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Forest Sci., Vol. 29, No. 1, 1983, pp; 127-137
Copyright 1983, by the Society o f American Foresters
L o n g - t e r m Growth Response o f Douglas-fir
to A m m o n i u m N i t r a t e Fertilizer
RICHARD E. M I L L E R
ROBERT F. TARRANT
ABSTRACT. The effect 0 f a single application o f a m m o n i u m nitrate fertilizer on diameter, height,
a n d v o l u m e growth o f a Site IV plantation o f Pseudotsuga menziesii was measured repeatedly
during a 15-year period. Fertilizer dosages o f 157, 314, a n d 471 kg N/ha increased gross v o l u m e
growth during the 15-year period by an average o f 5 I, 88, a n d 111 percent, respectively (67, 116,
and 146 m~/ha). Quadratic equations best described the relationship between fertilizer dosage
a n d gross and net v o l u m e growth during the eiatire period a n d during three intermediate periods.
V o l u m e growth on fertilized plots exceeded that on unfertilized plots in all periods. Response tO
all N dosages did not change significantly dUring the 15-year period o f obserVation. The positive,
10ng-term effect o f fertilizer on stand growth is attributed b o t h tO i m p r o v e d tree nutrition arid to
secondary effects o f increased tree growth on stand structure. FOREST SCl. 29:127-137.
ADDITIONAL KEY WORDS. PseudotsUga menziesiL forest fertilization, silviculture.
IN APRIL 1964, we a p p l i e d a m m o n i u m n i t r a t e to e x p e r i m e n t a l p i o t s in a 3 7 - y e a r o l d D o u g l a s - f i r [Pseudotsuga menziesii ( M i r b . ) FranCo] p l a n t a t i o n g r o w i n g o n Site
iV land in the W i n d River Experimental Forest near Carson, Washington. The
p u r p o s e o f t h e s t u d y w a s to o b s e r v e t h e e f f e c t i v e n e s s o f a c h e m i c a l fertilizer in
t h e s a m e D o u g l a s - f i r p l a n t a t i o n w h i c h h a d r e s p o n d e d s t r o n g l y to a b i o l o g i c a l
s o u r c e o f N ( T a r r a n t 1961). I n t h i s p a p e r , we r e p o r t a n d d i s c u s s a n u n u s u a l l y
s t r o n g effect o f a m m o n i u m n i t r a t e o n tree a n d s t a n d g r o w t h Over a 1 5 - y e a r p e r i o d
after t r e a t m e n t .
U n t i l r e c e n t l y , a m m o n i u m n i t r a t e h a s b e e n g e n e r a l l y i g n o r e d for forest use in
t h e D o u g l a s - f i r r e g i o n largely becaUse o f its l o w e r N c o n c e n t r a t i o n (34 p e r c e n t IN-)
a n d t h u s , g r e a t e r a p p l i c a t i o n cost vs. u r e a (46 p e r c e n t N ) . T h e few s t u d i e s i n
w h i c h a m m o n i u m nRrate h a d been used in Pacific N o r t h w e s t forest fertilization
e i t h e r i n v o l v e d t w o or m o r e fertilizers i n c o m b i n a t i o n (Gessel a n d o t h e r s 1969)
or e m p h a s i z e d effects o n c o n e p r o d u c t i o n (Ebell 1962). D u r i n g t h e t i m e t h i s s t u d y
has been underway, however, additional research has been reported (Harrington
a n d M i l l e r 1979, M i l l e r a n d H a r r i n g t o n 1979, M i l l e r a n d R e u k e m a 1974, Brix
1981). F u r t h e r , r e s e a r c h o v e r t h e p a s t d e c a d e Or m o r e ( M a r s h a l l a n d D e B e l l 1980)
i n d i c a t e s t h a t Under c e r t a i n c o n d i t i o n s , a m m o n i a c a n be v o l a t i l i z e d f r o m u r e a
a n d lost to t h e a t m o s p h e r e , t h u s l e s s e n i n g t h e cost a d v a n t a g e o f ux~ea.
S c a n d i n a v i a n foresters also h a v e d e v e l o p e d a s t r o n g i n t e r e s t in a m m o n i u m
n i t r a t e . I n n o r t h e r n l a t i t u d e s , p r i m a r i l y o n s h a l l o w , i n f e r t i l e m i n e r a l soils, fertili z i n g forests w i t h a m m o n i u m n i t r a t e p r o d u c e s g r e a t e r tree g r o w t h r e s p o n s e t h a n
t h a t g a i n e d f r o m u r e a ( B e n g t s o n 1977). As o f 1967, a l m o s t all forest fer/iliza'don
The authors are, respectively, .Principal S0il Scientist, U S D A Forest Service, Forestry SCiences
Laboratory, Olympia, Washington 98502, and Professor, D e p a r t m e n t o f Forest Science, Oregon State
University, Corvallis, Oregon 97331. The authors acknOwledge assistance o f J o h n H a z a r d and T i m o t h y
Max in statistical analyses and helpful technical reviews by D o n R e u k e m a , T i m o t h y Max, a n d anoriytri0us referees, Paper i628, Forest ResearCh Laboratory, Oregon State University. ManUSCript received 20 October 1981 .
.
.
.
VOLUMI~ 29; NLrMBER 1, 1983 / 1.2.7
FIGURE 1.
A n u n f e r t i l i z e d p l o t in t h e s t u d y a r e a (1971 p h o t o ) .
i n S w e d e n w a s w i t h u r e a , b u t b y 1 9 7 5 , a b o u t 80 p e r c e n t o f all N f e r t i l i z e r so u s e d
w a s a m m o n i u m n i t r a t e ( M o l l e r 1974).
MATERIALS
AND METHODS
Site.--The
s t u d y site (Fig. 1) w a s p l a n t e d i n 1929 w i t h 1-1 D o u g l a s - f i r
s e e d l i n g s f r o m a n o n l o c a l s e e d s o u r c e . I n i t i a l s p a c i n g w a s 2.4 b y 2.4 m (8 b y 8
ft), o r a b o u t 1,680 t r e e s / h a . A t f e r t i l i z a t i o n i n 1964, t h e 12 p l o t s i n t h e s t u d y
a v e r a g e d 1,547 t r e e s / h a , 123.2 m 3 / h a i n t o t a l s t e m v o l u m e a n d 2 6 . 2 m i n site
i n d e x ( 1 0 0 - y e a r base).
E l e v a t i o n o f t h e s t u d y a r e a is a b o u t 6 0 0 m e t e r s . A n n u a l p r e c i p i t a t i o n a v e r a g e s
a b o u t 2 , 5 0 0 m m , o n l y a q u a r t e r o f w h i c h falls d u r i n g A p r i l - O c t o b e r , t h e app r o x i m a t e growing season. P o t e n t i a l e v a p o r a t i o n greatly exceeds s u m m e r precipi t a t i o n , so v e g e t a t i o n at t h e s t u d y site a n n u a l l y u n d e r g o e s s e v e r a l m o n t h s o f
m o i s t u r e stress.
T h e u n c l a s s i f i e d soil h e r e is d e r i v e d f r o m e a r l y T e r t i a r y a n d e s i t i c o r r h y o d a c i t i c
rock. T h e gravelly l o a m surface h o r i z o n s also c o n t a i n p u m i c e a n d basaltic gravel
o f P l e i s t o c e n e o r i g i n . B e l o w 15- to 4 0 = c m d e p t h s , t h e soil is o f clay l o a m t e x t u r e
a n d h a s a s t r o n g , s u b a n g u l a r b l o c k y s t r u c t u r e . R o o t s a b o u n d i n t h e s u r f a c e layer,
b u t f e w e x t e n d b e l o w 1 m . T h e soil c o n t a i n s a p p r o x i m a t e l y 3 , 4 0 0 k g N / h a to a
d e p t h o f 1 m ( T a r r a n t a n d M i l l e r 1963), a b o u t a v e r a g e for f o r e s t l a n d s o f l o w to
m o d e r a t e p r o d u c t i v i t y in w e s t e r n W a s h i n g t o n .
Study
T r e a t m e n t s . - - A m m o n i u m n i t r a t e w a s b r o a d c a s t b y h a n d i n A p r i l 1964 o v e r 0 . 0 6 h a ( 0 . 1 5 - a c r e ) c i r c u l a r p l o t s a t r a t e s o f 157, 3 1 4 , a n d 471 k g / h a . T r e a t m e n t s ,
i n c l u d i n g unfertilized controls, were r e p l i c a t e d three t i m e s in a c o m p l e t e l y randomized experimental design.
- - D i a m e t e r at b r e a s t h e i g h t ( d b h ) w a s m e a s u r e d at 1.4 m a b o v e
g r o u n d o n all t r e e s 4.1 c m (1.6 i n c h e s ) a n d l a r g e r o n 0 . 0 2 - h a ( 0 . 0 5 - a c r e ) c i r c u l a r
Measurements.
128 / FOREST SCIENCE
T A B L E 1.
Fertilizer
application
(kg N / h a )
Average change in dbh during 15 years after fertilization.
Average dbh
1963
1978
Annual change'
......................................................................................
0
157
314
471
f
t
13.7
13.5
13.5
13.7
c m
18.5
19.3
22.1
23.1
Annual adjusted
growth of
surviving trees 2
.....................................................................................
0.32
0.39
0.57
0.63
0.28
0.38
0.47
0.56
' A v e r a g e d b h o f live trees in 1978 minus" a v e r a g e d b h o f l i v e t r e e s in 1963; w e i g h t e d a v e r a g e (by
b a s a l area) o f t h r e e p l o t s p e r t r e a t m e n t .
z A v e r a g e d b h g r o w t h o f s u r v i v i n g t r e e s in e a c h t r e a t m e n t w a s a d j u s t e d f o r d i f f e r e n c e s a m o n g t h e
t r e a t m e n t s in initial d b h . T h e a d j u s t e d m e a n s ' w e r e l i n e a r l y r e g r e s s e d o v e r N d o s a g e ; the q u a d r a t i c
r e s p o n s e w a s n o n s i g n i f i c a n t ( P < 0.425).
plots c e n t e r e d w i t h i n each t r e a t m e n t area. T o t a l h e i g h t was d e t e r m i n e d on a
s u b s a m p l e o f 14 to 17 t r e e s p e r p l o t ( p l o t s a v e r a g e d 30 t r e e s i n 1963). T h e s e t r e e s
were d i s t r i b u t e d over the d b h range, b u t t w o - t h i r d s of the s u b s a m p l e h a d largerthan-average dbh. First measurements of both height and dbh were made after
t h e 1963 g r o w i n g s e a s o n a n d b e f o r e f e r t i l i z a t i o n . A f t e r - t r e a t m e n t m e a s u r e m e n t s
w e r e m a d e a f t e r y e a r 2, t h e n at 1- t o 4 - y e a r i n t e r v a l s t h r o u g h 1 9 7 8 , 15 y e a r s a f t e r
fertilization.
Volume Growth Determination. m T o t a l s t e m v o l u m e , i n c l u d i n g t o p a n d s t u m p o f
e a c h h e i g h t - s a m p l e t r e e w a s e s t i m a t e d f r o m a r e g i o n a l v o l u m e e q u a t i o n for D o u g las-fir ( B r u c e a n d D e M a r s 1974). T h e s e s a m p l e t r e e v o l u m e s w e r e t h e n u s e d t o
c a l c u l a t e a s e p a r a t e v o l u m e e q u a t i o n f o r e a c h p l o t a n d m e a s u r e m e n t year. T h e
form of the equation was
log V---- a +
blogD
where g = cubic-foot volume of total stem and D = dbh (inches). The volume of
e a c h t r e e i n e a c h m e a s u r e m e n t y e a r w a s c o m p u t e d b y s p e c i f i c e q u a t i o n s for e a c h
plot a n d year. T h e difference b e t w e e n the c o m b i n e d v o l u m e o f live a n d o f d e a d
t r e e s at o n e i n v e n t o r y a n d t h e v o l u m e o f t h e l i v e s t a n d at t h e p r e v i o u s i n v e n t o r y
is g r o s s g r o w t h . T h e d i f f e r e n c e i n v o l u m e o f l i v e t r e e s at s u c c e s s i v e i n v e n t o r i e s
is n e t g r o w t h m g r o s s g r o w t h m i n u s t h e t o t a l v o l u m e i n t r e e s t h a t d i e d b e t w e e n
inventories.
Statistical A n a l y s e s . - - A n a l y s i s o f c o v a r i a n c e w i t h o r t h o g o n a l p o l y n o m i a l s w a s
used to e s t i m a t e a r e s p o n s e surface or c u r v e s o f c h a n g e in a v e r a g e dbh, height,
T A B L E 2.
ization.
Average change in n u m b e r s o f live trees during 15 years after fertil-
Fertilizer
application
(kg N / h a )
1963
.................................................................
0
157
314
471
1,515
1,515
1,614
1,549
:
1978
l i v e t r e e s / h a
1,087
1,137
1,038
907
Loss
...................................................................
428
378
576
642
VOLUME 29, NUMBER 1, 1983 / 129
T A B L E 3. Average annual height growth o f surviving trees during 15 years after
fertilization.
Fertilizer
application
(kg N / h a )
0
157
314
471
Average height
1963
!
~,
Adjusted average heights 2
1978
Height growth
m
m
m/yr
13.4
13.0
13.8
13.7
18.1
19.0
21.3
22.6
0.31
0.40
0.50
"0 . 5 9
Gain from treatment
• m/yr
percent
-0.09
0.19
0.28
-30
61
90
' T r e a t m e n t averages are based on 3 0 - 3 4 trees.
2 T r e a t m e n t a v e r a g e s w e r e a d j u s t e d f o r i n i t i a l d i f f e r e n c e s in h e i g h t t h e n l i n e a r l y r e g r e s s e d o v e r N
dosage; the q u a d r a t i c response surface was n o n s i g n i f i c a n t (P < 0.228).
a n d s t a n d c u b i c v o l u m e d u r i n g t h e 1 5 - y e a r p e r i o d . T h e c o v a r i a t e for c o m p u t i n g
a d j u s t e d t r e a t m e n t m e a n s w a s i n i t i a l d b h , h e i g h t , or v o l u m e , r e s p e c t i v e l y . T o
c o m p a r e t r e n d s o f g r o w t h , t h e 1 5 - y e a r p e r i o d w a s d i v i d e d i n t o p e r i o d s o f 5, 4,
a n d 6 years; t h e a v e r a g e a n n u a l v o l u m e g r o w t h d u r i n g e a c h o f these p e r i o d s w a s
c o m p a r e d as t h e s p l i t - p l o t effect in c o v a r i a n c e a n a l y s e s . U s i n g t i m e - a f t e r - t r e a t m e n t as t h e s p l i t - p l o t is suggested b y Bliss (1967). I n all a n a l y s e s , we u s e d t h e
5 - p e r c e n t p r o b a b i l i t y level to j u d g e s t a t i s t i c a l s i g n i f i c a n c e o f results; we also offer
t h e a c t u a l level o f significance for e a c h test.
RESULTS AND DISCUSSION
D i a m e t e r Growth. m D u r i n g t h e 1 5 - y e a r p e r i o d a f t e r t h e fertilizer a p p l i c a t i o n , increases in s t a n d a v e r a g e d b h a n d g r o w t h i n d b h o f s u r v i v i n g trees were l i n e a r l y
r e l a t e d to t h e a m o u n t o f N a p p l i e d ( T a b l e 1). P a r t o f t h e i n c r e a s e in a v e r a g e d b h
o f b o t h t h e u n f e r t i l i z e d a n d t h e e s p e c i a l l y h e a v i l y f e r t i l i z e d s t a n d s was d u e to
m o r t a l i t y (Table 2). M o r t a l i t y n o t o n l y p r o v i d e d m o r e g r o w i n g space for s u r v i v i n g
trees, w h i c h i n c r e a s e d t h e i r rate o f g r o w t h , b u t it also r e m o v e d m a n y s m a l l e r t h a n - a v e r a g e trees w h i c h a r i t h m e t i c a l l y i n c r e a s e d a v e r a g e d b h o f t h e s t a n d . F o r
e x a m p l e , m o r e t h a n twice as m a n y trees o f b e l o w - a v e r a g e d b h d i e d d u r i n g t h e
first 7 y e a r s after f e r t i l i z a t i o n w i t h 314 a n d 471 kg N / h a ( M i l l e r a n d P i e n a a r 1973).
T A B L E 4. Average annual gross growth a n d mortality by treatment during 15
years after fertilization.
Fertilizer
application
(kg N / h a )
Gross growth
Initial
volume
Unadjusted
.....................................................................................
0
157
314
471
119.4
119.6
131.5
122.3
8.6
12.2
17.9
t8.I
Mortality
Adjusted'
mJ/ha
Unadjusted
Adjusted
2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~......
8.8
13.2
16.5
18.4
2.0
1.9
1.9
2.9
1.9
1.8
2.1
2.1
' T r e a t m e n t .averages w e r e a d j u s t e d f o r i n i t i a l d i f f e r e n c e s in s t a n d v o l u m e a n d s m o o t h l y c u r v e d
o v e r N dosage. The covariate a n d the q u a d r a t i c r e s p o n s e surface for gross growth were significant:
P < 0.000 and 0.039, respectively. T h o s e for net g r o w t h were also significant: P < 0.000 a n d 0.034,
respectively.
"
2 T o t a l s t e m v o l u m e o f all t r e e s 4. l - c m d b h a n d l a r g e r .
1 3 0 / FOREST SCIENCE
4" - A d j u s t e d
g r o s s m e a n -- 8 . 7 3 4" 0 . 0 3 3 N
- 0.000025N
* - A d j u s t e d n e t m e a n -" 6 . 8 3 4 - 0 . 0 3 5 N
- 0.000035N
Net
. . . .
=.
¢l
"6
i
>
cO
~
~2z- -.-2 *f,r,-Z; io"o--'G
@
-_.--._-
....
-_-_--::__-_-__-_:.. --
8
-.-%g.-. .
.
.
.
-:--.:.:
c
......
i-.. -
.
.
;....-2
:_--.y-._
. : - . -_::
f,
....
!':.":'-"
.-.--.:
?.:_-':.
.....
a=°:°° -
"---'-'.'.t .',- ~-'.'2 ..--.---.-_-
-.-.
".'--'?'-'."
,.ooB.
:-:..¢,-:,__,. ~.:-.-'..-:--:2.--:
":...-._ ;-.-;.-! : -'.';._."
. ....
:':'::::
:_::.:::. !::.:':3.:
,-::-.-- "-.-?:~-
:._.._.:....-.:._-...
~.:~.;:
. . . . . . .
i'-'N
Plot ~
7
1
0
5
3
--:---- - ""-::
9
157
8
2
12
314
11
4
6
471
10
Nitrogen applied (kg/ha)
FIGURE 2. A v e r a g e a n n u a l g r o s s a n d n e t v o l u m e g r o w t h d u r i n g a 1 5 - y e a r p e r i o d b y p l o t s a n d
treatment; plots are arranged within treatment in ascending order of initial stand volume.
A v e r a g e a n n u a l d b h g r o w t h o f s u r v i v i n g t r e e s w a s d o u b l e d b y t h e 471 k g
N / h a t r e a t m e n t , p r e s u m a b l y a r e s u l t o f t h e c o m b i n e d effect o f r e d u c e d n u m b e r
o f s t e m s (or r e d u c e d c o m p e t i t i o n ) a n d o f i m p r o v e d n u t r i t i o n a f f o r d e d b y t h e
highest N dosage. A l t h o u g h the r e s p o n s e per p o u n d o f N a p p e a r e d greatest at the
157-kg-N/ha dosage, the response surface was not significantly curvilinear. Thus,
e v e n t h e h e a v i e s t f e r t i l i z e r a p p l i c a t i o n a p p a r e n t l y w a s n o t s u f f i c i e n t l y l a r g e to
establish the dosage which would provide maximum dbh growth.
HeightGrowth.--Average
h e i g h t g r o w t h o f s u r v i v i n g f e r t i l i z e d t r e e s w a s 30 t o 90
p e r c e n t g r e a t e r t h a n t h a t o f u n f e r t i l i z e d t r e e s d u r i n g t h e 1 5 - y e a r p e r i o d ( T a b l e 3).
M o r t a l i t y o r d a m a g e to n o n s u r v i v i n g t r e e s d i d n o t r e m o v e m o r e s m a l l e r - t h a n average height trees on the fertilized plots a n d t h u s di d not a r i t h m e t i c a l l y exagg e r a t e t h e g r e a t e r h e i g h t g r o w t h a f t e r f e r t i l i z a t i o n . A g a i n , as w i t h d i a m e t e r , g r o w t h
w a s l i n e a r l y r e l a t e d to t h e a m o u n t o f N a p p l i e d ( P < 0 . 0 0 1 ) .
Volume Growth (Gross).--During
t h e 15 y e a r s a f t e r f e r t i l i z a t i o n , t o t a l c u b i c v o l u m e g r o w t h p e r h e c t a r e o f all t r e e s 4 . 1 - c m d b h a n d l a r g e r w a s p o s i t i v e l y r e l a t e d
to t h e a m o u n t o f N a p p l i e d ( T a b l e 4).
G r o s s g r o w t h o n f e r t i l i z e d p l o t s a v e r a g e d 50 to 109 p e r c e n t g r e a t e r t h a n t h a t
o n c o n t r o l plots; c o r r e s p o n d i n g g a i n s i n v o l u m e g r o w t h d u r i n g t h e 1 5 - y e a r p e r i o d
a v e r a g e d 6 7 . 4 to 145.5 m 3 / h a . T h e r e s p o n s e s u r f a c e w a s s i g n i f i c a n t l y c u r v i l i n e a r
(P < 0.039). T h i s fall-off in v o l u m e r e s p o n s e at the highest N dosage p r o b a b l y
reflects t h e c o m b i n e d effect o f (1) s m a l l b u t s t a t i s t i c a l l y n o n s i g n i f i c a n t f a l l - o f f i n
b o t h d b h a n d h e i g h t r e s p o n s e o f s u r v i v i n g t r e e s w i t h 471 kg N / h a a n d (2) t h e
g r e a t e r n u m b e r o f t r e e s l o s t to m o r t a l i t y ( T a b l e 2 ) I a r g e l y f r o m i n c r e a s e d w i n t e r
d a m a g e i n t h e first f e w y e a r s a f t e r f e r t i l i z a t i o n ( M i l l e r a n d P i e n a a r 1 9 7 3 ) .
Average annual volume growth during the 15-year period varied among the
t h r e e r e p l i c a t i o n s o f e a c h t r e a t m e n t (Fig. 2). S o m e Of t h i s w i t h i n - t r e a t m e n t v a r i -
VOLUME 29, NUMBER 1, 1983 / 131
500
cm
4.1
450
14.2
400
I
19.3
24.4
350
I
29.5
e4~a~
-
-
I
I
¢o 3 0 0
•; ; : . - ~
. . . . .,.--m--~
N~....--~
~-~
250
~o:.-'.:..~;:-'~;~qt~.~
.C-.-'-.
,.'4":'0?8
.'~%-:
. . . . . i~:.~tt.~:5.'
:--y
"--2"-'.."
200
+..,
>,
150
•.~ ..-~ ;~'.'+,~'
'i:q"ii N \\ NN
::-.:."
.++.
-, ".'.~Sw
100
\'4
\q
\ \
50
3
Plot
0
9
157
8
2
12
314
,j
\\1
11
4
\
\
\
\
\
\
\
\
\k \
6
10
471
Nitrogen applied (kg/ha)
FIGURE 3. V o l u m e o f t r e e s e x c e e d i n g s p e c i f i e d d b h a t a g e 52 y e a r s b y p l o t a n d t r e a t m e n t , U n a d j u s t e d
for initial diffferences before fertilization.
a t i o n i n growth w a s e v i d e n t l y r e l a t e d t o d i f f e r e n c e s i n b e g i n n i n g v o l u m e ; r e p l i c a t e s
with greatest initial volume also had greatest growthi Covariance analysis substantiated that growth was positively related to stand volume (for gross and net
growth, P < 0.000).
V o l u m e G r o w t h (Net). -- D e p e n d i n g o n t h e a m o u n t o f N a p p l i e d , a d j u s t e d
v o l u m e g r o w t h o n f e r t i l i z e d p l o t s a v e r a g e d 4 . 5 t o 8.6 m 3 / h a / y r o r 65 t o
p e r c e n t m o r e t h a n o n c o n t r o l p l o t s d u r i n g t h e 15 y e a r s a f t e r f e r t i l i z a t i o n .
response surface was significantly curvilinear (P < 0.034), again indicating
the dosage for maximum
v o l u m e p r o d u c t i o n d u r i n g t h e first 15 y e a r s w a s
t h a n 4 7 1 k g N / h a (Fig. 2).
net
125
The
that
less
Current Volume By Tree Size.--Fifteen years after fertilization, and without adjustment for relatively small differences in tree sizes and numbers prior to fertili z a t i o n , t o t a l s t a n d v o l u m e o n f e r t i l i z e d p l o t s a v e r a g e d 25 t o 69 p e r c e n t g r e a t e r
than that on control plots; moreover, a much larger proportion of this volume
w a s i n l a r g e r d b h c l a s s e s (Fig. 3). F o r e x a m p l e , a n a v e r a g e o f 2 4 a n d 27 p e r c e n t
of total stand volume on plots fertilized with 314 and 471 kg N/ha, respectively,
was in trees of 29.5-cm (11.6-inches) and larger dbh. In contrast, 0-N and
157-N plots contained no volume in these sawtimber-size classes. Clearly, fertilization increased merchantable yields and economic value of this Douglas-fir
plantation. Moreover, the trends indicate that further enhancement of both stand
volume and value are likely.
G r o w t h R e s p o n s e T r e n d s . - - A l t h o u g h t h e t r e e s w e r e n o t r e m e a s u r e d a f t e r t h e first
growing season, increment cores revealed a definite response in dbh growth during
132 / FOREST SCIENCE
J¢
30
~'~ 25C
E 2o
L_
o
c
""15
a
¢1
¢
¢
157
10-o
0
0
a n
L_
a.
5-
%
65
66
67
68
69
70
Measurement
72
74
78
Years
Trends of average gross periodic annual growth, all species trees 4.1-cm dbh and larger.
Unadjusted for initial differences before fertilization.
FIGURE 4.
t h e 1st y e a r a f t e r f e r t i l i z a t i o n . T w o g r o w i n g s e a s o n s a f t e r t r e a t m e n t , a s t a t i s t i c a l l y
s i g n i f i c a n t r e s p o n s e i n v o l u m e g r o w t h w a s e v i d e n t f o r all l e v e l s o f a m m o n i u m
n i t r a t e ( M i l l e r a n d P i e n a a r 1973).
T h e 1 st y e a r r e s p o n s e o f t h i s s t a n d t o f e r t i l i z a t i o n is l i k e l y r e l a t e d t o i m p r o v e d
p h o t o s y n t h e t i c activity per unit o f foliage r a t h e r t h a n to i n c r e a s e d m a s s o f foliage
w h i c h w a s still e v i d e n t 15 y e a r s a f t e r f e r t i l i z a t i o n . H . B r i x 1 h a s c o n c l u d e d f r o m
h i s i n v e s t i g a t i o n s o f N f e r t i l i z a t i o n o f D o u g l a s - f i r t h a t b o t h p h o t o s y n t h e t i c efficiency and the amount and distribution of foliage in the crowns can be affected
b y f e r t i l i z a t i o n . O n N - d e f i c i e n t sites, B r i x f o u n d t h a t i n c r e a s e d g r o w t h d u r i n g t h e
first 2 - 3 y e a r s a f t e r f e r t i l i z a t i o n w a s r e l a t e ~ m a i n l y to i m p r o v e m e n t o f f o l i a g e
efficiency. T h e r e f o r e , f o l i a g e e f f i c i e n c y d e c l i n e d t o t h a t o f u n t r e a t e d l e v e l s , o r e v e n
less, b y y e a r 7. T h e g r o w t h r e s p o n s e w h i c h still e x i s t e d w a s c a u s e d s o l e l y b y t h e
difference in foliage mass.
D u r i n g t h e 15 y e a r s a f t e r f e r t i l i z a t i o n , v o l u m e r e s p o n s e t o 157 kg N / h a w a s
n o t as r a p i d n o r a s . l a r g e as t h a t a f t e r 3 1 4 o r 471 kg N / h a (Fig. 4). M o r e o v e r ,
v o l u m e r e s p o n s e to 157 kg N / h a m a y h a v e p e a k e d at y e a r 3 o r a b o u t 2 y e a r s
b e f o r e p e a k r e s p o n s e to h i g h e r N d o s a g e s .
T h e s e g r o w t h t r e n d s f r o m m e a s u r e m e n t s at 1- t o 4 - y e a r i n t e r v a l s , h o w e v e r ,
c o n t a i n erratic fluctuations a n d do n o t reflect initial differences in stand v o l u m e
before fertilization. To facilitate statistical analysis of these trends, we c o m b i n e d
data f r o m several short intervals b e t w e e n m e a s u r e m e n t s to d i v i d e the 15-year
p e r i o d i n t o 4- t o 6 - y e a r parts. I n a s u b s e q u e n t c o v a r i a n c e a n a l y s i s i n w h i c h t i m e a f t e r - t r e a t m e n t w a s t r e a t e d as a s p l i t - p l o t effect a n d s t a n d v o l u m e p r i o r to fertilization was the covariate, the r e s p o n s e surface was q u a d r a t i c (P < 0.045). M o r e over, both period and period-X-treatment interaction were nonsignificant (P <
0.069 and P < 0.191, respectively). This indicated that response trends of the
t h r e e d o s a g e s d i d n o t v a r y s i g n i f i c a n t l y o v e r t i m e (Fig. 5A).
i Personal communication with H. Brix, Canadian Forestry Service, Victoria, B.C., April 1982.
VOLUME 29, NUMBER 1, 1983 / 133
21.0
A
14.0
kg/ha
.................
.
.
.
.
"-~
.
47 1
31o,
.
..................
E
7.0
N
~ "
157
0
rE
IlJ
I
5
0.C
I
I
10
15
¢J
i
;~ 2 1 . 0
cc,,,-
B
.ca
0
14.0
~.~...~L-L
~.-~.. -.._.
Ik=
a.
FIGURE 5. T r e n d s in a d j u s t e d gross p e r i o d i c
a n n u a l i n c r e m e n t b y t r e a t m e n t : (A) P.A.I. in
7.0
0.c
Years
I
I
I
5
10
15
after fertilization
each period adjusted to a common 1963 (initial) volume; (B) P.A.I. in each period adjusted to a common volume at the start of
each period.
D u r a t i o n o f G r o w t h R e s p o n s e . - - R e s p o n s e is c l e a r l y e v i d e n t d u r i n g t h e 15 y e a r s
a f t e r t r e a t m e n t (Fig. 4). I n y e a r s 10 t h r o u g h 15 a f t e r f e r t i l i z a t i o n , t h e g a i n i n g ro s s
v o l u m e g r o w t h a v e r a g e d 34, 93, a n d 107 p e r c e n t , r e s p e c t i v e l y , w i t h i n c r e a s i n g N
dosage.
Based on the trends of past growth and the more vigorous crowns observed on
f e r t i l i z e d t r e e s a f t e r y e a r 15, w e b e l i e v e r e s p o n s e w i l l c o n t i n u e b e y o n d t h e p e r i o d
we have observed. Although increased N dosage increased the amount of response
during the 15-year period, we must await future measurement to determine whether d u r a t i o n o f r e s p o n s e will v a r y by N dosage.
Dosage Efficiency.--Although the response surface computed for the three periods
after fertilization has b e e n c u r v i l i n e a r , the m o s t efficient dosage (that p r o v i d i n g
the greatest amount of additional wood per kilogram N) may have shifted slightly
t o w a r d t h e h i g h e r a p p l i c a t i o n r a t e s (Fig. 6). B a s e d o n c u m u l a t i v e g ro ss v o l u m e
g r o w t h d u r i n g t h e first 5 y e a r s a f t e r t r e a t m e n t , t h e m o s t e f f i c i e n t d o s a g e f o r v o l u m e
r e s p o n s e w a s a b o u t 157 kg N / h a . B y t h e s e c o n d a n d t h i r d p e r i o d s a f t e r f e r t i l i z a t i o n ,
h o w e v e r , t h e e f f i c i e n t d o s a g e o f a m m o n i u m n i t r a t e w a s n e a r 3 1 4 kg N / h a .
It o b v i o u s l y w o u l d b e p r e m a t u r e to d r a w a c o n c l u s i o n as to b e s t d o s a g e b e f o r e
t h e t r e a t m e n t effect h a s c e a s e d . T h e e f f i c i e n t d o s a g e f o r v o l u m e p r o d u c t i o n o r
gain per kilogram N clearly depends on the period of observation and the duration
of response. Moreover, the most efficient dosage for maximizing the net economic
r e t u r n s f r o m f e r t i l i z a t i o n w i l l f u r t h e r d e p e n d o n t r e e size a n d q u a l i t y o f t h i s
increased volume and on the fertilization costs per kilogram N.
R e l a t i v e I n f l u e n c e o n G r o w t h - - I m p r o v e d N u t r i t i o n vs. A l t e r e d S t o c k i n g . - - T o u n d e r s t a n d t h e n a t u r e o f r e s p o n s e t o f e r t i l i z e r , w e b e l i e v e it n e c e s s a r y t o r e c o g n i z e
the interaction between improved tree nutrition and alteration in stand stocking
after supplemental nutrients have been added. Fertilizer can temporarily reduce
'"
134 / FOREST SCIENCE
Years
300
~
200
•
O
¢~
-"'-
""-"
1-9"
100
1-,.5
!
157
I
I
314
471
Nitrogen ( k g / h a )
FIGURE 6. A v e r a g e c u m u l a t i v e gross g r o w t h at specified y e a r s after fertilization. A d j u s t e d for differences in initial s t a n d v o l u m e b e f o r e fertilization.
o r e l i m i n a t e n u t r i e n t d e f i c i e n c i e s t h a t l i m i t t r e e g r o w t h ( n u t r i t i o n a l effect) a n d
c a n also p e r m a n e n t l y a l t e r t h e n u m b e r a n d size o f t r e e s i n t h e s t a n d ( s t o c k i n g
effect). C h a n g e s i n s t a n d s t o c k i n g t h a t m a y r e s u l t f r o m f o r e s t f e r t i l i z a t i o n a r e m o r e
s u b t l e t h a n t h o s e c r e a t e d b y p h y s i c a l t h i n n i n g , y e t t h e c o n s e q u e n c e s are t h e s a m e - a n e w s t a n d s t r u c t u r e is c r e a t e d a n d t h e c a p a c i t y f o r f u t u r e g r o w t h is affected.
I n o u r s t u d y area, b i o l o g i c a l c h a n g e s i n d u c e d b y f e r t i l i z a t i o n a f f e c t e d t r e e g r o w t h
rate and, therefore, c o m p e t i t i v e r e l a t i o n s b e t w e e n trees. Large trees r e s p o n d e d in
d b h g r o w t h m o r e strongly t h a n d i d s m a l l trees. F u r t h e r , in plots treated with 314
o r 471 k g N / h a , m o r e t h a n t w i c e as m a n y t r e e s o f b e l o w - a v e r a g e d b h d i e d d u r i n g
t h e first 7 y e a r s a f t e r f e r t i l i z a t i o n . T h i s t h i n n i n g effect a t t h i s l o c a t i o n w a s d u e
p r i m a r i l y to i n c r e a s e d s n o w a n d ice breakage, n o t to s u p p r e s s i o n (Miller a n d
P i e n a a r 1973). B e c a u s e o f t h e s e c h a n g e s , t h e v a r i o u s l y t r e a t e d s t a n d s b e c a m e
d i f f e r e n t i n n u m b e r a n d size o f t r e e s w h i c h a f f e c t e d s u b s e q u e n t s t a n d v o l u m e
T A B L E 5:
Split-plot covariance analysis.
Source of variation
W h o l e plot e r r o r
Regression
Treatment
Linear
Quadratic
Cubic
Split p l o t e r r o r
Regression
Period
P×T
•
D.F.
P<
7
1
0.000
I
1
1
.000
.206
.082
15
1
.2
.6
.458
.219
.171
VOLUME 29, NUMBER 1, 1983 / 135
g r o w t h i n a m a n n e r i n d e p e n d e n t o f c u r r e n t n u t r i t i o n a l s t a t u s o f t h e trees. T h u s ,
i f w e a r e to u n d e r s t a n d t h e g r o w t h r e s p o n s e f r o m a l t e r e d n u t r i t i o n a l s t a t u s vs.
that from physical changes in stand structure, the two sources of altered production
must be considered separately.
In the plantation we studied, growth on the variously treated plots was positively
r e l a t e d t o s t a n d v o l u m e b e f o r e f e r t i l i z a t i o n (Fig. 2). L i k e w i s e , t h e g r e a t e r v o l u m e
growth attained in fertilized stands presumably resulted in part from progressively
g r e a t e r s t a n d , ¢ o l u m e o r t r e e size i n t h e s e s t a n d s . T h e r e f o r e , for e a c h o f t h e t h r e e
g r o w t h p e r i o d s o f t h e t o t a l 15 y e a r s w e a d j u s t e d a v e r a g e p e r i o d i c g r o w t h o f e a c h
t r e a t m e n t t o a n a v e r a g e v o l u m e o f all t r e a t m e n t s at t h e b e g i n n i n g o f e a c h p e r i o d
(Fig. 5B). T h u s , w e m a t h e m a t i c a l l y r e m o v e d s o m e o f t h e effects o f g r e a t e r cum u l a t i v e s t a n d v o l u m e i n f e r t i l i z e d p l o t s . T h e " r e s i d u a l , " t h e n , is c o n s i d e r e d to
b e t h e c o n t r i b u t i o n o f c o n t i n u e d i m p r o v e d n u t r i t i o n o n f e r t i l i z e d plots. F o r m o f
t h e s p l i t - p l o t c o v a r i a n c e a n a l y s i s is s h o w n i n T a b l e 5.
Results of this analysis support three inferences:
(1) G r o w t h w a s l i n e a r l y r e l a t e d t o N d o s a g e , b o t h w h e n t h e a v e r a g e a n n u a l
v o l u m e g r o w t h for e a c h N d o s a g e w a s p o o l e d f o r all p e r i o d s a n d w h e n g r o w t h
w a s a d j u s t e d to t h e s a m e s t a n d v o l u m e at t h e s t a r t o f e a c h p e r i o d . T h i s i n d i c a t e s
that the greater cumulative tree volume on fertilized plots only partially explained
the long-continued growth response.
In previous covariance analyses of gross volume growth during the 15-year
p e r i o d (Figs. 2, 5A), t h e r e s p o n s e s u r f a c e w a s q u a d r a t i c w h e n t h e t r e a t m e n t m e a n s
were a d j u s t e d only for prefertilization differences in stand v o l u m e . W e attribute
the linear r e s p o n s e surface in the s u b s e q u e n t analysis to o u r h a v i n g r e m o v e d the
effects o f c h a n g e s i n c u m u l a t i v e l i v e s t a n d v o l u m e t h a t o c c u r r e d a f t e r f e r t i l i z a t i o n .
Thus, growth after the 314-kg N/ha dosage was mathematically adjusted downward in the second and third measurement periods because cumulative live stand
v o l u m e w a s h i g h e s t a f t e r t h a t t r e a t m e n t . F o r t h e 471 -kg N / h a t r e a t m e n t , h o w e v e r ,
g r o w t h was a d j u s t e d u p w a r d in these s a m e p e r i o d s because o f greater losses in
t r e e n u m b e r s a n d v o l u m e f o l l o w i n g t h i s t r e a t m e n t ( T a b l e 4).
(2) G r o w t h d i d n o t differ s i g n i f i c a n t l y a m o n g p e r i o d s ( P < 0 . 2 1 9 ) w h e n a v e r a g e
g r o w t h o f all t r e a t m e n t s w a s p o o l e d f o r e a c h p e r i o d .
(3) T h e s h a p e o f t h e r e s p o n s e s u r f a c e d i d n o t c h a n g e s i g n i f i c a n t l y d u r i n g t h e
15-year p e r i o d o f o b s e r v a t i o n (P × T i n t e r a c t i o n w a s s t a t i s t i c a l l y n o n s i g n i f i c a n t ) .
W e i n t e r p r e t this to m e a n that i m p r o v e d n u t r i t i o n c o n t r i b u t e d significantly to
r e s p o n s e i n all t h r e e m e a s u r e m e n t p e r i o d s .
CONCLUSIONS
A p p l y i n g a m m o n i u m nitrate fertilizer to a 3 5 - y e a r - o l d Douglas-fir p l a n t a t i o n
resulted in significant increases in diameter, height, and volume growth. Response
t o o n e a p p l i c a t i o n o f 157; 314, a n d 471 kg N / h a w a s still e v i d e n t 15 y e a r s a f t e r
t r e a t m e n t . D u r i n g t h e 10- t o 1 5 - y e a r p e r i o d a f t e r t r e a t m e n t , f e r t i l i z e d s t a n d s w e r e
still p r o d u c i n g 35 t o 107 p e r c e n t m o r e c u b i c v o l u m e t h a n w e r e u n f e r t i l i z e d s t a n d s .
Both the trend of response data and the vigorous crowns observed on fertilized
t r e e s s u g g e s t to us t h a t g r o w t h r e s p o n s e w i l l c o n t i n u e b e y o n d t h e 15 y e a r s t h u s
far o b s e r v e d .
A q u a d r a t i c e q u a t i o n b e s t d e s c r i b e d t h e r e s p o n s e s u r f a c e f o r gross a n d n e t
v o l u m e g r o w t h b y 5, 9, a n d 15 y e a r s a f t e r f e r t i l i z a t i o n . U n t i l g r o w t h r e s p o n s e h a s
c e a s e d , h o w e v e r , a r e l i a b l e e s t i m a t e o f o p t i m u m l e v e l o f a p p l i c a t i o n for v o l u m e
p r o d u c t i o n c a n n o t b e offered.
T h e l o n g - t e r m effects o f a s i n g l e f e r t i l i z a t i o n a t t h i s l o c a t i o n a r e e x p l a i n e d b y
initial and continued improvement in tree nutrition and by major changes in
stand stocking and structure that resulted from this improved nutrition.
13 6 / FOREST SCIENCE
In view of the unusually strong and long-continuing response of Douglas-fir to
ammonium
n i t r a t e f e r t i l i z e r at t h i s s t u d y a r e a , it a p p e a r s n e c e s s a r y h e r e a n d
e l s e w h e r e i n t h e P a c i f i c N o r t h w e s t to s y s t e m a t i c a l l y c o m p a r e t h e e f f i c i e n c y o f
various fertilizer N sources.
LITERATURE
CITED
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N e t h e r l a n d s . 32 p.
BLmS, C . I . 196~: Statistics in B i o l o g y . V o l 1. M c G r a w - H i U , N e w Y o r k . 558 p.
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S e r v R e s N o t e P N W - 2 3 9 , 5 p.
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n i t r o g e n fertilizer in w e s t e r n W a s h i n g t o n . T h e s e c o n d r e p o r t , C o i l F o r R e s o u r R e s Bull, U n i v
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losses o f n i t r o g e n f o l l o w i n g u r e a f e r t i l i z a t i o n o f f o r e s t soils. C a n J Soil Sci 6 0 : 5 4 9 - 5 6 3 .
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in a n 8 0 - y e a r - o l d D o u g l a s - f i r s t a n d . U S D A F o r e s t S e r v R e s N o t e P N W - 3 3 0 , 5 p.
MmLER, R. E., a n d L. V. PIENAAR. 1973. S e v e n - y e a r r e s p o n s e o f 3 5 - y e a r - o l d D o u g l a s - f i r to n i t r o g e n
fertilizer. U S D A F o r e s t S e r v R e s P a p P N W - 1 6 5 , 23 p.
MILLER, R. E., a n d D. L. REUKEMA. 1974. S e ' ~ e n t y - f i v e - y e a r - o l d D o u g l a s - f i r o n h i g h - q u a l i t y site
r e s p o n d to n i t r o g e n fertilizer. U S D A F o r e s t S e r v R e s N o t e P N W - 2 3 7 , 8 p.
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TARRANT, R. F. 1961. S t a n d d e v e l o p m e n t a n d soil fertility in a D o u g l a s - f i r - r e d a l d e r p l a n t a t i o n .
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a p l a n t a t i o n o f r e d a l d e r a n d D o u g l a s - f i r . Soil Sci S o c A m P r o c 2 7 : 2 3 1 - 2 3 4 .
VOLUME 29, NUMBER 1, 1983 / 137