United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

General Technical 

Report

PNW-GTR-794

August 2009

Canopy Structure on Forest 

Lands in Western Oregon: 

Differences Among Forest 

Types and Stand Ages

Anne C.S. McIntosh, Andrew N. Gray, and 

Steven L. Garman 
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Authors

Anne C.S. (née Fiala) McIntosh was an M.S. student, and Steven L. Garman

was a research scientist, Department of Forest Science, Oregon State University, 

Corvallis, OR 97331. Andrew N. Gray is a research ecologist, U.S. Department 

of Agriculture, Forest Service, Pacific Northwest Research Station, 3200 SW 

Jefferson Way, Corvallis, OR 97331. McIntosh is currently a Ph.D. candidate 

in the Department of Renewable Resources, University of Alberta, Edmonton, 

AB T6G 2H1, email: amcintos@ualberta.ca. Garman is currently with the U.S. 

Geological Survey, Rocky Mountain Geographic Science Center, Box 25046, 

Denver Federal Center Mail Stop 516, Lakewood, CO 80225.

Cover photographs by Andrew Gray.
Abstract

McIntosh, Anne C.S.; Gray, Andrew N.; Garman, Steven L. 2009. Canopy 

structure on forest lands in western Oregon: differences among forest types and 

stand ages. Gen. Tech. Rep. PNW-GTR-794. Portland, OR: U.S. Department of 

Agriculture, Forest Service, Pacific Northwest Research Station. 35 p.

Canopy structure is an important attribute affecting economic and ecological values 

of forests in the Pacific Northwest. However, canopy cover and vertical layering 

are rarely measured directly; they are usually inferred from other forest measurements. In this study, we quantified and compared vertical and horizontal patterns of 

tree canopy structure and understory cover along a successional gradient of forests 

and among stands with different thinning histories on nonfederal lands in western 

Oregon. Analyses focused on three dominant forest type groups: wet conifer, wet 

hardwood, and dry hardwood. We used data from 917 systematically located, 

forested Forest Inventory and Analysis plots measured between 1995 and 1997. 

On each plot, canopy cover by layer and species was measured on line-intercept 

transects, and cover of understory species was measured on five subplots. Trends 

in canopy structure with stand age did not always follow the patterns predicted by 

common successional models. Most of the cover in moist stands was in the upper 

tree layer, but cover in dry hardwood stands was more evenly distributed among 

layers. Contrary to expectations of canopy closure, mean canopy cover by age class 

rarely exceeded 85 percent, even in unthinned productive young conifer forests. 

Possibly as a result, effects of stand age on understory vegetation were minimal, 

except for low levels of forbs found in 20- to 40-year-old wet conifer stands. Shadetolerant tree species rarely made up more than 20 percent of canopy cover, even 

in the lower canopy layers and in stands >100 years old. Although heavily thinned 

stands had lower total cover, canopy structure did not differ dramatically between 

thinned and unthinned stands. Our findings suggest potential limitations of simple 

stand succession models that may not account for the range of forest types, site 

conditions, and developmental mechanisms found across western Oregon.

Keywords: Canopy structure, Douglas-fir, succession, canopy cover, 

understory vegetation, forest inventory, Pacific Northwest, hardwood.

Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

1

Introduction

Characterization of canopy structure is important for the management of forests in 

the Pacific Northwest (PNW). Amount of cover and the vertical structure of forest 

canopies can determine key ecological attributes: wildlife habitat (Hayes et al. 1997, 

Johnson and O’Neil 2001, MacArthur and MacArthur 1961, Maguire and Bennett 

1996, North et al. 1999, Thomas and Verner 1986), disease and insect susceptibility (Mathiasen 1996, Winchester and Ring 1996), fire hazard (Latham et al. 1998), 

atmospheric fluxes of energy and gases (Rose 1996), and microclimate (Gray et 

al. 2002, Yang et al. 1999). Forest management goals that include conservation of 

wildlife, reduction of fire hazard, or control of insect/pathogen outbreaks usually 

set criteria for canopy structure, either implicitly or explicitly.

Canopy structure is the horizontal and vertical distribution of tree crowns in 

a forest stand. Vertical canopy structure is often simplified by dividing canopy 

cover into height layers. Horizontal canopy structure is commonly quantified as the 

vertically-projected percentage of cover of plant canopies, and the abundance and 

size of canopy gaps. Additional attributes used to describe canopy structure include 

the number of vertical canopy layers, heights of the vertical layers, and the proportions of cover contributed by different species groups (Fiala 2003).

It is generally understood that canopy structure changes as forests develop 

with age (Bond and Franklin 2002, Franklin et al. 2002, Oliver 1981, Van Pelt and 

North 1996), but these changes have rarely been quantified. Forest succession has 

nevertheless been a useful template for describing changes in canopy structure and 

for comparing canopies between managed and natural environments (Franklin et 

al. 2002, McCook 1994). Managing forests based on simple characterizations may 

not adequately replicate the stand structure of natural forests, including canopy 

structure. The challenge for forest managers and biologists is to describe canopy 

structure as a meaningful forest indicator that is also repeatable, efficient, and reliable across a wide range of field-tested conditions. Characterizing canopy structure 

attributes across a stand development gradient can help achieve this objective.

Multiple stand development models are used to describe forest succession patterns and processes (e.g., Carey and Curtis 1996, Franklin et al. 2002, Oliver 1981, 

Spies and Franklin 1991). The most commonly cited stand development model is 

Oliver’s (1981) four-stage model, which comprises stand-initiation, stem-exclusion, 

understory-reinitiation, and old-growth phases, all of which include canopy cover 

criteria. Franklin et al. (2002) proposed an alternative stand development model for 

natural stands. Their model highlights eight commonly encountered development 

stages: disturbance and legacy creation, cohort establishment, canopy closure, 

biomass accumulation/competitive exclusion, maturation, vertical diversification, 

Forest succession has 

been a useful template 

for describing changes 

in canopy structure 

and for comparing 

canopies between 

managed and natural 

environments.
2

GENERAL TECHNICAL REPORT PNW-GTR-794 

horizontal diversification, and pioneer cohort loss, with canopy attributes described 

for each of these stages. Quantifying canopy structure attributes across a successional gradient can aid in evaluating these stand development models. 

The development of understory plant communities is usually related to changes 

in the overstory (Franklin et al. 2002, Henderson 1981, Naesset and Okland 2002, 

Oliver 1981, Stewart 1988, Zamora 1981). According to Connell and Slatyer’s (1977) 

“tolerance” model of succession, shade-tolerant species are generally present in all 

stages of succession, but invade the understory and increase in abundance across 

the gradient of development stages. To assess successional patterns, it is crucial 

to understand how variance of foliage and its patterns of distribution in the forest 

canopy impact understory conditions (Van Pelt and Franklin 2000). Therefore, it is 

important to examine patterns in both overstory and understory cover across a successional gradient, including the quantities and proportions of shade-tolerant cover 

vertically distributed in the canopy. 

The progression of forest canopy structure development among successional 

stages differs with forest type, species composition, disturbance history, and 

management practices. In the coniferous forests of the Pacific Northwest, the 

longevity of some tree species results in slower successional change than in 

forest types with more rapid seral replacement of species (e.g., red alder hardwood 

stands) (Ishii et al. 2000). However, the differences in canopy structure along 

forest succession gradients for multiple forest types have rarely been described.

Forest management has an impact on canopy structure. In western Oregon, 

clearcut logging, tree planting, and short stand rotation lengths have greatly reduced 

the structural variability of forests (Garman et al. 1992; Hansen et al. 1991, 1995; 

Smith et al. 1996). The vertical distribution of foliage can be altered by multiple 

silvicultural treatments, including pruning, fertilization, and thinning (Berg et al. 

1996, Maguire and Bennett 1996, but see Gillespie 1994). With increasing awareness of the value of structural heterogeneity for multiple wildlife species, there has 

been a growing emphasis on silvicultural techniques that promote heterogeneity 

in stand structure throughout the rotation interval (Berg et al. 1996). Retention of 

overstory trees during even-age regeneration harvest is an effort to better represent 

patterns of disturbance and the structural complexity of natural forests (Hansen 

et al. 1995). It is not known, however, how different management regimes affect 

canopy structure.

Much ecological research has described similarities and differences in patterns 

and processes associated with succession (McCook 1994), but we are unaware of 

any study that examines canopy structure attributes across a large range of forest 

types and successional stages.
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

3

The purpose of this study was to enhance understanding of canopy structure 

development patterns across the forests of western Oregon using inventory data. 

Specifically, we quantified and compared total cover of trees and understory vegetation, vertical canopy layering, and abundance of shade-tolerant cover across seral 

stages and forest types, and assessed the impact of thinning on canopy structure. 

Our expectations were that most stands would fall along the successional trajectory for productive forests after severe disturbance: an initial period of low tree 

cover and moderate understory cover; an early period of rapid increase to full tree 

closure in a single layer and sparse understory cover; an intermediate period of slow 

development of multiple tree canopy layers and slow increase in understory cover; 

and a later period of reduced tree cover in the tallest layer, with tree cover dispersed 

among multiple layers, and moderate to high understory cover. 

Methods

Data Collection

This study used data collected during the inventory of western Oregon forests conducted by the USDA Forest Service Forest Inventory and Analysis (FIA) Program 

from 1995 through 1997 (Azuma et al. 2004). Study sites were a permanent grid of 

systematically located plots located throughout western Oregon. Western Oregon 

was defined as the area west of the crest of the Cascade Mountain Range, delimited 

by county boundary lines. The study sites included all private and public forested 

lands, except for lands managed by the USDI Bureau of Land Management and the 

USDA Forest Service national forests. A separate inventory system was used on 

those federal lands in the 1990s that did not collect comparable canopy structure 

data. The systematic-grid design of the FIA inventory allows for statistical inferences to the population from which the grid points were sampled. Information 

compiled and distributed from the FIA inventory is comprehensive, has minimal 

bias, is scientifically sound, and has known precision (Azuma et al. 2004). The 

forested plots are representative of the entire population of nonfederal forest lands 

in western Oregon.

The study area encompassed five physiographic provinces: The Oregon Coast 

Range, The Willamette Valley, Oregon Western Cascades, Klamath Mountains, and 

High Cascades (Franklin and Dyrness 1973) (fig. 1). The forest zones included in 

the FIA inventory were Sitka spruce, western hemlock, Pacific silver fir, mountain 

hemlock, oak woodland, interior valley, mixed-evergreen, mixed-conifer, white 

fir, and Shasta red fir (Franklin and Dyrness 1973). A total of 1,127 FIA plots were 

measured for the inventory of western Oregon forests. Plots consisted of a cluster of 
4

GENERAL TECHNICAL REPORT PNW-GTR-794 

Figure 1—Locations of the 917 forested Forest Inventory and Analysis 1995–97 inventory plots analyzed in this study and the boundaries 

of the Franklin and Dyrness (1973) physiographic provinces.
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

5

five 0.09-ha subplots systematically placed over a 2.5-ha area. Portions of plots in 

different land class or stand types were delineated in the field, and all data collected 

identified to the type being sampled. For this study, data were only used for forested 

stands that were sampled by a minimum area of 0.27 ha (three subplots) to ensure 

an adequate sample of stand characteristics. Of the original 1,127 plots, 934 met 

this criterion. We eliminated another 17 plots classified as non-stocked by FIA (<10 

percent stocking of trees), leaving 917 plots for this study. We refer to each FIA plot 

as a ”stand” throughout this paper, and treat a stand as the experimental unit.

Ground-based canopy cover estimates were made on the forested portions of 

each plot. Canopy cover was measured on three 17-m (horizontal distance) transects 

originating at subplot center and radiating out at 0, 135, and 225 degrees. Trees 

≥ 1.4 m in height were assigned to as many as three canopy layers (upper, middle, 

lower), with discrete layers differing by a minimum of 5 m in mean height. Canopy 

layers were relative to conditions within a stand; actual layer heights differed 

among stands. The line-intercept method was used to measure canopy cover in each 

layer (Canfield 1941, Fiala et al. 2006, O’Brien 1989). For every tree species (see 

app. 1) within a canopy layer, generalized crown boundaries (i.e., ignoring minor 

gaps and openings) were vertically projected onto transects using a clinometer (see 

USDA Forest Service 1995 for detailed procedures). The distance along a transect 

line that the crown intercepted was recorded. The proportion of transect lengths 

intercepted by the crowns was the ground-estimated canopy cover for a stand.

Selected tree and understory plant attributes measured by FIA crews were also 

used for this study. Composition, cover, and height of all trees ≥ 0.5 m in height and 

< 2.5 cm diameter at breast height (d.b.h.), all shrubs, and forbs and grasses with 

> 3 percent cover were measured on 5-m fixed-radius plots around each subplot 

center. Separate estimates of total shrub and forb cover were also made. Age, d.b.h., 

and height of trees were measured with a combination of variable- and fixed-radius 

plots. Trees 2.5 to 12.5 cm d.b.h. were measured on 2.35-m-radius plots, whereas 

trees > 12.5 cm d.b.h. were measured using a 7-m2/ha basal area factor prism to a 

maximum distance of 17 m from subplot center.

Analyses

We calculated multiple cover values for each of the 917 stands. For trees, we 

calculated cover for individual species for each of the vertical layers (< 3) as the 

proportion of the transect lengths covered by them. Total cover of a species was the 

combination of its cover in each of the three layers. Combining of layers did not 

double-count cover from multiple layers that intercepted the same horizontal areas 
6

GENERAL TECHNICAL REPORT PNW-GTR-794 

along each transect; cover could not exceed 100 percent. We classified tree species 

as either shade-tolerant or shade-intolerant, wherein Pacific silver fir, white fir, 

grand fir, Pacific dogwood, western redcedar, Pacific yew, western hemlock, English holly, mountain hemlock, and Sitka spruce (see appendix for species names) 

were classified as shade-tolerant. We then quantified canopy cover separately 

for shade-tolerant and shade-intolerant cover, for both individual layers and total 

combined cover. Finally, we combined all tree species to calculate cover by layer 

and total cover, accounting for overlap within and among layers so cover could not 

exceed 100 percent. We summarized tree cover levels at the stand level, accounting 

for transects and/or layers within transects that had no cover recorded by assigning 

them cover values of 0. We averaged total shrub and total forb cover values across 

the three to five subplots. 

Stand age, forest type, and forest type group were classified using the tree 

data for each stand in the FIA inventory (Azuma et al. 2004). Stand age was based 

on the ages of the dominant trees in each stand (excluding residual trees from 

previous stands) and grouped into 10-year age classes up to age 200, lumped into 

a 100-year age class for ages 200 to 300 (labeled as age “250” in this study), and 

combined into a single age class stands >300 years old (labeled as age “400” in this 

study). Forest type was assigned based on the dominant tree species in the stand. 

We assigned each stand to a forest type group, which was defined by the dominant 

species type (conifer, hardwood) and relative site moisture characteristic (wet or 

dry). To determine site moisture characteristics, each stand was overlaid on a map 

of mean annual precipitation generated with the PRISM climate model (1971-2000 

means, 800-m grid size; Daly et al. 1994). Four forest type groups were generated 

with this procedure: wet conifer, dry conifer, wet hardwood, dry hardwood (table 

1). Douglas-fir was the dominant species in the wet conifer group, red alder was the 

dominant species in the majority of wet hardwood stands, and Oregon white oak 

and Pacific madrone dominated most of the dry hardwood stands. Because the FIA 

sample is representative of distributions on the landscape, sample sizes were not 

evenly distributed among forest type groups (hereafter referred to as forest groups) 

and age classes (table 2).

Canopy structure patterns were evaluated for all forest groups combined and 

separately for three forest groups. The dry conifer group had few samples (n = 33) 

and was not analyzed separately (see Fiala (2003) for additional analyses). Because 

70 percent of the stands were in the wet conifer group (table 1), results for all forest 

groups combined paralleled trends of the wet conifer group. Therefore, beyond 

describing total canopy cover and canopy cover for the three vertical layers, we 

focused our analyses on cover for the three dominant forest groups in our data set. 
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

7

Table 1—The four forest groups included in the 1995–97 Forest Inventory and Analysis inventory of western 

Oregon that met the criteria for inclusion in this study, and the number of plots in each category

Forest groupa Precipitation (SE) Number Dominant tree species in the stand

Centimeters

Wet conifer 185.4 (7.3) 645 Douglas-fir (n = 558), western hemlock (n = 57), Sitka spruce (n = 15), 

western redcedar (n = 7), noble fir (n = 4), Pacific silver fir (n = 2), 

Port-Orford cedar (n = 2)

Dry conifer 124.2 (21.6) 33 Grand fir (n = 9), incense-cedar (n = 9), lodgepole pine (n = 6), 

white pine (n = 6), ponderosa pine (n = 3) 

Wet hardwood 188.4 (5.1) 137 Red alder (n = 99), bigleaf maple (n = 25), black cottonwood (n = 5), 

willow spp. (n = 3), California laurel (n = 3), Oregon ash (n = 2)

Dry hardwood 125.6 (12.4) 102 Oregon white oak (n = 42), Pacific madrone (n = 32), tanoak (n = 18), 

California black oak (n = 6), canyon live oak (n = 3), golden 

chinkapin (n = 1)

Note: SE = standard error.

a Stands were grouped by hardwood and conifer dominance and mean precipitation levels from the PRISM model (Daly et al. 1994).

Table 2—Sample sizes for each forest group and stand-age 

class combination in the 1995–97 Forest Inventory and Analysis 

inventory stands used in this study

Sample size by forest group

Stand-age Wet Wet Dry All forest

class conifer hardwood hardwood groups

Years

5 67 13 12 92

15 90 14 12 122

25 118 23 14 161

35 89 31 8 131

45 107 27 16 154

55 66 16 11 96

65 36 8 7 54

75 22 0 5 29

85 9 2 3 16

95 9 0 4 16

105 13 1 4 19

115 9 0 4 13

125 2 0 0 2

145 0 0 1 1

155 1 1 0 2

165 0 0 1 1

175 1 0 0 1

250a 4 1 0 5

400b 2 0 0 2

 Total 645 137 102 917

a All stands between ages 200 and 300 were combined into the 250-year age class.

b All stands aged > 300 years were combined in the 400-year stand-age class.
8

GENERAL TECHNICAL REPORT PNW-GTR-794 

We calculated means and standard errors of multiple canopy structure attributes 

by stand-age class for the forest groups. Although age is not a predictor of successional stage, it serves as a proxy for stand-development pattern (Spies and Cohen 

1992). Overstory attributes were canopy cover of shade-tolerant, shade-intolerant, 

and all tree species combined, cover in each of the three vertical canopy layers, 

heights of the three vertical layers, and the number of vertical canopy layers. Mean 

shrub and forb cover for each stand were calculated to compare understory and 

overstory vegetation cover along the stand-age development gradient. For shrub and 

forb species present in > 20 percent and > 10 percent of the 917 stands, respectively, 

we calculated mean species cover by stand age for stands with observations of the 

species (“characteristic cover”). Shrub species richness also was derived by stand 

age for all stands combined.

Analysis of variance (General Linear Model procedure, SAS Institute Inc. 1999) 

were computed for selected canopy attributes by stand age, forest group, and the 

age × forest-group interaction. Effects with an estimated type 1 error of p < 0.05 

were considered significant, and comparison of means for significant effects was 

done with the Tukey-Kramer procedure. The data were approximately normally 

distributed. Although an arcsin-square-root transformation of the percentage of 

cover data somewhat improved normality, the impact on parameter estimates 

and p-values for effects was minor. Thus, analyses used the untransformed cover 

data for greater simplicity of interpretation of results. Statistical analyses with 

forest group were restricted to stands <75 years old to ensure data were available 

for each combination of stand age × forest group (n = 785). We assessed patterns 

qualitatively for data from stands >75 years old.

Two vertical structural diversity measures were calculated for each wet conifer 

stand: Simpson’s Diversity Index (SDI, Simpson 1949), and the Canopy Height 

Diversity Index (CHDI, Spies and Cohen 1992). The indexes were not calculated 

for hardwood stands because the numbers of stands by age class were less evenly 

distributed than for wet conifer stands and because CHDI was developed for wet 

conifer stands, and its reliance on maximum tree height makes it less appropriate 

for hardwood stands. For SDI, trees ≥ 2.5 cm d.b.h. were grouped into 5-m vertical 

intervals based on their heights. Stem density weighted by basal area was used as 

a substitute for species to compare diversity among height intervals. Proportions 

of total basal area were used because the number of stems gave higher weight to 

smaller trees. However, we expected the larger trees to be taller with larger crowns, 
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

9

and thus contribute more to stand vertical diversity than smaller trees. Previous 

studies have also used basal area instead of density to better represent the use of 

resources, recognizing that larger trees will have more influence (e.g., Staudhammer and Lemay 2001). The CHDI equation requires crown area estimates, which 

were calculated using equations developed to estimate crown width from d.b.h. 

(Moeur 1985).

Effects of recent management on canopy cover were assessed by comparing 

Douglas-fir forests 15 to 45 years old that were unthinned, lightly thinned, or 

heavily thinned in the 10 years prior to measurement, as estimated or verified 

by FIA crews. There were 245 unthinned stands without evidence of harvest 

or wildfire. Twenty-one stands had evidence of light harvest levels, defined as 

remaining trees constituting > 25 percent crown cover with <20 percent of live 

trees >12.5 cm d.b.h. harvested. Heavy partial harvest, with > 25 percent crown 

cover and > 20 percent of live trees >12.5 cm d.b.h. harvested, was identified for 

20 stands. 

Results

Tree Cover

Tree canopy cover differed significantly by age class and forest type group (fig. 

2; F6,764 = 118.3, p < 0.001 and F2,764 = 5.47, p = 0.004, respectively), and for 

their interaction (F12,764 = 4.76, p < 0.001). For all forest groups, mean total cover 

significantly increased with increasing stand age up to age 35 (canopy closure) and 

subsequently remained high (60 to 96 percent) after canopy closure was reached, 

although it rarely exceeded 85 percent. Mean cover of the upper layer contributed 

the most to total cover and thus closely mirrored trends in total canopy cover, 

except in stands >200 years old. Mean cover of the middle layer also increased 

with stand age (F6,764 = 8.21, p < 0.001). Mean cover of the lowest layer was 

nominal (< 20 percent) except in stand ages >200 years. Canopy cover values and 

trends for wet conifer and wet hardwood forests paralleled those for all forest types 

combined, except that total canopy cover was significantly higher in 1- to 10-yearold wet hardwood stands than in wet conifer stands (as seen in the significant 

interaction term). In contrast, the dry hardwood forest type had the lowest total 

cover levels, and the upper and middle cover layers contributed similar amounts 

of cover across stand ages. 

Mean total cover 

increased with 

increasing stand 

age up to age 35 

and subsequently 

remained high, 

although it rarely 

exceeded 85 percent.
10

GENERAL TECHNICAL REPORT PNW-GTR-794 

Figure 2—Mean canopy cover (±1 SE) for three tree canopy layers and all layers combined across a chronosequence of stand ages 

(grouped into 10-year intervals and labeled using midpoints of intervals) by forest group (see table 1) in western Oregon. Sample sizes 

differed among stand ages (see table 2).

Shade-tolerant canopy cover was quite low across stand age classes for all 

three forest groups (fig. 3). The wet conifer stands had significantly higher levels 

of shade-tolerant cover than the other two groups (F2,764 = 17.0, p < 0.001), with 

about 20 percent total shade-tolerant cover between stand ages 35 to 85, primarily 

concentrated in the upper layer. However, total shade-tolerant cover was unexpectedly low in the older wet conifer stands. The wet hardwood stands were even lower 

in shade-tolerant cover, with fairly consistent and similar levels for all three layers 

among stand ages, except in the older single samples. The dry hardwood stands 

had only nominal total shade-tolerant cover in all layers for all stand ages.
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

11

Figure 3—Mean percentage of shade-tolerant canopy cover (± 1 SE) for three tree canopy layers and all layers combined across a chronosequence of stand ages (grouped into intervals and labeled using midpoints of intervals) by forest group (see table 1) in western Oregon. 

Sample sizes differed among stand ages (see table 2).

In contrast with absolute abundance, the proportions of cover that were shadetolerant were generally highest in the lower cover layer, compared with the middle 

and upper layers across all three forest groups (fig. 4). In the wet conifer stands, the 

proportions of shade-tolerant cover were consistently highest for the lower cover 

layer with the exception of the 5-year age class. For the wet hardwood stands, trends 

in the proportions of shade-tolerant cover were generally similar to wet conifer 

stands. The dry hardwood stands had minimal proportions of shade-tolerant cover, 

consistent with their nominal shade-tolerant cover levels. Across all three forest 

groups, the proportion of shade-tolerant cover rarely exceeded shade-intolerant 

cover in any of the three vertical cover layers.
12

GENERAL TECHNICAL REPORT PNW-GTR-794 

Vertical Structure

Heights of canopy layers increased with increasing stand age (fig. 5; F6,764 = 34.4, 

p < 0.001). The upper and middle layer heights for wet conifer forests increased 

with stand age to 55 years, and then remained fairly constant in older stands. The 

wet hardwood forest group followed a similar pattern, except that the height of the 

upper layer was similar for stands aged 5- to 25-years, and heights of the upper and 

middle layers increased only to age 35 (forest type group by stand age interaction 

F12,764 = 6.77, p < 0.001). The lower layer heights for both wet conifer and wet 

hardwood forests were consistently short among stand ages. The dry hardwood 

group differed in that mean heights of its upper and middle canopy layers were 

shorter than for the other two forest groups (interaction effect for middle layer 

F12,764 = 2.28, p = 0.008), and all three canopy layer heights increased minimally 

across the 165-year chronosequence.

Figure 4—Mean proportion (± 1 SE) of shade-tolerant canopy cover across a chronosequence of stand ages (grouped into intervals 

and labeled using midpoints of intervals) by forest group (see table 1) in western Oregon. Sample sizes differed among stand ages 

(see table 2).
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

13

The number of canopy layers increased with stand age in all forest groups 

(F6,764 = 23.2, p < 0.001), indicating differentiation of vertical canopy structure 

(fig. 6). For the wet conifer stands, the mean number of layers developed from 

one in 5-year stands to greater than two layers for ages 15 to 55 and then leveled 

off at three layers. Layer development in the wet hardwood stands paralleled 

trends in the wet conifer stands, except that even the youngest (5- to 15-year) 

stands had greater than two layers of cover (group by age interaction F12,764 = 

3.56, p < 0.001). Vertical layering in the dry hardwood stands fluctuated between 

two and three layers for stands greater than 5 years old.

Figure 5—Mean heights (± 1 SE) for three tree canopy layers across a chronosequence of stand ages by forest group (see table 1) in 

western Oregon. Sample sizes differed among stand ages (see table 2).
14

GENERAL TECHNICAL REPORT PNW-GTR-794 

The SDI of tree heights for wet conifer forests increased across age classes 

(fig. 7). Diversity increased up to age 65, and then leveled off. Similarly, CHDI 

for the wet conifer stands increased with stand age up to age 155, but leveled 

off around age 55 (fig. 8). Unlike SDI, CHDI showed lower values for stand age 

classes 175 and 250. 

Understory Vegetation

Cover of shrubs and forbs differed among forest groups and stand age classes, 

and some of the variation was associated with overstory tree cover (fig. 9, table 

3). Forb cover was lower for ages 25 to 35 than for ages 5 to 15 for the wet 

conifer forest type group (group by age interaction: F12,764 = 2.78, p = 0.001). 

Shrub cover was higher in the wet hardwood group than in the other types (F2,764

= 5.48, p = 0.004). There was no significant relationship between shrub cover and 

stand age (F6,764 = 0.97, p = 0.442), although cover levels were slightly elevated 

at age 15 in the wet conifer and wet hardwood types. Forb cover was negatively 

correlated with tree cover in the wet conifer and dry hardwood types (table 3), 

indicating that some of the stand-age effect may have been due to differences in 

tree cover. This analysis also revealed a small negative correlation between shrub 

cover and tree cover for the wet hardwood type.

Figure 6—Mean number of canopy layers (± 1 SE) across a chronosequence of stand ages by forest group (see table 1) in western 

Oregon. Sample sizes differed among stand ages (see table 2).

Forb cover was 

negatively correlated 

with tree cover in the 

wet conifer and dry 

hardwood types.
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

15

Figure 7—Mean Simpson’s Diversity Index (± 1 SE) of tree heights weighted by their proportion of 

total basal area, within 5-m vertical height intervals among a chronosequence of stand ages for the 

wet conifer stands (n = 645) in western Oregon. Sample sizes differed among stand ages (see table 2).

Figure 8—Mean Canopy Height Diversity Index (CHDI (± 1 SE)) of the wet conifer stands (n = 645) 

among the chronosequence of stand ages. Sample sizes differed among stand ages (see table 2).
16

GENERAL TECHNICAL REPORT PNW-GTR-794 

Figure 9—Mean percentage of cover (± 1 SE) for total tree canopy, shrub, and forb across a chronosequence of stand ages (grouped 

into intervals and labeled using midpoints of intervals) by forest group (see table 1) in western Oregon. Sample sizes differed among 

stand ages (see table 2).
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

17

Table 3—Pearson correlation coefficients and p-values 

between tree cover and forb cover, and tree cover and 

shrub cover, by forest group

Forb cover Shrub cover

Forest group R p R p

All -0.251 <0.0001 -0.037 0.2647

Wet conifer -0.340 <0.0001 -0.024 0.5370

Wet hardwood 0.117 0.1745 -0.187 0.0286

Dry hardwood -0.360 0.0002 -0.105 0.2921

There were 18 shrub species present in >20 percent of the stands and 5 forb 

species present in >10 percent of the stands that were analyzed for cover trends 

with age (figs. 10 and 11). Of the shrub species analyzed, the stand age effect 

was significant (p < 0.05) for vinemaple, red huckleberry, dwarf Oregon grape, 

oceanspray, trailing blackberry, and snowberry (fig. 10). For the first four shrub 

species, cover was lower in stands aged 5 and 15 than in stands aged 35 to 65, 

whereas the reverse was true for the last two species. Of the forb species analyzed, 

the stand age effect was significant (p < 0.05) for sword fern and bracken fern 

(fig. 11). For swordfern, cover was lower in stands aged 5 and 15 than in stands 

aged 35 to 65; the reverse was true for bracken fern.

Mean shrub species richness peaked just prior to crown closure (age 15) and 

revealed a small but significant drop in richness coincident with age of canopy 

closure at ages 25 to 35 (fig. 12; F18,896 = 2.94, p < 0.001). Shrub species richness 

appeared to rise somewhat with older age classes, but variability in the low sample 

size obscured patterns in the oldest age classes.

Tree and Understory Cover After Thinning

Canopy cover differed significantly among the levels of thinning in the 15- to 

45-year-old Douglas-fir stands (table 4; F2,292 = 6.38, p = 0.002). Undisturbed (ND) 

and lightly thinned (LT) stands had similar (78 to 79 percent) total mean canopy 

cover, whereas mean cover was lower (62 percent) in heavily thinned (HT) stands. 

Trends in canopy cover for the upper layer were similar to those of total cover. 

Mean canopy cover of the middle layer appeared lower in the HT stands than in the 

ND and LT stands, but the difference was not significant (F2,292 = 1.08, p = 0.340). 

Mean cover of the lower canopy layer was nominal for all three thinning intensities. 

Mean shrub species 

richness peaked just 

prior to crown closure 

(age 15) and revealed 

a small but significant 

drop in richness 

coincident with age 

of canopy closure at 

ages 25 to 35.
18

GENERAL TECHNICAL REPORT PNW-GTR-794 

Figure 10—Mean percentage of cover (± 1 SE) of the 18 most commonly occurring shrubs in stands in which the particular species was 

found (“characteristic cover”) from the Forest Inventory and Analysis inventory across a chronosequence of stand ages (grouped into 

intervals and labeled using midpoints of intervals) in western Oregon. The dashed line at age 35 represents canopy closure.

Mean layer heights also differed among the three levels of thinning. The upper layer 

of the ND stands was on average shorter (20 m) than for thinned stands (24 to 25 m) 

(F2,292 = 5.17, p = 0.006). The heights of the middle and lower layers were consistent 

among thinning intensities. All three thinning levels had a mean of approximately 

2.5 canopy layers. There were no evident trends between understory cover and 

thinning history; both shrub and forb cover were fairly similar among the three 

thinning intensities. The difference in mean shade-tolerant canopy cover was not 

significantly lower in LT and HT than in ND stands (F2,292 = 1.54, p = 0.216).
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

19

Figure 10—Continued.
20

GENERAL TECHNICAL REPORT PNW-GTR-794 

Figure 11—Mean percentage of cover (± 1 SE) of the five most commonly occurring forbs in stands in which the particular species was found (“characteristic cover”) from the Forest Inventory and Analysis inventory across a chronosequence of stand ages 

(grouped into intervals and labeled using interval midpoints in western Oregon. Sample sizes differed among stand ages (see table 

2). The dashed line at age 35 represents age of canopy closure.

Figure 12—Mean species richness of shrubs (± 1 SE) across a chronosequence of stand ages 

(grouped into intervals and labeled using midpoints of intervals) in western Oregon (all forest 

types combined). Sample sizes differed among stand ages (see table 2).
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

21

Discussion

Stand Development Canopy Structure Patterns

The differences in canopy cover and layering found over the chronosequence of 

stand ages in western Oregon were largely consistent with the stand development 

models of Oliver (1981) and Franklin et al. (2002). The Oliver (1981) and Franklin 

et al. (2002) models proposed different starting points for the initiation of stand 

development. The Oliver model generally describes stand development after a 

“major” disturbance, whereby all living stems in an area are killed. The Franklin 

et al. model is more flexible as it offers a gradient of starting points for a stand, 

from disturbance regimes in which large remnant trees remain in the overstory, to 

the major stand-clearing disturbances described by Oliver. For the wet conifer 

stands in western Oregon, where clearcut harvesting or high-intensity stand 

replacement fire are the dominant stand-initiation disturbances, both models 

suggest a similar starting point for live trees. The wet conifer stands in the 5-year 

age class in our study had single layers with low levels of cover, indicating full 

stand replacement. However, the young hardwood stands had unexpectedly high 

cover levels, two layers, and tall heights of the upper canopy layer. The tree tally on 

many of these plots indicated postdisturbance residual overstory trees (i.e., >20 cm 

d.b.h.) were retained in these hardwood stands. Possible explanations for the residual hardwoods include state forest practice rules that require retention in riparian 

areas, which are likely to be dominated by hardwoods, and tree retention for 

amenity values by nonindustrial private owners that tend to be located at lower 

elevations where hardwood stands are most common. Also, many of the hardwood 

species (e.g., Pacific madrone, red alder, bigleaf maple, Oregon white oak) can 

vegetatively reproduce (sprout) and undergo rapid juvenile growth (Burns and 

Table 4—Mean total and shade-tolerant percentage of cover levels (± 1 SE) and mean heights of the canopy 

layers for three levels of thinning in 15- to 45-year-old Douglas-fir stands 

Canopy layer Height of layer

Thinning Combined

intensitya layers Upper Middle Lower Shrub Forb Upper Middle Lower

- - - - - - - - - - - - - - - - - - - - - Percent - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Meters - - - - - - - -

None Total 78.3 (1.3) 64.4 (1.6) 19.4 (1.2) 5.1 (0.5) 46.4 (1.3) 27.0 (1.3) 20.5 (0.5) 11.1 (0.4) 4.2 (0.2)

(n = 254) Shade tolerant 12.3 (1.0) 8.1 (0.8) 3.5 (0.4) 1.8 (0.3)

Light Total 79.1 (2.9) 61.0 (6.6) 18.5 (3.9) 8.4 (2.7) 46.5 (5.1) 22.2 (3.2) 25.3 (1.0) 12.4 (1.2) 4.1 (0.5)

(n = 21) Shade tolerant 7.8 (2.3) 3.5 (6.4) 1.9 (0.7) 2.8 (1.3)

Heavy Total 61.9 (5.0) 48.7 (5.9) 14.6 (3.1) 8.3 (3.0) 42.7 (5.4) 28.4 (3.0) 24.2 (1.0) 12.6 (1.2) 4.5 (0.8)

(n = 20) Shade tolerant 7.7 (2.4) 4.2 (1.8) 1.8 (0.5) 1.9 (0.9)

a No disturbance (ND), light thin (LT), and heavy thin (HT). 
22

GENERAL TECHNICAL REPORT PNW-GTR-794 

Honkala 1990), and have a broader spreading form than the upright conical conifers. Given the varied starting points for the hardwood stands included in the FIA 

inventory, the Franklin et al. model appears more suitable for describing the initial 

phases of hardwood stand development in western Oregon. 

Canopy closure after a new cohort of trees is established is a stage proposed 

by both the Oliver and Franklin et al. models. In this stage, the cohort of trees 

is primarily within a single height class. In our study, mean canopy cover rarely 

exceeded 85 percent for each of the forest groups. Therefore, it may be an oversimplification to conclude that full use of available growing space occurs only when 

adjacent plant crowns are touching, as it assumes that sunlight is the limiting factor 

in stands (Oliver and Larson 1996). However, moisture and nutrients can also be 

limiting variables across the range of FIA sites. In addition, the average stand 

condition is not that of a fully stocked “normal” stand (or portion of a stand) such 

as those used to study growth and yield (e.g., McArdle et al. 1961). As Meyer (1930) 

noted for mature Douglas-fir forests (60 to 80 years old), real stands have about 85 

percent of the basal area of fully stocked “normal” stands of the same site index 

because of patchy tree establishment, shrub-dominated openings, streams, rock 

outcrops, and isolated tree mortality. Our results similarly suggest that the term 

“closed canopy” may be misleading. Although canopy openness could be caused 

by previous thinning, relatively few stands had been thinned in the prior decade 

(between plot measurements), and the total cover of unthinned stands was below 

85 percent.

Total canopy cover in young stands in the wet hardwood and wet conifer 

forest groups was dominated by the upper canopy layer. In contrast, dry hardwood 

stands were characterized by similar contributions from the upper and middle 

canopy cover layers across stand ages. The dry hardwood stands were dominated 

by Oregon white oak and Pacific madrone; although they are not generally classified as shade tolerant, they adequately reproduce under their own shade (Burns 

and Honkala 1990). Trees of dry hardwood stands also retain few shade leaves 

(Oliver and Larson 1996) allowing more light penetration to reach the forest floor. 

Aber (1979) proposed two potential successional pathways for hardwood stands of 

the Northeastern United States older than age 30: (1) the canopy would continue 

to grow upwards in a concentrated form; or (2) the canopy would grow taller but 

elongate as it grew, with a more evenly distributed canopy. In western Oregon dry 

hardwood stands, it appears that the second pathway is the more likely scenario 

given the lack of a dominant upper canopy layer and the lack of a true canopy 

closure stage.

It may be an 

oversimplification to 

conclude that full use 

of available growing 

space occurs only 

when adjacent plant 

crowns are touching, 

as it assumes that 

sunlight is the limiting 

factor.
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

23

Canopy cover trends in wet hardwood and wet conifer stands after canopy 

closure were consistent with Oliver’s stem-exclusion phase and Franklin et al.’s 

biomass accumulation/competitive exclusion stage. As stand age increased, cover 

leveled off, followed by slight decreases across older stands. However, contributions 

from the upper canopy layer remained dominant. These changes likely resulted 

from the horizontal and vertical expansion of tree crowns that continues after 

canopy closure. Vertical tree growth would lead to differentiation of crown classes; 

eventually the shorter trees would be overtopped by more dominant trees, resulting 

in the separation of discrete layers. This process was demonstrated by the increase 

in number of canopy layers with increasing stand age among forest groups. Horizontal expansion of tree crowns could cause increased competition for finite light 

and nutrient resources and could result in tree mortality among the layers. These 

developments would lead to the slight decreases in total canopy cover seen in the 

older aged stands.

Easter and Spies (1994) documented canopies of mature stands (ages 90 to 

145) in their study on the western slopes of the Cascade Range as monolayers of 

Douglas-fir with minimal amounts of shade-tolerant hardwoods and conifers in the 

lower story. This finding of a dominant single layer was expected because of the 

relative shade-intolerance of the dominant species (Douglas-fir and red alder), and 

the lack of shade-tolerants in the understory. With a dominant cohort of trees in the 

upper layer, the crowns of the shade-intolerant trees in the lower layers were unable 

to develop. 

Shade-Tolerant Canopy Layers

As described in Franklin et al.’s vertical diversification and Oliver’s understoryreinitiation successional stages, the number of canopy layers and the contribution 

from shade-tolerant tree species appeared to be greater for stands over age 45 than 

those under age 45. We expected the light-restricted middle and lower canopy layers 

to be dominated by shade-tolerant cover.

Although the lower layers had more shade-tolerant cover than the upper layer, 

the mean shade-tolerant proportion of total cover rarely exceeded 40 percent. 

The contribution by shade-tolerant species appeared to be even lower in the oldest stands than in those 30 to 75 years old. However, the systematically placed 

inventory plots fell in relatively few stands over 75 years of age, and the estimated 

annual precipitation at those plots (132 cm, SE = 7.0) was significantly lower than 

the precipitation at stands ≤75 years of age (174 cm, SE = 2.2; t915 = 5.47, p < 0.001). 

Nevertheless, it is possible that shade-tolerant cover does not greatly increase in 

naturally developing stands until substantial loss of the initial shade-intolerant 
24

GENERAL TECHNICAL REPORT PNW-GTR-794 

cohort, which might not occur until age 200 or more without intermediate disturbance (Van Pelt and Nadkarni 2004). Because most of the species classified as 

shade tolerant also have higher optimum moisture levels than the shade-intolerant 

species (Minore 1979), the differences in shade-tolerant cover between stands <75 

and >75 years old are probably more related to moisture availability than to stand 

age. A more robust analysis of canopy structure and composition would only be 

possible if comparable data were available for older forests that are more typical of 

federal lands. 

Understory Vegetation

Trends in understory shrub and forb cover levels among stand ages were not 

consistent with the expectation that understory cover would dramatically decrease 

during stem exclusion (e.g., Alaback 1982). Although forb cover was lower in 

25- to 45-year-old wet conifer stands than in younger and older stands, mean forb 

cover was 20 percent, and shrub cover did not display much of a pattern with stand 

age. There were no apparent patterns of understory cover in the other forest type 

groups. The lack of a strong effect of crown closure on understory cover may be 

related to our finding that mean crown cover did not exceed 85 percent. Similarly, 

Hanley (2005) found that few stands in southeastern Alaska are uniformly dense 

and depauperate of understory plants, but young hardwood stands had much greater 

cover of understory vegetation than did young conifer stands. 

Changes in vertical structure with stand age may also play a role in understanding our results. Shrub levels in old-growth stands are highly correlated with 

horizontal and vertical variation in canopy cover (Van Pelt and Franklin 2000). 

Although the leaf-area-index values of wet conifer forests of the PNW are among 

the highest in the world, this cover is spread out over the vertical dimension of the 

stand (Van Pelt and Franklin 2000), as shown by the frequency of more than two 

vertical layers early in stand development in our study. Brown and Parker (1994) 

found that photosynthetically active radiation (PAR) was not strongly correlated 

with many simple measures of canopy structure, including canopy cover. Instead, 

PAR was highly correlated with variables that described the vertical distribution 

of foliage, and PAR transmittance increased during the first 50 years of stand 

development. These findings again suggest it is the vertical arrangement of foliage 

in a stand that influences light levels, rather than percentage of cover. In our study, 

the heights many of the stands attained, canopy closure at 85 percent cover, and 

the layering of tree cover appeared to allow sufficient light penetration to sustain 

understory shrubs and forbs.
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

25

Plants adapted to shaded habitats are less responsive to changes in light quality 

than those adapted to open habitats (Ross et al. 1986). Although the composition 

of shrubs and forbs may change from shade-intolerant to shade-tolerant species 

during early stand development, the absolute levels of understory cover differed 

little during stem exclusion. Several understory species displayed either increased 

or decreased cover with increasing age and corresponding high tree canopy cover. 

Although a few studies documented the shade tolerance of understory species, 

trends in cover of understory species here indicated that with 85 percent canopy 

closure many species were still shade tolerant enough to remain at consistent levels 

of cover under these reduced light conditions. However, cover is a relatively coarse 

measure of plant abundance; it is possible for plants to maintain similar cover in 

shaded environments yet have less leaf area (canopy density) and productivity than 

in more open environments. An analysis of invasive plants (Gray 2005) found that 

presence and absence of Himalayan blackberry and thistle were strongly correlated 

with canopy cover and stand density, whereas our analysis of cover found few 

relationships. Both cover and presence/absence metrics would appear to be useful in 

understanding the distribution and abundance of understory species.

Total cover of shrubs remained relatively consistent with age, but species richness peaked and then decreased slightly with canopy closure (age 35). This finding 

may have been a result of a shift in species composition with the reduction in light 

levels, suggested by the lower cover of vine maple, red huckleberry, dwarf Oregon 

grape, and oceanspray, and higher cover of trailing blackberry and snowberry with 

increasing stand age.

Tree and Understory Cover After Thinning 

Cover in the upper tree canopy layer of young (15- to 45-year-old) Douglas-fir 

stands was lower in those that had been heavily thinned than in those that were 

unthinned or lightly thinned within 10 years prior to measurement. Commercial 

thinning generally coincides with crown closure to ensure steady radial log growth 

(Berg et al. 1996). With large gaps between trees in the heavy-thin stands, remaining trees were probably not able to extend their foliage into the newly available area 

in the time since thinning. In contrast, for lightly thinned stands, the remaining 

trees could probably quickly fill in the newly available space through lateral branch 

development.

Tree heights were greater in the light- and heavy-thin stands than in the 

unthinned stands. Although the unthinned stands were 3 years younger on average 

than the thinned stands, the 4- to 5-m difference in height suggests that thinning 
26

GENERAL TECHNICAL REPORT PNW-GTR-794 

tended to occur on the more productive sites in this region. The difference in 

productivity among thinning treatments could also reflect a difference in ownership, because productivity tends to be higher and active stand management more 

common on forest industry lands than on other private or public lands (Azuma et al. 

2004). Differences in shade-tolerant cover with thinning levels were inconclusive, 

but suggested that thinning favors more commercially desirable shade-intolerant 

species (e.g., Douglas-fir) over shade-tolerant species. 

We expected greater cover of understory vegetation in thinned than in 

unthinned stands but did not detect significant differences in this analysis. It is 

possible that light levels did not differ sufficiently among thinning levels to have 

a dramatic effect. Nevertheless, thinning has been found to have significant 

effects on individual species, particularly weedy invasive plants (Bailey et al. 

1998, Gray 2005).

Limitations 

The FIA inventory design provides an unbiased representative sample of the 

population (i.e., nonfederal forests in western Oregon). However, the plots were 

not part of an experiment designed to control for variation to test the effects of 

a few factors. As a result, the number of plots in different age classes and forest 

types differed, and site attributes like species composition, site productivity, and 

disturbance history likely differed among plots within a forest type. The lack of 

balance in plot numbers has the potential to confound statistical comparisons of 

trends among groups, so statistical results are primarily descriptive and do not 

imply cause and effect. 

A noticeable limitation of our analyses was the paucity of plots in older forests. 

Given the population of nonfederal lands sampled by this FIA inventory and the 

land use history of western Oregon, fewer older forests were available in the data 

set. Detailed canopy measurements were not collected for inventories of federal 

lands, where the majority of older stands in this region are located (Campbell et al. 

2002). Therefore, results for the few measured stands greater than 75 years of age 

should not be interpreted as representative of western Oregon as a whole, rather of 

nonfederal forest lands in western Oregon.
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

27

Conclusions

Canopy cover and layering differed by stand age and forest type across 917 forested 

inventory plots representative of nonfederal lands in western Oregon. Our results 

support both Oliver’s (1981) and Franklin et al.’s (2002) models of stand succession 

in wet conifer forest types, but the Franklin et al. model better represented the 

effects of residual trees in regenerating stands found in hardwood forests. Although 

both models predict a period of canopy closure in young stands, mean canopy 

cover in 20- to 50-year-old stands rarely exceeded 85 percent, even in productive, 

unthinned conifer forests. Cover of shade-tolerant tree species was also lower 

than expected, even in the lowest canopy layer and in mature (50- to 100-year-old) 

stands. As expected, the wet conifer and wet hardwood stands were dominated by 

cover in the upper layer in most stand age classes, but the dry hardwood stands 

had a more even vertical distribution of canopy cover. Although most successional 

models associate the period of crown closure in young stands with suppression of 

understory plants, there was little evidence for this in our study, except for forbs in 

wet conifer forest types. Further assessment of the patterns of canopy structure and 

underlying mechanisms on representative samples of our forest lands will enhance 

our understanding of forest-canopy development, and aid in the construction of 

realistic models of forest succession.

Acknowledgments

This project was funded through a joint-venture agreement between the USDA 

Forest Service Pacific Northwest Research Station and Oregon State University. 

We are grateful to Travis Belotte, Nalini Nadkarni, and W. Keith Moser for their 

valuable reviews of the manuscript.

English Equivalents

When you know: Multiply by: To get:

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Hectares (ha) 2.47 Acres

Square meters per hectare (m2/ha) 4.37 Square feet per acre
28

GENERAL TECHNICAL REPORT PNW-GTR-794 

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GENERAL TECHNICAL REPORT PNW-GTR-794 

Appendix: List of All the Species Included in This Paper

Common name Scientific name

Trees:

bigleaf maple Acer macrophyllum Pursh.

black cottonwood Populus balsamifera L. ssp. trichocarpa (Torr. & Gray ex Hook.) Brayshaw

California black oak Quercus kellogii Newb.

California laurel Umbeullularia californica (Hook. & Arn.) Nutt.

canyon live oak Quercus chrysolepis Liebm.

Douglas-fir Pseudotsuga menziesii (Mirb.) Franco

English holly Ilex aquifolium L.

golden chinkapin Chrysolepis chrysophylla (Dougl. ex Hook.) Hjelmqvist

grand fir Abies grandis (Dougl. ex D. Don) Lindl.

incense-cedar Calocedrus decurrens (Torr.) Florin

lodgepole pine Pinus contorta Dougl. ex Loud.

mountain hemlock Tsuga mertensiana (Bong.) Carriere

noble fir Abies procera Rehd.

Oregon ash Fraxinus latifolia Benth.

Oregon white oak Quercus garryana Dougl. ex Hook 

Pacific dogwood Cornus nuttallii Audubon ex Torr. & Gray

Pacific madrone Arbutus menziesii Pursh 

Pacific silver fir Abies amabilis (Dougl. ex Loud.) Dougl. ex Forbes

Pacific yew Taxus brevifolia Nutt.

Ponderosa pine Pinus ponderosa Dougl. ex Laws.

Port-Orford cedar Chamaecyparis lawsoniana (A. Murr.) Parl.

red alder Alnus rubra Bong. 

Shasta red fir Abies magnifica shastensis (Lemmon) Lemmon

Sitka spruce Picea sitchensis (Bongard) Carriere 

tanoak Lithocarpus densiflorus (Hook. & Arn.) Rehd.

western hemlock Tsuga heterophylla (Raf.) Sarg.

western redcedar Thuja plicata Donn ex D. Don

western white pine Pinus monticola Dougl. ex D. Don

white fir Abies concolor (Gord. & Glend.) Lindl. ex Hildebr. 

willow Salix spp.

Shrubs:

beaked hazelnut Corylus cornuta Marsh

California laurel Umbellularia californica (Hook. & Arn.) Nutt.

dwarf Oregon grape Mahonia nervosa (Pursh) Nutt.

dwarf rose Rosa gymnocarpa Nutt.

Himalayan blackberry Rubus discolor Weihe & Nees

Honeysuckle Lonicera spp.

oceanspray Holodiscus discolor (Pursh) Maxim.

Pacific poison oak Toxicodendron diversilobum (Torr. & Gray) Greene

Pursh’s buckthorn Frangula purshiana (DC.) Cooper

red elderberry Sambucus racemosa L.

red huckleberry Vaccinium parvifolium Sm.

salal Gaultheria shallon Pursh

salmonberry Rubus spectabilis Pursh

snowberry Symphoricarpos spp.
Canopy Structure on Forest Lands in Western Oregon: Differences Among Forest Types and Stand Ages

35

Common name Scientific name

thimbleberry Rubus parviflorus Nutt.

trailing blackberry Rubus spp.

vinemaple Acer circinatum Pursh

whitebark raspberry Rubus leucodermis Dougl. ex Torr. & Gray

Forbs:

bedstraw Galium spp.

bracken fern Pteridium aquilinum (L.) Kuhn

redwood sorrel Oxalis oregana Nutt.

swordfern Polystichum munitum (Kaulf.) C. Presl

thistle Cirsium spp.

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