United States

Department of

Agriculture

Forest Service

Southern 

Research Station

General Technical 

Report SRS–127

Descriptive Statistics of Tree 

Crown Condition in the United 

States Interior West 

KaDonna C. Randolph and Mike T. Thompson
July 2010 

Southern Research Station 

200 W.T. Weaver Blvd. 

Asheville, NC 28804

Authors 

KaDonna C. Randolph, Mathematical Statistician, USDA Forest 

Service, Southern Research Station, Forest Inventory and Analysis, 

Knoxville, TN 37919; and Mike T. Thompson, Resource Analyst, 

USDA Forest Service, Rocky Mountain Research Station, Forest 

Inventory and Analysis, Ogden, UT 84401.

Cover photo: A stand of lodgepole pine and Douglas-fir in the 

Rocky Mountain Region. (photo courtesy of Rocky Mountain 

Research Station)
Descriptive Statistics of Tree Crown Condition 

in the United States Interior West

KaDonna C. Randolph and Mike T. Thompson
II

Contents

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

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

 Data Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

 Data Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

 Tree Crown Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

 Sapling Crown Vigor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

 Variations Due to Species Differences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

 Variations Due to Site Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

 Crown Condition Stressors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

 Statistical Characteristics and Hypothesis Testing . . . . . . . . . . . . . . . . . . . . . . . . 12

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Appendix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Page
Abstract

The U.S. Forest Service Forest Inventory and Analysis (FIA) Program 

uses visual assessments of tree crown condition to monitor changes 

and trends in forest health. This report describes four crown condition 

indicators (crown dieback, crown density, foliage transparency, and 

sapling crown vigor) measured in Colorado, Idaho, Nevada, Utah, and 

Wyoming between 1996 and 1999. Descriptive statistics are presented by 

species and FIA species group. Inter- and intra-species variation, crown 

condition stressors, and statistical concerns that should be considered 

when analyzing and interpreting the crown condition data are discussed.

Keywords: Crown density, crown dieback, FIA, foliage transparency, 

forest health, sapling vigor.

Introduction

Tree crown condition is an important visual indicator of tree 

and forest health. A tree’s crown is its principal engine for 

energy capture. Therefore, trees with full, vigorous crowns 

are generally associated with higher growth rates due to 

an increased capacity for photosynthesis. When crowns 

become degraded, photosynthetic capacity is reduced. 

Crown degradation is typically the result of past and present 

stressors such as insects, diseases, weather events (e.g. frost, 

wind, and ice storms), drought, senescence, and competition 

or other stand conditions (Kenk 1993), and when severe 

enough, may result in tree mortality (Lawrence and others 

2002). 

Broad-scale assessment of tree crown condition was 

initiated by the U.S. Forest Service Forest Health 

Monitoring (FHM) Program when ground inventory 

plots were established in six Northeastern States in 1990 

(Riitters and Tkacz 2004). Plots were added throughout 

the 1990s and by the end of the decade ground plots had 

been established in 32 States. In 1999, the network of FHM 

ground plots was integrated as the “phase 3” effort of the 

U.S. Forest Service enhanced Forest Inventory and Analysis 

(FIA) Program (Riitters and Tkacz 2004). Since that time, 

FIA has continued to assess tree crown condition as well as 

many of the other variables initiated by FHM.

At the State level, the 5-year FIA reports mandated by 

the 1998 Farm Bill [Agricultural Research, Extension, 

and Education Reform Act of 1998] (Public Law 105-185) 

are a primary outlet for reporting tree crown condition. 

These reports describe the current status and trends in 

forest extent and condition, and typically present data 

summaries in tabular format by species or species group 

(e.g. Donnegan and others 2008). The purpose of this crown 

condition summary is to document the species-specific 

crown conditions collected by FHM in the U.S. Interior 

West (fig. 1) so that the FIA State-level summaries can 

be understood in their regional historical context. Rogers 

and others (2001) presented frequency histograms for 

three crown condition indicators (crown dieback, foliage 

transparency, and crown density) by hardwood and softwood 

taxonomic groups as an appendix of their summary of 

forest health monitoring in the Interior West between 1996 

and 1999. Though based on the same data, this report goes 

beyond their summary by presenting detailed descriptive 

statistics at the species level. Similar regional summaries 

for the West Coast (Randolph and others 2010a), Southern 

(Randolph 2006), North Central (Randolph and others 

2010b), and Northeastern (Randolph and others 2010c) 

States are also available.

Descriptive Statistics of Tree Crown Condition 

in the United States Interior West

KaDonna C. Randolph and Mike T. Thompson

Figure 1—Interior West States included in the crown condition summary 

Figure 1—States in the United States interior are shaded gray. West included in the crown 

condition summary are shaded gray.
2

Methods

Data Collection

In order to have complete statewide coverage for as many 

Interior West States as possible, we elected to summarize 

the crown condition data collected by FHM between 

1996 and 1999 to serve as a baseline against which more 

recent data can be referenced. No modifications were 

made to the data collection protocols in the transition from 

FHM to FIA administration for the four crown condition 

indicators being summarized, so the data from the FHM 

period is compatible with the data now collected by 

FIA. The data for this summary consisted of the crown 

condition assessments from all forested FHM plots in 

Colorado, Idaho, Nevada, Utah, and Wyoming (table 1). 

Each inventory plot is a cluster of four 1/24-acre circular 

subplots with subplot centers located 120 feet apart (U.S. 

Department of Agriculture Forest Service 1999). The four 

crown condition indicators included in this summary are: 

(1) crown density—the amount of crown branches, foliage, 

and reproductive structures that blocks light visibility 

through the projected crown outline; (2) crown dieback—

recent mortality of branches with fine twigs, which begins 

at the terminal portion of a branch and proceeds inward 

toward the trunk; (3) foliage transparency—the amount of 

skylight visible through the live, normally foliated portion 

of the crown, excluding dieback, dead branches, and large 

gaps in the crown; and (4) sapling crown vigor—a visual 

measure designed to categorize saplings into three broad 

classes based on the amount and condition of the foliage 

present (Schomaker and others 2007). Crown density, crown 

dieback, and foliage transparency (fig. 2) were measured 

for every live tree ≥5.0 inches in diameter at breast height 

(d.b.h.) or, for woodland species, diameter at root collar 

(d.r.c.) on each subplot. Sapling crown vigor was assessed 

for every live tree (sapling) with d.b.h. or d.r.c. ≥1.0 inch but 

<5.0 inches on a 1/300-acre microplot located 12 feet east 

from each subplot center.

All four indicators were visually assessed by two-person 

field crews. Crown density, crown dieback, and foliage 

transparency were measured in 5-percent increments and 

recorded as a two-digit code: 00, 05, 10... 99, where the 

code represents the upper limit of the class, e.g. 1 to 5 

percent is code 05 and 96 to 100 percent is code 99. Sapling 

crown vigor was recorded in one of three classes: good 

(vigor class 1), fair (vigor class 2), and poor (vigor class 3). 

Though foliage transparency and crown density are similar 

measures, they cannot be interpreted as exact inverses. 

Crown density measures the amount of sunlight blocked 

by all biomass produced by the tree (both live and dead) 

in the crown, whereas foliage transparency measures the 

amount of sunlight penetrating only the live, foliated portion 

of the crown. Deductions are made from the maximum 

possible crown density for spaces between branches and 

other large openings in the crown. However, large gaps 

in the crown where foliage is not expected to occur are 

excluded from consideration when foliage transparency 

is rated. Within a species, higher crown density values, 

lower foliage transparency values, and lower crown dieback 

values typically are associated with better tree health. More 

detailed descriptions of the crown condition indicators are 

available in Schomaker and others (2007).

Data Summary

Ratio-of-means (ROM) estimators (Cochran 1977) were 

used to estimate the tree crown condition means and 

standard errors for all species combined, hardwood and 

softwood groups, FIA species groups, and individual 

species with at least 25 observations. Some of the FHM 

plots were measured more than once between 1996 and 

1999, but only the latest measurement was included in the 

summary. Estimates were made with the SAS® procedure 

SURVEYMEANS (An and Watts 1998) and the following 

statement options: (1) CLUSTER—to designate the primary 

sampling unit of the survey, i.e., the plot; (2) RATIO—to 

request ROM estimates; and (3) DOMAIN—to identify 

the subpopulations, or domains, of interest, e.g. hardwoods 

and softwoods. Other descriptive statistics (minimum, 

maximum, and median or 90th percentile) also were 

Table 1—Number of FHM plots with at least one 

accessible forested condition by State and year

State

Year

1996 1997 1998 1999 Total

number

Colorado 23 36 37 57 153

Idaho 19 37 39 47 142

Nevada — — — 67 67

Utah — — — 142 142

Wyoming — 26 16 25 67

All States 42 99 92 338 571

FHM = Forest Health Monitoring.

— = no sample.
3

Figure 2—The dashed line is the projected crown outline against which crown density is assessed. 

The dash-dot line within the projected crown outline defines the area of crown dieback. The 

striped areas are areas where foliage is not expected to occur and are not included in the foliage 

transparency estimate. Adapted from Millers and others (1992).

calculated for the trees. Summaries by FIA species group 

are presented for completeness (tables A.1 through A.3) 

and to allow flexibility in future reporting. However, 

discussion of observed tree crown condition primarily 

focuses on individual species. ROM estimators also 

were used to estimate the percentage of saplings in each 

vigor class and associated standard errors for all species 

combined, hardwood and softwood groups, and FIA species 

groups. Sample sizes were not adequate to summarize the 

saplings at the individual species level.

Results 

Tree Crown Condition

Tree crown condition was assessed for 12,361 trees on 542 

of the 571 forested plots. A total of 34 species was observed, 

and of these, 26 species had 25 or more observations. For 

all trees combined, the range of possible values, from 0 to 

99 percent, were observed for each of the crown condition 

indicators, though the majority of crown dieback and foliage 
4

transparency values tended to concentrate in a small portion 

of the total possible range. Ninety-six percent of the trees 

exhibited <15 percent crown dieback (fig. 3) and 94 percent 

had foliage transparency < 25 percent (fig. 4). Crown 

densities were concentrated in the middle of the range; 79 

percent of the trees had a crown density of 30 to 60 percent 

(fig. 5). Mean crown conditions were 3.3 percent crown 

dieback, 14.4 percent foliage transparency, and 44.1 percent 

crown density (table 2).

On average, the absolute difference between the softwood 

and hardwood crown condition means was greatest for 

crown density. Mean crown density was 45.3 percent for 

the softwoods and 37.9 percent for the hardwoods (table 2). 

Figure 3—Crown dieback frequency histogram and cumulative frequency distribution for all trees combined 

for Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99.

Crown dieback (percent)

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Frequency (percent of trees)

0

6

12

18

24

30

36

42

48

54

60

Cumulative frequency (percent of trees)

0

10

20

30

40

50

60

70

80

90

100

Frequency

Cumulative frequency

0 5

Foliage transparency was higher for the hardwoods (18.5 

percent) than for the softwoods (13.7 percent), whereas 

crown dieback was <5 percent for both groups (table 2).

A broad range of average conditions was exhibited for each 

of the crown condition indicators among the species. Mean 

crown dieback ranged from 1.3 percent for corkbark fir 

(Abies lasiocarpa) to 7.3 percent for Gambel oak (Quercus 

gambelli) (table 3). Mean foliage transparency ranged from 

9.9 percent for western juniper (Juniperus occidentalis) 

to 19.4 percent for quaking aspen (Populus tremuloides) 

(table 4), and mean crown density ranged from 33.0 percent 

for Gambel oak to 52.0 percent for grand fir (A. grandis) 

(table 5).
5

Foliage transparency (percent)

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Frequency (percent of trees)

0

5

10

15

20

25

30

35

40

45

50

Cumulative frequency (percent of trees)

0

10

20

30

40

50

60

70

80

90

100

Frequency

Cumulative frequency

Figure 4—Foliage transparency frequency histogram and cumulative frequency distribution for all trees 

combined for Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99.

0 5

Figure 5—Crown density frequency histogram and cumulative frequency distribution for all trees combined 

in Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99.

Crown density (percent)

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Frequency (percent of trees)

0

2

4

6

8

10

12

14

16

18

20

Cumulative frequency (percent of trees)

0

10

20

30

40

50

60

70

80

90

100 Frequency

Cumulative frequency

0 5
6

Table 2—Mean crown attributes and other statisticsa for all live trees ≥ 5.0 inches diameter by crown condition 

indicator and species group for Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99

Crown condition indicator 

and species group Plotsb Trees Mean SE

95% confidence

Minimum MedianMaxiLower Upper mum

- - - number - - - - - - - - - - - - - - - - - - - - - - - - percent - - - - - - - - - - - - - - - - - - - - -

Crown density

Softwoods 508 10,397 45.3 0.5 44.4 46.2 0 45 99

Hardwoods 167 1,964 37.9 1.0 35.9 39.9 5 35 85

All trees 542 12,361 44.1 0.4 43.3 45.0 0 40 99

Crown dieback

Softwoods 508 10,397 3.1 0.2 3.0 3.7 0 0 99

Hardwoods 167 1,964 4.4 0.5 3.4 5.4 0 0 95

All trees 542 12,361 3.3 0.2 3.0 3.7 0 0 99

Foliage transparency

Softwoods 508 10,397 13.7 0.2 13.3 14.1 0 15 99

Hardwoods 167 1,694 18.5 0.7 17.1 19.8 5 15 95

All trees 542 12,361 14.4 0.2 14.0 14.9 0 15 99

SE = standard error.

a The mean and SE calculations consider the clustering of trees on plots.

b

 Total number of forested plots on which trees were measured. Plot totals are not cumulative because multiple species may occur on 

any given plot.

Sapling Crown Vigor

Crown vigor was assessed for 2,142 saplings on 361 of 

the 571 forested plots. Overall, 59.2 percent of the sapling 

crowns were categorized as good (table 6). Although the 

percentage of hardwood and softwood saplings categorized 

as poor was about the same, 65.8 percent of the softwoods 

were in the good category, compared to only 52.5 percent 

of the hardwoods. Among the softwood species groups 

with at least 25 observations, the ponderosa and Jeffrey 

pines (Pinus ponderosa and P. jeffreyi) group had the 

highest percentage of saplings in the good category (79.5 

percent) and the western redcedar (Thuja plicata) group 

had the lowest percentage of saplings in the good category 

(52.9 percent). The Engelmann and other spruces (Picea 

engelmannii and P. spp.) group had the lowest percentage 

of saplings in the poor category (2.9 percent), whereas 

the western redcedar group had the highest percentage 

of saplings in the poor category (11.8 percent). Among 

the hardwood species groups, only the cottonwood-aspen 

(Populus spp.) and western woodland hardwoods groups 

had 25 observations or more. Both of these groups had about 

the same percentage of trees in the good category, but more 

of the western woodland hardwoods were categorized as 

poor (7.1 percent) (table 6). 

Discussion

A number of factors should be considered when analyzing 

and interpreting the crown condition data. These include 

variations due to species and site differences, impacts 

of biotic and abiotic stressors, the general statistical 

characteristics of the data, and the inventory sample design. 

We present a brief overview of each factor. 

Variations Due to Species Differences 

Average crown conditions are expected to vary by species 

due to differences in leaf and branch morphology and 

underlying shade tolerance. This expectation held true for 

species in this region where, for example, crown density 

averages ranged between 33.0 and 52.0 percent. On average, 

the species with the highest crown densities and lowest 
7

Table 3—Mean crown dieback and other statisticsa for all live trees ≥ 5.0 inches diameterb by species for Colorado, 

Idaho, Nevada, Utah, and Wyoming, 1996–99

Speciesc Plotsd Trees Mean SE

95% confidence

Minimum90th

percentile

MaxiLower Upper mum

- - - number - - - - - - - - - - - - - - - - - - - - - - - - - - percent - - - - - - - - - - - - - - - - - - - - -

Softwoods

Corkbark fir 10 64 1.3 0.7 0.0 2.5 0 5 10

Western larch 15 64 1.6 0.8 0.2 3.1 0 5 10

Western hemlock 12 96 1.8 1.1 -0.5 4.0 0 5 10

Bristlecone pine 8 29 1.9 0.4 1.1 2.7 0 5 10

Douglas-fir 162 1,283 2.0 0.3 1.5 2.5 0 5 99

Engelmann spruce 109 1,104 2.2 0.3 1.5 2.9 0 5 99

Ponderosa pine 56 404 2.3 0.7 0.9 3.6 0 5 60

Grand fir 43 323 2.6 0.4 1.8 3.3 0 5 70

Lodgepole pine 111 2,096 2.7 0.5 1.7 3.7 0 10 80

White fir 17 171 3.0 1.4 0.3 5.7 0 10 95

Subalpine fir 112 1,157 3.1 0.4 2.3 3.9 0 10 99

Singleleaf pinyon (w) 62 760 3.2 0.6 2.0 4.4 0 5 80

Western redcedar 17 187 3.2 1.1 1.1 5.3 0 5 60

Limber pine 40 122 3.4 0.9 1.6 5.2 0 5 60

Rocky Mountain juniper (w) 48 202 3.7 1.0 1.8 5.7 0 10 70

Common pinyon (w) 96 597 3.8 0.4 2.9 4.7 0 5 99

Whitebark pine 18 90 4.2 2.1 0.1 8.2 0 5 90

Western juniper 3 39 4.4 0.9 2.6 6.1 0 10 25

Oneseed juniper (w) 8 102 4.7 0.9 3.0 6.4 0 10 30

Utah juniper (w) 154 1,467 5.4 0.3 4.8 6.1 0 10 90

Hardwoods

Narrowleaf cottonwood 5 46 2.9 0.4 2.2 3.7 0 10 35

Quaking aspen 84 1,233 3.1 0.5 2.1 4.0 0 5 95

Rocky Mountain maple (w) 24 65 4.2 1.0 2.3 6.2 0 10 30

Bigtooth maple (w) 6 49 6.6 2.0 2.8 10.5 0 10 75

Curlleaf mountainmahogany (w) 38 289 7.0 1.4 4.2 9.9 0 15 85

Gambel oak (w) 22 250 7.3 1.8 3.9 10.8 0 10 80

SE = standard error.

a The mean and SE calculations consider the clustering of trees on plots.

b Diameter measured at root collar for woodland species, designated with a (w), and at breast height for all other species.

c See appendix table A.4.

d Total number of forested plots on which the species was measured.
8

Table 4—Mean foliage transparency and other statisticsa for all live trees ≥ 5.0 inches diameterb by species for 

Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99

Speciesc Plotsd Trees Mean SE

95% confidence

Minimum MedianMaxiLower Upper mum

- - - number - - - - - - - - - - - - - - - - - - - - - - - - percent - - - - - - - - - - - - - - - - - - - - - -

Softwoods

Western juniper 3 39 9.9 0.2 9.5 10.3 5 10 15

Bristlecone pine 8 29 10.3 0.3 9.8 10.9 5 10 15

Corkbark fir 10 64 10.5 0.8 9.0 12.1 5 10 20

Utah juniper (w) 154 1,467 11.6 0.4 10.9 12.4 0 10 30

Subalpine fir 112 1,157 11.7 0.5 10.8 12.6 0 10 99

Engelmann spruce 109 1,104 11.8 0.5 10.7 12.8 5 10 40

Rocky Mountain juniper (w) 48 202 12.8 0.8 11.2 14.5 5 10 65

Common pinyon (w) 96 597 12.9 0.3 12.2 13.5 0 15 50

White fir 17 171 12.9 1.0 10.9 14.9 5 15 25

Singleleaf pinyon (w) 62 760 13.0 0.4 12.2 13.8 0 15 45

Oneseed juniper (w) 8 102 13.5 1.3 11.1 16.0 5 15 20

Ponderosa pine 56 404 14.9 0.6 13.7 16.0 5 15 55

Douglas-fir 162 1,283 14.9 0.4 14.1 15.6 0 15 99

Grand fir 43 323 15.0 0.4 14.3 15.8 5 15 35

Limber pine 40 122 15.2 0.8 13.6 16.8 0 15 25

Lodgepole pine 111 2,096 15.7 0.3 15.1 16.4 0 15 75

Western hemlock 12 96 17.2 2.0 13.3 21.1 5 15 35

Western larch 15 64 17.6 1.1 15.3 19.8 10 15 40

Whitebark pine 18 90 17.9 0.9 16.0 19.7 5 15 50

Western redcedar 17 187 18.3 1.0 16.4 20.3 5 20 40

Hardwoods

Bigtooth maple (w) 6 49 14.5 0.7 13.2 15.8 10 15 25

Curlleaf mountainmahogany (w) 38 289 14.7 0.7 13.4 16.0 5 15 95

Narrowleaf cottonwood 5 46 16.8 1.2 14.6 19.1 15 15 30

Gambel oak (w) 22 250 18.8 2.4 14.1 23.6 10 15 90

Rocky Mountain maple (w) 24 65 19.0 0.9 17.3 20.7 10 20 50

Quaking aspen 84 1,233 19.4 0.9 17.6 21.3 5 20 90

SE = standard error.

a The mean and SE calculations consider the clustering of trees on plots.

b Diameter measured at root collar for woodland species, designated with a (w), and at breast height for all other species.

c See appendix table A.4.

d Total number of forested plots on which the species was measured.
9

Table 5—Mean crown density and other statisticsa for all live trees ≥ 5.0 inches diameterb by species for 

Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99

Speciesc Plotsd Trees Mean SE

95% confidence

Minimum MedianMaxiLower Upper mum

- - - number - - - - - - - - - - - - - - - - - - - - - - - - percent - - - - - - - - - - - - - - - - - - - - -

Softwoods

Grand fir 43 323 52.0 1.8 48.4 55.5 10 50 90

Western hemlock 12 96 51.6 6.6 38.6 64.6 10 55 85

Western larch 15 64 50.7 3.4 44.0 57.4 15 45 80

Rocky Mountain juniper (w) 48 202 50.7 2.0 46.7 54.6 10 50 90

Subalpine fir 112 1,157 49.3 1.0 47.3 51.3 0 50 90

Western juniper 3 39 48.6 1.0 46.7 50.5 5 55 80

Common pinyon (w) 96 597 48.4 0.9 46.5 50.2 5 50 90

Douglas-fir 162 1,283 47.2 0.8 45.6 48.8 0 45 85

Corkbark fir 10 64 47.0 2.0 43.1 51.0 30 45 80

Singleleaf pinyon (w) 62 760 46.8 1.2 44.5 49.1 5 45 85

Western redcedar 17 187 46.8 2.8 41.3 52.3 5 45 85

Utah juniper (w) 154 1,467 46.2 1.0 44.4 48.1 5 45 95

Engelmann spruce 109 1,104 45.2 1.1 43.1 47.4 5 45 99

White fir 17 171 44.2 2.9 38.5 49.9 5 40 80

Ponderosa pine 56 404 42.1 1.2 39.8 44.3 10 40 85

Oneseed juniper (w) 8 102 41.7 2.6 36.6 46.7 5 45 75

Limber pine 40 122 40.5 2.6 35.4 45.7 5 40 80

Lodgepole pine 111 2,096 39.1 0.7 37.8 40.5 5 40 80

Bristlecone pine 8 29 38.8 2.4 34.2 43.4 25 35 70

Whitebark pine 18 90 34.2 2.3 29.6 38.7 10 35 50

Hardwoods

Rocky Mountain maple (w) 24 65 50.9 3.0 45.0 56.9 25 50 85

Curlleaf mountainmahogany (w) 38 289 45.2 2.6 40.1 50.3 5 45 80

Bigtooth maple (w) 6 49 37.1 0.7 35.8 38.5 10 40 50

Narrowleaf cottonwood 5 46 36.5 3.7 29.3 43.8 15 35 70

Quaking aspen 84 1,233 36.4 1.1 34.2 38.6 5 35 80

Gambel oak (w) 22 250 33.0 1.9 29.1 36.8 5 35 70

SE = standard error.

a The mean and SE calculations consider the clustering of trees on plots.

b Diameter measured at root collar for woodland species, designated with a (w), and at breast height for all other species.

c See appendix table A.4.

d Total number of forested plots on which the species was measured.
10

Table 6—Distribution of sapling crown vigor class for all live saplings 1.0 to < 5.0 inches diametera by FIA species 

group, Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99

Species group Plotsc Saplings

Crown vigor rating

Good Fair Poor

Percent SEd Percent SEd Percent SEd

- - - number - - - percent percent percent

Softwoods

Douglas-fir 45 108 62.0 7.2 27.8 6.0 10.2 3.1

Ponderosa and 

Jeffrey pines 19 44 79.5 7.3 20.5 7.3 0.0 —

True fir 101 349 67.3 3.5 26.6 3.2 6.0 1.9

Western hemlock 3 12 58.3 — 25.0 — 16.7 —

Western white pine 1 7 71.4 — 14.3 — 14.3 —

Engelmann and 

other spruces 49 104 55.8 6.9 41.3 6.8 2.9 1.5

Western larch 1 1 100.0 — 0.0 — 0.0 —

Lodgepole pine 42 160 56.3 5.8 36.3 5.1 7.5 2.2

Western redcedar 7 34 52.9 9.7 35.3 6.4 11.8 4.1

Western woodland 

softwoods 98 222 74.8 3.9 21.6 3.6 3.6 1.2

Other western softwoods 21 51 70.6 9.3 23.5 8.1 5.9 2.6

All softwoods 285 1,092 65.8 2.3 28.3 2.1 6.0 0.9

Hardwoods

Cottonwood and aspen 55 183 56.8 5.1 39.9 4.9 3.3 2.3

Other western hardwoods 2 9 11.1 — 88.9 — 0.0 —

Western woodland 

hardwoods 81 858 52.0 4.7 40.9 4.3 7.1 1.1

All hardwoods 132 1,050 52.5 4.0 41.1 3.6 6.4 1.0

All trees 361 2,142 59.2 2.4 34.6 2.2 6.2 0.7

FIA = Forest Inventory and Analysis; SE = standard error (Standard error calculations consider the clustering of saplings on plots.); — = not 

presented due to insufficient sample.

a Diameter measured at root collar for woodland species groups and at breast height for all other species groups.

b See appendix table A.4.

c

 Total number of forested plots on which saplings were measured. Plot totals are not cumulative because multiple species may occur on any 

given plot.

d SE is not presented for species groups with number of saplings < 25.
11

foliage transparencies were those of the firs (Abies spp.) 

and junipers (Juniperus spp.), species which tend to have 

reasonably symmetrical growth forms with closely- or 

many-branched stems. The species with the least dense 

and most transparent crowns were those of the hardwoods, 

whose crowns tend to be more broad-spreading and open. 

Such great variability inhibits direct comparisons of 

species because some species clearly tend to have denser 

crowns than others. For example, a western hemlock (Tsuga 

heteropyhlla) tree with a crown density of 35 percent may 

indicate that the tree is under stress; however, a bristlecone 

pine (Pinus aristata) tree with the same crown density may 

not be under stress (table 5). 

If comparisons among species or across mixed-species 

plots are required, Zarnoch and others (2004) propose 

standardizing the crown condition indicators to a mean 

of 0 and standard deviation of 1. This adjusts the crown 

indicators for species differences by expressing the 

indicators in terms of standard deviation units from the 

mean for a given species. This allows an indicator to be 

combined across species or for direct comparison of an 

indicator among species.

Variations Due to Site Factors

In addition to varying among species, average crown 

conditions may vary within individual species due to other 

factors such as stand density, stand age, or site moisture, or 

to the relative location of the species to its natural range. 

One way to accommodate stand and site influences is 

stratification, i.e., grouping together sets of homogenous 

observations and making comparisons only among those 

sets. Stratification, e.g. by physiographic class or stand 

origin, reduces variation in descriptive statistics and 

summaries, but it does not necessarily facilitate further 

inferential analyses. In broadscale surveys such as the FIA 

phase 3 program, complete stratification leads to small and 

unbalanced sample sizes that complicate analyses, limit 

interpretations of the results, or have both of these effects. 

One way to avoid these drawbacks of stratification and still 

account for stand influences is to “residualize” the crown 

condition indicators by redefining them as the residuals 

from a model that predicts crown condition based on tree 

and stand conditions (Zarnoch and others 2004). Following 

residualization, observations from many different plots 

within a given species can be combined or compared. 

Crown Condition Stressors

Average crown conditions are impacted by a variety of 

biotic and abiotic stressors that directly or indirectly damage 

foliage and branches. Numerous insects and diseases 

damage trees in the forests of the U.S. Interior West. These 

include a variety of mistletoes, root diseases, and bark 

beetles, as well as defoliators like the Douglas-fir tussock 

moth (Orgyia pseudotsugata McDunnough), western spruce 

budworm (Choristoneura occidentalis Freeman), and the 

fall cankerworm (Alsophila pometaria Harris). 

Between 1997 and 1999, increasing mountain pine beetle 

(Dendroctonus ponderosae Hopkins), western spruce 

budworm (Choristoneura occidentalis Freeman), and 

Douglas-fir beetle (D. pseudotsugae Hopkins) activity 

was observed in Colorado, Nebraska, South Dakota, and 

Wyoming (Forest Health Monitoring Staff 2001); but in 

general, insects and diseases were at endemic levels between 

1996 and 1999. Since the 1990s, however, there has been an 

increase in damage to species vulnerable to the mountain 

pine beetle, particularly lodgepole pine (P. contorta) (U.S. 

Department of Agriculture Forest Service 2004), and an 

increase in mortality among many forest types including 

aspen stands (Worrall and others 2008), pinyon-juniper 

woodlands (Shaw and others 2005), and subalpine fir forests 

(Rogers and others 2001). These increases in mortality 

are likely due to general declines resulting from the 

interaction of predisposing stress factors (e.g. defoliating 

insects, drought, frost or ice damage, poor site quality, 

unbalanced soil nutrition, and advanced tree age) and 

secondary diseases or insects (e.g. root fungi, canker fungi, 

and insect borers) (Thompson 2009). As a result of these 

ongoing declines, crown conditions observed in the affected 

forest types after 1999 may differ significantly from those 

reported here.

In addition to these biotic stressors, tree crown conditions 

are influenced by fire, drought, and periodic weather events 

such as snow storms, ice storms, and tornadoes. Together 

with the biological stressors, these factors may have a 

multiplicative, rather than a simply additive, impact on 

crown condition. On a regional level, drought undoubtedly 

has been the most notable stressor. Coulston (2007) noted 

that compared to historical records the 1995 to 2004 decade 

was more droughty than expected for several ecoregion 

sections in the Western United States.
12

Statistical Characteristics and Hypothesis Testing

A statistical power analysis by Bechtold and others (2009) 

demonstrated the statistical rigor of the crown condition 

indicator and determined the spatial scale at which the 

indicator is functional for hypothesis testing. For most 

plausible scenarios, about 100 plots (or 50 paired plots) 

are adequate for detecting differences between two sets of 

observations. Given the FIA phase 3 sampling network, an 

area of 4.8 million acres of forest provides the necessary 

50 plots (Bechtold and others 2009). The eight States in 

the Interior West region (fig. 1), each have enough forested 

area to supply the minimum sample size individually, and 

when combined to the regional level, <4 percent of the total 

forested area would need to be impacted in order to detect a 

significant change in crown condition (Bechtold and others 

2009).

In addition to having an adequate sample size, any data used 

in hypothesis testing must meet the underlying assumptions 

of the tests being used. Many hypothesis tests applicable 

to the crown condition data (e.g. the t-test) require an 

assumption of normality. When normality cannot be 

assumed, other avenues for analyzing the crown condition 

indicators, such as nonparametric techniques or categorical 

methods for ordinal data, should be explored. For instance, 

because the distribution of the crown dieback indicator 

resembles a log-normal distribution, Bechtold and others 

(2009) suggest using the ROM rather than the difference 

of the means when comparing two sets of data. Randolph 

(2006) examined the distributional characteristics of the 

crown condition data from the Southern United States 

and determined that the crown density indicator met the 

assumption of normality and that given the robustness of the 

t-test and ANOVA (analysis of variance), the assumption of 

normality could be applied to foliage transparency as well, 

as long as the sample sizes of the groups being compared 

are about equal and sufficiently large. Deviation from 

normality was determined to be too extreme, however, for 

such tests to be applied to crown dieback (Randolph 2006). 

Normality diagnostics (skewness and kurtosis values, and 

normal probability plots) indicated that the distributional 

characteristics of crown dieback and crown density in the 

Interior West were similar to those in the South; however, 

the distribution of foliage transparency was more skewed in 

this region than in the South. 

Conclusion

With continued damage by the mountain pine beetle, 

increasing mortality in several forest types, and 

uncertainties about climate change (Solomon 2008), forest 

health monitoring in the Interior West is increasingly 

important. Because a tree’s health is generally reflected 

in the amount and condition of its foliage (Anderson 

and Belanger 1987, Innes 1993), tree crown condition is 

included as one of the FIA forest health indicators. We 

have provided an overview of several factors to consider 

when analyzing and interpreting the crown condition data 

so that valid inferences can be drawn from the results. 

Integrating crown condition data with aerial damage 

surveys (e.g. Morin and others 2004), other forest health 

indicators (e.g. Will-Wolf and Jovan 2009), or both, may 

provide more powerful analyses for investigating changes in 

forest health. Such analyses are encouraged so that as FIA 

continues assessments in the Interior West, calculation of 

changes in the crown measurements will indicate whether 

crown condition—and by extension, forest health—is stable, 

improving, or declining.

Acknowledgments

Appreciation is extended to William Bechtold and Sally 

Campbell for their comments on the initial draft of the 

manuscript that helped improve the quality of this report.

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14

Table A.1—Mean crown dieback and other statisticsa for all live trees ≥ 5.0 inches diameterb by FIA species group 

for Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99

Species groupc Plotsd Trees Mean SEe

95% confidence

Minimum90th

percentile

MaxiLower Upper mum

- - - number - - - - - - - - - - - - - - - - - - - - - - - - - percent - - - - - - - - - - - - - - - - - - - - - -

Softwoods

Douglas-fir 162 1,283 2.0 0.3 1.5 2.5 0 5 99

Ponderosa and 

Jeffrey pines 57 407 2.2 0.7 0.9 3.6 0 5 60

True fir 170 1,715 2.9 0.3 2.3 3.6 0 5 99

Western hemlock 12 96 1.8 1.1 -0.5 4.0 0 5 10

Western white pine 8 15 6.0 — — — 0 10 50

Engelmann and 

other spruces 112 1,118 2.2 0.3 1.5 2.8 0 5 99

Western larch 15 64 1.6 0.8 0.2 3.1 0 5 10

Lodgepole pine 111 2,096 2.7 0.5 1.7 3.7 0 10 80

Western redcedar 17 187 3.2 1.1 1.1 5.3 0 5 60

Western woodland 

softwoods 227 3,128 4.5 0.3 3.9 5.0 0 10 99

Other western softwoods 66 288 3.6 0.8 2.0 5.2 0 5 90

Hardwoods

Cottonwood and aspen 89 1,288 3.1 0.5 2.2 4.0 0 5 95

Other western hardwoods 8 21 9.0 — — — 0 10 95

Western woodland 

hardwoods 84 655 6.8 1.0 4.9 8.7 0 15 85

FIA = Forest Inventory and Analysis; SE = standard error; — = not presented due to insufficient sample.

a The mean and SE calculations consider the clustering of trees on plots.

b Diameter measured at root collar for woodland species groups and at breast height for all other species groups.

c See appendix table A.4.

d Total number of forested plots on which trees were measured.

e SE is not presented for species groups with number of trees < 25.

Appendix
15

Table A.2—Mean foliage transparency and other statisticsa for all live trees ≥ 5.0 inches diameterb by FIA species 

group for Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99

Species groupc Plotsd Trees Mean SEe

95% confidence

Minimum MedianMaxiLower Upper mum

- - - number - - - - - - - - - - - - - - - - - - - - - - - - percent - - - - - - - - - - - - - - - - - - - - - -

Softwoods

Douglas-fir 162 1,283 14.9 0.4 14.1 15.6 0 15 99

Ponderosa and 

Jeffrey pines 57 407 14.8 0.6 13.7 16.0 5 15 55

True fir 170 1,715 12.4 0.4 11.7 13.2 0 10 99

Western hemlock 12 96 17.2 2.0 13.3 21.1 5 15 35

Western white pine 8 15 19.3 — — — 15 20 30

Engelmann and 

other spruces 112 1,118 11.9 0.5 10.9 12.9 5 10 40

Western larch 15 64 17.6 1.1 15.3 19.8 10 15 40

Lodgepole pine 111 2,096 15.7 0.3 15.1 16.4 0 15 75

Western redcedar 17 187 18.3 1.0 16.4 20.3 5 20 40

Western woodland 

softwoods 227 3,128 12.3 0.3 11.8 12.9 0 15 65

Other western softwoods 66 288 14.7 1.1 12.5 16.8 0 15 50

Hardwoods

Cottonwood and aspen 89 1,288 19.3 0.9 17.6 21.1 5 20 90

Other western hardwoods 8 21 20.7 — — — 10 20 85

Western woodland 

hardwoods 84 655 16.7 1.0 14.7 18.7 5 15 95

FIA = Forest Inventory and Analysis; SE = standard error; — = not presented due to insufficient sample.

a

 The mean and SE calculations consider the clustering of trees on plots.

b

 Diameter measured at root collar for woodland species groups and at breast height for all other species groups.

c

 See appendix table A.4.

d

 Total number of forested plots on which trees were measured.

e

 SE is not presented for species groups with number of trees < 25.
16

Table A.3—Mean crown density and other statisticsa for all live trees ≥ 5.0 inches diameterb by FIA species group 

for Colorado, Idaho, Nevada, Utah, and Wyoming, 1996–99

Species groupc Plotsd Trees Mean SEe

95% confidence

Minimum MedianMaxiLower Upper mum

- - - number - - - - - - - - - - - - - - - - - - - - - - - - percent - - - - - - - - - - - - - - - - - - - - - -

Softwoods

Douglas-fir 162 1,283 47.2 0.8 45.6 48.8 0 45 85

Ponderosa and 

Jeffrey pines 57 407 42.2 1.2 39.9 44.5 10 40 85

True fir 170 1,715 49.2 0.9 47.5 50.9 0 50 90

Western hemlock 12 96 51.6 6.6 38.6 64.6 10 55 85

Western white pine 8 15 47.7 — — — 20 45 70

Engelmann and 

other spruces 112 1,118 45.3 1.1 43.2 47.4 5 45 99

Western larch 15 64 50.7 3.4 44.0 57.4 15 45 80

Lodgepole pine 111 2,096 39.1 0.7 37.8 40.5 5 40 80

Western redcedar 17 187 46.8 2.8 41.3 52.3 5 45 85

Western woodland 

softwoods 227 3,128 46.9 0.7 45.6 48.2 5 45 95

Other western softwoods 66 288 40.1 2.3 35.5 44.7 5 40 80

Hardwoods

Cottonwood and aspen 89 1,288 36.5 1.1 34.3 38.6 5 35 80

Other western hardwoods 8 21 46.2 — — — 5 45 65

Western woodland 

hardwoods 84 655 40.5 2.0 36.6 44.4 5 40 85

FIA = Forest Inventory and Analysis; SE = standard error; — = not presented due to insufficient sample. 

a The mean and SE calculations consider the clustering of trees on plots.

b Diameter measured at root collar for woodland species groups and at breast height for all other species groups.

c See appendix table A.4.

d Total number of forested plots on which trees were measured.

e SE is not presented for species groups with number of trees < 25.
17

Table A.4—Common and scientific name for tree species included in the FHM survey in Colorado, Idaho, Nevada, Utah, and 

Wyoming, 1996–99a

Species group 

and common name Scientific nameb

Species group 

and common name Scientific nameb

Douglas-fir Pseudotsuga menziesii Other western softwoods

Ponderosa and Jeffrey pines Western juniper Juniperus occidentalis

Jeffrey pinec Pinus jeffreyi Whitebark pine Pinus albicaulis

Ponderosa pine P. ponderosa Bristlecone pine P. aristata

True fir Coulter pinec P. coulteri

White fir Abies concolor Limber pine P. flexilis

Grand fir A. grandis Pacific yew Taxus brevifolia

Subalpine fir, corkbark fir A. lasiocarpa Mountain hemlockc Tsuga mertensiana

Western hemlock Tsuga heterophylla Cottonwood and aspen

Western white pine Pinus monticola Narrowleaf cottonwoodc Populus angustifolia

Engelmann and other spruces Black cottonwoodc P. balsamifera

Engelmann spruce Picea engelmannii Plains cottonwood P. deltoides ssp. monilifera

Blue spruce P. pungens Quaking aspen P. tremuloides

Western larch Larix occidentalis Other western hardwoods

Lodgepole pine Pinus contorta Paper birch Betula papyrifera

Western redcedar Thuja plicata Bur oak Quercus macrocarpa

Western woodland softwoods Western woodland hardwoods

Oneseed juniper Juniperus monosperma Rocky Mountain maple Acer glabrum

Utah juniper J. osteosperma Bigtooth maple A. grandidentatum

Rocky Mountain juniper J. scopulorum Curlleaf mountain-mahogany Cercocarpus ledifolius

Common pinyon Pinus edulis Alderleaf mountain-mahogany C. montanus

Singleleaf pinyon P. monophylla Gambel oak Quercus gambelli

FHM = Forest Health Monitoring.

a

 Species group, common, and scientific names of species occurring in the FHM sample as saplings (1.0 to < 5.0 inches diameter) and trees (≥ 5.0 

inches diameter) unless otherwise noted by footnote c.

b

 Little (1979).

c Tree only.

Randolph, KaDonna C.; Thompson, Mike T. 2010. Descriptive statistics of tree crown 

condition in the United States Interior West. Gen. Tech. Rep. SRS–127. Asheville, NC: 

U.S. Department of Agriculture Forest Service, Southern Research Station. 17 p.

The U.S. Forest Service Forest Inventory and Analysis (FIA) Program uses visual 

assessments of tree crown condition to monitor changes and trends in forest health. This 

report describes four crown condition indicators (crown dieback, crown density, foliage 

transparency, and sapling crown vigor) measured in Colorado, Idaho, Nevada, Utah, 

and Wyoming between 1996 and 1999. Descriptive statistics are presented by species 

and FIA species group. Inter- and intra-species variation, crown condition stressors, and 

statistical concerns that should be considered when analyzing and interpreting the crown 

condition data are discussed.

Keywords: Crown density, crown dieback, FIA, foliage transparency, forest health, 

sapling vigor.
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