United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General

Technical Report

PNW-GTR-488

March 2000

Effects of Forest Management

on Understory and

Overstory Vegetation:

A Retrospective Study

David R. Thysell and Andrew B. Carey
Authors David R. Thysell was a botanist and Andrew B. Carey is a principal research wildlife biologist, Forestry

Sciences Laboratory, 3625 93d Avenue S.W., Olympia, WA 98512-9193.
Abstract Thysell, David R.; Carey, Andrew B. 2000. Effects of forest management on understory

and overstory vegetation: a retrospective study. Gen. Tech. Rep. PNW-GTR-488.

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest

Research Station. 41 p.

Management for wood production through long rotations and thinning (MT) and

management for biodiversity through legacy retention, protection, and no subsequent

manipulation (ML) are two approaches to managing second-growth forests in the

Pacific Northwest. We describe how these two strategies have affected tree size, tree

density, and native and exotic vascular plant diversity and abundance.

Keywords: Diversity, second-growth, Douglas-fir, Pseudotsuga menziesii, understory

vegetation, species richness, forest development, thinning, legacies.

Summary We examined understory and overstory vegetation in two proximate tracts of Douglas-fir

(Pseudotsuga menziesu) forests with divergent management histories. The two tracts

regenerated from harvest of old growth in the 1920s (MT) and 1930s (ML): MT

subsequently was thinned twice and contained little coarse woody debris (CWD) and

few snags; ML received no management after harvest with retention of some live and

dead trees and contained moderate CWD, snags, and residual trees. We compared

tree density, tree size, tree size-class diversity, understory species richness and

abundance, and Shannon index of vascular plant diversity (H') in the two tracts. We

performed a Bray-Curtis ordination to more fully illustrate variation in the understory

communities.

The MT tract had fewer and larger trees and had higher tree size-class diversity than

ML. Species richness and diversity were significantly higher in MT than in ML. Total

vascular plant species richness in MT (94 species) was twice that in the more densely

stocked ML (47 species). Average total understory cover was much higher in MT (88

percent) than in ML (34 percent). Both tracts lacked shade-tolerant understory trees

and had minimal foliage height diversity. Numerous exotic species were found in MT,

but these were virtually absent in ML.

Plant communities in the two tracts differed qualitatively and quantitatively, and current

conditions will differentially influence plant composition and structure after future

silvicultural entries. Forest processes such as crown differentiation, decadence, and

understory development are amenable to management and suggest hypotheses about

development of late-seral attributes in such stands. Due to lack of legacy trees and

CWD in MT, crown differentiation will occur solely within the current cohort; CWD

enhancement by tree felling would reduce stand density to unacceptably low levels.

Future thinnings in MT may result in further increase of already-dominant clonal

understory species that, along with lack of CWD as nurse logs, could delay establishment of a shade-tolerant understory and retard development of a diverse foliage height

profile. In ML, crown differentiation is less than in MT but is augmented by residual

trees. The amount of CWD in ML is greater than in MT; unlike MT, live-tree densities

in ML permit CWD augmentation while retaining high stocking levels. After future

thinnings, shade-tolerant trees, shrubs, and herbs should be more likely to establish in

ML because of less competition and more CWD as nurse logs, thereby resulting in a

more rapid increase in foliage height diversity and relatively more seedling than clonal

expansion in the understory in ML than in MT. Late-seral
Contents

development in both tracts will be hindered by absence of shade-tolerant understory

trees. Future thinnings will increase vascular plant species richness in both tracts, but

many new species will be exotics. Major challenges in forest management include

minimizing spread and impact of exotics, fostering late-sera) conditions in managed

stands, and, above all, ensuring continuity of natural legacy-perpetuating processes

and systems.

1 Introduction

3 Study Area

3 Site Selection

3 Physical Environment

3 Vegetation and Management History

6 Methods

6 Sampling Design

6 Field Sampling

7 Analysis

9 Results

9 Overstory Composition, Density, Size, and Size-Class Diversity

10 Understory Species Richness and Diversity

13 Understory Species Abundance and Frequency

18 Ranked Abundance and Species Indicator Values

19 Bray-Curtis Ordination

24 Discussion

24 Limitations

25 Community Composition and Structure

27 Autecological Considerations

28 Globalization and Homogenization of the Understory

29 Management Implications

33 Acknowledgments

33 Metric Equivalents

33 Literature Cited
Introduction A great deal of attention has been paid to management of biological diversity in forested

ecosystems of the Pacific Northwest and elsewhere (see papers in Gilliam and Roberts

1995). Emerging and related concerns are the homogenization of the world's floras and

the threat to native species posed by exotic species that spread in response to human

activities (Soule 1990; United States Congress, Office of Technology Assessment 1993;

Wilcove and others 1998; Wiser and others 1998). Recent shifts in thinking about forest

management coincide with increases in human population, decreases in timber

harvests, and concerns about sustainability of managed forests (Carey 1998; Kohm and

Franklin 1997; McGinnis and others 1996, 1997; Perry 1998) and are manifested by a

divergent paradigm of active, adaptive, and site-specific management versus passive,

protective management in efforts to hasten development of late-seral forest attributes

and to increase native biodiversity (Carey 1998, Carey and Curtis 1996). Though

certainty of outcome is impossible, combining knowledge of past management practices,

contemporary manipulations, and previously accumulated research with retrospective

and experimental investigations can increase the likelihood of meeting specific

objectives while retaining a wide range of future options (Carey and others 1996a,

1996b, 1999b; Spies 1997; Walters and Holling 1990).

Numerous silvicultural tools are available to forest managers and include a wide variety

of harvest systems, retention levels and patterns, regeneration methods, and rotation

lengths that can be applied to meet site-specific objectives (Carey and others 1996b,

Curtis 1997, DeBell and others 1997, Franklin and others 1997, McComb and others

1993). However, information relating past harvest practices to current understory and

overstory conditions in older, managed forests is lacking (Halpern and Spies 1995;

North and others 1996; Spies 1991, 1997). Such information is needed to better predict

the effects of future manipulations on biodiversity, stand development, and long-term

ecosystem structure and function.

Many low- to mid-elevation forested landscapes of western Washington and Oregon

currently are dominated by young (< 80 years old), even-aged stands of a single tree

species (usually Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco]). Such landscapes

resulted primarily from clearcuts and have created negative public perceptions as well

as scientific concerns about forest management (Curtis 1997, Curtis and Carey 1996).

Past management often simplified forest composition and structure. Clearcutting,

burning, planting of genetically uniform trees, application of herbicides, and

precommercial thinning have diverted many ecosystem resources to rapid growth of

timber and fiber; fewer resources were intentionally directed to maintenance of biodiversity (Carey and others 1996a, 1996b, 1999a; Perry 1998). Nonetheless, because

such young, managed stands are so widespread, they may provide the greatest short--

term opportunity for management and study as they develop. With appropriate and welldesigned management intervention, differentiation in such stands can probably be

expedited and directed to meet a variety of specific late-successional goals (Carey and

others 1996a, 1996b; Curtis and others 1998; DeBell and others 1997). With or without

management, these stands are the raw material from which many old forests of the

future likely will develop.

Simulation modeling (Carey and others 1996a, Carey and Curtis 1996, Lippke and

others 1996), early results from young-stand manipulation experiments (Carey and

others 1996b, 1999b), and retrospective studies (Carey 1995, Carey and Johnson 1995,

Carey and others 1996b, 1999b) indicate that compared to no management,

1
management could enhance biodiversity while hastening development of late-seral

forest conditions. These studies suggest that integrated strategies which include legacy

retention, alternative harvest and thinning regimes, and direct habitat improvement

may be necessary to enhance native biodiversity, minimize the spread of exotic species,

accelerate development of late-seral conditions, and restore balance to landscapelevel stand-age distributions, while providing high-quality wood, increased levels of

carbon sequestration, and increased employment (Carey and Curtis 1996; Carey and

others 1996a, 1999b; Curtis and Carey 1996; Lippke and others 1996).

Several replicated silvicultural experiments in developing late-seral conditions in

managed stands are underway in the Pacific Northwest but are in the early phases of

their response monitoring1 (Aubry and others 1999; Carey and others 1999b). Though

there is little published information on the efficacy of silvicultural interventions in

accelerating forest development in young stands (Hayes and others 1997, McComb and

others 1993), preliminary data suggest that thinning can stimulate understory

development and the establishment and growth of shrubs and conifer seedlings,

increase diversity of understory vascular plants (Bailey and Tappeiner 1998; Carey and

others 1996a, 1999b; North and others 1996) and hypogeous ectomycorrhizal fungi (

Colgan 1997, Colgan and others 1999), and increase abundance of some forest floor

small mammals (Carey and others 1996b).

Our purpose in this paper is to consider the effects at >55 years in stand age of two

commonly proposed management strategies on tree size, density, and size-class

diversity and on current plant species diversity and abundance in forests of financial

rotation-age, and to reflect on how past management and current conditions may affect

the development of native biodiversity and late-seral attributes and the establishment

of exotic species after future silvicultural entries. Our data are baseline information for

a 20-year silvicultural experiment subsequently imposed on the stands.

We identified two proximate forested tracts of similar age, substrate, topography, and

composition, but markedly different management histories, that illustrate commonly

occurring end-of-rotation conditions in the Pacific Northwest; both were past the common age at harvest for industrial forests and approaching a historically common age of

harvest on public lands. The tracts differed in amount of legacy tree, snag, and coarse

woody debris (CWD) retention from the previous harvest and in subsequent tree density

control through thinning. We contrast the results of the two strategies by tree size,

density, and size-class diversity and plant species abundance and diversity at four spatial

scales: plot, stand, block, and track levels. Plots represent within-stand variation owing

to stochasticity in stand initiation and development. Among-stand variation incorporates

minor effects of slope, aspect, and stochasticity in application of management. Blocks

represent large-scale site differences and differences that arise as a result of timing of

treatments. Between-tract variation should represent effects of management strategy.

 _________

1Harrington, C.; Carey, A. Manuscript in preparation. Authors may

be contacted at the Forestry Sciences Laboratory, Olympia, WA.

2
Study Area After a regional search for retrospective study sites in western Washington and

Site Selection Oregon (Thomas and others 1993), we further inspected potential sites to determine

suitability as to forest homogeneity, sufficient size to allow replication of treatments,

and histories representative of currently recommended strategies of legacy retention

and management with thinnings. We located such an area and interested managers

at the Fort Lewis Military Reservation in Thurston County, Washington. The area is

part of about 6000 ha of 40- to 70-year-old second-growth Douglas-fir stands, thus

allaying concerns about the potentially confounding effects that could arise due to

close proximity to older forests. Within the 6000 ha were two large tracts that had

markedly different management histories. Both tracts contained two management

units, each large enough to accommodate four 13-ha stands. The two management

units per tract, although sharing general management history, were potentially different in site, aspect, slope, time of implementation, initial forest conditions, and implementation by different operators. Differences among the four management units,

except those distinguishing the two tracts, appeared small.

Physical Environment Fort Lewis Military Reservation is in the southern Puget Trough Physiographic Province

(Franklin and Dyrness 1973). Present-day land forms and soils were deposited during

the recession of the Puget Lobe of the Vashon Stade of the continental glacier 13,000

to 15,000 years ago (Kruckeberg 1991). Topographic relief is moderate to rolling with

occasional steeper areas surrounding small kettle depressions; topography is typical of

the south-central portion of the Puget glacial lobe. The study sites are at latitude 46° 55'

42" to 46° 59' 00" N. and longitude 122° 40' to 42' E. The elevation ranged from 120 to

165 m, slopes were moderate, and aspect was variable, though tending southeast (

table 1). No perennial creeks, ponds, or wetlands were in the study area. The sites

were on glacial uplands, terminal moraines, and glacial till and outwash plains where

root penetration into the soil is moderately shallow and drainage is classified as

somewhat excessive. Soils are coarse-textured gravelly and gravelly-sandy loams of the

Everett and Tenino soil series (Pringle 1990). Annual precipitation for the Fort Lewis

area is 800 to 900 mm with only 10 to 15 percent of the annual precipitation falling

during the peak growing months of June through September (Pringle 1990).

Vegetation and The two tracts, about 3 km apart, have experienced substantially different manageManagement History ment, both at the time of original harvest and subsequently (table 1). Both tracts

resemble the Pseudotsuga menziesiii/Holodiscus discolor association of the Tsuga

heterophylla zone of Franklin and Dyrness (1973) or the Pseudotsuga menziesiii/

Gaultheria shallon association of Henderson and others (1989), though Douglas-fir

forests of the Puget lowlands have not been studied in great detail and may eventually

be assigned to their own separate vegetative associations (Franklin and Dyrness 1973).

Both tracts were even-aged Douglas-fir forests that had established by natural seedfall.

Neither tract had experienced natural or prescribed fire since harvest, and we did not

attribute current differences to fire. The tracts thus illustrate two possible developmental

pathways and two potential endpoints in a management cycle of about 60 years, while

also representing two conditions where future management to enhance biodiversity and

develop late-seral conditions could occur. We refer to the two strategies as

management with thinnings (MT) and management with legacies (ML).

7
Table 1-Characteristics of forests managed with legacies versus managed with

thinning in the Puget Trough of Washington, 1991-92

Old growth in the two management units in the MT tract was clearcut in 1923-27; the

subsequent regeneration was commercially thinned in the early 1970s and again in the

late 1980s. During harvests in the two MT units, live and recently dead trees were

removed and slash was piled and burned; few snags or residual trees, and little cover of

CWD, remained in 1992 (table 1). At the time of our sampling, MT (fig. 1) was

dominated by well-spaced Douglas-fir 55 to 65 cm in diameter at breast height (d.b.h.)

with small numbers of black cottonwood (Populus balsamifera ssp. trichocarpa [Torr. &

Gray] Brayshaw) and red alder (Alnus rubra Bong.). Based on 50-year site curves for

Douglas-fir from King (1966), the MT tract, with stand site indices of 39.6 to 40.5 m, was

site class II. The vascular plant understory was well developed and moss cover was low

to moderate (Carey and others 1999b).

In contrast, old growth in the two management units in ML was cut in 1935-39 and was

not further manipulated; moderate CWD and numerous legacy trees and snags

remained in 1992 (table 1). In 1992, ML (fig. 2) was a closed-canopy forest dominated

by closely spaced Douglas-fir 30 to 45 cm d.b.h., with a few western redcedars (Thuja

plicata Donn ex D. Don) and Pacific yews (Taxus brevifolia Nutt.). Trees killed by

suppression and trees with small crowns were abundant. Based on 50-year site curves

for Douglas-fir from King (1966), the MT tract, with stand site indices of 36.0 to 36.3 m,

also was site class II. Understory vascular vegetation in ML was sparse and forest floor

mosses were abundant (Carey and others 1999b).

8
Figure 1-Typical understory vegetation and tree size and density conditions in the managed-with-thinning

tract, summer 1992.

Figure 2-Typical understory vegetation and tree size and density conditions in the managed-with-legacies

tract, summer 1992.

9
Methods

Sampling Design

Field Sampling

Four 13-ha stands were located in each of the four management units (hereafter

referred to as blocks) in each of the two tracts (Carey and others 1999b). The two ML

blocks were Star (stands 101-104) and Stellar (stands 201-204); the two MT blocks

were Farley (stands 301-304) and Hill (stands 401-404). Stand size was chosen to be

as large as possible but was constrained by topography, changes in vegetation, and

past management activities, including roads; 13 ha was the maximum contiguous area

of homogeneous vegetation we could regularly circumscribe. An 8 by 8 sampling grid

with 40-m spacing between grid points (e.g., 280 by 280 m), encompassing 7.8 ha,

was surveyed in the center of each stand. Though management had been applied to

blocks that were relatively homogeneous, there was within- and among- block variation

in time, space, and implementation of management. We expected to capture much of

the variation due to site and management differences with the four stands in each

block and the two blocks in each tract. The four stands per block were selected as

distinct segments of each block, and we expected greater similarity within than among

blocks. We also used replication of plots within stands to assess within- and amongstand variation and block-level variability. The two blocks per tract allowed us to

evaluate interblock variation that may have been caused by spatio-temporal and

treatment implementation differences within the same general management strategy.

Our retrospective investigation required the use of circumscribed blocks in which the

same management had been applied throughout. Within each block, the four stands

were, by necessity, placed to optimize use of available space, thus assuring even,

complete, and representative coverage across the blocks. Such coverage was further

ensured at the stand level by sample plots being systematically located in each stand.

We recognize that our sampling design lacked random assignment of treatments and

disallows formal hypothesis testing, thus we used statistical analyses and tests primarily for

exploratory purposes and to describe the variation encountered in our data set.

Overstory and understory vegetation was sampled at or near 15 systematically located

interior grid points within each of the 16 stands (240 points total), with the six interior

north-south grid lines alternately containing two or three sample points. Sample points

were >56 m apart. Overstory trees were measured during fall and winter 1991-92;

understory vegetation was measured from June through August 1992. At each sample

point, a variable-radius prism plot tally (basal area factor 30 [English]) was made. We

recorded tree species and d.b.h. in centimeters for each tallied tree, measuring trees >5

cm d.b.h. For the two largest, current-stand, Douglas-fir trees per prism plot (dominants),

d.b.h., height, height to live crown, and crown width were recorded. Using the volume

of a cone as an idealized approximation, we derived crown volume estimates of the

dominant trees based on crown width as the diameter of the cone base, and tree height

minus height to live crown as the cone height.

Understory vegetation was recorded from a 2.82-m radius circular plot (25-m2) offset 5

m at a random bearing from each overstory sampling point. On understory plots we

recorded percentage of cover by species, defining cover as the portion of ground

covered by the vertical projection of the vegetation onto a horizontal plane. For our

analysis, we grouped plants into the following life forms or functional groups: (1) tall

shrubs and understory tree species typically >2 m in height but below the forest canopy,

(2) low shrub species 0.5-2 m in height, (3) trailing shrubs and vine species <0.5 m in

height or scandent on other plants, (4) herbs and subshrubs, (5) ferns, (6) graminoids (

members of the families Poaceae, Cyperaceae, and Juncaceae), and (7) forest-floor

mosses.

10
Analysis

Vascular plant nomenclature follows Kartesz (1994), and vascular plant species

concepts and identifications are based on Hitchcock and Cronquist (1973); moss

nomenclature follows Schofield (1992). Vascular plant acronyms are from the

PLANTS database (USDA 1997). Vascular plant species origins (e.g., native or

nonnative), autecology, and disturbance responses were based on regional guides (

Hickman 1993, Hitchcock and Cronquist 1973, Klinka and others 1989, Pojar and

MacKinnon 1994) and previously published studies (Dyrness 1973; Halpern 1988,

1989; Halpern and Spies 1995; Long 1977; Schoonmaker and McKee 1988; Spies 1991;

Spies and Franklin 1991).

Overstory tree-size diversity (Shannon index) was examined based on the abundance of

trees in each of nine d.b.h. classes: >5-19.9 cm; >20-29.9 cm; >30-39.9 cm; >40-49.9 cm; >

50-59.9 cm; >60-69.9 cm; >70-79.9 cm; >80-89.9 cm; and >90 cm. Measured trees were

used to compute average density (for trees >15 cm d.b.h.), d.b.h., and height by stand,

block, and tract. Understory vascular plant species diversity was calculated based on

percentage of cover (as the measure of abundance) for each species. For understory

vascular plant species, we examined species richness (the average number of species

per plot), and the Shannon index of diversity which incorporates both species richness

and abundance. The Shannon diversity index (H') is calculated as:

where piis the proportional abundance of the ith of m groups and In is the natural

logarithm (Magurran 1988). For the overstory, piis the number of trees in the ith size

class divided by the sum of all trees in the m size classes in the group under consideration (e.g., stand, block, tract). For understory species, pi

is the ratio of the cover of

species i to the summed cover of all m species in the group under consideration. We

examined understory species richness and diversity and tree size-class diversity at the

plot, stand, block, and tract levels.

To estimate species richness and to illustrate the range of variability among stands,

blocks, and tracts, we generated species-area curves. To improve the precision of our

estimates of stand-, block-, and tract-level species richness, we used the first-order jackknife estimator (JN) for species richness:

where S = the number of species found in n plots, and r1 = the number of species

found in only one plot (Heltshe and Forrester 1983, Palmer 1990).

To illustrate major gradients of plot-level variation within and among blocks and tracts,

we performed a Bray-Curtis (BC) ordination. We chose BC due to its ease of ecological interpretation and its specific relevance to phytosociological data (Beals 1984). We

used the Sorenson distance measure (in the form of percentage of dissimilarity or PD)

to generate the ordination distance matrix and we used the variance-regression method

to select reference points for the ordination axes (Beals 1984, McCune and Mefford

1997). The Sorenson distance (PD) between two plots is:

11
where PDij = the Sorenson distance between plots i and j, m = the number of species,

xjk = the abundance (percentage of cover) of the kth species in plot l, and xik = the

abundance (percentage of cover) of the kth species in plot j (Greig-Smith 1983, McCune

and Mefford 1997). We used raw quantitative data (percentage of cover) on all vascular

plant species (e.g., without transformation or relativization) from all 240 plots for the final

ordination data set, calculating the first three ordination axes. The amount of variance

explained by each axis was computed by subtracting the ratio of the sum of squares of

the residual distance matrix to the sum of squares of the original distance matrix from

one and expressed as a percentage (McCune and Mefford 1997). For frequently

encountered species and for variables such as total understory cover, total moss cover,

and tree density, we examined the correlation (Pearson's product-moment correlation

coefficient, r) between the variable or species and each of the first three ordination axes

(McCune and Mefford 1997). Using n = 240 (df= 238) to describe variation across the

full plot-by-species data set, the critical value (two-tailed) of r at a = 0.01 was 0.167 (Zar

1984). This value was used to test whether correlations differed significantly from zero.

To illustrate within- and among-stand patterns of abundance for dominant species,

species groups, and for tree attributes, we generated 95-percent confidence interval

graphs of the attributes for the 16 stands based on the 15 plots per stand. In an

exploratory analysis, we examined differences between 2 or >2 groups with parametric t-tests or one-way analysis of variance (ANOVA), respectively, where assumptions of

homoscedasticity and normality were met or were able to be met through data

transformations. We used the Levene statistic (Norusis 1993) at a = 0.05 to test for

homogeneity of variance. Because of the small sample size (n = 4 stand means per

block) used in the ANOVA groups, normality of distribution for some blocks was difficult to ascertain though such distributions approach normality under the central limit

theorem. We also confirmed that assumptions were met for the eight stand means per

tract. We performed block-wise one-way ANOVA to detect differences among blocks

and used the Tukey's B post hoc test (at a = 0.05) to identify which blocks differed.

Nested ANOVAs based on plot-level data from stands within blocks within tracts were

characterized by inequality of variance and lacked normal distributions that we could not

correct with transformations. Nonetheless, nested ANOVAs supported our one-way

ANOVAs based on stand means and demonstrated the plot- and stand-level variation

that we present graphically. Though we assumed, a priori, that the overstory and

understory would differ between ML and MT, we used ANOVAs and post hoc tests to

assess among-block variation within and between tracts and to relate present

conditions to previous management. Between-tract t-tests were performed by using

the eight stand means per tract. Where data were intractable to transformation, as in

testing for between-tract differences in individual species abundances and for testing

for tract-level effects on frequency of individual species occurrence, we used the

nonparametric Mann-Whitney U-test, and X2test, respectively (Norusis 1993, Zar

1984).

12
Results

Overstory Composition,

Density, Size, and SizeClass Diversity

We calculated the indicator value (IV) for species with high frequency or abundance, or

both, to identify species characterizing one or the other management strategy (Dufrene

and Legendre 1997). The species IV, in percent, is its relative abundance times its

relative frequency. Relative abundance is the average abundance of a species in a

group of plots divided by its average abundance in all plots. Relative frequency is the

percentage of the plots within a group in which a species is present (Dufrene and

Legendre 1997, McCune and Mefford 1997). We then evaluated the statistical

significance of the maximum IV (IV-max) for the two tracts by using a Monte Carlo

method employing 1,000 permutations of the data where the resultant P-value was the

proportion of permutations in which the IV-max from the randomized data set equaled

or exceeded the observed IV-max. Thus, a significant IV-max indicates a species that is

characteristic of a tract and that the indicator value is larger than would be expected by

chance in the permutation set (Dufrene and Legendre 1997, McCune and Mefford 1997)

. All calculations were made with the SPSS statistical program release 6.1.2 (Norusis

1993) or with PC-ORD, a program for multivariate analysis of ecological data (McCune

and Mefford 1997).

Of 1,441 live trees sampled in the two tracts, 98 percent were Douglas-fir. In ML (n

= 777) 99 percent were Douglas-fir; the remaining 1 percent were western redcedar and western hemlock. In MT (n = 664) 96 percent were Douglas fir; the

remaining 4 percent were red alder, bigleaf maple, black cottonwood, and western

hemlock.

Within- and among-stand variability in mean densities was higher in ML than in MT (

fig. 3A). Tree density was greater in ML blocks than in MT (df= 3, 12; F= 13.9;

P < 0.01); blocks differed significantly between, but not within, tracts. Tree density in

ML was significantly greater than in MT (fig. 3B).

 9
Within- and among-stand variability in d.b.h. was less in ML than in MT (fig. 4A).

Blocks differed in mean d.b.h. (df= 3, 12; F= 25.0; P< 0.01) with blocks differing

significantly between but not within tracts. Mean d.b.h. in ML (33.9 ± 1.2 cm) was

significantly less than in MT (52.7 ± 1.8 cm) (df = 14, t = 8.7, P < 0.01). Were a

constant growth rate for ML trees used, an additional 12 years (the age difference

between tracts) would result in 7.4-cm increase in d.b.h., or 39 percent of the

observed between-tract difference in d.b.h. Tree size-class diversity (H') was

significantly greater in MT than in ML (fig. 4B).

There were significant differences among the four blocks in dominant tree d.b.h.,

height, crown diameter, and crown volume but not in height-to-live-crown (table 2).

Again, blocks differed significantly between but not within tracts. Dominant trees in MT

were larger in diameter, taller, had wider crowns, greater heights-to-live-crown, and

greater crown volume (table 2). The live-crown ratio of the dominant trees averaged

50 percent in MT and 44 percent in ML. The ratio of the d.b.h. of dominants to the d.

b.h. of all prism plot trees was 1.30 ± 0.02 in ML vs. 1.18 ± 0.02 in MT.

There were significant differences among blocks in average number of species per plot,

average total species per stand, and first-order jack-knife species per stand; in all cases

post hoc tests showed that blocks differed significantly between but not within tracts (

table 3). Ranking of mean number of species per plot in all 16 stands showed the MT

stands to rank 1-8 and the ML stands to rank 9-16. Across all 240 plots, species

richness per plot ranged from 3-15 in ML and from 5-29 in MT Understory species

accumulated at distinctly lower rates in ML than in MT (fig. 5). Species

Understory Species

Richness and

Diversity

14
Table 2-Block- and tract-level dominant tree attributes for forests managed with legacies (

ML) and managed with thinning (MT) with block-wise ANOVAs and between-tract t-tests'

Table 3-Average species richness per plot, total species per stand, 1st-order jack-knife

estimate of total species per stand, and Shannon-Weiner Diversity Index for forests managed

with legacies (ML) and managed with thinning (MT), with block-wise ANOVA and between-tract

t-testsa

15
richness per stand, block, and tract were substantially less in ML, as was the amongstand, within-tract variation (fig. 5A). Jack-knife estimates of species richness were 26

to 40 species per stand in ML and 49 to 86 species per stand in MT Species-area

curves for the two blocks in each tract were nearly coincident (fig. 5B).

Figure 5-Species area curves for stands (A), blocks (B), and tracts (C) in

forests managed with legacies (ML) and managed with thinning (MT), summer

1992. Numbers in parentheses are the first-order jack-knife estimates of stand-

, block-, or tract-level species richness.

16
Understory Species

Abundance and

Frequency

At the tract level, similar patterns also were evident (fig. 5C): 94 of 98 (jack-knife

estimates 111 of 114) total species were found in MT and 47 species were in ML (

jack-knife estimate = 56). MT averaged more species per plot and more species

per stand and was significantly more diverse than ML (table 3).

MT had greater average percentages of cover than ML for total understory, total tall

shrubs, dominant low shrubs (Gaultheria shallon), and common ferns (Polystichum

munitum and Pteridium aquilinum). For the ferns, among-plot, within-stand variation

was higher in MT than in ML; similarly, among-stand within-tract variability was higher

in MT than ML. For Gaultheria, among-plot within-stand variation was similar in both

tracts, and the among-stand within-tract variation was much greater in MT than in ML.

In general, cover of Gaultheria was substantially higher in MT than in ML. For moss

cover, among-plot and among-stand variabilities were similar in the two tracts, with

total moss cover being much higher in the ML stands than in MT (fig. 6, A-F).

17
Table 4-Block-wise one-way ANOVA of percentage of cover for major understory

species or groupsa

The two ferns, Polystichum munitum and Pteridium aquilinum, were much more abundant in MT than in ML as was total understory cover; mosses were more abundant in

ML. For each of these four variables, blocks differed significantly between but not within

tracts. Although Gaultheria was about twice as abundant in MT blocks as in ML blocks,

the small number of stands and high interstand variability precluded detection of a

significant difference between blocks for this species (table 4). Between-tract

differences in abundance of Gaultheria, Polystichum, Pteridium, total moss cover, and

total understory cover were highly significant (table 5).

Of the 94 vascular plant species found in MT, 51 were not found in ML. Of those 51

species, 33 were native species and 18 were nonnative. The 18 nonnative species

included 1 tall shrub, 2 low shrubs, 13 herbaceous species, and 2 grass species (

nonnative species are indicated by lower-case acronyms in table 5). At least one

nonnative species was found in 44 of 120 plots in the MT tract. Mycelis muralis was

the most frequently encountered nonnative plant species (found in 22 percent of MT

plots), followed by Senecio jacobaea (8 percent) and flex aquifolium (7 percent).

Though many of the non-native species were found infrequently in MT, collectively

they accounted for 35 percent of the 51 species not found in ML. Of the 33 native

species found only in MT, 9 were tall shrubs or understory trees, 1 was a low shrub,

18 were herbaceous, 2 were ferns, and 3 were graminoids (table 5).

Of the 47 vascular plant species found in ML, 4 were not found in MT. All four were

native; two were understory shrubs or trees (Taxus brevifolia and Thuja plicata) and two

were herbaceous species (Disporum hookeri and Maianthemum stellatum). Only one

nonnative species, Ilex aquifolium, was encountered in ML and was found on only one

plot. Of the 43 species found in both tracts, 21 were significantly more abundant and

23 were significantly more frequently encountered in MT than in ML. Two

18
Table 5-Percentage of cover and percentage of frequency of occurrence of

vascular plant species and cover of mosses and vascular plant groups in forests

managed with legacies versus managed with thinning in the Puget Trough, 1991-

92

19
Table 5-Percentage of cover and percentage of frequency of occurrence of

vascular plant species and cover of mosses and vascular plant groups in forests

managed with legacies versus managed with thinning in the Puget Trouqh, 1991-

92 (continued)

20
Table 5-Percentage of cover and percentage of frequency of occurrence of

vascular plant species and cover of mosses and vascular plant groups in forests

managed with legacies versus managed with thinning in the Puget Trough, 1991-

92 (continued)

21
Ranked Abundance

and Species Indicator

Values

Of the commonly encountered tall shrubs and understory tree species, all except Corylus

cornuta var. californica and Vaccinium parviflorum were significantly more frequent and

abundant in MT than in ML. Corylus was encountered at similar frequencies and

abundances in both tracts. Vaccinium parviflorum was found at similar cover in both

tracts but at significantly higher frequency in ML. Of the low shrubs, all except Mahonia

nervosa were found more frequently and at higher cover in MT. Mahonia nervosa was

found at about the same frequency and abundance in both tracts. Except for Linnaea

borealis, all trailing shrub and vine species were significantly more frequent and

abundant in MT than in ML. In particular, Rubus ursinus and Symphoricarpos hesperius

were much more frequent and abundant in MT than in ML; their combined cover in MT

was nearly seven times that in ML, summing to over 7 percent cover in MT. The

frequency and abundance of Linnaea borealis was similar in the two tracts.

Herbaceous species accounted for a large portion of the increased species richness and

abundance in the MT tract; herbaceous cover totaled nearly 9 percent in MT but <2

percent in ML. Thirty-one of 51 herbaceous species were found only in MT; 2 of 51 only

in ML. Of the 18 herbaceous species found in both tracts, 10 were significantly more

abundant in MT; only Goodyera oblongifolia was significantly more abundant in ML. Of

the 31 herbaceous species found only in MT, 18 were native and 13 were nonnative.

Nonnative herbaceous species were primarily members of the family Asteraceae and

included species of such widespread and weedy genera as Cirsium, Crepis,

Hypochaeris, Lapsana, Mycelis, Senecio, Sonchus, and Taraxacum (table 5).

The two fern species, Polystichum munitum and Pteridium aquilinum, were common (>60

percent frequency) in both tracts but were much more abundant in MT. Their combined

cover averaged 25 percent in MT but <5 percent in ML. More graminoids occurred in

MT (7 taxa) than in ML (2 taxa), and the two species common to both tracts (Bromus

vulgaris and Luzula parviflora) were significantly more frequent and more abundant in MT

than in ML (table 5).

The two tracts were qualitatively similar in species composition with 9 of the 10 most

abundant understory species found in both tracts. Vaccinium parviflorum, 8th most

abundant in ML, was 13th in MT and was found in similar abundance in both tracts.

Symphoricarpos hesperius, 9th in MT, was 11th in ML but was nearly three times as

abundant in MT than ML. Of the 13 vascular plant species in MT and the 9 in ML with

>40 percent frequency, 8 species were in common (Corylus cornuta var. californica,

Gaultheria shallon, Mahonia nervosa, Pteridium aquilinum, Rubus ursinus, Polystichum

munitum, Trientalis latifolia, and Vancouveria hexandra). Gaultheria shallon was the most

abundant understory species in both tracts but was nearly twice as abundant in MT than

in ML. In ML, the combined cover of the 10 most abundant species was 31 percent and

constituted 93 percent of total understory cover, whereas in MT the 10 most abundant

species totaled 74 percent cover and accounted for 84 percent of total understory (table

6).

Many common species had high indicator value in one or both tracts. Understory

species characteristic of both tracts were Gaultheria shallon, Mahonia nervosa, Corylus

cornuta var. californica, and Vaccinium parviflorum. Polystichum munitum, Pteridium

aquilinum, and Rubus ursinus had the highest indicator values of any species and

reached their IV-max in the MT tract. Species with IV-maxima that were

22
Table 6-Ranked abundance (by absolute cover) for top 10 species in forests

managed with legacies versus managed with thinning in the Puget Trough, 1991-

92

significant (P< 0.01) and >20 percent were all found in the MT tract and included

mostly native but one nonnative (Mycelis muralis) species. One native species,

Goodyera oblongifolia (a shade-tolerant orchid [Klinka and others 1989]), attained its

IV-max in the ML tract. Though its IV-max was only 12 percent, this value was significant (P < 0.05) (table 7).

We calculated the first three Bray-Curtis ordination axes by using the full data set of

240 plots and 98 species from both tracts. The three axes accounted for 45.3 percent

of the variance (18.9 percent, 20.8 percent, and 5.6 percent, respectively, for axes 1, 2,

and 3). The two tracts exhibited substantially different data swarms when graphed

against axes 1 and 2 (fig. 7A) and axes 2 and 3 (fig. 7B), with the two blocks in each

tract clustering similarly.

Of the most common species, abundance of Gaultheria shallon and Polystichum

munitum were most highly correlated with axis 1; Gaultheria was negatively correlated

and Polystichum was positively correlated. No other frequently encountered species

was strongly correlated with axis 1 (table 8). Thus, we interpreted axis 1 to represent

Bray-Curtis

Ordination

t

23
Table 7-Indicator values for selected vascular plant species in forests managed

with legacies versus managed with thinning in the Puget Trough. 1991-92

a gradient of increasing available soil moisture from left to right. Our interpretation

was strengthened by other plant species concentrating in their expected locations

along such a gradient. Moist-site indicators, such as Athyrium filix-femina, Blechnum

spicant, and Sambucus racemosa (Franklin and Dyrness 1973, Klinka and others

1989, Pojar and MacKinnon 1994), were most frequent and achieved high abundances only at the right or moist end of axis 1 and were not found in plots at the left

or dry end of the axis. Dry site or water-shedding-site species, such as Amelanchier

alnifolia, Holodiscus discolor, and Rosa gymnocarpa (Klinka and others 1989), were

most frequent and most abundant at the dry end of axis 1. The range of variation

along axis 1 for the MT tract was much greater than for the ML tract, with ML plots

concentrating from the middle or mesic portion of the illustrated range to the dry end (

fig. 7A).

Axis 2 further separated ML from MT. Most ML plots clustered toward the upper end

of axis 2 where these plots also had minimal variation along axis 1. Though most

individual species correlated poorly with axis 2, the widespread species Gaultheria

shallon, Polystichum munitum, and Pteridium aquilinum were negatively correlated with

axis 2. Total vascular plant understory cover was negatively correlated with axis

24
Figure 7-Bray-Curtis ordination of the managed-with-thinning (MT) and managed-with-legacies (ML) tract: and

blocks along axes 1 and 2 (A) and along axes 2 and 3 (B). The Star and Stellar blocks are in the ML tract; the Farley

and Hill blocks are in the MT tract. Data are from summer 1992.

2, as was the summed cover of Gaultheria shallon, Polystichum munitum, Pteridium

aquilinum, and Rubus ursinus (the four most abundant species in MT). Plot-level tree

density and moss cover were positively correlated with axis 2 (table 8). We interpreted

axis 2 to represent a gradient of increasing tree density with associated increasing

overstory canopy cover and decreasing understory--a competitive exclusion axis.

21
Table 8-Frequency of occurrence of selected vascular plant species; correlations

(r) between species abundance or other variables with the 3 Bray-Curtis ordination

axesa for all plots in forests managed with legacies and managed with thinning in

the Puget Trough, 1991-92

Certain plant species did, however, cluster strongly or exclusively within the distal

portion of axis 2. In particular, Goodyera oblongifolia was nearly restricted to the

closed-canopy conditions of the ML tract at the top of axis 2 and Corallorhiza maculata

was most frequent there-both species are mycotrophs. Other widespread species, such

as Mahonia nervosa, Trientalis latifolia, Trillium ovatum, Vaccinium parvifolium, and Viola

sempervirens, were distributed evenly along axis 2.

Variation along axis 3 was associated with a positive and significant correlation

with the abundance of Pteridium aquilinum (fig. 7B, table 8). The abundances of

Symphoricarpos hesperius and Vancouveria hexandra also were correlated positively

and significantly with ordination axis 3 and achieved their maxima in the distal portion

26
Figure 8-Bivariate plots of percentage of cover of Gaultheria shallon (GASH), Polystichum munitum (POMU),

and Pteridium aquilinum (PTAQ), and of the value of the Shannon-Weiner diversity index (H") from forests

managed with thinning (MT) and managed with legacies (ML). The graphs below and to the left of the

diagonal cells (+) are from the MT tract. Those above and to the right (x) are from the ML tract. Data are

from summer 1992.

of axis 3. Combined abundance of these three species was positively and significantly

correlated with axis 3. The plots with high abundances of these three rhizomatous

species were all located in MT and did not contain any obligate mycotrophic members of

the families Pyrolaceae, Monotropaceae, and Orchidaceae (Cronquist 1981) (e.g.,

Chimaphila umbellata, Corallorhiza maculata, Goodyera oblongifolia, Listera caurina,

Monotropa uniflora, and Pyrola asarifolia). Total understory cover was positively correlated

with axis 3 and the number of trees/ha was negatively correlated with axis 3. We

interpreted axis 3 to represent a gradient of increasing past ground disturbance

associated with the repeated logging and yarding in MT and suggest that these plots are

a legacy of previous thinning-related disturbances.

Particularly in MT, there were three understory conditions characterized by high abundance of Gaultheria shallon, Polystichum munitum, or Pteridium aquilinum. These taxa

were by far the most abundant understory species in the MT tract, and our ordination

suggests these three species attained their optima where light was not limiting and under

dry, moist, or previously disturbed conditions, respectively. High abundance of any one

of these three species also was associated with reduced plot-level diversity (H'). In the

ML tract, similar patterns were apparent but less pronounced owing to the low cover of

vascular vegetation (fig. 8).

27
Discussion

Limitations

Our study is retrospective and quasi-experimental (Cook and Campbell 1979). We

have treatments and measured outcomes but lack random treatment assignment

necessary to infer treatment-induced causality. In addition, our comparisons are confounded because the study sites differed in ways other than the treatments of interest.

Thus, we strove to separate treatment effects from those due to other differences.

Besides the treatments (management strategies), we recognize two potentially

confounding influences on the plant community.

First, the two tracts differed slightly in measured site quality, age, and soil substrate.

Because stand density may influence tree height-based site-quality estimates where

substrate and climate are otherwise comparable (King 1966), we attribute observed

differences in site index primarily to variation in tree density. Age as a determinant of

tree size accounted for <40 percent of the observed difference in tree d.b.h.; we attribute the remaining differences to effects of tree density management. We do not feel

that age difference alone is a major component of current between-tract plant community differentiation. Because of the high permeability, low-moderate water capacity, and

surface-layer droughtiness characteristic of the glacially derived sandy and gravelly

soils in the study area (Pringle 1990), we also do not regard variation in substrate as a

major cause of between-tract differentiation.

Second, we lacked complete histories of fire intensity and frequency and data on

operational (spatial, temporal, and logger variation) differences during previous harvests. We recognize that Puget Lowland Douglas-fir forests have burned frequently in

the past, but there was no evidence that either tract had burned since stand initiation,

and we do not attribute current between-tract differentiation to fire-related causes---

the differences we observed are of the sort we would expect as a result of management. We also were aware that differences in implementation of treatments at the time

of timber harvest would have been inevitable, thus we chose to contrast block- with

tract-level management effects. However, our systematic sampling template allowed us

to characterize plot-, stand-, and block-sources of variability, and thus to evaluate

differences likely due to management strategy.

Having chosen stands on the basis of their homogeneity in areas of contiguous forest,

and having used a systematic sampling template, it is unlikely that bias would have

been introduced by oversampling or undersampling of canopy gaps. At 47° N. latitude

and with heights-to-live-crown of 20 m, maximum mid-day summer insolation to the

forest floor occurs where canopy openings are >9 m south, therefore confounding one-toone relations between the understory and the canopy immediately overhead. Thus,

though our overstory and understory plots were 5 m apart, we nonetheless chose to

present plot-level correlations between ordination axes (based on understory data) and

tree density (based on overstory data) while also recognizing the potential weakness of

this association.

Our analyses were exploratory rather than hypothesis testing. We characterized

patterns in our data set, but caution must be used in extrapolating our results beyond

our study area, though most species from our study sites are common species

throughout western Oregon and Washington and our findings pertaining to the

dominant understory species Gaultheria shallon, Polystichum munitum, and Pteridium

aquilinum were consistent with previous research. Thus, our study has furthered

understanding of the ecology of managed stands by providing a rather unique

28
Community

Composition and

Structure

opportunity to examine influences of past forest management on (1) plant community

structure and composition, (2) the autecology of some native species, and (3) the

spread of exotic species and of native species favored by management. These factors,

in turn, suggest how forests such as MT and ML may continue to develop after

additional management.

Of the macroscopic species and species groups in conifer-dominated forests of the

Pacific Northwest, vascular plants contain a large portion of total species diversity. In

conifer forests of western Washington and Oregon, the understory contains, or has the

potential to contain, most of the vascular plant diversity while also supporting much of

the faunistic diversity (Carey and others 1996b, 1999b; Franklin and Dyrness 1973;

Halpern and Spies 1995). The extremely diverse mycorrhizal fungal community (

Kendrick 1992, Molina and others 1992) is chiefly associated with the Douglas-fir

overstory (Carey and others 1999b; Trappe 1962, 1977), whereas most understory

species are associated with the much less diverse vesicular arbuscular or endomycorrhizal fungi (Kendrick 1992, Malloch and others 1980, Perry and others 1990, Trappe

1987). Our study of two forested tracts of about the same age demonstrates some of

the effects forest management can have on plant composition and structure in the

space of only 55 to 70 years. Through its effects on forest plant communities, management also can affect lichen (Neitlich and McCune 1997), mycorrhizal fungi (Colgan

1997, Colgan and others 1999), avian (Carey and others 1991; Chambers and McComb

1997; Hansen and others 1995; Huff and others 1991; B. Haveri and A. Carey2), and

mammal (Carey 1995; Carey and Johnson 1995; Carey and others 1999a; Corn and

Bury 1991; S. Wilson and A. Carey3) communities as well, thus confirming the

importance of evaluating plant community responses to forest management.

We consider current plant community structure and composition in ML and MT to be a

reflection of management-induced vertical partitioning of solar energy absorption (and

associated forest floor disturbances) and differences in legacy retention. In ML, insolation is primarily intercepted by the dense tree canopy. Because of the dense ML

canopy, there was minimal structural and vertical vegetative development below the tree

crowns. The extremely high diversity of ectomycorrhizal symbionts in these Douglas-fir

forests (Carey and others 1999b, Colgan 1997, Colgan and others 1999, Trappe 1977)

ensures that photosynthetically fixed carbon, in addition to contributing directly to tree

girth, height, and foliage growth, also contributes to mycelial and sporocarpic growth.

Because of the relatively depauperate angiosperm community in ML, vertebrate primary

consumers should be skewed toward mycophagy (rather than herbivory or granivory) as

we have observed, although the conifer-seed specialist, Tamiasciurus douglasii, was

common in both ML and MT (Carey and others 1999b).

In ML, retention of some legacy trees, snags, and CWD during the previous harvest and

subsequent lack of active management are now associated with a high density of smalldiameter trees and a relatively sparse and species-poor understory vascular plant

community. Competitive-exclusion forests such as ML often provide unfavorable

conditions to most wildlife while also supporting the lowest understory vascular plant

2 Haveri, B.; Carey, A. Manuscript in preparation. Authors may be

contacted at the Forestry Sciences Laboratory, Olympia, WA.

3 Wilson, S.; Carey, A. Manuscript in preparation. Authors may be

contacted at the Forestry Sciences Laboratory, Olympia, WA.

29
diversity (Brown 1985, Carey 1995, Carey and Johnson 1995, Carey and others 1999b,

Ruggiero and others 1991), though remnant old trees and snags in ML and other

similar forests can provide important habitat for sciurids (Carey 1995, Carey and others

1997) and other old-growth-associated organisms (Neitlich and McCune 1997). The

reduced plant species richness and abundance in the ML tract understory were as

expected following a lengthy competitive-exclusion phase, as was the high cover of

forest floor mosses (Alaback 1982, Halpern and Spies 1995, Long 1977, Oliver and

Larson 1996, Schoonmaker and McKee 1988, Turner and Long 1974). In such densely

stocked stands, the sparse understory will probably persist until widespread changes

occur in the now minimally differentiated canopy. A gradual increase in understory

diversity and abundance may not occur until 100 or even 200 years of age without

intervention (Carey and others 1999a, Franklin and others 1981). Thus, nativedominated, well-developed, botanically diverse understory stages (Carey and others

1996a, Oliver and Larson 1996) may not develop for a century or more following

ecosystem initiation, especially in forests that lack shade-tolerant tree species, as did

ML and MT (Carey and others 1999b).

Compared to ML, the MT tract contained fewer, but larger, trees and a greater diversity

of tree sizes. Below the more open MT canopy was a diverse and abundant low

vascular understory that was frequently dominated by one or more of the common and

widespread species, Gaultheria shallon, Polystichum munitum, or Pteridium aquilinum.

In our study area, as throughout much of the range of coastal Douglas-fir forests (

Franklin and Dyrness 1973, Halpern 1989, Halpern and Spies 1995, Klinka and others

1989), these three species typically reach their optima under dry, moist, or previously

disturbed conditions, respectively. Optimal conditions for each of the three species was

most likely to be found in the MT tract, where high abundance of any one of the three

was typically associated with low plot-level diversity (H'). The MT flora also contained

substantial numbers of nonnative species that persist or continually establish under that

type of management. The prevalence of exotic species in the MT flora was in marked

contrast to ML, and substantially increased the overall MT species richness--more than

one-third of the species unique to MT were exotics.

Though species richness and abundance for all vascular plant groups were greater in

MT than in ML, both tracts lacked appreciable vertical differentiation (foliage height

diversity). The general absence of shade-tolerant conifers or deciduous tree species

contributed to the minimal structural development in both tracts making it unlikely

these stands will develop high structural diversification without underplanting (Carey

and others 1999b). Shade-tolerant tree species are found in surrounding areas and

were present in small numbers in stands formerly occupying these sites, suggesting

clearcut harvesting followed by burning and heavy natural regeneration may have

contributed to producing these Douglas-fir monocultures.

The combination of widespread and evenly applied thinning activities and removal of

CWD in MT was associated with an increase in low shrubs and ferns, primarily the

strongly rhizomatous species Gaultheria shallon and Pteridium aquilinum, as well as

Polystichum munitum. In addition to increasing light to the forest floor, canopy opening

during thinning would be associated with localized increases in evapotranspiration,

precipitation, and disturbance. We hypothesize that thinning, such as has taken place

in MT, may promote development of three broad site types that are otherwise rare in

densely stocked, unmanaged forests. In our study area, such site types were typified

30
Autecological

Considerations

by high abundance of (1) Gaultheria shallon on well-drained sites or where evapotranspiration was elevated, (2) Polystichum munitum where soils were poorly drained or able

to store precipitation not intercepted by the canopy, and (3) Pteridium aquilinum where

soils were most disturbed during thinning activities. Through the use of alternative

methods, such as variable-density thinning (Carey and others 1999b), we believe

silviculture could promote a much wider range of vegetation site types (Carey and others

1999a) than are currently found in stands such as ML and MT.

Though the tall shrub-understory tree layer was significantly more abundant in MT than

in ML, many constituents of that layer, such as the shade-intolerant and relatively shortlived Alnus rubra, Populus balsaminifera, and Salix scouleriana (Burns and Honkala 1990)

, probably became established shortly after logging in the 1920s and are not expected to

persist or reestablish under the current canopy. As such, their contribution to plant

diversity will diminish as the stand ages, though they will then become short-lived snags

and cavity trees for primary and secondary cavity dwellers (Carey and others 1997). The

more shade-tolerant tall shrubs such as Corylus cornuta var. californica have not

increased significantly in response to the MT thinnings, possibly due to damage during

repeated thinning activities or to poor seedling survival below the strongly GaultheriaPolystichum-Pteridium dominated understory.

Goodyera oblongifolia and Vaccinium parvifolium were more frequent in ML than in MT.

Goodyera is often found associated with well-decayed woody debris or among mosses in

decayed litter, and V. parvifolium is often found in decaying wood and on stumps and

logs (Klinka and others 1989, Pojar and MacKinnon 1994), conditions that were more

frequent in ML. Two common and widespread species, Corylus cornuta var. californica

and Mahonia nervosa, were found at similar abundance and frequency in the two tracts.

Corylus is ectomycorrhizal (Agerer 1987-95, Molina and others 1992), and so C. cornuta

may benefit from shared ectomycorrhizal symbionts with Douglas-fir as has been

demonstrated with Douglas-fir and Betula papyrifera (Simard and others 1997). This

association could partially explain the abundance of C. cornuta beneath the mostly

closed ML canopy and suggests that retained Corylus could provide ectomycorrhizal

inoculum to Douglas-fir after timber harvest as has been suggested for other hardwoods

(Amaranthus and Perry 1994, Borchers and Perry 1990, Jones and others 1997, Smith

and others 1995). Alternatively, the similarity in Corylus abundance in ML and MT could

have resulted from (1) comparable establishment in the two tracts after the previous

harvests and ability to persist under variable light conditions for the subsequent 55 to 70

years or (2) mediation by small-scale variation in organic and mineral soils. Nonetheless,

because of the potential for shared ectomycorrhizae between Corylus and Douglas-fir,

further research into the ecology of Corylus is warranted.

Unlike the dominant low shrub Gaultheria shallon, abundance of Mahonia nervosa did not

differ between tracts. Though both species are rhizomatous, evergreen, perennial

shrubs, Mahonia probably has much less potential for clonal spread (Huffman and

Tappeiner 1997). Because of greater recruitment into an apparently wider range of

acceptable seedling microsites for the larger-seeded Mahonia than for Gaultheria (

Huffman and Tappeiner 1997, USDA 1974), Mahonia may be able to increase more by

seedlings after further forest management in ML than in MT owing to less

31
Globalization and

Homogenization of

the Understory

competition from Gaultheria and other understory species, though its subsequent clonal

growth would be slower than that of Gaultheria (Bailey and Tappeiner 1998, Huffman

and Tappeiner 1997). In MT, however, additional management could result in

Gaultheria becoming even more widespread and dominant through its strong clonal

growth. After further increase, Gaultheria and other clonal shrubs persist for many

years, even into old growth (Bailey and Tappeiner 1998, Huffman and others 1994)

providing an enduring legacy, but perhaps an unwanted homogeneity, in some

intensively managed stands. In contrast, stem-exclusion conditions as in ML may be

useful for suppressing those species favored by human-caused disturbance regimes

that differ substantially than from historic disturbance regimes (Hobbes and Huenneke

1992, Roberts and Gilliam 1995). For example, the ML tract was almost totally lacking

in nonnative plant species--Ilex aquifolium was the only introduced species observed

there-and the clonal natives Gaultheria shallon and Pteridium aquilinum were much less

abundant there than in MT. Stem-exclusion forests also may encourage seedling

rather than vegetative reproduction after subsequent management (Bailey and

Tappeiner 1998, Tappeiner and Zasada 1993) and may provide better habitat for some

shade-tolerant or other minor species such as mycotrophs.

In the Puget Trough and in the vicinity of the MT and ML tracts, there are many wellestablished exotic species (e.g., Agrostis capillaris, Crataegus monogyna, Cytisus

scoparius, Digitalis purpurea, Geranium robertianum, Hedera helix, Holcus lanatus,

Hypochaeris radicatus, flex aquifolium, Leucanthemum vulgare, Polygonum cuspidatum,

Rubus discolor, R. laciniatus) (Carey and others 1999b; personal observation of the

senior author). Many of these species are members of the "pan-temperate ruderal

flora"'--a relatively few highly invasive species attaining global distribution and

dominance (Hobbs and Humphries 1995). In addition, an expanding fraction of newly

introduced species in the rapidly changing Pacific Northwest flora are trees and shrubs

(Toney and others 1998). Because time lags are common between initial introduction

and noticeable impacts (United States Congress, Office of Technology Assessment

1993), and because woody and herbaceous exotic species have become unwanted

components of forested ecosystems elsewhere (Walker and Vitousek 1991, Wiser and

others 1998, Woods 1993), it is important to consider how forest management

practices could affect establishment of these potentially deleterious species. Nonnative

plant species on the nearby Olympic Peninsula increased from 40 in 1900 to 143 in

1936 to 333, or nearly 25 percent of the 1,452 reported plant taxa (Buckingham and

others 1995) in 1995, demonstrating the potential for rapid and extensive

transformation of a nearby bioregional flora. Though the effects of a rapidly expanding

exotic species presence on Pacific Northwest forest ecosystem structure and function

remain largely unknown, the differences we observed between ML and MT lead us to

believe that in contrast to the essentially fixed native species pool, the exotic species

presence will continue to increase with subsequent management entries.

Additional forest management can be expected to be accompanied by an influx of

exotic species proportional to the amount of long-lasting ground-disturbing activity on

the landscape and to the abundance of invasive nonnative species in the surrounding

flora-such species find a ready avenue of dispersal where road density is high, as it

often is in managed landscapes (Forman and Alexander 1998, Perry 1998). The

4Update as PNW-GTR-488 goes to press: Heckman, C.W. 1999.

The encroachment of exotic herbaceous plants into the Olympic

National Forest. Northwest Science. 73: 264-276.

32
ecological effects of the rapid (about 100 years) expansion of the exotic flora into Pacific

Northwest forests are difficult to predict, but the combined effects of a high rate of exotic

species introductions (Toney and others 1998) and further management and other forms

of disturbance across the landscape will surely increase the nonnative component in the

vascular flora and continue to homogenize ecological communities (Soule 1990)

regardless of current forest conditions, while also increasing threats to the native flora

and fauna from alien species (Wilcove and others 1998).

Native species also may be favored by forest management. Many of the species with

high indicator value in the MT forest are characteristic of disturbed or cutover land or

open-canopy Douglas-fir forests (e.g., Galium aparine, Gaultheria shallon, Mycelis muralis,

Polystichum munitum, Pteridium aquilinum, Rosa gymnocarpa, Rubus ursinus, Sambucus

racemosa, and Symphoricarpos hesperius [Klinka and others 1989, Pojar and

MacKinnon 1994]). In forests such as MT where these common species are already

abundant, additional systematic human-caused disturbances can be expected to

promote their continued expansion, which in turn may impede forest and shade-tolerant

shrub regeneration because of competition (Huffman and others 1994) or changes in

the humus form and associated mycorrhizal community (Ponge and others 1998).

Rapid expansion of already-common clonal species also could reduce the chance for

obligate mycotrophs or other minor or specialized species to find suitable regeneration

niches (Grubb 1977). Members of the mycotrophic genera Chimaphila, Corallorhiza,

Goodyera, Listera, Monotropa, and Pyrola or closely related species may be extirpated

for 40 years or more following clearcutting and burning of old growth (Halpern and

Spies 1995, Schoonmaker and McKee 1988)-a period of time that usually coincides

with major expansion of Gaultheria shallon, Polystichum munitum, and Pteridium

aquilinum (Halpern 1989, Long 1977, Schoonmaker and McKee 1988). Our work also

suggests that high abundances of these three species are associated with low plot-level

species diversity, similarly to what other studies have shown (Long 1977, Schoonmaker

and McKee 1988). Because infrequently occurring herbaceous, fern, and graminoid

species constituted a substantial portion of species richness in MT and ML, further

expansion of the three dominant species could be accompanied by a reduction in native

understory diversity. In general, however, there is a lack of research on the autecology

of minor native understory species or that relates effects of management (including

spread of nonnative species and dominance by clonal native ferns and shrubs) to the

distribution and abundance of native understory species. Recent demographic research

has shown, however, that clearcutting and site preparation can reduce seedling

recruitment of the common Trillium ovatum and result in localized extirpations (Jules

1998). Further research will be necessary to determine management effects on other

species in other areas.

Though the MT and ML stands certainly do not represent the potential range of variability in structure or biodiversity that could exist (Spies 1997), even among forests of

this age, they nonetheless show some of the differences that can accrue during only

one rotation cycle. We view the between-tract differences in tree size and density,

legacy retention, exotic species presence, and understory composition and abundance

as representing only part of a continuum of forest expression. We also suggest that

the range of potential forest conditions and native-dominated vegetation site types

could be even greater and more rapidly developed with active, adaptive, site33
Management

Implications

specific, and intentional management (Carey 1998, 2000), with deliberate efforts to

reduce forest-floor disturbance, and through the use of variable-density thinning to

augment site-type variation in the understory vegetation (Carey and others 1999a,

1999b).

The patterns of vegetation structure and diversity we observed in ML and MT are as

would be expected under such management and stocking scenarios, and suggest

hypotheses that can be tested by experiments that focus on manipulation of factors

crucial to the future development of late-seral conditions in such stands. Site-specific

knowledge such as we have gained about the two tracts allows refinement of

hypotheses and of expected responses to management activities within each forest.

Among processes that are alterable through active management (and as such allow

testing of hypotheses) are crown-class differentiation, decadence, canopy stratification,

gap formation, understory development, and development of habitat breadth (Carey

and others 1999a). Current conditions in ML and MT have very different implications for

future management of the two tracts and for forest-specific hypotheses concerning

these factors.

In MT, past management has reduced CWD and snags to a low level while increasing

both the diameter of the retained trees and their size-class diversity. The use of sizeclass diversity and crown volume magnitude and variability as surrogates for amount of

crown differentiation indicates that the process of crown differentiation within the

current cohort of trees is more advanced in MT than in ML. However, because of the

near-absence of legacy trees and snags in MT, crown differentiation must take place

entirely among the current cohort of about 230 trees per hectare. Because CWD is

depauperate in MT, these trees are the source of future forest-floor woody debris as

well as snags. Fifty trees averaging 50 cm in diameter and 40 m tall would be necessary to create 5 percent cover on the ground-removal of 22 percent of the current stand

solely for woody debris augmentation would probably be unacceptable in most

pragmatic management scenarios. Some habitat attributes associated with late-seral

form and function may require >10 percent cover of CWD (Carey and Johnson 1995,

Carey and others 1999a), thus management for increased decadence in MT is

constrained by lack of woody debris and by tree densities that resulted from past

management solely for optimal tree growth and periodic financial return.

Management legacies in MT also include high cover of rapidly growing and long-lived

clonal shrubs and ferns such as Gaultheria shallon and Pteridium aquilinum and a welldeveloped exotic flora. The abundant MT understory and lack of CWD may hinder

development of the shade-tolerant conifers and midstory that are currently lacking.

Ground-disturbing activities associated with future thinning in MT may aggravate this

condition, although a well-developed shade-tolerant strata could begin to reduce dominance of understory species with high light requirements, while also favoring other

shade-tolerant taxa. This process of expanding the amount of canopy stratification and

spatial partitioning should result in a greatly expanded multidimensional niche space,

even if absolute vascular plant species richness does not increase.

Previous thinnings in MT have presumably obscured incipient gap formation processes--again legacies of past management complicate formulation of future management

scenarios. Where natural gap-forming processes such as Phellinus weirii (laminated

root rot) pockets could have begun to create patchiness in a forest of this

34
age (Thies and Sturrock 1995), thinning entries would have removed dead or dying

trees that could serve as patches of decadence, crown differentiation (through lateral

spread into open areas), stratification (through hardwood or root-rot resistant species

recruitment into the patch), and variation in understory composition and structure.

In ML, however, lack of active density management and retention of legacy trees,

snags, and CWD imply different future management alternatives. The use of size--

class diversity and crown volume as surrogates for crown differentiation suggests

a greater crown uniformity within the current cohort in ML. However, because of the

presence of some snags and legacy trees in ML (the contribution of which to crown

differentiation was not quantified because previous-stand trees were excluded from the

dominant tree sampling in the prism plots) and because of the much greater density of

current cohort trees, there are more opportunities for multiple-entry manipulations to

increase levels of crown-class differentiation, although relatively low live-crown ratios and

crown volumes suggest conservative expectations.

Opportunities for decadence management are also greater in ML than MT. ML enters the

next round of management with substantial CWD and snags. In addition, the current

cohort is experiencing widespread suppression mortality resulting in numerous 15- to

25-cm diameter trees becoming woody debris on the forest floor. For trees averaging 35

cm diameter and 34 m tall, 83 of the 620 per hectare could be felled to augment woody

debris cover by an additional 5 percent-removal of that many trees for CWD

enhancement is more likely to be acceptable to managers. Thus future management for

decadence in ML will benefit from previous legacy retention and from the high tree

density in the current stand.

Management legacies in ML also include a poorly developed understory, a lack of

exotics, and a near-total absence of shade-tolerant conifers and midstory constituents.

The depauperate ML understory may mean that shade-tolerant conifers (planted or

natural), midstory species, and other understory species will more rapidly establish after

future management entries (because of a wider range of regeneration niches [Grubb

1977] on the more open forest floor, more abundant nurse logs [Christie and Mack

1984], and with less competition from Gaultheria shallon, Pteridium aquilinum, and

Polystichum munitum). Ground-disturbing activities associated with future thinning can

be expected to result in a larger proportional increase in weedy plant species

abundance in ML than in MT, at least in the short term, though we also expect a rapid

increase in seedling establishment and clonal expansion by Mahonia nervosa, Gaultheria

shallon, Pteridium aquilinum, Polystichum munitum, Rubus ursinus, Vaccinium

parvifolium, Corylus cornuta, and other native species frequently found in or near ML. In

ML as in MT, increasing the amount of canopy stratification and spatial partitioning by

the plant community should result in a greatly expanded multidimensional niche space.

In ML we expect a proportionately greater increase in vascular plant richness and

diversity after future management than in MT, though again, the absolute numbers are

probably less important than the process of niche expansion.

A lack of previous thinnings in ML has allowed the expression of incipient gap formation processes--the ML tract contained a number of gaps created by Phellinus weirii (

laminated root rot) infection (Carey and others 1999b). Thus, natural patch-forming

processes have begun in ML that otherwise would be obscured by repeated thinning.

35
Where such patches occur in a forest of this age, they provide opportunities to capitalize on preexisting areas of decadence, early crown differentiation, stratification, and

variation in understory composition and structure. Naturally occurring patches also

suggest that silvicultural management of tree density should emphasize variable relative densities on 0.2- to 0.5-ha scale to reduce homogeneity in the understory and the

overstory and to stimulate the development of a wide variety of vegetation site types (

Carey and others 1999a).

If forested communities are richest in species adapted to the most commonly occurring

patch type (Denslow 1980, Roberts and Gilliam 1995), past management endeavors (

especially "broad-brush," extensively applied techniques such as clearcutting, burning,

single-species planting, subsequent density control for optimal tree growth, and short

rotations) may represent both a departure from historical disturbance regimes and a

shift to larger, more homogeneous, and more frequently and predictably created patch

types. Widespread use of such extensive management tools creates a new and

different selective filter on the landscape (Perry 1998), and it will increase abundance of

species most able to establish and reproduce in such managed environments, just as

we observed with invasives and abundant clonal species in MT. If natural forests serve

as legacy-perpetuating systems wherein the evolutionary memory of genes, cells,

species, and communities is preserved and reproduced, then a major challenge of

future silvicultural management will be to retain as much of that memory as possible

while minimizing the effects of exotics and of novel or unexpected human-related

selective filters.

Management-induced disturbances are now the most common type of disturbance

in many Pacific Northwest forests and have initial and lasting effects that differ from

historical disturbance patterns. Many managed, Douglas-fir forests in the Pacific

Northwest differ along continua defined by (1) amount of coarse woody debris, snags,

and previous-stand trees retained after harvest and (2) initial and subsequent tree

density. These two factors also typically are associated with continuous variation in

ground-disturbance intensity (e.g., harvest-related disturbances and site preparation).

Past management that resulted in forests definable along such continua also brought

about variability in the operational factors (Spomer 1972) affecting species distributional

patterns. Our ordination implies that availability of water and light for understory plants

(axes 1 and 2, fig. 7a) are the major operational factors affected by such management

and thus affecting distribution and abundance of the understory species. Variation along

the less precisely defined axis 3 (fig. 7b) may have resulted from a range of initial

disturbance intensity associated with management, differences in colonization by

appropriate species (particularly Pteridium aquilinum), and variation in subsequent

disturbance during thinning entries. The ML and MT tracts represent different positions

along the two management-created continua and thus along the three ordination axes.

Intentional manipulation of snag, woody debris, and legacy retention, and of variation in

tree density will thus directly influence the operational factors that determine patterns of

understory vascular plant species distribution and abundance.

The MT and ML tracts present different conditions that we view as having developed

primarily in response to two commonly applied strategies of management for wood only.

The two tracts now provide distinct opportunities for hypothesis testing relative to

important factors in the development of late seral forests such as crown-class

differentiation, decadence, crown stratification, gap formation, and understory

36
Acknowledgments

Metric

Equivalents

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development. Conditions in the two tracts are expected to affect forest development in

different but quite predictable, ways after future entries. Associated with future forest

management are major scientific challenges that include (1) ascertaining how best to

ensure continuity of evolutionary legacy-perpetuating systems in managed forests, (2)

assessing the use of management to foster development of habitat for late-seral

associated species and reduce spread of exotic species, and (3) determining effects of

the increasing numbers of exotics on site productivity and ecological function of

managed forests. A complete randomized block experiment is now in progress in the

MT and ML tracts that uses the same experimental template reported herein to begin

addressing aspects of these three challenges. It incorporates variable-density thinning,

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tests of hypotheses concerning enhancement of species and patch-type diversity, and

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and intentional management (Carey and others 1999b).

This research was funded by the Pacific Northwest Research Station, USDA Forest

Service; the Environmental and Natural Resources Division, Department of Army, Fort

Lewis; and the USDA National Research Initiative. We thank the U.S. Department of

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Denise Liguiri, Yana Valachovic, and Stacey Ball. David Ford, Andrew Gray, Charles

Halpern, Connie Harrington, Charles Heckman, Jan Henderson, and Tom Spies provided valuable comments on earlier drafts of the manuscript.

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