

characteristic spatial arrangement of trees in many areas 

is becoming increasingly difficult to attain as former 

openings regenerate, many now with trees over 16 inches 

in diameter (Moore et al. 2004). Also, since no trees 

over 16 inches would be cut to reach density objectives, 

the pressure to harvest more or all trees under 16 inches 

is likely to result in more homogenous structure, as 

trees under 16 inches are targeted to achieve the desired 

conditions for basal area. Regular interspersion of 

tree groups of differing ages is likewise important for 

biodiversity:

 According to Reynolds and others (1992), high 

interspersion is of moderate to high value for 11 

of 14 goshawk prey species. The authors also 

report that an herb-shrub component characteristic 

of forest gaps is important for a separate set of 

11 prey species. The limitations of diameter 

cap management are likely to inhibit tree group 

interspersion and the sustenance of an understory 

with characteristic composition.

 Reductions in overstory density to more 

heterogeneous patterns of openings among 

trees groups of moderate density levels are 

commensurate with objectives to increase 

understory plant diversity (Griffis et al. 2001, 

Laughlin et al. 2006, Moore and Dieter 1992).

 Even moderately dense tree cover reduces native 

plant abundance (Griffis et al. 2001, Sabo et al. 

2008) and diversity (Bataineh et al. 2006)(Laughlin 

et al. 2004).

Similar studies on and near the Kaibab National Forest 

in northern Arizona show a strong correlation between 

overstory density and understory species diversity and 

production (Abella 2009, Bataineh et al. 2006, Moore 

and Deiter 1992). Deiter (1989) showed that the basal 

area of trees must be below about 65 ft2/ac (36 m2/ha) to 

produce a robust understory response, defined as at least 

50 percent of site potential.

In DMC, as forests become denser under a diametercap policy, their ability to regenerate ponderosa pine 

sustainably is frustrated by the regeneration of shade 

tolerant conifers favored under scenarios of limited 

and infrequent disturbance (Heinlein et al. 2005) (see 

“Species Composition in Dry Mixed Conifer”). Though 

intervention is still necessary to control the growth of 

shade tolerants, canopy gaps facilitate the regeneration 

of ponderosa pine, the development and maintenance 

of uneven-aged forest structure, and the diversity and 

abundance of understory vegetation.
14

In summary, stand density provides 

strong inference for ecosystem dynamics 

and associated structure and habitat 

characteristics. Irregular forest density, 

for instance with canopy gaps and 

nonforest openings, can facilitate the 

reintroduction and maintenance by 

frequent surface fire and allow land 

managers more control over fire (Pollet 

and Omi 2002). This type of forest 

structure can also limit the transition 

of surface fire to crown fire during 

severe burning conditions, reducing the 

probability of uncharacteristically severe 

fire effects (DeBano et al. 1998, Scott 

2003). The restoration of historic forest 

spatial patterns, including canopy gaps 

and tree groups, is critical to proper ecosystem function 

and related biodiversity. 

Species Composition 

in Dry Mixed Conifer

The composition of tree species in dry mixed conifer 

forests is of particular interest to forest managers in the 

West (Arno and Fiedler 2005, North et al. 2009), and adds 

an important restoration component equal to tree density 

and vertical stratification. For analysis on the effects of a 

diameter cap on the restoration and maintenance of forest 

composition in DMC, we turn to related work by Moeur 

(2011) and to key concepts of stand density index (SDI).

First, in her recent analysis Moeur (2011) modeled 

changes in tree species composition in dry mixed conifer 

systems as the result of a standard set of management 

activities (table 4). Each management activity was 

evaluated by simulating silvicultural and fuels treatment 

prescriptions with FVS, using subsets of FIA plots 

associated with each VDDT model state. The FIA tree 

list data were compiled from samples occurring in DMC 

forests across USFS lands of the Southwest. Simulations 

were generated for each management theme, the results 

analyzed according to the effects by management on 

structure and composition.

Using white fir as a key successional indicator, results 

are summarized in figure 4 for all management activities. 

White fir has low resistance to fire, is shade tolerant 

(Burns and Honkala 1990), and is favored by infrequent 

fire and closed canopy conditions. Historically it was 

a subordinate component in DMC (as with desired 

conditions) and is now much more abundant, due 

primarily to fire suppression and the capacity of white fir 

to thrive in high stem densities (see following discussion 

on stand density index). Figure 4 shows the considerable 

disparity in the density of large white fir trees between 

the 16-inch diameter cap and uneven-aged management 

treatments. After one treatment, the density of white fir 

in the diameter cap scenario is about three times that of 

the group selection immediately following treatment. The 

diameter cap scenario points to the inability of managers 

to meaningfully affect composition of the residual stand.

This assessment provides an example of how a shade 

tolerant species is retained in higher than desired 

proportions relative to desired conditions as a result of the 

cutting constraint. This analysis does not examine species 

composition of tree species regeneration following 

treatment, and the subsequent cumulating effects on stand 

composition. Large white fir and other undesired species 

that are retained by this cutting constraint become seed 

trees that perpetuate shade tolerant species and jeopardize 

true restoration objectives. Stand density index can be 

Table 4. Standard management activities evaluated using FVS, as 

represented by FIA samples across dry mixed conifer forests of the 

Southwest

Code Management Activity

B Free thin, all sizes to target basal area, 50 ft2/ac (11 m2/ha)

C Thin from below to target basal area, 80 ft2/ac (18 m2/ha)

D Thin under a 16-inch diameter cap to target basal area, 60 ft2/ac 

(14 m2

/ha)

E Group selection with matrix thin to target basal area, 70 ft2/ac (16 

m2

/ha)

F Shelterwood seed cut to target basal area, 30 ft2/ac (7 m2/ha)

J Prescribed fire, low intensity burning conditions

K Prescribed fire, moderate intensity burning conditions

L Prescribed fire, high intensity burning conditions

M Thin under a 9-inch diameter cap, 150 ft2/ac (34 m2/ha)
15

used to show that diameter cutting constraints are likely to 

result in profound shifts in tree species composition over 

time, favoring the regeneration and sustenance of shade 

tolerant and fire intolerant species.

The stand density index (SDI1

) is a conceptual framework 

that relates an absolute density measure to forest stand 

developmental dynamics (detailed discussion in appendix 

B). The SDI provides a long accepted, operational 

methodology for stand dynamics. Long (1985) divided 

the stand density index into four zones based on the 

percent of the overall density of a tree stand relative to 

the biological maximum density, a species-specific value. 

Table 5 displays the four zones, and includes descriptions 

of the dynamics of tree regeneration establishment, 

competition, and growth based on stand density 

percentages relative to the maximum SDI values specified 

for each tree species.

Figure 4. Large white fir tree density (TPA) immediately before and after each management 

treatment scenario (table 4) simulated by FVS, as an indication of successional status and 

consistency with desired conditions (where white fir is subordinate).

The stratification of stand density index in table 5 

provides a useful means for discussing stand dynamics 

relative to species composition, and the implications 

of varying the timing, scale, and intensity of density 

management to affect the variety of stand and tree 

characteristics (Long 1985, Long et al. 2004, Shaw and 

Long 2010):

 Regeneration of desired species can be initiated 

by maintaining stand density in zone 1, based on 

maximum SDI of desired species. The regeneration 

model utilized in FVS was based on this 

assumption (see “Methods”).

 Open canopy stands with grassy understories 

and large diameter trees with long, heavy-limbed 

crowns can be developed by targeting densities in 

zones 1 and 2.

 Stands of moderate crown closure and intermediate 

sized trees with thrifty, well-pruned crowns can be 

1 Measure of stand density in relationship to number of trees in a stand to quadratic mean diameter expressed as a percentage of the 

maximum (SAF 1998).
16

Table 5. Percent of maximum SDI (Langsaeter 1941) by density zone according to key stand dynamics 

(Long 1985)

Percent 

Maximum SDI* Zone Stand and Tree Characteristics

0–24 percent

Low density 1

—Less than full site occupancy, maximum understory forage production.

—No competition between trees, little crown differentiation.

—Maximum individual tree diameter and volume growth.

—Minimum whole stand volume growth.

—Tree regeneration freely establishes.

25–34 percent

Moderate density 2

—Less than full site occupancy, intermediate forage production.

—Onset of competition among trees, onset of crown differentiation.

—Intermediate individual tree diameter and volume growth.

—Intermediate whole stand volume growth.

35–60 percent

High density 3

—Full site occupancy, minimum forage production.

—Active competition among trees, active crown differentiation.

—Declining individual tree diameter and volume growth.

—Maximum whole stand volume growth.

—Upper range of zone marks the threshold for the onset of density-related mortality.

>60 percent

Extremely high 

density

4

—Full site occupancy, minimum forage production.

—Severe competition among trees, active competition induced mortality.

—Minimum individual tree diameter and volume growth, stagnation.

—Declining whole stand volume growth due to mortality

Percentages based upon individual species (Shaw and Long 2011). Examples include ponderosa pine maximum SDI = 450, Douglas-fir 

maximum SDI = 570, and white fir maximum SDI = 640. Note that the absolute density measure is calculated for the entire stand, but that 

developmental characteristics differ for each species within the stand based on the established maximum density for the species.

developed by targeting densities in the upper half 

of zone 2 and the lower half of zone 3.

 Clumpy, irregular stands containing groups of 

varying ages can be developed through periodic 

creation of canopy openings (zone 1), where 

growing space for tree regeneration is made 

available for seedling establishment.

 Longevity of existing large diameter trees could 

be enhanced by thinning adjacent smaller trees to 

create zone 2 or 3 growing conditions.

 Avoiding density related mortality and maintaining 

forest vigor can be achieved by maintaining 

densities at or less than the lower half of zone 3.

By considering SDI with these management strategies, 

stand density can be modified to favor the regeneration of 

shade intolerant conifers in dry mixed conifer, as a means 

of achieving desired conditions.

Fire Behavior

While fire risk and postfire effects associated with high 

density forests in the western U.S. are well established, 

little is known about the direct responses of fire behavior 

to a diameter cap. What diameter cap treatments have 

been implemented in the Southwestern Region have 

occurred at project scales and only within the last 10 

years so that observations on the potential impacts to 
17

fire behavior and resulting fire regimes are problematic. 

An unpublished study by Nicolet (2011) was recently 

conducted on the Apache-Sitgreaves National Forests in 

Arizona, and considered the effects of a 16-inch cap on 

forest ecosystems including the potential for passive and 

active crown fire. The study was part of an environmental 

analysis to evaluate the effects of the proposed Rim Lakes 

Forest Restoration Project. In this study, the potential 

fire behavior was modeled for a range of management 

alternatives with the following assumptions:

 No action – No management other than fire 

suppression.

 Uneven-aged forest management – Areas outside 

of Mexican spotted owl habitat managed for large 

trees and the northern goshawk by implementing 

combinations of group selection cuttings to 

spatially distribute forest age classes and canopy 

openings. Mechanical treatments are followed with 

prescribed fire. Slash created from tree harvest 

is piled and burned, or lopped and scattered and 

burned. Broadcast burns are applied on a 2- to 15-

year return interval over time in ponderosa pine 

forest types and 5 to 25 years in dry mixed conifer 

forest types.

 16-inch diameter cap – The analysis area is 

managed as with the uneven-aged alternative, but 

cutting is limited to trees less than 16 inches in 

diameter. To the extent possible, similar spatial 

distribution of age classes is promoted, though the 

cutting constraint resulted in different outcomes. 

The two alternatives were similar in all other ways.

Potential fire behavior was modeled using FVS 

and the Fire and Fuels Extension (Reinhardt 

and Crookston 2003) and the FLAMMAP 

model (Finney 2006). Results assume that the 

time since treatment allowed for the recovery 

of fine fuels in treated areas, 2 to 4 years post 

treatment. Analysis results are reported in table 

6.

Crown fire would be expected over about two 

thirds of the analysis area as a result of the “no 

action” scenario (23 percent active, 44 percent 

passive), suggesting higher tree mortality in a given fire 

event compared to the other two management alternatives. 

Results for the uneven-aged alternative showed a 

significant reduction in passive crown fire compared 

to the no action alternative (16 percent vs 44 percent), 

inferring reduced tree mortality and impacts to forest 

structure. The 16-inch cap alternative also exhibited a 

reduction in passive crown fire, but still had nearly double 

the potential for passive crown fire compared to unevenaged management.

As with our analysis of basal area, the effects of diameter 

cap cutting are deceptively short lived (Abella et al. 

2006). There are likewise implications for fire behavior, 

not only in the false sense of hazardous fuels reduction 

that such treatments provide for the longer term, but in 

the way such stands are modified to make restoration 

more difficult. The removal of one or more tree age class 

cohorts may require many decades to replace through 

recruitment and growth. The analyses reported here 

indicate that a diameter cap substantially effects stand 

structure, species composition, and ecological function, to 

constrain future management options and to lengthen the 

time necessary to achieve desired conditions.

Economics

A diameter cap comes at a lower cost efficiency that, 

in turn, may impact the rate of restoration for a given 

administrative unit. Restoration treatments often cost 

more to implement than the value of the products 

produced by them—in a range of about $300 to $730 

per acre ($740 to $1,800/ha) for cutting alone, and a 

Table 6. Projected fire behavior within the Rim Lakes study 

area as a proportion of the project area resulting from each 

alternative—no action, uneven-aged management, and 16-inch 

diameter cap.

Fire Behavior No Action Uneven-aged 16-Inch Cap

No Fire 1% 1% 1%

Surface Fire 32% 66% 51%

Passive Crown Fire 44% 16% 30%

Active Crown Fire 23% 17% 18%
18

cost of around $830 per acre ($2,050/ha) for associated 

fuels treatments and administrative costs (Hjerpe et al. 

2009). Diameter caps affect the profitability of restoration 

projects, according to the merchantability of trees less 

than 16 inches, usually subordinate and codominants with 

inferior quality, and by the limit on the amount of wood 

that such a limit indirectly imposes. Larson and Mirth 

(2001) found that a “cap resulted in implementation cost 

increases of 5 to 19.4 percent, harvested fiber decreases 

of 10 to 39 percent on a volume basis, and reductions in 

operator net returns ranging from 22.3 to 176 percent. 

And this doesn’t account for treatment longevity in 

relation to fuels and other restoration objectives. Even 

if the cost to implement diameter cap and uneven-aged 

treatments were equal, our analysis suggests that a more 

evenly distributed canopy (less gaps) resulting from a 

cap will close after the first decade or so, as opposed to 

uneven-aged management with irregular stand density 

(and gaps) that closes after 20 to 30 years. As a result, 

stands treated with a cap will require more frequent 

entries, as long as treatments are viable, leading to higher 

service costs. 

Because Federal budgets are relatively limited, projects 

that require subsidies are also limited, and competition 

for available funding may well increase as other areas in 

the West seek to undertake large-scale restoration projects 

(USDA Forest Service 2011, WGA 2001). Even with a 

long-term contract in place, such as the White Mountain 

Stewardship Project (Sitko and Hurteau 2010), a taxpayer 

subsidy of about $480 per acre ($1,240/ha) is required 

(pers. comm.. Paul Fink, White Mountain Stewardship 

Project forester). With other factors being equal, lower 

costs to restore forests can translate to more acres restored 

in a given time period.
19

Conclusion

Fire-adapted forest systems were historically driven by 

frequent fire burning through an herbaceous understory 

to maintain open, uneven-aged conditions in ponderosa 

pine and dry mixed conifer forests of the Southwest. 

Restoration treatments on today’s landscapes are used to 

lower the overstory density and canopy continuity and 

reestablish forest openings to provide for recruitment 

of younger tree cohorts sufficient to reestablish these 

conditions and prevent uncharacteristic fire effects. 

Ultimately, the goals of restoration are to provide 

ecosystem function, including plants and animal habitat, 

largely as a byproduct of providing for economic and 

social benefits including lower fire risk to surrounding 

communities. The results of this study discredit the 

efficacy of thinning treatments under a preservation 

strategy that imposes diameter caps, and the sustainability 

of such a policy given that:

 The plurality of stands would trend toward a large 

diameter, single story, closed-canopy condition;

 Closed canopy forest stands do not allow for 

the sustainable growth of shade intolerant (fire 

resistant) tree species into the future (not to 

mention continued recruitment for the large 

diameter trees of the future);

 Closed canopy forest stands do not provide canopy 

gaps to support robust understory vegetation for 

plant diversity and wildlife habitat;

 In dry mixed conifer forests, a diameter cap favors 

the retention and regeneration of uncharacteristic 

proportions of shade tolerant, non-fire resistant 

conifer species;

 Without openings and canopy gaps, closed canopy, 

single-storied forest stands are more susceptible 

to crown fires and changes to fire regimes, along 

with long-term conversion of forested plant 

communities to shrub- and herb-dominated 

vegetation types (Savage and Mast 2005); and

 Subsidies are required that reduce the cost 

efficiency of forest management, and limit the 

amount of area that can be ultimately treated. 

This analysis shows that within 3 decades, nearly all 

stands managed under the constraints of a cutting size cap 

would be converted to a functional even-aged condition. 

This level of landscape-scale homogeneity lacks 

biological diversity, and indicates an unstable ecosystem 

subject to synchronous large-scale disturbances.

Where diameter caps are applied, the continued growth 

of residual trees following treatments will favor greater 

tree densities and basal area, increased homogeneity 

of structure, and higher frequencies of uncharacteristic 

crown fires. If a diameter cap is considered for political 

or social reasons, then it should be prescriptive and not 

programmatic—narrowly focused, to achieve specific 

local objectives. Such treatments should be followed with 

deliberate long-term monitoring to enable the assessment 

and reporting of effects. Our results also suggest that a 

diameter cap would limit future flexibility in management 

in terms of the narrowing range in conditions of forest 

structure and composition. 
20

Abella, S. R. 2009. Tree canopy types constrain plant 

distributions in ponderosa pine—Gambel oak 

forests, northern Arizona. USDA Forest Service 

Research Note RMRS-RN-39. Rocky Mountain 

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Abella, S. R., P. Z. Fulé and W. W. Covington. 2006. 

Diameter caps for thinning southwestern 

ponderosa pine forests: Viewpoints, effects, and 

tradeoffs. Journal of Forestry 104:407–414.

Arno, S. F., and C. E. Fiedler. 2005. Mimicking Nature’s 

Fire: Restoring Fire-Prone Forests in the West. 

Island Press, Washington, DC, USA.

Bailey, J. D. 2008. Forty years later at Taylor Woods: 

merging the old and new. Pp. 100-105 in S. D. 

Olberding and M. M. Moore, tech coords, Fort 

Valley Experimental Forest—A Century of 

Research 1908-2008, 7-9 August 2008. USDA 

Forest Service proceedings RMRS-P-55. Rocky 

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Baisan, C. H., and T. W. Swetnam. 1990. Fire history 

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Wilderness, AZ, USA. Canadian Journal of 

Forest Research 20:1559-1569.

Bataineh, A. L., B. P. Oswald, M. M. Bataineh, H. 

M. Williams, and D. W. Coble. Changes in 

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Forest Ecology and Management 235:283-294.

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processes. Ecoscience 8:115-126.

Burns, R. M., and B. H. Honkala. 1990. Silvics of North 

America: Vol. 1. Conifers. USDA Forest Service, 

Agriculture Handbook 654. Washington Office, 

Washington, DC. 675 pp. 

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26

Overview (USDA Forest Service 2010)

As outlined in the provisions of the 1982 Planning 

Rule, desired conditions are descriptions of goals to be 

achieved at some time in the future. They are normally 

expressed in broad general terms and are timeless in 

that they have no specific date by which they are to be 

completed. Goals and desired conditions are the focus of 

the plan and are the basis for developing objectives and 

other plan components. Desired conditions, together with 

the other plan components, constitute a framework for 

sustainability and should clearly articulate management 

intent over the life of the forest plan. 

Tabled Desired Conditions

The following tables provide quantitative desired 

condition values for seral stages of major ecosystems 

of the Southwestern Region. Seral stages are defined by 

model states developed with the Vegetation Dynamics 

Development Tool (VDDT) (ESSA 2006) software 

used to simulate ecosystem dynamics. The seral stage 

proportions are a standard expression of the narratives 

from the R3 Vegetation Desired Conditions Working 

Group, and reflect the approximate mid-points of the 

ranges, as described but not quantified in the narratives. 

Only desired conditions for forest and woodland systems 

have been identified at this time. Reference conditions can 

be assumed for the remainder of systems.

The tabled seral stage values are used primarily as a 

performance measure for modeling management scenarios 

in VDDT, and as a starting point for modifications based 

on local circumstances. For instance, the R3 desired 

conditions state for ponderosa pine forest: “Denser tree 

conditions exist in some locations such as north-facing 

slopes and canyon bottoms.” Since the proportion of 

acres on north-facing slopes and canyons varies from one 

forest to the next, desired conditions may also vary so that 

modifications by individual forests may be necessary to 

reflect their local conditions. The proportion in each seral 

Appendix A: Desired Conditions for 

Ponderosa Pine and Dry Mixed Conifer (Mixed 

Conifer - Frequent Fire) Systems of the U.S. 

Forest Service Southwestern Region 

stage (model state) would be described in terms of the 

actual desired conditions and not in terms of management 

strategies and limitations. Desired condition values are 

updated in the accompanying tables, which are also 

used for area-weighting subtypes of some ecosystems. 

For example, on each forest, seral stage conditions for 

ponderosa pine forest are determined by weighting 

desired conditions for the ponderosa pine/Gambel oak and 

the ponderosa pine/bunchgrass subtypes, according to the 

amount of area within each subtype. The desired condition 

for ponderosa pine forest may, in effect, be specific for 

each forest according to their ratio of subtypes.

In phase II analysis of forest plan revision, desired 

condition values are enumerated as targets to shoot for 

and to be measured against. The objective of phase II 

modeling is to project and evaluate how conditions 

change over time under each management scenario. Each 

alternative is evaluated quantitatively, and compared 

among alternative management scenarios in terms of the 

numerical indicators of desired conditions. So, beginning 

with current conditions, alternative management themes 

are represented in VDDT. Each management theme has 

a unique combination of plan components such as plan 

objectives (which identify management actions), special 

areas, standards and guidelines (which put limitations 

on the use of management actions), and management 

opportunities (e.g., percent area in MSO PACs) that 

can vary in amount and timing. The end result of model 

simulations is the degree to which desired conditions 

are apparently achieved, usually measured in terms of a 

similarity index. If conditions are not trending toward the 

desired conditions from where they are now, either the 

plan alternative strategy or the desired conditions need to 

be modified. And, even if conditions are trending toward 

desired conditions, a given management scenario may be 

further refined to accelerate the trend.

Seral stage proportions for modeled states should be 

assessed only at landscape scales of 6th-code HUC units 

and greater, depending on the ecosystem. Applying seral 
27

stage values for spruce-fir forests, for instance, that 

typically have long stand replacement intervals and large 

patch dynamics, may only be appropriate at subregional 

scales. A landscape perspective may not be appropriate 

for certain standards and guidelines and regulatory 

requirements, where seral stages are sometimes assessed 

only at the scale of the planning unit (e.g., for MSO 

habitat requirements).

The following tables are accompanied by abstracts from 

the R3 desired condition narratives. 

Current Trends Model Baseline Desired Conditions

Source State Description

Ponderosa Pine/Gambel 

Oak

Ponderosa Pine/

Bunchgrass

Ponderosa Pine Forest - All 

(PPO/PPG)

S Stage Description S Stage Description S Stage Description

USFS 

R3 

Model 

PPF

A, N GFB/SHR = 0% Reference condition 0% Reference condition 0% *Reference condition

B

F

SSO

SSC

=

=

2%

*Conditions indicative of 

occasional even-aged 

stand dynamics and the 

development of MSO 

habitat.

1%

*Conditions indicative of 

occasional even-aged 

stand dynamics and the 

development of northern 

goshawk nesting habitat.

1.5%

*Conditions indicative of 

occasional even-aged 

stand dynamics and the 

development of closed 

mature forest habitat.

C SMO = 2%

*Conditions indicative of 

occasional even-aged 

stand dynamics and the 

development of MSO 

habitat.

1%

*Conditions indicative of 

occasional even-aged 

stand dynamics and the 

development of northern 

goshawk nesting habitat.

1.5%

*Conditions indicative of 

occasional even-aged 

stand dynamics and the 

development of closed 

mature forest habitat.

D, J MOS, MOM = 79%

(Primarily 

as 

J and K)

**Based on reference 

condition, and the 

predominance of unevenaged dynamics and open 

forest. The plurality of 

stands on low-productivity 

sites likely to occur as 

state J, versus highproductivity sites where 

state K is more likely.

94%

(Primarily 

in 

J and K)

Based on reference 

condition, and the 

predominance of unevenaged dynamics and open 

forest. The plurality of 

stands on low-productivity 

sites likely to occur as 

state J, versus highproductivity sites where 

state K is more likely.

86.5%

(Primarily

 in 

J and K)

Based on reference 

condition, and the 

predominance of unevenaged dynamics and open 

forest. The plurality of 

stands on low-productivity 

sites likely to occur as 

state J, versus highproductivity sites where 

state K is more likely.

E, K VOS, VOM =

Contemporary landscapes only

USFS 

R3 

Model 

PPF

G SMC = 2%

*Conditions indicative of 

occasional even-aged 

stand dynamics and the 

development of MSO 

habitat.

1%

*Conditions indicative of 

occasional even-aged 

stand dynamics and the 

development of northern 

goshawk nesting habitat.

1.5%

*Conditions indicative of 

occasional even-aged 

stand dynamics and the 

development of closed 

mature forest habitat.

H, L

I, M

MCS, MCM

VCS, VCM

=

=

15%

Conditions indicative 

of MSO habitat, and 

occasional even-aged 

dynamics that occurred 

in the reference condition 

(Romme et al. 2010), 

particularly on northfacing slopes and 

canyons. The plurality of 

stands on low-productivity 

sites likely to occur as 

state H/L, versus highproductivity sites where 

state I/M is more likely.

3%

Conditions indicative of 

northern goshawk nesting 

habitat, and occasional 

even-aged dynamics that 

occurred in the reference 

condition (Romme et al. 

2010), particularly on 

north-facing slopes and 

canyons. The plurality of 

stands on low-productivity 

sites likely to occur as 

state H/L, versus highproductivity sites where 

state I/M is more likely.

9%

Conditions indicative 

of mature closed forest 

habitat and occasional 

even-aged dynamics that 

occurred in the reference 

condition (Romme et al. 

2010), particularly on 

north-facing slopes and 

canyons. The plurality of 

stands on low-productivity 

sites likely to occur as 

state H/L, versus highproductivity sites where 

state I/M is more likely.

Ponderosa Pine Forest

* Reflects percentage of seral forests necessary to sustain at least 15 percent MSO habitat in mature closed forest.

** Based on residual proportion of the landscape not including mature closed forest (states H, L, I, M) and early-mid even-aged states (B, F, C, G).
28

Dry Mixed Conifer Forest (AKA Mixed Conifer – Frequent Fire)

R3 Desired Conditions 

(abstracted from R3 DC narratives, 

landscape and mid-scale)

At the landscape scale, the ponderosa pine forest 

vegetation community is composed of trees from 

structural stages ranging from young to old. Large trees 

are well distributed in the landscape. Forest appearance 

is variable but generally uneven-aged and open; 

occasional areas of even-aged structure are present. The 

forest arrangement is in individual trees, small clumps, 

and groups of trees interspersed within variably-sized 

openings of grass, forb, and shrub vegetation associations 

similar to historic patterns. Openings typically range from 

10 percent of the total stand/state area in more productive 

sites to 70 percent in the less productive sites. Size, shape, 

number of trees per group, and number of groups per 

area are variable across the landscape. In the Gambel oak 

subtype, all sizes and ages of oak trees are present. Denser 

tree conditions exist in some locations such as northfacing slopes and canyon bottoms.

At the mid-scale the ponderosa pine forest vegetation 

community is characterized by variation in the size and 

number of tree groups depending on elevation, soil type, 

aspect, and site productivity. The more biologically 

productive sites contain more trees per group and more 

groups per area, resulting in less space between groups. 

Openings typically range from 10 percent in more 

productive sites to 70 percent in the less productive sites. 

Tree density within forested areas generally ranges from 

20 to 80 square foot basal area per acre (5–18 m2/ha). 

The mosaic of tree groups generally comprises an unevenaged forest with all age classes present. Infrequently 

patches of even-aged forest structure are present. 

Disturbances sustain the overall age and structural 

distribution. 

Forest conditions in goshawk post-fledging family areas 

(PFAs) are similar to general forest conditions except 

these forests contain 10 to 20 percent higher basal area 

in mid-aged to old tree groups than in goshawk foraging 

areas and the general forest. Goshawk nest areas have 

forest conditions that are multiaged but are dominated by 

large trees with relatively dense canopies.

Current Trends Model Baseline Desired Conditions

Source State Description S Stage Description

USFS 

R3 

Model 

MCD

A, N GFB/SHR = 9% *Reference condition and conditions indicative of even-aged stand 

dynamics and the development of MSO habitat. B, F SSC, SSO =

C SMO Intolerant = 3% **Reference condition and conditions indicative of even-aged stand 

dynamics and the development of MSO habitat.

D, J MOS, MOM 

Intolerant =

60%

**Based on reference condition and the predominance of unevenaged dynamics and open forest. The plurality of stands on lowproductivity sites likely to occur as state J, versus high-productivity 

E, K sites where state K is more likely. VOS, VOM 

Intolerant =

G SMC Mixed 

Tolerant = 3% **Reference condition and conditions indicative of even-aged stand 

dynamics and the development of MSO habitat.

H, L MCS, MCM 

Mixed Tolerant =

25%

Conditions indicative of MSO habitat and occasional even-aged 

dynamics that occurred in the reference condition (Romme et 

al. 2010), particularly on north-facing slopes and canyons. The 

plurality of stands of low-productivity sites likely to occur as state 

H/L, versus high-productivity sites where state I/M is more likely. I, M VCS, VCM 

Mixed Tolerant =

* Reflects percentage of seral forests necessary to sustain at least 15 percent MSO habitat in mature closed forest.

** Based on residual proportion of the landscape not including mature closed forest (states H, L, I, M) and early-mid even-aged states (B, F, C, G).
29

R3 Desired Conditions 

(abstracted from R3 DC narratives, 

landscape and mid-scale)

At the landscape scale, the dry mixed conifer vegetation 

community is a mosaic of forest conditions composed 

of structural stages ranging from young to old trees. 

Large trees are well distributed in the landscape. Forest 

appearance is variable but generally uneven-aged and 

open; occasional patches of even-aged structure are 

present. The forest arrangement is in small clumps 

and groups of trees interspersed within variably-sized 

openings of grass, forb, and shrub vegetation associations 

similar to historic patterns. Openings typically range from 

10 percent of the total stand/state area in more productive 

sites to 50 percent in the less productive sites. Size, shape, 

number of trees per group, and number of groups per area 

are variable across the landscape. Where they naturally 

occur, groups or patches of aspen and all structural stages 

of oak are present. Denser tree conditions exist in some 

locations such as north-facing slopes and canyon bottoms.

At the mid-scale the dry mixed conifer forest vegetation 

community is characterized by variation in the size and 

number of tree groups depending on elevation, soil type, 

aspect, and site productivity. The more biologically 

productive sites contain more trees per group and more 

groups per area. Openings typically range from 10 

percent in more productive sites to 50 percent in the 

less productive sites. Tree density within forested areas 

generally ranges from 30 to 100 square foot basal area per 

acre (7–23 m2

/ha). 

The mosaic of tree groups generally comprises an 

uneven-aged forest with all age classes and structural 

stages. Occasionally small patches (generally less than 50 

acres (20 ha)) of even-aged forest structure are present. 

Disturbances sustain the overall age and structural 

distribution.

Forest conditions in goshawk post-fledging family areas 

(PFAs) are similar to general forest conditions except 

these forests contain 10 to 20 percent higher basal area 

in mid-aged to old tree groups than in goshawk foraging 

areas and in the general forest. Goshawk nest areas have 

forest conditions that are multiaged but are dominated by 

large trees with relatively dense canopies.
30

Appendix B: Stand 

Density Dynamics

Density management is the manipulation and control of 

growing stock (trees) in order to achieve management 

objectives. The most generally effective indices of 

growing stock are those that combine some expression of 

mean diameter and density (Long and Smith 1984). Stand 

density is a quantitative measure of stocking expressed 

as trees per acre (TPA) or square feet of basal area per 

acre (BA). Despite widespread use, density measures like 

TPA or BA have limited utility, as they have no inherent 

biological or physiological frame of reference (Smith 

1986). Respectively, TPA and BA convey only the average 

distance between trees of unknown size and species, 

and the cross-sectional area of the tree at 4.5 ft (1.4 m) 

above the ground. Reineke’s (1933) stand density index 

(SDI) was considered with our analysis, according to 

parameters described by Shaw and Long (2010), because 

SDI accounts for tree size, can be related to a species’ 

maximum physiological density, and is independent of 

age and of site quality (Daniel, Helms, and Baker 1979). 

Percent of maximum stand density is determined based on 

Reineke’s stand-density index (Reineke 1933). Reineke 

discovered a predictable relationship between quadratic 

mean diameter-squared (QMD2

) and trees per unit area 

in dense even-aged stands. Stand density index is a value 

based on the number of trees per acre at an average stand 

diameter of 10 inches (25 cm).

The relationship between average size and density of 

trees in populations experiencing density related 

or suppression mortality has been shown to 

be predictable for a number of tree species. 

This “self-thinning rule” (Yoda et al. 1963) 

is a fundamental relationship found to be 

independent of both stand age and site quality. 

Because of this relationship, a stand density 

that is desired in the context of a specific set 

of management objectives can be projected 

forward or backward to a different stage of stand 

development.

Those who use SDI—or any index of stand density—as 

an estimate of growing stock, must assume that the index 

is proportional to site utilization (Long and Smith 1984). 

Since the contribution of individual stand components to 

both total SDI and total site utilization is additive (Stage 

1968), SDI can also be used to assess control of growing 

stock in uneven-aged stands as well as even-aged stands 

(Long and Smith 1984). Although SDI and the maximum 

size-density relationship were originally described for 

pure, even-aged stands, Long and Daniel (1990) have 

proposed the extension of its utility to uneven-aged and 

multiaged situations. 

Basal area is a widely used measure of stocking; however, 

SDI is a more descriptive means of expressing stand or 

group density because SDI is related to average diameter 

(quadratic mean diameter) and trees per acre. Table 7 is 

used to demonstrate the usefulness of SDI over basal area, 

taking two groups or stands of equal basal area, at 60 

ft2

 (14 m2/ha). Basal area alone reveals little difference. 

However, if stand A has an average tree diameter of 6 

inches (15 cm) and stand B has an average diameter of 

18 inches (46 cm), it becomes apparent that stand A is 

considerably more dense, with nine times the number 

of trees per acre. The more dense condition of stand A 

provides more canopy cover, hiding cover, and tree-totree competition than the condition in stand B. These 

stand structural differences cannot be detected by basal 

area stocking alone.

Table 7. Example showing two stands of equal tree basal area 

but significantly different density

Stand Metric Stand A Stand B

Basal area 60 ft2/ac (14 m2/ha) 60 ft2/ac

Quadratic mean diameter 6" (15 cm) 18" (46 cm)

Density 306 TPA (756 TPH) 34 TPA (84 TPH)

Stand density index 135 87
31

The SDI in the Southwestern Region of the USDA Forest 

Service is calculated based on all live trees larger than 

1 inch in diameter (2.5 cm) using one of the following 

formulas:

 (1a) Total stand SDI = total number of trees ≥1" X 

(quadratic mean diameter/10)1.6

(1b) Total stand SDI = (sum of all tree diameters 

≥1"/10)1.6 

Note: Formulas 1a and 1b yield similar results in 

even-aged stand conditions. Formula 1b is used to 

determine SDI in uneven-aged or multiaged stands.

In even-aged stands of a given quadratic mean diameter 

(QMD), Reineke (1933) observed that there was an 

upper limit to the number of trees per acre present in the 

densest stands. As QMD increases, the upper density 

limit decreases exponentially. Drew and Flewelling 

(1979) reached similar conclusions with regard to 

mean tree volume and density. The exponential slope 

of the upper limit curve is the same for all tree species. 

The intercept of that curve, however, is both regional 

and species specific—higher for tolerant species and 

lower for intolerant species. The upper limit curve is 

referenced to the theoretical maximum number of 10-inch 

diameter trees per acre. The published maximum SDI 

for ponderosa pine is 450, indicating that the upper limit, 

or reference, curve passes through 450 TPA when QMD 

equals 10 inches.

Stand density index is more often expressed as a percent 

of maximum (see table 5). An SDI of 180, for example, 

would be 40 percent of the maximum for local ponderosa 

pine. One would expect the densest possible stands to 

be clustered between 80 and 100 percent of maximum. 

Figure 5 illustrates these SDI concepts.

Knowledge of these relationships, along with basic 

knowledge of plant physiology and resource allocation, 

permits informed decisionmaking with regard to desired 

stand structure and development. 

Plant foliage produces organic sugars that are allocated 

for growth and reproduction in this general priority: (1) 

maintenance respiration; (2) production of fine roots 

and foliage; (3) flower and seed production; (4) height 

growth; (5) crown expansion and root extension; and (6) 

diameter growth and defense mechanisms (Oliver and 

Larson 1996). To enable biochemical and physiological 

processes, including 

the production of 

carbohydrates, requires 

sunlight, water, mineral 

nutrients, suitable 

temperature, oxygen, 

and carbon dioxide. 

These growth factors, 

in aggregate, form an 

abstract, non-dimensional 

“growing space” on 

any given site. A site’s 

productivity, also known 

as carrying capacity, will 

be limited by the scarcest 

of the six growth factors.

Figure 5. The SDI for ponderosa pine in the southwestern U.S. is described by this family 

of curves derived from Reineke’s equations (Reineke 1933). The top curve is the theoretical 

“upper density limit.” Lower curves indicate percentage of the maximum. Any point on a given 

curve represents an equal level of physiological density.
32

Implications of the maximum size-density relationship are 

as follows:

1. As mean tree size increases, the upper 

physiological limit of TPA decreases.

2. As a stand’s size density trajectory approaches the 

maximum:

• Tree competition increases

• Tree growth slows

• Trees lose vigor

• Tree mortality increases

The prospects of individual trees improve as their share 

of growing space increases, either through superior 

competitive advantage, or the decline and death of 

their neighbors. As a tree grows, so does its volume of 

respiring tissue and the weight of its crown. In order to 

fuel this new tissue and support more weight, a tree must 

capture additional growing space. If it fails, the priorities 

Figure 6. Relationship of annual whole stand growth as a 

percentage of potential (max SDI), based on Langsaeter 

(1941), as described in Long (1985). 

Figure 7. Relationship of annual individual tree growth as 

a percentage of potential (max SDI), based on Langsaeter 

(1941), as described in Long (1985). 

of photosynthate allocation are engaged, reducing tree 

function in reverse order of priority. When a tree controls 

insufficient growing space to sustain even maintenance 

respiration, the tree dies (Oliver and Larson 1996). 

Site Occupancy and Growth

Site occupancy refers to the degree to which growing 

space is utilized by trees. The relationship of site 

occupancy to stand and individual tree growth is familiar 

to silviculturists as that depicted by the three zones of 

Langsaeter’s growth curves (Langsaeter 1941). Long and 

Shaw (2010) have proposed the percent of maximum 

SDI that correlates with the zone thresholds within 

growth functions (Long 1985). Figures 6 and 7, and 

table 3 illustrate the relationship of Langsaeter’s growth 

curves and Long’s percent of maximum SDI values to 

site occupancy, competition among trees, diameter and 

volume growth in trees, volume growth in stands, species 

composition, and stem quality. 
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