Washington Oregon 

Cascades Cascades 

Oregon Coast 

Range 

140 

120 

’ 

‘\ 

100 / \ 

/ 

80 ’ 

Q 

6O’-y 

__,. 

.-._. ‘\ 

-_ > 

::. . . 

40 ‘. N 2.. ‘-1 

..‘/‘- 

20 . .::.. 

I ;’ 20’ 

50 100 150 200 0 50 100 150 200 250 O- 50 100 150 200 250 

.a*.** = Young DCA 1 

--- = Mature 

- q Old 

this moisture-class are characterized by Douglas-fir/ 

H&discus discolor* and western hemlocWGaultheria 

shalkm plant associations (F’ranklin 1979). Moderate sites 

are on deeper soils of mid to low topographic position. Plant 

communities of this moisture-class were characterized by 

western hemlocW&rberis nervosa, western hemlock/Berberis 

nervosa/Polystichm munitum and related associations. Moist 

sites occur on lower northerly slopes, wet benches, and river 

terraces where plant communities include western hemlock/ 

Polysdchum munitum, wcstem hcmlock/Oxalis oregana, and 

western hemlock/Oplopanax horridurn and other related 

tpes. 

After field sampling was completed, the site-moisture classifications were refined for each province using detrended COTrespondence analysis (DECORANA) (Gauch 1982), a multivariate data-reduction and ordination program for ecological 

data. Moisture classifications also were done separately for 

each of three subprovinces (north, central, and south) in the 

Oregon Cascade province, because it spanned a very large 

and diverse area. The moisture classifications consequently 

relate to specific provinces or subprovinces. Retinements to 

the classification were made subjectively, based on the 

environmental interpretation of the ordination axes. 

DECORANA was also used to help reduce differences in 

environment among the age-classes. Analysis of understory 

vegetation data from all stands indicated that young and 

mature stands sampled a broader range of environment than 

originally intended. Scatter plots of the DECORANA stand 

scores were used as a guide for deleting stands within an 

age-class that were outliers from the general range of stands 

of the other age-classes. This editing procedure, which was 

done subjectively, based on both the DECORANA analyses 

and knowledge of the site conditions, resulted in selection of 

177 stands from the 196 for comparing age-classes. Although 

the distributions of the strands differed somewhat with respect 

to the ordination scores, the stand scores of the age-classes 

still overlapped considerably (fig. 1). 

93 
Stand Selection and Plot Measurements

Stand size ranged from about 4 to 20 ha. Within each stand,

five circular plots were systematically located. In some of the

very small stands, only three or four plots were established.

Plots were spaced either 100 or 150 m apart, depending on

stand size, with wider spacing in large stands. The locations

of the plots were determined before the field sampling by the

size and shape of the stand as seen on aerial photographs.

Each plot was a nested set of circular plots of six sizes:

0.002 ha, 0.02 ha, 0.05 ha, 0.1 ha, and 0.2 ha. In the 0.1 -ha

plot, trees over 50 cm in d.b.h. and snags over 0.1 m tall with

upper diameters >l0 cm were measured (10 cm in d.b.h. for

snags>1.4 m tall) and recorded by species, diameter, heightclass, and decay-class using the five-class system of Cline

and others (1980). Trees were classified into one of the following height-classes: 1 to 2 m; 2 to 4 m; 4 to 8 m; 8 to

16 m; 16 to 32 m; 32 to 48 m; 48 to 64 m; >64 m. Additional measurements on trees included coding of condition

of crowns and boles, presence of disease and disturbance

indicators, and presence of natural and excavated cavities

on the readily visible lower third of the bole. Additional

measurements on snags included the presence of natural

cavities and excavations, and heights that were estimated

visually after heights of a few snags or trees on each plot

were measured with a clinometer and tape. The 0.2-ha plot

was used during the second field season to record information on snags >50 cm in d.b.h. and >15 m tall, which occurred with low densities in most forests. The 0.05-ha plot

was used to record the information described above for trees

50 cm in d.b.h. All logs >10 cm in diameter (large end) that

projected into the 0.05-ha plot were measured. Log measurements included length (within plot), horizontal diameter at

both ends, species and decay-class using a five-class scheme

developed for Douglas-fir logs (Fogel and others 1973,

modified by Sollins 1982). The decay-classes were used for

all species because no classification was available for the

other species.

Understory vegetation information was recorded by species

in the three smallest plots. The number, species, and height

by class of all tree seedlings and saplings 1 m tall to 5 cm in

d.b.h. was recorded in the 0.05-ha plot. The percentage cover

of tall shrubs was estimated by species over the 0.05-ha plot.

The percentage cover of all vascular plants, mosses, and

lichens (including foliose lichens from the canopy that had

fallen to the forest floor) was estimated by species in the

0.02-ha plot. All vascular plants were identified to species or

genus, but only selected mosses and lichens were identified

to species. The number, species, and height-class of all tree

seedlings <1 m tall were recorded in four 0.002-ha plots

systematically located in the 0.05-ha plot. The depth of the

L, F, and H litter layers was recorded to the nearest millimeter at three points in each 0.002-ha plot (12 points in each

0.05-ha plot) by use of a trowel and ruler.

Data on slope, aspect, topographic position, and shape of

topographic cross section were collected from the 0.l-ha

plot. The heights of two to four trees on each plot were

measured with a clinometer and tape. Tree ages were determined for young and mature stands by increment coring at

breast height of at least one dominant Douglas-fir per plot,

adding 5 to 7 years for growth to breast height (depending

on site), and averaging the plot estimates to obtain an age

estimate for the stand. For old-growth stands, age estimates

were made from stumps in nearby clearcuts, along trails or

roads, or, sometimes, from increment cores on dominant

trees. Data collected by Hemstrom (1979) were used to

estimate ages of the stands in Mount Rainier National Park.

Calculations and Analyses

The density of trees, and volume and number of logs and

snags, were computed by species, decay-class, and various

size-classes for each plot in a stand. Diversity of tree heights

(H') (Pielou 1977) was calculated for the height-classes,

based on either density of trees or basal area of trees in each

class. Volume was computed by using the formula for a

cone. Stand values were then calculated as the means and

variances of the plot values. Logs in decay-classes 1 to 4

were considered round in cross section. For decay-class 5

logs, which are flattened in cross section, vertical diameters

were estimated by multiplying the horizontal diameter by

0.439, a ratio determined from a sample of 20 logs (J. Means,

unpubl. data). We estimated the upper diameters of snags by

using a taper value of 0.12 cm/dm determined from diameters and lengths of class 1 to 3 logs. Plot areas were not

area-corrected for slope because such corrections result in an

overestimate of stand characteristics as compared with level

ground (Mueller-Dombois and Ellenberg 1974).

The potential number of variables available to characterize

the forest stands is extremely large-perhaps over 2000 if

characteristics of species are included. Therefore, the analysis

began with a multi-step process of variable selection and

data reduction, with aggregation of species information into

life-form-group variables (appendix table 10) to reduce the

number of variables. The only exception was Douglas-fir,

which retained its species identity for several tree characteristics. Variables were then divided subjectively into four

attribute-sets: overstory, stand condition (tree and snag

characteristics related to vigor, damage, disease, or animal

use), understory, and debris. In each of these sets, 15 to 30

variables were subjectively chosen, based on documented or

hypothesized importance to wildlife habitat, ecosystem function, and successional development (appendix table 11).

Four separate but related analyses were used for each

attribute-set and the combination of all attributes. First, a

stepwise discriminant analysis (SAS 1987) was conducted on

each set with either the three age-classes for the entire study

area, or nine province/site moisture-classes. A significance

94
level of P < 0.15 was used to include or exclude a variable

from the model. The distributions of all variables were examined before analyses, and square-root or logarithmic transformations were used where appropriate to produce normal

or close-to-normal distributions. The stepwise analyses typically selected 8 to 12 variables for each attribute-set with

significant discriminatory power for the particular class

models. These new smaller attribute-sets were then used in

the next three analytic steps.

Canonical discriminant analysis (SAS 1987), a data-reduction

technique, was the next step, which extracted a small set of

new variates that maximized differences between the classes.

The correlations of the original variables with the new variates was used to evaluate the dependence structure of the

variables associated with differences in the classes. The stand

scores of the variates were plotted to evaluate the relative

distinctness or degree of overlap among the age- or oldgrowth province/moisture-classes.

Discriminant analysis was then used to determine the classification error rate by a jackknife procedure based on the

variables selected in the stepwise analysis (Lachenbruch and

Mickey 1968). This procedure generates a relatively unbiased

estimate of the error or success rate of the discriminant

models. Within-class covariate matrices were used in the

discriminant analysis of the age-classes, and pooled covariate

matrices were used in the analysis of the Province/moistureclasses.

The fourth and last analysis on the variables from the stepwise analysis consisted of combining the best variables from

each of the four attribute-sets and conducting another series

of stepwise discriminant analyses, canonical discriminant

analyses, and error-rate analyses.

Results

How Do Old-Growth Forests Differ From Younger

Forests?

Overstory (trees >5 cm in d.b.h..)-Tree density, mean

stand diameter, and basal area were most important in discriminating among age-classes (table 1). Tree density was

about twice as high in young stands as it was in mature and

old-growth stands (table 2). Basal area increased with ageclass, and mean tree diameter was highest in mature stands,

which lacked the smaller diameter, shade-tolerant trees common in the old growth (table 2). The high variability in oldgrowth stand structure was evidenced by old growth’s having

the highest standard deviation of tree diameter at breast

height (table 2). Diversity of tree heights (H’), which had

weak discriminating power, was actually highest in mature

stands, probably because tree densities were more evenly

distributed across the height-classes in mature stands than in

young or old-growth stands, where tree densities are concentrated in the shortest height-classes.

Most of the variance between age-classes could be reduced

to a single canonical variate related to the standard deviation

of tree diameter and the density of large trees (>lOO cm in

d.b.h.) (table 3). A second minor variate was related to mean

tree diameter and tree density. Ordination of the stands,

based on their canonical variate scores, revealed relatively

distinct age-class means but considerable within-age-class

variation and moderate overlap between the mature stands

and the old-growth and young stands (fig. 2a).

Stand condition-Percentage of snags with natural cavities,

percentage of Douglas-fir boles with resinosis, and percentage of tree crowns with broken tops were important in the

discriminant model (table 1). Nearly all measures of tree

decadence were much higher in old growth than in the

younger age-classes (table 2). The percentage of Douglas-firs

with broken-topped crowns was nearly 10, and it was nearly

3 times higher in old growth than in young and mature

stands. The percentage of broken tops in old growth is

probably much higher than we measured because many

smaller breaks or older, obscured breaks were not counted.

The percentage of trees with excavated cavities was <1 in all

age-classes, although the percentage in old growth was

several times that of the other age-classes. Both old-growth

and young stands had similar percentages of fall scars. The

bark of small, shade-intolerant trees in young stands is easily

scarred; in old-growth, thin-barked, shade-tolerant species

also scar relatively easily, and falling trees and large limbs

can cause scarring even on thick-barked shade-intolerant

trees.

Most of the between-class variation could be accounted for

by the first canonical variate, which is related to tree crowns

with broken and dead tops (table 3). The second, minor variate was related to fall scars and dead tops. Age-class means

were not well separated, and considerable overlap occurred

between the mature stands and the old and young stands,

although the distributions of the old and young age-classes

overlapped only slightly (fig. 2b).

Understory vegetation-Cover of herbs, density of shadetolerant saplings, and cover of deciduous shrubs were important in the discriminant model (table 1). Herb and deciduousshrub cover increased with stand age-class. As expected, the

density of shade-tolerant saplings was highest in old growth,

but, unexpectedly, shade-tolerant saplings were lowest in

mature stands and intermediate in young ones (table 2).

95
Table l-Variables and their F values in final models selected by stepwise discriminant analyses of forest age-classes based on

different subsets of variables (all variables significant at P < 0.05)

Variable set

Overstory F Stand conditionaF Understory F Debris F Combined set F

Tree density 22.1

Tree d.b.h. mean 20.3

Total basal area 18.4

>lOO-cm-d.b.h. Douglas-fii

density 10.3

Tree spp. richness

in stand 1.5

Shade-tolerant species

density 5.2

Tree d.b.h. standard

deviation 5.0

Broad-leaved species

basal area 4.8

Shade-intolerant

species density 4.0

Diversity of tree

heights (H’) based

on density 3.6

3.6

Snags with natural

cavities

Resinosis

Douglas-fir

Broken-topped

croyts

Sweeping boles,

all spp.

Trees with

excavated cavities

Snags >50-cmd.b.h. with

naNEd cavities

Broken-topped

crowns, Douglas-fir

Trees with rootcollar cavities

Douglas-firs

with fall scars

Trees with

leaning boles

Snags >50-cm-d.b.h.

with excavated

cavities

7.6

6.4

6.4

4.6

4.1

3.9

3.6

3.4

3.4

3.2

3.2

Percentage herb

cover

Density shadetolerant saplings

Percentage cover

deciduous shrubs

Density of subcanopy saplings

Percentage total

vegetation cover

Coefficient of

variation of

herb cover

8.6

7.7

6.1

4.7

4.0

3.4

5-to 15-m-tall

snag density

Decay class 2

log volume

>5-m-tall and

>50-cm-d.b.h.

snag density

50-cm-d.b.h.

snag density

Snag volume

Decay class 4

snag volume

1 S- to S-m-tall

snag density

Decay class 5

snag volume

Log volume

22.2

14.3

9.8

6.7

6.3

5.9

5.1

4.5

3.7

Shadeintolerant

tree species

density

100~cm-d.b.h.

Douglas-fii

density

>5-m-tall and.

>50-cmd.b.h.

snag density

1.5- to 5-m-tall

snag density

Decay-class

2 log volume

Tree d.b.h.

standard

deviation

Snag volume

Decay class 5

snag volume

Percentage herb

cover

Broken-topped

tree crowns

Decay class

4 snag

volume

Broad-leaved

tree species

basal area

>SO-cm-d.b.h.

snag density

14.8

7.4

7.1

5.7

5.5

5.1

5.0

4.9

4.3

4.2

4.0

3.6

3.6

’ All variables are percentages.

The first canonical variate, which explained most of the variation, was related to the density of shade-tolerant saplings,

density of subcanopy saplings, and density of herb cover

(table 3). The second, minor variate was related to graminoid

cover and herb cover. All of the age-class distributions overlapped in the ordination (fig. 2c). Only a portion of the oldgrowth distribution was distinct from the young and mature

stands; these old-growth stands had high densities of shadetolerant saplings and high cover of herbs.

Debris-Density of snags 5 to 15 m tall, decay-class 2 logvolume, and density of large snags were important variables

in the discriminant model (table 1). The density of snags 5 to

15 tall decreased with age-class (table 2), but decay-class 2

log-volume and density of large snags increased with ageclass. In general, small snags and logs were more numerous

in young stands and large ones were most numerous in old

growth. Although not selected by the stepwise discriminant

analysis, the combined-depth of the F and H litter layers was

deeper in old growth (x-= 21 mm) than in young (x = 15

mm) or mature stands (x = 14 mm).

The density of snags less than 15 m tall was related to the

first canonical variate, which explained most of the between

group variation (table 3). The ordination along this variate

separated the old-growth from young and mature forest,

which overlapped almost entirely (fig. 26). The second,

minor variate, which was related to log volume and the

coefficient of variation of numbers of large logs per ha

(table 3), distinguished mature stands from both old-growth

and young stands, although the overlap among classes was

high (fig. 2d). The high coefficient of variation for large logs

in mature stands may result from the scarcity amounts of

logs of this size and the patchy mortality in these stands

from root pathogens and bark beetles, or from an occasional

remnant old-growth tree or a large snag that falls to the

forest floor.

96
Table 2--Means’ and confidence limitsb

 of characteristics selected in stepwise discriminant analyses of young, mature, and

old-growth forests

Age-class Age-class

Characteristic’ Young Mature Old-growth

Overstory:

Tree density

Tree d.b.h. mean

Total basal area

>lOO-cm-d.b.h. Douglas-fir

density

Tree spp. richness (no./stand)

Shade-intolerant spp. density

Shade-tolerant spp. density

Tree d.b.h. standard deviation

Broad-leaved spp. basal area

Diversity (H’) of tree heights 2.6

based on density (2.4-2.9)

Stand condition:

Boles with fall scars (%)

(3.Z.Y)

0.3

(O-1.2)

Resinosis, Douglas-fir (%)

Broken-topped crowns,

all spp. (%)

Sweeping boles, all spp. (%)

Trees with excavated

cavities (%)

Snags >50-cm-d.b.h. with

natural cavities (%)

Broken-topped crowns,

Douglas-fir (%)

Root-collar cavities (%)

Boles with fall scars,

Douglas-fir (%)

935

(758-l 154)

(l&5)

(394450)

(O&573)

(4E.6)

490

(333-719)

277

(165-416)

(lE4)

0.01

(0.0-0.08)

(O.G.9)

(9!:6)

co.001

(0-O)

(92:6)

1.3

(0.1-3.8)

3.4

(1 .0-7.3)

3.7

(1.6-6.5)

452

(373-548)

(3E9)

(535p65)

(0.9E.73)

5.1

(4.3-5.9)

243

(171-346)

132

(64-223)

(2:;4)

0.01

(0.0-0.08)

2.8

(2.5-3.0)

4.3

(2.7-6.3)

1.0

(0.2-2.3)

(3.E.4)

(1EO)

0.06

(O-0.2)

7.8

(2.0-17.4)

4.2

(1.6-7.9)

(3.3Yl.3)

(0.?:.6)

448

(394-5 11)

(293-134)

(646874)

(lQp23)

(4.E.O)

(4F79)

270

(199-353)

(3E4)

0.001

(0.0-0.004)

(2.::.8)

6.6

(5.3-8.3)

2.2

(1.2-3.4)

6.3

(5.2-7.7)

(11-16)

0.1

(0.06-0.3)

(3.f?2.8)

$5)

(9’?7)

(1 E.5)

CharacteristicC Young Mature Old-growth

Boles with lean (%)

Snags >50-cm-d.b.h. with

excavated cavities (%)

Understory:

Herb cover (%)

Shade-tolerant saplings

density

Deciduous shrub cover (%)

Subcanopy tree saplings

density

Total understory cover (%)

C.V. herb cover

Debris:

5-to 15-m&l snag density

Decay-class 2 log volume

>50-cm-d.b.h. and >5-m-tall

snag density

>50-cm-d.b.h. snag density

Snag volume

Decay-class 4 snag volume

1.5-to 5-m-tall snag density

Decay-class 5 snag volume

Log volume

(8!:4)

(8?5)

(1.5-4.1)

228

(104-400)

(4.p-;81.2)

(1?6)

(33q662)

(6:-;7)

(59-1O3)

2.0

(0.9-4.5)

5.6

(3.1-8.9)

(182-52)

132

(92-189)

(36-83)

(41-72)

(14-41)

223

(163-305)

(9’?5)

(&O)

(3.35::s)

(2281486)

(5.6Y4.1)

(OY7)

(445;2)

(6&4)

(44qcjl9)

(3.98f7.8)

6.1

(3.6-9.3)

(li-to)

(628:19)

(13-42)

(31-5s2)

(6-23)

124

(93-165)

(G7)

8.9

(4.1-15.7)

6.4

(4.7-8.7)

335

(233-456)

(9’;6)

(295-384)

(435-262)

(667:4)

(8-18)

(9.61i6.9)

(1E5)

(2:;8)

159

(128-199)

(23-45)

(2::8)

(9-22)

(2 1?!24)

’ Means are back-transformed to linear values from either a logarithmic or

square root transformation.

* 95% confidence limits of the mean; Scheffe’s procedure. Limits are

asymmetrical about the mean because of transformations.

’ Units: basal area in square meters per hectare density in number/per

hectare; volume in cubic meters per hectare and diameter in centimeters.

97
Table 3-Percentage explained variance and correlation

coefficients of the first two canonical variates of analyses of

age-classes based on different sets of characteristics (only the

2 highest correlations are shown for each canonical variate,

except for the combined analysis where the variables with the

4 highest correlations are shown)

Characteristic Variate 1 Variate 2

Overstory:

Percentage of variance explained

Correlation coefficients

Tree d.b.h. standard deviation

>lOO-cm-d.b.h. Douglas-fir density

Tree diameter mean

Tree density

Stand condition:

Percentage of variance explained

Correlation coefficients

Broken-topped crowns, all spp. (%)

Broken-topped crowns, Douglas-fir (%)

Trees with conks on lower bole (%)

Dead-topped crowns, all spp. (%)

Understory:

Percentage of variance explained

Correlation coefficients

Shade-tolerant tree sapling density

Subcanopy tree sapling density

Graminoid cover (%)

Herb cover (%)

Debris:

Percentage of variance explained

Correlation coefficients

5- to 15-m-t& snag density

d-m-tall snag density

Log volume

%O-cm-diameter log C.V. density

Combined:

Percentage of variance explained

Correlation coefficients

Tree d.b.h. standard deviation

>100-cm-d.b.h. Douglas-fir density

Shade-intolerant tree spp. density

>5-m-tall snag density

Tree d.b.h. mean

%O-cm-diameter log density

>50-cm-d.b.h. snag density

Snag volume

91 9

0.89 0.20

0.85 -0.12

0.33 0.63

-0.50 0.63

85 15

0.56 0.40

0.56 0.04

0.19 0.37

0.54 -0.34

74 26

0.54 -0.56

0.53 -0.09

-0.11 0.70

0.50 0.60

82 18

0.83 -0.10

0.61 0.04

-0.32 0.63

0.09 -0.60

89 11

0.88 0.18

0.85 -0.10

-0.79 0.08

-0.54 0.04

0.33 0.55

0.04 0.50

0.08 -0.43

0.20 -0.39

Combined variable set-When all classes of variables are

considered together, overstory and debris variables are the

most important in the discriminant model (table 1). Density

of shade-intolerant overstory trees appeared to separate the

age-classes (table 2). The density of large-diameter Douglas

firs increased dramatically with age-class; snag volume distinguished mature stands, which had the lowest volume, from

young and old-growth stands, which had relatively high snag

volumes (table 2).

A high percentage of between-class variation was accounted

for by the first canonical variate, which was positively correlated with the standard deviation of tree diameter and density

of large Douglas-firs and negatively correlated with density

of shade-intolerant trees (table 3). The second variate was

related to log and snag volume. The age-classes were relatively distinct when the stands were ordinated along the two

variates, although overlap still occurred (fig. 2e).

The scores of the first canonical variate were closely related

to stand age in an approximately logistic relation (fig. 3a)

illustrating the continuous nature of habitat variation. The

changes with stand age begin to level-off by 400 to 500

years, reflecting changes in variation in tree diameter and

density of large Douglas-fir. The scores of the second

canonical variate followed a different pattern with stand

age, rising to a peak between 100 and 200 years, and then

declining (fig. 3b). This pattern reflects changes in both

average tree diameter, which is high between 100 and 200

years, and amounts of coarse woody debris which are high

both early and late in succession.

Classification error among attribute types-The variable

sets differed in their capacity to distinguish among the ageclasses (table 4). The highest classification success was

achieved by overstory- and combined-attribute sets. Tree

condition- and understory-attribute sets had the lowest

success percentages. The overstory discriminant model

achieved a slightly higher classification success for oldgrowth stands than did the combined-attribute model. In the

combined-attribute model, however, no young stands were

classified as old-growth; in the overstory model, 5 percent

of the young stands were classified as old growth. In all but

the understory- and debris-attribute models, mature stands

were the least successfully classified age-class, which is not

surprising because mature stands are intermediate in the

more or less continuous process of stand development and

succession.

How Does Old-Growth Structure Differ Among

Geographic Provinces and Site Moisture-Classes?

Overstory-Basal area of shade-tolerant tree species and

basal area of broad-leaved tree species were most important

in the discriminant model (table 5). Old-growth stands in the

Washington Cascades generally had the highest basal areas

of shade-tolerant trees of all provinces, although old growth

on moderate sites in the Oregon Cascades had the highest

total basal area of any single province moisture-class (table

6). In general, moist sites had higher basal areas of shadetolerant tree species than did moderate or dry sites. The basal

area of broad-leaved tree species, although typically less than

0.1 m2

 per ha, was highest in the Coast Range. The broadleaved trees in old growth in the Coast Range primarily are

bigleaf maple, which is most common on the east side of the

mountain range. The density of large-diameter Douglas-firs

98
, Overstory a

-6 -4 -2 0 2 4

'."'.E Young

- - - = Mature

-= Old

3

2

1

0

-1

-2

-3

-4

Decadence

[Qb ij$

. . I

\.’ -4

1 -2 0 2 4 6 -3 -2 -1 o I 2 3

Canonical Variate 1

51 Debris d

4- e*.

, *.*.

.

3- .

.

:

. .

.

2- .

. .

.

1. :,

/-4

.

0. #- ;

-2.

v -

\ *z .jr

. .* . ..' I

\

\

_---

-1.

-3A

-4 -2 0 2 4 6

Canonical Variate 1

4. Corn bined e

3. I’\ I \

2. I \

I

l - /

:: I.. . :

.

:4\ * I

0. .: \;

-,.i :

. :

-2. ; ,J-

-3. 

. 

i,..=

a 0

-41, ,

-6 -4 -2 0 2 4 6

?igure 2-Ordiiations of old-growth, mature, and young stands along fit two canonical variates from analyses of overstory attributes (a), stand condition

tttributes (b), understory attributes (c), debris attributes (d), and a combination of all attribute sets (e).

was typically lowest in the Washington Cascades and did not

exhibit a strong pattern related to moisture-class, except for a

decrease in density with increasing moisture in the Coast

Range. The density of subcanopy trees generally decreased

with increasing site moisture, probably as a result of increasing basal area of shade-tolerant trees in the upper canopy that

creates low-light conditions unfavorable to understory trees.

Basal area of shade-tolerant species and density of shadeintolerant species were related to the first canonical variate,

which accounted for 49 percent of the variation (table 7).

This variate essentially distinguished dry sites of the Oregon

Cascades and Coast Range from all other old-growth sites

(fig. 4a). The second variate was related to variation in tree

diameter, density of subcanopy trees, and basal area of

broad-leaved trees (table 7). Stands positioned along this

variate ranged from moist Washington and Oregon stands

with high standard deviations of tree diameter to dry Washington stands with lower variation in tree diameter and high

densities of subcanopy trees (table 7).

Stand condition-Percentages of natural root-collar cavities,

of trees with arching boles, and of trees with fungal conks

on the lower bole were important in the discriminant model

(table 5). Natural root-collar cavities were most abundant

in stands in the Washington Cascades, where western

hemlock-which often roots on rotten wood and forms

stilted roots-was very common; root-collar cavities were

least common in the Coast Range stands (table 7). The percentage of arching boles, perhaps an indicator of steep slopes

and soil creep or disease-weakened stems, was highest in the

Coast Range. The percentage of trees with fungal conks in

99
5- a 

4- 

. 

. . . . 

3- . . : . : 

*. " . . . * * 

2- . . . . . 

. . . . 

.- -* 

l- 1. - : ..;.I.: . 

. . 

. . 

O- o- :I*. . * 

-l- -1. *.* ** * 

-5. 

.;-, ( 

. . . . 

-2. **:. 

-3. . -** 

-4. .:j 

-5. :"- 

I I , 

0 200 400 600 800 1000 

b 

0 200 460 660 860 -GO0 

Stand age (years) 

Percentage classified buo ageclass by discriminmt function 

Variable set 

Fmm ageclass 

overstory Tree condition LIndastory Debris Combined 

Y M 0 Y M 0 Y M 0 Y M 0 Y M 0 

Young@=37) 81 14 5 68 21 5 54 32 5 6s 30 5 84 16 0 

Mature@=44) 16 73 11 32 43 25 23 64 14 18 68 14 11 73 16 

Ol&growth(n=96) 0 6 94 6 17 77 IS 27 55 5 13 82 0 10 90 

the stands, an indicator of disease incidence, was highest in other classes when plotted against the two variates (fig. 4b), 

the Coast Range on all sites, and generally highest on dry although considerable variation was found within groups and 

sites in aU provinces. overlap among groups. 

The percentages of trees with arching boles and fimgal conks 

were related to the fast canonical variate, which explained 

51 percent of the variance (table 7). The second variate was 

related to the percentage of large snags with excavated 

cavities and Douglas-fir boles with sweep. Coast Range 

stands and the dry Oregon stands were separate from all 

Understory vegetation-Important variables in the discriminant modal included percentage cover of graminoids, total 

vegetation, evergreen shrubs, and deciduous shrubs (table 5). 

Coast Range stands had the highest cover of graminoids on 

all sites (table 6). Total cover by understory vegetation also 

tended to be higher in the Coast Range and did not exhibit a 

strong relation to site moisture, except perhaps in the Oregon 
Table S-Variables and their F values in final models selected by stepwise discriminant analyses of province-moisture classes based

on different subsets of variables (all variables significant at P <0.05)

Variable set

Overstory F Stand condition’ F Understory F Debris F Combined set F

Shade-tolerant tree

spp. basal area 7.6

Broad-leaved tree spp.

basal area 4.3

>lOO-cm-d.b.h. Douglas-fir

density 3.1

subcanopy tree spp.

density 3.1

Total basal area 2.9

>lOO-cm-d.b.h. tree

Shade-intolerant tree

spp. basal area 2.1

Trees with rootcollar cavities

Arching boles

Conks on lower bole

Dead-topped crowns

Snags with natural

cavities

Broken-topped

crowns

Resinosis,

Douglas-fir

Multiple-stem

crowns

Snags with excavated

cavities

>50-cm-d.b.h. snags

with excavated

cavities

Sweeping boles,

Douglas-fir

6.0

4.8

3.8

3.8

3.3

2.9

2.6

2.6

2.4

2.8

2.1

Graminoid cover

(%I

Total vegetation

cover(%)

Evergreen shrub

cover (%)

Decidous shrub

cover(%)

Herb cover (%)

Subcanopy tree

spp. sapling

density

Evergreen shrub

cover C.V.

11.0

9.8

8.3

7.6

5.8

F and H litter

layer depth

30- to 6@cmdiameter

log density

Snag volume

Decay class 5

snag volume

3.1

2.8

8.7

4.9

3.9

2.7

Shade-tolerant

tree spp.

basal area

Deciduous shrub

cover (%).

100~cm-d.b.h.

tree density (%)

Total vegetation

cover (%)

Subcanopy tree

spp. density

>lOO-cm-d.b.h.

Douglas-fir

density

F and H litter

layer depth

Sweeping boles,

Douglas-fir (%)

Evergreen shrub

cover (“I)

Herb cover (%)

Broken-topped

crowns (%)

Conks on lower

bole (%)

Dead-topped

crowns(%)

Broad-leaved

tree spp. basal

area (%)

&Q-cm-diameter

logs C.V.

density

Snag volume

7.5

6.1

5.3

4.9

4.5

4.3

3.8

3.7

3.6

3.2

3.2

310

2.9

2.5

2.4

2.1

’ All variables are percentages.

Cascades, where it increased with site moisture. Evergreen

shrub cover tended to be higher on dry sites in all provinces.

Canonical variates analysis indicated that two variates were

required to account for most of the variation between the

classes (table 7). Cover of graminoids was most closely related to the first canonical variate, and cover of evergreen

shrubs was related to the second variate. Ordination of the

stands along the first two variates separated the Coast Range

stands from the Cascade stands and suggested a gradient of

increasing moisture from top to bottom (fig. 4c).

Debris-The combined depth of the F and H litter layers and

the density of medium-diameter logs were important in the

discriminant model (table 5). The litter layers were generally

thinnest in the Coast Range and thickest in the Washington

Cascades, except on moist sites, where the litter layers of the

Oregon Cascades were thickest (table 6). The forest floor

depth was typically highest on moist sites and lowest on dry

sites, except in the Coast Range. Snag volumes were highest

in the Washington Cascades on all sites and highest on moist

sites in all provinces.

Depth of the F and H litter layers and the density of mediumdiameter logs were related to the first and second canonical

variate (table 7). Ordinations of stands along the first canonical variate (fig. 4d) separated Coast Range stands and dry

Oregon stands, which had relatively thin litter layers and low

amounts of coarse woody debris, from all other classes. As

with the other discriminant models, dry old growth in the

Washington Cascades was more similar to moderate sites in

Washington and Oregon than to dry sites in the other

provinces.

101


Table ‘I-Percentage of explained variance and correlation

coefficients (r)of the first 3 canonical variates of analyses of

province-moisture-classes based on different sets of characteristics (only the 2 highest correlations are shown for each

canonical variate except for the combined analysis sets where

variables with the 4 highest correlations are shown)

Characteristic Variate 1 Variate 2 Variate 3

Overstory:

Percentage of variance explained

Correlation coefficients

Shade-tolerant tree spp. basal

area

Shade-intolerant tree spp. density

Tree d.b.h. standard deviation

Subcanopy tree spp. density

Broad-leaved tree spp.

basal area

>lOO-cm-d.b.h. Douglas-fir density

Stand condition:

Percentage of variance explained

Correlation coefficients

Conks on lower boles (W)

Arching boles (%)

Snags SO-cmd.b.h. with

excavated cavities (%)

Sweeping boles, Douglas-fir (%)

Snags with natural cavities (%)

Trees with swollen knots (%)

Understory:

Percentage of variance explained

Correlation coefficients

Graminoid cover (%)

Herb cover (%)

Evergreen shrub cover (%)

Total vegetation cover (%)

Evergreen shrub cover C.V. (%)

Debris:

Percentage of variance explained

Correlation coefficients

F and H litter layer depth

30- to 60-cmd.b.h. log density

Snag volume

Decay class 5 snag volume

>6O-cm-diameter log density C.V.

Combined set:

Percentage of variance explained

Correlation coefficients

Shade tolerant spp. basal area

F and H litter layer depth

Broad-leaved tree spp. basal

area

Conks on lower bole (4”)

Subcanopy tree density

Evergreen shrub cover (%)

Snag volume

Sweeping boles (Q)

Herb cover (%)

Broken-topped crowns (%)

>lOO-cm-d.b.h. tree density

49 2 s 11

0.97 0.09 -0.06

0.50 0.16 0.14

0.16 0.53 0.29

-0.34 -0.53 0.07

-0.44

-0.3 1

0.48

0.20

0.45

0.29

51 18 1 1

0.63 -0.26 0.23

0.56 0.33 0.003

0.27 0.47 0.02

0.11 0.45 0.15

0.11 0.23 0.52

0.27 -0.10 -0.46

44 30 13

0.82 0.08 -0.30

-0.46 -0.24 0.21

0.11 0.69 0.47

.O.lS -0.02 0.44

0.02 -0.55 -0.61

4 8 30 12

0.83 -0.54 -0.03

0.80 0.25 0.14

0.68 0.32 0.07

0.44 0.10 -0.64

-0.55 -0.03 0.46

37 2 2 16 ’

0.72 -0.58 0.14

0.70 0.04 0.42

-0.63 -0.14 -0.13

-0.53 0.34 0.13

0.06 0.50 -0.24

-0.25 0.47 0.03

0.43 -0.44 0.03

0.10 0.53 -0.48

0.36 -0.13 0.47

0.02 -0.16 0.42

-0.17 -0.22 0.41

104

results also suggest that the broad moisture-classes developed

in each province are not comparable across provinces or

even within large provinces, such as the Oregon Cascades.

Of the three provinces, the Coast Range old-growth types

were most successfully identified in the discriminant model

based on combined attributes (table 9). The Washington

Cascade old-growth types were the next most-successfully

classified types-except for Washington dry old-growth

types, which were often classified as dry or moderate Oregon

Cascade types. The moderate and moist Oregon Cascade

types were not distinctive in the models; the moderate types

often were classified as other Cascade moisture types or

Coast Range types, and the moist types often were classified

as other Oregon Cascade types or Washington Cascade

types. These results suggest that the Oregon old-growth types

are somewhat intermediate in habitat structure between the

Washington types to the north and the Coast Range types to

the south and west.

Discussion

Limitations of the Chronosequence

In characterizing habitat structure in relation to stand age, the

assumption has been that space can be substituted for timean assumption that is violated to different degrees in most

chronosequence studies because of differences in site and

stand history. In this study, the effects of site differences

were controlled somewhat by maintaining similar levels and

ranges of site characteristics across the age-classes. Stand

history could not be controlled except by restricting the

stands primarily to those that regenerated naturally after

stand-replacing wildfire. Differences in climate, establishment, and subsequent minor and moderate rates of disturbance also could not be controlled and can leave a strong

imprint on stand structure (Spies and others, unpubl. manuscript). Given these caveats, successional interpretation of

differences in age-classes must be made with caution..

Differences Amcmg Age-classes

Multivariate analyses indicated that forest habitat structure

differs among age-&asses in numerous ways. The variables

selected by stepwise discriminant analyses were not the only

variables that exhibited significant differences among ageclasses. They have high discriminating power, but are not

necessarily the most discriminating set (Legendre and

Legendre 1983). Other variable sets that were not selected

may have equal or possibly somewhat greater discriminating

power, and the subset of variables selected by the stepwise

process is not stable against small changes in the, data set

(SAS 1987). Variables with the highest F values, however,

typically stable against small changes in the sample set, and

the general kind of variables selected were very stable

against changes in sample sets. Several different stepwise

analyses run on different subsets of the data, for example,
Overstory a

.

.

.

. .

-2.

. . . .

. .

*. WD :

.,i. t , , , , , ,

-4 -3 -2 -1 0 1 2

Decadence b Understory ___ c

Canonical variate 1

Debris d Combined e

1 ,:y=. ___

Canonical variate 1

Figure 4-Ordinations of province-moisture class means (with standard deviations) of old-growth stands along fit two canonical variates from analyses of

overstory attributes (a), stand-condition attributes (b), understory attributes (c), debris attributes (d), and a combination of all attribute-sets (e). First letter

I

indicates province: Oregon Cascades (0); Washington Cascades (W); Coast Range (C). Second letter indicates moisture class: Dry (D), Moderate (&I), Moist

(w). Dotted lines encompass province distributions.

still indicated that overstory attributes had the highest discriminating power. These characteristics of the analyses

suggest generalizing the results not on the basis of the discriminating power of a single specific structural attribute or

exact size-class of attributes, but rather on the general type

or gross features of the attributes.

The high discriminating power of overstory attributes for

age-classes is not surprising because the age-classes were

based on the age of overstory trees. Within the overstory set,

two general processes contribute to the structural changes.

The first is associated with stand development: the decline in

stand density and the increase in tree size. The second is

more related to successional changes: the increase in variation in tree diameter as the density of large-diameter Douglasfirs increases and the density of smaller-diameter, Shadetolerant trees increases. Variation in tree diameter increased

strongly with succession, but diversity of tree heights (H’)

did not, which probably reflects the fact that the Heightclasses were too broad to be sensitive to height diversity.

105
Table S-Summary of discriminant classifications of province-moisture-classes based on different sets of attributes

Percentage classified into class by discriminant function

Variable set

From classes Overstory

Nine province- and moisture-classes 44

Three province-classes alone 66

Three moisture-classes alone 57

Stand condition Understory Debris Combined

33 42 30 56

71 61 56 81

43 58 55 65

Table 9-Summary of discriminant classifications of province- and moisture-classes based on combined sets of attributes”

Percentage classified into class by discriminant function

From class Dry Moderate Moist

Moist Province Washington Oregon Coast Washington Oregon Coast Washington Oregon Coast

DRY Washington 44 44 0 0 11 0 0 0 0

DRY Oregon 0 61 33 0 0 0 0 0 0

DRY Coast 0 11 67 0 0 22 0 0 0

Moderate Washington 20 5 0 50 10 0 10 5 0

Moderate Oregon 0 15 0 0 35 10 0 30 10

Moderate Coast 0 0 0 0 0 91 0 0 9

Moist Washington 0 0 0 17 8 8 58 8 0

Moist Oregon 5 5 0 5 14 5 14 55 0

Moist Coast 0 0 0 0 20 0 0 0 80

0 Values in bold type indicate correctly classified percentages.

The developmental and successional changes of stands, site are likely to play an equally important role. The difwhich are described above, account for most of the variation ficulty of observing tall canopies from the ground may also

in overstory structure associated with stand age. have contributed to the poor discrimination.

Debris attributes were not as powerful in discriminating the

age-classes. This difference is probably because the time

since disturbance provides a better measure of debris amounts

than does stand age (Spies and others 1988). Woody debris

attributes operate with a different cycle than overstory attributes. Woody debris amounts actually are highest early in

succession and lowest in mid-succession for many Douglasfir ecosystems. This difference produces a more U-shaped

pattern of development with stand age, which tends to reduce

the discriminating power of these characteristics for natural

forest age-classes. Because natural disturbances typically do

not destroy much of the debris biomass in large pieces of

wood, these features are present in natural stands of all ages

to some degree (Spies and others 1988).

Stand-condition attributes alone did not exhibit good discriminatory power, probably because of high variability

associated with non-age-related factors. Characteristics such

as percentage of broken-topped crowns increased with stand

age; however, topography and the history of windstorms at a

Understory characteristics exhibited the lowest discriminatory

power for age-classes. Although forest understories are likely

to be strongly controlled by the overstory, the important

overstory characteristics are not necessarily closely linked to

stand age. Overstories casting dense shade, for example, may

occur in dense, young Douglas-fir stands and in old-growth

stands with dense hemlock canopies. Despite their low discriminatory power, many of the understory characteristics

exhibited trends associated with the age-classes.

Among the strongest trends in understory characteristics was

the increase in percentage of herb cover with stand age-class.

An increase in herb cover in late-successional forests has

been observed in Sitka spruce-western hemlock forest in

southeastern Alaska (Alaback 1982). Higher herbaceous

cover in old growth might be explained by changes in understory light intensity and microclimate. Light intensity under

old-growth forest canopies dominated by western hemlock is

lower than under mature or young forest canopies dominated

by Douglas-fir (Spies and others 1990). Low light might

106
favor more shade-tolerant herbs over evergreen shrubs such

as salal that often form dense clonal patches in mature- and

young-forest understories. The more competitive, lightdemanding shrubs such as salal could limit the development

of herbaceous plants in the understories of younger forests

(Long and Turner 1974). In addition, a more moderated

microclimate in old-growth understories might favor more

mesic understory species.

The high discriminatory power of overstory attributes does

not mean that other attributes are unimportant in characterizing the habitat structure of natural young, mature, and oldgrowth forests. Other attributes with more subtle changes or

greater variability can play important roles in the habitat and

ecosystem function of these forests. These characteristics

must be considered along with the characteristics of the overstory when habitat relations and land-management objectives

are identified. The discriminatory power of the overstory attributes, however, can be used advantageously when simple

indicators of forest habitat condition are sought, such as when

aerial-photo inventories of habitat conditions over large areas

are needed. We can infer that overstory characteristics such

as densities and sizes of trees might be reasonably successful

in identifying old-growth habitat from aerial photographs.

Regional and Site Differences in Old-Growth Forests

The study indicates that regional gradients exist in the structure of old-growth forest habitat, and to a lesser degree, gradients in structure occur within provinces in relation to site

moisture. Given the large geographic area covered in the

study, finding regional patterns stronger than within-region

site differences is not surprising.

I

One of the prime drivers of habitat differences among the

provinces was the basal area of shade-tolerant trees, primarily

western hemlock. The study area ranged from near the southem limit of the species toward the middle part of its range

(Harlow and Harrar 1969). This area encompasses relatively

steep gradients in species composition and stand structure

(Franklin and Dymess 1973).

Disturbance history also plays a role in the regional differences. The southern and eastern Coast Range, where most of

the Coast Range stands were located, probably has experienced more fires in the last several hundred years than the

two Cascade provinces (Juday 1976). Fire disturbance is

evidenced by the fact that fire-sensitive subcanopy trees and

shade-tolerant trees such as western hemlock are less common in the Coast Range than in the Cascades; forest floor

layers are much thinner there, and the percentage of Douglasfirs with fire scars on moderate sites in the Coast Range was

1.6 but only 0.4.and 0 on similar sites in the Oregon and

Washington Cascades. Stand ages of old growth in the Coast

Range are younger, averaging 330 years compared with over

420 years for the Cascade Range stands (Spies and others

1988).

Differences in climate and site productivity also played a

role in the regional patterns that were observed. Coast Range

stands averaged almost 400 meters lower in elevation than

Cascade stands, and winter minimum temperatures are 2 to

5

oC higher than in the Cascades (Franklin and Dymess

1973). Trees can grow faster in the Coast Range than in the

Cascades, which is evidenced by the relatively high densities

of large-diameter (>100 cm in d.b.h.) trees in the Coast

Range, despite the younger average stand age.

Differences in the moisture-related habitat structure of sites

were not as great as regional patterns, but several general

trends were apparent. The overstories on dry sites generally

have a low abundance of shade-tolerant species, which

probably reflects drier conditions, greater fire frequencies,

and younger stand ages. The low density of shade-tolerant

species on these sites suggests that canopies are more open,

which allows greater understory development. The greater

density of subcanopy trees such as chinkapin or possibly

Pacific dogwood on these sites, and the relatively high cover

of total understory vegetation and graminoids supports this

conclusion. Moist old-growth sites, by contrast, have relatively high cover of herbaceous species. The relatively high

cover of evergreen shrubs on drier sites follows a similar

finding by Zobel and others (1976) for the central Oregon

Cascades, although they primarily associated the higher

evergreen understory cover with low nitrogen availability.

Differences in debris characteristics associated with moisture

conditions of sites have been identified by Spies and others

(1988). Amounts of debris and numbers and sixes of debris

pieces generally increase with site moisture, probably as a

result of higher site productivity, lower fire frequencies

(which can consume some of the wood), and possibly because of lower rates of decay resulting from excessively high

moisture in the wood (Harmon and others 1986). Thicker

litter layers on moist sites probably reflect the infrequency of

fire and higher litter-fall rates associated with dense canopies

of western hemlock and other shade-tolerant species.

Differences in stand condition across moisture-classes were

associated with either species-composition differences,

age/disturbance factors, disease, or tree vigor. The higher

percentage of root-collar cavities on moderate and moist sites

is related to the higher abundance of western hemlock on

these sites. The higher incidence of broken tops on moderate

and moist sites, which contain the oldest trees, may be

related to the fact that with increasing age, trees are more

likely to experience wind storms that cause crown damage.

The higher incidence of fungal conks on dry sites, which are

107
characterized by steep south and west-facing slopes, is

similar to the findings of Boyce and Wagg (1953) who

observed a higher incidence of conk rot in old-growth

Douglas-fir on steep slopes and southerly aspects. Our

percentages of fungal conk incidence (1 to 6 percent) are

much lower than the percentage incidence of conk rot

(Boyce and Wagg 1953), but we surveyed only the lower

one-third of the tree boles for fungal conks, which are only

one indicator of the disease.

Conclusions and Management Implications

The structure of natural Douglas-fir forests in western

Oregon and Washington is extremely diverse because of

numerous processes operating at different spatial and

temporal scales. Stand development and succession are

important processes that determine forest habitat; however,

regional flora, disturbance, local site potential, and climatic

patterns are also important. We should not assume that the

old-growth forest structure will be the same at one location

or area as at another until we have a better understanding of

how these processes interact and affect structure. Nor should

we assume that the young and mature forests of today will

produce the same old-growth forest structure in the future as

the one that we see today. Management of old growth in

western Oregon and Washington should be sensitive to the

regional diversity of old-growth conditions. It should recognize the fact that natural young and mature forests have also

provided important habitat and ecosystem functions that may

be lost if only old-growth forest areas are used to provide

habitat diversity for an entire landscape.

These results indicate that differences in forest structure

associated with stand age and regional and local geography

are numerous; they are characterized by multiple continua in

forest structure rather than discrete classes. Changes in forest

habitat structure do not stop when stands become 200 years

old, the age often used for the onset of old-growth conditions.

The results of this study suggest that structural changes continue until stands are at least 400 to 500 years. Additional

changes likely happen after this time, but they may be more

gradual and may move toward less structural diversity as

stands become dominated by western hemlock and other

shade-tolerant species.

The variability in old-growth forest structure strongly suggests varied developmental histories. Many, if not most,

old-growth stands have been affected by low to moderate

amounts of disturbance during their histories. Such disturbances have contributed to the development of multiple sizeand age-classes. Structurally distinguishing old-growth stands

with continuous age-class distributions from those composed

primarily of a few (3 to 5) tree cohorts may be difficult, but

this difficulty suggests that one method for creating stands

that resemble old growth is to leave an overstory of dominant green trees at the time of final cutting.

The young and mature stands examined in this study are not

the equivalent of intensively managed, even-aged plantations.

Many natural young and mature stands have some of the

attributes of old-growth stands that may not be present in

young, managed stands. Perhaps the greatest difference

between natural and managed stands is the lower number

and volume of large snags and logs in managed plantations

(Spies and Cline 1988). Many young natural forests less than

80 years old have high amounts of carry-over of woody

debris, although some young natural stands have little carryover (Spies and others 1988). Other structural differences

between young and mature natural forests and their managed

equivalents are less well known. Managed plantations, however, generally will have fewer tree species, more uniform

tree sizes and spacing, and no large remnant overstory trees.

Understories of young natural and managed stands may be

less spatially heterogeneous if overstory density is controlled.

Over several rotations, a decline in the population of some

understory species may occur if those species are sensitive to

the short interval between major canopy and forest-floor

disturbances in short-rotation plantations.

The concept of the “intensively managed stand” is, however, an ideal that has not yet been realized in many forests.

It is also subject to change if management objectives are

broadened to include a greater array of ecological values.

Lessons from the natural forests in this study will be useful

as our concept of forest management evolves with society’s

needs.

Acknowledgements

Field work could not have been completed without the efforts

of several dedicated field crews. Gody Spycher, Diane

Chung, and Janice Harmon provided valuable assistance in

managing and analyzing of the large data set. We thank Jim

Agee, Paul Alaback, and an anonymous reviewer for their

helpful comments.

108
Appendix Table 10-continued

Table 10-Tree life form groups used in analysis Subcanopy trees:

Pacific madrone

Golden chinkapin

Pacific dogwood

Western hemlock

Broad-leaved dominant trees:

Bigleaf maple

Red alder

Oregon ash

Tanoak

Bitter cherry

Oregon white oak

Willow

California-laurel

Shade-intolerant dominant trees:

Subalpine fir

California red fir

Noble fir

Incense-cedar

Engelmann spruce

Sitka spruce

Sugar pine

Western white pine

Douglas-fir

Shade-tolerant dominant trees:

Pacific silver fir

White fir

Grand fii

Port-Orford-cedar

Alaska-cedar

Western redcedar

Western hemlock

Mountain hemlock

Table ll-Variables used in stepwse discriminant analyses

Overstory (>5-cm-d.b.h.) Stand condition* Understory Debris

Total basal area

Tree density

Basal area, standard deviation

Tree density, standard deviation

Tree d.b.h. mean

Tree d.b.h. standard deviation

Number of overstory trees

Trees >lOO-cm-d.b.h. density

Tree height diversity (H’) based

on basal area

Tree height diversity (H’) based

on stem density

Douglas-fir basal area

Douglas-fir >ltQcm-d.b.h.

density

Shade-tolerant tree spp. basal area

Shade-intolerant tree spp. basal

area

Subcanopy tree spp. basal area

Broad-leaved tree spp.

basal area

Shade-intolerant tree spp. density

Shade-tolerant tree spp. density

Subcanopy tree spp. density

Trees with root-collar cavities

Trees with natural cavities

Snags with excavated cavities

Broken-topped crowns, Douglas-fir

Broken-topped crowns, all spp.

Sweeping boles, Douglas-fir

Sweeping boles, all spp.

Multiple-stemmed crowns, all spp.

Multiple-stemmed crowns, Douglas-fir

Dead-topped crowns, Douglas-fir

Dead-topped crowns, all spp.

Broken-topped crowns with upturned

leaders, Douglas-fir

Broken-topped crowns with upturneds

leaders, all spp.

Forked bole

Arching boles.

Boles with fall scars, Douglas-fir

Boles with fall scars, all spp.

Trees with swollen knots, Douglas-fir

Conks on lower boles, Douglas-fir

Conks on lower bole, all spp.

Resinosis, Douglas-fir

Resinosis, all spp.

Trees with excavated cavities

Snags with natural cavities

Snags >50-cm-d.b.h. with excavated

cavities

Snags >50-cm-d.b.h. with natural

cavities

Shade-tolerant sapling density

Subcanopy sapling density

Cover vegetation c2 m tall (%)

C.V. herb cover

Lichen cover (%)

Moss cover (%)

Fern cover (%)

Herb cover (%)

Evergreen shrub cover (%)

Deciduous shrub cover (%)

Total understory cover (%)

C.V. evergreen shrub cover

C.V. deciduous shrub cover

C.V. >2-m-tall vegetation cover

F and H litter layer depth

Snags SO-m-tall density

10- to 30-cm-diameter log C.V. density

30- to 60-cm-diameter log C.V. density

Snag >5-m-tall and 50-cm-d.b.h. density

5-to 15-m-tall snag density

SO-cm-d.b.h. snag density

Snags >50-cm-d.b.h. density

10- to 30-cm-diameter logs density

30- to 60-cm-diameter log density

60-cm-diameter log density

Decay-class 1 log volume

Decay-class 2 log volume

Decay-class 3 log volume

Decay-class 4 log volume

Decay-class 5 log volume

Decay-class 1 snag volume

Decay-class 2 snag volume

Decay-class 3 snag volume

Decay-class 4 snag volume

Decay-class 5 snag volume

Ratio log-volume decay-class 4 + 5

to total log volume

Log volume

Snag volume

Snag volume (C.V.)

LI All variables are percentages.

109
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Plant Species Diversity and Occurrence in

Young, Mature, and Old-Growth Douglas-Fir

Stands in Western Oregon and Washington

Thomas A. Spies

Author

THOMAS A. SPIES is a research forester, USDA Forest

Service, Pacific Northwest Research Station, Corvallis,

Oregon 97331.

Abstract

The objective of the study was to characterize differences in

vascular plant diversity and species abundance among young

(30-80 years old), mature (80-195 years old), and old-growth

(195-900 years old) Douglas-fir forests. A chronosequence of

Douglas-fir stands was sampled in each of three physiographic provinces: southern Washington Cascade Range; Oregon

Cascade Range; and southern Oregon Coast Range. The

cover of all vascular plants was recorded in 177 stands, each

consisting of 4 to 5200 m2

 plots. Measures of species

diversity were calculated for the overstory and understory

strata of each stand. Species diversity in the three provinces

showed an age-class effect (P < 0.1) in 6 of the 12 tests

performed. Diversity tended to remain constant or increase

slowly with age-class. The frequency of occurrence of

species showed an age-class effect (P < 0.1) in 15 to 20

percent of understory species tested in the three provinces.

Similar results were obtained in analyses based on species

cover, although relative differences between old growth and

the other age-classes were greater. Species occurring with

greatest frequency and abundance in old growth included

Pacific yew, an understory tree; Cornus canadensis’ and

Tiarella trifoliata var unifoliata, understory herbs; and

Lobaria spp., foliose canopy lichens. Four factors contributing to late-successional change in these communities are

hypothesized: changes in resources, changes in horizontal

heterogeneity, changes in vertical heterogeneity, and long

periods without intense fire.

Introduction

The plant compositional changes during the middle to late

stages of succession in Douglas-fir forests are poorly known.

Most research has concentrated on early successional changes

after catastrophic disturbances (Dyrness 1973, Halpern 1987,

Isaac 1940, Long 1977). These studies have focused on the

first 50 years of a potentially 1000-year-long ecological

process-less than a twentieth of the sere!

Plant species diversity within stands appears to be highest in

the early stages of succession in western coniferous forests

(Habeck 1968, Schoonmaker and McKee 1988), and plant

community changes are rapid during the early stages of

succession (Long 1977). After canopy closure, less dramatic

I In this botanical chapter, herbaceous plants and lichens are referred to by

their scientific names for accuracy and the convenience of readers.

111

--
but distinct changes may continue for many centuries. For

example, in Sitka spruce-hemlock forests, where stand development requires hundreds of years, understory communities

continue to change for at least 300 years (Alaback 1982).

Stewart (1986a, in press) reported that canopy structure and

composition influenced understory development in several

mature and old-growth Douglas-fir stands in the Oregon

Cascades.

Few examples of plants that are restricted to old-growth

Douglas-fir forests or find optimal habitat in old growth have

been reported. Franklin and others (1981) suggest that some

mycotrophic plants, such as pinesap, and lichens, such as

Lobaria oregana find optimal habitat in old growth, but they

do not present any data. Moir and others (1979) reported that

Tiarella unifoliata had higher cover in old than in young

growth in a chronosequence of stands on similar sites from

Mount Rainier National Park. No plant community studies

have reported how diversity measures change with time in

the later stages of Douglas-fir forest succession.

The objective of this study was to contrast the composition

of natural young, mature, and old-growth Douglas-fir stands

in western Oregon and Washington, emphasizing distinctive

features of old growth. The specific objectives were to characterize the species diversity in overstories (tree stratum) and

understories (herb, shrub, and seedling stratum) of the three

forest age-classes, and to identify plant species that differ in

frequency of occurrence and abundance among the forest

age-classes.

Study Area

The study was conducted in Douglas-fir-dominated stands in

Washington and Oregon in three physiogmphic provinces:

the southern Washington Cascade Range, the Oregon

Cascade Range, and the southern half of the Oregon Coast

Ranges (Franklin and Dymess 1973). These provinces are all

characterized by steep, deeply dissected terrain with welldeveloped soil. Parent materials are Tertiary basalts and andesites in the Cascade Range and early Tertiary sedimentary

rocks in the Coast Range. The climate is mild and wet in

winter and cool and dry in summer. Annual precipitation is

heavy, ranging from 800 to over 3000 mm. Highest amounts

of precipitation occur near the upper western slopes of the

Coast Range and in the Cascade Range in Washington and

northern Oregon (Franklin and Dymess 1973). Lowest precipitation occurs on the eastern slopes of the Coast Range and

in the southern Oregon Cascades.

The study area primarily encompasses the Western Hemlock

Zone and the lower elevational portion of the Pacific Silver

Fir Zone (Franklin and Dymess 1973). Western hemlock and

Pacific silver fir are the climax species on most sites in these

zones; on dry sites, Douglas-fir may be climax. In southern

Oregon, the northern fringes of the Mixed Conifer Zone

(Franklin and Dymess 1973) were sampled. In the Coast

Range, the eastern margin of the Sitka Spruce Zone was

sampled.

Most Douglas-fir stands in the region originated after catastrophic wildfire (Franklin and Hemstrom 1981); see Franklin

and Dymess (1973) for a generalized, natural successional

sequence. Young stands originating from wildfires are typically dominated by Douglas-fir, although western hemlock or

red alder may dominate in some areas. By 200 years, many

stands exhibit old-growth characteristics (Franklin and others

1981; Spies and Franklin, this volume), such as codominance

of western hemlock in the overstory, diverse vertical foliage

distribution, and large accumulations of woody debris (Spies

and others 1988). True “climax” forests composed entirely of

shade-tolerant species are rare because pioneer Douglas-fir

can persist in stands for over 1000 years (Franklin and DeBell

1988), and wildfires have occurred more frequently than this

on most sites.

Methods

General Design

A total of 196 Douglas-fii stands in Washington and Oregon

representing different ages (40 to 900 years) and site conditions were sampled during 1983 and 1984 (for a map of the

stand locations, see Spies and Franklin, this volume). All

stands originated after wildfires, which killed most or all of

the overstory trees. Where remnant old-growth trees occurred

in young and mature stands, their densities were typically

less than 1 per 10 ha. Similar geographic distribution among

ages of sampled stands was maintained by sampling in areas

in each province that contained all three of the following

broad age-classes: young (<80 years), mature (80-195 years),

and old-growth (>195 years). These age-classes were then

used in the analyses.

To control site variation across the age-classes, a subset of

177 stands were selected based on field observations, site

data, and DECORANA ordinations (Spies and Franklin, this

volume). In general, the age-classes in a province had similar

means and ranges of site conditions (table 1). Drier and

wetter plant associations were sampled in old-growth stands

(Spies and Franklin, this volume) but were not included in

the chronosequence analysis. Old-growth stands occurred on

slightly drier aspects than mature or young stands in the

Washington Cascades and were intermediate to young and

mature stands in the other provinces. Old growth did tend to

occur at slightly lower topographic positions, but elevations

and slope percentages were intermediate to young and mature stands. Plots from DECORANA ordinations (fig. 2, in

Spies and Franklin, this volume) indicated that the overall

vegetation composition was similar among the age-classes.

112
Table I-Means and ranges of age and site characteristics for young (Y), mature (M), and old-growth (0) stands in 3 physiographic 

provinces 

Given the large geographic area of the study and the large 

size of sample stands (see below), the analysis could not be 

stratified by individual plant associations (Hall 1988). Each 

age-class in the chronosequence, however, encompassed a 

similar range of mesic to dry-mesic plant-association groups 

of the Western Hemlock Series (Hall 1988). The major association groups were: Western Hemlock/Rhododendron-Salal 

(CHS?); Western Hemlock/Salal-Oregongrape (CHSI); Western Hemlock/Rhododendron, Cool (CHSC); Western HemlcckiForb, Dry (CHF2); Western Hemlock/Rhododendron, 

Mesic (CHSM); Western HemlocldSalal-Oregongrape, Dry 

(CHSD); and Western Hemlock/Shrub, Dry (CHC2). In 

addition, no significant difference among age-classes was 

found in the percentage cover (P < 0.05) of the following 

key plant association indicator species: Gaultheria shallon; 

Xerophyllum tenax; Rhododendron macrophyllum: Holodisus discolor; Berberis nervosa: Athyriumfelix-femina: Rubus 

spectabilis; Opiopanax horridum; Oxalis oreganum: and 

Polystichum munitum. 

Every effort was made to control site variation. True chronosequences should have similar soils, sites, climates, climate 

histories, and disturbance and establishment histories (Oliver 

1982), but these assumptions are violated to various degrees 

in most cbronosequence studies. Given tie wide geographic 

range of this study, the inferences about successional change 

should be viewed with caution, and more as working hypotheses than as established facts. 

Stand selection and plot measurements 

Stand size ranged from about 4 to 20 ha. Within each stand, 

five circular plots were establisbcd systematically. In a some 

very small stands, only three or four plots were established. 

Plots were spaced either 100 or 150 m apart, depending on 

stand size, with the wider spacing used in large stands. The 

locations of the plots were determined before the lield 

sampling by the size and shape of the stand as seen on aerial 

photographs. 

Each plot consisted of a set of nested circular plots of four 

sizes: 0.002 ha, 0.02 ha, 0.05 ha, and 0.1 ha. Each plot six 

sampled different aspects of forest structure (see Spies and 

Franklin, this volume). Within the 0.1.ha plot, trees over 

50 cm in diameter at breast height (d.b.h.) were measured 

and recorded by species, d.b.h., and height. The0.05.ha plot 

was used to record the information described above for w.xs 

5 to 50 cm d.b.h. 

113 
Information on understory vegetation was recorded by species in the three smallest plots. The percentage cover of tall

shrub species and trees less than 8 m tall was estimated by

species over the 0.05ha plot. The percentage cover of all

vascular plants, mosses, and lichens (including canopy foliose

lichens, Lobaria oregana and L. pulmonaria, that had fallen

to the forest floor) was estimated by species in the 0.02-ha

plot. Species occurring in the 0.05-ha plot were also recorded

and used in presence/absence analyses. All vascular plants

were keyed to species or genus. For mosses and lichens, only

a selected subset were identified to species or genus.

Data on slope, aspect, topographic position, and shape of

topographic cross section were collected from the 0. l-ha plot.

Tree ages were determined for young and mature stands by

increment coring of 1 to 4 dominant Douglas-fir per plot (5

20 per stand) at breast height, adding 5-7 years for growth to

that height (depending on site), and averaging the plot estimates to obtain an age estimate for the stand. For old-growth

stands, age estimates were made from stumps in nearby clearcuts, along trails or roads, or sometimes from increment cores

on dominant trees. Data collected by Hemstrom (1979) were

used to estimate ages of the stands in Mount Rainier National

Park.

Calculations and Analyses

Species richness and diversity were calculated separately for

the overstory (trees >5 cm d.b.h.) based on basal area, and

for the understory vascular plants based on percentage cover.

Because about 25 percent of the stands were sampled with

fewer than 5 plots, a smaller subset, 144 stands, was used to

calculate mean species richness and diversity for each ageclass. Stands were selected for each age-class so that the

mean number of plots per stand or area was the same for each

age-class in a province. Species diversity was calculated as

No and N2 (Hill 1973), the reciprocal of Simpson’s index. No

is simply the total number of species in a stand. N2 is calculated as:

l/pi2

where pi is the proportional abundance of the ith species.

Analyses of species frequency and abundance in the ageclasses were conducted for overstory and understory species

occurring at least five times within a province. Preliminary

analyses of species distributions by latitude and elevation

were conducted in each province. Many species were elevationally and latitudinally restricted in the provinces. Consequently, each province was divided into ecological zones

(table 2), based on inspections of sample and species distributions. The zones were typically large enough to include several stands of different age-classes. If a species did not occur

in any stands within a zone, then none of the sample stands

from that zone were used in the age-class analyses for that

species. Consequently, sample sizes differ among species.

The objective of the stratification was to restrict the analysis

Table 2-Latitudinal-elevation zones used in determining

species distributions for age-class analyses

Province

Oregon Cascades

Zone

1

2

3

Elevational range Latitudinal

(meters) range (degrees)

1000-1500 42.5-43.5

400-1000 42.5-43.5

950-1200 43.6-44.8

400-950 43.6-44.8

300-950 44.9-45.6

Oregon Coast Range 1 50-800 43.0-44.0

2 100-800 44.1-44.5

3 800-100 44.1-44.5

Washington Cascades 1 350-l 100 45.8-46.6

2 600-1200 46.6-41.0

of species to age-class relationships within zones where the

species could occur, based on their ranges and environmental

conditions. It had the effect of reducing the number of zero

values for some species and increasing the power of the

statistical analyses. Chi-square analyses were used to test for

an age-class effect in the frequency of occurrence of each

species. Kruskal-Wallis nonparameteric analyses were used

to test for differences in species abundance (either percentage

cover or basal area) among the age-classes. All analyses

were conducted using SAS (SAS Institute Inc. 1987).

Results

Species Diversity

Overstory-Species richness (No) was significantly different

(P < 0.05) among the age-classes only in the Coast Range,

where old-growth stands had the highest numbers of overstory species, and mature stands the lowest numbers (fig. 1).

Overstory species diversity (N2), on the other hand, showed

an age-class effect only in the Cascade provinces, where old

growth had the highest diversity.

Understory-Species richness was marginally significant

(P < 0.09) among age-classes in the Coast Range (fig. 2); old

growth had the highest number of species and young stands

had the lowest. Understory species diversity (N2

) showed a

significant age-class effect in the Washington Cascades and

the Coast Range. Old growth had the highest values in both

provinces; young stands had the lowest values in the Washington Cascades, and mature stands had the lowest values in

the Coast Range.

Species Occurrence

Overstory-Species differing significantly (P < 0.1) in basal

area among the age-classes were either shade-tolerant species

that had highest basal areas in old-growth stands or shadeintolerant species with highest basal area in young stands

(table 3). Western hemlock, Pacific yew, and bigleaf maple

114
Table 3-Rasal area (m’iha) of trees species (>5 cm d.h.h.) in Young (Y), mature (M), and old-growth (0) Dou&xs-fir stands in 

3 physiographic provinces 

**t 

*** 

<“.I 0.2 0.1 

0.5 3.2 2.” 

1.0 0.3 0.3 

0.3 0.1 0.1 

0.4 1.4 2.” 

0.1 <o. 1 cu.1 

25.6 48.8 47.8 *** 

0.2 0.1 0.67 ** 

2.1 1.3 7.4 

7.9 1.5 14.0 *** 

I 

*P values i” Kmskd-wallis test: + = 4.10; * = <0.05; +* = co.01; *** = 4w1. 

were the only species with significant differences among 

age-classes in all three provinces. The basal area of Pacific 

yew, an understory tree, typically was 5 to 74 times higher 

in old growth than in the younger age-classes in the two 

Cascade provinces. In the Coast Range, no overstory-size 

stems (that is 5 cm d.b.h.) of yew were encounlered in any 

stands younger than 200 years. The basal area of Douglas-fir 

was significantly different among age-classes only in the 

Oregon Cascades. Bigleaf maple, an early to mid-successional 

species in the Cascade provinces, had highest basal area in 

old growth in the Coast Range. 

Understory-A total of 26 of 129, 35 of 171, and 13 of 98 

species were found to differ in percentage frequency among 

the age-classes (P < 0.1) in the Washington, Oregon, and 

Coast provinces, respectively (table 4). Most species attained 

their maximal age-class frequency in old-growth or mature 

9.9 

* 

+ 

+ 

stands. Only two plants, Pacific yew and Lobaria spp., were 

significant in all provinces: they both reached their highest 

frequency of occurrence in old growth. Other species with 

maximal frequency of occurrence in old growth included: 

Corner canadensis, Linnaea borealis var longiflora, and Tiarella trifoliala var unifoliora in the Cascade provinces, and 

Synlhyris reni~‘ormis in the Coast Range province. Species 

occurring more frequently in mature and young stands than 

in old-growth stands included: H&discus discolor, Anemone 

lyallii, and Pteridium apilinum. Altbough differences were 

significant for many species, most species were relatively 

common, occurring 25 percent or more of tbe time in at Ieat 

two of the age-classes. This overlap includes the old-growtbassociated species mentioned above, except for Pacific yew 

and S. reniformis in the Coast Range province, where these 

species either did not occur or occurred infrequently in 

mature and young stands. 

115 
Table ‘t-Percent frequency of occurrence of species showing an age-class effect (P < 0.1) in old-growth, mature, and 

young stands io 3 physiographic provinces (species me listed from top to bottom in order of decreasing frequency 

in old growth) 

Species 0 M Y Sig.” Species 0 M Y Sig.” 

84 64 50 

74 80 1W 

26 52 33 ^. .- 

20 

l&l 

80 

10 

23 

44 61 

68 78 

59 54 

17 20 

*I* 

+ 

I 

* 

** 

* 

I. 

+ 

* 

* 

* 

+ 

*** 

*t 

t* 

+ 

** 

* 

l * 

t 

t* 

+ * 

* 

* 

98 80 89 

98 80 89 

62 46 20 

58 36 33 

58 28 56 

30 23 0 

81 1w 56 

63 92 56 

24 5s 53 

58 72 39 

18 29 0 

11 32 7 

9 36 22 

6 36 0 

7 33 2s 

5 23 40 

4 27 8 

4 27 17 

2 20 6 

0 0 17 

49 

43 

17 

9, 

22 

88 

38 

64 

20 

lea 

0 

39 

* 

+ 

* 

+ 

* 

+ 

**I 

** 

* 

+ 

+ 

+ 

* 

**I 

+, 

** 

+ 

+ 

* 

* 

+ 

*** 

+ 

*** 

91 78 33 

55 0 0 

102 78 67 

91 56 50 

91 67 50 

87 56 50 

86 57 33 

74 56 17 

78 67 17 

64 14 0 

64 29 17 

68 14 83 

87 33 17 

** 

** 

* 

* 

* 

: 

t 

** 

** 

+ 

.I 

*I* 

L1 44 + 

5 5 ;i + n Significmce level (P) in *i-square test for age-class 

3 23 27 ’ effect: + =<O.l, * =co.os, ** =cQ.o*. *** = co.Wl. 

0 0 17 * 

116 
The cover of many understory species also differed among 

the age-classes, with at least marginal significance (I’ < 0.1) 

(table. 5). Thirty species, 45 species, and 14 species differed 

among the age-classes (P < 0.1) in the Washington, Oregon, 

and Coast provinces (table 5). Mean species cover was highest in old-growth 37 times, highest in mature 30 times, and 

highest in young stands 22 times in the three provinces. Eight 

species differed significantly in all three provinces. Five of 

these, Pacific yew, Goodyera oblongz@lia, Linnaea borealis 

var longiflora, Achlys triphylla, and Lobnria spp. had highest 

cwa in old growth; the remaining three, Adenocaulon bicolor, Anemone lyallii, and Pteridium aquilinum had higher 

cover primarily in mature stands. The cover by Pacific yew 

and Lobaria spp. in young and mature stands was typically 

less than one-tenth of the cover in old-growth stands (table 

4). Other species with highest cover in old growth included: 

Chimaphilla umbellata, Berberis nzrvom, Corms canadensis, and Tiarella trifoliata var unifoliata. 

Table .S-Ratios of mean species cover in mature 

(M) and young (Y) stands to mean specks cover 

In old growth for species showing an age-class 

effect (P < 0.1) (species are listed from top to 

bottom in order of decreasing mean cover in old 

growth) 

0.09 

0.10 

0.24 

0.48 

3.8 

,100 

0.09 

0.13 

0.13 

0.36 

0.50 

0.85 

1.6 

1.7 

3.5 

4.9 

6 

12 

19 

0.03 

>I00 

Sig.” 

*** 

*** 

* 

* 

** 

*** 

*** 

** 

*t 

*** 

+ 

** 

: 

* 

*** 

*** 

* 

I) 

: 

* 

* 

** 

: 

** 

Table S-continued 

Smcies M Y Sk.” 

0.02 

0.33 

0.63 

7 

12 

>I00 

0.22 

0.95 

1.7 

1.7 

2.7 

<a.01 

44 

66 

0 

o.to 

0.54 

2.3 

9.8 

22 

,100 

0.27 

0.24 

0.95 

4 

0.85 

18 

44 

16 

>100 

<O.O, 

0.01 

0.31 

0.03 

0.27 

0.4, 

0.09 

0.14 

0.04 

2.47 

4.2 

0.59 

0.24 

CO.01 

u.99 

0.37 

0.95 

14 

<O.O, 

0.93 

CO.01 

2.4 

2.3 

0 

>100 

18 

*** 

*** 

*** 

** 

* 

* 

* 

* 

: 

1 

** 

* 

* 

+ 

: 

* 

** 

* 

*I 

+ 

*** 

** 

** 

*** 

** 

** 

* 

: 

** 

* 

*** 

* 

* 

* 

*** 

: 

+ 

* 

*** 

117 
Table S-continued 

Species M Y SirP 

Several species differed in cover among age-classes (P < 0.1) 

only in one province (table 5). For example, in the Washigton Cascades, Vaccinium spp. and Clintonia uniflora had 

many times more cover in old growth than in mature or 

young stands. In the Oregon Cascades, Copfis laciniata and 

Pyrola asarifolia had much higher cover in old growth relative to the younger age-classes. In the Coast Range, Acer 

circinatum and Synthyris reniformis had highest cover in old 

growth 

The analyses based on percentage cover differed in two ways 

from the analyses based on percentage occurrence. First, more 

species showed significant differences among the age-classes 

in tests based on cover (90 species) than in tests based on 

occurrence (76 species). Second, for many species, such as 

Pacific yew and Timella trifoliata var unifoliata, relative 

differences among age-classes were greater for percentage 

cover than for percentage occurrence. The results indicate 

that the significant species are not strongly restricted to one 

age-class but differ in their development in the age-classes. 

Discussion 

Species Diversity 

Various patterns of plant species diversity during succession 

have been hypothesized (Odum 1969, Pielou 1966, Whittaker 

1965) including increases and decreases, and multiple peaks 

in diversity. Because this study did not examine the earliest 

stages of forest succession, the entire pattern of changes in 

diversity in Douglas-fir forests cannot be examined with 

these data. Other studies (Halpem 1987, 1989; Schoonmaker 

and McKee 1988), however, show that after stand-replacing 

disturbance in old-growth Douglas-fir stands, plant species 

diversity (both No and N2) increases to a peak early in 

succession as shade-intolerant herbs and shrubs invade the 

surviving community of shade-tolerant forest species. These 

studies indicate that as a forest canopy develops and closes, 

between 20 and 40 years, diversity declines to a low point 

when the dense tree canopy shades-out all but the most shade 

tolerant of the understory species. 

The results of this study indicate that after canopy closure. 

the trend in understory diversity was to increase slightly from 

young to old growth. The increase may, in fact, be stronger 

than indicated by this study. The wide age range of the oldgrowth age-class may mask a steeper increase, followed by a 

decline in diversity as the stand becomes entirely composed 

of shade-tolerant tree species such as western hemlock. In 

the Coast Range, where old growth was almost 100 years 

younger than the Cascade old growth and had a lower basal 

area of hemlock, understory richness and diversity showed 

the strongest increase with ageclass. Habeck (1968) found 

that forests dominated by western hemlock in Montana had 

lower species richness than earlier stages containing a mix of 

early- and late-successional species. This finding suggests 

that within the old-growth age-class, diversity may be high 

in the early stages of old growth (from 200 to 400 years) 

and low in the later stages (after 400 years), when western 

hemlock reaches dominance in the stand (Spies and others, 

unpubl. manuscript). 

The overstory component of diversity appears to follow the 

general pattern described above, with peaks early and late in 

succession. The tendency for overstory diversity to be higher 

in young stands than in mature stands in the Oregon Cascades 

and Coast Range suggests that early successional species, 

such as red alder, biuer cherry, and madrone persist in young 

stands and drop out during the mature stage. During the 

old-growth stage, shade-tolerant species reach the canopy, 

contributing to an increase in canopy tree diversity during 

this period. A decline in overstory diversity probably occurs 

as Douglas-fir drops out of the stands. 

118 
Species Occurrence

The list of species associated with succession should be

viewed as a first approximation, requiring further verification

and more controlled study. Given that 398 individual nonparametrical statistical tests were conducted, about 40, or

10 percent of the tests, might be expected to yield significant

results at P < 0.1 by chance. Consequently, the statistical

tests should be viewed with caution. The results do suggest,

however, that many species were different among the ageclasses because about 20 percent of the tests in both analyses

were significant"and many of those were significant at

P < 0.01. The results should also be viewed with caution

because some species that increase or decrease in abundance

with stand age may have occurred too infrequently to show

statistical significance. The influence of site and historical

factors in this geographically broad chronosequence study

can never be fully evaluated; however, the similarities in

average site and vegetation characteristics (see Methods) do

not indicate that these factors played a strong role in the

community and species patterns among the age-classes.

Several of the species most common in old growth have

been associated with late-successional coniferous forests in

other studies. Tiarella unifoliata was most common in latesuccessional forests in Montana (Habeck 1968) and Washington (Moir and others 1979). Pacific yew reached highest

abundance in late-successional cedar forests in Montana

(Habeck 1968, McCune and Allen 1985). Alaback (1982)

found that C. canadensis and Vaccinium alaskaense had highest biomass in old growth in a chronosequence of Sitka

spruce/western hemlock stands in southeastern Alaska.

Some species had different successional relationships than

were reported in other studies. Habeck (1968) found that

Vaccinium membranaceum, a species associated with old

growth in this study, was most frequent in early-successional

stands in Montana, and Adenocaulon bicolor, which was most

abundant in mature stands in this study, was most frequent in

late-successional stands in Montana (Habeck 1968). Alaback

(1982) found that V. parvifolium, which had highest percentage cover in old growth in the Washington Cascades, had

high biomass in mature stands and low biomass in old growth

in southeastern Alaska. Many factors, such as climate, genetics, competition, and disturbance regime, could lead to geographically different successional distributions for a single

species.

Even within this study, some species did not show consistent

successional patterns. For example, bigleaf maple had lowest

basal area in old growth in the Cascade provinces but highest

basal area in old growth in the Coast Range. This pattern may

be because bigleaf maple has not yet dropped out of Coast

Range old-growth stands, which are younger than the stands

in the Cascade provinces. In addition, the lower basal area of

western hemlock, a competitor for light, in old growth in the

Coast Range may allow bigleaf maple to persist longer than

it does in the Cascade stands.

Factors Contributing to Successional Change

Assuming that many of the patterns of vegetation and stand

age are a consequence of forest succession, four related

mechanisms of change in understory communities and plant

species abundance are hypothesized.

The first mechanism is change in resources, particularly

light, and competitive abilities to capture those resources.

Old-growth forests develop many areas of dense hemlock

canopies in addition to the relatively open Douglas-fir canopies and canopy gaps. Small, very shade-tolerant herbs and

shrubs such as Clintonia, Syntheris, Tiarella, and Cornus

are favored in the deep-shade areas. In mature and young

forests”with more uniform, open canopies”aggressive,

less shade-tolerant shrubs such as Gautheriu shallon often

form large, dense patches that may exclude smaller, lesscompetitive herb and shrub species (Long and Turner 1974).

The increase of shade-tolerant herbs and shrubs in coniferous

forest development has been observed by Alaback (1982)

and Stewart (1988). Some shade-tolerant understory herbaceous plants and tree species such as Pacific yew and westem hemlock may also be favored in old growth by cool,

moist microclimates during the dry, warm season. Although

the relative microclimates of the age-classes have not been

quantified, old-growth understories are likely to be relatively

cool and humid during the dry season because they are protected from radiation and drying winds by deep, multiple

canopy layers. In addition, soil moisture may be higher as a

consequence of the large accumulations of litter on the forest

floor (Spies and Franklin, this volume) and pieces of decomposed wood that store moisture in the soil.

A second mechanism of understory change may be an increase in the horizontal spatial heterogeneity of resources

and environments. As Douglas-fir stands develop, they become a mosaic of canopy gaps, and dense hemlock canopies

overlying a forest floor with accumulations of woody debris,

tip-up mounds, and shrub patches. This structural heterogeneity may create a shifting mosaic of resources and environments that favors species that can both tolerate low light and

spread laterally to take advantage of increased light. In addition, if canopy gaps also represent areas of low root occupancy, then soil moisture and nutrients may be relatively more

available. Species that can take advantage of resource heterogeneity include Linnaea borealis, Rubus lasiococcus and

Clintonia unifrora, which spread rapidly by stolons or rhizomes (Antos and Zobel 1984) to occupy gaps that form

above or near the plant. These species can apparently persist

with low abundance in shaded areas but may require canopy

gaps to survive for long periods in some stands.

119
A third mechanism in plant community change is the increase

in vertical environmental diversity, specifically the increase

in the height and number of canopy layers. The canopies of

old-growth trees provide moist, cool environments that favor

the growth of foliose lichens such as Lobaria oregana (Pike

and others 1975). Because the canopy flora was not sampled

directly, other epiphytic plant species may also be associated

with tall, structurally and environmentally diverse canopies.

A fourth mechanism may be sensitivity to fire and slow rates

of reestablishment and growth after fire. Fire-sensitive species

such as western hemlock and Pacific yew are typically killed

by moderate to high-severity fires and can take relatively long

periods to reach maturity, growing slowly in the understory

for many years. They are often present as scattered seedlings

and saplings in young and mature stands but do not reach

maximum size and density until the old-growth stage, one

hundred to several hundred years later. This distribution does

not mean that old-growth stands where these species occur

have been free of fire during their entire development. Many

old-growth stands have had a history of patchy low-intensity

fires (Morrison and Swanson 1990), which miss individuals of these species or only injure them. Best development

of these species appears where a stand has not been subjected

to low-intensity fire for several centuries, however.

Conclusions and Management Implications

The majority of species did not show any differences in their

frequency or abundance within the age-classes. At least

20 percent of the species did differ among the age-classes,

and many of those were more frequent and abundant in old

growth. Most species associated with old growth, however,

were not restricted to old-growth forests and could be found

with moderate frequencies and abundances in at least one

younger age-class. Of all the species showing an age-class

effect, only Pacific yew and Lobaria spp. showed strong preference for old-growth in all three provinces, suggesting that

these species would suffer the greatest decline in regional

populations if most of the current old growth is clearcut and

converted to short-rotation plantations. Many of the ecological conditions that favor old-growth-associated plant species

can be found in lesser amounts in younger unmanaged forests. Gaps, deeply shaded and cool understories, and accumulations of woody debris on the forest floor can be found in at

least small areas in many unmanaged young and mature

stands. Conditions such as long periods since catastrophic disturbance, development of massive tree crowns, and buffered

canopy microclimates, which favor Pacific yew and Lobaria

are not typically found in unmanaged young or mature stands

that regenerated after stand-replacing fire, however.

The suggestion that the composition of the forest understory

community changes with overstory succession has important

implications for using vegetation to assess site conditions and

for maintaining biological diversity in managed landscapes.

120

The abundance (cover) of many understory species is used to

classify forests into plant associations used for management.

If the influence of succession on understory plant communities is not taken into account, then understory plant communities may incorrectly estimate site conditions and potential

(Spies and Barnes 1985). Stewart (1988) has found that

many understory species in Douglas-fir forests are influenced

by tree canopies and warns that understory communities may

reflect canopy-disturbance history more than site quality.

Species such as Tiarella, which may be affected by changes

in light intensity and understory microclimate, will have

limited value in identifying plant associations and indicating

physical site conditions. A better understanding of the autecologies of many understory shrubs and herbs, will help

resolve uncertainty of the indicator value of some understory

species.

The effect of forest management on understory and overstory

species composition is not well known. This lack is partly a

consequence of the limited research studies but is also a

result of the varied nature of “managed” stands. Managed

forests range from young stands that originated from railroad

logging in the early 1900’s to second- and third-rotation

stands on non-Federal lands that have been burned, sprayed

with herbicides, planted, fertilized, or thinned. Speculating

on how old-growth-associated plant species might be affected by some specific management practices may be

useful, however. Management effects can be divided into

effects on the initial plant population in an old-growth stand

from activities such as clearcutting and site preparation, and

effects on recovering plant populations from activities such

as stand-density control and short rotations. Initial old-growth

populations of many species will suffer at least temporary

declines as a consequence of clearcutting and site preparation, especially broadcast burning. The current practice

of clearcutting removes all the overstory trees, leaving no

habitat for canopy epiphytes such as Lobaria. In landscapes

where no large canopy trees occur and no plans are made to

grow them, Lobaria and other canopy plant species will

probably decline to very low populations. Where site preparation includes broadcast burning, fire-sensitive species such

as Pacific yew, Chimaphila, and Linnaea will show strong

declines (Halpern 1987) without return to old-growth populations for at least 20 years. If stands are planted to high

densities and not thinned, then recovery of understories may

be delayed if very dense canopies develop and shade-out the

understory. On the other hand, if canopy density is controlled

by planting density, precommercial thinning, or both, then

forest understories may recover more quickly. If species

composition is strongly controlled during this process to

favor one species such as Douglas-fir, however, then both

canopy and understory diversity will be reduced. The

practice of short rotations may reduce populations of

understory and overstory species associated with old growth

before their populations can recover from the effects of
clearcutting and burning. In the long term, the combination

of clearcutting, site preparation, and short rotations could

greatly reduce populations of some old-growth-associated

plant species.

By creating and managing microsites, however, managers

have many opportunities to maintain populations of plant species associated with old growth in managed stands and landscapes. Leaving some live canopy trees in cutting units can

maintain canopy species that might recolonize the young forest canopy more rapidly than if no refuge trees were present.

Leaving large Pacific yew trees in cutting units may provide

for this species, but no studies have evaluated how well such

trees survive broadcast burning, hotter drier microclimate,

and increased herbivory from ungulates. Using cooler, more

patchy site-preparation fires, or not using fire on some sites,

could help maintain fire-sensitive species. Creating more

diverse canopies by planting a mix of conifers, allowing

hardwoods to regenerate, thinning dense stands, and

cutting canopy openings could help create a more heterogenous understory environment and allow plant species populations to recover more rapidly. Finally, growing some stands

on long rotations would help maintain species such as Pacific

yew that require long recovery after catastrophic disturbance.

Possibilities for maintaining populations of old-growth-associated species in managed stands and landscapes appear to be

good, but additional information is needed about the autecologies of many understory species and the effects of management practices on those species to develop more specific

management recommendations.

Acknowledgments

I thank Gody Spycher and Janice Harmon for their assistance

with data analysis. Jim Agee and two anonymous reviewers

provided helpful suggestions.

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