Control

Development of a Mixed Shrub–

Ponderosa Pine Community in a

Natural and Treated Condition

Philip M. McDonald Gary O. Fiddler

United States

Department

of Agriculture

Forest Service

Pacific Southwest

Research Station

Research Paper

PSW-RP-224-Web
Abstract

McDonald, Philip M.; Fiddler, Gary O. 1995. Development of a mixed shrub–ponderosa

pine community in a natural and treated condition. Res. Paper PSW-RP-224-Web.

Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of

Agriculture; 19 p.

On a medium site in northern California, a mostly shrub community was treated by

two manual release techniques and by two herbicides, to study its development in both a

natural (control) and treated condition. Survival and growth of planted ponderosa pine

seedlings were quantified for 8 to 11 years after initial treatment applications. Treatments

included manual release in a 4-foot radius around pine seedlings one, two, and three times;

grubbing the entire one-seventh acre plot two times; applying 2,4-D and Velpar herbicides

to the entire plot one time; and a control. Data are presented for the most abundant species

(greenleaf manzanita), second most abundant species (snowbrush), by the two species

combined, and by all 10 shrub species combined. At the end of the study in 1990, manzanita

was the most abundant species with 15,267 plants per acre, cover of 24,800 ft2, and height

of 5.4 feet. Ponderosa pine developed best in plots where the entire area was grubbed twice

(mean diameter at 12 inches above ground of 6.3 inches) and in the Velpar-treated plots

(mean height of 14.2 feet). The cost of grubbing the entire area twice was almost $1,700 per

acre. Applying Velpar, including cost of the the chemical, was about $100 per acre. Site

preparation without subsequent release led to a brushfield similar to that present before

the study began.

Retrieval Terms: cost, greenleaf manzanita, manual and chemical release, northern

California, ponderosa pine seedlings, shrub community

The Authors

Philip M. McDonald is a research forester assigned to the Station’s Vegetation

Management Research Unit, with headquarters at Redding, Calif. (2400 Washington Ave.,

Redding, CA 96001). Gary O. Fiddler is a silviculturist assigned to the Timber Resource

Planning Unit, Pacific Southwest Region, USDA Forest Service, with headquarters in San

Francisco, and stationed at Redding, Calif. (2400 Washington Ave., Redding, CA 96001).

Publisher:

Albany, California

Mailing address:

PO Box 245, Berkeley CA

94701-0245

510 559-6300

http://www.pswfs.gov

May 1995

Pacific Southwest Research Station

Forest Service

US Department of Agriculture
Development of a Mixed Shrub–Ponderosa

Pine Community in a Natural and Treated Condition

Philip M. McDonald Gary O. Fiddler

Contents:

In Brief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

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

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

Location and Site Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Study and Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Plant Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Greenleaf Manzanita . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Snowbrush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Greenleaf Manzanita and Snowbrush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

All Shrubs Combined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Ponderosa Pine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Plant Moisture Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Production and Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Discussion and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
In Brief

Conifer plantations characteristically occupy areas that have been severelydisturbed by wildfire or timber harvest, often where slash was windrowed

and usually burned, and where most remaining vegetation was uprooted and

removed. Soil surfaces in these areas often are bare and temporarily devoid of

most plants and animals. Available site resources, however, are high because

large quantities of organic material are incorporated into the soil through

harvesting and site preparation, and large amounts of moisture are absorbed by

this organic matter. Warm temperatures and added moisture cause a rapid

buildup of microorganisms that decompose organic material and liberate

nutrients. In this unstable and rapidly changing environment, plants of many

species often become abundant. Competition the first 5 years on productive sites

often is intense and early densities of some species cannot be sustained.

Consequently, the normal progression in the revegetation of disturbed areas is

toward increasingly smaller numbers of taller and wider plants—a condition

noted for shrubs in the untreated control in this study.

In treated plots, the amount of shrubs and other vegetation is a product of

the kind and number of treatments. In turn, the amount of growth by planted

ponderosa pine seedlings is a function of the amount of competing vegetation

that remains and its rate of recovery. Amount and competitive potential of new

vegetation also is important. Consequently, the treatment that best controls

competing vegetation soon after planting almost always is best for ponderosa

pine seedling growth. Early treatment denies competing plants the chance to

recover or to utilize the high levels of site resources typically present after

disturbance.

The four manual and two chemical treatments evaluated in this study created

a plant community dominated either by planted pines or greenleaf manzanita

and a few other shrubs. Forbs and grasses were virtually absent after 5 years. The

most effective treatments (applying Velpar in liquid form or grubbing the entire

area two times) created the pine community; the other treatments led to

communities of pines and shrubs. Doing nothing (no treatment after site

preparation) was rapidly leading to a plant community much like that before the

study began—essentially a brushfield—with the exception that planted pine

seedlings were present.

The most effective treatments with rapidly growing pines have high potential

to create a forest similar to that present before the man-caused fires 50–120 years

ago caused the brushfield. A forest probably is the more natural, stable, and

desired condition. Certainly, the forest has more opportunities to provide a

broad array of amenities and commodities needed by society than a brushfield.

In ecosystem management, however, various amounts of shrubs, which provide

berries, forage, and cover for wildlife, could be valuable. If mixes of pines and

shrubs are desired, the study treatments show how to attain them and the costs

of doing so. Costs ranged from $37 per acre for applying 2,4-D one time to almost

$1,700 per acre for grubbing the entire plot two times. Cost-effective treatments

that promote rapid growth of conifer seedlings have a place in ecosystem

management. A mature forest, for example, may be needed a century from now

in a particular area, and knowledge on how to get it established and growing at

the potential of the site will be useful.

McDonald, Philip M.; Fiddler,

Gary O. 1995. Development of a

mixed shrub–ponderosa pine

community in a natural and

treated condition. Res. Paper

PSW-RP-224. Albany, CA: Pacific

Southwest Research Station, Forest Service, U.S. Department of

Agriculture; 19 p.

Retrieval Terms: cost, greenleaf

manzanita, manual and chemical

release, northern California, ponderosa pine seedlings, shrub

community

USDA Forest Service Gen. Tech. Rep. PSW-GTR-157-Web. 1995. iii
The Forest Service, U.S. Department of Agriculture, is responsible for Federal

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• Protection and management of resources on 191 million acres of National Forest System

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to help protect and manage non-Federal forest and associated range and watershed

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to improve living conditions in rural areas

• Research on all aspects of forestry, rangeland management, and forest resources

utilization.

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Samoa, and the western Pacific.

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PSW-RP-224-WebResearch PaperResearch StationPacific SouthwestForest Serviceof AgricultureDepartmentUnited States

Community in a Natural and Treated ConditionDevelopment of a Mixed Shrub–Ponderosa Pine
USDA Forest Service Res. Paper PSW-RP-224-Web. 1995. 1

Introduction

Recent surveys indicate that Americans are vitally concerned about the use ofnatural resources and the environment. Most people regard the forest as a

valuable resource and want it to be present and to provide a full range of

amenities and commodities for future generations. To this end, increased

emphasis is being placed on ecosystems, and particularly on their sustainability.

This presupposes, probably with merit, that if the ecosystem is sustained, all the

organisms in it, from the smallest to the largest, will also be sustained.

As forest management shifts toward forest ecosystem management, the need

for knowledge on plant species and plant communities that develop naturally or

after a deliberate manipulation is increasingly important. Manipulation could be

for a variety of reasons: to create a future forest, to provide an economic crop, to

grow plants along with trees whose seeds would be critical to wildlife, or simply

to provide a broad base of species and age classes and thus be in position to

provide amenities and commodities that will be needed in the future.

In forestry, unfortunately, much knowledge has been generated on economic

species (the trees), but little is available on the uneconomic species, which often

are disparagingly referred to as “weeds.” However, as Ralph Waldo Emerson

observed in 1879: a weed is “a plant whose virtues have not yet been discovered.”

In 1994, Jack Ward Thomas, the chief of the Forest Service, acknowledged in his

first testimony to Congress that we have much to learn about ecosystems, and

that “we have no option but to move forward on the basis of what we know.”

Furthermore, “We must quickly adapt to new information and new

understanding as it becomes available” (Thomas 1994). Acquiring new

information on uneconomic species is necessary for implementing ecosystem

management.

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 bulldozed brushfield in northern California. Planted

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

part of the plant community. As part of a National Forest Service Administrative

Study on alternative release methods (Fiddler and McDonald 1984), data were

gathered from 1981–1990 on vegetation developing naturally in an untreated

control and on vegetation manipulated deliberately by several manual and

chemical treatments.

This paper reports the results of these treatments and denotes the density

and development of the two most abundant shrubs in the plantation both alone

and combined. It also quantifies the diameter and height of the pine seedlings

and shows when these parameters in each treatment differed statistically from

those in the control and other treatments. Treatment costs and biological

responses then are combined to show cost effectiveness—a measure that provides

managers with options suited to their particular goals, constraints, and budgets.
2 USDA Forest Service Res. Paper PSW-RP-224. 1995.

Methods

Location and Site Characteristics

The study area, located on the Hat Creek Ranger District of the Lassen National

Forest, is about 50 airline miles east of Redding, California. It also is located

about 1 mile west of Logan Lake, a shallow meadow-lined ephemeral lake that

historically has been a favorite camping place of hunters and stockmen. Fire

scars and District records indicate that fire has been frequent in the area,

sometimes originating near the lake. Fire probably was responsible for changing

the predominately ponderosa pine forest (similar to that nearby) to a 250-acre

brushfield with only an occasional pine. Before harvest of these remnant trees

and site preparation on about 90 acres, most of the area was occupied by a dense

mass of shrubs that averaged 10 feet tall and was nearly impenetrable to man or

beast (fig. 1). Scattered California white firs (Abies concolor var. lowiana [Gord.]

Lemm.) were slowly emerging from the shrubs.

After harvest in 1975 the woody shrubs and a little slash were pushed into

contoured windrows with a brushrake-equipped bulldozer and burned in fall

1976. Very little soil was windrowed, and after 3 or 4 years, the windrows were

barely visible.

Ponderosa pine seedlings from a local seed source were raised in the USDA

Forest Service Nursery at Placerville and outplanted as 1–0 seedlings in mid–

May 1977. Spacing was 8 by 10 feet. An excellent job of planting was done, and

survival in the fall of 1979, when we began the study, was about 80 percent.

The plant community when the study began consisted mostly of plants that

originated from dormant seeds in the soil and from windblown seeds. A few

large Arctostaphylos and Ceanothus burls escaped the site preparation and

contributed rapidly growing sprouts to the community as well. In addition to the

planted ponderosa pine seedlings, the most abundant vegetation in the plantation

was greenleaf manzanita (Arctostaphylos patula Greene) with snowbrush

(Ceanothus velutinus Dougl. ex Hook.) a distant second. Scattered individuals and

small aggregations of Sierra chinkapin (Castanopsis sempervirens [Kell.] Dudl.)

and snowberry (Symphoricarpos spp.) plus six other species of woody shrubs also

were present. Other vegetation included six species of forbs and two grasses,

none of which were abundant.

Figure 1—Part of the Logan Lake

study area in 1984. Note the mature

brushfield with scattered California

white firs in the background, the 8-

year-old shrub community typical of

the study area and treated plot in the

middleground, and tops of ponderosa

pine seedlings in the foreground.
USDA Forest Service Res. Paper PSW-RP-224. 1995. 3

Site quality of the study area is medium with height of dominant ponderosa

pines averaging about 55 feet in 50 years (Biging and Wensel 1984). The soil,

which is closely related to the Crozier series, was formed from andesitic parent

material. It has a cobbly loam texture and is about 3 feet deep. Stones, cobbles,

and large boulders are abundant throughout. The elevation is about 5,000 feet,

the slope is 15 percent, and the aspect is east. The area is remarkably uniform

with respect to slope, aspect, and soil. Precipitation averages about 36 inches per

year, with about 70 percent falling as snow. Temperatures range from 19 oF to 80 o

F with the average annual temperature being 43 oF. The growing season is 122

days.

Study and Design

The experimental design was completely randomized with one-way treatment

structure. Seven treatments, including the control, were each replicated three

times. A replicate (plot) consisted of about one-fifth acre on which were about 40

ponderosa pine seedlings surrounded by two to three rows of buffer (seedlings

receiving similar treatment). Treatments and treatment dates were:

Manual grub

4-foot radius one time, installed September 1979,

4-foot radius two times, September 1979, November 1982;

4-foot radius three times, September 1979, November 1982, June 1985;

Entire plot August 1980, July 1984;

Chemical spray

2,4-D, September 1982;

Velpar, March 1983;

Control

Treatments were not considered complete until repeat applications had been

performed.

The manual release treatments were designed to test the effect of removing

competing vegetation one or more times from a fixed radius around crop trees

and from the entire plot. Vegetation was grubbed below the root crown and

sheared if leaning into the 4–foot radius or into the plot. The chemical treatments

were the herbicides 2,4-D and Velpar, which for various reasons were not applied

until before the 1983 growing season.1 When the study began, 2,4-D was the

foliage-active herbicide most often applied to forest plantations in California,

and Velpar was a soil-active herbicide (possibly with some foliage activity) just

beginning to be used in conifer plantations in the state.

The 2,4-D was applied directly in ester form from a backpack apparatus and

a carbon-dioxide pressurized boom at the rate of 3 pounds acid equivalent per

acre. Nozzles on the boom were the same as those used in helicopter application,

hence rate of application and droplet size were similar. The boom, which covered

a 9-foot swath, was held about 12 inches above the shrubs and the spray was

directed downward. Trial runs with water determined the proper walking speed

needed to apply the correct amount of herbicide to each plot. The entire

seventh-acre plot and half the buffer were sprayed. Guides were used to ensure

even coverage and no overlap. Because the buds were hard, the likelihood of

spray damage was slight, and the seedlings were not covered. Spraying took

place in clear, calm weather between 7 and 9 a.m. PST.

1This paper neither recommends

the pesticide uses reported nor

implies that the pesticides have

been registered by the appropriate state governmental agencies.
4 USDA Forest Service Res. Paper PSW-RP-224. 1995.

Velpar L was applied to entire plots at a rate of 2 pounds acid equivalent per

acre from standard backpack sprayers. Care was taken to insure that spray

coverage was uniform without skips or overlaps. Substantial rain fell on May 22,

1983, which helped carry the chemical into the soil.

The cost of installing each treatment both initially and for retreating was

calculated from hourly records and the current wage—$8.74 per hour—the rate

for a WG-1 laborer, U.S. Department of Labor, as of June 1993.

Sampling

In each plot, 30 of the best ponderosa pine seedlings, identified as potential crop

trees, were given a metal tag that was moved several times during the study to

keep from strangling tagged branches. As their name implies, these were well

developed seedlings that had good potential of becoming harvestable trees. On

each of the sample seedlings, stem diameter and height were measured.

Diameters were recorded at 12 inches above mean groundline. The seedlings

were checked for injury from chemicals, animals, and insects.

Sampling intensity for evaluating other vegetation and foliar cover of pines

in the control was five randomly selected subplots in each plot. Subplots were

centered around ponderosa pine seedlings in all treatments. For plots where

the entire area was treated (manual, 2,4-D, Velpar, and control), a square 1

milacre (0.001 acre) frame was used. For radius plots, sampling took place only

in the treated area. Vegetation was measured 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 [Daubenmire 1968]), and average

dominant height (average of the three tallest stems measured from mean

ground line to bud).

Data were statistically analyzed as soon as possible after each measurement.

To test for treatment effects and significant differences among treatments,

one-way analysis of variance of treatment means (fixed model, Steel and Torrie

1980) and Tukey tests were applied (SAS Institute Inc. 1988). Statistical

significance in all tests was at α = 0.05. Data were gathered from permanent plots

measured each year, and where analyses of means from repeated measurements

are concerned, the data are not truly independent. The α levels or type I errors

given for various tests apply to each measurement and year separately.

To quantify plant diversity, all species were noted when the study began,

after 5 years, and at the end of the study.

Internal moisture stress of ponderosa pine, greenleaf manzanita, and

snowbrush seedlings was determined to help explain growth differences among

treatments that became apparent early in the study. Xylem sap tension was

measured with a Scholander pressure chamber (Scholander and others 1965)

on August 17, 1982, a typical cloudless day near the end of summer. At this

time of year, the seedlings were physiologically stressed because of high

moisture deficits.

Sampling of sap tension began well before dawn and continued through late

afternoon. Because of time and distance considerations, only one replication was

sampled. For this reason and because of the ever-changing amount of xylem sap

tension in the plant, no statistical analysis of differences among treatments was

attempted. Sampling intensity was three randomly selected seedlings of each

species in each treatment. Consequently, three twigs of each species were tested

in the pressure chamber at five measurement times throughout the day. Each

twig was placed in a sealed plastic bag. The elapsed time between cutting and

placement in the chamber did not exceed 3 minutes. Every third sample was

measured twice in the chamber as a check on technique and working order of the

equipment. “Plant moisture stress” in atmospheres is used in this paper to
USDA Forest Service Res. Paper PSW-RP-224. 1995.

5

express pressure chamber values (Waring and Cleary 1967) because it is positive,

direct reading, and most familiar to foresters and biologists.

Results

Plant Diversity

The plant community at the beginning of the study consisted of planted

ponderosa pine, 10 species of shrubs, 6 forbs, and 2 grasses. Numerous tall dead

stems of bull thistle (Cirsium vulgare [Savi] Ten.) and woolly mullein (Verbascum

thapsus L.) indicated that plants of these species had been abundant the third and

fourth years after site preparation but had then declined (fig. 2). After 5 years the

community consisted of the same species with three additional forbs. By the end

of the study in fall 1990, these same species were present. However, the forbs and

grasses were so few and small that one had to search to find a single plant. This

was true of some of the shrub species as well. The pines, greenleaf manzanita,

and snowbrush, plus occasional snowberry, chinkapin, and two species of

rabbitbrush (Chrysothamnus spp.), so dominated the plantation that plants of

other species were becoming both diminutive and scarce.

One forb species that became abundant for a short but critical time was

woolly nama (Nama Lobbii Gray), a low, spreading, shallow-rooted forb. It

becomes established in sunny, exposed environments and is intolerant of shade

(Nord and Leiser 1974). In a previous study, McDonald and Oliver (1984)

noticed that woolly nama invaded shortly after an effective treatment removed

almost all competing plants, quickly became dense, and excluded shrub

seedlings. In this study, it became present in the plots where the entire area was

grubbed twice. Many plants were noticed after the first grubbing, but it was not

until after the second in July 1984 that the species became abundant. Thus, it

probably excluded new shrub seedlings germinating from buried seeds and

those recently distributed from nearby plants by birds and animals for about 5

years. It disappeared as quickly as it appeared, probably because of shade from

pine crowns.

Greenleaf Manzanita

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

plantation. After site preparation, viable seeds in the soil germinated by the

thousands. Because greenleaf manzanita is a widespread and commonplace

competitor in many pine plantations in California and southern Oregon, several

relationships are presented for it. To show its recovery after site preparation,

density, foliar cover, and height are presented in the control for 1981–1990. To

show its response to the various treatments, density, cover, and height are

presented for 1990 or after 14 growing seasons.

Density of greenleaf manzanita in the control was highest in 1981 at 26,000

plants per acre and declined thereafter (table 1). Foliar cover and height steadily

increased from the beginning to the end of the study. From 1981 through 1990,

the number of manzanita plants in the control had decreased an average of 41

percent, but cover had increased by 174 percent and height by 157 percent.

By fall 1990, manzanita density was significantly higher in the control than in

any other treatment (table 2). Density also was significantly higher in plots

grubbed only once to a 4-foot radius than in radius plots treated three times, with

the entire area grubbed two times, or treated with Velpar one time. In the latter

two treatments, number of manzanita plants numbered 400 or less with virtually

no foliar cover and little height. These were observed to be newer, younger
6 USDA Forest Service Res. Paper PSW-RP-224. 1995.

Figure 2—(A) Numerous tall stems of woolly mullein and a few bull thistle (foreground) are present in the control in 1980, but in 1981

(B) few could be seen.

A

B
USDA Forest Service Res. Paper PSW-RP-224. 1995.

7

plants that had less time to grow and in many instances were dominated by taller

pines or shrubs along plot edges. The trend in foliar cover was generally similar

to that of density with large amounts of cover in the control and in plots grubbed

to a 4-foot radius one time. Cover of greenleaf manzanita was intermediate in

plots grubbed two and three times or treated with 2,4-D, and lowest where the

entire area was grubbed twice or sprayed with Velpar. Manzanita plants were

significantly shorter if the entire area was grubbed twice or sprayed with Velpar.

Snowbrush

Although snowbrush is a widespread and commonplace competitor in many

conifer plantations in California and southern Oregon, ecological information,

particularly density and development data, are needed for it (Conard and others

1985). Snowbrush is a hardy, fast-growing shrub with many adaptations (Hanes

1981, McDonald 1982) that make it a serious competitor to conifer seedlings in

plantations. Mature plants form large burls with dormant buds that sprout

vigorously if the aboveground plant parts are damaged. Moreover, the

deep-rooted burl is resistant to removal by site preparation (McDonald and

Fiddler 1989). The species also produces large and frequent seedcrops.

Competition with pine seedlings in the study area was in the form of scattered

root crown sprouts, seedlings from seeds in the soil, and seedlings from new

seeds carried into the area by birds and rodents.

In fall 1981, density of snowbrush plants in the control was more than 1,800

plants per acre with more than 5,500 ft

2

 of foliar cover and 2.1 feet of height (table

1). By fall 1990, density had decreased 21 percent and cover was 55 percent

lower, but height had increased by 57 percent.

By fall 1990, density of snowbrush was highest in plots treated with 2,4-D

followed by plants in the control (table 2). Density was lowest in plots where the

entire area was treated two times. For foliar cover, the only significant difference

was between plants treated with 2,4-D and all the other treatments. Snowbrush

developed best in 2,4-D plots, probably because manzanita cover had been

reduced substantially there. Snowbrush height did not differ significantly among

treatments.

Greenleaf Manzanita and Snowbrush

Because these two species often are found together, are evenly distributed

throughout the plantation, and frequently produce large seedcrops having high

value to wildlife, they are presented as a combination.

In the control, density of manzanita and snowbrush decreased 40 percent

from 1981 through 1990, and foliar cover and height increased 87 and 120 percent

respectively (table 1).

In treated plots, statistically significant differences among treatments for

density included: (a) plants were more numerous in the control than in any other

treatment; (b) plants in radius plots grubbed one and two times, and treated with

2,4-D were more abundant than in other treatments; and (c) plants in entire area

grubbed or Velpar plots were fewer than in other treatments (table 2). Statistically

significant differences among treatments for cover were quite similar to those for

density. For height, only plants in plots with the entire area grubbed twice or

treated with Velpar differed from all other treatments.

All Shrubs Combined

Most early seral vegetation in young plantations in California and Oregon almost

always consists of a mixture of shrubs, forbs, and grasses, sometimes with an

admixture of hardwoods. Rarely is this vegetation dominated by a single class of

plants (McDonald and Redosevich 1992). Thus it is of interest to ecologists to
8 USDA Forest Service Res. Paper PSW-RP-224. 1995.

have knowledge of a plant community dominated only by shrubs, albeit from

two regeneration strategies (Grime 1979). These were rapid aboveground

expansion by vegetatively produced sprouts and regeneration from a persistent

seed bank (dormant seeds in soil). Combining all shrubs also allows inclusion of

poorly distributed species such as snowberry and Sierra chinkapin, and fairly

rare species such as gooseberry (Ribes spp.) and silktassel (Garrya fremontii

Torrey).

In fall 1981, density of combined shrubs in the control was 28,600 plants per

acre with more than 14,700 ft2 of foliar cover and 1.8 feet of height (table 1). By fall

1990, density had decreased 38 percent, but cover and height had increased 85

and 106 percent, respectively (table 1).

Among treatments in fall 1990, the same relationships noted earlier were

generally present: for density, the treatment having the most shrubs was the

control; the least, Velpar. However, many combinations of treatments differed

from other combinations that had not occurred before (table 2). For foliar cover,

the relationships among treatments were similar to those for manzanita and

snowbrush combined. Only minor increases in cover were noted. This same

similarity also was reflected in the average dominant height of combined shrubs.

However, height was either less than or only slightly larger than greenleaf

Table 1—Density, cover, and height, with standard errors (SE), of greenleaf manzanita,

snowbrush, manzanita and snowbrush, and combined shrubs in the control, Lassen

National Forest, 1981-1990

Year Density SE Cover SE Height SE

---- Plants/acre ---- ------- Ft2/acre ------ ------- Ft ------

Greenleaf manzanita

1981 26,000 2,731 9,067 794 2.1 0.2

1984 16,800 1,825 19,800 1,112 4.0 0.3

1985 16,533 1,576 22,467 1,556 4.3 0.2

1990 15,267 1,092 24,800 1,610 5.4 0.3

Difference (pct) -41 +174 +157

Snowbrush

1981 1,867 370 5,533 1,288 2.1 0.2

1984 1,733 392 7,733 1,275 3.2 0.2

1985 1,533 110 7,400 1,099 3.3 0.2

1990 1,467 139 2,467 628 3.3 0.3

Difference (pct) -21 -55 +57

Manzanita and Snowbrush

1981 27,867 2,885 14,600 877 2.0 0.1

1984 18,533 1,943 27,533 772 3.6 0.2

1985 18,067 1,607 29,867 1,404 3.8 0.2

1990 16,733 1,148 27,267 1,915 4.4 0.3

Difference (pct) -40 +87 +120

Combined Shrubs

1981 28,600 3,245 14,733 1,042 1.8 0.1

1984 19,200 3,156 27,533 803 3.1 0.2

1985 19,400 2,070 29,933 1,491 3.2 0.2

1990 17,733 1,773 27,267 1,907 3.7 0.3

Difference (pct) -38 +85 +106
USDA Forest Service Res. Paper PSW-RP-224. 1995. 9

Table 2—Density, cover, and height of greenleaf manzanita, snowbrush, manzanita and snowbrush, and combined shrubs, Lassen National Forest, 1990

Treatment Density Cover Height

Plants/acre Ft2/acre Ft

Greenleaf manzanita

Manual

4-ft radius 1 time 6,585.b1 16,348.b 4.3.b

4-ft radius 2 times 2,368.ab 8,087.cd 4.1.b

4-ft radius 3 times 693.a 3,177.ac 4.6.b

Entire area 2 times 400.a 0.a 0.5.a

Chemical

2,4-D 1 time 4,600.ab 7,400.acd 4.4.b

Velpar 1 time 333.a 67 a 1.1.a

Control 15,267.c 24,800.e 5.4.b

Standard error 1,092. 1,610. 0.3.

.

Snowbrush

Manual

4-ft radius 1 time 924.bc1 2,080.a 3.7.a

4-ft radius 2 times 520.ab 1,098.a 3.7.a

4-ft radius 3 times 289.ab 578.a 2.8.a

Entire area 2 times 0.a 0.a —.

Chemical

2,4-D 1 time 1,600.d 5,133.b 3.1.a

Velpar 1 time2 0.a 0.a —.

Control 1,467.cd 2,467.a 3.3.a

Standard error 139. 628. 0.3.

Manzanita and Snowbrush

Manual

4-ft radius 1 time 7,510.b1 18,428.bc 4.0.b

4-ft radius 2 times 2,888.ab 9,185.ac 4.0.b

4-ft radius 3 times 982.ac 3,755.ad 4.2.b

Entire area 2 times 400.a 0.a 0.5.a

Chemical

2,4-D 1 time 6,200.bc 12,533.cd 3.7.b

Velpar 1 time2 333.a 67.a 1.1.a

Control 16,733.d 27,267.b 4.4.b

Standard error 1,148. 1,915. 0.3.

Combined Shrubs

Manual

4-ft radius 1 time 10,745.cd1 18,428.bc 3.2.b

4-ft radius 2 times 7,221.abc 9,416.cd 3.0.b

4-ft radius 3 times 4,448.abc 4,044.ad 3.0.b

Entire area 2 times 2,867.abc 0.a 0.6.a

Chemical

2,4-D 1 time 10,000.bd 12,600.cd 2.5.bc

Velpar 1 time2 467.a 67.a 1.0.ac

Control 17,733.d 27,267.b 3.7.b

Standard error 1,773. 1,907. 0.3.

1

For each species and combination, treatment means in each column followed

by the same letter do not differ statistically at the 0.05 level 2

No snowbrush present in plots before or during study
10 USDA Forest Service Res. Paper PSW-RP-224. 1995.

manzanita or snowbrush alone or combined—indicating the shorter stature of

the additional shrubs.

Ponderosa Pine

Death of ponderosa pine seedlings was less than one per treatment during the

study period. Despite the presence of numerous deer, and a few sightings of

rabbits and ground squirrels early in the study, damage to ponderosa pine

seedlings also was negligible.

Statistically significant differences among treatments for ponderosa pine

stem diameter, measured at 12 inches above mean ground line, first showed up

in fall 1985. Mean stem diameter of pines treated with 2,4-D was significantly

smaller than counterparts in plots that had been entirely grubbed two times

(table 3). In fall 1990, pine seedlings in entire-grubbed plots and those treated

with Velpar had the largest average diameter. These two treatments, which did

not differ from each other, differed from most other treatments (fig. 3). Significant

differences among treatments for stem height were not present until fall 1990.

Seedlings in plots treated with Velpar, entire area grubbed, and 4-foot radius

grubbed three times differed significantly from seedlings in the control, 2,4-D

plots, and those grubbed to a 4-foot radius one and two times (table 3).

The ratio of height to diameter helps explain the effect of treatment. In those

plots where the entire area was grubbed, for example, the height-diameter ratio

decreased from initial values and then held steady throughout the study period;

in the control, the ratio generally increased; and in the 4-foot radius treatments

the ratio decreased and then remained steady. The treatments with the lowest

values at the end of the study period were entire area grubbed and Velpar—

suggesting that stem diameter was expanding faster than stem height. The

Table 3—Stem diameter and height of ponderosa pine seedlings, by treatment, Lassen

National Forest, 1981-1990

Treatment 1981 1984 1985 1990

Diameter ------------------------------ inches ---------------------------------

Manual

4-ft radius 1 time 1.09.a1 2.21.a 2.52.ab 3.67.b

4-ft radius 2 times —. 2.31.a 2.65.ab 4.04.b

4-ft radius 3 times —. —. 2.70.ab 4.38.bc

Entire area 2 times —. 2.58.a 3.18.a 6.33.a

Chemical

2,4-D 1 time —. 1.61.a 1.89.b 3.20.b

Velpar 1 time —. 2.30.a 2.94.ab 6.08.ac

Control 0.97.a 1.91.a 2.14.ab 2.92.b

Standard error 0.13. 0.23. 0.26 0.35.

Height -------------------------------- feet -----------------------------------

Manual

4-ft radius 1 time 2.81.a 5.37.a 6.16.a 8.77.b

4-ft radius 2 times —. 5.37.a 6.23.a 9.86.b

4-ft radius 3 times —. —. 6.35.a 10.16.ab

Entire area 2 times —. 5.65.a 6.86.a 14.00.a

Chemical

2,4-D 1 time —. 4.07.a 4.81.a 8.34.b

Velpar 1 time —. 5.04.a 6.28.a 14.19.a

Control 2.74.a 5.21.a 5.90.a .66.b

Standard error 0.26. 0.50. 0.58. 0.84.

1

For each year, treatment means followed by the same letter do not differ significantly

at the 0.05 level.
USDA Forest Service Res. Paper PSW-RP-224. 1995. 11

Figure 3—Ponderosa pine seedlings

in the various treatments in August

1984: (top) entire plot grubbed two

times, (middle) 4-foot radius grubbed

two times, and (bottom) control.
12 USDA Forest Service Res. Paper PSW-RP-224. 1995.

highest ratios were in the control and in plots treated with 2,4-D—suggesting

that stem diameter was expanding slower than stem height.

Plant Moisture Stress

Predawn minimum moisture stress of greenleaf manzanita was lowest (6.4

atmospheres) in the grub entire area treatment, intermediate (6.5 atmospheres)

in the control, and highest (10.2 atmospheres) in plots grubbed to a 4-foot radius.

Maximum moisture stress followed a similar trend and was lowest in plots

treated by grubbing the entire area (17.0 atmospheres), intermediate in the

control (20.7 atmospheres), and highest in plants within the 4-foot radius (23.8

atmospheres) (fig. 4). Maximum stress was noted first (10:21 a.m. PST) in

manzanita plants in the control, next in plants within the 4-foot radius (1:35 p.m.

PST) and last in plants in which the entire plot was grubbed (2:17 p.m. PST).

Manzanita plants in plots where the entire area had been grubbed twice had to

endure less than half as long a period of high stress (more than 15 atmospheres)

than plants in the other two treatments.

For snowbrush plants, predawn minimum moisture stress was lowest (6.3

atmospheres) in the grub entire area treatment, intermediate (6.5 atmospheres)

in the control, and highest (8.0 atmospheres) in plots grubbed to a 4-foot radius.

Maximum moisture stress rankings were similar. From lowest to highest,

treatments were: entire area (16.3 atmospheres), control (17.8 atmospheres), and

4-foot radius (18.7 atmospheres) (fig. 4).

Predawn minimum moisture stress of ponderosa pine seedlings was lowest

(5.8 atmospheres) in the grub entire area treatment, intermediate (7.7

atmospheres) in the control, and highest (8.8 atmospheres) in plots grubbed to a

4-foot radius. The ranking of maximum moisture stress among treatments was

similar in the entire area (14.8 atmospheres), 4-foot radius (17.2 atmospheres),

Figure 4—Diurnal trend of internal

moisture stress on August 17, 1982

in (A) greenleaf manzanita plants, (B)

snowbrush plants, and (C) ponderosa

pine seedlings. Standard errors

applicable to differences between

treatments could not be estimated.
USDA Forest Service Res. Paper PSW-RP-224. 1995. 13
14 USDA Forest Service Res. Paper PSW-RP-224. 1995.

and control (17.7 atmospheres) (fig. 4). The onset of maximum stress occurred

first in pine seedlings in the control (10:12 a.m. PST), next in plants within the

4-foot radius (1:22 p.m. PST) and last in plants in which the entire plot was

grubbed (2:05 p.m. PST). Indeed, by being under 15 atmospheres of moisture

stress (Cleary 1970), pine seedlings in the plots grubbed entirely two times

probably achieved positive net photosynthesis the entire day while seedlings in

the control did so from the time the stomates first opened shortly after first light

(5:30 a.m. PST) until about 10:00 a.m. PST.

Production and Cost

Grubbing a 4-foot radius two or three times or grubbing the entire plot twice

proved expensive. Waiting 4 years before grubbing the entire plot the second

time allowed the shrubs to develop large rootstocks that were difficult and costly

to remove. Spraying chemicals was the least expensive as shown in the following

tabulation:

Treatment: Laborer Dollars/acre

hours/acre

Manual

4-ft radius, one time 24 210

4-ft radius, two times 59 516

4-ft radius, three times 99 865

entire plot, two times 194 1,696

Chemical

2,4-D, one time 3 27

Velpar L, one time 3 27

Costs are for labor only and do not include overhead or the cost of the chemical.

If the cost of the chemical was included, $10 should be added to the cost per acre

for 2,4-D and $75 for Velpar L.

Discussion and Conclusions

In spring 1977 or the first growing season after site preparation, visual evidence

and notes by Ranger District personnel indicated that revegetation began quickly.

Buried, dormant seeds of greenleaf manzanita, snowbrush, and other shrub

species germinated and produced many thousands of seedlings. A few remaining

but damaged plants of snowbrush, snowberry, and Sierra chinkapin produced

sprouts as well. Seeds of bull thistle and woolly mullein probably blew in on the

wind in the fall and settled in pockets in the soil or caught on minor protrusions

from it. Seedlings from these species and a few others formed the forb component

of the plant community. Some grass seeds also were carried into the study area,

probably by animals or the wind. A few tree seedlings, particularly of ponderosa

pine and California white fir, also struggled to become established.

During the 1978 and 1979 growing seasons, competition for light and growing

space intensified with inherently shorter species falling behind. Plants that could

not compete well for soil moisture, the limiting environmental factor, were

dying. Bull thistle (McDonald and Tappeiner 1986, Randall 1990) and woolly

mullein (Semenza and others 1978) are biennials that typically produce plants

that peak in density during the third and fourth growing seasons. At the end of
USDA Forest Service Res. Paper PSW-RP-224. 1995. 15

the fourth growing season in 1980, the forbs, grasses, naturally-seeded conifer

seedlings, and smaller shrubs were becoming short and scarce. They were too

few and too poorly distributed to be analyzed statistically. By the end of the

study, only 18 plant species had been found, causing this plantation to rank next

to last in terms of species diversity (range of 17–41 species) in our National

Administrative Study in young conifer plantations (McDonald and Fiddler 1993).

Analysis of the first full data set in 1981 showed that greenleaf manzanita

constituted more than 90 percent of total plant density and 62 percent of foliar

cover in the control. Density declined and then steadied at 86 percent. Foliar

cover of manzanita steadily increased to 91 percent of total shrub cover in 1990.

By 1984, average dominant height of manzanita was highest of any shrub in the

plantation—an advantage that it maintained throughout the study. Snowbrush

density ranged from 7 to 9 percent of total plant density in the control, but foliar

cover, which initially amounted to 38 percent of total cover, decreased to 25

percent in 1985 and to 9 percent in 1990. Thus, while the number of snowbrush

plants per acre remained almost constant, foliar cover decreased dramatically

during the 10-year study record, especially during the last 5 years.

After subtracting manzanita and snowbrush from combined shrubs, the

dynamics of the eight other shrub species in the control were discernible. Other

shrubs increased from 3 to 6 percent of total shrub density, but contributed

virtually nothing to foliar cover during the study period. That they were able to

increase their number during the time of rapid manzanita expansion suggests

that they have the capability to become established in small uncolonized areas

and in the shade, but not to develop well either horizontally or vertically.

In the heavy competition and reduced soil moisture availability of the control,

ponderosa pine seedlings typically sacrifice crown and needle development for

height growth to avoid being overtopped. Consequently, their form tended to be

tall with relatively narrow crowns. When foliar cover was expressed as a

percentage of each acre occupied by vegetation, the proportion at the end of the

study was manzanita, 57 percent; snowbrush, 6 percent; ponderosa pine, 28

percent; and rocks/bare ground, 9 percent.

On treated plots, we noted that shrub densities grubbed once early in the

study tended to be fairly high and then to decrease with time, probably as a

consequence of competition. If shrubs were treated later, as in the 2,4-D plots,

density, which initially was high, also tended to decline naturally. If plots were

treated two or three times, plant densities would build and then be reduced in a

cycle created by each treatment. Foliar cover and height showed only one

pattern—increase until treated, decline, and then increase again.

By fall 1990, the various treatments had all reduced the shrub component of

the plant community to some degree. Except for 2,4-D, treatments that affected

the entire plot area were more effective than grubbing to a 4-foot radius. From

previous studies, we have found that the minimum effective radius is 5 feet—

any lesser radius does not give the conifer seedlings time to develop the deep

and extensive root system necessary to secure adequate soil moisture for growth

at the potential of the site (Fiddler and McDonald 1984, McDonald and Fiddler

1989). Grubbing three times during a 6-year period significantly reduced

manzanita density and foliar cover compared to grubbing one time (table 2). But

multiple grubbing did not significantly improve pine growth over that in the

control.

For all shrubs combined, the chemical treatments provided a noteworthy

contrast in effectiveness. Basically, 2,4-D was ineffective and Velpar was very

effective. More than 21 times more shrubs were present in 1990 in 2,4-D plots

than in Velpar plots and they had 188 times more foliar cover.
16 USDA Forest Service Res. Paper PSW-RP-224. 1995.

The cost and production data gathered in this study reflect the interplay

between size of woody shrubs and the cost of eliminating most of them. Almost

all of the shrubs on study plots originated from buried, dormant seeds in the

spring after site preparation. Consequently, most shrubs were 3 years old when

the first 4-foot radius was grubbed in September 1979, 4 years old when the

entire plots were first grubbed in August 1980, and 6 years old when sprayed

with Velpar early in 1983. For manual release, grubbing to a 4-foot radius cost

more each time in spite of the fact that fewer shrubs were present in the

increasingly shaded (by the pines) subplots. However, more and more vegetation

along the sides of the grubbed area were leaning in and these required increased

clipping. For those plots where the entire area was grubbed two times, waiting

four growing seasons before performing the second round of treatment was

costly. The crew foreman noted that shrubs were 3 feet tall and rootstocks were

large and tough to grub. In contrast, spraying chemicals on 3-foot-tall shrubs was

much less arduous.

Purely from a vegetation management viewpoint, the best conifer release

treatment is one that effectively controls competing vegetation and does so in a

timely manner—right after the conifer seedlings are planted and in the process of

becoming established (McDonald and Oliver 1984, McDonald and Fiddler 1989).

The first 3 years are important; the first year is critical. Seedlings must be free to

vigorously expand their root systems in an ever-increasing volume of soil. In a

southern Oregon study for example, roots of Douglas–fir (Pseudotsuga menziesii

[Mirb.] Franco) seedlings were excavated and carefully measured relative to the

amount and position of roots from competing vegetation. Results showed that

the Douglas-fir roots expanded greatly in biomass and volume of soil occupied

in the near-absence of competing vegetation, but developed scarcely at all if

roots of competing species were present (Tesch and Hobbs 1989). In our study at

Logan Lake, no treatment was applied until after the competing vegetation (and

the planted seedlings) was at least 3 years old. Indeed, in one treatment (Velpar),

the competing shrubs were 6 years old. Delaying treatment for 2 or 3 years is too

long. Roots of competing plants not only become extensive and capable of

capturing most of the available soil moisture, but they also cause manual

grubbing to be much more expensive.

An ongoing objective of our National Administrative Study on vegetation

management is to determine the timing of treatment effectiveness. In one trial

with ponderosa pine in northern California, we were able to attain a statistically

significant difference in ponderosa pine seedling diameter relative to the control

two growing seasons after treatment of 4-year-old shrubs (McDonald and Fiddler

1990). This is the fastest that we have found statistical significance in our studies.

Seedling diameter almost always provides a significant difference before seedling

height. Although differences often are present, having a statistical difference at

the 5 percent level is much harder to attain. In general, the longer the time the

conifer seedling grows with a high level of competition, the longer it takes the

difference between an effective treatment and the control to become statistically

significant.

The interaction between level of competition and the effectiveness of the

treatment also is important. Two related but different interactions occur. The

first is caused by only moderate competition and the second by only a moderately

effective treatment. If the competing vegetation is developing at only a moderate

rate, the conifer seedlings in the control grow fairly well. Differences in conifer

seedling growth between these seedlings and faster growing seedlings in

effective treatments take a long time to become statistically significant. And, if

the competition is strong but the treatment is only moderately effective, an

interval of many years may be required before a statistically significant difference

becomes apparent.
USDA Forest Service Res. Paper PSW-RP-224. 1995. 17

In this study, the interval between time of planting and first significance,

which was for grubbing the entire area two times, occurred at 6 years for pine

diameter and at least 7 years for pine height. The initial application was applied

early enough, but the interval between the first and second applications was too

long. Consequently, the total treatment is best described as moderately effective

for strong competition.

“Replacement” vegetation, which becomes established after treatment, is of

particular interest to ecologists and vegetation management specialists. It may

contain some beneficial plants as in the case of woolly nama, or it may encourage

a detrimental species that would vector in a destructive animal like pocket

gophers (Thomomys spp.). Pocket gophers can destroy a young conifer plantation

(Crouch 1986). Woolly nama was beneficial because it appeared to offer little

competition to ponderosa pine seedlings, and it apparently interfered with the

establishment of highly competitive shrub species. Consequently, it probably

functioned as a biological control during a short but critical time after the

treatment was applied. The fact that it did not appear after the third grubbing in

the radius plots probably can be explained by the high amount of shade

contributed by the “wall” of shrubs surrounding the subplots and by the

expanding pine crowns. It did not become present on the bare ground in the

Velpar plots either, probably because of the residual chemical in the soil and

because of shade from tree crowns. Identifying beneficial replacement species,

learning their biology, and developing propagation techniques for future use is a

facet of vegetation management that has much promise.

Assuming that good site preparation fosters good pine seedling growth is

unrealistic. Data for combined shrubs in the control and relative growth rates

show this dramatically. In 1990, shrub density was more than 17,700 plants per

acre, 3.7 feet tall, with interwoven crowns that covered more than 62 percent of

the area so closely that they cast a solid shadow. Average pine height and

diameter in the most effective treatments (Velpar, grub entire area two times)

was 63 and 112 percent greater, respectively, than for counterparts in the control.

Although almost all pine seedlings are alive in the control, their future is

uncertain and probably precarious. Many have sparse crowns with only tufts of

short current-year needles. Although ponderosa pine seedlings are noted for

their capability to survive for two decades or more with high levels of

competition, the odds for decreased growth and survival are high (McDonald

and Oliver 1984).

Manual and chemical release created several vegetative assemblages, each

with different amounts of the principal species, and with different developmental

potentials. If the treatment was Velpar or grubbing the entire area two times, the

future plant community would likely consist of ponderosa pines with a few

scattered shrubs in the understory. The pines in these treatments should continue

to grow rapidly as sunlight and soil moisture are fully utilized. If no treatment

followed site preparation as in the control, the plant community of the future most

likely would be dominated by large numbers of vigorous shrubs and scattered

slow-growing pines. The other treatments would produce plant communities with

various mixes of shrubs and pines having variable growth rates.
18 USDA Forest Service Res. Paper PSW-RP-224. 1995.

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