Introduction

The establishment of conifer plantations in California is often an arduous and

continuing task. Almost all new plantations will be invaded by competing plant

species in the form of hardwoods, woody shrubs, grasses, and forbs.1 The onset of

competition is often rapid and its intensity formidable.2 The composition of the

plant communities affects the magnitude of competition in young conifer

plantations and has been shown to influence conifer seedling survival and growth

in a negative manner.3,4,5 In order to ensure acceptable growth, and indeed the

survival of the conifer seedlings in these plantations, this competition needs to be

controlled.

Release is the common control method used in plantations in California. The

timing of the release treatment is an important component in the release program.

Numerous research findings indicate that in order for planted conifers to develop

at or near the potential of the site, the conifer seedlings must be released from

competing vegetation early in the life of the plantation. “Early” usually is defined

as the first or second year after outplanting.6,7,8

Despite the need for early release, this activity often is postponed until the

third or fourth growing season. Lack of adequate budgets and shrinking resources

are the most common causes of these postponements. Is this delayed release

efficient and cost effective? Is releasing several years in a row later in the life of a

plantation as effective as releasing for the first 1 or 2 years after plantation

establishment? Release after 4 years is generally not cost-effective because the

competing vegetation has become large and well established with well developed

root systems.2

To increase knowledge on the ecology of several widespread plant species

and on a plant community that is commonplace in California and Oregon,

information was collected on the growth dynamics and competitive relationships

of vegetation reoccupying a timber-harvested site in northern California. Planted

ponderosa pine (Pinus ponderosa Dougl. ex Laws. var. ponderosa) seedlings were

part of the plant community. As part of the USDA Forest Service’s National

Administrative Study on alternative release methods,9 data were gathered from

1985-1995 on vegetation developing naturally in an untreated control and on

vegetation manipulated deliberately by grubbing to create different time spans.

The time spans, or more specifically vegetation recovery times after disturbance

by site preparation and release, are the treatments.

This paper reports the results of testing whether there are significant

differences in plant community development and recovery between two release

treatment schedules 4, 7, and 10 years after manipulation.

Fiddler, Gary O.;

McDonald, Philip M.

1999. Treatment duration

and time since disturbance

affect vegetation development

in a young ponderosa pine

plantation. Res. Note PSWRN-424. Albany, CA:

Pacific Southwest Research

Station, Forest Service,

U.S. Department of

Agriculture: 8 p.

The effect of early and

delayed release treatments

(designated as “Treat first

3 years” and “Treat second

3 years,” respectively) on

diameter, height, and foliar

cover of ponderosa pine

seedlings, and density,

foliar cover, and height

of competing vegetation

was evaluated in a

young northern California plantation. Manual

grubbing created vegetation

recovery times (the period

of time from the last release

treatment until the end of

the study) that lasted 4 to

10 years. Duration and

timing of the grubbing

operations constituted the

treatments. Development

of species other than the

planted pines was

evaluated directly for

density, cover, and height,

and indirectly for effect on

pine survival and growth.

By 1995 greenleaf manzanita had higher values

for density, cover, and

height in the treatments

allowing the longest

recovery times. Survival of

the ponderosa pines over

all treatments after 1

growing season was 99

percent and after 10

seasons was 96 percent.

Both release treatments

Treatment Duration and Time Since

Disturbance Affect Vegetation

Development in a Young Ponderosa

Pine Plantation

Gary O. Fiddler Philip M. McDonald

United States

Department

of Agriculture

Forest Service

Pacific Southwest

Research Station

http://www.psw.fs.fed.us

Research Note

PSW-RN-424

December 1999

(continued on page 2)
USDA Forest Service Research Note PSW-RN-424. 1999. 2

Methods

The study site was located on the Goosenest Ranger District of the Klamath

National Forest about 21 airline miles northeast of Weed, California. It was

harvested as part of the Trapper Springs Timber Sale in 1984. Before harvest, the

forest was predominately California white fir (Abies concolor var. lowiana [Gord.]

Lemm.), Shasta red fir (Abies magnifica A. Murr.), and ponderosa pine, with

occasional Sierra lodgepole pine (Pinus contorta var. murrayana [Grev. & Balf.]

Engelm.) and sugar pine (Pinus lambertiana Dougl.).10,11 Sierra chinquapin

(Castanopsis sempervirens [Kell.] Dudl.), and to a lesser extent greenleaf manzanita

(Arctostaphylos patula Greene) and snowbrush (Ceanothus velutinus Dougl. ex

Hook.), were well developed shrubs in small openings and on rocky outcrops.

The forest cover type is white fir.12 Vegetation in the study area has been placed

in the montane and subalpine region of the Sierra Nevada and Cascade Ranges.13

Site quality of the study area is medium-poor with height of dominant

California white fir averaging about 60 feet in 50 years.14 Soils are of the ScheldIller complex from colluvium, tuff, and igneous parent material. Texture is sandy

loam, drainage is excessive, and water holding capacity is low. Stones and

occasional aggregations of boulders are abundant throughout the area. The

elevation is about 6,200 feet, the slope ranges between 10 and 20 percent, and the

aspect is northwest. The study area is uniform with respect to slope, aspect, and

soil. Annual precipitation averages 25 inches, with most falling as snow.

Thunderstorms often provide critical moisture in the spring and through late

summer. Temperatures range from 20o

F to 80oF with an average annual

temperature of 43oF. The growing season is about 100 days.

The study site was prepared by a brushrake-equipped tractor. The windrows

of logging slash and brush were burned in the fall of 1984. Very little soil was

displaced, and the windrows could hardly be seen at the start of the study.

Ponderosa pine seedlings from a local seed source were raised at the Stone

Nursery in Medford, Oregon, and outplanted as 2-0 seedlings in April 1985.

Spacing was 8 by 8 feet. An excellent job of planting was done. Although little

precipitation fell on the study area for several months after planting, the pine

seedlings looked healthy at the end of the first growing season.

The plant community before the study began consisted mostly of species that

originated from sprouts. Chinquapin and, to a lesser extent, snowbrush and

greenleaf manzanita, were the most abundant sprouting species. A few manzanita

seedlings were the only shrub species from seed that were observed. Grasses

were almost absent from the study area, although a few plants were noticed in a

plantation nearby. Several species of forbs were present, but none were

particularly abundant.

Evidence of deer (Hemionus spp.) and black bear (Ursus americanus) was

noticed. Small rodents and numerous birds were seen.

The study area was located in a large, homogeneous area free of overstory

vegetation. The experimental design was completely randomized with a one-way

treatment structure. Three treatments, including the control, were each replicated

three times. A replicate (plot) consisted of about one-fifth of an acre containing

about 40 ponderosa pine seedlings surrounded by 2 to 3 rows of buffer (seedlings

receiving similar treatment). The study began in 1985 and ended in 1995.

Treatments and treatment dates were:

T-first-3: Treat first 3 years (1986-1988) and develop naturally the last 7

years.

T-second-3: Delay 3 years, treat the next 3 years (1989-1991), and develop

naturally for the last 4 years.

Control: Not treated after site preparation and develop naturally the

entire 10 years of the study.

resulted in greater conifer

diameter, height, and foliar

cover than in the control at

the end of the study.

Conifer values between the

two release treatments did

not differ significantly at

the end of the study.

Retrieval Terms: competing

vegetation, northern

California, ponderosa

pine seedlings, release

treatments, timing of

release

(continued from page 1)
USDA Forest Service Research Note PSW-RN-424. 1999. 3

Plots in the T-first-3 and the T-second-3 treatments were grubbed to a 5-foot

radius. Vegetation was grubbed below the root crown with hand tools. The work

was performed during the summer at a time when growth of the ponderosa pine

seedlings had stopped but that of the shrubs was continuing.

In each plot, 25 healthy-looking ponderosa pine seedlings, identified as having

good potential of becoming harvestable trees, were selected and tagged before

treatment began. Chlorotic and misshapen seedlings that would be removed in the

first precommercial thinning were excluded. On each of the sample seedlings,

stem diameter (measured at 12 inches above mean ground line) and height were

measured. Seedlings were measured annually for the first 7 years of the study with

a final measurement being made at the end of the study. The seedlings were

periodically checked for injury from animals and insects.

Sampling intensity for evaluating competing vegetation consisted of five

randomly selected subplots in each plot. Subplots were centered around

ponderosa pine seedlings and encompassed 1 milacre (0.001 acre). Vegetation

was evaluated for density, foliar cover (the sum of shadows that would be cast by

leaves and stems of individual species expressed as a percentage of the land

surface),15 and average dominant height (average of the three tallest stems

measured from mean ground line to bud). Competing vegetation was evaluated

annually for the first 7 years of the study followed by final measurements at the

end of the study. More specifically, number of plants in each subplot were

counted, foliar cover was visually estimated, and height was measured with a

graduated pole. The most abundant vegetation, greenleaf manzanita, was

analyzed separately. All other shrubs were combined and analyzed as a group.

This group was composed of Sierra chinquapin, snowbrush, and rabbit brush

(Chrysothamnus spp.). Grass was analyzed as a separate category of vegetation.

To test for treatment effects and significant differences among treatments,

one-way analysis of variance of treatment means (fixed effect model)16 and

Tukey tests were applied.17 Statistical significance in all tests was at a= 0.05. Data

were gathered from permanent plots measured each year, and for analysis of

means from repeated measurements, the data are not truly independent. The a

levels or type I errors given for various tests apply to each measurement period

and year separately.

To quantify plant diversity, all plant species were tallied on study plots

when the study began and when it ended.

Results

Ponderosa Pine

Mortality of ponderosa pine during the study period was low. Despite receiving

little moisture after planting in spring 1985, first year survival for the entire

study was 99 percent. The control and T-first-3 treatment each lost one seedling

during this first year. Mortality was attributed to drought. Overall survival of

ponderosa pine seedlings on all study plots in all treatments was 96 percent. At

the end of the study, percent survival by treatment was:

Treatment Percent

Control 95

T-first-3 97

T-second-3 97

Despite numerous sightings of deer in the study area, only minor amounts of

browsing to the ponderosa pine seedlings were detected. Gophers (Thomomys spp.)

killed one ponderosa pine seedling in 1988. The pine seedlings suffered winter

wind-burn during several years of the study, but overall growth did not seem to be

affected. During the winter of 1994, some of the branches of the larger pines were

broken or pulled out by snow. No terminals were damaged during this event.
USDA Forest Service Research Note PSW-RN-424. 1999. 4

Table 1—Stem diameter, height, and foliar cover of ponderosa pine seedlings, by

treatment, northern California, 1985-1995.

Treatment 1985 1989 1990 1991 1995

Diameter (inches)

Control - 0.63 a1 0.78 a 0.98 a 1.63 a

T-first-3 - 0.91 a 1.34 b 1.59 b 3.00 b

T-second-3 - 0.63 a 1.00 a 1.28 ab 2.80 b

Standard error 0.09 0.10 0.09 0.15

Height (ft)

Control 0.46 a 1.92 a 2.44 a 2.93 a 4.77 a

T-first-3 0.45 a 2.46 a 3.43 a 4.32 a 7.88 b

T-second-3 0.42 a 2.01 a 2.77 a 3.56 a 7.35 b

Standard error 0.02 0.23 0.31 0.34 0.45

Cover (ft2/acre)

Control - 1,400 a 1,800 a 2,067 a 2,733 a

T-first-3 - 2,800 b 4,333 b 5,867 b 14,200 b

T-second-3 - 1,800 ab 2,867 ab 3,600 ab 10,867 b

Standard error - 306 515 564 1,806

1For each year, treatment means followed by the same letter do not differ statistically at the 0.05

level.

Average height of the ponderosa pine seedlings increased steadily in all

treatments, and by the end of the study in 1995, it ranged from 7.9 feet in the Tfirst-3 treatment to 4.8 feet in the control (table 1). Statistically significant

differences among treatments for ponderosa pine stem height were not present

until fall 1995. Seedlings in the T-first-3 and T-second-3 treatments were

significantly taller than those in the control.

Stem diameter, measured at 12 inches above mean ground line, increased

throughout the study period and by fall 1995 ranged from 3.0 inches in the T-first3 treatment to 1.6 inches in the control. Significant differences among treatments

for ponderosa pine stem diameter were first found in 1990, when mean stem

diameter of pines in the T-first-3 treatment was significantly larger than

counterparts in the control or T-second-3 treatment (table 1). By fall 1991 conifer

stem diameter differed only between the T-first-3 treatment and the control. By the

end of the study in 1995, ponderosa pine seedlings in the T-first-3 and the Tsecond-3 treatments had the largest average stem diameters. These two treatments,

which did not differ from each other, differed significantly from the control.

The first significant difference in ponderosa pine parameters that could be

attributed to treatment effects occurred in foliar cover. By fall 1989, foliar cover of

the pine seedlings was significantly larger in the T-first-3 treatment than in the

control. This difference continued through 1995. By study end in fall 1995, foliar

cover of the ponderosa pine seedlings in the T-first-3 and T-second-3 treatments

differed significantly from that of counterparts in the control.

Greenleaf Manzanita

This evergreen, fast-growing shrub was the most abundant species in the control in

1986 and remained abundant throughout the study (table 2). Although a few

manzanita sprouts were noted, most of these plants originated from viable seeds in

the soil that germinated after site preparation. We also noted that manzanita was

browsed in 1988 and 1990, probably by deer, and that it suffered from winter windburn in 1990.

Average density of greenleaf manzanita in the control was highest in 1986

(80,533 plants per acre) and declined 25 percent to 60,267 plants per acre in 1995.

Foliar cover and height steadily increased from the beginning to the end of the

study. Foliar cover increased 82 times from 333 to 27,200 ft2 per acre and height 13

times from 0.15 to 2.0 feet tall in 1995.
USDA Forest Service Research Note PSW-RN-424. 1999. 5

Table 2—Density, cover, and height, with standard errors (SE), of greenleaf

manzanita, shrubs, and grass in the control, northern California, 1986-1995.

 Year Density SE Cover SE Height SE

 plants/acre ft2/acre ft

Greenleaf manzanita

1986 80,533 21,259 333 196 0.15 0.00

1989 67,467 12,643 9,267 1,714 0.71 0.04

1990 71,800 12,845 12,600 2,075 0.97 0.07

1991 53,800 8,234 17,133 2,281 1.20 0.10

1995 60,267 8,977 27,200 1,943 2.00 0.17

Change (pct) -25 huge huge

Shrubs

1986 3,733 1,416 600 570 0.19 0.05

1989 5,200 1,100 1,533 657 0.56 0.07

1990 7,066 1,603 1,867 615 0.58 0.09

1991 4,733 1,187 2,333 944 0.68 0.19

1995 4,067 1,203 1,733 732 1.24 0.23

Change (pct) +9 +189 +565

Grass

1986 2,000 792 200 121 0.21 0.05

1989 3,200 1,094 400 230 0.50 0.12

1990 5,867 1,675 467 214 0.44 0.13

1991 3,867 1,254 267 101 0.63 0.07

1995 8,333 4,423 67 86 0.48 0.17

Change (pct) +317 -67 +129

Among treatments, manzanita density in 1995 differed significantly between

the T-second-3 treatment and the control (table 3). Foliar cover and height were

significantly less in both T-first-3 and T-second-3 treatments than in the control.

Density, cover, and height values of greenleaf manzanita reflect the ability of

this species to rebound from disturbance. The longer the time since disturbance,

the higher the values. The control (10 years since disturbance) had 70 times more

foliar cover than did the most recent treatment (T-second-3, 4 years since

disturbance). Density and height followed the same pattern with the control

showing almost eight times more density and almost four times more height

than in the T-second-3 treatment. Without treatment, this species tended to

decrease in density but increase in foliar cover and height throughout the study.

Surviving plants were becoming taller and more robust. This development

meant that competition to the planted ponderosa pines increased.

Shrubs

Density of the combined shrubs was much less than that of the most

numerous shrub, greenleaf manzanita (table 2). This group of shrubs suffered

from winter wind-burn in 1988, 1989, and 1990. Grazing damage was noted on

snowbrush in 1990.

Density increased from 1986 (3,733 plants per acre) through 1990 (7,066

plants per acre) and then decreased to 4,067 plants per acre by the end of the study.

This represented an increase of 9 percent during the study. Foliar cover followed a

similar pattern, peaking in 1991 at 2,333 ft2 per acre and declining to 1,733 ft2 per

acre by 1995, which was an increase of 189 percent. Height increased throughout

the study from 0.20 feet in 1986 to 1.2 feet in 1995, which was a gain of 565 percent.

Among treatments in fall of 1995, average density of combined shrubs

ranged from 300 to 4,067 plants per acre, foliar cover from 200 to 1,733 ft2 per

acre, and height from 1.0 to 1.2 feet (table 3). Statistical differences were not

found among treatments for density, cover, or height. Similar to greenleaf

manzanita, this group of shrubs tended to decrease in density and increase in
USDA Forest Service Research Note PSW-RN-424. 1999. 6

height throughout the study, but unlike manzanita, foliar cover was decreasing

at the end of the study. Competition from the taller manzanita and the ponderosa

pine probably was negatively impacting this group.

Grass

Almost all plants in this category were from the genera Bromus or Achnatherum

(formerly Stipa).11 Almost absent from the study area at the start of the study,

density increased to 2,000 plants per acre by fall 1986 (table 2). And during the

study, density increased from 2,000 to 8,333 plants per acre for a gain of over 300

percent. Cover increased from 200 ft2 per acre in 1986 to 467 ft2 per acre in 1990. It

decreased from this amount to 67 ft2 per acre by the end of the study, which was

an overall decrease of 67 percent. Height increased from 0.2 feet in 1986 to 0.5 feet

in 1995 for gain of 129 percent. Damage from grazing was noted in 1990.

Among treatments in fall of 1995, average density of grass ranged from 5,533

plants per acre in the T-first-3 treatment to 8,333 plants per acre in the control

(table 3). Cover ranged from 67 to 200 ft2 per acre, and height was basically the

same in all treatments with a value slightly less than 0.5 feet. No statistical

differences were found among treatments for density, cover, or height.

Discussion

This study at Trapper Springs differs from most studies in the Forest Service’s

National Administrative Study on alternative release methods in that it is one of

five studies designed to document the recovery of vegetation after a series of

manipulations that were applied at different times and continued for various

durations. The hypothesis tested was that the amount of vegetation and the time

that it was present, relative to release, would affect various species in the

community, and in turn affect the conifer seedlings. The actual release methods—

grubbing in the case of Trapper Springs—accomplished the treatments. The

different timings and durations were the treatments.

Plantations in California need site preparation and release in order for the

planted conifer seedlings to survive in adequate numbers and grow at acceptable

rates to meet management objectives. Timing of this release is critical. Conifers

Table 3—Density, cover, and height of greenleaf manzanita, shrubs, and

grass after 10 growing seasons, by treatment, northern California.

Treatment Density Cover Height

plants/acre ft2/acre ft

Greenleaf manzanita

 Control 60,267 a1 27,200 a 2.00 a

 T-first three 25,867 ab 2,400 b 1.10 b

 T-second three 7,933 b 400 b 0.54 b

 Standard error 8,977 1,943 0.17

Shrubs

 Control 4,067 a 1,733 a 1.24 a

 T-first three 1,200 a 200 a 0.56 a

 T-second three 300 a 500 a 0.98 a

 Standard error 1,203 732 0.23

Grass

 Control 8,333 a 67 a 0.48 a

 T-first three 5,533 a 67 a 0.43 a

 T-second three 6,533 a 200 a 0.43 a

 Standard error 4,423 86 0.17

1For each species, treatment means in each column followed by the same letter do not

differ statistically at the 0.05 level.
USDA Forest Service Research Note PSW-RN-424. 1999. 7

released early (within the first 3 years after planting) are able to capture enough

site resources to become established and grow at rates near the potential of the

site. In this study, the T-first-3 and T-second-3 treatments were designed

specifically to test early versus delayed release for ponderosa pine.

Results from numerous studies have shown that early release is more cost

effective (including degree of vegetation response and cost of release treatment)

than delayed release.18,19 This study supports that finding. In 1990, 5 years after

release, ponderosa pine seedlings from the T-first-3 treatment had significantly

larger diameters than those seedlings in the T-second-3 treatment. In 1991

seedlings in the T-first-3 treatment, although still larger than those in the Tsecond-3 treatment, did not differ from them at the 0.5 percent level—a finding

that did not change for the next 4 years. Conifer height and foliar cover in the Tfirst-3 treatment were greater than their counterparts in the T-second-3 treatment

throughout the study, although not statistically. At the end of the study, conifer

diameter, height, and foliar cover in the two release treatments differed

significantly from those in the control. Thus, release enhanced growth of

ponderosa pine seedlings on this poor site, regardless of when initially

performed.

The fact that ponderosa pine seedlings can survive with severe competition

was demonstrated again at Trapper Springs. Survival of conifers in the control

was almost as high (95 versus 97 percent) as it was in the treated plots, despite

larger amounts of shrubs and grass. However, their ability to grow at the

potential of the site is questionable because of competition.

Cost of treatment is an important factor when evaluating the timing of

release. The longer the delay before release, the larger the competing plants and

their root systems. Field notes indicated that it took almost twice as long to

install the T-second-3 treatment as the T-first-3 treatment. It was much more

time consuming to remove the competing vegetation after four seasons of growth

(T-second-3) than during the first season of growth (T-first-3). Although early

and delayed release did not result in statistically different conifer growth, the

cost of early release made it a much more desirable treatment than delaying

release for 4 years. Economically, early release (T-first-3) was superior to delayed

release (T-second-3) in that it maximized conifer growth while being cheaper to

install.

Time since disturbance can be an important factor affecting the amount of

natural vegetation that develops in a given area. In this study, grubbing resulted

in plant community recovery times of 4 years in the T-second-3 treatment, 7

years in the T-first-3 treatment, and 10 years in the control. Greenleaf manzanita

had the highest density, cover, and height in the treatment allowing the longest

recovery time (control). Shrubs and grass also had the highest density values in

the control.

With the exception of greenleaf manzanita, time since disturbance did not

greatly affect the species composition of the plant community during the study.

The same species that were present initially are present at the end of the study,

and their overall density and development were similar. A few forbs, their

numbers too few to be statistically analyzed, were also present. Shrub densities

are declining with surviving plants becoming more robust. Greenleaf manzanita

will be the dominant shrub in the plant community. Although grass numbers

were at an all time high at the end of the study, grass cover has declined greatly

since 1986. Thus, the grasses will occupy a place in the community, but will be

relegated to a minor role.

The dominant species in the community of the future will be the planted

ponderosa pines. The early release treatments have allowed these conifers to

more fully occupy the site and to establish a young forest at an earlier age.
USDA Forest Service Research Note PSW-RN-424. 1999. 8

End Notes and References

1 Fiddler, Gary O.; McDonald, Philip M. 1990. Manual release contracting: production rates, costs,

and future. Western Journal of Applied Forestry 5(3):83-85.

2 McDonald, Philip M.; Fiddler, Gary O. 1997. Treatment duration and time since disturbance affect

vegetation development in a young California red fir plantation. Res. Paper PSW-RP-233.

Berkeley, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture;

14 p.

3 McDonald, Philip.; Anderson, Paula J.; Fiddler, Gary O. 1997. Vegetation in group-selection

openings: early trends. Res. Note PSW-RN-421. Berkeley, CA: Pacific Southwest Research Station,

Forest Service, U.S. Department of Agriculture; 8 p.

4 Gjertad, Dean H., ed. 1993. Proceedings of the international conference on forest vegetation

management; 1992 April 27-May 1; Auburn, AL. Auburn, AL: Auburn School of Forestry; 310 p.

5 Stewart, Ronald E.; Gross, Larry L.; Honkala, Barbara H. 1984. Effectiveness of competing vegetation

on forest trees: bibliography with abstracts. Gen. Tech. Rep. WO-43. Washington, DC: Forest

Service, U.S. Department of Agriculture [loose-leaf].

6 McDonald, Philip M. 1982. Adaptations of woody shrubs. In: Hobbs, S.D.; Helgerson, O.T., eds.

Proceedings of a workshop on reforestation of skeletal soils; 1981 November 17-19; Medford, OR.

Corvallis, OR: Forest Research Laboratory, Oregon State University; 21-29.

7 McDonald, Philip M.; Fiddler, Gary O. 1986. Release of Douglas-fir seedlings: growth and treatment

costs. Res. Paper PSW-182. Berkeley, CA: Pacific Southwest Forest and Range Experiment

Station, Forest Service, U.S. Department of Agriculture; 9 p.

8 Newton, Michael; Preest, David S. 1988. Growth and water relations of Douglas-fir seedlings under

different weed control regimes. Weed Science 36: 653-662.

9 Fiddler, Gary O.; McDonald, Philip M. 1984. Alternatives to herbicides in vegetation management:

a study. In: Proceedings of the 5th annual forest vegetation management conference; 1983

November 2-3; Sacramento, CA. Redding, CA: Forest Vegetation Management Conference; 115-

126.

10 Little, Elbert L. 1979. Checklist of United States trees. Agric. Handb. 541. Washington, DC: USDA

Forest Service; 375 p.

11 Hickman, James C., ed. 1993. The Jepson manual, higher plants of California. Berkeley: University

of California Press; 1400 p.

12 Gordon, Donald T. 1980. White fir. In: Eyre, F.H., ed. Forest cover types of the United States and

Canada. Washington, DC: Society of American Foresters; 92-93.

13 Rundel, Philip W.; Parsons, David J.; Gordon, Donald T. 1977. Montane and subalpine vegetation

of the Sierra Nevada and Cascade Ranges. In: Barbour, Michael G.; Major, Jack, eds. Terrestrial

vegetation of California. New York: John Riley and Sons; 559-599.

14 Biging, Greg S.; Wensel, Lee C. 1984. Site index for young-growth mixed conifers of northern

California. Res. Note 8. Berkeley: Department of Forestry and Resource Management, University

of California; 13 p.

15 Daubenmire, R.F. 1968. Plant comminities: a textbook on plant synecology. New York: Harper

and Row; 300 p.

16 Steel, Robert C.D.; Torrie, James H. 1980. Principles and procedures of statistics. 2nd ed.New York:

McGraw-Hill; 218-220.

17 SAS Institute, Inc. 1988. SAS procedures guide, release 6.03 edition. Cary, NC: SAS Institute, Inc.

18 McDonald, Philip M.; Oliver, William W. 1984. Woody shrubs retard growth of ponderosa pine

seedlings and saplings. In: Proceedings, 5th annual forest vegetation management conference;

1983 November 2-3; Sacramento, CA. Redding, CA: Forest Vegetation Management Conference;

65-89.

19 McDonald, Philip M.; Fiddler, Gary O. 1989. Competing vegetation in ponderosa pine plantations:

ecology and control. Gen. Tech. Rep. PSW-GTR-113. Berkeley, CA: Pacific Southwest Forest and

Range Experiment Station, Forest Service, U.S. Department of Agriculture; 26 p.
The Authors

Gary O. Fiddler and Philip M. McDonald are Silviculturist and Research

Forester, respectively, with the Pacific Southwest Research Station’s Western

Forests Ecology and Silviculture Program, Silviculture Laboratory, 2400

Washington Avenue, Redding, CA 96001.

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an equal employment

opportunity employer.

Federal Recycling Program

Printed on Recycled Paper