Interim Site-index Curves

for Longleaf Pine Plantations1

William D. Boyer

SUMMARY

No single set of site-index curves can be

uniformly applied to young longleaf pine plantations without a sacrifice in reliability. A recent

study using plantation remeasurement data

indicated that planting-site condition (old fields

and mechanically prepared or unprepared cutover forest sites) has a major impact on early

plantation height growth. Stand density (surviving trees per acre) also affected form of

height-over-age curves. On both prepared and

unprepared forest sites, site quality affected

curve form, so polymorphic site-index curves

resulted for these two conditions. Longleaf pine

plantation site-index curves for these three

planting-site conditions were derived from 660

plots remeasured through ages 15 to 17 or 20

to 22 years. Modification of the curves may be

necessary as more data become available.

INTRODUCTION

The early growth of southern pine plantations,

including those of longleaf pine (Pinus palustris

Mill.), is so variable that any single set of siteindex curves is of limited value. Site index

at age 25 (SI,,) usually the mean height of

dominant-codominant trees at that age, is normally

considered the principal indicator of plantation

productivity. Site-index estimates derived from

current height of young plantations are often

used to evaluate the effectiveness of cultural

treatments. If treatments affect the form of

height-over-age curves, then estimates of site

index from one set of curves will be unreliable.

Economic evaluation of treatment effects on

future volume yields would also be untrustworthy.

Successive remeasurement3 of young plantations, particularly from 10 to 20 years of age,

often result in dramatic changes in estimated

SI,,. Such shifts mean that the site-index curves

being used are wrong for that plantation. Even

small errors in site-index estimates can cause

large miscalculations of expected volume growth

of a plantation. A change from 60 to 55 ft in SI,,

reduced by 25 percent the projected cubic-foot

volume yield of a slash pine plantation at age

20 (Bennett et al. 1959).

Past studies suggest that height-over-age curves

for southern pines established on old fields may

differ not only between plantation and natural

stand but also from similar stands on cutover

forest sites (Chapman 1938, Allen 1955, Bailey

et al. 1973). Form of height-over-age curves

may also be affected by other stand and site

variables, particularly stand density (Bennett

1975, McClurkin 1976) and site quality (Beck

and Trousdell 1973, Graney and Burkhart 1973).

Using remeasurement data provided mainly

by the Southwide Pine Seed Source Study

(SPSSS), I examined factors associated with

‘For the metric equivalent for measurements In this paper,

multiply number of feet by 0 3048 to obtain meters

Southern Forest Experiment Station/T-10210 U.S. Postal Services Bldg., 701 Loyola Avenue, New Orleans, La. 70113

Forest Service, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Louisiana, Mississippi, Eastern Oklahoma, Tennessee, Eastern Texas.
differences in the form of height-over-age

curves of longleaf pine plantations. Analyses

identified several factors related to differences

in height-growth patterns. Some of these relationships were used for the sets of longleaf

plantation site-index curves presented here

(Fig. l-3). These are more closely associated

with specific site and stand conditions than are

any existing site-index curves. Refinements can

be made with later remeasurement data.

DATA BASE

Of 660 plots in this study, 637 were from the

SPSSS and 23 from two studies in north Florida.

All plots were scheduled for measurement at

plantation ages 3 and 5 and at 5-year intervals

thereafter, although this schedule was not always

kept. Some plots (136) were last measured at

age 20, 21, or 22:The rest were last measured

at age 15 except for 67 plots last measured at

age 16 or 17. In all, there are 2737 heightover-age observations.

SPSSS Series 1 and 2 (planted during winter

1952-53) and Series 4, 5, and 6 (planted during

winter 1956-57) were all represented in this

study. The 34 plantings were located in all

coastal states from Texas to North Carolina. Two

of the 34 plantings were replicates of 2 others

and were combined. So pines were planted

in 32 locations. Wells and Wakeley (1970) detail

the longleaf phase of SPSSS.

At each examination, number of surviving

trees and height of each survivor were recorded

for individual plots. The mean height of the

tallest half of surviving trees on each plot

was determined and represents the dominantcodominant fraction of the stand.

ANALYSES

All plantations were classified into three groups

according to planting-site condition: old fields

(263 plots and 1172 observations), mechanically

prepared cutover forest sites (116 plots and 488

observations) and unprepared cutover forest

sites (261 plots and 1077 observations). All

observations combined wnre given a stepwise

regression analysis of the form: Log,,Height=

b,+b, (Age)“+b, (Age)‘+b, (Age)‘*+b, (Ager3

+b, (Age)d. The analysis determined which

independent variables would give the best

single-variable regression.

When fitted to the height-over-age observations

for each individual plot, the best single-variable

regression for all observations combined gave

660 regression equations. Later analyses explored the relationship of the slope coefficient

for each plot, as a dependent variable, to recorded

site and stand variables. These were mainly

stand density (surviving trees per acre at age

lo), site quality (height of tallest half of trees

per plot at age 15) and the three planting-site

conditions (table 1).

Table 1. -Distribution of plots among stand density’ and

site quality2 classes

Stand density

(trees/acre)

Site quality (height in feet)

10 20 30 40 50

Old Fields

Total

200 0 0 17 2 0 19

400 0 0 15 14 2 31

600 0 2 14 33 1 50

800 0 1 24 56 5 86

1000 0 1 20 59 8 88

1200 0 0 0 7 2 9

Prepared Sites

200 1 11 6 1 0 19

400 9 9 7 2 0 27

600 5 21 3 2 0 31

800 4 13 6 4 0 29

1000 0 4 5 0 0 9

1200 0 1 0 0 0 1

Unprepared Sites

200 2 10 8 4 0 24

400 2 10 26 11 1 5 0

600 3 8 38 29 0 76

800 3 9 41 27 2 82

1000 0 4 11 10 1 26

1200 0 0 0 1 0 1

Totals 2 9 104 243 262 2 2 660

Surviving trees per acre at age 10.

2Mean height of tallest half of trees at age 15.

William D. Boyer is Principal Silviculturist at the George W. Andrews Forestry Sciences Laboratory, Auburn, Alabama,

maintained by the Southern Forest Experiment Station, Forest Service-USDA, in cooperation with Auburn University.

Thanks are due to 0.0. Wells and Southern Forest Tree Improvement Committee for providing seed-source data.

2
Coefficients for all 136 plots remeasured through

age 20-22 were compared with coefficients

derived from the same plots for observations

through age 15 only. Values of slope coefficients

from plots remeasured through age 20-22

differed from age-15 values by an average of

only 0.7 percent. Apparently plantation heightgrowth patterns are well established by age

15. So all slope coefficients were pooled for

analyses without regard to plantation age at

last measurement.

RESULTS

The best single-variable regression for all

2737 height-over-age observations was: Log,,HT

=1.8844-6.1764 (Age)‘. The coefficient of

determination (r*) was 0.8484: standard error of

the slope coefficient (b,) was 0.0499. The only

other variable contributing significantly (.05

level) to the regression was (Age)-4, which, when

included in the equation, resulted in an I?*

of 0.8497.

The model Log,, kT=b,+b, (Age)” was fitted

to each individual plot: 520 (79 percent) of the

resulting equations had r* values of 0.99 or

better. Slope coefficients (b,), with negative sign

omitted, became the dependent variable in

analyses of how planting-site condition, stand

density, and site quality affect form of early

plantation height growth.

Planting-to-planting and plot-to-plot variation

in slope coefficients was high. Classification

of the 32 SPSSS plantation locations into the

three planting-site conditions accounted for

70 percent of the variation, among plantings,

in average slope coefficient. The remaining

variation was probably caused by factors such as

two different years of plantation establishment,

varying sets of seed sources (Series), and geographic location with its associated climatic and

soil-site conditions.

Planting-site condition, stand density, and site

quality accounted for 48 percent of the plotto-plot variation in growth-curve coefficients.

Stand density significantly affected coefficients

in all three planting-site conditions. Site quality,

however, affected curve coefficients only on prepared and unprepared forest sites. Regressions

for each planting-site condition show how stand

density and site quality affect coefficients. The

predicted coefficient is Y; (TlO) is trees per

acre at age 10, and (H15) is mean height of

tallest half of trees per plot at age 15.

Old fields: Y =5.831 O-0.000732 (Tl 0)

Prepared sites: Y=7.7162-0.001057 (TlO)

-0.027189 (H15)

Unprepared sites: Y=8.2746-0.000976 (TlO)

-0.021189 (H15) .

Differences among planting-site conditions

are apparently related to the type and intensity

of competition from ground cover on the site.

Overtopping hardwoods and brush were eliminated when plantations were established.

If all planting-site conditions have the same

site index at age 25, height of old-field plantings

and prepared forest sites will exceed the height

of plantings on unprepared sites. The greatest

difference will be reached at plantation age

8-10 years. For example, with SI,, of 70 ft and a

density of 800 trees per acre, old-field plantings

at age 9 will average 5% ft taller than similar

plantings on unprepared sites. Prepared-site

plantings will average 4 ft taller. On good sites,

little difference exists between curves for old

field and prepared forest sites, but differences

in these two planting-site conditions increase as

site quality declines.

Such differences in growth-curve form will

greatly affect site-index predictions from one set

of curves. Consider, for example, a 15-year-old

plantation on an old field with 800 trees per

acre and a height of 40 ft. Longleaf pine siteindex curves from U.S. Department of Agriculture, Forest Service (1976) predict a SI,,

of 69 ft. This value is much higher than SI,,

predicted by curves from this study-60 ft for

unprepared, 57 ft for prepared sites and only

55 ft for old fields. Here, use of curves other

than those for old fields resulted in inflated

estimates of SI,,.

Errors in site-index estimations will be even

greater for plantations close to 10 years old

when height differences related to planting-site

conditions near their maximum. For a 1 O-year-old

plantation with a height of 30 ft and 800 trees

per acre, the predicted SI,, for unprepared sites

is 72 ft; for prepared sites is 65 ft; and for old

fields, 61 ft.

Clearly, because of the way site and stand

variables affect early height growth of longleaf pine plantations, use of a single set of

site-index curves is impractical for adequate

prediction of SI,, 5-15 years in advance. If

3
site-index predictions are wrong, then volume

yield projections will also be wrong, as will

economic evaluations based on these projections. Someday we may be able to develop

site-specific site-index curves. Meanwhile, the

site-index curves developed from this study will

account for some factors affecting early height

growth in longleaf pine plantations.

The site-index curves here (Figs. l-3) are for

the three planting-site conditions each with 600

trees per acre at age 10. Curves for higher

stand densities will be slightly above, and lower

densities slightly below, the curves shown.

Curves converge at index age. Because site

quality is so important, site-index curves for

prepared and unprepared sites are polymorphic.

6 0 -

6 0 -

i:

ti so -

4

:

8

$ 4 0 -

AGE (YEARS)

Figure 1 - Site-mdex curves forlongleafprneplanfations on Figure 2. - Site-index curves for longleaf pine plantations

old fields. on mechanically prepared cutover forest sites.

‘The growth-curve coefficient and resulting

site-index curves can be obtained from the

three equations above for old fields and also

for prepared and unprepared forest sites if

plantation height at age 15 is known. If it is

not known, the coefficient and appropriate siteindex curve for a specified index height can be

determined with a computer using a trial and

error iterative procedure.

If the curve coefficient (b,) is known or

approximated, then Siz5 for a given age and

height, or height for a given age and site index

can be obtained from:

LOglo (Sl~@LogIo (HT)-b, (.04-l/Age)

LOgto (HT)=Loglo (SI,,)-b, (l/Age-.041

6 0 -

6 0 -

;:

$j 6 0 -

h

h -

2

s 4 0 -

E

SITE

INDEX

AQh (YEARS)

SO

4 0

30

2 0

4
Bailey, R. L., W. F. Mann, Jr., and T. E. Campbell.

1973. Slash pine site index in the West Gulf.

U.S. Dep. Agric. For. Serv. Res. Note

SO-169, 4 p. South. For. Exp. Stn., New

Orleans, La. I

Beck, D. E., and K. B. Trousdell.

1973. Site index: accuracy of predictionUS.

Dep. Agric. For. Serv. Res. Pap. SE-107,

7 p. Southeast For. Exp. Stn., Asheville, N.C.

Bennett, F. A.

1975. Slash pine: some intriguing growth and

yield relationships. U.S. Dep. Agric. For.

Serv. Res. Pap. SE-133, 12 p. Southeast.

For. Exp. Stn., Asheville, N.C.

Bennett, F. A., C. E. McGee, and J. L. Clutter.

1959. Yield of old field slash pine plantations.

U.S. Dep. Agric. For. Serv. Stn. Pap. 107, 19

p. Southeast. For. Exp. Stn., Asheville, NC.

Chapman, R. A.

1938. The effect of origin of stand on the site

index of longleaf pine. J. For. 36:75-77.

Graney, D. L., and H. E. Burkhart.

1973. Polymorphic site-index curves for shortleaf pine in the Ouachita Mountains. U.S.

Dep. Agric. For. Serv. Res. Pap. SO-85 12 p.

South. For. Exp. Stn., New Orleans, La.

McClurkin, D. C.

1976. Influence of spacing on growth of

loblolly pines planted on eroded sites. U.S.

Figure 3. -Site-index curves for/ong/eafpinep/antations on Dep. Agric. For. Serv. Res. Note SO-209,

unprepared cutover forest sites. 4 p. South. For. Exp. Stn., New Orleans, La.

U.S. Department of Agriculture Forest Service.

LITERATURE CITED

1976. Volume, yield and stand tables for

second-growth southern pine, U.S. Dep.

Agric. Misc. Pub. 50, 202 p. [Revision of

1929 edition]

Allen, R. M. Wells, 0. 0. and P. C. Wakeley.

1955. Growth of planted longleaf pine on 1970. Variation in longleaf pine from several

cutover land and old fields. J. For. 53587. geographic sources. For. Sci. 16:28-42.

5

fr U.S. GOVERNMENT PRINTING OFFICE: 1980-771-081