Vegetation Trends in a 31-Year-Old

Ponderosa Pine Plantation: Effect of

Different Shrub Densities

Philip M. McDonald Celeste S. Abbott

United States

Department

of Agriculture

Forest Service

Pacific Southwest

Research Station

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

Research Paper

PSW-RP-231
Abstract

McDonald, Philip M.; Abbott, Celeste S. 1997. Vegetation trends in a 31-year-old ponderosa

pine plantation: effect of different shrub densities. Res. Paper PSW-RP-231. Albany,

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

Agriculture; 35 p.

On a poor site in northern California, a brushfield community was treated in various

ways which left initial densities of no shrubs, light, medium, and heavy shrubs. Density

and development (height, foliar cover, crown volume) for three shrub species (alone and

combined), one grass, and planted ponderosa pine in these categories were quantified

from 1966 to 1992. Successional trends (ascendance and decline) are presented for these

species and for forbs from 1962 (the date pines were planted) through 1992. Regression

equations that model density and development are presented for the shrubs and pine. In

general, greenleaf manzanita prospered during the study; snowbrush initially developed

well, but then declined; Sierra plum endured, but was relegated to the understory;

needlegrass invaded rapidly, peaked early, and was mostly gone by the end of the study.

Only a trace of forb species remained by study end. Needlegrass displayed strong

environmental preference, becoming dense and developing well in shrub-free areas, but

was scarcely present in heavy shrubs. Ponderosa pine grew well in no-shrub plots, fairly

well in light-shrub plots, and poorly in medium- and heavy-shrub plots. Extensive testing

showed that shrub foliar cover and crown volume per acre explained more variation in

several pine parameters than shrub height or density.

Retrieval Terms: density, development, grasses, plant succession, ponderosa pine, shrubs

The Authors

Philip M. McDonald and Celeste S. Abbott are research forester and forester,

respectively, assigned to the Vegetation Management Research Unit, located at the Station’s

Silviculture Laboratory, 2400 Washington Avenue, Redding, CA 96001.

Publisher:

Albany, California

Mailing address:

PO Box 245, Berkeley CA

94701-0245

510 559-6300

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

April 1997

Pacific Southwest Research Station

Forest Service

US Department of Agriculture
Vegetation Trends in a 31-Year-Old

Ponderosa Pine Plantation: Effect of

Different Shrub Densities

Philip M. McDonald Celeste S. Abbott

Contents:

In Brief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

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

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Location and Site Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Study and Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Plant Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Greenleaf Manzanita . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Snowbrush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Sierra Plum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Combined Shrubs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Needlegrass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Ponderosa Pine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Discussion and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Biological Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Vegetation Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Entomology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Pacific Southwest

Research Station

USDA Forest Service

Research Paper

PSW-RP-231

 April 1997

ii USDA Forest Service Research Paper. PSW-RP-231. 1997.

In Brief...

McDonald, Philip M.; Abbott, Celeste S. 1997. Vegetation trends in a 31-year-old

ponderosa pine plantation: effect of different shrub densities. Res. Paper

PSW-RP-231. Albany, CA: Pacific Southwest Research Station, Forest Service,

U.S. Department of Agriculture; 35 p.

Retrieval Terms: density, development, grasses, plant succession, ponderosa pine,

shrubs New conifer plantations characteristically are located in areas that have been

severely disturbed by wildfire or timber harvest, or where the plant

community consists of woody or herbaceous species with no economic value. If

the area is a brushfield, the goal often is to restore tree species that traditionally

were present there. In the area of this study, a mixed conifer forest had been

present in the past. But after at least a decade of logging and fire (1887-1897), all

the trees had disappeared and a large area had become clothed with a dense

mass of vigorous shrubs. In the next 60 years or so, these shrubs undoubtedly

produced many seed crops, each of which contributed to the seedbank in the soil.

It is likely that subsequent fires burned the brushfield, as well, with each slightly

modifying the soil and its nutrient- and water-conducting capability. Because the

study area was part of a much larger area, no seed trees remained. The shrubs

fully occupied the site and virtually no chance for the natural establishment of a

forest existed.

In 1962, when the study area was cleared and planted, conversion from

brush to trees was the land management objective. But because the growth

potential of the site was low and the odds for success of a mixed conifer plantation

also were low, only ponderosa pine was planted. This pine survives and grows

well in hot, dry conditions on poor sites. Foresters recognized that conversion to

the original mixed conifer forest involved two steps: (1) getting the forest started

and creating a forest environment, and (2) in the less harsh forest environment,

planting shade-tolerant trees characteristic of the mixed conifer type.

Revegetation on the bare soil of recently disturbed areas usually is rapid and

highly variable. In this study, greenleaf manzanita and snowbrush originated

from seeds in the soil, Sierra plum sprouted from roots and root crowns, and

several species of forbs appeared from windblown seeds. In 1966, the natural

vegetation consisted of 6 species of shrubs, 10 forbs, and one graminoid. In 1967,

a native needlegrass became an important component of the plant community.

Previous herbicide trials had left the study area with various amounts of

shrubs. In 1966, these were segregated into no-, light-, medium-, and heavyshrub categories, and these became the “treatments” in this study. Many

developmental characteristics (height, foliar cover, volume) of shrubs (alone and

together), grass, and ponderosa pine were quantified in each category through

1992. Plant density also was quantified for shrubs and grass, and diameter at

breast height was measured for pine.

The ascendance and decline of the various plant species portray successional

trends in the study area. In general, greenleaf manzanita prospered in the

plantation. It retained a relatively high density level and developed well in foliar

cover and height. It was virtually undamaged by abiotic agents. Snowbrush was

much less dense to begin with, developed well for the first 15 years, declined for

the next 15 years, and clearly was fading from the plant community at the end of

the study. Frost, drought, and wind contributed to its decline. Sierra plum,

which developed only from vegetative propagules, will be a part of the future

plant community, but mostly in the understory. It existed in the shade and

sprouted in the shade—something the other shrub species did not do.
USDA Forest Service Research Paper. PSW-RP-231. 1997. iii

Needlegrass proved to be an ephemeral species that invaded the study area

late and departed early. Its density and development keyed inversely to shrub

density: the fewer the shrubs, the greater the needlegrass density and height. In

1983, for example, needlegrass density was 65,900 plants per acre in no-shrub

plots and 450 per acre in heavy-shrub plots. About 540 plants per acre resided in

no-shrub plots at the end of the study; none at all in other shrub categories.

Because needlegrass invaded after the ponderosa pines had become established,

its competitive effect was not as great as if it had been present earlier.

Furthermore, its roots were not as deep as those of shrubs, and hence it was less

competitive than them.

Ponderosa pine height and diameter, in general, differed statistically among

all shrub categories during most of the study. Pines in the shrub-free category

averaged 30.4 feet tall at study end, 21.6 feet tall in light shrubs, 15.2 feet tall in

medium shrubs, and 9.0 feet tall in heavy shrubs. Furthermore, the pines were

growing at an average rate of 1.1, 0.7, 0.5, and 0.3 feet per year in the no-, light-,

medium-, and heavy-shrub categories, respectively. Plainly, even a small amount

of shrubs cost the pines an appreciable amount of growth. Similar trends were

present for diameter as well. After 31 years, ponderosa pines with any amount of

shrubs are falling farther behind those with no shrub competition.

Because natural resource managers may want to mathematically express the

long-term development of the shrubs, grass, and pine, regression equations are

presented that model each species’ density, foliar cover, height, and crown

volume. In addition, several pine parameters were regressed against many shrub

variables. Shrub foliar cover and volume per acre explained the most variation in

most pine parameters. Consequently, shrub foliar cover is probably the most

practical shrub variable to measure.
USDA Forest Service Research Paper. PSW-RP-231. 1997.

1

Introduction This paper reports a long-term study on secondary succession in an ecosystem

consisting of three shrub species, one species of grass, a few forbs, and planted

ponderosa pines. Because the study presents trends on the presence, density, and

development of these species and their ascendance and decline, it has strong

successional overtones. Because it demonstrates that even small amounts of an

aggressive shrub species can significantly reduce pine seedling growth, it suggests

at least one vegetation management principle. And because it creates an

environment in which a less aggressive species excludes a more aggressive species,

it suggests a biological means for manipulating unwanted vegetation.

Recent polls, newspaper articles, and proposed legislation seem to be

emphasizing that the American people want the inherent productivity of the

forest to be maintained, and that it provide a full range of amenities and

commodities for future generations. Emphasis is being placed on ecosystems

and especially on ecosystem components and processes. Sometimes, ecosystems

need to be nudged a bit. Large burned areas that once were forest, for example,

have no tree seed source and centuries can pass before a forest reoccupies the

land. Here, ecosystem processes can be speeded up by manipulating a key

component like plant species. 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 be in position to capitalize on values and

commodities that will be needed in the future (McDonald and others 1994). In

forestry, much knowledge has been generated on economic species (primarily

conifer trees in the western United States), but little is available on the currently

uneconomic hardwoods, shrubs, forbs, and grasses.

Stewart (1987) identified several major areas in forest vegetation management

that needed research. One specific need was to evaluate “the effect of density and

composition of the weed community on treatment response and degree of

control.” Only data from a long-term study can fully evaluate this need.

Abundant historical and scientific evidence shows that competing plants cause

major losses in conifer seedling survival and subsequent growth (McHenry 1985).

That competing vegetation must be controlled to provide site resources to conifer

seedlings is almost universally agreed upon (Gjerstad and Glover 1992, McDonald

and Fiddler 1989, Stewart and others 1984). In almost all plantations, even those with

good site preparation, competing plant species either kill seedlings outright or

reduce their growth below the potential of the site (McDonald and Fiddler 1986a).

This paper documents long-term (1962-1992) trends in shrub, grass, and

ponderosa pine seedling development for four different initial shrub densities:

no shrub, light, medium, and heavy shrubs. All were studied for 27 years. The

fourfold objectives of this paper are both ecological and silvicultural: (1) to

quantify density, foliar cover, height, and crown volume of shrubs and grass;

diameter, foliar cover, height, and crown volume of ponderosa pine, (2) to

denote growth dynamics of these species and trends in succession, (3) to relate

shrub development to ponderosa pine survival and growth, and (4) to develop

regression equations for each species in terms of the above variables, and to

ascertain which shrub variable best quantifies competition to pine seedlings.

Methods

Location and Site Characteristics

The study site is located on the Mt. Shasta Ranger District of the USDA Forest

Service’s Shasta-Trinity National Forests, about 3 airline miles north of the city of
2 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Mt. Shasta, and about an equal distance southwest of the mountain itself. The

Township and Range survey notes of 1879 portray the vegetation in what is now

the study area. The entire township was described as being “very heavily timbered”

with five species of conifers and “some” oak. As close as can be determined, at

least part of the study area supported this type of forest. Part of it also supported

“dense chaparral undergrowth,” which implies an overstory of trees. The recorded

diameter of bearing trees (those marked to denote the location of section corners or

quarter corners) varied between 4 and 60 inches at breast height. The largest

breast-height diameter of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) was 50

inches, sugar pine (Pinus lambertiana Dougl.) 60 inches, and ponderosa pine (Pinus

ponderosa Dougl. ex Laws. var. ponderosa) 38 inches. Trees of California white fir

(Abies concolor var. lowiana [Gord.] Lemm.), incense-cedar (Libocedrus decurrens

Torr.), and California black oak (Quercus kelloggii Newb.) were present but smaller.

In December 1886, the Central Pacific Railroad, which was extending its lines

north from Redding, California and south from Medford, Oregon, met—forming

an extensive West Coast rail network. The railroad was key to utilizing the vast

timber resource in the area, and the logging and lumbering industries virtually

exploded (Zanger 1992). By 1889, at least 10 sawmills were located near Sisson

(renamed Mt. Shasta in 1923). Intense logging and lumbering, plus the inevitable

spark lead to one fire after another. By 1898, a large area near Black Butte, which

is very near the study area, was noted as being a brushfield (Barrett 1935).

By 1960, the brushfield consisted of a mass of vegetation 5 to 7 feet tall, that was

virtually impenetrable to man or beast. Greenleaf manzanita (Arctostaphylos patula E.

Greene), snowbrush (Ceanothus velutinus Hook.), and Sierra plum (Prunus subcordata

Benth.) were the most abundant species. Conifer trees and seedlings were absent.

The 30-acre study area was cleared with a bulldozer in 1961 with shrubs

being pushed into windrows (fig. 1) along with an estimated 4 to 6 inches of soil.

Ponderosa pine seedlings, grown in the nursery for 2 years, were machine

planted in April 1962 at an 8- by 10-foot spacing. The seed source was from zone

Figure 1—A ponderosa pine

plantation similar to that in the study

area. Notice the shrub development

in the windrows. Mt. Shasta is in the

background, circa 1970’s.
USDA Forest Service Research Paper. PSW-RP-231. 1997. 3

“X,” which was entitled “North Coast Pine,” and included the Klamath

Mountains Province (Fowells 1946).

A pioneering study by Jay Bentley began in 1961 when he and coworkers

installed extensive trials for controlling shrub species that resprouted from root

crowns and rootstocks, or arose from seeds buried in the soil. These trials

involved dividing the area into many rectangular “bays,” each of about 0.2 acres,

sandwiched between windrows, and randomly applying various herbicides,

dosages, application techniques, and timings of application (Bentley and others

1971). Some trials ended in 1962, some in 1964. Some were effective and some

were not. The net effect was to leave a large area covered with a wide range of

shrub densities (including no shrubs), ideal for a study whose aim was to

quantify subsequent plant community-ponderosa pine relationships.

The study began in summer 1966. All bays were surveyed and the shrub

density in each was recorded as no shrubs, light, medium, or heavy shrubs (fig. 2)

based on Bentley’s density data and relative visual differences. Values of about

5,000, 10,000, and 15,000 plants per acre separated the light, medium, and heavy

designations. The number of bays in each shrub category ranged from 9 to 15. We

then randomly chose four bays in each category as our sample. Each category

was in effect a “treatment.”

Site quality of the area is low (Conlin 1963) with dominant mixed conifers

averaging 40 feet in height in 50 years (Biging and Wensel 1984). Fertility is rated as

borderline for conifers (Conlin 1963). The slope of the study area is gentle (5 percent)

with only minor dissection. The aspect is west and the elevation is about 4,100 feet.

The soil is 42 to 54 inches deep, and of outwash material derived from andesitic rock.

Soil texture grades from sandy loam near the surface to very gravelly sandy loam at

the lowest horizon. The coarse soil tends to be well drained or excessively well

drained. Mean annual precipitation is 38 inches with most falling in winter, about

half as snow. The mean annual temperature is 49 oF and the growing season is about

130 days.

The plant community when we began the study consisted of the same shrub

species noted earlier, the planted ponderosa pines, and a few other shrub and forb

species. No grasses or natural conifer seedlings were present. Most vegetation was

from seeds that were dormant in the soil or blew in on the wind. However, a few

snowbrush plants sprouted from root crowns and all Sierra plum plants originated

from roots and root crowns that had escaped the bulldozer.

Several species of insects besieged the plantation and affected the growth and

form of the ponderosa pines. The principal species were the pine needle-sheath

miner (Zelleria haimbachi Busch) (Stevens 1959) and the gouty pitch midge

(Cecidoymia piniinopis Osten Sacken) (Bedard and others 1989). Because they were

part of the ecology of the plantation and influenced the development of the pines,

their damage was surveyed and extensive notes taken on their effect. Damage was

recorded separately for death of the terminal shoot and death of lateral twigs.
4 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Figure 2—Competing vegetation

and ponderosa pine seedlings in (A)

no-shrub, (B) light-shrub, (C)

medium-shrub, and (D) heavy-shrub

plots, Mt. Shasta brushfields, 1966.

A

B
USDA Forest Service Research Paper. PSW-RP-231. 1997. 5

C

D
6 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Study and Design

The experimental design was completely randomized with four treatment

categories and four replications, each referred to as a “plot.” Small manzanita

and snowbrush seedlings were hand pulled in the no-shrub plots for the first two

years. None appeared after that.

Each plot was rectangular and consisted of about 60 pine seedlings

surrounded by at least two rows of buffer (seedlings receiving similar treatment).

Forty healthy seedlings, identified as potential crop trees, then were chosen for

treatment. No small, misshapen, or discolored seedlings that would have been

removed in the first thinning were chosen. On each of the sample seedlings, stem

height and stem diameter at breast height were measured. The seedlings also

were checked for possible injury from wind, drought, or insects.

Sampling intensity of shrubs and other vegetation was five randomly selected

subplots in each plot. Subplots were square and contained 1 milacre (0.001 acre).

The most abundant species were 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 groundline to top

of plant). Less abundant species were not measured, but noted in a species list.

Crown volume for shrubs and grass was calculated by multiplying foliar cover in

square feet times height in feet. Crown volume for ponderosa pine, which was

the space occupied by both crown and stem, was calculated using the formula of

a cone (1.047 r

2

h). Volume in both instances was expressed on a per-acre basis.

Vegetation was measured in late summer or fall during various intervals,

which were close together early in the study and widened at the study end. 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 the analytical tools (SAS Institute 1988). Because natural resource managers

may wish to express these species’ development on similar sites, regression

equations are presented for each species’ density, foliar cover, height, and crown

volume (SAS Institute 1988) on the basis of 1966-1992 data. One equation was fitted

for each treatment and species. The general regression model was: Yij = a + bxij + errorij

where: Y = average response per plot;

a = intercept;

b = coefficient;

x = function of age (age

2

, ln of age, or age);

i = 1,2,3,4; replication (number of plots/replication = 4);

j = 1,2, . . . y (y = number of yearly intervals).

The regression analyses ignore the fact that measurements were taken on

permanent plots measured at yearly intervals and are not truly independent.

This may underestimate the standard errors and possibly increase the

significance of the estimated coefficients (intercept and slope).

And because we wished to determine which shrub variable explained the

most variation in the various pine parameters, we regressed the pine parameters

against the shrub variables. For this test, “shrub” was all shrub species in all

categories. The general regression model was linear as before, where:

Y = average pine diameter, height, foliar cover per acre, or crown volume

per acre response per plot,

a = intercept,

b = coefficient, and
USDA Forest Service Research Paper. PSW-RP-231. 1997. 7

x = shrub density, height, foliar cover per acre, or crown volume per acre

at ages 10, 20, and 29. Three forms of each shrub variable (numerical

value, value2

, and natural log of value) were tested.

Each pine parameter was regressed separately against each shrub variable, each

treatment, each year, and each form.

Significance in all tests was at α = 0.05. The α levels (type I errors) given for

comparing means apply to each measurement time separately.

Results

Plant Diversity

The natural plant community on all plots at the beginning of the study (1966)

consisted of 6 shrubs, 10 forbs, and one graminoid (table 1). One year later, a few

plants of needlegrass (Achnatherum nelsonii [Scribner]) were noticed in the noshrub and light-shrub plots. One other forb was found on study plots in 1975 and

another in 1979. These were the only increases in species over the initial plant

community. Two ephemeral forb species are noteworthy because of their

contrasting habits. Baker’s globe mallow (Iliamna bakeri [Jepson] Wiggins) is a

relatively tall (2.5 feet) perennial that had invaded the study plots in 1966,

peaked in 1971, and disappeared by 1979. It occupied only the most open places.

American vetch (Vicia americana Willd. var. americana) is an annual that is both

short (4 inches) and sprawling (up to 1.2 feet tall in deep shade). It invaded the

study plots in 1975, peaked in 1979, and was gone in 1983. It tended to occupy the

most dense plots although it was present in one light-shrub plot for some time.

Total number of species declined over time from the initial 17 members to 13

species in 1979, 11 species in 1983, and 8 species in 1992. By the study’s end, the

dominance of the shrubs and pines was nearly complete. Only two plants of one

forb, three of another, and some short plants of needlegrass could be found.

Table 1—Natural vegetation in study plots, Mt. Shasta brushfield, ShastaTrinity National Forests, California, 1966-1992

Species 1966 1977 1979 1983 1992

Shrubs

Amelanchier pallida XXXXArctostaphylos patula X X X XX

Ceanothus prostratus X X X XX

Ceanothus velutinus X X X XX

Chrysothamnus nauseosus X X X XX

Prunus subcordata X X X XX

Forbs

Carduus nutans X - - --

Eriogonum spp. X X X X X

Gayophytum spp. X - X - -

Hypericum perforatum X - - --

Iliamna bakeri XX - --

Lilium spp. X X X X -

Polygala cornuta X X X XX

Stephanomeria paniculata - -X --

Tragopogon dubius XX - --

Vicia americana - XXXViola lobata XX - --

Unknown forb X X - - -

Graminoids

Carex spp. X - - - -

Achnatherum nelsonii - X X XX
8 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Greenleaf Manzanita

Recorded observations of greenleaf manzanita during the study period showed a

consistent trend of increasing development with virtually no damage from wind,

frost, drought, or insects. The only damage noted was on July 26, 1977 (the

second of 2 consecutive years of severe drought) when greenleaf manzanita was

noted as having lost more than half of its leaves. No lasting effect from this leaf

fall was recorded, however. One other botanically interesting event was that

manzanita first began to layer in 1973—a process uncommon for this species.

Layering is a process in which a stem, usually on an open-grown plant, touches

the ground and develops roots at that point. Because layered plants remained

attached to the parent, they were not considered as new plants and hence did not

increase plant density.

In 1966, density of this evergreen shrub ranged from almost 1,500 plants per acre

in the light-shrub category to almost 6,500 per acre in the heavy-shrub category—a

statistically significant difference that continued through 1969, but not thereafter (table

2). From 1969 to 1992, manzanita density in the three shrub categories was relatively

stable—gaining a little in the light-shrub category from new seedlings and decreasing

a little from mortality in the medium- and heavy-shrub categories. At the end of the

study, manzanita density ranged from 2,550 plants per acre to 4,750 plants per acre.

Table 2—Average density, cover, height, and volume of greenleaf manzanita by shrub category,

Mt. Shasta brushfield, Shasta-Trinity National Forests, 1966-1992

Year Category Density Cover Height Volume

plants/acre ft2/acre ft ft3/acre

1966 Light 1,499.a1 0 a 0.3 a 0 a

Medium 4,246 ab 832 b 0.8 b 1,107 a

Heavy 6,494 b 3,580 b 1.3 c 5,194 b

Standard error 1,015 489 0.1 833

1969 Light 2,150 a 950 a 0.9 a 1,345 a

Medium 4,900 ab 4,350 a 1.9 b 9,845 a

Heavy 5,650 b 8,450 b 2.4 c 20,905 b

Standard error 800 861 0.1 2,267

1971 Light 2,200 a 1,550 a 1.4 a 2,765 a

Medium 4,800 a 5,300 a 2.6 b 15,110 b

Heavy 5,350 a 11,250 b 2.9 b 33,290 c

Standard error 649 1,034 0.2 2,691

1975 Light 2,450 a 2,750 a 1.9 a 6,665 a

Medium 4,750 a 5,750 a 3.0 b 18,370 a

Heavy 4,800 a 12,300 b 3.3 b 41,845 b

Standard error 886 1,074 0.2 3,592

1979 Light 2,300 a 4,350 a 2.7 a 13,825 a

Medium 4,700 a 7,650 a 3.8 b 30,325 b

Heavy 4,750 a 15,200 b 4.0 b 61,355 c

Standard error 836 1,060 0.2 4,107

1983 Light 2,400 a 6,950 a 3.4 a 27,445 a

Medium 4,600 a 13,400 a 4.5 b 63,060 b

Heavy 4,750 a 23,100 b 4.4 ab 102,780 c

Standard error 811 1,835 0.3 7,525

1992 Light 2,550 a 9,150 a 4.4 a 47,445 a

Medium 4,550 a 19,800 b 5.7 b 114,310 b

Heavy 4,750 a 29,100 c 5.6 ab 163,355 c

Standard error 783 2,215 0.3 11,513

1

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

significantly according to a Tukey test (α = 0.05).
USDA Forest Service Research Paper. PSW-RP-231. 1997. 9

Foliar cover of greenleaf manzanita was dynamic. It increased in all shrub

categories and differed statistically in various combinations among them for the

entire study. By study end, foliar cover ranged from 9,150 ft2

 per acre in the light

category to 29,100 ft2

 per acre in the heavy-shrub category.

Manzanita height followed much the same pattern of steady increase in all

shrub categories with statistically shorter plants in the light-shrub category. By

study end, plants averaged 4.4 to 5.7 feet tall (table 2).

Above-ground volume consistently increased in all shrub categories

throughout the study period. By the end of the study, manzanita volume ranged

from 47,445 ft3 per acre in the light-shrub category to 163,355 ft3 per acre in the

heavy-shrub category (table 2). Crown volume per acre was always statistically

larger in the heavy-shrub category relative to light shrubs and differences were

statistically significant among all categories from 1979 through 1992.

The relationship of manzanita density, foliar cover, height, and volume over

the 1966 through 1992 period are portrayed in figure 3. The regression equations

are shown in the appendix.

Figure 3—Relationship of greenleaf manzanita to shrub density category for (A) density, (B) foliar cover, (C) height, and (D) crown volume, Mt.

Shasta brushfields, 1962-1992.
10 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Snowbrush

Development of snowbrush during the study period was characterized by rapid

growth, periodic damage by abiotic agents, and finally by rapid senescence (table 3).

One event in early October 1971 was notable because it did not happen. This was

the year when one of the largest outbreaks of the California tortoiseshell

(Nymphalis californica Boisduval) on record took place. On warm sunny days,

tens of thousands of butterflies passed through the plantation in a given day.

Their flight began about 12 p.m. and ended about 6 p.m. They flew just above the

trees and shrubs in a southeast direction, generally quartering the gentle south

wind. Although they defoliated several species of Ceanothus at mid elevations

throughout much of northern and central California, they did not pause and

consume snowbrush leaves in the study area.

Based on recorded observations, the winter of 1972-1973 was severe and

many upright stems of snowbrush froze and died back to the root crown. New

sprouts were prolific, however, and regrowth was rapid through 1975. Like

manzanita, the 1976-1977 drought caused early loss of snowbrush leaves. It also

killed upright snowbrush stems, and foliar cover declined. The cold, windy,

virtually snowless winter of 1978-1979 exacerbated the decline of snowbrush,

Table 3—Average density, cover, height, and volume of snowbrush by shrub category, Mt. Shasta

brushfield, Shasta-Trinity National Forests, 1966-1992

Year Category Density Cover Height Volume

plants/acre ft2/acre ft ft3/acre

1966 Light 1,082.a1 1,415 a 1.0 a 1,715 a

Medium 1,249 a 1,665 a 1.2 ab 2,231 a

Heavy 1,498 a 1,998 a 1.5 b 3,230 a

Standard error 275 615 0.1 900

1969 Light 1,050 a 3,650 a 1.7 a 7,200 a

Medium 1,400 a 5,450 a 2.3 a 13,410 a

Heavy 1,650 a 4,300 a 2.1 a 9,535 a

Standard error 298 830 0.2 2,616

1971 Light 1,100 a 5,350 a 2.6 a 16,505 a

Medium 1,400 a 7,850 a 3.1 a 26,300 a

Heavy 1,700 a 5,900 a 2.7 a 18,220 a

Standard error 254 1,495 0.3 6,204

1975 Light 1,050 a 5,950 a 2.9 a 17,990 a

Medium 1,450 a 8,500 a 3.0 a 27,340 a

Heavy 1,500 a 5,950 a 2.8 a 18,695 a

Standard error 267 1,634 0.3 6,313

1979 Light 900 a 4,450 a 3.5 a 16,985 a

Medium 1,250 a 6,800 a 3.8 a 27,605 a

Heavy 1,400 a 4,650 a 3.5 a 18,165 a

Standard error 270 1,025 0.3 6,369

1983 Light 800 a 4,350 a 3.8 a 19,300 a

Medium 1,050 a 8,300 a 4.1 a 39,275 a

Heavy 1,200 a 5,000 a 4.1 a 22,815 a

Standard error 278 2,077 0.4 11,560

1992 Light 400 a 2,600 a 5.8 a 15,485 a

Medium 600 a 2,250 a 6.1 a 14,065 a

Heavy 950 a 1,750 a 5.6 a 11,305 a

Standard error 237 823 0.5 4,958

1

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

according to a Tukey test (α = 0.05).
USDA Forest Service Research Paper. PSW-RP-231. 1997. 11

and mortality—not just of upright stems—but of entire plants, was noted in all

shrub categories. In 1983, some new sprouting from the root crown was recorded,

but entire plants continued to die. Individual stems died also, but this time, the

lower (shaded) stems expired, rather than the exposed (upright) ones. The form

of snowbrush plants also changed as remaining stems were forced to become

more upright to stay in sunlight. By 1992, many plants and stems had died. Those

left were generally tall and spindly.

Density of snowbrush in 1966 varied from more than 1,000 plants per acre in

light-shrub plots to almost 1,500 plants per acre in heavy-shrub plots (table 3). For

all shrub categories, mean density stayed relatively constant through 1971 and

then decreased from 1975 through 1992. By the end of the study, density of

snowbrush had decreased 65 percent to 400 plants per acre in light-shrub plots

and by 44 percent to 950 plants per acre in heavy-shrub plots. At no time during

the study were differences in snowbrush density among shrub categories

statistically significant.

Foliar cover of snowbrush increased from 1966 through 1975 and then

decreased through the end of the study. In 1975, average foliar cover peaked at

5,950 ft2

 per acre in the light-shrub category, at 8,500 ft2 in the medium-shrub

category, and at 5,950 ft2

 per acre in the heavy-shrub category (table 3). By the end

of the study, foliar cover had decreased 56, 74, and 71 percent, respectively, for

the light-, medium-, and heavy-shrub categories. Differences in foliar cover

never were statistically significant among shrub categories during the study.

Average height of snowbrush generally increased with time in all shrub

categories, reaching 5.6 to 6.1 feet in 1992. Other than in 1966, differences in

average height were not statistically significant among categories (table 3). Some

of the variation in foliar cover and height occurred when old stems died and new

stems replaced them.

Average crown volume of snowbrush generally increased from 1966 through

1983, and then decreased markedly (table 3). At the study end in 1992, snowbrush

volume by shrub category (decrease from 1983 in parentheses) was light, 15,485

ft3 per acre (20 percent); medium, 14,065 ft3 (64 percent); and heavy 11,305 ft3 per

acre (50 percent), respectively. No statistically significant differences in crown

volume occurred at any measurement date during the study.

The relationship of snowbrush density, foliar cover, height, and volume over

the 1966 through 1992 period are portrayed in figure 4. The regression equations

are shown in the appendix.
12 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Figure 4—Relationship of snowbrush to shrub density category for (A) density, (B) foliar cover, (C) height, and (D) crown volume, Mt. Shasta brushfields,

1966-1992.
USDA Forest Service Research Paper. PSW-RP-231. 1997. 13

Sierra Plum

This species was damaged by abiotic agents and suffered from suppression

almost from the beginning of the study. In 1967, a 2- to 6-inch dieback of exposed

shoots was noted in all shrub categories; wind and drought were the probable

cause. In 1969, many stems were bent or broken, apparently from snow. Exposed

shoots also died back. By 1969, most plants had become shorter than other shrub

species. In 1973, a surge of new sprouts from horizontal roots was recorded, but

from 1975 to the end of the study, Sierra plum generally was declining, especially

in foliar cover. By 1992, most plants had died back several times and either had

died outright or become shorter. Because a few plants put forth a tall vigorous

shoot, especially where snowbrush plants had died, this shortening is not

reflected in the height values for 1992, which actually increased over 1983 values

in every shrub category (table 4).

Although this plum produced many fruits and seeds, new seedlings

were never found. After an initial decrease, mean density of Sierra plum

in the light-shrub category increased from 150 plants per acre in 1969 to

1,150 plants per acre in 1992—all from roots or root crowns (table 4). In the

medium-shrub category, density initially decreased and then remained

relatively stable at 3,400 to 3,750 plants per acre. Density of Sierra plum in

the heavy-shrub category also decreased initially, remained relatively

Table 4—Average density, cover, height, and volume of Sierra plum by shrub category, Mt. Shasta

brushfield, Shasta-Trinity National Forests, 1966-1992

Year Category Density Cover Height Volume

plants/acre ft2/acre ft ft3/acre

1966 Light 500.a1 0.a 0.7 a 0 a

Medium 5,994 b 2,414.ab 1.6 b 3,888 ab

Heavy 7,326 b 2,913.b 1.7 b 5,661 b

Standard error 1,310 731 0.1 1,405

1969 Light 150 a 0 a 1.0 a 0 a

Medium 3,750 ab 1,850 a 1.5 a 3,800 a

Heavy 5,900 b 1,900 a 1.6 a 3,535 a

Standard error 1,141 546 0.2 1,150

1971 Light 150 a 0 a 0.7 a 0 a

Medium 3,550 ab 1,250 a 1.6 a 2,535 a

Heavy 5,850 b 1,250 a 1.3 a 2,200 a

Standard error 1,080 466 0.2 910

1975 Light 200 a 0 a 0.9 a 0 a

Medium 3,750 ab 950 a 1.6 a 2,095 a

Heavy 5,750 b 1,050 a 1.4 a 1,915 a

Standard error 1,000 352 0.2 859

1979 Light 200 a 50 a 1.8 a 110 a

Medium 3,750 ab 700 a 1.8 a 1,495 a

Heavy 5,850 b 900 a 1.4 a 1,765 a

Standard error 1,024 325 0.2 723

1983 Light 400 a 100 a 1.0 a 185 a

Medium 3,750 ab 650 a 1.9 a 1,620 a

Heavy 5,700 b 950 a 1.4 a 2,435 a

Standard error 1,062 280 0.3 972

1992 Light 1,150 a 100 a 2.1 a 350 a

Medium 3,400 a 250 a 3.2 a 1,060 a

Heavy 4,150 a 150 a 2.0 a 615 a

Standard error 793 108 0.5 457

1

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

according to a Tukey test (α = 0.05).
14 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Figure 5—Relationship of Sierra plum to shrub density category for (A) density, (B) foliar cover, (C) height, and (D) crown volume, Mt.

Shasta brushfields, 1966-1992.

constant at about 5,800 plants per acre, and then decreased to 4,150 plants

per acre at the end of the study. Statistically significant differences

generally occurred between the fewer shrubs in the light-shrub category

and the many more shrubs in the heavy-shrub category.

Although trends in mean foliar cover differed among shrub categories, no

difference among categories was statistically significant at any measurement

date. For the light-shrub category, cover began with zero and increased slightly

to 100 ft2 per acre at the end of the study (table 4). For the medium- and heavyshrub categories, cover decreased consistently during the 27-year study to 250

and 150 ft2 per acre, respectively. Overall decreases were 90 and 95 percent.

Mean height of Sierra plum varied inconsistently during the study, and other than

initially, differences were never statistically significant among shrub density classes.

At the end of the study, plum height ranged from 2 to 3 feet (table 4). Like snowbrush,

the height of Sierra plum varied over the years from dieback and resprouting.

Crown volume of this shrub closely paralleled the trends in foliar cover and

height noted above. In the light-shrub category, volume was absent through 1975

and then increased slightly throughout the study (table 4). Volume was larger in

the other categories and generally decreased throughout the study period. The

decrease was particularly sharp in 1992.

The relationship of Sierra plum density, foliar cover, height, and volume

over the 1966 through 1992 period are portrayed in figure 5. The regression

equations are shown in the appendix.
USDA Forest Service Research Paper. PSW-RP-231. 1997. 15

Table 5—Average density, cover, height, and volume of combined shrubs by shrub category, Mt. Shasta

brushfield, Shasta-Trinity National Forests, 1966-1992

Year Category Density Cover Height Volume

plants/acre ft2/acre ft ft3/acre

1966 Light 3,080.a1 1,415 a 0.7 a 1,324 a

Medium 11,488 b 4,912 b 1.2 b 6,088 b

Heavy 15,318 b 8,491 c 1.5 b 13,262 c

Standard error 1,253 573 0.1 882

1969 Light 3,350 a 4,600 a 1.2 a 6,802 a

Medium 10,050 b 11,650 b 1.9 b 23,321 b

Heavy 13,200 b 14,650 b 2.1 b 30,618 b

Standard error 1,420 926 0.2 2,758

1971 Light 3,450 a 6,900 a 2.1 a 14,979 a

Medium 9,750 b 14,400 b 2.4 a 35,439 b

Heavy 12,900 b 18,400 b 2.4 a 45,161 b

Standard error 1,352 1,183 0.2 4,293

1975 Light 3,700 a 8,700 a 2.3 a 21,052 a

Medium 9,950 b 15,200 b 2.5 a 38,500 b

Heavy 12,050 b 19,300 b 2.6 a 50,419 b

Standard error 1,333 1,227 0.2 4,126

1979 Light 3,400 a 8,850 a 3.0 a 28,700 a

Medium 9,700 b 15,150 b 3.1 a 47,930 b

Heavy 12,000 b 20,750 c 3.1 a 64,837 b

Standard error 1,358 1,174 0.2 4,838

1983 Light 3,600 a 11,400 a 3.2 a 40,851 a

Medium 9,400 b 22,350 b 3.4 a 79,437 b

Heavy 11,650 b 29,050 b 3.4 a 100,928 b

Standard error 1,256 1,803 0.2 8,989

1992 Light 4,100 a 11,850 a 4.3 a 60,152 a

Medium 8,550 b 22,300 b 4.7 a 107,540 ab

Heavy 9,850 b 31,000 b 4.5 a 140,442 b

Standard error 1,009 2,490 0.3 12,982

1

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

according to a Tukey test (α = 0.05).

Combined Shrubs

Combined values for greenleaf manzanita, snowbrush, and Sierra plum reflect

the contribution of the shrubs—a very important component of the total plant

community in the Mt. Shasta brushfields. In 1966, density of combined shrubs

ranged from 3,080 plants per acre in the light-shrub category to 15,318 plants per

acre in the heavy-shrub category (table 5). Shrub density was significantly lower

in the light-shrub category than in the medium- or heavy-shrub category, which

did not significantly differ from each other throughout the study. Mean density

of combined shrubs in the light-shrub category generally increased from the

beginning of the study (3,080 plants per acre) to the end of the study (4,100 plants

per acre). This contrasted with the trend of density in the other categories where

it declined (11,488 shrubs per acre to 8,550 per acre in the medium-shrub category

and 15,318 plants per acre to 9,850 per acre in the heavy-shrub category).

Mean foliar cover of combined shrubs generally increased throughout the

study in all shrub density categories. From the beginning of the study to the end,

the increases were: light shrubs 1,415 to 11,850 ft2 per acre; medium shrubs 4,912

to 22,300 ft2, and heavy shrubs 8,491 to 31,000 ft2 per acre (table 5). Significantly
16 USDA Forest Service Research Paper. PSW-RP-231. 1997.

more cover was present in the medium- and heavy-shrub categories than in the

light-shrub category, although in 1966 and 1979 shrub cover differed significantly

among all three categories.

Mean height also increased in all shrub categories during the study. By the

end of the study, the height of combined shrubs ranged from 4.3 to 4.7 feet and

did not differ statistically among categories (table 5).

To further understand the performance of these shrubs, periodic annual

height increment (PAI), defined as the difference in height between succeeding

years divided by the number of years in the period, was calculated for each shrub

species and for combined shrubs. Individually, the PAI of manzanita was least

variable and that of snowbrush most variable. When presented for combined

shrubs, PAI, which was quite variable through 1980, became consistently small

(less than 0.2 feet) in light-, medium-, and heavy-shrub categories in 1992 (fig. 6).

Figure 6—Periodic annual height

increment (PAI) of combined shrubs,

Mt. Shasta brushfields, 1966-1992.
USDA Forest Service Research Paper. PSW-RP-231. 1997. 17

Figure 7—Relationship of combined shrubs to shrub density category for (A) density, (B) foliar cover, (C) height, and (D) crown volume, Mt. Shasta

brushfields, 1966-1992.

The average crown volume of combined shrubs, which differed significantly

among all shrub categories in 1966, significantly differed only between light shrubs

and the other two categories from 1969 through 1983 (table 5). In 1992, volume

differed significantly only between the light-shrub and heavy-shrub categories.

Total combined shrub volume at the end of the study by category was: light, 60,152

ft3

 per acre; medium, 107,540 ft3 per acre; and heavy, 140,442 ft3 per acre.

The relationship of combined shrub density, foliar cover, height, and volume

over the 1966 through 1992 period are portrayed in figure 7. The regression

equations are shown in the appendix.
18 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Table 6—Average density, cover, height, and volume of needlegrass by shrub category, Mt. Shasta

brushfield, Shasta-Trinity National Forests, 1966-1992

Year Category Density Cover Height Volume

plants/acre ft2/acre ft ft3/acre

1966 No shrubs - - - -

Light - - - -

Medium - - - -

Heavy - - - -

Standard error - - - -

1969 No shrubs 17,800.a1 1,950 a 1.7.a 3,725 a

Light 4,300 ab 550 a 1.0.a 940 a

Medium 2,550 b 50 a 1.6.a 55 a

Heavy 0 b 0.a- 0.a

Standard error 3,563 497 0.2. 924

1971 No shrubs 32,000 a 2,550 a 2.5.a 6,850 a

Light 9,000 b 450 b 1.4.b 1,020 b

Medium 6,850 b 150 b 1.5.ab 385 b

Heavy 100 b 0 b 0.3.b 0.b

Standard error 3,561 451 0.2. 1,252

1975 No shrubs 44,950 a 4,300 a 2.4.a 10,240 a

Light 13,050 b 600 b 1.4.b 1,080 b

Medium 9,650 b 200 b 1.3.b 385 b

Heavy 200 b 0 b 0.8.b 0.b

Standard error 4,530 338 0.1. 608

1979 No shrubs 50,000 a 4,450 a 2.5.a 11,360 a

Light 17,650 b 600 b 2.0.ab 1,355 b

Medium 8,200 bc 150 b 1.6.b 355 b

Heavy 400 c 0 b 0.6.c 0.b

Standard error 3,798 326 0.2. 812

1983 No shrubs 65,900 a 4,750 a 2.9.a 14,175 a

Light 16,800 b 700 b 1.9.b 1,575 b

Medium 5,750 b 150 b 1.2.c 240 b

Heavy 450 b 0 b 1.1.c 0

Standard error 4,673 434 0.1. 1,281

1992 No shrubs 12,850 a 400 a 0.7.a 540 a

Light 0 a 0 a -.a 0 a

Medium 0 a 0 a -.a 0 a

Heavy 0 a 0 a -.a 0 a

Standard error 6,027 200 0.3. 270

1

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

according to a Tukey test (α = 0.05).

Needlegrass

Formerly classified as Stipa columbiana, this grass now has been placed in the genera

Achnatherum and given the specific epithet nelsonii (Hickman 1993). Beginning in

1967, needlegrass rapidly invaded the study area. This grass first colonized bare

areas in no-shrub and light-shrub plots and by 1969 had moved into medium-shrub

plots. It occurred in all plots in these categories and one heavy-shrub plot in 1971

(table 6). In 1979 and 1983, needlegrass had expanded to three of the four heavyshrub plots. Where it was free of shrubs and not directly under pines, its

developmental pattern over several years was from a few single stems per acre, to

thousands of single stems, to a combination of well-defined clumps and many

single stems (fig. 8). When the needlegrass population declined, it followed a lastin/first-out pattern. By 1992, it was present only in the no-shrub plots.

In terms of density, 17,800 plants per acre were present in the no-shrub

category in 1969 (table 6). Average number of plants in this category increased to

65,900 per acre in 1983, but decreased markedly to 12,850 per acre in 1992.

Number of plants in the light-shrub category was about one-fourth this amount

throughout this same timespan, but fell to zero in 1992. Density of needlegrass in
USDA Forest Service Research Paper. PSW-RP-231. 1997. 19

the medium-shrub category increased from 1969 (2,550 plants per acre) through

1975 (9,650 plants per acre), declined from 1979 through 1983, and was zero in

1992. Needlegrass in the heavy-shrub category ranged from 100 plants per acre

in 1971 to 450 plants per acre in 1983, to zero plants in 1992.

Average foliar cover of needlegrass, although never large, corresponded

well to the trend in density. It was largest in the no-shrub category and decreased

dramatically (table 6) as the amount of shrubs increased. In fact, needlegrass

never developed well enough to have measurable cover in the heavy-shrub

plots. In general, foliar cover in the no-shrub category differed significantly from

all other categories.

Average height of needlegrass generally followed the same trends noted

above. It tended to increase with time through 1983, reaching its maximum

height of 2.9 feet in no-shrub plots in that year (table 6). Average crown volume

also peaked in 1983, reaching 14,175 ft3 per acre in no-shrub plots (table 6). It then

decreased to 540 ft3 per acre in these plots in 1992, and to zero in all others.

Because nearly all needlegrass plants had died by the end of the study.

regression equations for density, foliar cover, height, and volume were not derived.

Figure 8—Typical needlegrass density

and development in the no-shrub

category, Mt. Shasta brushfields, 1973.
20 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Ponderosa Pine

Because we did not begin this study until summer 1966, we quantified mortality

well after its typical early-age peak. Consequently, during our study a few

ponderosa pine seedlings died each year through 1975, with no single year

having concentrated mortality. By 1979, survival of ponderosa pine saplings by

treatment was:

No shrubs 99 percent

Light shrubs 98 percent

Medium shrubs 98 percent

Heavy shrubs 81 percent

At the end of the study in 1992 only one additional pine had died, this being in a

medium-shrub plot.

Damage to pine seedlings was almost entirely from winterburn and insects.

Winterburn, as its name suggests, occurs when the ground is frozen and the

wind pulls the moisture from the needles. This moisture cannot be replaced

because the roots are inactive. This happened for several years, especially when

the pines were younger. It caused many needles to turn brown and look

unhealthy, but it did not kill any seedlings.

The gouty pitch midge and the pine needle-sheath miner did not kill any

seedlings either, but their damage caused a loss of height growth on some

seedlings and deformed others so badly that they will never become crop trees

(fig. 9). Some pine trees were attacked repeatedly, others periodically, and still

others only rarely. Insect damage was chronic. Rare was the year that at least a

Figure 9—(A) This 12-year-old

ponderosa pine in heavy shrubs has

at least four new terminals, (B)

Although an extreme case, this 31-

year-old pine shows the negative

effects of heavy shrubs and repeated

insect attacks.

A
USDA Forest Service Research Paper. PSW-RP-231. 1997. 21

few buds were not killed and also rare was the year that both of these insects

were not present together in the plantation.

Insect damage was first recorded in 1968 but did not materially affect tree

form until 1970. Death of terminal shoots was manifest primarily from 1970

through 1979 (table 7). By 1983, few trees were affected in this manner, although

lateral shoots continued to be killed by these insects. Number of dead terminal

shoots varied from year to year depending on the intensity of the insect attack. A

general trend was that damage increased with increasing shrub density. The

average number of pines with dead terminal shoots in each plot during the 1970-

1979 period, for example, was 8 in no shrubs and 42 in heavy shrubs—or more

than five times as many killed in the heavy-shrub category.

Table 7—Average number of ponderosa pines with dead terminal shoots per plot in each

shrub category, Mt. Shasta brushfield, Shasta-Trinity National Forests, 1970-1979

Category Year Total

1970 1971 1973 1975 1977 1979

No shrubs 0 1 1 33 0 8

Light shrubs 2 6 1 8 9 0 26

Medium shrubs 1 4 5 10 6 1 27

Heavy shrubs 5 7 11 8 9 2 42

B
22 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Death of lateral shoots from insects also first showed up in 1970 but extended

throughout the study period. After 1970 this form of damage worsened with

1971 through 1977 being a period of heavy attacks and much damage (table 8).

For the 27-year period that we collected these data, 1975 was the worst. The

statistics that follow quantify the effect of an epidemic by the pine needle-sheath

miner and the gouty pitch midge. In 1975, the number of pines in the study, after

mortality, totalled 613 trees. Of these, 598 or 98 percent had one or more dead

lateral branches from insect damage. The average per tree was 22 dead laterals in

the range of 2 to 200. One 14-foot-tall tree of good form was described as having

100 percent of its 200 lateral shoots infested; another 8.9-foot-tall pine had 80

laterals infested, which also was 100 percent. In spite of this, both trees were alive

and growing well at the end of the study. In 7 of the 16 plots, 100 percent of the

trees were infested to some degree. Even seedlings that were overtopped by

shrubs were attacked. A general trend, which held from 1970 through 1979, was

that in a given year, the average number of infested pines per plot was roughly

similar regardless of shrub-density category. The average number of infested

pines per plot in the heavy-shrub category is lower than that in other categories (table 8) from 1975 through 1979 because the average was influenced by mortality.

About six trees per plot had died in this category, leaving fewer trees present to

become infested. After 1979, the number of trees with dead lateral branches

increased as shrub density increased.

Average breast-height diameter of ponderosa pine was measured from 1979

through 1992. During this period it increased from 5.08 inches to 7.85 inches in

no-shrub plots, from 3.89 to 6.11 inches in light-shrub plots, from 2.91 to 4.56

inches in medium-shrub plots, and from 1.35 inches to 2.14 inches in heavyshrub plots (table 9). Statistically, pine diameter differed among all shrub

categories during this timespan.

Mean foliar cover of this pine increased in all shrub categories from 1966

through 1992. At the end of the study foliar cover was 24,050 ft

2

 per acre in noshrub plots, 12,400 ft2

 in light-shrub plots, 11,200 ft

2

 in medium-shrub plots, and

4,150 ft

2

 per acre in heavy-shrub plots (table 9). Statistically significant differences

in general were consistent between the no-shrub and heavy-shrub categories and

often between no-shrub and most of the other categories.

Mean ponderosa pine height differentiated rapidly among shrub categories

after 1969. From 1975 through 1992 pine height at each measurement date was

significantly different among all shrub categories (table 9). At the end of the

study, pine height by shrub category was: no shrubs, 30.4 feet; light shrubs, 21.6

feet; medium shrubs, 15.2 feet, and heavy shrubs, 9.0 feet.

Average crown volume of ponderosa pine increased in all shrub categories

during the study, and in general differed significantly between the no-shrub and

all other categories (table 9). Average volume of pines ranged from 237,894 ft

3

 per

acre in the no-shrub category to 15,125 ft

3

 per acre in the heavy-shrub category at

the end of the study.

The relationship of ponderosa pine foliar cover, height, and crown volume

over the 1966 through 1992 period are portrayed in figure 10. The regression

equations are shown in the appendix.

To ascertain which shrub variable explained the most variation in a given

ponderosa pine sapling parameter, we regressed four pine parameters against

four shrub variables, three timespans, and three forms of numerical values. Of

the 132 regressions that we examined, 81 showed a significant relationship

among tested parameters at the 0.05 level. Here one should remember that

although a relationship may be significant, it may not explain much of the

variation in a given parameter.
USDA Forest Service Research Paper. PSW-RP-231. 1997. 23

Table 9—Average breast-height diameter, cover, height, and volume of ponderosa pine by shrub

category, Mt. Shasta brushfield, Shasta-Trinity National Forests, 1966-1992

Year Category Diameter Cover Height Volume

inches ft2/acre ft ft3/acre

1966 No shrubs - - 2.9 a -

Light - 916.a1 2.9 a 761 a

Medium - 1,415 a 2.8 a 1,393 a

Heavy - 500 a 2.4 a 430 a

Standard error - 326 0.2 299

1969 No shrubs - 5,750 a 6.8 a 12,856 a

Light - 3,250 ab 6.2 ab 5,370 b

Medium - 3,650 ab 5.5 b 6,804 b

Heavy - 1,450 b 3.9 c 2,027 b

Standard error - 628 0.3 1,318

1971 No shrubs - 9,400 a 9.3 a 28,442 a

Light - 5,100 b 7.8 b 11,855 b

Medium - 5,150 b 6.8 b 11,844 b

Heavy - 2,000 b 4.5 c 3,411 b

Standard error - 774 0.3 2,315

1975 No shrubs - 14,000 a 13.9 a 65,497 a

Light - 7,800 b 10.6 b 24,013 b

Medium - 6,450 bc 8.7 c 18,883 b

Heavy - 2,400 c 5.3 d 5,147 b

Standard error - 1,144 0.4 5,072

1979 No shrubs 5.08 a 20,050 a 16.5 a 113,128 a

Light 3.89 b 12,600 ab 12.0 b 46,343 b

Medium 2.91 c 9,800 bc 9.3 c 31,820 bc

Heavy 1.35 d 3,400 c 5.8 d 8,554 c

Standard error 0.15 1,775 0.4 8,785

1983 No shrubs 5.93 a 21,700 a 20.9 a 157,273 a

Light 4.54 b 12,750 ab 14.9 b 59,856 b

Medium 2.96 c 10,200 ab 10.8 c 39,501 b

Heavy 1.30 d 4,200 b 6.4 d 12,664 b

Standard error 0.24 3,162 0.6 19,789

1992 No shrubs 7.85 a 24,050 a 30.4 a 237,894 a

Light 6.11 b 12,400 ab 21.6 b 82,325 b

Medium 4.56 c 11,200 b 15.2 c 57,225 b

Heavy 2.14 d 4,150 b 9.0 d 15,125 b

Standard error 0.31 2,950 0.9 28,694

1

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

significantly according to a Tukey test (α = 0.05).

Table 8—Average number of infested ponderosa pines and number of dead lateral shoots per plot

in each shrub category, Mt. Shasta brushfield, Shasta-Trinity National Forests, 1970-1992

Category Year

1970 1971 1973 1975

S1 L SL SL SL

No shrubs 1 1 17 348 18 219 39 760

Light shrubs 1 5 20 387 17 200 39 1,160

Medium shrubs 1 2 21 302 23 258 39 894

Heavy shrubs 1 7 20 216 20 162 32 528

1977 1979 1983 1992

S L SL SL SL

No shrubs 29 254 18 103 7 62 3 43

Light shrubs 25 222 20 191 9 71 3 26

Medium shrubs 29 313 16 122 13 128 6 40

Heavy shrubs 22 225 15 81 14 100 8 34

1

S = Number of infested pines; L = number of dead lateral shoots on them.
24 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Figure 10—Relationship of

ponderosa pine to shrub density

category for (A) foliar cover, (B)

height, and (C) crown volume, Mt.

Shasta brushfields, 1966-1992.
USDA Forest Service Research Paper. PSW-RP-231. 1997. 25

The following shrub variables explained the most variation in ponderosa

pine parameters on the basis of highest adjusted coefficients of determination

(R2

) and P values:

Ponderosa pine Combined shrubs P R2

parameter variable age

Foliar cover Foliar cover 29 0.0008 0.66

Volume/acre 29 0.0028 0.57

Volume/acre Foliar cover 29 0.0001 0.76

Volume/acre 29 0.0004 0.70

Height Ln density 20 0.0046 0.52

Foliar cover 29 0.0001 0.79

Volume/acre 20 0.0017 0.61

Diameter at Ln density 20 0.0060 0.50

breast height (Foliar cover)2 29 0.0001 0.76

Volume/acre 20 0.0023 0.58

In general, mean shrub height was never significant, mean shrub density

was relatively insignificant, numerical values were better than squared values or

natural logarithms, and 29-year-old shrub data were best. Overall, foliar cover

and crown volume per acre of combined shrubs explained the most variation in

most pine parameters. These shrub variables are the ones that the vegetation

manager may want to evaluate when determining competition in ponderosa

pine plantations. And because foliar cover is much easier to estimate and relate

to, it may be the most useful variable from a practical standpoint.

Discussion and Conclusions

Ecology

Before this study and its predecessor (Bentley and others 1971) began, the mature

brushfield had little plant diversity. After site preparation, the area gained in

diversity, and at study end it had lost what it had gained. However, an important

native tree component—ponderosa pine—had been restored to the landscape.

And in the no-shrub plots, the odds are that these pine trees will continue to

grow well and reach maturity. By forming a forest, they bring to the landscape an

increased opportunity for higher plant diversity in the future.

Cooper (1922) classified the Arctostaphylos patula-Ceanothus velutinus

community as subclimax in successional state, but Frenkel (1974) noted that “the

present chaparral type on Mount Shasta shows great stability, which possibly

reflects changes in the soil brought on by severe fire.” Thus, planted pines in the

no-shrub environment not only have high potential to form a future forest, but

they also bring this potential to the landscape much sooner than would have

occurred naturally.

The long-term trend in density and development data developed for each

species in this paper also has successional implications. Greenleaf manzanita, by

virtue of its relatively high density, strong development, and lack of damage, has

high potential to be a major component of this plant community for many more

years. In contrast, the declining density, foliar cover, and volume of snowbrush

indicate that it has low potential of being a major component of the future

community. Its susceptibility to frost, drought, and wind reinforce the likelihood

of this potential. Snowbrush will probably be a minor component overall, but
26 USDA Forest Service Research Paper. PSW-RP-231. 1997.

perhaps strong in local areas where competition from trees and manzanita is less.

The future role of Sierra plum in the plant community is not yet certain. This

species has two attributes that the other shrubs did not have—capability to exist

in the shade and ability to sprout in the shade, albeit with a single stem and little

foliar cover. Its ability to develop vegetatively from roots and root crowns

allowed it to produce new shoots that kept many plants alive. Although damaged

by many abiotic agents, Sierra plum was able to recover from most of them and

to actually gain in density in areas having a light amount of shrubs. Overall, this

species seems likely to maintain a strong presence in the understory and to

sprout when opportunities arise.

Needlegrass, like the various forbs, was an ephemeral species. Its role in

succession, however, is interesting both from the standpoint of where it happened

and what it caused not to happen. It first invaded bare areas, became most dense,

developed best, and absented last from these areas. Its entire record in this study

strongly portrays its strong presence and dynamic nature in habitats free of

shrubs, and its much reduced presence and development in areas where shrubs

were present. For example, if even a light amount of shrubs had existed,

needlegrass density would have decreased by about 70 percent and foliar cover

by about 80 percent from values in no-shrub plots.

Because of the ascending and descending trends in the various components

of the vegetative community in the study plots, we surmised that after 31 years, a

similar biological sum or carrying capacity (Ford-Robertson 1971) might be

reached in all shrub categories. But which of our measured variables would best

show this? Density was dominated by the huge number of needlegrass plants.

Crown volume and height reflected mostly pine characteristics. Consequently,

foliar cover seemed the only plausible descriptor of total vegetative development.

When foliar cover of grass, shrubs, and ponderosa pines in each shrub

category was totaled (table 10), some major differences were evident. Foliar cover

in all categories, except medium-shrub, reached its maximum after 22 years. This

maximum ranged from 57 to 90 percent of that possible (43,560 ft2

—the area of an

acre). Nine years later, or 31 years after planting, foliar cover actually decreased:

by 8 percent in no-shrub plots; by 3 percent in light-shrub plots, and by 10

percent in heavy-shrub plots. As noted in table 1, no new plant species took

advantage of this space. But as noted in tables 2 and 4, the density of greenleaf

manzanita and Sierra plum increased in the light-shrub category.

That large differences in total foliar cover in the various shrub density

categories exist after 31 years is puzzling. Perhaps foliar cover is not a good

estimator of carrying capacity. Certainly it does not reflect growth in the third

dimension—height. Even more important is that carrying capacity probably is not

Table 10—Total foliar cover of all vegetation by shrub categories, Mt. Shasta

brushfield, Shasta-Trinity National Forests, 5-31 years after planting, 1966-1992

No. of No shrubs Light shrubs Medium shrubs Heavy shrubs

years

-------------------------------------- ft2/acre ----------------------------------------

5 - 2,331 6,328 8,991

14 8,300 17,100 21,850 21,700

22 26,450 24,850 32,700 39,250

31 24,400 24,200 33,500 35,150
USDA Forest Service Research Paper. PSW-RP-231. 1997. 27

governed by density and development of vegetation above ground. Belowground

is where the real quantifier of carrying capacity probably resides. Because soil

moisture is the limiting environmental variable in summer-dry California, it

follows that a moisture-related value, like root volume, would be a better quantifier

of carrying capacity. Research on the belowground ecosystem is needed.

Belowground research also is needed to better define the competitive

relationships of grasses, and needlegrass in particular. For grasses in North

American grasslands, 85 percent of the total standing crop of live plants is below

ground (Trappe 1981). The cumulative length of roots on grass plants can be

huge. A single wild oat plant excavated after 80 days of growth had developed a

total root system that measured more than 50 miles (Radosevich and Holt 1984).

Logically, the impact of competition is below ground, and the agent of

competition is the roots. Aboveground values of grass density, and especially of

foliar cover and height, may or may not, but probably do not, reflect the true

competitive ability of this grass.

Biological Control

Biological control of weeds is defined as the use of living organisms to lower

plant pest populations to the point where the pests are no longer economic

problems. Besides animals and microorganisms, biological control agents can be

other plants that are either parasitic, competitive, or allelopathic (Rosenthal and

others 1985).

The windrows surrounding the plots in this study were covered with tall

vigorous shrubs that produced many seed crops. Seedlings from these crops plus

those from seedbanks in the soil should have resulted in many young manzanita

and snowbrush plants, especially on the bare ground of the no-shrub and lightshrub areas. Indeed, young plants of these species were present in no-shrub plots

and were physically removed before needlegrass became established. Tiny

manzanita plants were present after the grass became established as well. They

were noticed early in the spring, but had disappeared by late summer. Physical

or chemical interference by the grass kept them away.

That grasses can control woody shrubs by excluding new plants has been

documented in California (Griffin 1982, Schimke and others 1970, Schultz and

others 1955) and in Oregon (Klingler 1982). However, in these studies, the grass

was seeded by hand or machines. Reports of control by naturally occurring

species are scarce.

Biological control of aggressive competing species by a less aggressive species

has special appeal to forest vegetation managers. Long-term costs may be lower,

especially if additional release treatments are not needed. And in sensitive areas,

biological control may be the only method available.

Grass can be a formidable agent in young plantations and many have failed

because of it (Rietveld 1975, Roy 1953). However, if the pines become established

before the grass, the pines usually dominate. And if the soil is deep, and the pines

become established first, the grass roots do not grow as deep and the pine roots

have a competition-free zone deep in the soil. We observed this in a pit dug in a

no-shrub plot. In this study, the needlegrass did not invade until 1967 or about 5

years after planting and the soil was deep. Consequently, naturally occurring

needlegrass excluded the deeper-rooted shrubs and in so doing indirectly aided

pine growth (McDonald 1986). Of course, it also used site resources and cost the

pines some growth. The tradeoff is unknown, but that there is a tradeoff, not just

a cost, is now recognized.

Vegetation Management

Early control of competing vegetation is fundamental to rapid and continuing

growth by ponderosa pines. Increasing evidence suggests that seedling growth
28 USDA Forest Service Research Paper. PSW-RP-231. 1997.

is closely related to amount of resources, especially to available soil moisture.

Any moisture used by competing vegetation is that much less available for

conifer growth. Consequently, the advantage lies with those plants or species

that are present first. Their roots are unimpeded by competitors’ roots or toxins

or mycorrhizae, and they are free to expand into surrounding soil. In turn, shoot

growth also benefits. Thus it is important to accomplish complete or nearcomplete control of competing vegetation during the first growing season or

within the first three seasons (McDonald and Fiddler 1986b, 1989, 1990).

Comparing the average diameter and height of ponderosa pine in the various

shrub categories demonstrates the relationship of increasing shrubs and

decreasing pine development (table 11). Even a light amount of shrubs costs the

pines some growth. Medium and heavy amounts of shrubs cost progressively

more. Are the differences made up? Periodic annual height increment (1983-

1992) for pines in the no-shrub category was 1.1 feet per year; light shrub, 0.7;

medium shrub, 0.5; and heavy shrub, 0.3 feet per year. These values plus the

slope coefficients from the regression equations show that differences in pine

height after 31 years are still widening.

How much loss of pine growth is too much? Fiske (1986) developed

minimum size standards for average potential crop trees needed to meet future

goals. Ponderosa pines, at age 30, on a site like that of the study should average 6

inches in breast-height diameter and be 20 feet tall. Thus, pines in the mediumand heavy-shrub categories in this study do not meet the standard, and treatment

to control the shrubs or start the plantation over needs to be considered.

Visual observations plus the developmental trends presented in this study

give insight to the health and future of the pine stand in the four shrub categories.

In the no-shrub category, most pines are tall, full crowned, and growing well (fig. 11).

Virtually none have been deformed by insects. In the light-shrub category, some

variation in pine size and growth rate is apparent. Several pines are permanently

deformed. In the medium-shrub category, much more variation in size and

growth rate is present. Some pines are tall, others are short. Some are deformed.

Many trees look stressed and support only current-year needles. Although almost

all trees are taller than the shrubs, most are growing slowly. In the heavy-shrub

category, the majority of surviving pines are above the shrubs but extend only a

few inches in height each year. Many are deformed.

Although mortality of ponderosa pines virtually ceased after 1979, it is a

tribute to the tenacity for life that additional trees have not died. In the heavyshrub plots, several overtopped pines with a few living needles have survived

for years beneath manzanita crowns. Another pine seedling has consisted of one

Table 11—Relationship between diameter and height of ponderosa pine and

shrub category, 1992

Shrub Average Loss in Average Loss in

category diameter diameter1 height height1

----------- inches ------------- -------------- feet------------

No shrubs 7.85 .0. 30.4 .0.

Light shrubs 6.11 1.74 21.6 8.8

Medium shrubs 4.56 3.29 15.2 15.2

Heavy shrubs 2.14 5.71 9.0 21.4

1

Relative to no-shrub category.
USDA Forest Service Research Paper. PSW-RP-231. 1997. 29

living lateral branch for many years. This branch “snakes” through manzanita

stems into a small opening among the shrubs. In both instances the pines receive

just enough light and water to stay alive.

Entomology

The timing and pattern of pine needle sheath miner and gouty pitch midge

damage give clues to the environment preferred by these insects and have

vegetation management implications. Insect damage did not begin to be serious

until the pine seedlings were about 8 years old. Death of terminal shoots then

became a major factor affecting growth for the next 10 years. More than five

times more terminal shoots were killed on pines in heavy shrubs than in light

shrubs during this timespan. The general trend was an increasing number of

dead terminals with increasing shrub density. Apparently the shrub

environment, or perhaps the weaker trees in the shrub environment, is favorable

to the insects. After the trees reached a height of 12 feet, damage to terminals

lessened appreciably.

Figure 11—Tall, full crowned, and

growing well, ponderosa pines in the

no-shrub category have a good

chance of becoming a future forest.
30 USDA Forest Service Research Paper. PSW-RP-231. 1997.

Mortality of lateral pine shoots followed a similar trend. Damage to lateral

branches began 1 year after the terminals were infested and ended about 3 years

after damage to terminals declined. 1975 also was the year in which peak damage

was observed. After the trees reached a height of about 12 feet, damage above

this height lessened but continued in the middle and lower portions of the crown

in larger trees. Number of trees damaged seemed to be independent of shrub

category through 1979. After 1979, the number of trees damaged became

disproportionally higher for trees in the medium- and heavy-shrub categories.

The shrub density/ponderosa pine/insect interaction became a negative

synergism. By slowing pine growth, the shrubs hold the trees below a certain height.

The insects then attack the trees, deform them, and together with the shrubs continue

to hold the trees to a certain size where they are attacked repeatedly. The insects and

the shrubs together do more damage to the pines than each alone.

Application

The material in this paper should have appeal to a broad range of specialists.

Ecologists could find the developmental and successional relationships

worthwhile. Wildlife biologists might be interested in the amount of browse and

potential for nutritious berries and seeds. Fire and fuel specialists could use the

height and volume relationships to calculate potential flame lengths and rates of

spread. And landscape architects might note the time it took for the vegetation to

limit the roadside view.

Similarly, the finding that the variation in most pine parameters was best

explained by shrub foliar cover and crown volume per acre will be useful to

forest managers when planning vegetation manipulation activities in plantations

on similar sites.

In the introduction to their book on weed ecology, Radosevich and Holt (1984)

noted: “By analyzing the way crops, weeds, and environment interact, perhaps it

will be possible to grow more food in our fields and trees in our forests.” This

paper, which provides some long-term developmental relationships among shrubs,

grass, and planted ponderosa pines, does not illustrate how to grow more trees,

but it does show how to maximize growth on the young trees that are present, and

the negative consequences of leaving various amounts of competition. Ultimately,

this knowledge may be useful as well.

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Appendix

The fitted regression equations, which allow forest vegetation managers to model

various parameters of greenleaf manzanita, snowbrush, Sierra plum, combined

shrubs, and ponderosa pine on similar sites, should be useful to a broad audience.

However, they should be used with caution. In some instances, the fit was good;

in others, poor. Also, users should remember that individual species were

influenced by varying amounts of competition from other species in the plant

community, and this influence may differ from that elsewhere.

Greenleaf Manzanita

Density

Light shrubs: Y = 1,793.3 + 33.0 * age

Medium shrubs: Y = 4,675.2 - 1.2 * age

Heavy shrubs: Y = 5,819.3 - 53.3 * age

Standard errors for the intercept and slope coefficients were 474.3 plants

per acre and 33.0; the squared multiple correlation coefficient (R2) was 0.40;

the standard error of the estimate (Sy.x) was 1,582 plants per acre; F = 16.88;

and P value < 0.0001.

Foliar cover

Light shrubs: Y = - 5,721.4 + 3,863.6 * ln age

Medium shrubs: Y = - 8,951.2 + 7,018.1 * ln age

Heavy shrubs: Y = - 9,627.0 + 10,064.0 * ln age

Standard errors for the intercept and slope coefficients were 1,745.5 ft2

per acre and 730.4; the squared multiple correlation coefficient (R2) was 0.82;

the standard error of the estimate (Sy.x) was 3,126.4 ft2 per acre; F = 106.22; and

P value < 0.0001.

Height

Light shrubs: Y = - 2.1 + 1.8 * ln age

Medium shrubs: Y = - 1.8 + 2.1 * ln age

Heavy shrubs: Y = - 0.9 + 1.8 * ln age

Standard errors for the intercept and slope coefficients were 0.2 foot and 0.1;

the squared multiple correlation coefficient (R2) was 0.92; the standard error of

the estimate (Sy.x) was 0.42 foot; F = 263.31; and P value < 0.0001.

Volume

Light shrubs: Y = - 830.7 + 59.2 * age2

Medium shrubs: Y = 2,106.4 + 134.7 * age2

Heavy shrubs: Y = 15,001.0 + 185.2 * age2
USDA Forest Service Research Paper. PSW-RP-231. 1997. 33

Standard errors for the intercept and slope coefficients were 2,154.5 ft3 per

acre and 6.8; the squared multiple correlation coefficient (R2) was 0.93; the

standard error of the estimate (Sy.x) was 10,359.9 ft3

 per acre; F = 308.30; and P

value < 0.0001.

Snowbrush

Density

Light shrubs: Y = 1,112.0 - 0.8 * age2

Medium shrubs: Y = 1,426.3 - 0.9 * age2

Heavy shrubs: Y = 1,616.4 - 0.8 * age2

Standard errors for the intercept and slope coefficients were 96.5 plants

per acre and 0.3; the squared multiple correlation coefficient (R2

) was 0.26;

the standard error of the estimate (Sy.x) was 464 plants per acre; F = 9.52; and

P value < 0.0001.

Foliar cover

Light shrubs: Y = 2,136.6 + 842.7 * ln age

Medium shrubs: Y = 2,764.3 + 1,381.8 * ln age

Heavy shrubs: Y = 3,272.9 + 440.6 * ln age

Standard errors for the intercept and slope coefficients were 1,682.3 ft2

per acre and 704; the squared multiple correlation coefficient (R2) was 0.08;

the standard error of the estimate (Sy.x) was 3,013.2 ft2

 per acre; F = 2.97; and

P value < 0.0148.

Height

Light shrubs: Y = 0.7 + 0.2 * age

Medium shrubs: Y = 1.1 + 0.2 * age

Heavy shrubs: Y = 1.1 + 0.2 * age

Standard errors for the intercept and slope coefficients were 0.2 foot and 0.01;

the squared multiple correlation coefficient (R2) was 0.8; the standard error of the

estimate (Sy.x) was 0.63 foot; F = 96.56; and P value < 0.0001.

Volume

Light shrubs: Y = - 4,558.7 + 7,866.9 * ln age

Medium shrubs: Y = - 4,542.0 + 11,158.0 * ln age

Heavy shrubs: Y = - 335.4 + 6,299.3 * ln age

Standard errors for the intercept and slope coefficients were 6,832.7 ft3

per acre and 2,859.3; the squared multiple correlation coefficient (R2) was

0.21; the standard error of the estimate (Sy.x) was 12,238.4 ft3

 per acre; F = 7.41;

and P value < 0.0001.

Sierra Plum

Density

Light shrubs: Y = - 16.9 + 157.1 * ln age

Medium shrubs: Y = 6,718.3 - 1,117.5 * ln age

Heavy shrubs: Y = 8,415.7 - 1,098.3 * ln age

Standard errors for the intercept and slope coefficients were 1,094.7 plants

per acre and 458.1; the squared multiple correlation coefficient (R2

) was 0.60;
34 USDA Forest Service Research Paper. PSW-RP-231. 1997.

the standard error of the estimate (Sy.x) was 1,960.8 plants per acre; F = 35.96;

and P value < 0.0001.

Foliar cover

Light shrubs: Y = - 79.8 + 47.9 * ln age

Medium shrubs: Y = 3,996.1 - 1,165.7 * ln age

Heavy shrubs: Y = 4,344.0 - 1,250.7 * ln age

Standard errors for the intercept and slope coefficients were 499 ft2 per acre

and 208.8; the squared multiple correlation coefficient (R2) was 0.51; the standard

error of the estimate (Sy.x) was 893.8 ft2 per acre; F = 26.09; and P value < 0.0001.

Height

Light shrubs: Y = 0.5 + 0.05 * age

Medium shrubs: Y = 1.2 + 0.05 * age

Heavy shrubs: Y = 1.5 + 0.01 * age

Standard errors for the intercept and slope coefficients were 0.1 foot and 0.01;

the squared multiple correlation coefficient (R2) was 0.37; the standard error of

the estimate (Sy.x) was 0.5 foot; F = 12.94; and P value < 0.0001.

Volume

Light shrubs: Y = - 219.2 + 125.9 ln age

Medium shrubs: Y = 7,270.3 - 1,944.8 ln age

Heavy shrubs: Y = 8,284.0 - 2,348.0 ln age

Standard errors for the intercept and slope coefficients were 1,164.6 ft3

per acre and 487.4; the squared multiple correlation coefficient (R2) was

0.41; the standard error of the estimate (Sy.x) was 2,086 ft3

 per acre; F = 17.38;

and P value < 0.0001.

Combined Shrubs

Density

Light shrubs: Y = 2,635.3 + 369.0 * ln age

Medium shrubs: Y = 12,945.0 - 1,260.1 * ln age

Heavy shrubs: Y = 17,133.0 - 2,013.5 * ln age

Standard errors for the intercept and slope coefficients were 1,315.2 plants

per acre and 550.4; the squared multiple correlation coefficient (R2

) was 0.73;

the standard error of the estimate (Sy.x) was 2,355.7 plants per acre; F = 66.76;

and P value < 0.0001.

Foliar cover

Light shrubs: Y = - 3,664.5 + 4,754.2 * ln age

Medium shrubs: Y = - 2,190.7 + 7,234.1 * ln age

Heavy shrubs: Y = - 2,009.7 + 9,254.3 * ln age

Standard errors for the intercept and slope coefficients were 1,580.3 ft2 per

acre and 661.3; the squared multiple correlation coefficient (R2

) was 0.86; the

standard error of the estimate (Sy.x) was 2,830.5 ft2 per acre; F = 145.87; and P

value < 0.0001.

Height

Light shrubs: Y = 0.6 + 0.1 * age

Medium shrubs: Y = 1.1 + 0.1 * age
USDA Forest Service Research Paper. PSW-RP-231. 1997. 35

Heavy shrubs: Y = 1.3 + 0.1 * age

Standard errors for the intercept and slope coefficients were 0.1 foot and 0.01;

the squared multiple correlation coefficient (R2) was 0.88; the standard error of

the estimate (Sy.x) was 0.3 foot; F = 180.55; and P value < 0.0001.

Volume

Light shrubs: Y = - 5,652.6 + 2,264.3 * age

Medium shrubs: Y = - 2,191.8 + 3,738.4 * age

Heavy shrubs: Y = - 361.4 + 4,713.6 * age

Standard errors for the intercept and slope coefficients were 3,534.3 ft3 per

acre and 246.1; the squared multiple correlation coefficient (R2) was 0.88; the

standard error of the estimate (Sy.x) was 11,789.4 ft3 per acre; F = 172.59; and P

value < 0.0001.

Ponderosa Pine

Foliar cover

No shrubs: Y = 3,009.7 + 845.4 * age

Light shrubs: Y = 1,080.2 + 509.7 * age

Medium shrubs: Y = 1,705.2 + 391.2 * age

Heavy shrubs: Y = 609.7 + 148.5 * age

Standard errors for the intercept and slope coefficients were 1,023.9 saplings

per acre and 71.3; the squared multiple correlation coefficient (R2

) was 0.74; the

standard error of the estimate (Sy.x) was 3,415.4 saplings per acre; F = 61.85; and P

value < 0.0001.

Height

No shrubs: Y = - 12.4 + 11.2 * ln age

Light shrubs: Y = - 6.9 + 7.4 * ln age

Medium shrubs: Y = - 3.3 + 4.9 * ln age

Heavy shrubs: Y = - 0.7 + 2.5 * ln age

Standard errors for the intercept and slope coefficients were 0.9 foot and 0.4;

the squared multiple correlation coefficient (R2) was 0.92; the standard error of

the estimate (Sy.x) was 1.7 feet; F = 264.89; and P value < 0.0001.

Volume

No shrubs: Y = - 51,079.0 + 10,037.0 * age

Light shrubs: Y = - 12,671.0 + 3,369.1 * age

Medium shrubs: Y = - 5,571.3 + 2,197.2 * age

Heavy shrubs: Y = - 1,474.9 + 601.6 * age

Standard errors for the intercept and slope coefficients were 6,472.1 ft3 per

acre and 450.7; the squared multiple correlation coefficient (R2) was 0.83; the

standard error of the estimate (Sy.x) was 21,589.3 ft3 per acre; F = 106.11; and P

value < 0.0001.
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PSW-RP-231

Research PaperResearch StationPacific SouthwestForest Serviceof AgricultureDepartmentUnited States

Plantation: Effect of Different Shrub DensitiesVegetation Trends in a 31-Year-Old Ponderosa Pine