United States

Department

of Agriculture

Forest Service

Pacific Southwest

Research Station

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

Research Paper

PSW-RP-220

Seedfall, Regeneration, and

Seedling Development in

Group-Selection Openings

Philip M. McDonald Celeste S. Abbott

1970

1973

1965
McDonald, Philip M.; Abbott, Celeste S. 1994. Seedfall, regeneration, and seedling development

in group-selection openings. Res. Paper PSW-RP-220. Albany, CA: Pacific Southwest

Research Station, Forest Service, U.S. Department of Agriculture;13 p.

Forty-eight openings of 30, 60, and 90 feet in diameter were created in previously unmanaged

mixed conifer-hardwood stands on a high-quality site in northern California in 1963. They were an

attempt to convert an essentially even-aged forest to an uneven-aged arrangement and comprised the

initial cut in the group-selection system. Five conifers and three hardwoods produced 36 seed crops

that ranged from very light to heavy during the 10-year study period (1964-1973). Sound seed for

four conifer species combined (not sugar pine) for 10 years amounted to the equivalent of more than

302,900 seeds per acre and, in general, did not differ significantly among opening sizes. Shade and

roots from bordering trees were major influences on seedling survival and development in the

openings. Density of established conifer and hardwood seedlings (at least 4 years old) ranged from

12 per acre for incense-cedar in 90-foot openings to about 9,600 ponderosa pines per acre in 60-foot

openings after 10 years, and did not differ significantly among opening sizes. Conifer seedling

heights ranged from 0.8 to 1.7 feet after 9 years. Average height of ponderosa pines and hardwoods

differed significantly between the largest and smallest opening size with height being best in the

largest. Normally multi-stemmed and wide-crowned shrubs were conspicuous by their single

spindly stems.

Retrieval Terms: conifers, hardwoods, seed crops, seedling density, seedling development, management implications

The Authors:

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

assigned to the research unit studying regeneration of California forests, and stationed at the

Station’s Silviculture Laboratory (2400 Washington Avenue, Redding, CA 96001).

Acknowledgments:

We are grateful for the constructive comments of reviewers: Michael Shelton, Southern Forest

Experiment Station, USDA Forest Service, Monticello, Ark.; Russell Graham, Intermountain Forest

and Range Experiment Station, USDA Forest Service, Moscow, Idaho; and Phillip Reynolds,

Forestry Canada, Sault Ste. Marie, Toronto, Ontario.

Publisher:

Pacific Southwest Research Station

Albany, California

(Mailing address: P.O. Box 245, Berkeley, CA 94701-0245

Telephone: 510-559-6300)

January 1994
Seedfall, Regeneration, and

Seedling Development in

Group-Selection Openings

Philip M. McDonald Celeste S. Abbott

Contents

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

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

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

Species and Site ............................................................................................ 1

Treatments, Measurements, and Analysis ................................................... 2

Results ................................................................................................................. 2

Seed Crops .................................................................................................... 2

Seed Production ............................................................................................ 4

Survival and Density of Early Seedlings ..................................................... 4

Density of Established Seedlings ................................................................. 8

Seedling Growth ........................................................................................... 8

Summary and Discussion ................................................................................ 11

References ......................................................................................................... 13
18 USDA Forest Service Res. Paper PSW-RP-220. 1994.

In Brief . . .

McDonald, Philip M.; Abbott, Celeste S. 1994. Seedfall,

regeneration, and seedling development in groupselection openings. Res. Paper PSW-RP-220. Albany,

CA: Pacific Southwest Research Station, Forest Service,

U.S. Department of Agriculture; 13 p.

Retrieval Terms: conifers, hardwoods, seed crops, seedling

density, seedling development, management implications

The group-selection cutting method promotes a landscape that has the appearance of continuous forest cover.

Closer examination, however, reveals a number of small

openings created by timber harvest. These are scattered

throughout, at least initially, and suggest small-scale natural

disturbances such as those caused by small hot fires. In this

study on the Challenge Experimental Forest in northern

California, 48 openings—30-, 60-, and 90-feet in diameter—were created in 1963. These openings were located

such that groups of mostly mature trees were harvested

along with some individuals of poor form, slow growth, and

low development potential. After site preparation by bulldozer, the mineral soil seedbed provided an excellent

medium for conifer, hardwood, and shrub seeds to become

hidden, germinate, and reproduce the stand.

Once the seedlings are present, however, the environment

in which they grew was characterized by shade and a soil

moisture regime influenced by the roots of trees adjacent to

the openings. For those species whose seedlings are intolerant

of shade, like whiteleaf manzanita, deerbrush, California

black oak, and ponderosa pine, growth will be curtailed.

Some species, like tanoak, which is adapted to shady situations, will grow well. The other tree species that are common

in the study area (sugar pine, California white fir,

incense-cedar, Douglas-fir) tolerate shade, but will not grow

as well as they would in more sunlit environments.

The five conifer and three hardwood species surrounding

the group-selection openings produced 36 seed crops during

the 10-year study period—4 heavy, 9 medium, and 23 light

and very light. Total sound seed for 10 years from the four

conifer species that were quantified averaged more than

302,900 seeds per acre in each opening. After 10 years, the

equivalent of more than 9,800 conifer and 1,450 hardwood

seedlings per acre, at least 4 years old, were present in each

opening. Every seed crop contributed to the total. Height of

9-year-old seedlings from lowest to highest was incense-cedar,

ponderosa pine, sugar pine, Douglas-fir, California white

fir, hardwoods, and shrubs. Ponderosa pine seedlings were

significantly larger in 90-foot diameter openings than in

30-foot openings, and combined hardwoods in 60- and 90-foot

openings were significantly larger than counterparts in 30-foot

openings. Plants of all species were generally taller in

the largest openings because of better development near

the center.

Although the opening sizes studied here are at the lower

end of the size range associated with group selection, findings from them should contribute to the practice of this

regeneration cutting method. Stands can be regenerated with

a diversity of species while maintaining high forest cover;

the reproduction, although too numerous and developing

slowly, should continue to grow and even accelerate in

growth in the future—forming a spatially distinct age class;

and the normally aggressive shrub species, which are

negatively affected by the shady environment, pose little

threat to the survival and growth of the conifer and

hardwood seedlings.

ii
USDA Forest Service Res. Paper PSW-RP-220. 1994. 1

A crucial question is whether the desired species become

established and develop satisfactorily under the imposed

treatments (Guldin 1991).

This paper reports results 10 years after the first entry in

group-selection cutting. It quantifies sound seed from 20

seed crops of four conifer species and relates subsequent

seedling survival, density, and development to small openings of three different sizes. Establishment and development

of three hardwood and two shrub species also are quantified.

Results from this study indicate remarkable similarity in seedfall and regeneration among the three sizes of openings, but

striking differences in conifer and hardwood seedling height.

They have strong ecological and silvicultural implications.

Methods

Species and Site

The study was located on the Challenge Experimental

Forest in north central California. Here, site quality is high

and the dominant species, ponderosa pine (Pinus ponderosa

Dougl. ex Laws. var. ponderosa), will average about 95 feet

in height in 50 years (Powers and Oliver 1978). Other tree

species scattered over the Forest are Douglas-fir (Pseudotsuga

menziesii [Mirb.] Franco), sugar pine (Pinus lambertiana

Dougl.), California white fir (Abies concolor var. lowiana

[Gord.] Lemm.), and incense-cedar (Libocedrus decurrens

Torr.). Hardwoods, principally California black oak (Quercus

kelloggii Newb.), tanoak (Lithocarpus densiflorus [Hook. &

Arn.] Rehd.), and Pacific madrone (Arbutus menziesii Pursh),

are present throughout as individual trees, clumps, or groves.

Altogether, trees larger than 3.5 inches in diameter at breast

height (d.b.h.) number over 245 per acre and contain about

270 ft2/acre of basal area. The forest is a mosaic of even-aged

stands, depending on fire history, with the oldest dominant

and codominant trees being about 100 years old and averaging about 140 feet tall. Trees 170 feet and taller at the same

age are not unusual. Those in the dominant and codominant

crown classes average about 35 per acre, 70 percent of

which are ponderosa pines. In general, many trees of the

more shade-tolerant species (Douglas-fir, California white

fir, incense-cedar, sugar pine, and tanoak) are younger,

reflecting a decreasing incidence of fire in the area. Dormant

but viable seeds of whiteleaf manzanita (Arctostaphylos viscida

Parry) and deerbrush (Ceanothus integerrimus H. & A.) are

abundant in the soil almost everywhere on the Experimental

Forest. Apparently, these species were abundant in the past in

many areas where not a trace above ground remains today.

Herbivores, chiefly deer (Odocoileus spp.), woodrats

(Neotoma spp.), and deer mice (Peromyscus maniculatus),

are plentiful throughout the Experimental Forest.

Summers on the Experimental Forest are hot and dry;

winters cool and moist. The average midsummer maximum

Introduction

F orest managers have, and always will, need to choose

among different silvicultural and management systems to achieve a desired mix of commodities and

amenities for an ever-changing and ever-more-complicated

society. Today, as never before, sentiment is shifting away

from clearcutting toward the application of systems that

people perceive to be more “gentle” on the land, its vegetation, and its creatures. Proposed alternatives include managing for multiple-aged, multiple-species stands (Beuter 1992)

and having an ecosystem perspective that provides a wide

array of values and products (Salwasser 1992).

The group-selection method, which is part of an

uneven-aged silvicultural regeneration system, involves the

removal of groups of trees to create small openings that

range from 0.02 to 2.0 acres. Several of these openings,

scattered throughout a stand, usually are harvested together

on a cyclic basis. The cycle often is 10 or 15 years. There is

no rotation, and cutting continues in perpetuity, always

removing clusters of mature trees. Initial and early entries

into the stand, however, often have an element of “improvement” in them, whereby trees of younger ages are removed

(Hawley 1946). This gives opportunity to lower the proportion of less desirable species, trees with poor growth, those

that interfere with the growth of better trees, and poorly

formed, slow-growing, and diseased individuals (Smith 1962).

This cyclic cutting creates a stand of many age classes, often

with several species, whose crowns and roots fill nearly all

available space. If an equal proportion of all age classes is

present throughout the stand, a condition called a balanced

stand is achieved, and productivity is considered to be close

to the inherent capacity of the site. It has been proposed that

this system is particularly adapted to (i) maintaining the

health and resistance to damage of the forest; (ii) producing

the highest possible volume of valuable timber; (iii) promoting the protective effect of the forest in the widest sense

(protecting the soil, scenery, and native flora and fauna); and

(iv) functioning like a bank (being in a position to supply

critical material when needed without jeopardizing productive capacity) (Hann and Bare 1979).

In addition, the periodic creation of small openings, with

their particular environment, is one technique that aids conversion from an even-aged stand of primarily one species to

an uneven-aged arrangement with several species.

Uneven-aged silviculture is more an art than a science

(Hann and Bare 1979). Implementing it on a broad scale,

particularly where stands are even-aged, leads to significant

administrative, financial, and biological problems (Fiske and

others 1992). A major need is for knowledge on the species

composition, magnitude, persistence, and development of

regeneration and competing vegetation in group-selection

openings (McDonald and Fiddler 1991, Phares 1978).
2 USDA Forest Service Res. Paper PSW-RP-220. 1994.

temperature, based on 43 years of record, is 90 oF; the

midwinter minimum is 30 oF. The growing season is about

200 days. Precipitation averages 68 inches with 94 percent

falling between October and May. Moisture is the limiting

factor and drought the primary cause of mortality for conifer

and hardwood seedlings. Aspects are mostly northeast and

southwest, and slopes vary from 3 to 30 percent. Soils are of

clay loam-clay texture, deep, moderately well drained, and

quite fertile.

The overall environment of the small openings is quite

similar. Other than for occasional sunflecks, the sun shines

in the openings only when directly overhead. Trees on the

border cast shade, ameliorate temperatures, and lessen the

amount of wind in the openings. Warm temperatures, in

general, would not get as high, and cold temperatures would

not get as low as in less sheltered environments. Peak summer temperatures may be reduced by as much as 9 o

F and

winter extremes by an equal amount (Geiger 1966). Snow

accumulates in the openings and, because of shade, remains

longer than in more open stands. Relative humidity tends to

be higher in small openings, and the moisture content of the

surface soil may remain higher later into the summer. Because of cold air ponding in the openings, late spring frosts

sometimes occur.

Treatments, Measurements,

and Analysis

Groups of large and small trees in a natural stand were

harvested in the fall of 1963 to create openings of about 30,

60, and 90 feet in diameter (crown dripline to crown dripline).

The smallest size of opening was created by removing several medium to small trees, the largest by removing several

large and medium trees. Each opening size was replicated 16

times, and all openings were located within 1 mile of each

other on relatively homogeneous ground having similar soil

and growth potential. Location of openings was random

with the restriction that they be within 150 feet of skid roads.

Opening size was randomly assigned to the 48 chosen locations. In May 1964, unmerchantable conifers and hardwoods

were pushed over and the ground scarified by a bulldozer.

The scarification pattern resembled spokes radiating from a

central hub with a small flat pile of material located at the

end of each spoke. In only a few openings was the slash

heavy enough to require piling in the adjacent skid road for

eventual burning. Consequently, the surface in all openings

was mostly bare mineral soil with little or no competing

vegetation.

Seed traps were made of two sizes of galvanized wire

screen and were about 1-foot square (fig. 1). The 0.25-inch

top mesh often excluded sugar pine seed and was too small

to reliably sample seeds of hardwoods. For smaller seeds

from the other conifer species, this size of trap has provided

reliable estimates of seedfall (McDonald 1973, Roy 1959).

Number of randomly located seed traps in the 30-, 60-, and

90-foot openings was 5, 8, and 10, respectively. Collections

from the traps were made in late fall or early winter after

most seeds had been disseminated, and again in the subsequent summer when winter- and spring-fallen seeds were

collected. Seeds from these two collections made up the

sample for the preceding seed year. Contents of each trap

were placed in paper sacks. After drying, seeds were sorted

from the litter and the total number tallied. A cutting test on

all seeds determined soundness. Seedfall was quantified

every year in the study period (1964 through 1973).

Conifer and hardwood seed crops were evaluated over

the entire Forest and surrounding area during the study

period by means of a visual rating system (table 1) based on

the proportion of trees with fruit and the number of fruit per

tree (McDonald 1992). This system generally identified the

medium and heavy seed crops of the conifer species on the

Forest, but occasionally missed the light and very light crops

of species that produce small cones.

Conifer, hardwood, and shrub seedlings were surveyed in

late spring on randomly located, circular quarter-milacre

(0.00025-acre) plots from 1965 through 1974. Sampling

intensity reflected the variation in seedling density that occurred in similarly treated stands (McDonald 1966). Number of plots for each 30-, 60-, and 90-foot opening was 9, 17,

and 20, respectively. To trace the fate of each species of

seedling from each seed crop, a colored (for species) and

coded (for seed crop) plastic stick was placed next to each

seedling when initially surveyed. The plastic stick remained

with the seedling until it died. Individual seedlings were

tracked each year through age 4 and then considered as

established. Established seedlings from the 1964-1970 seed

crops were then accumulated and the total reported for each

species for the 10-year study period. Whiteleaf manzanita

and deerbrush were tallied as “shrubs,” and only presence

was recorded from 1964 to 1970. Height of all species was

recorded to the nearest 0.5 inch in 1973.

Experimental design was completely randomized, and

analysis was by analysis of variance (ANOVA) and Tukey

test (Wilkinson 1989). Data on shrub presence were first

transformed (arc sine) to produce an approximately homogeneous variance before analysis by ANOVA. Significance

in all tests was at α = 0.05. Because information was

gathered from permanent plots measured each year, the data

are not truly independent from year to year. The α levels or

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

Results

Seed Crops

Seed production during the 10-year study period ranged

from very light to heavy for conifer and hardwood species

(table 1). Ponderosa pine was the most reliable producer,
USDA Forest Service Res. Paper PSW-RP-220. 1994. 3

Table 1—Periodicity and magnitude of conifer and hardwood seed crops on the Challenge Experimental Forest, 1964-1973

Seed crop ratingx

__________________________________________________________________________________________________________________________________________________________________________

Species 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973

Ponderosa pine M - L H L - H VL - M

Douglas fir L L L VL L - - VL - H

California white fir M - - M - - - - -M

Incense cedar - VL - - - - - L - VL

Sugar pine M - - L - - M - - VL

California black oak - - - M L -----

Tanoak L - L L - H - L VL -

Pacific madrone - VL - - M VL - VL - -

xSeed crop ratings: VL = very light, L = light, M = medium, H = heavy

Figure 1—Three seed traps are visible in part of this 30-foot diameter opening. Note the center pin that anchors them to the ground.
4 USDA Forest Service Res. Paper PSW-RP-220. 1994.

yielding medium and heavy seed crops every third year with

at least one smaller crop in intervening years (fig. 2).

Douglas-fir and tanoak tended to produce frequent, but light

and very light crops. Ponderosa pine, Douglas-fir, and tanoak

yielded the most crops and California black oak the fewest.

All species except incense-cedar bore at least one medium

seed crop during the period. Little synchrony existed between

conifer and hardwood species. No stimulation of reproductive material due to group-selection cutting was observed.

Seed Production

Sugar pine seed and the fruits of hardwoods were too

large to fit in the small seed traps. For the four species that

were sampled (ponderosa pine, Douglas-fir, California white

fir, and incense-cedar), the amount of sound seed ranged

from more than 124,000 per acre for ponderosa pine in a

heavy seed year in a 60-foot opening to no sound seed for

incense-cedar in any opening in the very light seed year of

1965 (table 2). When seeds were totaled for all conifer

species in all openings over the 10-year period and averaged, each opening received the equivalent of slightly more

than 302,900 sound seeds per acre. Ponderosa pine constituted 87 percent of total sound seeds.

Amount of sound seed varied with opening size, but only

for the low-rated ponderosa pine seed crop of 1966 was a

statistically significant difference among opening sizes found.

For other seed crops, whether heavy or light, no significant

differences among opening sizes were discernible.

Survival and Density of Early

Seedlings

To ascertain trends in survival and establishment, numbers of conifer and hardwood seedlings of each species in

each size of opening were averaged and compared. Only

minor differences were found. Numbers of seedlings of each

species then were averaged over all openings combined, and

their densities plotted each year until they were considered

as established (after four growing seasons). Seedling mortalFigure 2—Numerous ponderosa pine cones in part of this 30-foot wide opening indicate heavy seed fall from this species.
USDA Forest Service Res. Paper PSW-RP-220. 1994. 5

ity, mostly from drought, followed the general trend of being

highest the first year or two and then leveling off. Consequently, with time, the trend in seedling density formed a

steep-to-shallow descending pattern (fig. 3). California white

fir, for example, numbered almost 1,540 seedlings per acre

in 1965 and about 850 seedlings per acre in 1968; tanoak

numbered 200 seedlings per acre in 1965 and 173 four years

later. Because ponderosa pine produced so much seed and so

many seedlings (table 3), density values are presented for

three seed crops (fig. 4). They give insight to regeneration

trends in group-selection cutting, especially for small openings with site preparation.

The most important trend was that every seed crop

resulted in many additional seedlings, which became established and accumulated to the point that each opening

became over-stocked with conifer and hardwood seedlings.

We noticed that the whole opening became a favorable

microsite that stayed favorable until the mineral soil was

covered with organic material. In addition, the environment

of the openings was such that the number of shrubs, forbs,

and grasses were few and their development slow. Over 40

species of plants, including conifer and hardwood seedlings,

were present in the openings after 10 years.

Table 2—Average number of sound seeds by species and size of group-selection opening, Challenge

Experimental Forest, 1964-1973

 Number per acre ________________________________________________________________________________________________________________________________________________

Opening 1964 1965 1966 1967 1968 1970 1971 1973

size

 ft

Douglas-fir

 30 2,725 1,090 545 4,355 1,090 1,090 1,090 23,413

 60 1,020 0 680 340 0 340 680 22,461

 90 2,725 250 740 745 495 250 495 22,770

 SEx 979 450 553 1,757 472 493 543 6,835

Ponderosa pine

 30 15,790 0 545 37,570 3,270 119,795 545 86,031

 60 24,495 0 1,360 54,780 1,700 124,185 4,424 77,591

 90 19,555 265 4,700y 36,140 2,475 105,445 4,207 65,092

 SEx 5,572 144 1,048 1,757 472 22,956 1,257 10,754

California white fir

 30 2,180 0 0 545 0 545 0 1,634

 60 4,925 0 0 6,805 340 0 0 2,723

 90 2,225 0 0 1,980 250 745 0 1,238

 SEx 1,608 - - 10,737 243 442 - 1,259

Incense-cedar

 30 0 0 0 0 0 0 545 0

 60 0 0 0 340 0 0 1,020 340

 90 0 0 0 250 0 0 495 250

 SEx - - - 470 - - 565 243

xSE = standard error.

yDiffers significantly at 5 percent level from values in 30- and 60-foot opening sizes. This was the only

significant difference observed.
6 USDA Forest Service Res. Paper PSW-RP-220. 1994.

a b

Figure 3—Average number of conifer and hardwood seedlings plus standard errors for first 4 years in group-selection openings, Challenge

Experimental Forest, for Douglas-fir (A); California white fir (B); sugar pine (C); California black oak (D); tanoak (E), and Pacific madrone (F).

c d

e f
USDA Forest Service Res. Paper PSW-RP-220. 1994. 7

Table 3—Average number of established conifer and hardwood seedlings by

species and size of group-selection opening, Challenge Experimental

Forest, 1974

Opening size (ft) ________________________________________________________________

Species 30 60 90 SEx

----------- Number per acre -----------

Conifers

Douglas-fir 250y 279 264 88

Ponderosa pine 6,379 9,625 8,244 1,972

California white fir 805 1,589 388 555

Incense-cedar 83 103 12 44

Sugar pine 778 441 274 202

Hardwoods

California black oak 166 588 550 209

Tanoak 1,111 544 550 330

Pacific madrone 222 500 175 143

xSE = standard error.

yNo significant difference was found among opening sizes for any species.

Figure 4—Average number of ponderosa pine seedlings plus standard

errors in group-selection openings for seed crops of 1964 (A), 1967 (B),

and 1970 (C), Challenge Experimental Forest.

a b

c
8 USDA Forest Service Res. Paper PSW-RP-220. 1994.

Density of Established Seedlings

Number of established conifer seedlings ranged from an

average of 12 incense-cedar per acre in 90-foot openings to

about 9,600 ponderosa pine per acre in 60-foot openings

(fig. 5). Similarly, an average of 166 California black oak

seedlings per acre became established in 30-foot openings

and 1,111 tanoak seedlings per acre in openings of similar

size. No significant difference among opening sizes was

detected for any species. A similar finding resulted when

aspect (north vs south) was included in the analysis. When

all seedlings of all species were averaged over all opening

sizes, the number of seedlings in each opening in 1974 was

very large—the equivalent of more than 9,800 conifers and

1,450 hardwoods per acre (fig. 6). Although ponderosa pine

constituted 82 percent of established seedlings, the more

shade-tolerant conifers and hardwoods were almost always

represented in each opening (fig. 7). If the species composition of the future forest was based solely on conifer and

hardwood seedling density after 10 years, it would closely

resemble that of the original stand.

Presence of combined whiteleaf manzanita and deerbrush

plants on the regeneration plots was 15, 27, and 24 percent

on the 30-, 60-, and 90-foot openings, respectively. No

statistical difference among opening sizes was found. Because no plants of either species were present before logging, it is likely that most plants originated from dormant

seeds in the soil. Almost all plants became present soon after

site preparation.

Seedling Growth

Mean height of established conifer, hardwood, and shrub

seedlings, measured in fall 1973 or after 9 years, ranged

from 0.8 feet for ponderosa pine in 30-foot openings to 3.8

feet for shrubs in 90-foot openings (table 4). No damage

Figure 5—Abundant California white fir and ponderosa pine seedlings are visible in part of this 60-foot wide opening.
USDA Forest Service Res. Paper PSW-RP-220. 1994. 9

Figure 6—(A) Ponderosa and sugar pines are becoming visible in part of this 90-foot opening in 1973. (B)

Hardwoods and ponderosa pines occupy most of this 90-foot opening in 1973. Note the heavy litter on the

ground in both photos.

a

b
10 USDA Forest Service Res. Paper PSW-RP-220. 1994.

from browsing animals was noted. Analysis by size of opening

revealed that ponderosa pine seedlings were significantly

larger in 90-foot openings than in 30-foot openings, and

hardwoods in 90- and 60-foot openings were significantly

larger than counterparts in 30-foot openings. In general,

plants of all species were tallest in 90-foot diameter openings, largely because of taller plants near the centers.

Relative height by species in descending order was shrubs,

hardwoods, California white fir, Douglas-fir, sugar pine,

ponderosa pine, and incense-cedar. This ranking reflects the

various species response to the environment of the small

openings, and specifically to the lower levels of light available there. It may or may not relate to seedling vigor. For the

shade-tolerant hardwoods like tanoak, and to a lesser extent

for Pacific madrone, their ranking accurately reflects the

natural ability of the seedlings to grow well in shady environments. The poor development of ponderosa pine (many

seedlings were only 5 inches tall at age 9) well portrays its

response to an inhospitable environment. The ranking of the

tolerant conifers (sugar pine, Douglas-fir, incense-cedar,

California white fir) reflects both good adaptation to the

Figure 7—This quarter-milacre circular plot in part of a 60-foot opening in 1973 contains 10 living and one dead

(horizontal stick) ponderosa pines, and one each of Douglas-fir, sugar pine, and California white fir. Note the relative

heights of the seedlings and specifically the taller Douglas-fir (upper left) and sugar pine (upper right).

Table 4—Average height of established seedlings by species or by

combination of species and opening size, Challenge Experimental Forest, 1973

Opening size (ft) __________________________________________________________

Species 30 60 90 SEx

--------------- Height (ft) ----------------

Ponderosa pine 0.8 ay 1.0 ab 1.3 b 0.2

Sugar pine 0.9 a 1.5 a 1.5 a 0.3

Douglas-fir 1.3 a 1.0 a 1.6 a 0.3

California white fir 1.2 a 1.5 a 1.7 a 0.2

Incense-cedar - - 0.9 a -

Hardwoods 1.5 a 2.5 b 2.8 b 0.2

Shrubs 2.6 a 2.2 a 3.8 a 0.3

x

SE = standard error.

y

For each species, treatment means followed by the same letter

do not differ statistically at the 0.05 level.
USDA Forest Service Res. Paper PSW-RP-220. 1994. 11

During the 10-year study period, 24 seed crops were

produced on five conifer species and 12 seed crops on three

hardwood species. Ponderosa pine, Douglas-fir, and tanoak

each produced six seed crops that ranged from very light to

heavy. All other species yielded at least two crops during the

study period. Ponderosa pine bore more medium to heavy

seed crops (four) than any other species and had the most

even periodicity (every third year).

Sound seeds were quantified for four conifer species and,

when totaled for all species over the study period, amounted

to the equivalent of more than 302,900 sound seeds per acre in

every opening. Eighty-seven percent of it was ponderosa pine.

Because of bare mineral soil and the shade-ameliorated

environment of the small openings, the entire area of each

opening became a favorable microsite for the establishment

of conifer and hardwood seedlings. Consequently, each seed

crop contributed to the pool of established seedlings to the

point that too many seedlings became present.

The survival pattern of seedlings of all species for the

first 4 years was descending and steep to shallow depending

on the species and timing of seed crop relative to amount of

organic material on the forest floor. Shade-tolerant species

and those from large seeds (acorns) in general had a less

rapid rate of mortality than intolerant species.

Established conifer seedlings (at least 4 years old) averaged more than 9,800 per acre and hardwoods over 1,450 per

acre in each opening. Eighty-two percent were ponderosa

pine. Virtually no statistically significant differences among

opening sizes were found for seed fall, survival and early

seedling density, and established regeneration of any conifer, hardwood, or shrub species.

Because conifer and hardwood seedlings were abundant,

few site resources will be available for the establishment of

future seedlings. Consequently, the odds are that these seedlings will be the only age class that will be present in all

these openings.

In general, the largest opening size was more amenable to

height growth for all species than was the smallest opening

size. Height of seedlings was significantly greater for ponderosa pine and combined hardwoods in larger openings.

But the overall height growth of ponderosa pine seedlings

was poor, which leads to the question of this species representation in future stands. In a longer-term study with

single-tree selection in the central Sierra Nevada, in a forest

type where the majority species of dominant trees were

shade tolerant, Lilieholm and others (1990) noted that

shade-tolerant species comprised the bulk of seedlings, but

“selection harvests have resulted in adequate quantities of

seedlings of all five conifer species, including pines.”

In the shady environment of the small openings, whiteleaf

manzanita and deerbrush seedlings were tall and slender

rather than “bushy”—their traditional form in areas with

more sunlight.

environment of the openings and to an inherently slow growth

habit as seedlings. The shrubs, however, almost all consisted

of a single spindly stem that reflected poor vigor. In a more

open sunlit environment they would be much taller, have

multiple stems, and be almost as wide as they were tall

(Cooper 1922, Hellmers and others 1955).

Summary and Discussion

The environment of 30- to 90-foot diameter openings in a

mixed-conifer and hardwood stand on a good site in northern California, cut as the first entry in a group-selection

regeneration system, is modified both above and below ground

by the adjacent trees. Above ground, the forest floor of the

openings is shaded and showered by needles, twigs, and

other organic material; below ground, roots of large trees

extend into the openings and probably intensify in volume

after harvest as roots capture at least some of the space

formerly occupied by cut trees (Smith 1962). In general, soil

moisture and light are less available to seedlings near the

forest edge and become more available with distance toward

the center of the openings.

Although the general environment of the small openings

may be similar, the specific environment actually differs,

depending on the size of the opening. In general, more light

is available in 90-foot openings than in 30-foot openings and

increases as aspect becomes more southerly. In a separate

study on a nearby area on the Experimental Forest, Ziemer

(1968) measured soil moisture around a forest-grown, but

isolated (all other trees removed), 28-inch diameter sugar

pine. He found that “soil moisture depletion extended outward to a distance of slightly over 20 feet from the base of

the tree and somewhat deeper than 15 feet under the tree.”

Presumably, this zone of depletion was caused by

moisture-absorbing tree roots. Consequently, tree roots probably extend throughout the 30-foot diameter openings, through

most of the 60-foot openings, and in all but the center of

90-foot openings. Less soil moisture would be available to

seedlings in areas where the roots were present.

Years of observation have indicated that most seeds are

distributed in the fall on days when winds are from the

northeast and the atmosphere is warm and dry (McDonald

1983, 1992). When warm temperatures and low humidity combine to lower the moisture content of cones and

scales and acorn cups, a shower of seeds takes place. If this

weather continues, most of the seeds fall, often within a

week. If it does not last, remaining seeds dribble out in warm

weather between storms, which are usually from the southwest, or again are distributed under the influence of northeasterly winds.
12 USDA Forest Service Res. Paper PSW-RP-220. 1994.

Because large ecological and silvicultural differences exist between group selection and clearcutting in terms of the

survival, density, and development of conifer, hardwood,

and shrub seedlings, the two methods are compared. In

general, in small openings cut as the initial entry in group

selection, shade-tolerant species are better represented, and

more tree seedlings per unit area become established. However, the shrub and herbaceous plant community is much

less diverse and less developed in group-selection openings

than in clearcuttings.

In clearcuttings, after site preparation by windrow and

burn or broadcast burn, vegetative reoccupation is much

more dynamic in terms of rapidity, diversity of species, and

plant density. In nearby clearcut areas on the Experimental

Forest, for example, plant species numbered more than 100

with a mean density of more than 57,500 per acre 5 years

after site preparation by windrow and burn (McDonald 1983).

Most favorable microsites became occupied by vegetation

after 1 year. After 3 years, few areas remained that were

receptive to the survival and establishment of conifer seedlings. Consequently, in clearcuttings, it is the first conifer

seed crop that provides seedlings for the future forest—

subsequent seed crops almost always are in vain. If no conifer seeds fall for 3 or 4 years after site preparation, the

chance for acceptable conifer seedling establishment is low.

Height (in feet) of 9-year-old shrub, hardwood, and conifer

seedlings in clearcuttings was 7.8 (shrubs), 12.6 (hardwoods),

4.0 (California white fir), 4.2 (Douglas-fir), 5.5 (sugar pine),

and 6.2 (ponderosa pine) (McDonald 1976), or 3 to 6 times

taller than seedlings in group-selection areas.

Can these small openings contribute to the practice of

group selection in the future, given their size and their

unique (shady) environment? More specific questions are:

(1) will the newly established conifer and hardwood species

grow to maturity as a spatially distinct age class, (2) will

intolerant species be present in the future, and (3) will the

normally-aggressive shrubs be a threat to the conifer

seedlings?

Although these questions cannot be answered directly by

this study, the data and relationships suggest possible answers: (1) Although the uncut forest and the small openings

are best described as “shady,” a critical, higher amount of

sunlight is present in the openings. This, plus the mineral

soil present there, promulgates seedling establishment and

slow but steady height growth. That the seedlings will continue to grow and form a distinct age class is likely. (2) The

high proportion of ponderosa pine seedlings, their relatively

low heights, and poor development potential suggest high

mortality and a much-reduced presence in the future. It is

likely that the more shade-tolerant species will replace ponderosa pine in all but the centers of the largest openings. (3)

Although the growth of both shrubs and tree seedlings, with

the exception of tanoak, is reduced greatly in the small

openings, the spindly nature of the shrubs suggests that they

will furnish only minor future competition to the tree seedlings. This cutting system offers good potential as an indirect vegetation management technique. A major goal of

future research will be to find that opening size where the

tree seedlings grow well and the shrubs do not.
USDA Forest Service Res. Paper PSW-RP-220. 1994. 13

References

Beuter, J. 1992. Comments presented at the Oregon Logging Conference

as reported in The Oregonian of February 22, 1992.

Cooper, William S. 1922. The broad-sclerophyll vegetation of California. An ecological study of the chaparral and its related communities. Carnegie Institution of Washington Publication 319. Washington,

DC: Technical Press; 119 p.

Fiske, John N., and others. 1992. Effects of a group selection strategy for

the Sierra Nevada mixed conifer. Internal Report. San Francisco:

Pacific Southwest Region, USDA Forest Service; 29 p.

Geiger, Rudolf. 1966. The climate near the ground. Cambridge, MA:

Harvard University Press; 611 p.

Guldin, James M. 1991. Uneven-aged BDq regulation of Sierra Nevada

mixed conifers. Western Journal of Applied Forestry 6(2): 27-32.

Hann, David W.; Bare, B. Bruce. 1979. Uneven-aged forest management: state of the art (or science?). Gen. Tech. Rep. INT-50. Ogden,

UT: Intermountain Forest and Range Experiment Station, Forest Service,

U.S. Department of Agriculture; 18 p.

Hawley, Ralph C. 1946. The practice of silviculture. 5th edition. New

York: John Wiley and Sons, Inc.; 354 p.

Hellmers, H.; Horton, J.S.; Juhren, G.; O’Keefe, J. 1955. Root systems of

some chaparral plants in southern California. Ecology 36(4): 667-678.

Lilieholm, R.J.; Davis, L.S.; Heald, R.C.; Holmen, S.P. 1990. Effects of

single tree selection harvests on stand structure, species composition, and understory tree growth in a Sierra mixed conifer forest.

Western Journal of Applied Forestry 5(2): 43-47.

McDonald, Philip M. 1966. Seed fall and regeneration from a group

selection cut . . . first year results. Res. Note PSW-113. Berkeley, CA:

Pacific Southwest Forest and Range Experiment Station, Forest Service,

U.S. Department of Agriculture; 6 p.

McDonald, Philip M. 1973. Cutting a young-growth, mixed-conifer

stand to California Forest Practice Act Standards. Res. Paper PSW-89.

Berkeley, CA: Pacific Southwest Forest and Range Experiment Station,

Forest Service, U.S. Department of Agriculture; 16 p.

McDonald, Philip M. 1976. Forest regeneration and seedling growth

from five major cutting methods in north-central California. Res.

Paper PSW-115. Berkeley, CA: Pacific Southwest Forest and Range

Experiment Station, Forest Service, U.S. Department of Agriculture; 10 p.

McDonald, Philip M. 1983. Clearcutting and natural regeneration . . .

management implications for the northern Sierra Nevada. Gen. Tech.

Rep. PSW-70. Berkeley, CA: Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture; 11 p.

McDonald, Philip M. 1992. Estimating seed crops of conifer and hardwood species. Canadian Journal of Forest Research 22(6): 832-838.

McDonald, Philip M.; Fiddler, Gary O. 1991. Vegetation in group selection openings: ecology and manipulation. In: Proceedings 12th Annual

Forest Vegetation Management Conference; 1991 May 14-16; Redding,

CA. Redding, CA: Forest Vegetation Management Conference; 97-106.

Phares, Robert E. 1978. Research gaps and research needs for the

western forest regime. In: Uneven-aged silviculture and management in

the United States. Washington, DC: Forest Service, U.S. Department of

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Powers, Robert F.; Oliver, William W. 1978. Site classification of ponderosa pine stands under stocking control in California. Res. Paper

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Roy, D.F. 1959. Small seed traps perform well in the Douglas-fir type

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14 USDA Forest Service Res. Paper PSW-RP-220. 1994.
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 improve living conditions in rural areas

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