FOREST SERVICE
U. S.DEPARTMENT OF AGRICULTURE
P. O. BOX 245, BERKELEY, CALIFORNIA 94701
NTERCEPT FOR EST
in young ponderosa pine p antations and natura
William
USDA Forest Service
Research Note PSW-276
1972
Abstrace: Site index is difficult to estimate with any
refiab3ity in ponderosa pine (Pinus po~derosa.Laws.)
stands below 20 y e a s old* A method of estimating
site index based on 4-year height intercepts (total
length of the fixst four i n t e m d e s above breast
height) is descdbed, Equations based on two sets of
pubfished site-imdex curves were developd. They
worked equally well for plantations and natural
stands in northern @&fornia.
Oxford: 541:174.7 Binus ponderosa (794),
Retvieval Terms: B3inus pnderosa; site-index estimation; CaEfornia (northern); heigh"cn"ircept method;
internodal measurement.
W.Oliver
Foresters find it difficult to make regable estimates of site index h young stands of ponderosa
pine (Pinus ponderosa Laws.). In stands under 20
years old, this estimate of site quality camot be
obtahed by conventional methods; in stands above
20 years, s m l l enors in estimting age often create
large errors in site index.' Fwthermore, managem n t decisions, such as plamhg cultural work in
plantations, often must be made where trees suitable for estimt ing site productivity by standard
means are lacking.
Height intercept offers a way of estimting site
hdex h stands too young for conventional methods. This concept is based on the total len@h of a
given number of consecuti
nodes beghning at or
just above breast height. In 1954, W&eley2 first
proposed that 5-year height intercept be used to
esthate growh in southern pines. Since then, height
intercept has been developed as an independent
measure of site producti~ty for red pine (Binus
resinom it.),^ and as a method of estimating site
index for stands of Douglas-fiir (fiezcdofsugamesaziesii
[Mirb.] ~ r a n c o ) ,western
~
hemlock (Tsuga heterophylla [Raf.] Sarg.)-Sitka spruce (Piem sitchensk
[Bong.] Car.), and eastern white pine (Pinusstrobus
L.).
Like red pine and eastern white pine, ponderosa
pine is ideally suiked to the height htercept method.
Height growth is uinodal. And height gro.dvth differences caused by site appear early-by the time the tree
reaches breast height.'
This note reports a method of uskg height intercept
to estimate site index in young, natural stands and
plantations of ponderosa pine in northern C&forda.
METHODS
Thirty-five even-aged psnderosa pine stands on the
westside Sierra Nevada, s o u ~ e r nCascade Range,
Warner Mountkns of northeastern Gafifomia, and
-
hterior Coast h n g e s were sampled. Nineteen stands
were natrardal,and 16 were plantations. N l stands m r e
untMmed, were currently free of bsect or disease
attack, and showed little pregoras stem breakage. Stand
ages ranged from Z 6 to 83 years.
WitGn each stand aueast five dodnants meeting
the usual crlIQeria for a site-index sample tree were
se9ec"cd in a smaU horngenesus area. Total height and
age were obtained for each tree, Planting records gave
total age for planted trees. Total age of natural trees
was estimted by adjusthg age masured at breast
height from bcrement cores for time to reach breast
hei&t.7
Site hdex was deterdned for each area ~ t two
h
sets of site curves, one developed by Dramkg and
Reineke QD & R ) , ~and the other by Arvanitis,
Lindqlaist, and Palley
Neither set of curves was
designed for use in the Warner Mountaim. I included
the Wuner Mountahs anyway because only t h e e of
the 35 plots were located there and because the pattern
of height growth on these plots fit the pattern s h o w in
the site index curves. Site hdices for stands less than 50
years (age at breast height) were adjusted according to
the m t h o d described by ~owers.'O Hei&t intercegts
one through six were recorded individudy to "&he
nearest 0.1 foot for each tree.
RESULTS
The relations&p of site index to 4-year height
intercept was linear, with no apparent dgference
between natural and plmted stands. Therefore, 1
cornbined the data.
How m a w hternodes need to be measured for an
acceptable estimte of site index? Most invest@atora;
have recomwnded five. But I was uncerbin whether
five intemdes were the most efficient number for
ponderosa pine in northlcrn Califorka. To test this, X
compared the coefficients of determination (r2) of the
linear regressions sf both D & R and ALP site indices
over 1- to 4-year heiglL intercepts (fig.I ). The 6-yea
height intercept accowted h 81 percent of ths:
variatbn between plots when estimating D (8, R site
index and 74 percent when estimathg ALP site index.
The r2 values dropped slowly as fewer internodes
were measured-down to 79 percent for D & R site
index and 72 percent for ALP site index at four
Figure I-The r2 values of linear r q r e s i o n s
estimating Dunning and ~ e i n e k and
e ~ Arvanilis,
eP a/,' site indices increase only slightly when
more than 4-year height intercepts are measured
in young p o d e r o s a pine g a n d s in noahern
California.
in"cmodes. Then the amount of vziation e x p l a e d
dropped more quicMy. But the drop was less than.
expected. Sunprishgly, when only one internode above
breast height was measured, 68 percent of the vafiation
between plots remahed accounted for when estimathg
D & R site index. Apparently, by "Ie time young
ponderosa pine reach 4.5 feet in hei&t, tbir hei&t
gowth drongiy reflects the Site potentid. $eve= b r u h
competiLion could m s k "cs g o w h response. In this
study, however, alP samples were in fully-stocked
stands where brush, although preseI-hl in vaying
arraounts, never had a full crown cover.
Little hprovernent in e s t h a t h g eitlzer D & R or
ALP site index can be expected by measuring more
"can four internodes (fig*1).Measuring four hstead of
six internodes m k e s field measurements easier and
lowers the fininnurn age at which site hdex can be
estimated.
The relations~pof 4-year height intercept to Dek.R
site index (fke2) is preli'ernred over ALP site indexffig.
3),Ntlnough ALP site hdex curves are m r e frequently
used, D & R site hdex curws (base age: SO years) more
accurately describe height g o W h in stands less than SO
years old.' O For example, 72 percent of the variation
in 4-yeas height intercept was accounted for by
ponderrosa pine site kdex curves developed by WLP
(base age: 100 years) compared with 79 percent for D
8t R curves. But 4-year height hkrcept stjill will p r o ~ d e
a atisfacborgr estimate oEAW site index (fk. 3).
.9
rvanitis, Lindquist & Palley
Internodes rneasu red (number)
Figure 2--Dunning a d ~ e i n e k esite
~ i d e x can
be e s i m a t d e i o s l y by 4-year height intercept
i n young ponderosa pine s t a d s i n noPihern
California.
site index can b e
Figure 3-Arvanitis, e t
e s t i m a t d by &year height intercept i n young
p n d e r o m pine stands i n n o a h e r n California.
20
4-year Height Intercept (feet)
The predicting equations I developed are:
1. Dunning and Reineke Site Index = 11.75 + 9.86
H.I"
2. Arvmi"cs, Lindquist, and Pdley Site Index =
36.55 + 12.44 H.I.
rin wEch H.I. = 4-year height htercept .
Mean square deviation from regression (Sy.x2) was
101-98 for D & R site index and 245.49 for ALP site
index.
When estimating D & R site index from 4-year
height intercepts, 90 percent of the individual estimates should fall d t f i n i: 18 site index units of the true
value. Since 4-year height intercept is a less accurate
e d i m t e of ALP site index, 90 percermL of these
esthates shodd faU w i t ~ -128
n site hdex wiLs ofthe
true value.
MPLIBCA'JTION
To use the height intercept method of site index
estimation:
@
Select at least four or five dodnant trees ~ t M n
a small, homogeneous area. The number of tree groups
40
4-year Height Intercept (feet)
necessary will depend on the variation in site quality
and the accuracy needed. Choose vigorous, wellh r m d trees with no e.didence of disea~se,insect attack
or stem damage-trees t h t , if older, would be candidates for conventional site index estimtes.
Measure the total length (nearest 0.11 foot) of the
four internodes beginning with the node at or just
above breast height.
Average the 4-year height intercepts for each
group.
@
Read site index from figtkres 2 or 3, or calculate
it from equations 1 or 2.
Estimates of site productivity will not be precise
with this method. Site index, itself, is only an estimate
of site productivity. Errors inherent in site index curves
are compounded by errors in the heighl intercept
method. Nevertheless, for young stands of ponderosa
pine below 20 years of age it offers a way to obtain site
index. And for stands at, or slightly above 20 years,
height intercept is probably as accurate and certainly
easier to obtain than by conventional means. The
method appears to work equally well for natural stands
and plantations.
NOTES
liver, William W., and Robert F. Powers. Eorly height growth
o f ponderosa pine forecasts dominance in plantations. USDA
Forest Serv. Res. Note PSW-250, Pac. Southwest For. and
Range Exp. Stn., Berkeley, Calif. 4 p. ,illus. 197 1.
'~akeley, Philip C. The g o w t h intercept method of sire
class$ication. Proc. 3rd Ann. Forestry Syrnp. La. State Univ.
School Forestry, p. 32-33. 1954.
%eree, M. J., 3'. D. Shearer, and E. L. Stone, Jr.A methodof
evaluating site qualify in young red pine plantations. J. For.
56(5): 328-332. ,illus. 1958.
Day, M. W., 6. F. Bey, and V. J. Rudolph. Site index for
planted red pine by the 5-yeargowth ktercepf method. J. For.
58(3): 198-202. ,illus. 1960.
warrack, G. C., and A. R. Fraser. Estimation ofsite qualify in
juvenile Doughs-@ stands. Brit, Columbia For. Serv. Res. Note
28,5 p. , illus. 1955.
5 Gregory, Robert A. Estimating site index in sapling and pole
stands in southeast Alaska. U.S. Forest Serv. Alaska For. Res.
Center Tech. Note 48,3 p. ,illus. 1960.
'Beck, Donald E. Growth intercept asan indimtor ofsite index
in natural stands of white pine in the southern Appalachians.
USDA For. Serv. Res. Note SE-154, Southeastern For. Exp.
Stn., Asheville, N.C. 6 p. , illus. 197 1.
' ~ e ~ e Walter
r,
H. Yield of even-aged stands of ponderosa pine.
Rev. USDA Tech. Bull. 630,59 p. ,illus. 1961.
'Dunning, Duncan, and L. H.Reineke.Pl.eliminary yield tables
for second-guowth stands in the Californk pine regzbn. USDA
Tech. Bull. 354,23 p., illus. 1933.
'Arvanitis, L. G., J. Lindquist, and M. Palley. Site index curves
fop. even-aged young-growth pondevosa pine of the westsge
S i e m Nevada. Univ. Calif., School of Forestry, Calif. For. and
For. Prod. 35,6 p., illus. 1964.
'Power$ Robert F.Estimting site index ofponderosa pine in
northern CalIjcornia. . .standard curves, soil iberies, stem amtalys&. UUSA For. Serv. Res. Note PSW-265, Pac. Southwest For.
and Range Exp. Stn., Berkeley, Calif. 8 p., Bus. 1972.
'
WlllLELlhM W. CBkIBrER is doing silvicultural research on Siena Nevada
conifer types, with headquarters at Reddhg, Cmornia. He earned a B.S.
degree (1956) in forestry from the University of New Bampshhe, and an
M.F. degree (1960) from the Unkersity of Michkan
OFFICE:
794-419-3715