Response of ponderosa pine 8 years after fertilization

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FOREST AND RANGE
EXPERIMENT STATION

. -USDA FOREST SERVICE RESEARCH NOTE]

PNW-301 August 1977

RESPONSE OF PONDEROSA PINE 8 YEARS AFTER FERTILIZATION

by

P, H. Cochran, Principal Research Sotl Scientist

Abstract

Diameter and volume growth of individual ponderosa
pine trees in thinned stands continues to respond to
fertilizer 8 years after application in three of four
study areas. Hetght growth ts not responding to ferttl-
ization. Removal of bitterbrush in one study area
decreased volume growth in the seventh and etghth
growing seasons. Apparently brush removal sttmulated

fescue growth, increasing water and nutrient competitton.

KEYWORDS: Fertilizer response (forest tree), fertilization
(forest), increment (volume), volume increment ,
increment (diameter), diameter increment,
thinning (tree), ponderosa pine, Pinus ponderosa, ¢
bitterbrush, brush control, growing season.

Metric Conversion Factors

pound/acre .121 kilograms/hectare
acre .405 hectare

foot .3048 meter

inch .54 centimeters

square foot
cubic foot
mile

.092903 square meter
LO2Z8317) cubic meter
.61 kilometers

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FOONOCOF

OREST SERVICE - U.S. DEPARTMENT OF AGRICULTURE - PORTLAND, OREGON

Introduction

Response of individual ponderosa pine (Pinus ponderosa) trees to fertil-
ization during the first four growing seasons after application was reported
in an earlier note (Cochran 1973). As then outlined, 1/10-acre areas around
dominant or codominant sample trees in thinned stands were fertilized. These
stands are located in Sumpter Valley near Baker in northeastern Oregon and
in central Oregon on the Fort Rock Ranger District, the Pringle Falls Experi-
mental Forest, and 5 miles west of the Experimental Forest along Road 1808.
The three central Oregon locations were chosen to represent areas differing
in effective moisture. The Fort Rock and Road 1808 locations were thought
to be the driest and wettest respectively.

In three of these four areas, 24 sample trees were selected so treatments
could be replicated eight times. Treatments equivalent to 0, 200, and 400
pounds of nitrogen (N) per acre in the form of urea were used in Sumpter Valley.
In addition to the control, the two levels of fertilizer application per acre
in central Oregon were: (1) 200 lbs N, 100 lbs phosphorus (P), 30 lbs sulfur
(S) and 1.88 lbs boron (B) and; (2) 400 lbs N, 200 lbs P, 60 lbs S, and 3.75
lbs B.

In the Fort Rock area 48 sample trees were selected so a shrub vegetation
removal treatment for the 1/10-acre area could be superimposed on the fertilizer
treatments used in the other two central Oregon areas. The experimental
design is a randomized complete block. In the Sumpter Valley area blocks
were chosen so that trees in each block were on the same slope and aspect
and were the same distance from an abandoned irrigation canal. Blocks in
the central Oregon areas were assigned on the basis of similar tree diameters.
Some test tree and stand parameters established at the beginning of the study
of the four areas are present in table l.

During the first 4 years after application, fertilization increased
height growth significantly at the Sumpter Valley and Road 1808 locations;

Table 1--Some test tree and stand parameters at the beginning of the study

Mean starting values

rl! |Height
2

Stand
spacing

Stand
age

Location

Diamete Volume

Inches Feet Feet Feet® Feet Years
Sumpter Valley 10.0 51 0.556 9.66 Sexe 64
Pringle Falls 1OsZ 52 -591 11.02 14 x 14 78
Road 1808 14.6 65 1.258 31.43 2 34 12 2/ 68 3/
Fort Rock WARC2 47 .836 13.25 variable— variable—

1/ This diameter is the actual mean diameter of the test tree and not the
diameter equivalent to the average basal area.

2/ 10 x 10 feet to very open.

3/ Stand consists of residual trees left after railroad logging in the
1920's and reproduction immediately after.

but there was no significant difference between the two levels of fertilization.
The effect of fertilization on height growth tended toward significance in

the other two areas, and results may have been confounded with top damage.

Tree diameter and basal area growth were increased by fertilization in all

the areas, but there was no difference among levels of application. Volume
growth was also increased significantly by fertilization except in the Fort
Rock area where the effect of fertilization was almost, but not quite,
Significant at the 5-percent level. This study was continued beyond the

first 4-year period after fertilization to obtain more information about
duration of response.

Current Methods of Study

Fire destroyed the Silviculture Laboratory and the records for this
study 2 years after measurements were taken for the previous report
(Cochran 1973). Fortunately, a copy of the study plan and increment core
Measurements taken at the end of the first 4-year period were available from
Pacific Northwest Forest and Range Experiment Station headquarters. Original
diameters and heights and those for the end of the 4-year period were lost.
Records of increment borings for each tree from all study areas, showing the
diameter growth 4 years previous to treatment and 4 years after treatment,
were retrieved. Trees were measured with optical dendrometers following
the sixth and eighth growing seasons after treatment. Increment borings
were again made on two opposite sides of every tree to determine diameter
growth during the 4-year period previous to treatment and the first and
second 4-year periods after treatment. The increment measurements were then
compared with those previously taken and used to identify the treatments
given each tree.

Because diameter growth was significantly increased by fertilization
during the 4 years after initiation of the study, identification of the
control trees for each replication in each area was certain. Separation of
the test trees for the two levels of fertilization was questionable for some
of the replications. Therefore, analyses of possible differences in responses
between levels of fertilization were not attempted. Identification of the
shrub removal treatments in the Fort Rock stand was certain because the
outline of the 1/10-acre area was clearly visible and no shrubs had reinvaded
the areas.

Height and volume growth for the seventh and eighth growing seasons after
treatment were obtained from dendrometer readings subjected to the STX program
(Grosenbaugh 1964). This program incorporated a modification of Brickell's
equation (Brickell 1970) for diameter outside bark to diameter inside bark
conversion developed by Cochran (1976). Diameter growth was determined from
the increment borings.

Diameter and volume growth were subjected to analysis of covariance

using diameter growth for 4 years previous to treatment as the covariate.
Height growth was subjected to a standard analysis of variance.

Results and Discussion

Height growth during the seventh and eighth growing seasons after
initiation of the study was not increased significantly (at the 5-percent

level of probability) by fertilization or shrub removal (table 2). A signif-
icant increase in height growth at the Pringle Falls location would have
occurred if the probability level was increased to 8 percent.

Table 2--Average annual height growth for the seventh and eighth
growing seasons

Wy

F values—

Fertilized |Control vs. fertilized

Treatments
Location

Control

- - - - Feet - - - -
Sumpter Valley OF7, 0.9 1 575
Pringle Falls .6 .85 3.8039
Road 1808 ay | .6 . 2588
Fort Rock .6 .6 .0040.
Shrubs Shrubs Shrubs Shrubs
present removed present vs. removed
Fort Rock .65 .6 ~ 121

y F values have 1 and 14 degrees of freedom except for the
Fort Rock location where both F values have 1 and 35 degrees of
freedom.

Diameter growth during the second 4 years after treatment was signif-
icantly increased by fertilization except in the Road 1808 stand (table 3).
The increase in diameter growth due to fertilization would have been signifi-
cant at the Road 1808 location if the probability level was raised to 11
percent. Shrub removal did not significantly influence diameter growth in
the Fort Rock area.

Volume growth during the seventh and eighth growing seasons after
treatment was increased significantly (at the 5-percent level of probability)
by fertilization except in the Road 1808 area (table 4). Even though in the
Road 1808 area large differences in adjusted means for volume growth occurred,
the probability level would have had to be 13 percent to be significant. This
is due to large variation among replicates within treatments. The dense
Road 1808 stand has more volume on each 1/10 acre and some of the test trees
are codominants with other trees on the 1/10 acre. The competition of the
other trees with the test tree for nutrients and water may be greater and
more variable than in the other study areas.

Removal of shrubs (all shrubs were bitterbrush (Purshita tridentata)) on
the Fort Rock area significantly decreased volume growth during the seventh
and eighth growing seasons, but no significant interactions of shrub removal
and fertilization occurred. A decrease in volume growth due to shrub removal
is difficult to explain and seems contrary to other published work. Barrett
(1970) found that removal of all understory vegetation (shrubs plus grasses
and sedge) increased growth of thinned pine in a study on the Pringle Falls

Table 3--Average annual diameter growth for the fifth, sixth,
seventh, and eighth growing seasons after treatment.
Values calculated ustng adjusted means obtained from
covartant analysts

Treatments

Fertilized

Location

Control Control vs. fertilized

- - - Inches - - -
Sumpter Valley 0.16 0.20 10.66192/
Pringle Falls 16 23 18.63772/
Road 1808 16 19 3.0100
Fort Rock ne) .18 33.05812/
Shrubs Shrubs Shrubs Shrubs
present removed present vs. removed
Fort Rock sili 16 0058

a F values have 1 and 13 degrees of freedom except for the
Fort Rock location where both F values have 1 and 34 degrees of
freedom.

2/ Indicates significance at the 5-percent level of
probability.

Table 4--Average annual volume growth for the seventh and
etghth growing seasons after treatment. Values
ealeulated using adjusted means obtained from
eovartant analysts

5 values-/

Treatments
Location

Fertilized | Control vs. fertilized
- - - Feet3 - - -

Sumpter Valley 0.47 0.83 9.51612/
Pringle Falls 1.22 2.00 5. 30552/
Road 1808 1:33 2.19 2.7515, ,
Fort Rock 1.02 1.43 4.54192

Shrubs Shrubs Shrubs Shrubs

present removed present vs. removed
Fort Rock 1.50 1.09 5.84422/

aE values have 1 and 13 degrees of freedom except for the
Fort Rock location where both F values have 1 and 34 degrees of
freedom.

ee Indicates significance at the 5-percent level of
probability.

Experimental Forest. Gordon (1962) found that understory vegetation adversely
affected growth of ponderosa and Jeffrey pine poles. He further concluded
that perennial grass had a greater effect than broad leaved plants. Although
production was not measured in the Fort Rock area, it appears that removal of
the bitterbrush stimulated the grass production, particularly fescue (Festuca
tdahoensts). An increase in the overall competitive affect may have occurred.

Summary and Conclusions

Diameter growth as well as the volume growth of trees in thinned stands
are still responding to fertilization 8 years after treatment in three of
four study areas. Height growth is no longer responding to fertilization
in the Sumpter Valley and Road 1808 areas where significant differences
occurred during the first 4 years after treatment.

Shrub removal alone in the presence of fescue did not reduce competition
to overstory but produced a situation where fescue apparently was competitive
to tree growth.

This study shows that foresters can expect significant responses, visibly
noticeable on increment cores, to fertilization in thinned stands of pines
which will last at least 4 to 8 years.

Further work is underway to correlate response with kinds and amounts of
fertilizer and various kinds of soils in different plant communities. Other
work in preparation for publication suggests that P, S, and B in combination
with N do not stimulate volume growth in the field. For land managers wishing
to fertilize thinned ponderosa pine stands now, the tentative recommendation
of 200 pounds of elemental nitrogen per acre in the form of urea is offered.

Literature Cited

Barrett, James W.
1970. Ponderosa pine saplings respond to control of spacing and understory
vegetation. USDA For. Serv. Res. Pap. PNW-106, 16 p., tidus. Pac.
Northwest For. and Range Exp. Stn., Portland, Oregon.

Brickell, James E.
1970. Test of an equation for predicting bark thickness of western Montana
species. USDA For. Serv.) Res. Note INI-107, 7 po.) wltus. ointerntes kor.
and Range Exp. Stn., Ogden, Utah.

Gochran? Ps He
1973. Response of individual ponderosa pine trees to fertilization. USDA
For. Serv. Res. Note PNW-206, 15 p. Pac. Northwest For. and Range Exp.
Stn., Portland, Oregon.

Cochranka Rests
1976. Predicting wood volumes for ponderosa pine from outside bark measure-
ments. USDA For. Serv. Res. Note PNW-283, 8 p. Pac. Northwest For. and
Range Exp. Stn., Portland, Oregon.

mse of eastside pine poles to removal of low vegetation.

RAN 4 program for estimates of tree populations from 3P
asurements. USDA For. Serv. Res. Pap. PSW-13, 49 p.
For. and Range Exp. Stn., Berkeley, California.

GPO 999-073

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