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III. Methods for Estimating Site Quality

III. Methods for Estimating Site Quality

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21 3

aged on short rotations, or young plantations. For most longer-lived species

in the western United States, 100 years is the index age. Site index curves

for many American species have been summarized by Hampf (1965) and

Carmean (1968, 1973). The Forestry Handbook (Forbes and Meyer,

1955) also contains many older site index curves.

Site index is not only the most commonly used method for directly estimating site quality but is also the standard commonly used in developing

indirect methods of site estimation, as described later, using features of

soil and topography, or understory vegetation. Because site index is so

important for both directly and indirectly estimating site quality, forest researchers and forest managers should understand that even when suitable

dominant and codominant trees are available, the accuracy of site index

estimates may be affected by several stand and tree conditions. Furthermore, an understanding of the methods used for constructing site index

curves also is important. The reason is that the kind of data and the computation methods used determine the accuracy of site index curves, thus determining the precision of site index estimations.

a. Stand and Tree Conditions Aflecting Site Index.

i. Stand density. Tree height growth is usually considered to be independent of stand density. However, tree height growth may be less in lightly

stocked upland oak stands than in more fully stocked stands (Gaiser and

Merz, 1951; McComb and Thomson, 1957). On the other hand, trees may

“stagnate” and have reduced height growth in certain very dense natural

stands of lodgepole, ponderosa, and slash pines (Parker, 1942; Smithers,

1956; Holmes and Tackle, 1962; Oliver, 1967; Collins, 1967). Reduced

height growth seems particularly serious for densely stocked stands on poor

sites. Because of this reduced growth, separate site index curves for different stand densities were developed for ponderosa (Lynch, 1958) and

lodgepole pines (Alexander, 1966; Alexander et al., 1967).

Planted trees have a uniformity of age and spacing unlike the more uneven nature of trees in most natural stands. When closely planted on poor

sites, some conifer species may not express dominance well, thus resulting

in reduced height growth. Red pine closely planted on a poor site in Michigan had reduced height growth (Ralston, 1954), as did closely planted

Douglas-fir on a poor site in western Washington (Curtis and Reukema,

1970). On the other hand, height growth of red pine was not affected

by spacing on a good site in Pennsylvania (Byrnes and Bramble, 1955).

Likewise, spacing did not affect the height growth of planted jack, slash,

loblolly, and longleaf pines (Rudolph, 1951; Ralston, 1953; Ware and

Stahelin, 1948).

ii. Crown class. Site index curves can be based either on dominant trees

only or on dominant and codominant trees combined. Site index estimates

based on dominant Douglas-fir and lodgepole pines are less subject to sam-



pling error, thus estimates are more precise than those based on dominant

and codominant trees combined (Ker, 1952; Dahms, 1966). The reason

is that dominants are less variable in height than are dominant and codominants combined, and therefore fewer dominant trees are needed to attain

a specified level of accuracy.

Site index may be somewhat overestimated if only dominant trees are

used with site curves constructed from both dominant and codominant tree

data. Staebler (1948) recommended using only dominant trees for estimating Douglas-fir site index even though the regional site curves were constructed from both dominants and codominants. It was possible to use only

the taller dominant trees because Staebler calculated an equation for reducing average dominant tree heights to average heights for dominant and

codominant trees combined.

iii. Tree sampling error. Site index estimates for large areas are sometimes based on only a few trees. However, these estimates may not be

reliable because trees vary in height growth even in relatively small areas

that appear to be similar in site. Ker ( 1952) and Johnson and Carmean

(1953) found Douglas-fir site index to be variable even on small study

plots. They recommended measuring enough site trees so that dependable

site index averages could be obtained and large sampling errors avoided.

iv. Tree and stand age. The rate and pattern of height growth is considered to be directly related to site quality, particularly after trees are

well established and after stands achieve full stocking. However, initial

height growth may be affected by many factors in addition to site quality.

For example, initial height growth can be affected by weed and brush competition, frost damage, animal and insect injury, or differences in stock

quality and planting techniques. Certain species, such as longleaf and

white pines, may have slow initial growth before beginning more typical

rapid height growth. In contrast, seedling-sprouts of upland oaks have

rapid initial height growth and seldom display the sigmoid growth pattern

of true seedlings. For some species the initial lag in height growth can

be almost eliminated when competition is reduced by cultivating or applying herbicides (Wittenkamp and Wilde, 1964; Byrnes et al., 1973).

Site studies for planted red pine reveal that initial height growth is slow

and erratic and has little relation to site quality or to later height growth

(Ferree et a/., 1958; Day et al., 1960; Richards et al., 1962). Accordingly,

site curves for red pine are usually based on age at breast height. When

early height growth is erratic, the accuracy of all site index curves based

on total tree age is reduced, thus more accurate site curves can be attained

using breast-height age (Husch, 1956). A common procedure is to determine tree age at breast height, and then add a few additional years, the

number depending upon the level of site quality.



Almost all naturally established even-aged stands exhibit some variation

in tree age-usually 10 years or less. And age differences among dominant

and codominant trees growing in even-aged stands might result in differences in height growth, and thus affect site index measurements. Site studies

for upland oak in Missouri indicate that measuring trees appreciably

younger or older than trees in the main stand may cause errors in site

index estimation (McQuilkin, 1975). Trees younger than those of the main

stand have more rapid height growth than other trees, and thus will have

higher site index values; conversely, trees older than the main stand have

slower height growth and lower site index values. Accordingly, such agedeviant trees should be avoided, and only trees from the main stand should

be measured because their growth is more indicative of site quality.

Even-aged eastern hardwood stands frequently have many dominant

trees that originate as stump sprouts after clearcutting. These stump sprouts

might have more rapid height growth than trees originating as seedlingsprouts or as true seedlings. However, studies of upland oaks and yellowpoplar indicate that site index values are similar whether trees originate

as single stems or as multiple sprout clumps (Trimble, 1968; Kulow and

Tryon, 1968).

v. Genetics. All soil-site studies have some unexplained residual variation that is not associated with measured site features, and genetic factors

might be the cause of some of this variation. Nevertheless, genetic variation

in height growth apparently is relatively small within small forest areas.

Possibly rapid height growth is an important survival factor for trees growing and competing in dense stands. For most wild populations, a slowgrowing genotype would be at a disadvantage and would tend to be eliminated from dense stands where severe competition occurs with other more

rapid-growing genotypes.

An exception is aspen, which regenerates mostly from clones that may

occupy areas as large as 1 or 2 acres. Crown competition within a clone

is between individual stems all having the same genetic nature, thus slowgrowing clones are more likely to survive because only at their perimeters

do they compete with other more rapid-growing clones. Bigtooth aspen

clones growing on areas apparently similar in site quality had large differences in both height growth and site index (Zahner and Crawford, 1963).

Selection of site trees from several different clones was recommended as

a means for partially minimizing errors in site index estimation caused by

these clonal differences,

b. Harmonized Site Index Curves. Most older site index curves included

with normal yield table studies are termed “harmonized” in reference to

the mensurational technique used for their calculation. A key point is that

these curves were not based on actual measurements of tree height growth.



Instead, total height and total age was measured from dominant and

codominant trees on many growth and yield plots scattered throughout

a particular forest region. These height and age measurements were used

for calculating a single average regional height-age (site index) curve. Then

curves for a range of good and poor sites were fitted proportionally to

this average guiding curve. For example, upland oak harmonized site index

curves were based on data from several oak species found on 404 yield

plots scattered from Missouri east to Maryland, and from Michigan south

to northern Georgia (Schnur, 1937). Height and age measurements from

these plots were used to calculate an average oak height-age curve, then

this guiding curve was used to fit other good and poor site curves, each

of which was proportioned to the average guiding curve.

These older harmonized site index curves were intended only for identifying broad classes of site quality. But forest management in the United

States is now more intensive, and we need site index curves that are more

precise than these older harmonized curves. The older curves were often

in error because good and poor site plots might not be normally distributed

through all age classes. Such an abnormal distribution of site plots could

result from logging operations in which good site stands having large trees

were cut at younger ages than were poor site stands having small trees.

Thus older-aged stands on good sites would be rare and, as a result, olderage classes would be represented by mostly poor-site stands. Accordingly,

the average guiding curve would have an inaccurate downwarping for older

ages. In contrast, early logging in some mountainous regions was concentrated at lower elevations where better sites are more likely. Thus young

stands would occur mostly on the better sites, younger age classes in the

yield study might be represented by mostly good-site stands, and the average guiding curve would have an inaccurate upwarping at younger ages.

Harmonized site index curves from yield studies usually are a family

of proportioned curves all having the same shape or pattern of tree height

growth. Thus the harmonizing technique is based on the assumption that

the pattern of tree height growth is the same for all site classes, localities,

and soil conditions included in the regional yield study. We now know

that this assumption is not valid and that height growth patterns vary

greatly (are polymorphic) for many species that grow on contrasting sites,

or that have a wide geographic distribution (Carmean, 1968; Beck and

Trousdell, 1973). For example, many important species, including upland

oaks, southern pines, Douglas-fir, ponderosa pine, quaking aspen, and

white spruce, have extremely wide ranges. We already know that differences in soil, topography, and climate within such large geographic areas

cause large differences in site quality. However, these same soil, topographic, and climatic differences also can cause differences in the pattern



of tree height growth. Trees may grow at different rates at different times

and yet arrive at the same height (site index) at 50 or 100 years of age.

Thus the shape of the height-age curves portrayed in older harmonized

site index curves may not accurately represent the diverse sites and heightgrowth patterns actually found over the range of a particular tree species.

As a result, site index estimations from these curves will be in error.

Much evidence confirms the existence of polymorphic height-growth patterns for forest species growing on contrasting sites, soils, or in different

portions of a forest region (Carmean, 1968). This evidence includes: ( 1 )

comparisons of different sets of harmonized site index curves for species

that range over large forest regions, ( 2 ) soil-site studies, ( 3 ) periodic

height-growth measurements from permanent growth study plots, and ( 4 )

newer site index curves based on stem analyses.

When harmonized site curves are compared from different portions of

a species’ range, we often find marked differences in height-growth patterns. Contrasting patterns are particularly evident when harmonized site

curves are compared for Douglas-fir, for ponderosa, red, Virginia, and

loblolly pines, and for upland oaks (Smith et al., 1962; Curtis, 1966; Curtis

et al., 1974b; Powers, 1972; Spurr, 1956; Kulow et al., 1966; Trousdell

et al., 1974; Graney and Bower, 1971; Carmean, 1971, 1972). Further

study of the harmonized site index curves for longleaf pine showed that

stands originating on old fields had better site quality than stands originating after cutting; also trees on old fields had a different pattern of height

growth than trees on cutover lands (Chapman, 1938).

Soil-site studies using regression analyses often include coefficients for

the tree age variable that can be used for constructing site index curves.

These studies frequently indicate that height growth patterns are different

from those predicted by the regional harmonized site curves and, in certain

cases, also indicate differing patterns for trees growing on different local

soil groups. Soil-site studies indicating differing height growth patterns have

been published for shortleaf and loblolly pines, Douglas-fir, and upland

oaks (Coile and Schumacher, 1953a; Zahner, 1962; Carmean, 1956, 1964,


Periodic height measurements from permanent plots often reveal growth

patterns much different from those predicted by harmonized site index

curves (Spurr, 1956). These measurements sometimes indicate that site

index “changes” as stands grow older (Lange, 195 1; Watt, 1960; Williamson, 1963). In some cases several years of drought or unusually high rainfall might result in changed patterns of tree height growth, thus indicating

changes in site. However, pronounced changes in site quality are unlikely

for older stands growing on forest soils never cleared and cultivated for

agriculture. Therefore, these apparent site index changes probably are due



mostly to tree height-growth patterns on these permanent plots that are

different from the patterns predicted by the regional harmonized site index


c. Refined Site Index Curves from Stem Analyses and Internode Measurements. Site index can be considered a convenient label for a specific

height-age curve portraying height growth throughout the life of the tree.

Height attained at 50 or 100 years (index age) is an important standard,

but equally important are the tree heights attained both before and after

index age. Moreover, height growth and volume growth are closely related,

thus when trees have polymorphic patterns of height growth, polymorphic

patterns of volume growth also are likely (Curtis et al., 1974b).

Intensive forest management requires better site index curves than were

possible using the harmonizing technique, or using age coefficients from

soil-site studies. Height growth records from permanent growth and yield

plots could be used for refined site index curves. However, long-term

records are not available for most species, and growth and yield plots may

not be established in stands representing the full range of site, soil, topography, and climate where the species occurs. Stem analysis is now the method

most favored for developing more accurate site index curves, and in recent

years many new site curves have been published based on this method

(Carmean, 1968, 1972). These new site index curves, together with internode studies have confirmed that tree height growth is usually polymorphic

(Table I).

Refined site index curves also have been constructed using internode

measurements. This technique is similar to stem analysis and is most easily

used with conifers having conspicuous limb whorls marking the course of

annual height growth. Using this technique with red pine, Bull (1931)

was one of the first American investigators to demonstrate polymorphic

height growth. Internode techniques also have recently been used for red

pine, Douglas-fir, and eastern white pine site index curves (Richards et

al., 1962; Van Eck and Whiteside, 1963; King, 1966; Beck, 1971a,b).

Methods for collecting stem analysis and internode data are now fairly

similar. But methods differ for correcting for certain biases in the data

and for computing site index curves. Dahms ( 1963) found that the relative

heights of dominant lodgepole pine shift as stands age; trees tallest when

they are sectioned may not have been tallest earlier. Another bias requiring

correction is that sectioning points do not coincide with the tip of annual

leaders (Carmean, 1972; Lenhart, 1972). Plots selected for sectioning may

have an abnormal distribution of site quality for certain age classes as did

plots in some of the older normal yield studies. This bias can be very

serious if older-aged plots are mostly on poor sites, but it can be removed

using proportional methods for extending the height growth curves of the

younger plots (Curtis, 1964).




Polymorphic Patterns of Tree Height Growth are Demonstrated by Site Index Curves

Based on Stem Analysis and Internode Methods



Red pine

Jack pine

Shortleaf pine

Eastern white, Scotch

pines; Norway

spruce; European


Eastern white pine

Ponderosa pine

Noble fir

Sugar maple

Northern hardwoods

Black walnut

Upland oaks

Quaking aspen

Siberian elm

Bull (1931), Richards er a/. (1962), Van Eck and Whiteside

(1963), Wilde (1964), Wilde et a/. (1965), Mader (1968)

Jameson (1 963)

Graney and Burkhart (1973)

Lorenz and Spaeth (1 947)

Beck (1971a,b)

Daubenmire (1961), Sander (1962), Minor (1964), Hermann

and Peterson ( I 969), Powers ( 1972)

DeMars er a/. ( I 970)

Farnsworth and Leaf ( I 963)

Solomon ( I 968)

Carmean (1966, 1968). Losche and Schlesinger (1975)

Carmean (1 972)

Zahner and Crawford (1963), Jones (1967)

Sander (1 965)

A variety of equation models can be used for computing tree height

growth curves (Curtis, 1964; Stage, 1963, 1966; Carmean, 1972). Many

height growth curves are based on an initial segregation of plot data into

preselected site classes, but Bailey and Clutter (1974) proposed equations

for polymorphic curves that retain the same shape regardless of the selected

base age or site class. Strand (1964) and Curtis et al. (1974a) have

pointed out that height growth curves are based on equations using height

as the dependent variable and tree age as the independent variable. In

contrast, site index estimation curves are based on equations using site

index as the dependent, and height and age as independent, variables. Variations of this latter approach have been used by Johnson and Worthington

(1963), Hegar (1968, 1971, 1973), McQuilkin (1974a), and Curtis et

al. ( 1974b). Height-growth curves are better adapted for showing polymorphism and height-growth patterns throughout the life of the tree. But

site index estimation curves may be more precise for estimating tree height

at index age (site index).

2 . Site Index Comparisons between Species

Many stands suitable for site index measurements may not contain the

tree species for which site estimates are desired. Suitable dominant and



codominant trees of several species may be present, but no usable trees

of the particular desired species may occur. For such stands we can use

the tree species actually present for estimating site index. Then species

comparison graphs can be used to convert the site index of the species

present to the site index of the desired species. Comparison graphs and

site index ratios have been developed for several forest species in various

parts of the United States (Table 11).

Site index comparisons are a very useful means for extending direct site

index estimations, particularly in forest areas where soil and site vary

greatly, and where, for each site, the forest manager has the problem of

selecting the most desirable species for management from among many

possible species. However, we should realize that estimating site index is

only the necessary,first step in choosing the most desirable species for a

particular site. We must also consider tree height growth before and after

index age. Certain short-lived species such as aspens may grow rapidly

in early years and thus, at 50 years (site index age), are taller than longlived species such as maples, which have slower initial height. But maples

maintain their height growth and eventually are taller than aspen, thus

maples would have the higher site index if 100 years were used as index

age. We must also compare volume and value of wood in addition to tree

height (site index). Species valued for high-quality veneer and saw logs

might be preferred on certain sites even though their total volume may

be less than the volume produced by species utilized mostly for pulp and


Site index comparison graphs may have built-in inaccuracies that limit

the precision of site estimation. These graphs are based on plots where

paired site index estimations are obtained from two or more tree species.

Regressions relating these paired site index estimations are then used for

constructing the species comparison graphs. However, in many studies site

index estimations for the various species are made using older harmonized site index curves. Earlier we discussed how site index estimations

using these older curves may be subject to error. Accordingly, when

two erroneous site index values are correlated, their errors may be compounded in the resulting regression equations. More precision is possible if stem-analysis methods are used on study plots for deriving the site

index values for the associated tree species.

Another possible source of error is that regression equations expressing

site index correlations between paired species are not generally suited for

solving both forward and backward. The equations use site index of one

species (species 1 ) as the dependent variable and site index of the associated species (species 2 ) as the independent variable. Such an equation

is suited for a forward solution of species 1 site index using observations


22 1


Site Index Comparisons for Forest Tree Species in the United States


Loblolly and shortleaf pines

Upland oaks, yellow-poplar,

sweetgum, loblolly pine

Upland oaks, yellow-poplar,

shortleaf pine

Upland oaks; yellow-poplar;

eastern white, shortleaf,

Virginia, pitch pines

Upland oaks

Black, scarlet, and white oaks

Sweetgum, cottonwood, green ash,

bottomland oaks

Eastern white pine, red maple

Northern hardwoods

Northern conifers and hardwoods

Red pine and quaking aspen

Western white pine, western larch,

Douglas-fir, grand fir

Western white and lodgepole pines,

Douglas-fir, grand fir,

western hemlock

Douglas-fir, ponderosa and sugar


Douglas-fir, redwood, western



Piedmont of North


Coastal Plains of

S. Arkansas,

N. Louisiana

Piedmont of


Piedmont of

Virginia, North

Carolina, South,

South Carolina

S. Appalachian Mts.

Appalachian Mts. of

West Virginia

Missouri Ozarks

Lower Mississippi

River Valley

N. Connecticut,

W. Massachusetts,

E. New York


N. Minnesota

N. Minnesota

N. Rocky Mts.


Coile ( 1948)

Zahner (1967a)

Nelson and Beaufait

( 1956)

Olson and Della-Bianca


Doolittle (1958)

Trimble and Weitzman


McQuilkin (1974a)

Broadfoot (1 970)

Foster (1959)

Curtis and Post (1 962)

Carmean and Vasilevsky



Copeland ( I 956)

N. Idaho

Deitschman and Green

( 1965)

S. W. Oregon

Hayes and Hallin (1962)

N. W. California

Wiant and Porter (1966)

a Alban, D. H. 1975. US. Forest Serv. Norih Cenr. Forest Exp. Sta. (unpublished


of species 2 site index. However, this same equation cannot be used for

a backward solution-that is, an estimation of species 2 site index using

observations of species 1 site index. A second equation using species 2

as the dependent variable is necessary, or a third equation can be calculated



that averages the trends of the first two equations (Carmean and Vasilevsky, 1971).

3. Growth Intercept

The growth intercept method uses a selected period of early height

growth as an index of site quality rather than the long-term height growth

portrayed in site index curves. It was first proposed by Wakeley (1954),

and is usually based on the total length of the first 5 internodes produced

after trees have reached breast height (454 feet). This method was developed for conifers in the United States having easily recognized internodes

marking the progress of annual height growth (Table 111). Additional

growth intercept studies have been published for Douglas-fir in British

Columbia and for Sitka spruce and western hemlock in Alaska (Warrack

and Fraser, 1955; Schmidt, 1954; Smith and Ker, 1956; Gregory, 1960).

Douglas-fir and black walnut site quality also has been estimated using

average annual height growth instead of the cumulative length of 3-5 internodes as used in most growth intercept studies (Stoate and Crossin, 1959;

Hansen and McComb, 1958).

The growth intercept method is useful for areas where trees are too

young for conventional site index estimation using height, age, and site

index curves. For areas with young trees the growth intercept method has

several advantages: ( a ) total age and total height need not be measured,

thus measurement errors are avoided and fieldwork is simplified; ( b ) measuring internodes above breast height eliminates the period of erratic early

height growth, thus errors are avoided that are sometimes associated with

site index curves based on total age; and (c) many site index curves do


Growth Intercept Studies for Estimating Site Index of Forest Trees in the United States


Loblolly, shortleaf, longleaf,

and slash pines

Red pine


N. Mississippi,

S. E. Louisiana

New York


S. Ontario

Eastern white pine

Ponderosa pine


N. Minnesota

S. Appalachian Mts.

N. California


Wakeley (1954), Wakeley

and Marrero (1958)

Ferree et al. (1958),

Richards et al. (1 962)

Day er at. (19601,

Schallau and Miller

(1966), Gunter (1968)

Wilde ( 1964, 1965)

Alban (1972a)

Beck (1971a)

Oliver (1972)



not extend to ages younger than perhaps 15 or 20 years, thus the growth

intercept is useful for very young stands.

However, the growth intercept method has the disadvantage of basing

site quality estimation only on early height growth, and this may not be

a very good indication of height growth in later years. Later height growth

may not be adequately predicted, particularly in areas where soil and other

site factors are much different than in the area where the study was made.

The method is most useful for short rotations where height growth is only

projected a few years beyond the growth intercept measurements. In contrast, serious errors may result if growth intercepts are used t o estimate

later height growth for species that are managed on 80 to 100 year


Internode measurements are usually begun at breast height because

height growth up to that point is often influenced by competition, stock

quality, planting method, or injuries (Ferree et al., 1958; Day et al., 1960;

Richards et al., 1962). But erratic height growth after breast height was

observed for red pine in Wisconsin that was planted on areas having

marked differences in weed competition, in soil texture, or that had ground

water tables within the soil volume occupied by tree roots (Wilde, 1964).

On such areas growth intercept was not dependable for predicting later

height growth or for site index estimation. Natural red pine in northern

Minnesota also has variable height growth after breast height, and Alban

(1972a) found that starting internode measurements at 8 feet resulted in

more precise site index estimates than were possible using breast height

as the starting point.

Most studies show that the cumulative length of the first 3 to 5 internodes above breast height is adequate for site estimation. Fewer internode

measurements are usually less precise probably due to year-to-year variations in height growth. However, we cannot assume that 5 internodes will

always be an adequate number, o r that breast height is the best starting

point for these measurements. Therefore, when making growth intercept

studies in new areas and for different species, precision of results should

be tested using various numbers of internode measurements and various

starting points in addition to breast height.




I. Mensurational Methods

Mensurational methods have been proposed for uneven-aged stands and

for stands that lack trees suitable for directly estimating site index using

conventional methods. For example, heights of mature dominant trees have

been used as an index of site for uneven-aged old-growth western conifers

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III. Methods for Estimating Site Quality

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