Development of Vegetation in a 

Young Ponderosa Pine Plantation: 

Effect of Treatment Duration and 

Time Since Disturbance

Philip M. McDonald and Gary O. Fiddler

United States 

Department of 

Agriculture

Forest Service

Pacific Southwest 

Research Station

Research Paper

PSW-RP-251

June 2007
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Authors 

Philip M. McDonald is a research forester (retired), Pacific Southwest Research 

Station, Western Forest Management Research Unit, 3644 Avtech Parkway, 

Redding, CA 96002; e-mail: pmcdonald@fs.fed.us. Gary O. Fiddler is a 

silviculturist, Ecosystem Conservation Staff; San Francisco and stationed at the 

above address in Redding, CA; e-mail: gfiddler@ fs.fed.us.
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

Abstract

McDonald, Philip M.; Fiddler, Gary O. 2007. Development of vegetation in a 

young ponderosa pine plantation: effect of treatment duration and time since 

disturbance. Res. Pap. PSW-RP-251. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 23 p. 

The density and development of deerbrush (Ceanothus integerrimus Hook. 

& Arn.), other shrubs, forbs, graminoids, and ponderosa pine (Pinus ponderosa

Dougl. ex Laws. var. ponderosa) seedlings were evaluated in a young plantation 

in northern California from 1988 through 1997. Treatment regimes consisted of 

manual release (grubbing) over 3 to 6 years and resulted in vegetation recovery 

times of 4 to 10 years. Revegetation was remarkably diverse, rapid, and vigorous. 

At the end of the study, the dominant shrub, deerbrush, had significantly lower 

average density and foliar cover relative to the control when treated either the first 3 

years or the second 3 years, but average height was similar regardless of treatment. 

Average density of the 11 other shrubs was statistically higher in the control relative 

to other treatments, but cover and height did not differ. Density, foliar cover, and 

height of the forbs and graminoids did not differ among treatments at the end of the 

study. In the control, deerbrush plants numbered 6,100 per acre; other shrubs, 2,550 

per acre; forbs, 27,300 per acre; and graminoids, 5,800 plants per acre in 1997. 

Foliar cover of deerbrush at 22,700 ft2 per acre was more than five times that of 

all other naturally established species combined and slightly exceeded that of pine 

seedlings in all treatments. Average height of pine seedlings ranged from 7.7 ft with 

intensive release to 5.6 ft with no release. In general, release statistically improved 

pine growth regardless of timing or duration, but delayed release was more costly. 

Keywords: Competing vegetation, northern California, plant succession, 

ponderosa pine seedlings, timing of release, vegetation management.
RESEARCH PAPER PSW-RP-251 

Summary

This paper presents the density and development of shrub and herbaceous vegetation that invaded a young conifer plantation in interior northern California on an 

above-average site at mid-elevation (4,500 ft). It also quantifies the survival and 

growth of planted ponderosa pine (Pinus ponderosa Dougl. ex Laws. var. ponderosa)seedlings relative to this vegetation. A major influence on all the vegetation 

was the environment of the site, which was characterized by long hot, dry summers 

and cool, wet winters.

Before this study was begun, the original plant community was one of mature 

shrubs and a few forbs and graminoids (grass plus sedges) under scattered conifers. 

After timber harvest and site preparation (broadcast burn in 1987), most of the 

original species and many new ones appeared from root-crown sprouts (shrubs), 

from dormant seeds in the soil (shrubs, forbs), and from windblown seeds (forbs, 

graminoids). In all, 49 species were found in treated plots and control during the 10-

year study, which ranked this study second of 21 study areas in number of species 

(range 17 to 61) in our National Forest Service administrative study on alternative 

release methods.

This study differs somewhat from typical studies that evaluate alternatives for 

controlling competing vegetation in young conifer plantations. Most studies examine the direct effect of the release treatments (manual grubbing, mulching, etc.), but 

in this study, we documented the recovery of the vegetation after a series of manipulations that were applied at different times and continued for several durations. We 

hypothesized that the kind and amount of vegetation and when it was present would 

affect the various species in the community over time and in turn affect the growth 

of the conifer seedlings. Thus, the effect of the different timings and durations was 

emphasized. The four release treatments were grubbing every year for the first 

6 years, grubbing for the first 3 years, grubbing for the second 3 years (years 4 

through 6), and no grubbing at all (control). Of course, the longer the delay before 

grubbing, the more the vegetation grew and the greater the cost incurred. 

Colonization, as quantified in the control, was rapid. After one growing season, the average number of deerbrush plants was 21,500 per acre; other shrubs, 

10,050 per acre; forbs, 33,450 per acre; and graminoids, 3,050 plants per acre. By 

the end of the study, deerbrush dominated, with foliar cover and height increasing 

more than 17 and 12 times, respectively. Density and foliar cover of other shrubs 

declined, as did forb density, but forb cover increased slightly. The height of vegetation in both categories increased appreciably. Average density, foliar cover, and 
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

height of graminoids, however, generally increased during the study, with mostly 

large increases from low values.

Among treatments, average density and foliar cover values were generally 

higher in the control for deerbrush and other shrub species, but lower for the 

graminoids. Average height of each species and category of vegetation differed 

little among treatments at the end of the study.

Time since disturbance, or time for the competing vegetation to recover, often 

is cited as being important, but in this study, the invasion of so much competing 

vegetation so soon after site preparation and release masked treatment differences. 

Thus, statistically significant differences among treatments for ponderosa pine 

seedlings were not found until 1991 (year 4) for pine diameter and foliar cover. By 

the end of the study, pine diameter and foliar cover did not differ among treatments 

but did differ from the control. In this study, all release treatments, no matter when 

applied or for how long, were beneficial to ponderosa pine seedling growth.
RESEARCH PAPER PSW-RP-251 

Contents

1 Introduction

3 Methods

3 Location and Site Characteristics

5 Study and Design

6 Sampling

8 Results

8 Plant Diversity

9 Deerbrush

10 Other Shrubs

12 Forbs

13 Graminoids

13 Ponderosa Pine

17 Production and Cost

17 Discussion and Conclusions

20 English Equivalents

21 References 
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

Introduction

Because of shrinking budgets and a lack of resources, the treatment of competing 

vegetation in young conifer plantations has often been postponed, sometimes for 

two or three growing seasons or even longer. After such a delay, the forest vegetation manager often wonders: “Where did all that grass and all those shrubs come 

from,” and “What will they do to the growth of my conifer seedlings?” A larger 

question often is: “Will it do any good to treat now?”

The early plant community, typical of young plantations, often is quite variable because of differences in plantation history (fire, windthrow), different 

regeneration strategies of plants in the community (seed and seedling banks, 

windborne seeds, and rapid growth above- and below-ground (Grime 1979)), and 

different successional mechanisms (facilitation, tolerance, and inhibition (Connell 

and Slatyer 1977)). Many species from many categories of vegetation, including 

conifers, hardwoods, shrubs, forbs, grasses, and ferns, comprise the plant community (McDonald 1999, McDonald and Fiddler 1993). Through natural selection over millions of years, many species are well adapted to dominate in newly 

disturbed areas. Land recently prepared for planting is nearly ideal because soil 

moisture levels are high and nutrients generally are plentiful. Competing species 

almost always develop quickly, often grow faster; and usually deny site resources, 

especially soil moisture (De Groot 1991), to newly planted conifer seedlings. 

Death of the young conifers or a slowed growth rate is the usual consequence of 

not treating the competing vegetation.

Plainly, measures need to be taken to lessen the impact of competing vegetation on young conifer seedlings, and one method for doing so is plantation release. 

Release is the practice of treating competing vegetation by reducing the number 

of plants or their growth. But when should release be employed and for how long? 

Should it take place immediately after planting or can it be delayed? At some 

point, delayed release might not be more effective than never releasing, or releasing each year for several years might not be more effective than releasing for 1 or 

2 years, depending on when the release takes place. One correctly timed release 

might eliminate the need for a second release and consequently provide an economic benefit.

Possible clues for answering questions about the critical period of weed control 

can be found in agriculture (Barrentine 1974, Hall and others 1992, Weaver and Tan 

1983). Hall and others (1992), working with grain corn (Zea mays L.),1 for example, 

1

Scientific and common names of trees are from Little (1979), and the scientific and common 

names of all other vegetation follows the nomenclature of Hickman (1993). 

At some point, 

delayed release 

might not be more 

effective than never 

releasing, or releasing each year for 

several years might 

not be more effective than releasing 

for 1 or 2 years, 

depending on when 

the release takes 

place. One correctly 

timed release might 

eliminate the need 

for a second release 

and consequently 

provide an economic 

benefit.
RESEARCH PAPER PSW-RP-251 

defined when the critical period began, how long it lasted, and the duration of 

treatment. They also noted that reduced corn leaf area and accelerated senescence 

of lower leaves were the primary mechanisms affecting yield. 

Additional clues can be found in forestry in North America and California. 

For example, Lauer and others (1993) found that the first 2 continuous years of 

herbaceous vegetation control gave significantly better loblolly pine (Pinus taeda

L.) seedling volume growth at age 9 than treating just the first year, and Wagner 

and others (1999) noted declines in the stem volume of several conifer species in 

the Great Lakes region of Canada with increasing duration of competition from 

herbaceous vegetation. 

On a good site in northern California, Radosevich (1984) studied the competitive interaction between greenleaf manzanita (Arctostaphylos patula E. Greene) 

and ponderosa pine seedlings in an experiment with varying proportions of seedlings and shrubs. During the first growing season, no interference was observed 

between shrubs and seedlings, even though about 60 times more growth in terms 

of biomass took place on the shrubs than on the pines. Interference apparently 

began during the second growing season. After three growing seasons, shrub 

growth was still about 35 times that of pine seedling growth. Pine growth had 

decreased on all plots where manzanita was present. Also, after three seasons, 

manzanita was beginning to limit its own growth, especially where it was more 

abundant.

On a poor site in northern California, Fiddler and McDonald (1999) found that 

treating competing vegetation for the first 3 years and delaying treatment for the 

first 3 years and treating for the second 3 years both provided statistically larger 

values for height, diameter, and foliar cover of ponderosa pine seedlings than 

counterparts in the control after 10 years. Competition came primarily from three 

aggressive species of shrubs. In another study with ponderosa pine seedlings and 

aggressive shrub species, again on a poor site in northern California, McDonald 

and Fiddler (2001) noted that delaying treatment for 3 years produced results 

equivalent to treating for the first 3 years and that both resulted in significantly 

larger pine heights and diameters than for seedlings in the control after 10 years. 

However, delayed release cost more than 2.5 times that of early release. 

As part of a National Forest Service administrative study on alternative release 

methods (Fiddler and McDonald 1984), data in this study, which is on a good site, 

were gathered from 1988 through 1997 on vegetation developing after site preparation in a control and on vegetation manipulated deliberately by manually grubbing to a 5-ft radius to create different timespans before and after treatment. The 

timespans or more specifically “vegetation recovery times after disturbance by site 

preparation and release,” were important considerations. 
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

This paper reports the recovery of several categories of vegetation after treatment and quantifies the survival and growth of ponderosa pine seedlings after 

immediate and delayed release.

Methods

Location and Site Characteristics

The study area was located in the northern Sierra Nevada Mountains on the 

Mount Hough Ranger District of the USDA Forest Service’s Plumas National 

Forest in T 24 N, R 9 E, sec. 31, Mount Diablo meridian, about 5 airline miles 

southwest of Quincy, California. Before the study began, the land was covered 

with scattered conifers and tall, dense shrubs, probably from a forest fire in the 

distant past. Vegetation in the area has been placed in the montane and subalpine 

region of the Sierra Nevada and Cascade Ranges (Rundel and others 1977) and in 

the Sierra Nevada Mixed Conifer forest vegetation type (Tappeiner 1980). In terms 

of ecological subregions of California, the area corresponds to section M 261 E. 

Sierra Nevada and the granitic and metamorphic hills subsection (USDA Forest 

Figure 1—Blooming deerbrush in an opening in the uncut stand, spring 1988.
RESEARCH PAPER PSW-RP-251 

Service 1997). Ponderosa pine was the most abundant species, with coast Douglasfir (Pseudotsuga menziesii (Mirb.) Franco) second, and incense-cedar (Libocedrus 

decurrens Torr.) and California white fir (Abies concolor var. lowiana (Gord.) 

Lemm.) scattered throughout. Several native California hardwoods, including 

California black oak (Quercus kelloggii Newb.), Pacific dogwood (Cornus nuttallii

Audubon), and blue elderberry (Sambucus mexicana C. Presl.) were represented in 

the study area, mostly as single trees. 

Before the study began, the lower vegetation consisted primarily of 5- to 7-ft 

tall deerbrush (fig. 1), with scattered plants of mountain whitethorn (Ceanothus 

cordulatus Kellogg), greenleaf manzanita, and bush chinquapin (Chrysolepis sempervirens (Kellogg) Hjelmq.). Creeping snowberry (Symphoricarpos mollis Nutt.) 

also was present beneath taller shrubs and in small openings. 

Site quality of the study area is above average, with the height of dominant 

mixed conifers averaging about 70 ft in 50 years (Dunning and Reineke 1933). The 

soil, which belongs to the Skalan family, is 5 to 6 ft deep and has a sandy loam 

texture with 30 percent rock fragments. Taxonomically, it is an Ultic Haploxeralf 

derived from greenstone parent material. The elevation is about 4,500 ft, the slope 

averages 35 percent, and the aspect is southwest. As a whole, the area is reasonably 

uniform with respect to slope, aspect, and soil. Precipitation averages 40 in annually, with about 40 percent falling as snow. Annual temperatures range from 5 °F to 

95 °F and average 50 °F. The growing season is about 140 days. 

The site was prepared for planting by broadcast burning in fall 1987. The area 

burned “hot,” and almost all the slash, shrub stems, and aboveground organic 

material were consumed, leaving only blackened stumps, logs, and stubs of the 

largest shrub clumps.

Ponderosa pine seedlings from a nearby seed source that had been raised for 

1 year in the nursery were outplanted in late April 1988. Spacing was 10 by 10 ft. 

The spring was abnormally cold and wet. Survival of seedlings in all study plots in 

mid-July was 90 percent.

When the study began, the plant community consisted of sprouts from shrub 

and hardwood root crowns, plants from dormant seeds in the soil, and plants from 

windblown seeds. In addition to the planted ponderosa pine seedlings (fig. 2), the 

early plant community consisted of many species of shrubs and forbs, with the most 

abundant being those that originated from belowground root collars or rhizomes. 

Not many graminoids were present during the first growing season. A few deer 

(Hemionus spp.) and several mounds of pocket gophers (Thomomys spp.) were 

noticed in the study area. The pocket gopher population increased slowly, peaked in 

1991–92, and declined thereafter.
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

Study and Design

The experimental design was completely randomized, with four treatments, including a control, each replicated four times. A replicate (plot) was rectangular and consisted of about 0.1 acre, each of which contained 35 to 40 ponderosa pine seedlings 

surrounded by one or two rows of buffer (seedlings receiving similar treatment). 

The study began in 1988 and ended in 1997. Release treatments and dates were:

• T-first-6: Release every year for first 6 years (1988–93), develop naturally last 

4 years.

• T-first-3: Release first 3 years (1988–90), develop naturally last 7 years.

• T-second-3: Delay 3 years, release next 3 years (1991–93), develop naturally 

last 4 years.

• Control: No release, develop naturally entire 10 years of study (1988–97).

Release consisted of grubbing all vegetation within a 5-ft radius around pine 

seedlings with handtools. The goal was to grub vegetation below the root crown 

Figure 2—A typical ponderosa 

pine seedling in the study area 

after one full growing season.
RESEARCH PAPER PSW-RP-251 

to a depth of about 4 ins and not damage pine seedling roots. The initial grubbing 

took place in May 1988, and subsequent grubbing was performed in late summer or fall. The cost of each treatment, both initially and for retreatment, was 

calculated from the payroll records of laborers paid at the Wage Grade 1 rate (U.S. 

Department of Labor 1993) of $8.74 per hour. No allowance for travel, equipment, 

or overhead was included. 

Sampling

Of the 35 to 40 ponderosa pine seedlings in each plot, 25 that appeared to have 

good potential of becoming future crop trees were selected and tagged. Chlorotic 

and misshapen seedlings that would be removed in a precommercial thinning 

were excluded. Stem diameter (measured at 12 in above mean ground line), stem 

height, foliar cover (the sum of shadows that would be cast by the leaves and stems 

of individual species expressed as a percentage of the land surface [Daubenmire 

1968]), and needle length were measured on each seedling. Unfortunately, limitations in budget and staffing affected the amount of sampling that was done, and 

some remeasurements were not taken. The seedlings were periodically checked for 

injury from animals, insects, and weather.

The sampling intensity for evaluating natural vegetation was five subplots in 

each plot. The subplots were square and centered around randomly selected pine 

seedlings. The subplots encompassed 1 milacre (0.001 acre or 43.56 ft2). The most 

abundant species and categories of vegetation were evaluated for density, foliar 

cover, and average dominant height (average of the three tallest stems per subplot 

measured from mean ground line to bud). More specifically, the number of plants 

in each subplot was counted, foliar cover was visually estimated, and height was 

measured with a graduated pole. Values of foliar cover were checked by a footsquare wire frame and occasionally with a metal tape. In some instances, “plant” 

was difficult to define. Thus, each fern frond and each clump of hardwoods or 

shrubs was counted as one plant. As for pine seedlings, all vegetation was checked 

periodically for injury and causal agent. 

To test for treatment effects and significant differences among treatments, oneway analysis of variance of treatment means (fixed effect 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 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 period and year separately.

To quantify plant diversity, new species were noted on study plots at each visit, 

and all species were recorded when the study began and ended.

The number of 

plants in each subplot was counted, 

foliar cover was 

visually estimated, 

and height was 

measured with a 

graduated pole. 
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

Table 1–Natural vegetation in study plots, 1988–1997

Species 1988 1997 Common names

Conifers: –1 –

Hardwoods:

Cornus nuttallii Audubon X1 X Pacific dogwood

Quercus kelloggii Newb. X X California black oak

Sambucus mexicana C. Presl. X X Blue elderberry

Shrubs:

Amelanchier utahensis Koehne X X Utah service-berry

Arctostaphylos patula E. Greene X X Greenleaf manzanita

Ceanothus cordulatus Kellogg X X Mountain whitethorn

Ceanothus integerrimus Hook. & Arn. X X Deerbrush

Ceanothus prostratus Benth. X X Mahala mat

Chrysolepis sempervirens (Kellogg) Hjelmq. X – Bush chinquapin

Penstemon spp. X X Beardtongue

Penstemon spp.2 X X Beardtongue

Ribes spp. X X Gooseberry

Rosa gymnocarpa Nutt. X X Wood rose

Rubus parviflorus Nutt. X X Thimbleberry

Symphoricarpos mollis Nutt. X X Snowberry

Forbs:

Adenocaulon bicolor Hook. X X Trail plant

Agoseris grandiflora (Nutt.) E. Greene X X Bigflower agoseris

Apocynum androsaemifolium L. X X Spreading dogbane

Aster radulinus A. Gray X X Roughleaf aster

Balsamorhiza deltoidea Nutt. X – Deltoid balsamroot

Castilleja spp. X X Indian paintbrush

Cirsium vulgare (Savi) Ten. X X Bull thistle

Clarkia rhomboidea Douglas X X Diamond clarkia

Convolvulus arvensis L. X X Bindweed

Epilobium spp. X X Willow herb

Erigeron inornatus A. Gray X X California rayless fleabane

Galium spp. X X Bedstraw

Gayophytum diffusum Torrey & A. Gray X X Spreading groundsmoke

Gnaphalium luteo-album L. X X Cudweed

Iris spp. X X Iris

Lactuca serriola L. X X Prickly lettuce

Lilium washingtonianum Kellogg X X Washington lily

Madia gracilis (Smith) Keck X X Slender tarweed

Mimulus spp. X – Monkeyflower

Montia parvifolia (DC.) E. Greene X X Littleleaf minerslettuce

Osmorhiza chilensis Hook. & Arn. X – Sweet cicely

Phacelia spp. X X –

Potentilla spp. X X Cinquefoil

Smilacina racemosa (L.) Link X – False Solomon’s seal

Stephanomeria lactucina A. Gray X X Lettuce wirelettuce

Tragopogon dubius Scop. X X Yellow salsify

Viola spp. X X Voilet

Unidentified X X –

Graminoids:

Achnatherum spp. – X Needlegrass

Bromus spp. X X Brome

Carex spp. X X Sedge

Elymus glaucus Buckley – X Blue wildrye

Unidentified – X –

Fern

Pteridium aquilinum (L.) Kuhn X X Bracken

 var. pubescens L. Underw.

1 X = present, – = absent.

2 Two species of Penstemon.
RESEARCH PAPER PSW-RP-251 

Results

Plant Diversity

The natural plant community on all study plots (including control) near the beginning of the study in 1988 consisted of no natural conifer species, 3 species of 

hardwoods, 12 shrubs, 28 forbs, 2 graminoids (grasses and sedges), and 1 fern, for 

a total of 46 species (table 1). By the end of the study, vegetation in the study plots 

included no natural conifer seedlings, 3 hardwoods, 11 shrubs, 24 forbs, 5 graminoids, and 1 fern, for a total of 44 species. Plant diversity also was evaluated by 

treatment at the end of the study, but only on the subplots. The number of species 

by treatment was T-first-6, 19; T-first-3, 31; T-second-3, 27; and control, 31. Data 

and diary records emphasized the rapidity of revegetation in the study area and the 

high diversity of the new plant community. Indeed, the diary record of July 14, 1988 

noted: “The amazing part of this study is the tremendous increase in plant cover” 

(fig. 3A, B). 

Deerbrush

Deerbrush colonized aggressively. As early as May 9, 1988, deerbrush plants 

from root crowns and dormant seeds in the soil were evident throughout the study 

area. Diary records in mid-July indicated that it was difficult to tell the difference 

between recently grubbed plots and the control. In spite of many deer sightings, 

deerbrush was never more than lightly browsed during the study. Average density 

of deerbrush in the control in 1988 was 21,500 plants per acre, which then declined 

to 6,100 plants per acre at the end of the study in 1997 (table 2), a 72-percent 

decrease. Foliar cover and height increased steadily and reached 22,700 ft2 per acre 

and 6.4 ft tall respectively, which were 17 and 13 times first-year (1988) values. 

Among treatments, deerbrush in the control plots had significantly higher 

density (6,100 plants per acre) than those treated for the first 3 years (1,100 plants 

per acre) or the second 3 years (850 plants per acre) at the end of the study (table 4). 

The trend in foliar cover was similar, with plants in the T-first-3 treatment (8,400 

ft2 per acre) and T-second-3 treatment (3,700 ft2 per acre) having significantly lower 

average values than those in the control (22,700 ft2 per acre). Although not statistically significant, the higher density and foliar cover of deerbrush in the T-first-3 

treatment relative to the T-second-3 treatment indicated vigorous recruitment and 

subsequent growth of new seedlings after the first 3 years. Almost all these plants 

originated from dormant seeds in the soil. Average height of deerbrush ranged from 

5.5 to 6.4 ft and did not differ statistically among treatments at the end of the study. 
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance



Other Shrubs

Of the 11 shrub species other than deerbrush that were present in the study area, the 

two species of beardtongues (Penstemon sp.) and creeping snowberry were the most 

abundant and the most evenly distributed. The beardtongues probably originated 

from dormant seeds in the soil, and some plants of creeping snowberry could have 

Figure 3—An overview of the 

study area on (A) May 9, 1988, 

and (B) July 9, 1988. Note the 

large increase in deerbrush 

density and cover.

3A

3B
10

RESEARCH PAPER PSW-RP-251 

as well. However, the large central stem and extensive root system of the creeping 

snowberry suggest that most plants probably resprouted after the broadcast burn. 

The other species in this category were barely represented and widely scattered. 

Indeed, one shrub, bush chinquapin, could not be found at the end of the study.

In the control, the initial average density of other shrubs was high (10,050 

plants per acre) and then declined erratically by 75 percent to 2,550 plants per 

acre at the end of the study (table 3). The trend in foliar cover was similar, with an 

overall slight decline (1,950 to 1,850 ft2 per acre). Average height increased from 0.4 

to 1.5 ft during the study.

Among treatments, the average densities of other shrubs in T-first-3 (700 plants 

per acre) and T-second-3 (900 plants per acre) were significantly lower than that in 

the control (2,550 plants per acre, table 3). Time since disturbance did not appear 

to be a factor affecting average foliar cover or height because they did not differ 

statistically from the control. Foliar cover ranged from 450 to 1,850 ft2 per acre, and 

height from 1.4 to 1.6 ft (table 4). 

Table 2–Average density, foliar cover, and height, with standard errors (SE), 

of deerbrush, other shrubs, forbs, and graminoids in the control, northern 

California, 1988–1997

Species Year Density SE Cover SE Height SE 

No. of plants/acre Square feet/acre Feet

Deerbrush 1988 21,500 2,636 1,300 238 0.5 0.1

1990 8,250 763 6,350 556 2.6 0.3

1991 8,350 750 10,750 1,438 2.6 0.3

1993 6,350 741 16,750 1,541 4.0 0.2

1997 6,100 794 22,700 1,343 6.4 0.5

Other shrubs 1988 10,050 3,331 1,950 574 0.4 0.1

1990 3,900 911 1,900 311 1.1 0.2

1991 4,750 1,466 2,050 427 0.8 0.1

1993 3,400 616 1,400 383 1.1 0.1

1997 2,550 126 1,850 506 1.5 0.1

Forbs 1988 33,450 9,362 2,150 512 0.8 0.1

1990 52,200 18,740 2,350 574 1.7 0.3

1991 69,650 16,528 2,800 712 1.9 0.1

1993 46,450 10,028 2,100 1,002 2.2 0.1

1997 27,300 10,908 2,450 1,109 2.4 0.2

Graminoids 1988 3,050 1,312 50 50 0.8 0.3

1990 1,150 544 50 50 1.4 0.5

1991 2,050 759 100 58 0.9 0.1

1993 1,800 627 150 96 2.0 0.8

1997 5,800 2,634 150 150 2.1 0.6
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

11

Forbs

The forb component of the plant community, in addition to being unusually 

diverse soon after site preparation, was initially composed of perennial species 

like roughleaf aster (Aster radulinus A. Gray) (fig. 4A) and lettuce wirelettuce 

(Stephanomeria lactucina A. Gray) that originated from rhizomes, and others like 

bindweed (Convolvulus arvensis L.) (fig. 4B) and spreading dogbane (Apocynum 

androsaemifolium L.) that had deep, persistent roots. A host of early annual 

Table 3–Average density, foliar cover, and height of deerbrush, other shrubs, 

forbs, and graminoids among treatments, northern California, 1997

Species Treatment Density Cover Height

No. of plants/acre Square feet/acre Feet

Deerbrush Treated first 6 years –1 – –

Treated first 3 years 1,100 a2 8,400 a 6.4 a 

Treated second 3 years 850 a 3,700 a 5.5 a 

Control 6,100 b 22,700 b 6.4 a

Standard error3 4.63 2,164.36 0.77

F ratio3 35.52 20.91 .49

P value3 .0001 .0004 .6268

Other shrubs Treated first 6 years – – –

Treated first 3 years 700 a 450 a 1.6 a

Treated second 3 years 900 a 700 a 1.4 a

Control 2,550 b 1,850 a 1.5 a

Standard error 4.44 440.33 .47

F ratio 9.78 2.88 .04

P value .0055 .1083 .9572

Forbs Treated first 6 years – – –

Treated first 3 years 19,350 a 1,100 a 2.1 a

Treated second 3 years 30,250 a 900 a 1.9 a

Control 27,300 a 2,450 a 2.4 a

Standard error 20.58 729.35 .27

F ratio .61 1.34 .80

P value .560 .3103 .4792

Graminoids Treated first 6 years – – –

Treated first 3 years 14,350 a 250 a 2.7 a

Treated second 3 years 5,550 a 200 a 2.7 a

Control 5,800 a 150 a 2.1 a

Standard error 29.19 147.20 .54

F ratio .21 .12 .48

P value .8139 .8923 .6358

1

 Vegetation that became established after 6 years was judged as having little or no competitive effect on the pine 

seedlings. Hence it was not quantified. 

2 For each species, treatment means in each column followed by the same letter do not differ statistically at the 

0.05 level.

3 Standard error, F, and P for density of all species and categories are transformed (sqrt) values.
12

RESEARCH PAPER PSW-RP-251 

4B

4A

species like bull thistle (Cirsium vulgare [Savi] Ten.), a species of willow herb 

(Epilobium spp.), spreading groundsmoke (Gayophytum diffusum Torrey and A. 

Gray), prickly lettuce (Lactuca serriola L.), slender tarweed (Madia gracilis (Smith) 

Keck), and others that arose from dormant or windblown seeds also colonized 

the area. Initially, the distribution of these species was clumpy, but with time the 

aggregations expanded widely.

In the control, large numbers of forbs were present initially (33,450 plants per 

acre), peaked in 1991 (69,650 per acre), and then decreased in 1997 (27,300 plants 

per acre) (table 3). Average foliar cover followed the same trend, peaking in 1991 at 

2,800 ft2 per acre, and then declining at the end of the study to 2,450 ft2 per acre. 

Figure 4—Perennial forbs that originated from (A) rhizomes (Aster radulinus) and (B) deep, persistent roots (Convolvulus arvensis). 
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

13

The overall gain in foliar cover during the study was 14 percent. Average height of 

forbs steadily increased from 0.8 ft in 1988 to 2.4 ft in 1997. 

Among treatments, the average density of forbs was highest in the T-second-3 

treatment (30,250 plants per acre) and lowest in the T-first-3 treatment (19,350 per 

acre), with the control being intermediate (27,300 plants per acre) (table 4). Foliar 

cover and height were both lowest in the T-second-3 treatment and highest in the 

control. No statistically significant difference among treatments was found for forb 

density, foliar cover, or height. 

Graminoids

Except for a few plants of Bromus and Carex, the number of graminoids and their 

development lagged behind most of the other plant species that invaded the study 

area. Many small plants that were present initially died, and graminoid density 

decreased during the first 3 years. More grass species became present about this 

time, especially a species of needlegrass (Achnatherum sp.) and blue wildrye 

(Elymus glaucus Buckley). By 1997, the graminoids had increased appreciably and 

were exceeding their initial (1988) levels of density, foliar cover, and height.

Graminoids in the control numbered more than 3,000 per acre in 1988, but 

had only a minor amount of foliar cover and were relatively short (table 3). After a 

62-percent decrease in 1990, graminoid density generally increased through 1993 

and then jumped threefold in 1997 to 5,800 plants per acre. Foliar cover and height 

generally increased from low levels initially to 150 ft2 per acre and 2.1 ft, respectively, in the control at the end of the study.

Among treatments in fall 1997, average density, foliar cover, and height of 

graminoids in the T-first-3 treatment were larger or equal to those in the T-second-3 

or control (table 4), although not statistically different at the 5-percent level. Average density ranged from 5,550 to 14,350 plants per acre, foliar cover from 150 to 

250 ft2 per acre, and height from 2.1 to 2.7 ft.

Ponderosa Pine

Early pine seedling mortality apparently was caused by low seedling vigor, middleyear mortality by pocket gophers, and later-year death by drought-induced suppression. Percentage of survival by treatment at the end of the study was: 

Treatment Average Standard error

T-first-6 74 7

T-first-3 74 3

T-second-3 76 6

Control 66 6
14

RESEARCH PAPER PSW-RP-251 

Altogether, 134 of approximately 450 seedlings died, resulting in a survival rate 

of 70 percent. Analysis of variance of transformed (arcsin) percentages indicated no 

significant difference in survival rate among treatments. 

 Statistically significant differences among treatments for ponderosa pine 

seedlings first became evident four growing seasons after planting (fall 1991) when 

average stem diameter of the seedlings in the T-first-6 and T-first-3 treatments differed statistically from that of the seedlings in the control (table 5). Annual height 

growth of treated seedlings was beginning to accelerate (fig. 5). Average foliar 

cover of seedlings in the T-first-3 treatment also differed from counterparts in the 

control. At the end of the study in 1997, average stem diameter and foliar cover of 

pines in all treatments differed statistically from those in the control. Neither average height nor needle length, which was used as an indicator of future treatment 

effectiveness (McDonald and others 1992), differed statistically among treatments 

during the 10-year study. 

At the end of the study, average height of ponderosa pine seedlings ranged from 

5.6 ft in the control to 7.7 ft in the T-first-6 treatment; stem diameter varied from 

2.0 in in the control to 3.7 in in the T-first-6 treatment; and average foliar cover 

ranged from 5,650 ft2 per acre in the control to more than 21,000 ft2 per acre among 

treatments (table 5). 

Figure 5—A representative 6-year-old 

ponderosa pine seedling that has been 

grubbed for the first 6 years, April 1994. 

At the end of the 

study in 1997, average stem diameter 

and foliar cover of 

pines in all treatments differed statistically from those in 

the control.
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

15

Table 4–Average stem height, diameter, foliar cover, and needle length of 

ponderosa pine seedlings, by treatment, northern California, 1988–1997

Treatment 1988 1990 1991 1993 1997

 Height (feet):

Treated first 6 years 0.73 a1 1.54 a 2.18 a 3.70 ab 7.68 ab

Treated first 3 years .68 a 1.52 a 2.16 a 3.60 ab 7.46 ab

Treated second 3 years .70 a 1.31 a 1.75 a 3.20 ab 7.24 ab

Control .70 a 1.30 a 1.74 a 2.84 b 5.56 b

Standard error .15 .09 .11 .21 .56

F ratio 1.08 2.12 4.53 4.17 3.08

P value .401 .128 .053 .018 .049

 Diameter (inches):

Treated first 6 years – – .74 a 1.62 a 3.68 a

Treated first 3 years – – .72 a 1.43 a 3.23 a

Treated second 3 years – – .56 ab 1.42 ab 3.50 a

Control – – .48 b .93 b 2.01 b

Standard error – – .05 .11 .27

F ratio – – 6.74 7.06 7.57

P value – – .003 .002 .002

 Foliar cover (square feet/acre):

Treated first 6 years – – – – 21,082 a

Treated first 3 years – – 2,650 a 7,800 a 20,250 a

Treated second 3 years – – – – 20,850 a

Control – – 1,000 b 1,850 b 5,650 b

Standard error – – 358 1,430 3,047

F ratio – – 10.64 8.85 5.08

P value – – .017 .025 .009

 Needle length (inches):

Treated first 6 years – .46 ab .39 ab .52 ab –

Treated first 3 years – .44 ab .40 ab .49 ab –

Treated second 3 years – .42 b .38 ab .52 ab –

Control – .42 b .36 b .44 b –

Standard error – .01 .01 .02 –

F ratio – 4.14 2.91 2.96 –

P value – .019 .054 .055 –

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

To better portray the relationship between ponderosa pine seedlings and 

deerbrush, the average heights of the two species were compared graphically for the 

T-first-3 treatment (fig. 6A), T-second-3 treatment (fig. 6B), and the control (fig. 6C). 

For the two treatments, deerbrush grew taller initially but then was surpassed by 

the pines. The respective slopes of the curves at age 10 suggest that the growth of 

pine will outstrip that of deerbrush in the near future. In the control, the deerbrush 

stems were taller than ponderosa pine seedlings throughout the 10-year period, 

but the respective slopes suggest that pine height may slowly catch up to, or even 

exceed, that of deerbrush.
16

RESEARCH PAPER PSW-RP-251 

Figure 6­—Relationship 

between ponderosa pine 

seedling height and deerbrush 

stem height for the T-first-3 

treatment (A), T-second-3 

treatment (B), and control (C).

A

B

C
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

17

Production and Cost

Initial grubbing to a 5-foot radius in May 1988 took only a small amount of time, 

but needed to be done twice. Time to grub thereafter was minimal. The fourth year 

(1991) was the most costly year for grubbing because many plants, particularly 

shrubs, had stems that leaned into the subplots and had expanded their root crowns 

into them. These required additional labor to remove. Later grubbing in 1992 and 

1993 took less time each year in each treatment. The most time-consuming treatment was the initial grubbing in the T-second-3 plots. It took 60 hours for the crew 

to remove the well-established vegetation. Total time to grub the treatments and the 

corresponding cost were: 

The high cost of the T-second-3 treatment was caused not only by the high 

density of the vegetation, but also by its development belowground. The large burls 

of California black oak and deerbrush, most of which had survived the broadcast 

burn, had to be split first and then the roots removed. Grubbing creeping snowberry 

and bindweed also was difficult and time consuming because the roots were large 

and the root systems were connected in a radius of 2.5 ft or more. Snowberry plants 

also were connected aboveground by runners. Bracken fern, Aster radulinus, and 

Stephanomeria lactucina had deep, extensive, and rhizomatous root systems, which 

were almost impossible to remove completely. Other forbs, especially from the 

genera Cirsium, Clarkia, Epilobium, Gayophytum, Lactuca, Madia, Mimulus, and 

Viola, had an inexhaustible seed supply, and grubbing seemed to stimulate more 

plants than were removed. Needlegrass (Achnatherum sp.) and brome (Bromus sp.) 

required loosening around the edges before the main clump could be removed.

Discussion and Conclusions

A major reason for analyzing and presenting the results of this study in terms of 

time since harvest and duration of treatments was to reinforce the hypothesis, 

based on findings in earlier studies (McDonald and Fiddler 1990, 1995), that early 

release is more cost-effective than repeated release or delayed release. Delayed 

release almost always results in more and larger competing plants, often with large, 

deep masses of roots. The T-first-3 and T-second-3 treatments in this study were a 

direct test of early versus delayed release. Biologically, these two treatments did not 

Cost per acre 

Treatment Time (hours) (dollars)

T-first-6 65.5 238

T-first-3 27.0 197

T-second-3 82.0 598

The high cost of the 

T-second-3 treatment 

was caused not only 

by the high density 

of the vegetation, but 

also by its development belowground. 
18

RESEARCH PAPER PSW-RP-251 

significantly differ from each other in terms of pine seedling growth. But economically they did. The cost of manual grubbing in the T-second-3 treatment was more 

than three times of that in the T-first-3 treatment. 

Why was the T-first-3 treatment not biologically more effective than the Tsecond-3 treatment for promoting ponderosa pine seedling growth? Strong competition in the T-first-3 treatment, particularly from deerbrush, is a likely answer. 

In spite of zealous grubbing, parts of some clumps sprouted and dormant seeds 

in the soil germinated year after year. The result was a relatively high density of 

plants soon after each treatment ended and a concomitant drain on site resources. 

When a competitive species invades each year for several years, and the duration of treatment is short, early treatment is only marginally effective because a 

competing species has time to become reestablished before all favorable microsites 

are occupied. As shown in table 4, the average density, foliar cover, and height of 

deerbrush at the end of the study was higher in the T-first-3 treatment than in the 

T-second-3 treatment, even though not statistically different. Plainly, the early, 

short-duration treatment with 7 years of recovery time (1991–97) provided no gain 

in pine seedling growth over the treatment that had a later beginning and 4 years 

of recovery time (1994–97).

Releasing each year for the first 6 years, as in the T-first-6 treatment, was 

statistically no more advantageous to pine seedling development than treating for 

the first 3 years and was 21 percent more expensive to implement.

No release, as in the control, cost nothing economically but permitted the plant 

community to thrive at the expense of the ponderosa pine seedlings. In the control, 

both average foliar cover and height of deerbrush were greater than that of pine 

seedlings (22,700 versus 5,650 ft2 per acre and 6.4 versus 5.6 ft, respectively).

When both the biological findings and cost information in this study are 

considered, the most cost-effective treatment for releasing ponderosa pine seedlings 

would be to treat every year for the first 3 years (T-first-3) and do nothing thereafter. 

Also, the remarkable similarity in pine diameter and foliar cover among treatments 

and their significant difference from the control are of note. And the widening 

difference in height growth rates between pine seedlings and deerbrush at age 10 

(fig. 6 A,B) should be considered. On the basis of this information, ponderosa pine 

seedlings in this environment should be released, but when, and for how long 

are unimportant.

During this 10-year study, the plant community was composed of a maximum 

of 49 species; 46 at the beginning of the study and 44 at the end. Relative to other 

young plantations in our National Forest Service administrative study, 49 speWhen a competitive 

species invades 

each year for 

several years, and 

the duration of 

treatment is short, 

early treatment is 

only marginally 

effective because a 

competing species 

has time to become 

reestablished 

before all favorable 

microsites are 

occupied.

No release, as in the 

control, cost nothing 

economically but 

permitted the plant 

community to thrive 

at the expense of 

the ponderosa pine 

seedlings.
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

19

cies places it second in terms of species richness. The range was 13 to 61 species 

(McDonald and Fiddler 2006).

Early and sustained colonization by a large number of species in several categories is not unusual on an above-average site, as in this study, where site resources 

are plentiful and the growing season is relatively long. At the end of the study, more 

than 41,000 plants per acre were present in the treatments from which the data 

were taken, with 65 percent being forbs. Although the forbs dominated in terms of 

density, deerbrush excelled in foliar cover, with 84 percent of total cover. Deerbrush 

also dominated in average height, being more than twice as tall as other categories 

of natural vegetation at the end of the study. 

Three trends in the plant community, one expected, and two not expected, were 

noteworthy. We anticipated the late surge of the graminoids but did not expect the 

contribution of the other shrubs to be as small as it was, and we did not anticipate 

the smaller number of species on the most intensively treated plots. 

By the end of the study in 1997, the graminoids had increased greatly, although 

erratically, in average density and height. This was caused by the invasion of taller 

and more aggressive grasses and by their ability to compete in an increasingly 

crowded plant community with decreasing levels of site resources. Similar findings 

were found on both an above-average site (McDonald 1999) and a below-average 

site (Fiddler and McDonald 1999).

Average density, foliar cover, and height of the 11 other shrubs combined were 

generally lower than for other categories of vegetation in all treatments at the end 

of the study. Although several of these species are notably aggressive, they may 

not be as aggressive as deerbrush. In a direct comparison of planted deerbrush and 

greenleaf manzanita in another study (McDonald and others 1998), deerbrush had 

significantly more stems, was taller, and had wider crowns than manzanita after 

two growing seasons. This relationship continued throughout the 4-year study. 

In addition, deerbrush developed a clump of stems 1 year sooner than manzanita 

and began producing flowers and setting seed at age 3, whereas manzanita did not 

flower during the study.

The lower species richness in the T-first-6 treatment relative to the other treatments and the control probably can be explained by the uninhibited ability of the 

ponderosa pine seedling roots to capture site resources belowground and the rapid 

enlargement of crowns aboveground. Propagules of other species apparently found 

it hard to invade this environment or difficult to become established once there.

The density and developmental trends in the control suggest the composition 

and growth of the plant community in the near future in the absence of release. 
20

RESEARCH PAPER PSW-RP-251 

English Equivalents

When you know: Multiply by: To get:

Inches (in) 2.54 Centimeters

Feet (ft) .3048 Meters

Acres .405 Hectares

Milacre 4.05 Square meters

Square feet per acre (ft2/acre) .23 Square meters per hactare

Degrees Fahrenheit (°F) Degrees Celsius

Many pines remain, and although not growing as fast as their counterparts in the 

treated plots, some will persist and gradually overtop the competing vegetation. 

Deerbrush will thrive. With 6,100 plants per acre, 22,700 ft2 per acre of foliar cover, 

and 6.4 ft of height after 10 years, deerbrush clearly is, and will continue to be, 

the dominant lesser vegetation in the plantation. A few of the other shrubs will be 

relegated to small openings among the deerbrush plants, but most will be under 

them. The forbs will share the same fate, with a trend toward fewer species and a 

slow increase in perennials (McDonald 1999). The role of the graminoids in the 

future plant community is unknown. That they have competed well so far suggests 

that they will at least maintain their presence in the near future. 

On released plots, the plant community of the near future will consist of more 

and larger pines than in the control, and they will place more stress on the other 

species faster. Consequently, the community of the future will be mostly pines and 

deerbrush, with a few other shrubs, forbs, and graminoids among them. 

1.8

F-32
Development of Vegetation in a Young Ponderosa Pine Plantation: Effect of Treatment Duration and Time Since Disturbance

21

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