Growth of Thinned and 

Unthinned Hardwood Stands 

on a Good Site in Northern 

California

Philip M. McDonald and Nicholas R. Vaughn

United States 

Department of 

Agriculture

Forest Service

Pacific Southwest 

Research Station

General Technical Report

PSW-GTR-204

July 2007
The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of 

multiple use management of the Nation’s forest resources for sustained yields of wood, 

water, forage, wildlife, and recreation. Through forestry research, cooperation with the 

States and private forest owners, and management of the National Forests and National 

Grasslands, it strives—as directed by Congress—to provide increasingly greater service 

to a growing Nation.

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Authors 

Philip M. McDonald is research forester (emeritus), and Nicholas R. 

Vaughn was a research technician, U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station, Western Forest Management 

Unit, 3644 Avtech Parkway, Redding, CA 96002: E-mail: pmcdonald@

fs.fed.us. Vaughn currently is a graduate research assistant at the University 

of Washington.
iii

Abstract

McDonald, Philip M.; Vaughn, Nicholas R. 2007. Growth of thinned and 

unthinned hardwood stands on a good site in northern California. Gen. Tech. 

Rep. PSW-GTR-204. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station. 23 p.

Pacific madrone (Arbutus menziesii Pursh), tanoak (Lithocarpus densiflorus (Hook. 

& Arn.) Rehd.), and California black oak (Quercus kelloggii Newb.) are three 

hardwood species commonly found in the Sierra Nevada of California, an area 

better known for its mixed-conifer forests. Hardwood stands in this region currently are unmanaged and underutilized for commodity production. However, some 

landowners are now asking “How fast do these hardwoods grow,” and “Will thinning increase growth and yield?” Twelve young-growth, mixed-hardwood stands 

on the Challenge Experimental Forest in north-central California were thinned 

from an average basal area of 202 ft2 per acre to different levels of residual basal 

area that ranged from 66 to 153 ft2 per acre. An additional stand (control) provided 

information on development in an untreated condition. Tanoak trees grew faster in 

diameter in thinned plots and control than Pacific madrone, which grew faster than 

California black oak. In general, having two to four members per clump did not 

hinder diameter growth in the thinned plots. Tanoak also grew significantly faster 

both in diameter and volume in a very wet year, but for the other two species a very 

wet or very dry year did not make a difference. Both diameter and volume growth 

were best if stands were thinned to less than 75 ft2 per acre, and net volume growth 

(gross growth minus mortality) compared favorably to eastern oak stands on good 

sites. A future thinning and management regime is suggested.

Keywords: California black oak, diameter growth, Pacific madrone, stand thinning, tanoak, volume growth, yield.
Summary

California’s economically promising hardwoods–Pacific madrone (Arbutus menziesii Pursh), tanoak (Lithocarpus densiflorus (Hook. & Arn.) Rehd.), and California 

black oak (Quercus kelloggii Newb.)­–occupy millions of acres of forest land and 

are an integral part of many ecosystems. These species provide important habitat 

for birds and other animals, visual diversity in the landscape, and many commercial 

wood products. They also are common in the rural-urban interface where management for fuel reduction has become a critical issue. But in spite of these attributes, 

these species are underutilized and virtually unmanaged.

Part of the reason for the lack of management is that little information is available about these species and specifically on their growth. How fast do trees of these 

species grow; do clumps of trees grow as well as single trees of the same size; and 

is growth sensitive to wet and dry years? What is the response of these stands to 

thinning, and if they do respond, how much basal area should be retained to maximize stand growth? What yield per acre can be attained? These questions, being 

asked by foresters and land managers, are addressed in this paper.

Thinning hardwoods is more involved than thinning conifers. The younggrowth hardwoods in this study originate mostly by sprouting from the root crowns 

of stumps or clumps of stumps and thus clothe the landscape in clumps or single 

trees. Some trees lean into openings in the canopy and some grow straight and tall. 

A sudden opening of the stand, which often results in too much light, is detrimental 

to all three species. Thus, thinning needs to be done carefully to provide both shade 

and room for residual trees to enlarge their crowns. In some instances, “guard” 

trees need to be left to shade “save” trees. Consequently, a crown thinning is the 

most effective thinning technique.

Twelve 1⁄4-acre circular plots and one uncut control plot were installed on 

the Challenge Experimental Forest in north-central California to study tree and 

stand growth. Data on species, diameter at breast height, clump density, mortality, 

ingrowth, and stem volume were recorded periodically over 10 to 15 years.

In the natural or unthinned control, tanoak trees grew faster in diameter than 

either Pacific madrone or California black oak. Mortality was confined mostly to 

black oak and specifically to smaller trees that found the unthinned control to be 

too dense and too shady. All three species combined increased their diameters by 

about 0.9 inch (in) per tree over 15 years, and stems in clumps of 2, 3, and 4 members grew slower than single stems.
In the thinned plots, tanoak also had the fastest diameter growth rate. The 

average diameter of combined species increased by 1.9 in per tree for the 15-year 

timespan. Mortality from suppression and unknown causes was distributed almost 

evenly over the three species, with root disease in tanoak and cankers on Pacific 

madrone as prime suspects. Stems in clumps of 2, 3, or 4 members grew about the 

same in diameter as single stems.

A mixed-model analysis showed that diameter growth of individual trees was 

statistically related to species, breast-height diameter, and residual basal area. More 

specifically, tanoak grew the most, trees with larger diameters had the highest 

growth rate, and residual basal areas of 66 to 75 ft2 per acre indicated the highest 

growth rate per tree. Growth generally declined if thinning left more than 114 ft2

per acre of basal area.

Results for volume growth were similar to those for diameter growth. In 

general, heavy thinning to a residual basal area of 66 to 75 ft3 per acre provided the 

highest rate of volume growth per tree and thinning to 124 to 153 ft2 per acre the 

lowest rate. Natural mortality reduced average annual growth from 0 to 22 percent 

per plot. After accounting for mortality, net cubic volume growth was highest in 

plots thinned to 114 and 113 ft2 per acre, respectively. Their average over 10 years 

was 109 ft3 per acre per year.

These three hardwood species are noted as being drought resistant, but are 

they? An analysis to determine the effect of very wet and very dry years showed 

that diameter and volume growth did not differ statistically between wet and dry 

years for Pacific madrone and California black oak, but did differ for tanoak, which 

grew significantly better during the wet year.
Contents

1 Introduction

2 Methods

6 Results

6 The Thinning

8 Diameter Growth

13 Volume Growth

16 Discussions and Conclusions

20 Metric Equivalents

21 References
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California

Introduction

In 1992, a series of publications on California’s hardwood resource in the forest 

zone was begun. In the first publication, Huber and McDonald (1992) detailed the 

history of use for three species of economically promising hardwoods (California 

black oak (Quercus kelloggii Newb.) tanoak (Lithocarpus densiflorus (Hook. and 

Arn. (Rehd.)), and Pacific madrone (Arbutus menziesii Pursh),1 especially for 

wood and wood products, and presented 22 reasons why the industry had not been 

sustained. In the second publication, they followed the progress of the industry 

and cautiously suggested that a viable wood processing industry was possible in 

the near future (McDonald and Huber 1994). By developing this industry, demand 

for products from the hardwood forest could be created, which could lead to active 

management of the resource. McDonald and Huber (1994) also recognized that 

wood products were just one of four major “yields” from the land and that wildlife, 

water, and pleasing scenery were more valuable than wood products. This led to an 

ecosystem management perspective that was followed a year later by a publication 

that described a philosophy and presented guidelines for managing for these four 

yields in the general forest zone, as well as in an agroforestry and urban interface 

setting (McDonald and Huber 1995).

Although these publications provided an overview and a management framework, they did not include specifics on the reproductive biology and silvicultural 

practices necessary for regenerating and managing forest stands. In the fourth 

publication, McDonald and Tappeiner (2002) brought together what is known about 

regeneration (seedlings, seedling sprouts, and root-crown sprouts) of these three 

species in both natural and artificial environments (plantations). In this fifth and 

last publication, the authors present the growth of trees and stands on the Challenge 

Experimental Forest in north-central California in both a natural and artificial 

(thinned) setting.

California black oak, tanoak, and Pacific madrone can be found southward 

from the Oregon-California state line almost to the Mexican border, and eastward 

from near the Pacific Ocean to the Nevada state line. These species are also 

extensive in southwest Oregon, but only Pacific madrone is found in Washington. 

Within California, they grow well to an elevation of about 7,500 ft and form many 

ecological types as pure hardwood stands, as components of mixed hardwood and 

conifer stands, and as occasional trees, clumps, and groves within the conifer forest 

(McDonald and Huber 1995).

1 Common and scientific names are from Little 1979.
GENERAL TECHNICAL REPORT PSW-GTR-204 

As a resource, the inventory of hardwood trees is large, contains every size 

class and structure possible, and even has a high proportion of old-growth trees. In 

California, Waddell and Barrett (2005) estimated that the Pacific madrone, tanoak, 

and California black oak forest types occupied more than 2.85 million acres; 2.5 

million acres as timberland and 0.35 million acres as woodland. The total net 

growing-stock volume of these species that were at least 5.0 inches in diameter at 

breast height (d.b.h.) in the 1990s in California was almost 6.8 billion cubic feet. 

In addition to the timber resource, hardwood stands have high value for wildlife 

(Ohmann and Mayer 1987), for aesthetics (McDonald and Litton 1998), and possibly for increased water yield (McDonald and Huber 1994).

In spite of this large and extensive resource, little information is available on 

the growth of these three hardwood species and stands, especially after manipulation. McDonald (1980a) presented findings for seven of the plots in this study and 

found preliminary relationships between tree diameter growth and species, diameter class, and number of stems per clump.

Although the data and analyses presented in this study are intensive, findings 

are directly applicable only to specific species at a particular stand age on a high 

site. Hardwoods growing on poorer sites or at different ages may differ in their 

response to thinning.

This paper quantifies the response of three hardwood species in a wide range 

of residual basal areas over 10 to 15 years in terms of diameter growth, number of 

stems per clump, ingrowth, mortality, and average annual and net volume growth.

Methods

This study was located on the Challenge Experimental Forest in north-central California (Yuba County), T 19 N, R 7 E, MDM, sec. 29. The forest cover type in the 

vicinity of the research plots is Pacific ponderosa pine-Douglas-fir (SAF type 244) 

(McDonald 1980b). Several conifer and hardwood species characterize this type. In 

terms of ecological subregions of California, the area corresponds to section M261 

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

Service 1997). Site quality is high, with dominant California black oaks averaging 

about 70 ft tall, for a site index of 60 at 50 years (Powers 1972).

Early logging (McDonald and Lahore 1984) and subsequent forest fires 

removed most of the conifers from part of the Challenge Experimental Forest and 

left a hardwood stand that had originated from sprouts and essentially was even 

aged. This was administratively designated as a hardwood compartment. The few 

conifers that were present were logged in spring 1966, leaving a 60-year-old pure 

hardwood stand. The slash was piled by hand and eventually burned.
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California

Summers on the experimental forest are hot and dry; winters are cool and 

moist. The mean annual temperature is 55 °F. Precipitation was recorded each 

month and totaled each year. Average annual precipitation is more than 67 in with 

94 percent falling between October and May. Soil moisture is the environmental 

factor that limits plant growth. The hardwood compartment where the study was 

located is quite homogeneous with an elevation of 2,750 ft, 15 percent slope, and 

southwest aspect.

To quantify the growth of individual species, as well as that of the mixed-hardwood stands, thirteen 1⁄4-acre circular growth plots, plus buffer, were established in 

the hardwood compartment. Before logging, basal areas ranged from 165 to 244 ft2

per acre and, after logging, from 66 to 153 ft2 per acre. Desired basal area density 

levels ranged from 60 to 150 ft2 per acre and were randomly assigned. One of the 

13 plots was randomly selected as an unthinned control.

Marking young-growth hardwood stands for thinning is difficult and different 

from marking conifer stands. First of all, the stands are dense, and the trees tend 

to be slender with poorly developed crowns (fig. 1). The trees often are in clumps; 

their roots can occupy one area and their crowns can influence another. Second, 

the crowns of tanoak and Pacific madrone weaken and decline because of the high 

light levels created by sudden opening of the stand, and California black oak trees 

tend to form undesirable epicormic branches along the bole (fig. 2) (McDonald and 

Ritchie 1994). Thus, “guard” trees were left around selected “save” trees where 

Figure 1—A typical young-growth stand of 

Pacific madrone, tanoak, and California black 

oak before thinning on the Challenge Experimental Forest.

Figure 2—After thinning, the narrow crown 

of this California black has been supplemented 

with a false crown of epicormic branches 

along the bole.
GENERAL TECHNICAL REPORT PSW-GTR-204 

possible. For California black oak, the primary purpose of guard trees was to shade 

boles, and their secondary purpose was to partially shade crowns. In terms of classical thinning regimes, this thinning closely resembled a crown thinning whereby 

residual trees were provided with some “sky area” into which their crowns could 

expand. Save trees, or those that were to be retained after thinning, were straight 

and tall with a minimum of crook and sweep, no forks, no logging wounds, and no 

evidence of rot. Almost all guard trees also met these criteria.

Local fuelwood cutters thinned the stand in return for the wood (fig. 3 A, B, C). 

However, they were not available every year, and the initial cutting of individual 

growth plots took place from 1968 through 1979. Cutting rules required the removal 

of all unmarked trees, stumps no higher than 8 in above mean ground line, the piling of slash, and leaving no more than four stems per clump. Short stumps promote 

healthy, rot-free sprouts from the root crown, rather than weak sprouts from the 

top or sides of taller stumps (McDonald 1980a). Slash from thinning was piled well 

away from marked trees.

In each growth plot, all trees larger than 2 in d.b.h. were recorded and their 

diameters measured. These constituted the before-thinning sample. Every save 

tree was tagged (numbered) 2 in above breast height and measured at breast height. 

Untagged trees were removed. Species, diameter to the nearest 0.1 in, and clump 

density (1, 2, 3, or 4 members per clump) were recorded every year for the first 6 

years and at ages 10 and 15. Unfortunately, data from five plots were not recorded 

after 10 years. Diameter growth was calculated on a per-tree basis with quadratic 

means for reporting on a per-stand basis. Total tree volumes were estimated from 

local volume tables applicable to each species on a high site (McDonald 1983).

These hardwood species can die in a single year or over several years. Trees 

of Pacific madrone usually die in 1 year, and those of all three species die in 1 or 2 

years if diseased. As noted earlier, the crown of tanoak trees dies back, but rarely 

do trees die from suppression. The crowns and false crowns (epicormic branches) 

of suppressed black oaks die from the top down, and thus death was not recorded 

until the bole was dead at breast height. Rarely did the boles of any species 

expand in diameter after crown decline, and sometimes they actually shrank. For 

these reasons, diameter and volume growth were calculated only for trees that 

lived throughout the study period. Year of death and suspected cause were noted 

at each survey.

After thinning, root-crown sprouts quickly appeared at the base of 

most stumps. These sprouts form clumps of stems and tend to grow rapidly 

(McDonald and Tappeiner 2002). When they reached 3.5 in d.b.h. they were 

termed ingrowth, given a tag, and measured as part of the stand.

In terms of classical 

thinning regimes, 

this thinning closely 

resembled a crown 

thinning whereby 

residual trees were 

provided with some 

“sky area” into 

which their crowns 

could expand.
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California



Figure 3—Thinning and harvesting the stand. (A) Gathering wood from the partially thinned stand, (B) stacked wood ready for shipment, and (C) smaller diameter 

wood sacked for sale to a local supermarket. 

A

B

C

Statistical analysis was by analysis of variance, linear regression, mixed-model 

analysis (SAS 2001), and paired t-test (R Development Core Team 2004).
GENERAL TECHNICAL REPORT PSW-GTR-204 

Results

The Thinning

After thinning, the desired basal area density levels, which ranged from 60 to 

150 ft2 per acre, were generally achieved, with an average variation of 2.5 ft2 per 

density level (range 0 to 6.1 ft2). Before thinning, the average stand contained 

more than 202 ft2 per acre of basal area and 575 trees per acre. After thinning, the 

average stand contained 110 ft2 per acre of basal area and 163 trees per acre. Thus, 

stand basal area was reduced by 45 percent and density by 71 percent (tables 1 and 

2). Thinning removed all trees smaller than 4.1 inches d.b.h. and many trees smaller 

than 8.1 in d.b.h., thus favoring larger trees of all three species (table 2). Thinning 

also increased the average tree diameter of every species in every plot relative to 

the control (table 3). In terms of crown classification, all trees in the suppressed 

crown class and many in the intermediate class were removed. Examination of the 

cut stumps showed a slow and generally declining diameter growth rate, with no 

evidence of heart rot.

Table 1—Basal area of growth plots and control before and after thinning on the 

Challenge Experimental Forest, Yuba County, California

Basal area

Plot Before thinning After thinning Reduction

- - - - - - - - - - - Square feet per acre - - - - - - - - - - Percent

1 165.1 101.7 38.4

2 194.2 109.8 43.5

3 207.7 124.9 39.9

5 229.8 141.3 38.5

6 205.6 136.0 33.9

7 187.8 85.4 54.5

8 231.5 153.1 33.9

9 225.1 113.3 49.6

10 175.1 103.7 40.8

11 244.3 114.1 53.3

12 181.8 75.5 58.5

13 179.9 66.2 63.2

Average 202.3 110.4 45.6

4 (control) 202.3 202.3 –

 – = no data.
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California

Table 2—Stand density by species and diameter class before and after thinning on the 

Challenge Experimental Forest, Yuba County, California

Diameter class (inches)

3.5–4.0 4.1–8.0 8.1–12.0 12.1–16.0 16.1–20.0 Total 

Species Ba Aa B A B A B A B A B A

 - - - - - - - - - - - - - - - - - - - - -Number of trees per acre - - - - - - - - - - - - - - - - - - -

Pacific 

madrone 8 0 66 3 28 14 5 4 0 0 107 21

Tanoak 48 0 185 10 83 46 36 31 1 1 353 88

California 

black oak 12 0 63 20 35 30 5 4 0 0 115 54

Total 68 0 314 33 146 90 46 39 1 1 575 163

a B = before thinning; A = after thinning.

Table 3—Quadratic mean tree diameter by species in thinned plots 

and control at the Challenge Experimental Forest, Yuba County, 

California

Plot

Pacific 

madrone Tanoak

California 

black oak

 - - - - - - - - - - - - - - - - - - - Inches - - - - - - - - - - - - - - - - - - -

1 – 11.6 10.1

2 12.2 11.3 9.2

3 11.2 9.3 8.8

5 10.3 11.1 9.1

6 – 11.9 8.2

7 9 11.1 7.5

8 10.6 11.2 8.7

9 11.2 10.8 10.9

10 9.3 12.2 7.5

11 11.6 12 10.4

12 11.7 12.1 9

13 9.7 11 9.1

Average 10.7 11.3 9

4 (control) 8.7 7.6 6.6

 – = no data.
GENERAL TECHNICAL REPORT PSW-GTR-204 

More than 1,235 trees were removed for an average yield of 1,650 ft3 per acre 

or 12.9 cords per acre. The average volume of trees retained on thinned plots was 

2,515 ft3 per acre or 19.7 cords per acre.

Thinning also modified the makeup of the stand by increasing the number 

of California black oaks at the expense of Pacific madrone and tanoak. Species 

composition as a percentage of average stand density before and after thinning was:

The changes in species composition were entirely the result of the thinning 

prescription and not because of species preference.

Diameter Growth

To ascertain relationships among species and tree sizes, data were first analyzed 

by 4-in diameter classes (from 4 to 20 in) for each species, and then for all species combined. Only Pacific madrone trees in the largest diameter class (17 to 

20 in) showed a period of strong early growth surge (0.20 to 0.47 in in diameter 

per tree per year at age 3), a phenomenon often termed “release.” Tanoaks in the 

largest diameter class indicated a decline in early growth rate (0.37 to 0.22 in per 

tree per year at age 3). For all other species and diameter classes, increases and 

decreases in average diameter growth after thinning tended to be small, and no 

release was indicated.

For all species in all diameter size classes combined, the average growth rate 

of trees in thinned plots remained fairly constant through age 15, but in the control 

declined slightly (table 4). Relative to the control, thinning increased the cumulative 

average diameter growth per tree from 63 to 133 percent.

Species Before After

Pacific madrone 19 13

Tanoak 61 54

California black oak 20 33

Table 4—Average annual diameter growth per tree at five measurements 

in thinned plots and control for all species combined and percentage 

increase of thinned plots over control, Challenge Experiental Forest, 

Yuba County, California

Years since Growth

treatment Control Thinned Increase

Inches/tree/year Percent

1 0.08 0.13 63

3 .07 .14 100

5 .06 .14 133

10 .06 .14 133

15 .06 .13 117
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California



The relationship of average breast-height diameter growth per tree to time since thinning in both the thinned 

plots and control (fig. 4 A, B, C) was analyzed with a weighted linear regression of the form:

Υij = b0i + b1i • tij +eij in which:

Figure 4—Relationship of individual tree diameter 

growth to time since thinning for Pacific madrone (A), 

tanoak (B), and California black oak (C) on the Challenge 

Experimental Forest, Yuba County, California. Bars 

around means depict standard errors.

BIndividual tree diameter growth (inches)

Time since thinning (years)

Time since thinning (years)

Υij = d.b.h. growth of tree j of species i (three 

levels: Pacific madrone, tanoak, or California 

black oak),

bni = coefficients estimated by the linear 

regression procedure for species i,

tij = number of years after thinning, and

eij = random error.

Individual tree diameter growth (inches)

Time since thinning (years)

A

CIndividual tree diameter growth (inches)
10

GENERAL TECHNICAL REPORT PSW-GTR-204 

We determined that the variance of diameter growth increased as time since 

thinning increased. Therefore, the regression was weighted with the square 

of time (years since thinning). Repeated measurements of the same plot were 

assumed to be independent of each other for this analysis. Given this assumption, 

the slope term for each species and treatment is an estimate of the annual growth 

rate per tree.

Tanoak grew faster than the other species in both the thinned plots and control 

during the study. Pacific madrone was intermediate in diameter growth rate, and 

California black oak had the lowest growth rate. Relevant statistics and regression 

procedures (SAS 2001) are noted (table 5).

Do trees in a clump of two, three, or four members grow faster or slower than 

single trees of the same diameter? The answer is important to foresters when 

determining which members to save when thinning a stand. Thus, diameter growth 

for clumps of up to four members was quantified for both thinned plots and control. 

Table 5—Fitted parameters for the weighted linear regressions relating cumulative diameter growth per tree to years 

since treatment for Pacific madrone, tanoak, and California black oak

Species Action Intercept Slope F-value Probability > F

Pacific madrone Thinned -0.0033 0.1264a 395.27 <0.0001

Control .0427a .0580a 94.56 <.0001

Tanoak

Thinned -.0056 .1617a 2,327.76 <.0001

Control .0187b .0715a 243.52 <.0001

California 

black oak

Thinned .0074 .1001a 1,302.18 <.0001

Control .0096 .0514a 368.63 <.0001

a Parameter is significantly different from 0 at α = 0.05.

b Parameter is significantly different from 0 at α = 0.10.

Table 6—F-tests (type III) of the parameter estimates for the relationship of clump density, species, and treatment to 

single-tree diameter growth 

Denominator

Factor Representation Estimate degrees freedom F-value Probability > F

Intercept b0 1.12023 – – –

Speciesa b1 0.57890 1 138.73 <0.0001

Trees in clump x treatment b21-b20 0.25369 2 59.03 <0.0001

Thinned b21 -0.06842 – – –

Control b20 -0.32211 – – –

a Pacific madrone and California black oak were compared with tanoak in this factor. The levels of species are 1 = tanoak and 0 = madrone or black oak.

 – = no data.
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California

11

Ten years after thinning was considered to be the time of most interest. Data were 

analyzed by species, and no significant difference was found between California 

black oak and Pacific madrone (a = 0.05), so they were combined and compared to 

tanoak by an analysis of covariance (SAS 2001) with the form:

Υijk = b0j + b1 • Sj + b2i • CxTijk + eijk

where:

Υijk = 10-year diameter growth for tree k of species j and treatment i,

bn(i) = coefficients estimated by the SAS GLM (General Linear Model) procedure,

Sj = species j = 1 for tanoak or 0 for Pacific madrone or California black oak,

CxTijk = interaction term, number of trees in the clump by treatment, and

eijk = residual error.

For thinned plots, an increase in one member per clump caused only a 0.068-in 

decrease in average diameter (table 6). Consequently, the forester can leave clumps 

of up to four members, with only an insignificant loss in diameter growth. In the 

control, an increase in one member per clump caused a 0.32 in decrease in average 

stem diameter.

Diameter growth was then evaluated by species and residual basal area density 

level to evaluate the stand response to thinning. After 10 years, average diameter 

growth of Pacific madrone ranged from 0.83 to 1.9 in per tree, that of tanoak from 

1.09 to 2.55 in, and that of California black oak from 0.77 to 1.6 in per tree. In 

general, each species alone or in a mixture grew best if heavily thinned (60 to 75 ft2

of basal area per acre) and poorest if lightly thinned (124 to 153 ft2 per acre) (fig. 5).

Figure 5—Effect of residual basal area on diameter growth per tree for Pacific madrone, 

tanoak, and California black oak.

(ft2 • acre-1)
12

GENERAL TECHNICAL REPORT PSW-GTR-204 

To better portray diameter growth relative to beginning diameter, diameter 

growth was evaluated as a function of species, basal area density level, and d.b.h. of 

individual trees by a mixed-model analysis (SAS 2001) with the form: 

Υijk = b0j + qi+ b1j • — + b2 • Dbhk + eijk

where:

Uijk = 10-year diameter growth for plot i, species j (1 for tanoak, 0 otherwise), and 

tree k,

bnj = coefficient estimated by mixed-model analysis,

qi = random plot effect i = 1 to 13 plots,

Rbai = residual basal area for plot i,

Dbhk = d.b.h. for tree k, and

eijk = residual error.

This analysis showed that residual basal area, d.b.h., and species explained a 

significant amount of variation in the diameter growth of these hardwoods (table 

7). More specifically, table 7 and some of the foregoing tables and figures show 

that results, significant at the 0.001 level, 10 years after thinning, were fivefold: 

(1) tanoak had a significantly higher growth rate per tree than Pacific madrone or 

California black oak, (2) the basal area density levels that promoted the highest 

growth rate were at the lower end of those studied (60 to 75 ft2/acre), (3) larger 

diameter trees had better growth, (4) the species-diameter interaction was statistically significant, and (5) the species-basal area interaction was significant. The 

significant interactions mean that as diameter increases and basal area decreases, 

Table 7—F-tests (type III) of the parameter estimates for the relationship of residual basal area, species, and 

beginning diameter to diameter growth 

Factor Representation Estimate

Denominator

degrees freedom F-value Probability > F

Intercept β00 -0.60250 – – –

Residual basal area (1/r.b.a.) β10 142.36 12 110.44 <0.0001

Diameter (d.b.h.) β20 .07148 666 161.6 <.0001

Speciesa β01 – β10 -.21280 666 1.46 .227

R.b.a. x speciesa β11 – β10 -93.20 12 34.55 <.0001

D.b.h. x speciesa β21 – β20 .06427 666 15.51 <.0001

a

 Other species were compared with tanoak in each of these factors. The levels of species are 0 = tanoak and 1 = other species. Therefore, these factors are the effect on the intercept and the residential basal area (r.b.a.) and diameter at breast height (d.b.h.) slopes that are 

caused by changing the species from tanoak to either of the other two species.

–= no data.

1

Rba







i


Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California

13

tanoak growth will increase faster than California black oak and Pacific madrone 

although the rate of increase in growth is not noticeable at smaller diameters or 

higher residual basal areas.

Volume Growth

Although volume growth was calculated from local volume tables, which are based 

on stem diameter (d.b.h.), the relationship of diameter growth to volume growth 

differs because volume is based on squared diameter values and the relationship is 

curvilinear. Also, foresters and land managers prefer volume estimates to arrive at 

yield values.

To better understand growth relationships, volume per tree and volume per 

stand also were examined. After 10 years, average volume growth for Pacific 

madrone in thinned stands ranged from 2.3 to 10.9 ft3 per tree; for tanoak, from 

3.3 to 9.8 ft3 per tree; and for California black oak, from 2.1 to 5.0 ft3 per tree. In 

general, volume growth was highest for heavily thinned stands (66 to 75 ft2 per 

acre) and lowest for lightly thinned stands (124 to 153 ft2 per acre) (fig. 6).

We then expanded the analysis of stand volume from a gross-growth to a 

net-growth basis. But first, mortality needed to be calculated (table 8). For thinned 

plots, mortality amounted to 19 trees and was about evenly divided among species. 

Pacific madrone lost the most volume (38.9 ft3•acre-1•year-1) and California black 

oak the least (17.8 ft3•acre-1•year-1). For the control, 26 trees were lost, with the 

majority of both trees and volume being California black oak.

Figure 6—Effect of residual basal area on volume growth per tree for Pacific madrone, 

tanoak, and California black oak. 10-year volume growth (ft3 • acre-1)

Residual basal area (ft2 • acre-1)

After 10 years, average volume growth 

for Pacific madrone 

in thinned stands 

ranged from 2.3 to 

10.9 ft3 per tree; for 

tanoak, from 3.3 to 

9.8 ft3 per tree; and 

for California black 

oak, from 2.1 to 5.0 

ft3 per tree.
14

GENERAL TECHNICAL REPORT PSW-GTR-204 

Ingrowth often is used as an offset to mortality if larger trees are present, but in 

this study all the trees that became ingrowth were very small and thus their volume 

was inconsequential. Ingrowth consisted of 49 saplings in five plots cut from 85 

to 125 ft2 per acre. More than 31 percent were Pacific madrone, 67 percent were 

tanoak, and slightly less than 2 percent were California black oak. Most became 

ingrowth at age 15.

Natural mortality reduced average annual (gross) volume growth from 0 to 22 

percent per plot in thinned stands and 26 percent in the control. Plots with residual 

basal area density levels of 85 and 141 ft2 per acre had the highest percentage 

losses. After subtracting mortality, net volume growth was greatest in plots thinned 

to 114 and 113 ft2 per acre (fig. 7). These two plots had the highest gross volumes 

(114.5 and 104.2 ft3 •acre-1• year-1, respectively) and no mortality.

Because Pacific madrone, tanoak, and California black oak are noted for 

tolerating drought (McDonald 1982, Morrow and Mooney 1974, Waring 1969), 

we compared average diameter and volume growth of each species and all species 

combined for the 2 wettest years (1974 and 1978) and the 2 driest years (1976 and 

1977) to test this assumption. The analysis attempted to relate full-year growth 

to the amount of precipitation for both the time of active growth (April 1 through 

July 30) and the full year. Relative to the 50-year average taken at the Challenge 

Ranger Station less than 1 mile away, precipitation during the active growth period 

decreased 48 percent during the average driest year and increased 36 percent during the average wettest year. Similar comparisons for the full year were driest year, 

60 percent decrease; wettest year, 42 percent increase:

Period 50-year average Dry year Wet year

 - - - - - - - - - - - - - Inches of water - - - - - - - - - - - - -

Active growth 7.97 4.18 10.86

Full year 67.31 26.84 95.38

Table 8—Mortality by species during 10 years in the 12 thinned plots and the control plot 

on the Challenge Experimental Forest, Yuba County, California

Category Species Number of trees Volume

ft3• acre-1• year

-1

Thinned plots Pacific madrone 6 38.9

Tanoak 6 23.1

California black oak 7 17.8

Total 19 79.8

Control Pacific madrone 4 3.5

Tanoak 0 0.0

California black oak 22 19.5

Total 26 23.0
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California

15

To minimize thinning effects, plots had to have been thinned at least 4 years 

before the year could be selected as a wet or dry year. Values are for a calendar 

year, rather than a water year (July 1 to June 30).

A paired t-test (R Development Core Team 2004) showed that whereas average 

diameter growth for Pacific madrone, tanoak, and California black oak was greater 

during the average wet year, it was significantly greater (p = 0.047) only for tanoak 

(table 9). Similar results were obtained for volume growth as well (table 10).

Figure 7—Relationship of average annual net volume growth and mortality to residual basal 

area density level.

Residual basal area (ft2 • acre-1)

Average annual volume (ft3 • acre-1)

Table 9—Diameter growth per tree by species for the average of 2 relatively dry years (1976 and 1977) and 2 wet years 

(1974 and 1978) at the Challenge Experimental Forest, Yuba County, California

Species Dry years Wet years Difference n T-value Prob > |T|

 - - - - - - - - - - - - - - - - - - - Inches per tree - - - - - - - - - - - - - 

Pacific madrone 0.060 (0.0075) a 0.065 (0.0097) 0.006 (0.0087) 42 0.68 0.49881

Tanoak .090 (.0052) .101 (.0049) .010 (.0051) 172 2 .04687

California black oak .067 (.0043) .072 (.0040) .006 (.0044) 184 1.28 .20124

All species .076 (.0032) .084 (.0031) .008 (.0031) 398 2.44 .01514

a The value in parentheses is the standard error of the given mean estimate.
16

GENERAL TECHNICAL REPORT PSW-GTR-204 

Discussion and Conclusions

Thinning young-growth (60-year-old) mixed-hardwood stands requires special 

considerations. Most hardwood trees in the Sierra Nevada of California originate 

as root-crown sprouts and hence form clumps of stems. At 60 years of age, natural 

mortality has reduced the number of stems per clump to fewer than 10, but almost 

all the clumps remain with one or more trees. This combination produces a dense 

stand of slender trees. These trees are striving to establish their canopies in sunlight 

and thus allocate most available resources to height growth rather than to crown 

development or bole width. Consequently, the typical young-growth hardwood 

stand consists of a variable number of slow-growing trees in a variable number of 

clumps. Usually, the tallest and straightest trees remain, but sometimes a tree will 

curve away from the clump, turn upward, and become part of the canopy. When 

several trees in a clump survive, one or more often develop a leaning or J-shaped 

lower bole. Multiple-stemmed clumps of tanoak and Pacific madrone tend to be 

more common than those of California black oak. Consequently, the silviculturist that prepares the thinning prescription must decide whether to leave whole 

clumps, parts of clumps, or single trees. Of paramount importance to this decision 

is whether trees in clumps grow as fast as single trees. In general, we found that for 

thinned stands, a tree of a given diameter in a clump of two to four members grew 

at about the same rate as a single tree of the same diameter. In the control, trees in 

clumps of up to four members grew slower than single trees.

Thinning hardwoods becomes even more complicated because the crowns of 

the various species have a different sensitivity to abrupt changes in light. Newly 

exposed crowns of tanoak become blighted, die back, and eventually die to that 

point on the bole where the branches are shaded. Crowns of Pacific madrone also 

Table 10—Volume growth per tree by species for the average of two relatively dry years (1976 and 1977) and two wet 

years (1974 and 1978) at the Challenge Experimental Forest, Yuba County, California

Species Dry years Wet years Difference n T-value Prob > |T|

 - - - - - - - - - - - - - - - - Cubic feet per tree - - - - - - - - - - - - - - - 

Pacific madrone 0.265 (0.0423)a 0.305 (0.0514) 0.039 (0.0404) 42 0.97 0.33532

Tanoak .283 (.0191) .340 (.0212) .057 (.0170) 172 3.35 .00100

California black oak .209 (.0173) .233 (.0173) .023 (.0146) 184 1.59 .11331

All species .247 (.0124) .286 (.0135) .039 (.0108) 398 3.64 .00031

a The value in parentheses is the standard error of the given mean estimate.

Thinning 

young-growth 

(60-year-old) 

mixedhardwood 

stands requires 

special 

considerations.
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California

17

become blighted and turn into a collection of branches with little tufts of leaves at 

the end. Crowns of California black oak withstand sudden exposure better than the 

other two species, but epicormic branches often proliferate along the bole. The 

tendency for epicormic branching is both genetically controlled and a function 

of tree and stand characteristics. Epicormic branching of California black oak on 

the Challenge Experimental Forest occurred on all but the most heavily shaded 

trees and increased significantly with decreasing stand density, proximity to 

openings, and location on south and east bole faces (McDonald and Ritchie 1994). 

These branches produced no acorns and degraded lumber quality. Thinning, then, 

necessitates opening up the stand carefully and removing a high percentage of 

small trees. It also necessitates leaving some trees as guard trees that normally 

would be removed.

Consequently, the most practical type of thinning was a crown thinning, 

whereby the crown of each save tree was provided space for expansion.

Periodic measurements of several hundred living trees in 12 growth plots and 

an uncut control provided insight into how trees and stands grow in a natural and 

thinned state. For all plots, before-thinning basal areas ranged from 165 to 244 

ft2 per acre and after thinning from 66 to 153 ft2 per acre. Somewhere within this 

range should be a basal area that would balance the level of growing stock with 

the most efficient use of available site resources and therefore maximize tree and 

stand growth.

Tanoak grew reasonably well in the dense natural (unthinned) stand. Even 

small trees grew well, and mortality was negligible. Pacific madrone ranked second 

for diameter growth with some mortality, and California black oak was third. 

Mortality of California black oak was high because many small trees could not 

tolerate the deep shade and died from suppression. However, for larger, healthier 

black oaks, the deep shade of the unthinned control helped curtail the production of 

epicormic branches.

The timing and magnitude of the response to thinning for Pacific madrone, 

tanoak, and California black oak in this study differed by species and tree size. 

The growth of some trees of some species increased immediately after thinning, 

some responded later, and still others never responded to the increase in growing 

space. This variation was not surprising because factors such as prethinning tree 

vigor, clump density, development of epicormic branches, crown size, and crown 

position, as well as the interactions of all these variables, could lead to differences. 

In thinned stands, all three species of hardwoods had twice the diameter growth 

rate of trees in the control 3 years after thinning and maintained this superiority 

through age 15. However, the overall difference between thinned trees and trees 
18

GENERAL TECHNICAL REPORT PSW-GTR-204 

in the control narrowed at 15 years, indicating that these stands could have been 

thinned a second time around age 10. Among species, tanoak grew faster than 

Pacific madrone, which in turn grew faster than California black oak. For all species, growth slowed if thinning left 124 to 153 ft2

 of basal area per acre.

Because forest managers are interested in yield, gross (average annual) volume growth was calculated, as was net volume growth after tree mortality was 

subtracted. Gross volume growth for trees in thinned stands ranged from 2.1 to 

10.9 ft3 per tree for the 10-year period. Mortality was about evenly divided among 

species in thinned plots and reduced gross volume growth from 0 to 22 percent per 

plot. In the control, mortality, mostly of small California black oaks, amounted to 

26 percent.

Among thinned plots, results for volume growth were quite similar to those for 

diameter growth. In general, volume growth per tree was highest in stands thinned 

from 66 to 75 ft2 per acre and lowest in stands thinned to 124 ft2 per acre or more. 

After mortality was subtracted, the plot with about 114 ft2 per acre had the highest 

net growth (114.5 ft3•acre-1•year-1). Net volume growth for trees in the control was 

65.6 ft3 per acre per year.

The volume growth of these three California hardwood species compared quite 

favorably with that of eastern hardwood species on sites of similar quality. For 

natural even-aged, young growth stands of the five principal upland oak species 

in the central United States, volume growth averaged 43 ft3•acre-1•year-1 (Schnur 

1937). In the same general area, thinned stands of upland oaks added about 43 

ft3 per acre per year at a residual basal area of 94 ft2 per acre (Dale 1972). For a 

mixed-hardwood stand on an excellent site in West Virginia, net volume growth for 

trees in the range of residual basal areas of 60 to 100 ft2 per acre averaged about 94 

ft3•acre-1•year-1over a 10-year period (Trimble 1968).

The finding that a very wet year or a very dry year did not make much difference in the diameter or volume growth of Pacific madrone and California black 

oak was not surprising. All three species of hardwoods in this study have ancient 

progenitors in the Miocene epoch of 12 to 26 million years ago (Axelrod 1973), and 

all were part of the arborescent community then as now. Two species, tanoak and 

Pacific madrone, are classified as broad sclerophylls; California black oak is not, 

probably because it is deciduous. Broad sclerophylls are noted for having many 

morphological and physiological adaptations for tolerating drought (Cooper 1922). 

Although tanoak has many of these adaptations, its natural range is characterized 

by a cool, moist climate with high levels of either precipitation, fog, or relative 

humidity (Tappeiner and others 1990). Plainly, tanoak differs from the other two 

species in its tolerance of drought, and therefore it was not surprising that it grew 

significantly better in a wet year.

The volume 

growth of these 

three California 

hardwood species 

compared quite 

favorably with 

that of eastern 

hardwood species 

on sites of similar 

quality.
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California

19

Purely in terms of wood production, volume (quantity) considerations are 

only half the decision for future management. The quality of the wood (bole) is 

the other half. Although speculative, a regime of careful thinning and pruning is 

suggested. A reasonable future thinning schedule would be to thin two more times 

at approximately 10-year intervals. A few guard trees would have to be kept at the 

second thinning, but none would be left at the third. After three thinnings, tree 

crowns should have become fully developed and be in better balance with the roots 

in terms of biomass and ability to capture site resources (fig. 8). More wood, acorns, 

and berries will result. Epicormic branching on California black oak also should 

become minimal because of this balance and because the well-developed crowns 

would be shading tree boles. Pruning at the time of first thinning or even 

earlier should be inexpensive because most trees in the characteristically dense 

stands prune naturally, and only a few branches would need to be removed.

In the longer term, the root-crown sprouts from the cut stumps will soon 

be ingrowth and then small trees. They, too, will need to be managed. Thus 

a cutting method resembling a two-stage shelterwood is suggested, with the 

thinned overstory being the first stage and the root-crown sprouts the second. The 

environment beneath the thinned stands should be favorable in terms of available 

light, and stem form should be 

straight and tall. After removal of 

the overstory, the crowns of the 

sprouts will be in a codominant 

position, which is best for these 

hardwoods. Eventually, they also 

will need to be thinned and management of the stands repeated.

Figure 8—After three thinnings, trees such as this 

California black oak with a straight bole, no epicormic 

branches, and a well-developed crown should be present.
20

GENERAL TECHNICAL REPORT PSW-GTR-204 

Metric Equivalents

When you know: Multiply by: To find:

Inches (in) 2.54 Centimeters

Feet (ft) .3048 Meters

Mile (mi) 1.609 Kilometers

Acre .405 Hectares

Square feet (ft2) .0929 Square meters

Cubic feet (ft3) .0283 Cubic meters

Degrees Fahrenheit (°F) Degrees Celsius F-32

1.8
Growth of Thinned and Unthinned Hardwood Stands on a Good Site in Northern California

21

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