Management of ponderosa pine in even-aged stands in the Southwest

Historic, Archive Document

Do not assume content reflects current
scientific knowledge, policies, or practices.

■

urn mm.

Management of Ponderosa Pine in
Even-Aged Stands in the Southwest

Robert R. Alexander and
Carleton B. Edminster

Research Paper RM-225
Rocky Mountain Forest and
Range Experiment Station
Forest Service

U.S. Department of Agriculture

Abstract

Potential production of ponderosa pine in the Southwest is
simulated for various combinations of stand density, site index, age,
and thinning schedule. Such estimates are needed to project future
development of stands managed in different ways. .

Plant a tree! Mark the 75th birthday of the Forest Service
by giving a living gift to future generations.

Cover Photo.— Second-growth ponderosa pine on site index 60
lands (Meyer 1961) thinned to a GSL of 80, Taylor Woods near
Flagstaff, Ariz. Stand was about 45-years old when thinned in
1962.

USDA Forest Service
Research Paper RM-225

December 1980

3L^

Management of Ponderosa Pine in
Even-Aged Stands in the Southwest *

Robert ^Alexander, Chief Silviculturist
and

Carleton B.jEdminster, Mensurationist
Rocky Mountain Forest and Range Experiment Station1

'Headquarters is in Fort Collins, in cooperation with Colorado State University.

Contents

Page

Silviculture of Southwestern Ponderosa Pine 1

Establishment of Regeneration 1

Need for Early Precommercial Thinning 2

Estimates of Growth Under Intensive Management 2

Diameter Growth 3

Height Growth 3

Basal Area Growth 3

Total Cubic-Foot Volume Increment 4

Board-Foot Volume Increment 5

Maximizing Board-Foot Volume Yields 6

Tradeoffs to Increase Values of Other Resources 8

Management Caution 9

Literature Cited 10

Appendix 11

Management of Ponderosa Pine in
Even-Aged Stands in the Southwest

Robert R. Alexander and Carleton B. Edminster

Silviculture of Southwestern Ponderosa Pine

Southwestern ponderosa pine2 (Firms ponderosa
Laws] cover type occupies the largest area of commer-
cial forest land in Arizona and New Mexico (Choate
1966. Spencer 1966). It is less extensive in south-
western Colorado and southern Utah (Choate 1965.
Miller and Choate 1964). Ponderosa pine forests in the
Southwest occur between 6.000 and 8.500 feet eleva-
tion, but reach maximum development between 7,000
and 7.800 feet, where they are the climax forests
(Schubert 1974).

Southwestern ponderosa pine forests were first cut
during the Gold Rush of the mid-1800's. Commercial
cutting began with construction of the transcontinental
railroad during the late 1800's. Since then, ponderosa
pine forests have provided a variety of wood products,
forage for livestock, and habitat for a variety of
wildlife. Today other uses are becoming important.

How these forests are managed affects all resources
and uses. For example, if timber production is the
primary objective, higher growing stock levels (GSL)3
should be maintained, but forage production and water
yields can be substantially increased only at lower
GSL's. Low to medium GSL's are generally considered
necessary to improve developed recreational oppor-
tunities and enhance foreground esthetics. Wildlife
habitat varies from uncut to open forests. Improvement
of middleground and background esthetics generally
requires a combination of open, low stocking and high
stocking levels that provide contrasts.

Although land managers must increasingly direct
their practices toward multiple uses, these practices
must be based on sound silvicultural principles of the
forest type involved. Land managers must understand
the tradeoffs between the timber resource and other
physical, social, and economic considerations.

In the past, southwestern ponderosa pine has been
under extensive management. Harvesting practices
have generally been limited to "loggers selection" or

2Southwestern ponderosa pine as described here does not in-
clude the Front Range of Colorado and Wyoming.

3Growing stock level (GSL) is defined as the residual square
feet of basal area when average stand diameter is 10 inches d.b.h.
or more. Basal area retained in a stand with an average diameter
of less than 10 inches is less than the designated level (Myers
1971, Edminster 1978). Tables A-1. A-2, and A-3 in the appendix
give the basal area, number of trees, and square spacing in
stands with average diameters after thinning of 2 to 10 inches, for
GSL levels 40 to 160.

sanitation salvage and improvement selection cutting
that removed trees in a series of cuts on an individual
or group basis. Cutover areas were allowed to restock
naturally regardless of the time required or the stock-
ing achieved. Today, management intensity has in-
creased, and managers are concerned with (1) prompt
restocking of cutover areas with a new stand, (2) in-
creasing the growth rate of the new stand by control of
stand density, and (3) improving quantity and quality of
yields by periodic thinning to maintain stocking control
and growth rates and reduce mortality.

In old growth stands, average annual net increment
varies from 25 to 90 fbm per acre because of under-
stocking or overstocking and high mortality associated
with old-growth timber (Pearson 1950). Under intensive
even-aged management, annual net growth can be in-
creased to 100 to 300 fbm per acre (Edminster 1978).

Stand density control offers the greatest opportunity
for increasing wood production by increasing growth
and reducing mortality, but harvested stands must be
replaced promptly to reduce time required to reach
maximum yields. Ponderosa pine regeneration in the
Southwest has been notoriously slow, and some areas
have remained unstocked or poorly stocked for 50
years or longer. Periods of 10 to 30 years are more
common, but they still are not compatible with rota-
tions of 80 to 120 years. Low stumpage values have also
hindered intensive management. Improving stumpage
values and better understanding of natural and/or ar-
tificial regeneration allows forest managers to do the
cultural work necessary to increase timber production.

Establishment of Regeneration

Southwestern ponderosa pine forests can be main-
tained as productive forests under an even-aged
management system. A two-cut shelterwood method is
most appropriate for converting even-aged, old-growth
stands to managed even-aged stands (Schubert 1973).
Uneven-aged, old-growth stands require at least a
three-cut shelterwood that may incorporate features of
sanitation-salvage and improvement selection methods
for conversion to managed, even-aged stands. An
uneven-aged management system which includes in-
dividual tree selection and group selection cutting
methods is also appropriate for use in ponderosa pine
stands. They are not discussed in this paper because
suitable growth and yield prediction tools are not
available for managed, uneven-aged stands.

1

Natural regeneration of ponderosa pine will be slow
to establish and poorly distributed under any cutting
method if any of the following requirements are not met
(Schubert 1974):

1. A large supply of viable seed.

2. A well prepared seedbed.

3. A site free of competing vegetation.

4. A low population of seed-eating animals.

5. Sufficient soil moisture.

6. Protection from trampling and browsing and cer-
tain insects.

If cutover stands remain unstocked or poorly stocked
more than 5 years after final harvest, the manager
must take action under the regulations of the National
Forest Management Act of 1976 to artificially regen-
erate the areas. Schubert (1974) summarized guide-
lines for planting and direct seeding of southwestern
ponderosa pine.

Schubert (1974) recommends planting at least 680
trees per acre. This should provide a stocking of 340
trees per acre when average stand diameter reaches
5 inches d.b.h., which is GSL 80. However, if ponderosa
pine is to be managed at higher GSL's, a minimum of
1,000 to 1,200 trees per acre should be planted.

Need for Early Precommercial Thinning

Establishing a new stand is only the beginning. Trees
must have room to grow to reach merchantable size in
a reasonable amount of time. Where ponderosa pine
has regenerated well naturally in the Southwest,
reproduction is often overly dense — the 1919 seedling
crop is a notable example. At Taylor Woods, on the
Fort Valley Experimental Forest in Arizona, stands
with an average of 5,800 stems per acre reached an
average stand diameter of 2.6 inches in 43 years, and
more than one-third of a 120-year rotation has passed
without any usable wood production (Schubert 1971).
For acceptable growth rates, precommercial thinning
is needed to reduce stand density to 1,000 to 1,200
stems per acre before age 10 years.

When adequate numbers of well distributed seed-
lings become established within 5 years after the seed-
cut of a shelterwood method, the removal cut should be
made promptly to avoid suppression. In stands infested
with dwarf mistletoe, the longer the overwood remains
in place, the greater is the probability of transmitting
the parasite to the new stand.

Estimates of Growth
Under Intensive Management

Intensive management of southwestern ponderosa
pine forests provides many opportunities for increas-
ing usable wood production, but estimates of future
stand development under various management regimes
are needed.

Information available on the growth of ponderosa
pine from sapling stage to final harvest under even-
aged management with a shelterwood cut is provided
by field and computer simulation procedures devel-
oped by Myers (1971) and Myers et al. (1976) and re-
fined by Edminster (1978). The procedures were
developed from field data on past growth as related to
stand density, age, and site quality.

The modeling concept used in these programs holds
that the whole stand is the primary model unit,
characterized by average values. The equations upon
which the growth and yield simulations are based are
given in Myers et al. (1976). The programs project
stand development by consecutive, 10-year periods and
include relationships to project average stand
diameter, average dominant and codominant height,
and number of trees per acre. Average diameter at the
end of a projection period is a function of average
diameter at the beginning of the period, site index, and
basal area per acre. Periodic average dominant and
codominant height growth at managed stand densities
is a function of age and site index. Periodic mortality
is a function of average diameter and basal area per
acre. Adjustments are made to the growth and mortal-
ity functions to account for the effects of dwarf
mistletoe infestation. Stand volume equations are used
to compute total cubic feet per acre; factors are com-
puted to convert this to merchantable cubic feet and
board feet. Prediction equations are included to
estimate the effects of differing intensities of thinning
from above and below on average diameter, average
dominant and codominant height, trees retained per
acre, and average dwarf mistletoe rating (Hawksworth
1977).

Yield simulations discussed in the following para-
graphs were made to the same hypothetical initial
stand conditions for all growth parameters:

1. Average total age at first thinning is 30 years.

2. Average stand diameter is 4.5 inches d.b.h.4

3. Stand density is 1,000 trees per acre.

4. Site index is 50, 60, 70. 80. and 90 at base age
100 years (Meyer 1961).

5. Dwarf mistletoe rating is 0.

6. Projections were made for 50 years (stand age
80 years) and 90 years (stand age 120 years).

7. Thinnings from below were made every 20 and
30 years to GSL's of 40, 60, 80, 100, 120, 140,
and 160. Initial and subsequent entries were
made to the same GSL.

8. A two-cut shelterwood option was used. The seed
cut was made 20 years before final cut and re-
tained 50% of the subsequent GSL.

9. Minimum size for inclusion in board-foot volume
determination was 10 inches d.b.h. to a variable
top diameter. Stand volumes were determined
from tables prepared by Myers (1963).

10. All entries were made as scheduled, even though
all thinnings could be precommercial.

* Average stand diameter is the diameter of the tree of average
basal area; it is not the average of all the tree diameters.

2

Diameter Growth

Periodic mean annual diameter growth of southwest-
ern ponderosa pine is related to stand density and site
quality, but is affected little by the cutting cycles
tested. Cutting cycles do influence average stand
diameter, however, because thinning from below in-
creases average diameter at each entry. Actual basal
area in a stand with an average diameter of less than
10 inches d.b.h. continues to increase, because peri-
odic thinning does not reduce basal area to a fixed
(GSL) amount until an average stand diameter of 10
inches d.b.h. is reached. Consequently, the rate of
diameter growth for a given GSL is not constant over
time and is essentially a negative exponential function
of basal area per acre in the program. In contrast, peri-
odic diameter growth is a linear function of site index,
so that differences in diameter growth resulting from
site quality are constant throughout the range of GSL's
and rotations examined.

26 -

4 -

CO
CD

O

c

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c

05

22

20

18

16

14

tS 12

w

CD

cn

CO

»—

>
<

10

30

50

SI 90

30

50

70

Age (years)

90 100

120

Figure 1.— Estimated average stand diameter of southwestern
ponderosa pine in relation to age for different site classes at
GSL 100, with a 20-year thinning interval and 120-year rotation.

70 90
Age (years)

100

120

Figure 2.— Estimated average stand diameter of southwestern
ponderosa pine in relation to age and GSL on site index 70
lands with a 20-year thinning interval and 120-year rotation.

Growth rates and changes in diameter resulting from
thinning frequency were examined to determine
average size of trees relative to rotation age. For ex-
ample, at GSL 100 with a 20-year cutting cycle, trees
reach average stand diameters of 12.3 to 14.4 inches
d.b.h. after 80 years; and 16.4 to 20.7 inches d.b.h.
after 120 years for the range of sites tested (fig. 1). On
an average site (index 70), with a 20-year cutting cycle,
mean stand diameters reached 10 inches d.b.h. at 50 to
88 years of age for the range of GSL's 40 to 160 (fig. 2).

Height Growth

Periodic mean annual height growth of ponderosa
pine increases with site index and decreases with age,
but is unaffected by GSL's or cutting cycles. However,
because fewer and, therefore, taller trees are left after
each thinning from below, the mean height of the domi-
nant and codominant trees is increased slightly at each
entry. The increase is positively correlated with thin-
ning frequency and negatively correlated with GSL.

Basal Area Growth

Periodic mean annual basal area increment is
related to growing stock level, site quality, frequency
of thinning, and rotation age. Because actual basal
area continues to increase in a stand until average
stand diameter reaches 10 inches d.b.h. and thinning
reduces basal area to a fixed amount (GSL), the rate of
basal area growth for a given GSL is not constant over

3

Table 1 . — Estimated total cubic foot volume production per acre
of southwestern ponderosa pine in relation to growing stock
level, rotation age. cutting cycle, and site index

Table 2. — Estimated mean annual total cubic foot volume incre-
ment per acre of southwestern ponderosa pine in relation to
growing stock level, rotation age, cutting cycle, and site index

Rotation
age

Cutting
cycle

Growing stock level

Rotation
age

Cutting
cycle

Growing stock level

40

60

80

100 120

140

160

40

60

80 100 120

140

160

thousand cubic feet

cubic feet

years

years

Site index 50

Site index 50

9 17

9 A'i

Z.40

2 fift

2 88 ft 00

£- .KJkJ \J .\J\J

ft OP.

o. 1 o

ftn
ou

9n

9fi fi

ftn a

OU.4

ftft R ftfi n ft7 R

OO.J OU.U O / .J

ftft 9
oO . c.

ftQ 1

oy. i

1 00

I £U

ft OA

ft R7
J.Dl

4.08

4.39 4.60

4. 1 4

A R9

1 00

I ^1U

9fi ft

C- O . O

ftn fi

OU.U

ft4 n ftfi fi ftft ft

04.U OU.U OO.O

ftQ R
oy.o

AO 9
4U.Z

P.0

OU

ftn

ou

2.20

2.53

2.72

2.87 3.00

3.10

3.13

ftn

ou

ou

27.5

31.6

34.0 35.9 37.5

38.8

39.1

120

3.14

3.76

4.21

4.45 4.60

4.67

4.69

120

26.2

31.3

35.1 37.1 38.3

38.9

39.1

Site

indpx fiO

Site index 60

80

20

0 Rft

o Qft

3.27

3.54 3.74

ft ftfi
o.oo

ft Q4
O.o4

an

ou

20

fti fi

O I . u

ftfi fi

OU.U

AO Q AA 0 4fi 7

4U.3 44.L 4U./

4ft 0

40.^_

AO, 9
4y

1 20

ft rr
o.oo

A AA

5.02

5.44 5.82

fi 00

o.uu

fi OR
O.UO

1 00

I £.U

ftn r

JU. J

ft7 0

O f .U

41 ft 4R ft 4ft R

4 I .O 4J.O 40.J

Rn o

OU.U

Rn 7

OU. /

80

ftO

2.62

3.03

3.36

3.57 3.73

3.89

3.99

RO

ftO

32.8

37.9

42.0 44.6 46.6

48.6

49.9

120

3.76

4.58

5.23

5.64 5.94

6.12

6.30

120

31.3

38.2

43.6 47.0 49.5

51.0

52.5

Site

index 70

Site index 70

80

20

ft nn

ft R9

3.91

4.19 4.49

A 70

4 . / U

a ftn

4 . OU

80

20

ft7 R
o / .o

AA 0

44. U

4ft Q R9 4 Rfi 1

40.3 Oil. 4 OU. I

Rft 7
oo. /

fin o

OU.U

120

a ftn

R 9ft

6.05

6.66 7.20

7 Rfi

7 fift
( -DO

1 20

ftR ft

OJ.O

Ad 0

44 . U

Rn 4 rr r fin n

OU.4 OO.O UU.U

fift n

uo.u

fiA O
U4 . U

80

30

3.14

3.61

3.99

4.30 4.56

4.76

4.84

80

10

39.2

45.1

49.9 53.8 57.0

59.5

60.5

120

4.52

5.45

6.28

6.86 7.37

7.73

7.97

120

37.7

45.4

52.3 57.2 61.4

64.4

66.4

Site

index 80

Site index 80

80

20

Q AA

4. IO

4.63

4.98 5.26

R 4fi

J.4D

R fift
J.DJ

80

20

4ft n

40.U

R1 fi
O ! .U

R7 Q R9 9 RR ft
o / .0 \jeL.iL OO.O

fift 9

7n a

l U.4

120

4 94

fi 1ft

u . i o

7.18

7.97 8.62

q nn

J.Uu

Q 94

120

41 0

R1 R

RQ ft fifi 4 71ft
Oo.O OU.4 / I .O

7R n
/ o .U

77 n

( ( .u

80

30

3.70

4.21

4.67

5.11 5.41

5.56

5.67

80

30

46.2

52.6

58.4 63.9 67.6

69.5

70.9

120

5.32

6.50

7.44

8.18 8.76

9.16

9.28

120

44.3

54.2

62.0 68.2 73.0

76.3

77.3

Site

index 90

Site index 90

80

20

3.95

4.69

5.35

5.87 6.24

6.46

6.56

80

20

49.4

58.6

66.9 73.4 78.0

80.8

82.0

120

5.63

7.08

8.35

9.35 10.15

10.51

10.75

120

46.9

59.0

69.6 77.9 84.6

87.6

89.6

80

30

4.24

4.95

5.59

6.06 6.33

6.48

6.52

80

30

53.0

61.9

69.9 75.8 79.1

81.0

81.5

120

6.10

7.49

8.69

9.55 10.08

10.56

10.99

120

50.8

62.4

72.4 79.6 84.0

88.0

91.6

time. Periodic basal area increment increases as GSL
increases from 40 to 140, but the rate of increase
diminishes as stand density increases. At GSL's above
140, basal area increment declines on all sites.
Periodic mean basal area growth also increases as site
index increases. Moreover, the differences in basal
area growth between site classes become progressive-
ly greater as GSL increases. Periodic mean basal area
increment is greater with a 30-year cutting cycle than
with a 20-year entry at all rotation ages and GSL's
examined.

Total Cubic-Foot Volume Increment

Cubic-foot volume production is related to stand
density, site quality, rotation age, and frequency of
thinning (table 1). Although mean annual cubic volume
increment increases as GSL and site index increase,
the rate of increase diminishes as GSL increases, while
the differences in growth between site classes becomes
greater (fig. 3) (table 2). Cubic volume increment will
apparently continue to increase slightly at GSL's above

CO

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CD

IE I 1 1 I I 1 I J

40 60 80 100 120 140 160
Growing stock level

Figure 3.— Estimated mean annual total cubic-foot volume incre-
ment per acre of southwestern ponderosa pine in relation to
GSL and site quality for a 120-year rotation with a 20-year thin-
ning interval.

4

Table 3. — Estimated total board foot volume production per acre
of southwestern ponderosa pine in relation to growing stock
level, rotation age, cutting cycle, and site index (trees 10 inches
d.b.h. and larger to a variable top diameter)

Rotation Cutting Growing stock level

age cycle 40 60 80 100 120 140 160

Table 4. — Estimated mean annual board-foot volume increment
per acre of southwestern ponderosa pine in relation to grow-
ing stock level, rotation age. cutting cycle, and site index (trees
10 inches d.b.h. and larger to a variable top diameter)

Rotation Cutting Growing stock level

age cycle 40 60 80 100 120 140 160

years thousand board feet years — board feet -

Site index 50 Site index 50

80

20

3.36

3.68

4.00 4.40 4.40

4.16

3.68

80

20

42

46

50

55

55

52

46

120

8.40

9.84

10.80 11.28 11.28

10.56

9.60

120

70

82

90

94

94

88

80

80

30

3.28

3.60

3 84 4 08 3 92

3.60

3.04

80

30

41

45

48

51

49

45

38

120

8.40

9.84

10.56 10.92 10.80

9.96

8.64

120

70

82

88

91

90

83

72

Site index 60

Site index 60

80

20

4.48

5.04

5.44 5.76 6.00

6.00

5.76

80

20

56

63

68

72

75

75

72

120

10.92

12.72

14.16 15.24 15.60

15.36

15.00

120

91

106

118

127

130

128

125

80

30

4.40

4.80

5 20 5 44 5 68

5 52

5 20

80

30

55

60

65

68

71

69

65

120

10.68

12.72

13.80 14.52 14.76

14.64

14.40

120

89

106

115

121

123

122

120

Site index 70

Site index 70

80

20

5.60

6.24

6.72 7.20 7.60

8.00

8.16

80

20

70

78

84

90

95

100

102

120

13.08

15.72

17.64 18.96 20.04

20.64

21.00

120

109

131

147

158

167

172

175

80

30

5.44

6.16

6.64 7.04 7.28

7.28

7.20

80

30

68

77

83

88

91

91

90

120

13.56

15.96

17.88 19.08 19.92

20.16

20.40

120

113

133

149

159

166

168

170

Site index 80

Site index 80

80

20

6.80

7.68

8.48 9.04 9.44

9.92

10.24

80

20

85

96

106

113

118

124

128

120

15.96

18.96

21.48 23.40 25.20

26.64

27.36

120

133

158

179

195

210

222

228

80

30

7.04

7.76

8.40 8.96 9.20

9.36

9.44

80

30

88

97

105

112

115

117

118

120

17.04

19.68

22.08 24.24 25.44

26.04

26.40

120

142

164

184

202

212

217

220

Site index 90

Site index 90

80

20

8.08

9.20

10.24 11.04 11.84

12.32

12.64

80

20

101

115

128

138

148

154

158

120

18.84

23.16

26.64 29.40 31.44

32.76

33.60

120

157

193

222

245

262

273

280

80

30

8.56

9.28

10.16 10.88 11.20

11.52

11.76

80

30

107

116

127

136

140

144

147

120

20.64

23.64

26.76 29.52 30.96

31.92

32.40

120

172

197

223

246

258

266

270

160 on sites 70 and greater, but levels off or declines on
site indexes less than 70 at GSL's greater than 160.
Cubic foot growth is generally unrelated to length of
rotation or cutting cycle at all GSL's tested when site
index is less than 70. On site index 70 and greater
lands, cubic-volume growth is greater on 120-year rota-
tion at GSL's greater than 60, but there are no practi-
cal differences between a 20- and 30-year cutting cycle
(table 2).

Board-Foot Volume Increment

Board-foot volume production is related to all stand
parameters evaluated (table 3). Mean annual sawtim-
ber volume growth increases as stand density in-
creases throughout the range of GSL's on site index 80
and 90 lands, but generally levels off on site index 70
lands at GSL 140, and declines on site index 50 and 60
lands at GSL's 100 and 120, respectively (fig. 4) (table
4).

Board-foot volume growth increases with site qual-
ity, and the differences in growth between site classes

g 50-
c

03
<V

^ I I 1 1 1 ' I

40 60 80 100 120 140 160
Growing stock level

Figure 4. — Estimated mean annual board foot volume increment
per acre of southwestern ponderosa pine in relation to GSL
and site quality for a 120-year rotation with a 20-year thinning
interval.

5

becomes greater as GSL increases. Throughout the
range of GSL's tested, average annual board-foot incre-
ment per acre is always greater for all site classes on a
120-year rotation than on 80-year rotation (fig. 5).
There are no practical differences in board-foot
volume growth between 20- and 30-year cutting cycles
for the range of site indexes and GSL's tested (table 4).

Maximizing Board-Foot Volume Yields

What yields can be expected with intensive manage-
ment of southwestern ponderosa pine to maximize
timber production? If the objective is to integrate
timber production with other resources uses, what are
the timber tradeoffs? How can these objectives be at-
tained with the fewest precommercial thinnings?

The largest volume production per acre (33,600 fbm)
is attained on site index 90 lands, at GSL 160, on a
120-year rotation, with a 20-year cutting cycle (table 3).
These stands will contain about 72 trees per acre with
an average d.b.h. of nearly 17 inches at rotation age
(table 5).

CD
CD

1 200

o

o 150

i_

cd
cl

| 100

CD

E

CD
O

E 50
as

C
C

CO

c

CO
CD

120 year rotation

80 year rotation

1 1

40 60 80 100 120
Growing stock level

140

160

Figure 5.— Estimated mean annual board-foot volume increment
per acre of southwestern ponderosa pine on site index 70 lands
with a 20-year thinning interval in relation to GSL and rotation
age.

Table 5. — Estimated average diameter (inches) and number of trees per acre of southwestern
ponderosa pine at final harvest in relation to growing stock level, rotation age, cutting cycle,
and site index

Growing stock level

40

60

80

100

120

140

160

Rotation Cutting No. of No. of No. of No. of No. of No. of No. of

age cycle trees Diameter trees Diameter trees Diameter trees Diameter trees Diameter trees Diameter trees Diameter

80

20

22

16.5

39

14.8

64

13.3

120

9

23.7

18

20.6

29

18.5

80

30

24

15.9

45

14.1

72

12.7

120

10

22.9

20

19.6

33

17.6

80

20

21

17.0

38

15.3

60

13.9

120

9

24.4

17

21.5

27

19.4

80

30

23

16.4

42

14.6

67

13.2

120

10

23.5

18

20.7

30

18.5

80

20

20

17.5

36

15.8

55

14.5

120

8

25.4

15

22.5

25

20.2

80

30

21

17.1

39

15.2

63

13.8

120

9

24.8

17

21.7

28

19.2

80

20

18

18.3

34

16.4

53

15.0

120

7

26.9

14

23.5

22

21.4

80

30

20

17.7

37

15.8

61

14.1

120

8

25.8

15

22.7

25

20.4

80

20

17

18.9

32

16.9

50

15.5

120

7

27.6

13

24.4

21

22.2

80

30

20

17.9

36

16.2

55

14.9

120

8

26.6

14

23.7

22

21.6

Site index 50

92

12.3

119

11.5

153

10.8

189

10.2

46

16.4

63

15.3

89

13.9

123

12.6

101

11.7

131

11.0

165

10.4

202

9.8

50

15.9

67

14.9

94

13.6

128

12.4

Site

index 60

83

13.0

111

12.1

145

11.3

180

10.7

40

17.6

56

16.3

77

15.0

105

13.7

92

12.4

124

11.5

155

10.9

192

10.3

45

16.7

62

15.6

82

14.6

109

13.5

Site

index 70

80

13.3

106

12.6

132

12.0

168

11.2

37

18.5

52

17.1

66

16.2

90

14.9

90

12.7

120

11.9

147

11.4

183

10.7

41

17.6

56

16.5

74

15.5

99

14.3

Site

index 80

77

13.7

100

13.1

130

12.3

154

11.9

34

19.4

46

18.2

62

16.9

79

15.9

85

13.2

110

12.6

142

11.8

174

11.2

38

18.5

50

17.5

68

16.2

87

15.3

Site

index 90

71

14.4

94

13.6

119

13.0

148

12.3

30

20.7

41

19.2

55

18.0

72

16.8

79

13.8

106

13.0

132

12.4

165

11.7

34

19.5

46

18.3

60

17.3

78

16.2

6

Volume production substantially declines when GSL
is reduced below 160 on site index 70, 80, and 90 lands.
The decline is greater with each successive reduction
in stand density. Maximum volume production is at
GSL's 100 and 120, on site index 50 and 60 lands, re-
spectively, with a 20-year cutting cycle (table 3) (fig. 6).

Table 3 also shows the amount volume given up as
GSL is reduced from the level of maximum production
to GSL 40 for all combinations of stand parameters ex-
amined. Moreover, it shows that more volume can be
produced over the same time span with 120-year rota-
tions than with 80-year rotations. For example, on site
index 90 lands, maximum board-foot volume produc-
tion per acre from two 120-year rotations, or 240
years, would be 67,200 fbm, compared with 37,900 fbm
on three 80-year rotations, also 240 years.

Whether the board-foot volume production poten-
tials can be achieved depends largely on how much
money can be invested in thinning. It is assumed that
once a stand reaches a minimum merchantable size of
10 inches average d.b.h., market conditions permit in-
termediate thinnings to be made as scheduled. If
economic constraints limit managers to only one pre-
commercial thinning in the life of the stand, their
options are severely restricted. For example, on site in-
dex 50 to 60 lands, stand density must be reduced to
GSL 40 and the cutting cycle increased to 30 years
(table 6). On site index 70 and 80 lands, a GSL of 60 can
be maintained with a 30-year cutting cycle, and on site
index 90 lands, a GSL of 100 can be maintained.

Table 6. — Number of precommercial thinnings of southwestern
ponderosa pine in relation to growing stock level, cutting cycle,
and site index

Growing stock level

cycle

index

40

60

80

100

120

140

160

\/c> pi re
y era / o

o

o

o

A

A

A

60

2

2

2

3

3

4

4

70

2

2

2

3

3

3

4

80

2

2

2

2

3

3

4

90

1

1

2

2

2

3

3

30

50

1

2

2

2

3

3

3

60

1

2

2

2

2

3

3

70

1

1

2

2

2

2

3

80

1

1

2

2

2

2

2

90

1

1

1

1

2

2

2

Thinnings to a constant GSL have been assumed up
to now. However, if only one precommercial thinning is
possible, managers can increase their flexibility by
changing GSL's with successive reentries. For exam-
ple, on site index 70 and 80 lands with a 30-year cutting
cycle, stand density is initially reduced to GSL 60.
At the time of the second thinning, GSL is increased to
80, and increased to GSL 100 with the third thinning.
Volume production will be less than maximum, but
reasonably close to the volume available from a stand
maintained at a constant GSL 100. Attempts to raise
the GSL to 100 at the time of the second entry into the

Figure 6.— Second growth southwestern ponderosa pine on site index 60 lands (Meyer 1961)
thinned to GSL 120, Taylor Woods near Flagstaff, Ariz. Stand was about 45 years old when
thinned in 1962.

7

stand would result in a second precommercial thin-
ning. By following this procedure, managers can in-
crease GSL on site index 50 and 60 lands from 40 to 80.

The manager has another option if only one precom-
mercial thinning is possible. The initial thinning can be
made on schedule and the second entry delayed until
the stand reaches minimum merchantable size. This
will increase the second thinning interval to 40 years
or more, increase the length of the rotation, and result
in less than maximum volume production.

Where economic conditions permit investment of
funds in two precommercial thinnings, the manager
has the opportunity to maximize timber production on
site index 50 to 90 lands, with 30-year thinning
schedule.

Tradeoffs to Increase Values of Other Resources

Understory vegetation in southwestern ponderosa
pine is an important forage source for livestock and big
game animals, but as overstory density increases, the
productivity of the understory decreases. This inverse
relationship is generally shown to be curvilinear (Pear-
son 1964, Jameson 1969, Clary 1969). Generally, herb-
age yields on productive sites can vary from 50 to 75
pounds per acre under dense timber stands (basal area
per acre of 140 square feet) to 1,000 to 1,200 pounds
per acre on moderately grazed open grasslands (Clary
1975) (fig. 7). High herbage production on these sites
can be expected in clearcut openings until new tree
regeneration becomes limiting. Actual changes in herb-
age production will vary considerably, however,
depending upon habitat type, successional stage, and
past grazing history, as well as overstory density.

In partially cut or thinned stands, herbage produc-
tion generally is substantially greater than under uncut
stands only when stand density is reduced to 70 square
feet or less of basal area per acre, and differences in
herbage production become progressively greater as

^1,000

600

20 40 60 80 100 120 140
Ponderosa pine basal area (square feet per acre)

Figure 7.— Relation of herbage production to basal area of
southwestern ponderosa pine on the Wild Bill range north of
Flagstaff, Ariz. (Clary 1975).

!k 500

CD
CL

(/)

T3
C
3

o

CL

400 -

o 300
o

2 200

Q-
CD

cn

.2 100-

CD

X

Thinned area
Unthinned area

J L

0 20 40 60 80 100 120 140 160
Basal area (square feet per acre)

Figure 8.— Relationship between herbage production and basal
area of southwest ponderosa pine on thinned and thinned
areas, Beaver Creek Watershed near Flagstaff, Ariz. (Clary and
Ffolliott 1966).

stand density in the thinned stands is reduced to 20
square feet of basal area per acre (Clary and Ffolliott
1966, Pearson 1967) (fig. 8). Moreover, herbage pro-
duction under partially cut or thinned stands usually
peaks about 5 years after treatment and will exceed
production in uncut stands for only 10 to 15 years
(Reynolds 1962).

Although no methods or data are available to quan-
tify changes in understory herbage production under
southwestern ponderosa pine for the range of GSL's,
site indexes, rotation ages, and cutting cycles ex-
amined here, some general conclusions can be drawn.
To increase average herbage production to even mod-
erate levels (350 to 400 pounds per acre), the manager
must be willing to reduce basal area stocking per acre
to GSL 60 or less (fig. 9). To maintain forage production,
the manager must be able to make additional cuts in
the stand at intervals of at least every 20 years.

Southwestern ponderosa pine forests yield less
water than subalpine and mixed conifer forests (Rich
and Thompson 1974, Leaf 1975). The proportion of
water available for streamflow (3 to 5 inches) to
precipitation (20 to 25 inches) is low because of high
evapotranspiration demand from vegetation during a
long, warm growing season, and the variability of
precipitation (Hibbert 1979). Water yield is derived
mostly from snowmelt, and snowfall regimes are highly
variable in the Southwest. Streamflow is greatest when
winter snowfall is sufficient to maintain a continuous
snowpack and soil moisture is recharged during the
spring melt. Major runoff also occurs when rain falls
on snow. Regimes that produce intermittent snowpack —
snowfall followed by dry periods that melt the snow, or
years of light snowfall — contribute little to streamflow.
Weather from snowmelt to July is usually dry, and late
summer rains only partially replenish losses from
evapotranspiration.

The potential for increasing streamflow in ponder-
osa pine forests is also low. The largest increases (1 to

8

2 inches) occur when timber is harvested by clearcut-
ting (Brown et al. 1974). The most effective pattern of
timber harvest for increased water yields in ponderosa
pine forests when precipitation is largely snow and
redistribution by wind is significant is to clearcut
about 30% to 40% of a drainage in small, irregular-
shaped patches about five times tree height across, in-
terspersed with uncut patches of about five to eight
tree heights across (Gary 1975). If snowfall is not
significant or redistribution of snow by wind is not a
factor, larger clearcut openings are more effective in
increasing streamflow. In this case, the increase in
streamflow is largely a result of the reduction in con-
sumptive use by vegetation. With harvest cutting
methods that leave a residual stand or thinning, the in-
crease in water yield will be less than with clearcut-
ting and generally in an inverse proportion to the
amount of basal area left.

Based on information available from research,
observations, and experience, it is clear that stand
density must be reduced to and maintained at low
stocking levels (GSL's of 60 or less) to benefit forage
and water resources. For example, on site index 80
lands, at GSL 80, with a 120-year rotation, and a
20-year thinning schedule, 5,880 fewer fbm per acre
will be produced than at GSL 160. If the GSL is reduced
to 40, the loss in volume production per acre is 11,400

fbm. Foreground landscape esthetics and developed
and dispersed recreation opportunities are generally
improved at moderate (GSL 80 to 100) stocking levels.
Considerable timber volume is given up, however, at
both low to moderate stocking levels.

Middleground and background landscapes require
combinations of cleared openings, high and low stock-
ing levels, and uncut timber to provide the variety and
contrast that is visually pleasing. Some wildlife species
require openings, others open-standing timber, while
the habitat still of others is devastated by any kind of
timber cutting. But until the habitat requirements of
specific wildlife species are better known, the benefits
and losses to wildlife cannot be determined for stand
parameters examined here.

Management Caution

This simulation program estimates growth responses
to different stand parameters that appear reasonable
and consistent within the limits of current knowledge,
but no southwestern ponderosa pine stand has been
under management for a long time, and simulation
extends beyond the limits of the available data base.
Comparisons of estimates with actual values from plots
established to provide growth information will be need-
ed to verify simulated responses.

Figure 9.— Second-growth southwestern ponderosa pine on site index 60 lands (Meyer 1961)
thinned to GSL 30, Taylor Woods near Flagstaff, Ariz. Stand was about 45 years old when
thinned in 1962.

s

Literature Cited

Brown, Harry E., Malchus B. Baker, James R. Rogers,
Warren P. Clary, J. L. Kovner, Frederic R. Larson,
Charles C. Avery, and Ralph E. Campbell. 1972. Op-
portunities for increasing water yields and other
multiple-use values of ponderosa pine forest lands.
USDA Forest Service Research Paper RM-129, 36 p.
Rocky Mountain Forest and Range Experiment Sta-
tion, Fort Collins, Colo.

Choate, Grover A. 1965. Forests in Utah. USDA Forest
Service Resource Bulletin INT-4, 61 p. Intermountain
Forest and Range Experiment Station, Ogden, Utah.

Choate, Grover A. 1966. New Mexico's forest resource.
USDA Forest Service Resource Bulletin INT-5, 60 p.
Intermountain Forest and Range Experiment Station,
Ogden, Utah.

Clary, Warren P. 1969. Increasing sampling precision
for some herbage variables through knowledge of the
timber overstory. Journal of Range Management
22:200-201.

Clary, Warren P. 1975. Range management and its eco-
logical basis in the ponderosa pine type of Arizona:
The status of our knowledge. USDA Forest Service
Research Paper RM-158, 35 p. Rocky Mountain
Forest and Range Experiment Station, Fort Collins,
Colo.

Clary, Warren P., and Peter F. Ffolliott. 1966. Dif-
ferences in herbage-timber relationships between
thinned and unthinned ponderosa pine stands. USDA
Forest Service Research Note RM-74, 4 p. Rocky
Mountain Forest and Range Experiment Station, Fort
Collins, Colo.

Edminster, Carleton B. 1978. RMYLD: Computation of
yield tables for even-aged and two-storied stands.
USDA Forest Service Research Paper RM-199, 26 p.
Rocky Mountain Forest and Range Experiment Sta-
tion, Fort Collins, Colo.

Gary, Howard L. 1975. Watershed management prob-
lems and opportunities for the Front Range
ponderosa pine zone: The status of our knowledge.
USDA Forest Service Research Paper RM-139, 32 p.
Rocky Mountain Forest and Range Experiment Sta-
tion, Fort Collins, Colo.

Hawksworth, Frank G. 1977. The 6-class dwarf mistle-
toe rating system. USDA Forest Service General
Technical Report RM-48, 7 p. Rocky Mountain Forest
and Range Experiment Station, Fort Collins, Colo.

Hibbert, Alden R. 1979. Managing vegetation to in-
crease flow in the Colorado River basin. USDA
Forest Service General Technical Report RM-66, 27
p. Rocky Mountain Forest and Range Experiment Sta-
tion, Fort Collins, Colo.

Jameson, Donald A. 1967. The relationship of tree
overstory and herbaceous understory vegetation.
Journal of Range Management 20:247-249.

Leaf, Charles F. 1975. Watershed management in the
Rocky Mountain subalpine zone: The status of our
knowledge. USDA Forest Service Research Paper
RM-107, 23 p. Rocky Mountain Forest and Range Ex-
periment Station, Fort Collins, Colo.

Meyer, Walter H. 1961. Yield of even-aged stands of
ponderosa pine. U.S. Department of Agriculture
Technical Bulletin 630, 59 p. Washington, D.C.

Miller, Robert L., and Grover A. Choate. 1964. The
forest resource of Colorado. USDA Forest Service
Resource Bulletin INT-3, 55 p. Intermountain Forest
and Range Experiment Station, Ogden, Utah.

Myers, Clifford A. 1963. Volume, taper, and related
tables for southwestern ponderosa pine. USDA
Forest Service Research Paper RM-2, 24 p. (Revised
1972). Rocky Mountain Forest and Range Experiment
Station, Fort Collins, Colo.

Myers, Clifford A. 1971. Field and computer pro-
cedures for managed-stand yield tables. USDA
Forest Service Research Paper RM-79, 24 p. Rocky
Mountain Forest and Range Experiment Station, Fort
Collins, Colo.

Myers, Clifford A., Carleton B. Edminster, and Frank G.
Hawksworth. 1976. SWYLD2: Yield tables for even-
aged and two-storied stands of southwestern ponder-
osa pine, including effects of dwarf mistletoe. USDA
Forest Service Research Paper RM-163, 25 p. Rocky
Mountain Forest and Range Experiment Station, Fort
Collins, Colo.

Pearson, G. A. 1950. Management of ponderosa pine in
the Southwest. U.S. Department of Agriculture
Monograph 6, 218 p. Washington, D.C.

Pearson, Henry A. 1964. Studies of forest digestibility
under ponderosa pine stands. Society of American
Foresters Proceedings 1964:71-73.

Pearson, Henry A. 1967. Forage and beef production
from ponderosa pine range in the Southwest, p. 50. In
Abstracts of Papers, 20th Annual Meeting, Ameri-
can Society of Range Management, [Seattle, Wash-
ington February 14-17, 1967]. Journal of Range
Management, 71 p.

Reynolds, Hudson G. 1962. Effect of logging on
understory vegetation and deer use in a ponderosa
pine forest of Arizona. U.S. Department of Agri-
culture, Forest Service, Rocky Mountain Forest and
Range Experiment Station, Research Note 80, 7 p.
Fort Collins, Colorado.

Rich, Lowell R., and J. R. Thompson. 1974. Watershed
management in Arizona's mixed conifer forests: The
status of our knowledge. USDA Forest Service Re-
search Paper RM-130, 15 p. Rocky Mountain Forest
and Range Experiment Station, Fort Collins, Colo.

Schubert, Gilbert H. 1971. Growth response of even-
aged ponderosa pine related to stand density levels.
Journal of Forestry 69:857-860.

Schubert, Gilbert H. 1973. Southwestern ponderosa
pine. p. 45-46. In Silvicultural systems for the major
forest types of the United States. U.S. Department of
Agriculture, Agricultural Handbook 445, 114 p.

Schubert, Gilbert H. 1974. Silviculture of southwestern
ponderosa pine: The status of our knowledge. USDA
Forest Service Research Paper RM-123, 71 p. Rocky
Mountain Forest and Range Experiment Station, Fort
Collins, Colo.

Spencer, John S., Jr. 1966. Arizona's forests. USDA
Forest Service Resource Bulletin INT-6, 56 p. Inter-
mountain Forest and Range Experiment Station,
Ogden, Utah.

10

Appendix

Table A-1.— Basal areas (square feet per acre) after intermediate cutting in relation to average
stand diameter (inches) and growing stock level

Average stand Growing stock level

d.b.h. after
cutting

40

50

60

70

80

90

100

110

120

140

160

2

6.0

7.5

9.1

10.6

12.1

13.6

15.1

16.7

18.2

21.2

24.2

3

11.8

14.8

17.7

20.6

23.6

26.6

29.5

32.4

35.4

41.5

47.4

4

17.6

22.0

26.4

30.8

35.2

39.6

44.0

48.4

52.8

61.6

70.4

5

23.4

29.2

35.0

40.9

46.7

52.5

58.4

64.2

70.0

81.9

93.6

6

28.3

35.4

42.4

49.5

56.6

63.7

70.8

77.8

84.9

99.0

113.2

7

32.7

40.9

49.1

57.3

65.5

73.7

81.9

90.1

98.2

114.4

130.8

8

36.2

45.3

54.4

63.4

72.5

81.6

90.6

99.7

108.8

126.9

145.0

9

38.8

48.4

58.1

67.8

77.5

87.2

96.9

106.6

116.2

135.6

155.0

10 +

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

120.0

140.0

160.0

Table A-2.— Number of trees per acre in relation to average stand diameter (inches) and grow-
ing stock level

Average stand Growing stock levels

d.b.h. after
thinning

40

50

60

70

80

90

100

110

120

140

160

2

277

345

418

488

553

626

692

767

836

968

1,107

3

241

301

360

420

481

542

601

660

721

843

964

4

202

252

302

353

403

454

504

554

605

707

808

5

172

214

257

300

342

384

428

471

513

601

687

6

144

180

216

252

288

324

361

396

432

505

577

7

122

153

184

214

245

276

306

337

367

428

489

8

104

130

156

182

208

234

260

286

312

364

415

9

88

110

132

154

175

197

219

241

263

307

351

10

73

92

110

128

147

165

183

202

220

257

293

Table A-3.— Average distance (feet) between residual trees in relation to average stand

diameter (inches) and growing stock level

Average stand Growing stock level

d.b.h. after
thinning

40

50

60

70

80

90

100

110

120

140

160

2

12.5

11.1

10.2

9.4

8.8

8.3

7.8

7.5

7.2

6.7

6.3

3

13.4

12.0

11.0

10.2

9.5

9.0

8.5

8.1

7.8

7.2

6.7

4

14.7

13.2

12.0

11.1

10.4

9.8

9.3

8.9

8.5

7.9

7.3

5

15.9

14.4

13.0

12.0

11.3

10.6

10.1

9.6

9.2

8.5

8.0

6

17.4

15.6

14.4

13.2

12.3

11.6

11.0

10.5

10.0

9.3

8.7

7

18.9

16.9

15.4

14.3

13.3

12.6

11.9

11.4

10.9

10.1

9.4

8

20.5

18.3

16.7

15.5

14.5

13.6

13.0

12.3

11.8

10.9

10.2

9

22.3

20.1

18.2

16.8

15.8

14.9

14.1

13.4

12.9

11.9

11.1

10

24.4

21.8

20.1

18.4

17.2

16.2

15.4

14.7

14.1

13.0

12.2

11

A.

T

I ■ i

CO s c
3 -d •
S 3 >> °

CO a K cn

+3 aod

i "O CO

3 c (h

0 3 3

M CD ^

CO DO

m j 03

CD *

-d 3 ft

c g

CD

•d c

E _2

TO co
W

-a

PQ S,

d
o

CD
Ch
CO
CJ

r-t O

^ CJ

ID tn

CN o
CM Ch

Pd .2

ft CO
CO h-h
d

CD

3
O

CD O

TO CD

d c

rv cc

^ co

h-h O

ch u

CD CD

C d

Oh O

. ft

Ch

CD o
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Rocky
Mountains

Southwest

Great
Plains

U.S. Department of Agriculture
Forest Service

Rocky Mountain Forest and
Range Experiment Station

The Rocky Mountain Station is one of eight
regional experiment stations, plus the Forest
Products Laboratory and the Washington Office
Staff, that make up the Forest Service research
organization.

RESEARCH FOCUS

Research programs at the Rocky Mountain
Station are coordinated with area universities and
with other institutions. Many studies are
conducted on a cooperative basis to accelerate
solutions to problems involving range, water,
wildlife and fish habitat, human and community
development, timber, recreation, protection, and
multiresource evaluation.

RESEARCH LOCATIONS

Research Work Units of the Rocky Mountain
Station are operated in cooperation with
universities in the following cities:

Albuquerque, New Mexico

Bottineau, North Dakota

Flagstaff, Arizona

Fort Collins, Colorado*

Laramie, Wyoming

Lincoln, Nebraska

Lubbock, Texas

Rapid City, South Dakota

Tempe, Arizona

* Station Headquarters: 240 W. Prospect St., Fort Collins, CO 80526