Reprinted from
ECOLOGY,
VoZ.
46,
No. 5.
Late Summer
1965.
Purchased by the Forest Service, U.S. Department of Agriculture,
for official use.
DIURNAL AND SEASONAL PATTERNS OF NET ASSIMILATION IN DOUGLAS-FIR, PSEUDOTSUGA lYIENZTESll (MIRB.) FRANCO, AS INFLUENCED BY ENVIRONMENT JOHN
A.
HELMS
School of Torrstr:;', University of California, Bc/'kcley, California
AI'straet. A 2-year study on net assimilation was carried out in a 38-year-old natural stand
of Douglas-fir. Five trees in each of the dominant, co-dominant, and suppressed crown classes
were studied using the cuvette method on intact branches and measuring the CO2 exchange
with an infrared gas analyzer. Light intensity, air temperature, and relative humidity were
monitored using selenium photocells, thermocouples, and a 2-m\', 24-line recorder. The net
gain in photosynthesis in 1962 was found to be two to three times that in the dder year of
1961. Expressed pel' unit weight of dry foliage pel' hour, suppressed foliage possessed higher
photosynthetic efficiency than co-dominants, which in turn were slightly more efficient than
dominants.
Douglas-fir could photosynthesize at low light intensities.
The CO2 compensation point
was commonly as low as 10 ft-c, and maximum rates of net assimilation wcre attained at 800 ft-c.
Net assimilation could not be predicted from specific levels of air temperature, light intensity,
or relative humidity, but was found to be directly related to light intensities below 750 ft-c
and to air temperatures above 30°C. Apart from these extreme situations, net assimilation
under natural conditions is apparently limited by the interaction of many internal and external
factors.
Diurnal patterns of net assimilation differed in trees of different crown class. The rates of
photosynthesis and nocturnal respiration commonly fluctuated within each diurnal pattern despite
apparently stable environmental conditions. The causes of midday depressions are complex as
depressed patterns occurred under hot conditions in summer and under cool, foggy conditions
in autumn. Bursts of CO2 evolution immediately after sundown, lasting up to 2 hours, were
c01111110nly observed in summer and autumn.
INTRODUCTION Growth characteristics of trees are influenced by
the relationship between rates of photosynthesis
and respiration, which in turn are affected by fac­
tors of the environment. Net photosynthesis is
commonly measured by determining the difference
in CO2 content of an air stream before and after it
has passed over a foliar sample. Vlhen the amount
of CO2 assimilated in photosynthesis is greater
than, equal to, or less than the amount of CO2
which is concurrently liberated in respiration, the
measurement represents, respectively, net photo­
synthesis, CO2 compensation point, or net respira­
tion. The aim of this study was to relate air tem­
perature, light intensity, and relative humidity to
net photosynthesis of naturally growing Douglas­
fi r trees occupying different positions within the
crown canopy.
Several studies have been made on seasonal
fluctuations in photosynthesis of trees (Polster
1950, Saeki and Nomoto 1958, Bourdeau 1959,
Negisi and Satoo 1961, McGregor and Kramer
1963, and others). In many studies either seed­
l ings or excised materials have been used, and
while these procedures overcome some of the tech­
nical problems of sampling large trees, they are
subject to disadvantages. Firstly, seedling mate­
rial has been shown to possess a photosynthetic
capacity which may be considerably greater than
that of mature foliage (Hodges 1962, Krueger,
personal C011tl/1Hnication) and secondly, excised
material often exhibits a decrease in photosynthetic
capability about 30 min after severing and placing
in water (Clark 1954, Koch and Keller 1961).
To avoid these disadvantages, the photosynthetic
behavior of trees in this study was monitored with
the foliage intact within the tree crowns. This
procedure presents disadvantages, especially those
involving the recording of just a few characteris­
tics of an uncontrolled and fluctuating field en­
vironment, and also the inevitable distortion of
the micro-environment surrounding the foliar sam­
ple by the sampling chamber or cuvette. The
problems of cuvette environment and design have
been discussed by Bosian (1955, 1959), who con­
siders that much of the previously reported data
describing "two-peaked" diurnal net assimilation
curves may have been due to over-heating within
the cuvette.
Lal(' S\1\111l1l'r I %S
Thl'
/,;1':'1'
.\SSI IILA'I'ION
Ill'ld Illl'aSttrl'nH'llts III thl' PI'l'S('lIt studv
IN I)()\:(;I.AS-FIR
to climl > ing
ClllI\'l'rsiulI of net photosyntliesis to
tr('(' stems were used.
gross
'
photo
­
sy nthe s is by correcting' for rl 'spirat iOIl loss during
the day is often I1nct'l'tain. d l 1( partly to possilile
,
chang-('s in respiration dl1ring' illtl1llillatioll (I )eckt'1'
]1)55.
h: mtkov,
l{unecklcs, and Thilllalln
1958,
Tailing- 1(1).
altl111illttm ladders strapped to thl'
M easuremC'nt
was accolllplished by placing a
cylindrical 15- l >y 2()-cm cuvette over a hranchld
(Fig,
1).
Air was continually drawn 01'('1' tIlt'
foliage at 30 liters/hoHr and passed through a
Hartmann-Braun U.R.A.S. infrared gas analyzer
the gas stream every () min.
The study was conducted dtlring' 2 years, in an
ewn-ag-ed 38-year-old natural stand of J)ol1glas-fir
located in the University of \Vashing-ton's Charles
Lathrop Pack Demonstration Forest, situated
()O
l11iles sOl1th of Seattle, \Vashington, at all eleva­
tion of 1.o(X) ft. Five represl ntati l l' trees in each
(If the dominant, co-dominant. and sl1ppressed
'
crown classes \\'ere selected.
'
As used throughol1t
this paper, a tree in the dominant crown class is
a tree which has a considerahle portion of its
cro\\'n ahove the general level of the canopy; a
co-dominant tree forms the general canopy of the
stand; and a sl1ppressed tree is one which is COI11­
pletely over-topped by sl1rrol1nding trees.
Measl11'ement of net CO2 assimilation of a given
branchlet continued for 1 week, after which time
the ovenclry weight of its foliage was determined.
Several such measurements were made on each
selected tree during each of the {Ottl' seasons of
both 1961 and 1962. Sampling was confined to two
whorls of branches in the middle of the crown to
avoid both the top of the tree, which has a high
of
,
which automatically recorded the CO2 content of
1\ lA'I'ER1ALS ANn lH 1':'1' I fllDS
proportion
To minimize damag(' d\1e
I(IS?, I(ozlowski 1%2),
han' I)('('n l'xprl'ssed as lid rates onll'. sillcl' thl'
young
needles
characterized
by
higher respiration and lower photosynthesis than
mature foliage (Saeki and Nomoto 1958, Clark
1961, Oshima 1961), and the bottom of the crown,
which in forest conditions has a high proportion
of decadent foliage and acids little, if anything, to
the over-all food economy of the tree (Kramer
The ct1\'ettes were
made frolll triacetate plastic which was found by
tests using a spectrophotometer to have little effect
Oil
the transmission of light radiation within the
range of fr0111 0.320 to 3 fL.
The gas analyzer was
fitted with a gas selector switch which automati­
cally permitted gas streams from six different lines
to be passed successively through the analyzer for
a period of 1 min each.
Towards the end of this
period, the chopper-bar of a six-line recorder was
activated, recording the CO2 content of the gas
Each unit
stream directly in volume percentage,
on the scale of the recorder represents 0.001 vol %,
permitting a
change
in
CO2
concentration
0.0005 vol % to be read with ease,
of
A more de­
tailed description of this instrument is given by
Egle
and
Ernst
(1949),
Huber
(1950).
and
Strugger and Baumeister (1951).
To record air temperature and relative humidity,
two hygrothermographs were installed within the
crowns of the trees.
These instruments were later
supplemented by sensing units placed within each
sampling cuvette and also in the external environ­
ment.
Each sensing unit consisted of a selenium
photocell of the type B21VI (International Rectifier
Corporation) to monitor light intensity, and a
thermocouple system which was wired to provide
a measure of air temperature and also the differ­
ence between "wet bulb" and "dry bulb" tempera­
ture from which an estimate of relative humidity
was made.
The thermocouples were made by
micro-welding a junction between 36-gauge cop­
per and constantan wires.
Stable temperature
reference junctions were obtained by burying ther­
mocouples to a depth of
of each sample tree.
2 111
in the soil at the base
A net radiometer was placed
in an open area to provide a recording of light
intensity outside the stand.
These sensing units
were connected to a 24-line, 2-mv recorder which
provided successive recordings from
line every 96 sec.
a
particular
Unless otherwise stated, all
references to light intensity and air temperature
in the figures and text refer to recordings obtained
FIG. 1. Sampling cuvette. Air is continuously drawn
through the base-plate on the left, over the foliage, and
through the small cylindrical ullit containing thermo­
couples and a selenium photocell.
within the sampling cuvette.
Net CO2 assimilation, expressed as milligrams
per gram dry weight of foliage per hour, was
tabulated every quarter hour for all data together
JOliN
70U
,\.
Ecolug-y, Vol. 4(', l\o. 5
lIEL JS
with the corresponding values of air temperaturc.
ahly stahle and that it fluctuated slightly in a dim­
humidity, and light intensity.
llnl manner between
I )aily net assimila­
tion patterns were then drawn for each sample
i\1l1hien t
average
annual
precipitation,
is
Summers are comparatively
dry with July and August receiving only
1-10
inches of rain, although coastal fogs may provide
significant amounts of moisture.
Ambient tem­
peratures are generally mild and the frost-free
growing season is between
100-120
days.
The
5°C,
and
the warmest months of July and August have an
average of
17°e.
The day-to-night and season­
to-season variations are not great, and the approxi­
mate maximum and minimum annual temperatures
35°C
usually recorded are
and
O°e.
generally high under the influence of moist marine
The monthly average relative humidity in summer
is 60%.
Tests indicated that the ambient CO2 concen­
10
"
0
t/. 90
80
M
v
'"
0
c
"
,. ...........
,
'
;, ,,'"
-'
,-
AIR
,
,
70
-'
60
50
TEM ERATURE
turnal respiration for dominant and co-dominant
1961 and 1962 were derived
150 sample days for each
crown class in each season (Fig. 3). These figures
trees in each season of
from approximately
demonstate that:
1.
'"
u
10
9
8
7
6
5
10
3
FIG.
"
........
,
"
......_ ..,. ... "'
W lnfer
2.
,
Spring
,
"
,//"
,
z·
-0 D­
o
0:·
wln
r
,
,
,
,
,
,
,
,
,
---
--
,
/,
,
,
-
,
-
,
'-
,
,
,
__
1
-
,
i
-
Summer
t :::-r-- ---
,
'
.....
I
I
:
Spring
-
't
O' I
0'4
I
r-----
CO-DOMINANTS
,
,
4
3
,
i
,
,
"
c
o
8
7
6:
5 -:;
PRECIP TATION
'''...
2
,
,
I
-
I
,
,
,
,
,
I
DOMINANTS
10'2
of the
,
60
,
50
"
1/6 and 1/20
1962 respectively.
and
I
0'3
80
HUMIDITY
50
4
1961
annual total in
1%1
Winter net assimilation
amounted to approximately
70
- ..,
1962.
was twice that in
90 % ......
.. ..
The mean net CO2 assimilation rate of domi­
nant trees in the wetter, warmer winter of
40
RELATIVE
_ ..
The
hours after midday.
Autumn
-=4
Summer
Spring
Winter
3
hours before and
tration within the canopy of the trees was remark­
80 fF
Both ambient air tempera­
average daily rates of net photosynthesis and noc­
air brought in by the prevailing westerly winds.
20
re­
Precipi­
of average conditions recorded during the period
2
Humidity is
·C
were
2).
(Fig.
ture and relative humidity data represent means
coldest months of January and February have an
average temperature of approximately
conditions
1962
and
from the study area.
which
erally in the form of moderate rain showers with
60
1961
a meteorological station situated one-quarter mile
mostly distributed in the winter months, is gen­
a few days of snow.
vol %.
tation was recorded by a standard rain gauge at
ENVIRONMENTAL CONDITIONS IN \iVESTERN
\iVASHINGTUN
70
cn vironl11ental
corded during
The
0.036
and
SEASONAL PHOTOSYNTHETIC BEHAVIOR
day together with the corresponding curves of air
temperature and light illtellsity plotted ngainst
timt'.
0.034
];
!!
:
!l
o·I
z·
-0
2
I
Summer
. Autumn
Ambient environmental conditions recorded during
study period. 1961 ( -- ) ; 1962 ( -------- ) .
/
Wln
,
,
:
- .... --
,
/
,
,
,
,
,
,,
,
,
,
,
,
,
...
,'
,
,
,
,,
,
,
,
,
1
'
,
I
,
,
I
Sprlna
.
I
-
SUmmel'
Autumn
.......
.....
O'I
,
,
,
,
I
i
l--t---
,
,
,
,
,
,
... 'i' ... ....
I
"
FIG. 3.
Average daily net assimilation within each
season of 1961 ( -- ) and 1962 ( -------- ) . The verti­
cal bars represent the range of values recorded.
Late Summer 1965 "ET
.\SSI M I L AT IO ;';
2. Under the considerably cooler. moister con­
ditions during the spring. stimmel'. and autumn of
1962. the mean rates of net photosynthesis (per
unit dry weight of foliage) of both dominant and
co-dominant trees were two to three times greater
than the rates recorded during the hotter, drier
conditions in 1%1.
3. Nocturnal respiration rates in summer were
approximately twice those in spring and autumn,
and four times those in winter. Seasonal respira­
tion rates in 1961 were similar to those in 1962.
4. In 1%1 the mean rates of net photosynthesis
of dominant and co-dominant trees were very
similar. In the more favorable year of 1962 co­
dominant trees assimilated at slightly higher rates
than dominants.
DIURNAL PHOTOSYNTHETIC BEHAVIOR OF DOMINANT TREES \iVITHIN EACH SEASON Winter
Considerable variation between daily patterns
was obtained. On exceptionally dark, rainy days
(light intensity less than 50 ft-c) no gas exchange
was recorded at all during the 24 hours or during
several consecutive days. No gas exchange was
recorded on 3% of the 221 sample days in winter,
and on an additional 12% of the days there was a
net loss of CO2. Cuvette and ambient air tem­
peratures recorded during the night were ex­
tremely stable, yet on many occasions respiration
rates fluctuated considerably between the CO2
compensation point and 0.02-0.05 mg CO2 /g per
hour.
On dull, cloudy, or rainy days when maximum
rates of net assimilation were approximately 0.1
mg CO2/g per hour, fluctuations in net assimila­
tion were found to correspond with changes in
light intensity below 100 ft-c. On brighter days
30
0
L
0
20
t.:i.
E
3
}
0
,
"1
I,
...,
2 0
' I
' I
, I
GI
I- 10
Q
1:\
when net assimilation rates of 0.3 mg CO2/g per
hour were recorded, fluctuations could not be ex­
plained in terms of the environmental parameters
studied. A typical winter pattern is shown 111
1·ig. 4.
The responsiveness of dominant crowns to
change in conditions was well demonstrated in the
first week of March 1962, when snow fell on 3
consecutive days. The cuvette air temperature
during this period was between 1° and 4.5°C, and
cuvette light intensities were less than 100 ft-c.
Little or no net photosynthesis was recorded dur­
ing this period, but the following day, which was
clear and bright (air temperature 4°-5°C, light
intensity up to 5,000 ft-c), resulted in an ll-hour
period of net photosynthesis in which the net
assimilation rates were among the highest recorded
in any season. The following day was again clear
and bright, although several degrees warmer;
however, on this and subsequent days, net assimi­
lation rates were again at the moderate-to-low
level typical for this time of year.
The average period of net photosynthesis
(length of time during the day in which net assimi­
lation rates in excess of the CO2 compensation
point were recorded) was 5 hours.
Spring
All of the 180 days sampled in spring exhibited
some periods of net photosynthesis, although 1 Yz %
of these recorded a net loss of CO2 for the day.
Low rates of net assimilation (0.05 mg CO2/ g per
hour) were recorded on cold, dark days (tempera­
ture less than 9°C, light intensity 500 ft-c, or days
of heavy precipitation). On such days, net photo­
synthesis appeared to be limited by light intensity.
Fig. 5 presents a typical diurnal net assimilation
0
0
c:i.
E
III
I-
"\
I
," ..... 1
20
L
2
,
,
10
0
....
.c
1:11
I
6
0·2
18
c.>
...:
0
0
Q
.
.J
12
6
:3
30 ....
..r;;;
UI
0
701
J)Ol"(;LAS-FIR
.J
12
18 SPRING
E
0·1
.... e
IIJ'z:l oc(
0
FIG. 4.
oCt
18
12
6
0'1
Ii>
Ii>
WINTER Time
( hrs.)
Typical diurnal net assimilation pattern for dominant trees in winter.
....
IV
Z
0'1
o
0'1
FIG. S.
Typical diurnal net assimilation pattern for domi­
nant tree s in spring.
702
TABLE 1.
HELTITS
A.
JOliN
Ecology, Vol. 46, No. 5
Quantitative description of net photosynthesis patterns which exhibit midday depressions
i
--
-
-
--------
A.i\l. CO,
compensntion
point.
Time
��-.---
8.5-12.5
Rnnge
Tempemt.ure (DC)
Mode
0645-0930
0715
1315-1700
0745
1430
1745
1845
11-30
11-30
15-40
15.5-28
13-25
15
17
17.5
400-4000
400-7000
��- --
-- ------- ----
-
20-150
Range
Light (ft-c)
---.--
1000
30
Mode
----
Range
Rate (mgCO,jg per
hour)
Mode
:d
1
:
'\.'
f
, "
'II
'"
" ,
•
V
:
:
0.5
0.4
L
, ,..
I' I
" "',
I
8
-;
0
20
1 0 .1---
2
....
12
.r::
Cl
...J
18
0 '4
SUMMER
0·3
c
0
.....
0'2
2
E
1/1
1/1
0·1
....
.,
0
«
z
0·1
FIG. 6.
Time
0.01-0.2
0.1
P.M. Net
nssimilation
maximum
Final
decline
HiOO-1930
1745-1930
-
--
-- -
18.5
---
18
----- -----
150-4000
2000
-'--
0.1-0.4
150-1000
400
------
0.1-0.4
0.25
0.2
P.M. CO,
compensation
point
---
-
1900-2000
1£)30
-
11-20
----
Hi
-- -
---
30-250
75
- ------
0
The average period of net photosynthesis
spring was 11Y; hours (range 11-12 hours).
6 0
0
0
4
6
2000-10000
5000
0.3-0.7
10
I
I
•
30
2000
0.3-0.8
0
4.
u
0545-0830
11
------ ----
o
End of
depression
--
0500
Mode
-----_.
Start of
depression
OH5-0545
---
Rnnge
_._---------
A.M. Net
nssimilntion
maximum
(hrs,)
Typical diurnal net assimilation pattern for domi­
nant trees in summer.
pattern. Greatest net photosynthesis occurred on
high overcast or sunny days when air temperatures
were between 10° and 18°C, and light intensities
were greater than 500 ft-c. Under these condi­
tions, fluctuations in diurnal patterns of net photo­
synthesis were not directly related to changes 111
the environmental parameters recorded.
111
Swmmer
Dominant trees did not commence net photo­
synthesis at consistent levels of light intensity and
temperature. This condition was probably not due
to varying rates of respiration, as in many in­
stances foliar samples with relatively high rates of
dark respiration before sunrise (0.1 mg CO2/g
per hour) were found to reach compensation point
at some of the lowest light intensities (30-40 ft-c).
Net photosynthesis patterns in summer commonly
exhibited a midday depression (Fig. 6). Table I
presents a quantitative description of patterns of
this type. A net loss of CO2 was recorded on
24% of the 144 sample days in summer.
The greatest net photosynthesis in summer oc­
curred under conditions of heavy morning fog fol­
lowed by an overcast sky. Under these cooler
moister conditions, very high rates of net assimi­
lation (0.5-0.8 mg COdg per hour) were attained
within an hour after sunrise. After mid-morning
these rates diminished to 0.3-0.6 mg CO2/g per
hour and remained fluctuating within this range
until light intensities became iess than 750 ft-c in
the late afternoon. The period of net photosyn­
thesis in summer was usually 13 hours (range 11­
15 hours).
Aut·umn
The typical autumn pattern for dominant trees
was relatively symmetrical about the noon position
(Fig. 7). Characteristically, maximum net assimi­
lation rates (0.2-0.8 mg CO2/g per hour) were
recorded within an hour or two after sunrise, and
the foliage continued to assimilate at high but fluc­
tuating rates throughout the day until sunset. The
Late Summer 1965
NET ASSIMILATION IN DOUGLAS-FIR
10
8
:.\I,..I
'" I L
\
Po 4 I 'I
"". : '
I "
, "
, '.
30
u
0
20
4J
10
l-
0
l
I
I
,
2
0·4
c
0
....
.,
E
....
....
ct
...
.s::
01
.J
18
12
6
0
0
0
4
I
I
Q.
E
6
u
.,.:
AU TUMN
0'3
0·2
0'1
....
4J
0
:z
12
0·1
FIG. 7.
Time
18
( hrs.)
Typical diurnal net assimilation pattern for domi­
nant trees in autumn.
typical net photosynthesis period was 100 hours
(range 7-12 hours), whi<;h was the period during
which light intensity was greater than 10-50 ft-c.
Greatest net assimilation was obtained on days
of foggy mornings followed by high cloud. During
these mornings air temperature was very stable
(at a constant temperature of 9°_12°C), and light
intensity rose slowly at a uniform rate; however,
rates of net photosynthesis continued to exhibit
marked fluctuations, particularly when light inten­
sities exceeded 500 ft-c. Midday depressions were
obsen'ed on many occasions in the autumn as well
as in the summer. Net loss of CO2 occurred on
15% of 161 sample days.
VARIATION IN DIURNAL PATTERN BETWEEN
CROWN CLASSES IN EACH SEASON
In winter the air temperature above and within
the tree canopy was essentially the same; however,
light intensity within the co-dominant crown
canopy was frequently 200-500 ft-c whereas the
intensity near the exposed dominant foliage was
several times this value. The patterns of net
photosynthesis of dominant and co-dominant trees
were essentially similar. Beneath the tree canopy,
light intensities near suppressed trees often did
703
not exceed 50 ft-c. Small suppressed trees showed
measurahle net respiration during most winter
clays and nights except for a period of 6-8 hours
during the middle of each day when net assimila­
tion coincided with the compensation point. Of
77 sample clays recorded for suppressed trees in
winter, 300/0 of the patterns did not depart from
the CO2 compensation point for the entire 24-hour
period. On 14 of the sample days, the small sup­
pressed trees exhibited isolated periods of net
assimilation separated by varying periods of no
apparent gas exchange. These erratic patterns
were associated with the incidence of light intensi­
ties greater than 50 ft-c filtering through the
canopy. The larger suppressed trees whose
crowns were within, but over-topped by the
crowns of co-dominant trees, often produced pat­
terns of net photosynthesis whose peak rates (per
unit weight of dry foliage) equalled or exceeded
those recorded by co-dominant and dominant trees
sampled on the same day.
In spring, co-dominant patterns differed from
those of dominant trees in the more rapid attain­
ment of maximum rates of net photosynthesis in
the mornings, the maintaining of these high rates
for a longer period in the afternoon, and a more
rapid and direct return to the compensation point
when light intensity decreased below 1,000 to 300
ft-c. Small, stunted suppressed trees exhibited
patterns similar to those described in winter. Of
87 sample days recorded using these small trees,
all but five patterns either remained· at the CO2
compensation point for all or part of the day or
else exhibited low rates of respiration. The five
exceptional patterns showed moderate rates of net
assimilation (0.05-0.10 mg CO2/g per hour) for
periods of from .% to 4.% hours. The larger sup­
pressed trees again maintained rates of net photo­
synthesis which frequently exceeded those attained
by concurrently sampled dominant and co-domi­
nant trees, particularly on overcast days.
In summer, light intensities are considerably
lower ,vithin the shaded co-dominant crown can­
opy than above it; cuvette air temperatures here
are usually several degrees lower than those in
dominant foliage, consequently the characteristic
co-dominant pattern of net photosynthesis is rela­
tively symmetrical about the noon position with
an infrequent occurrence of midday depressions.
Net photosynthesis in co-dominant trees com­
menced at light intensities of 15-200 ft-c (modal
value 125 ft-c), and the maximum net assimi­
lation rate of 0.4-0.8 mg C02/g per hour at­
tained was similar to that of dominant trees ex­
cept that it occurred 2 or more hours later in the
morning. Air temperatures and light intensities
E cology, Vol. 46, 1\ 0.5
JOlIN A. lIEI,1\IS 704 recorded at the poi nt of nlaX IIllUIll rate of net
assimilation varied widely hetween 11 D and 30De
and 200-2,000 ft-e. I n general, eo-dominant trees
attained hig-her rates of net photosynthesis than
dominants. In one ease, a rate of 1.2 mg e02/g
per hom was reached at () :30 AM at a light inten­
sity of ClOO ft-c and an air temperature of 11 DC.
I n another instance, a 1110mentary peak rate of
1.6 mg e02/g per hour was recordecl at 8 :30 AM
at 6,500 ft-c ancl 11DC: net photosynthesis imme­
diately fell to 0.9 mg e03/g per hour, and this
rate was maintained throughol1t the clay despite
ctlvette air temperatures reaching- a maximum of
30De at noon. Net photosynthesis of suppressed
trees was similar to that recorded in other seasons
with the larger suppressed trees assimilating as
well as or better than trees of other crown classes.
The highest net assimilation rate recorded by any
tree during the study was attained by a suppressed
tree with a rate of 1.77 mg eOdg per hour at
noon and at a light intensity on the suppressed
foliage of 300 ft-c.
In autumn, dominant and co-dominant patterns
were similar. Co-dominant trees began net photo­
synthesis at 9De (range 7. 5D-11.5DC) and at a
light intensity of 10 ft-c (range 0-80 ft-c) which
is lower than that for dominants. Maximum rates
of between 0.2-1.0 mg e02/g per hour were
reached at 9 :15-11 :00 AM at temperatures ranging
from 9D-29.5DC and light intensities of 125­
3,000 ft-c. No relation between maximum rates
and levels of light intensity or air temperature was
obtained. The period of net photosynthesis for
co-dominants was 9;;'; hours (range 70-100
hours) which is 1 hour less than that for domi­
nants, The photosynthetic behavior of suppressed
trees was similar to that described for other
seasons.
THE INFLUENCE OF ENVIRONMENTAL FACTORS
ON NET PHOTOSYNTHESIS
The relationship between net photosynthesis and
the four environmental factors studied varied con­
siderably. The same foliar sample on consecutive
days of similar light intensity, air temperature,
and relative humidity regimes occasionally ex­
hibited similar patterns of net photosynthesis, but
more frequently quite different patterns were ob­
tained. Using all available data within each sea­
son separately, the only relationships obtained
were those between net photosynthesis and low.
light intensity (less than 1,000 ft-c) or high air
temperature (above 30De). The effect of relative
humidity could not be examined in detail since
humidities lower than 70% were rarely recorded
inside the cuvettes due probably to transpiration.
The general lack of relationships l11l1st he e1ue, in
part, to the interdependence of the environl11ental
factors l11onitored. Increases in light intensity
are associated with higher temperatures which fre­
quently result in increased relative hUlllidity by
increasing respiration and transpiration rates
(K ramer 1957). Also, such factors as internal
water stress and stomatal behavior \\'ould have
considerahle influence on rates of net photosyn­
thesis. 1 t is also possible that there are complex
interactions between the tree itself and its external
environment together with possible rhythmic
physiological behavior (as described with the
photosynthetic capacity of marine diatoms by
Palmer, Livingston, and Zusy 196../-) and hys­
teresis effects (suggested by Myers 19..(6. working
vvith Chlarella).
Environmental influence is illustrated in Fig. 8.
Net assimilation patterns fr0111 four different trees
on the same day fluctuated in an essentially paral­
lel manner. Light intensity limited net photo­
synthesis in Douglas-fir below 500-1,000 ft-c, and
above this range net photosynthesis was relatively
independent of light intensity. On different incIi­
0·8
0'7
0'6
1',
0·5 ,: " ..;:
.... 0'4 !! :.
ii ,
c
I:
r. o
c
E
;ii
\i
0.3
I
I/)
<t
!
0.2
....
0·1
III
z
I
I
i'.i I:
iI1..i:1
i,-'1"
., 1/
j
'Wi11
o
12 0'1 ( h r s.>
FIG. 8. Net assimilation patterns of four dominant trees
recorded on the same cloudy and rainy day in September
1962. Air temperature was constant at 15°C apart from
the period between 2 :00 PM and 3 :30 PM when fluctua­
tions between 20° and 25°C were recorded. The major
depressions at 11 :00 AM and 4 :00 PM were due to rain
showers when light intensity diminished from 1,500 to 30
ft-c. Fluctuating rates of CO 2 evolution were recorded
at night despite stable nocturnal temperatures.
La te Summer 1 Y65
NET ASSIMILATION IN DOUGLAS-FIR
Fogg Y
0'8
.::
Sunny
..... 0'7
E
N
o
o
'"
E
.
c
..
o
'f
....
..
..
z
x X K
.. .
Cloudy
'io4
...
i3
705
ol
WINTER
&,2
O·!!
K
0'4
.
.
0·3
•
0·2
-'
,.: •
0
• ,,
K
.
;
.
....
0
3
20
}:4
3
Lloht
4
5
Intensity
6
(I.c.
7
K
8
100)
9
10
FIG. 9. Relationship between net assimilation and light
intensity during the summer under conditions of fog,
cloud, and sun. Each point represents the mean of ap­
proximately 20 observations.
•
'
10
30
20
30
20
30
dl
6
.
:
. . .. •
:•
.. : t : t
'I " : HI'l·fl"':· ! .
',:'111 t' I .: / ••
0,
.
z
.1
0"
0'
.!il
0·1
2
"
u
..
K
•
SPRIN G
3
"-
0'6
bl
4
AUTUMN
4
3
!:::2
..
c
10
20
30
40
Tempera tUrf
o
'e
10
FIG. 10. Seasonal relationship between daily net assimi­
lation and the average midday air temperature (mean
temperature between 10 :00 AM and 3 :00 PM). Figure (c)
illustrates the limiting effect of high average temperatures.
vidual days, responses varied considerably during
different atmospheric conditions in summer (Fig.
9). These data were obtained using early morning radiometer; however, as the plastic material was
light intensities. By using late evening light in­ opaque to radiation of longer wavelength than 3 IL,
tensities similar curves were obtained, but com­ there was a tendency for cuvette air temperatures
paratively lower rates of net photosynthesis at a to become higher than ambient. This effect only
given light intensity resulted in curves of slightly became apparent when the cuvette was exposed to
reduced slope.
direct sunshine and the increase was commonly
It is generally accepted that naturally growing 30 -5 ° C, although on an extreme occasion it rose
trees attain maximum rates of photosynthesis at to 15°C above ambient.
light intensities below full sunlight (Kramer and
In 24% of the summer sample days, 15% of
Clark 1947, Polster 1955). Maximum rates in the autumn, 12% of winter, and 10 '1'0 of spring
this study were attained at ;,i-i/IO full sunlight, sample days, the foliage liberated more CO2 in
and the compensation point was as low as 10 ft-c.
0·8
Highest rates were reached during foggy· condi­
..
tions. Wilson (1948) reported that on 10 foggy
.c
days sampled the CO2 content of the air was 20­ ..... 0'7
SUMMER
E
: ,00
25% greater than normal, which resulted in in­
......
"
:: : . '
0'6
creased rates of photosynthesis. In the present
,
. .,
• ,
! ••
study no evidence of increased CO2 content in the u
••
.
.
. .
. . ' :. :"
. .\\
atmosphere was recorded within the tree canopy
eo 0'5
•• • ·1. t· . . 1'. ••
.: \
•
during conditions of fog. The beneficial effects
:"
i·:
..
.. •
', : , '.', '
c
... " , : " ': :!': .. . .. ..
of fog are probably associated with the creation
0 o·
'f\
.
.. :. "",
...
.
of favorable moisture conditions in the foliage and
. '\
.:. t··,·....: . .'.. ... . :
t:I
. . •\
, •. f. t' • •'
..
with a more efficient distribution of light.
0'3
'
.
•
:
:
'.:
:.'
'!S
:
1:
:
t·
•
·
..
E
, \
. : .. t : ' ••• •
'i I.
, •• t . .
In each season of the year, net photosynthesis
•
OJ
' , , .
OJ
t·'
" ,t " .
\
appears to be largely independent of air tempera­
0'2
.
.
4:
:.:.
.. :;:' r "'1',' •.: ' • ,:
.' \
,/J":\
, •••, :'. ' •• ' :
ture (Fig. 10). Fig. 10 c shows the influence of
·
-·
s."
'
./
,
. I.·t
.
:
,
.
.' . \
.
." .. r..
high summer temperatures on the total daily net ...G.I 0'1
. : .. ':H, \
..:
". "
;:
z:
assimilation of CO2• :! :•• ,
When individual recordings of net assimilation. 10
30
20
40
were related to corresponding cuvette air tempera­
Temperature
°c
tures, it be ame apparent that at about 40°C Doug­
FIG. 11. Individual rates of net assimilation and cor­
las-fir net 'assimilation sank to zero (Fig. 11).
responding individual cuvette recordings of air tempera­
The cuvette itself had no influence on light in­ ture in summer. The limiting effects of high temperature
tensity as measured by the selenium photocells and can be seen.
·
.
.
·
.
.
·t
'f
'
.t
'
.
.
.
•
'
0'
'
•
•..
.
•
.
70(i
JOIIN A. IIELlIfS
respiration than was taken up in photosynthesis.
This effect may be partially attributed to high tem­
perature, particularly during the night, although
this does not explain the occurrence of relatively
high respiration in winter. These results indicate
that, particularly during prolongecl periods of heat
and drought, the exposecl foliage of Douglas-fir
may he drawing upon stored reserves during the
clay, rather than accumulating carbohydrate. The
fl uctuating nature of nocturnal respiration rates,
observed on occasions in all seasons of the year
when air temperatures were stable, may possibly
result fr0111 association with stomatal movements.
Ecology, Vol. 46, No. 5
cuvette air temperature exceeded 200 C, 7 days
resulted in no depression despite the fact that on
five of these occasions air temperature reached
29°C. Also, on one occasion of heavy morning
fog with morning rates of net assimilation of 0.6
l11g COd g per hour, the depression occurred 10
hours before the fog lifted and while the tempera­
ture within the cuvette was 10° C. Other causes
of midday depressions discussed by Polster (1950)
are high light intensity, decrease in CO2 in the
atmosphere, and physiological "tiring" of the
assimilatory apparatus. Kramer and Kozlowski
(1960) add that the effect may be due to an
accumulation of carbohydrate within the photo­
DIURNAL PATTERNS
synthetic tissues.
Rapid fluctuations in net assimilation within
During a period of hot conditions in summer,
diurnal patterns are characteristic and similar to net assimilation did not commence before 7 :00 AM
those obtained by Polster 1950, Miller 1959, and or 7 :30 AM for several days despite the fact that
others. They were observed in every pattern ob­ during more mild conditions both before and after
tained throughout the study, even when environ­ the hot period, the usual time for commencement
mental conditions were stable. These fluctuations of net photosynthesis was approximately 5 :00 AM
are apparently either an inherent function of the when positive light intensities were first recorded.
photosynthetic mechanism itself or a direct result This lateness did not appear to be due to a cuvette
of changes in the internal status of the tree. Tests effect nor to a deterioration of foliage by excessive
made using standard gas mixtures indicate that the heat since it was observed at the very beginning
fluctuations are not caused by experimental pro­ of a sampling run and did not become progres­
cedures.
sively more pronounced during the 5 consecutive
Midday depressions which commonly occur in days of the run. It is possible that with the rela­
summer patterns are usually assumed to be ini­ tively high nocturnal and diurnal rates of respira­
tiated by high temperatures and/or water stress tion, a larger amount of assimilation would be
in the plant (Polster 1950, Tranquillini 1954, required to reach CO2 compensation point and
Kramer and Kozlowski 1960). This concept is show positive net assimilation and that this would
supported by many instances in this study (Fig. account for the apparent lateness. However, in­
12, Table I). The effect is not necessarily due to spection of a number of patterns obtained at other
periods of the summer indicates that, following
EI:P. ATURE
nights which produced similar rates of respiration
as in the hot period, the compensation point
was
nevertheless reached between 4 :30 AM and
12
18
24
6
12
18
6
12
18
24
Augus' 25
I
AugHt 26
I
6 :00 AM. The air temperature at sunrise at these
NET ASSIMILATjON
different periods of summer did not differ by more
04
than a few degrees, and it is possible that the de­
layed recording of net assimilation may be asso­
ciated with adverse moisture relations or failure
of the stomates to open.
Characteristically, on the brightest days in stun­
FIG. 12. Apparent influence of air temperature on net
mer a large burst of respiration (up to 0.3 mg
assimilation patterns of a dominant tree on 3 consecutive
CO2/g per hour) occurred immediately after net
days in August.
assimilation ceased at sundown. The rapid rate
of
respiration was usually maintained for 0 to 2
cuvette environment as suggested by Bosian (1955,
hours
and then gradually diminished with time.
1959), since fresh foliage sampled during a depres­
This
burst
of liberated CO2, which was exhibited
sion period· immediately recorded "depressed".
rates of photosynthesis. Midday depressions were to a less!';r extent in autumn and spt:ing, may be
recorded in autumn as well as in summer, and it similar to a post-illumination evolution of CO2
is difficult to attribute some of these effects to described by Decker (1959), although in the pres­
ent study the phenomenon was observed for up to
temperature or water stress.
Of 14 occasions in the autumn of 1962 when the 2 hours after sundown. Decker used tobacco
3:0:
'r
_,
'"
AIJllusl2.7
:
Late SUllllller 1965 707
N ET ASSIMILATION IN DOU(iLAS-FIR lea\'cs ullder laboratory conditions and found that
the CO hurst illl'l"eased fourfold when light in­
tensity preceding the clark period was iIlcreasecl
from 500 to 2,500 ft-c.
underneath the canopy they remain open. Neu­
wirth (1963) states that suppresseu crowns in
spruce stands use radiation more economically
than the dominants.
SEASONAL AND CROWN CLASS EFFECTS
The appreciable amount of net photosynthesis
occurring in winter (in 19(j I, al most one-quarter
of the total net gain for the year) must contribute
significantly to stored fooel reserves which aCCllmu­
late prior to the flush of spring growth. Net
assimilation in winter may therefore be of con­
siderable importance to the overall food economy
of Douglas-fir, particularly when, as in 1961, pho­
tosynthesis is low in summer and autumn due to
hot, dry conditions. This conclusion supports a
statement by Kramer (1957) who reported Hep­
ting's findings that Pinlls echillata accumulated
considerable amounts of carbohydrate in winter.
Similar results were reported by Parker (1961)
using Pil1US cembra.
In each season of the year the rates of net
photosynthesis qf larger suppressed trees, ex­
pressed per unit weight of dry foliage per hour,
were as high as or higher than the rates attained
by co-dominant trees, which in turn were slightly
higher than rates of net photosynthesis exhibited
by dominant trees. Each crown class is exposed
to a different environment, especially with respect
to light intensity, and the efficient photosynthesis
of suppressed foliage supports the concept that
light requirements for optimal photosynthesis in
Douglas-fir are very low and that photosynthesis is
limited by other factors such as high temperature,
high light intensity; and water stress. However,
Kuroiwa (1960 a,b) reported that photosynthesis
of suppressed trees in a 20-year-old A bies stand
was lower than co-dominants and dominants. This
result was due to higher respiration losses in rela­
tion to dry weight of foliage, and photosynthesis
was closely associated with nitrogen content of the
foliage, which was highest in the more vigorous
dominant trees. Kuroiwa's low rates of net photo­
synthesis and relatively hireh resniration rr'tes for
suppressed trees correspond with the findings in
the present study for the smaller, stunted sup­
pressed Douglas-fir which will die within 5 or 10
years. In evaluating photosynthetic efficiency,
therefore, consideration must be taken of the rela­
tive tolerance of the trees involved and the relative
position of the trees in the stand. Foliage of larger
suppressed trees growing under the stand canopy
may be analogous to shade leaves which have su­
perior assimilatory capacity and higher chlorophyll
content (Pisek and Tranquillini 1954). These
authors state also that in unfavorable moisture
conditions at the tops of trees, stomates close, but
ACKNOWLEDGMENTS
This paper reports part of a study carried out at the
University of Washington in partial fulfillment of the
requirements for the Ph.D. degree. The author wishes
to thank Dr. D. R. M. Scott, College of Forestry; and
D:. R. B. Walker, Department of Botany, University of
vVashington, for their consultation. Acknowledgment is
made of co-operative funds supplied by the U.S. Forest
Service to purchase some of the equipment. The latter
part of the study was financed by Grant #G 18071 from
the National Science Foundation.
B Jsian, G.
LITERA TURE CITED
1955.
Dber die V ollautoma tisierung der
CO,,-Assindiations-bestinllllung und zur Methodik des
Kli ettenklimas. Planta 45: 470-492.
---. 1959. Zum Problem des Kiivettenklimas: Tem·
peratur und Feuchteregulierung. Bel'. Deut. Bot. Ges.
72: 391-397.
BGurdea.u, P. F.
1959. Seasonal variation of the pho­
tosynthetic efficiency of evergreen conifers. Ecology
40: 63-67.
Cla rk , J. 1954.
The immediate effect of severing on
the photosy nthetic rate of Norway spruce branches.
Plant Psvsiol. 29: 489-490.
---.
1961. Photosynthesis and respiration in white
spruce and balsam fir.
State Univ. Coil. of For.,
Sy racuse, N. Y. 72 p.
D cker, J. P. 1955. A rapid post-illumination decelera­
tion of respiration in green leaves.
Plant Physio!.
3 0 : 82-84.
--. 1959.
Comparative responses of CO2 outburst
and uptake in tobacco. Plant Physiol. 3 4 : 100-102.
Egle, K., and A. Erllst. 1949. Die Verwendung de'
U.R.A.S. HiI' die volJautomatische und fortlaufende
CO2-Analy se bei Assimilations- und AtmungsmessUI1'
gen an Pflanzen. Z. Naturforsch. 46: 351-360.
Hodges, J. D.
1962.
Photosynthetic efficiency and
patterns of photosynthesis of seven different conifers
under different natural environment conditions. Un­
p lbl. M.F. Thesis, Univ. of Washington, Seattle,
Wash.
Hub er, B. 1950. Registrierung des CO2-Gefiilles und
Perechnung des CO2-Stromes libel' PfianzengeseJl­
,chaften mittels Ultrarot·Absorptionsschreiber. Ber.
Deut. Bot. Ges. 63: 52-63.
Koch, W., und T. Keller.
1961. Der Einfiuss von
Alterung und Abschneiclen auf den CO2-Gaswechsel
von Pappclbliittern. Ber. Deut. Bot. Ges. 7 4 : 64-74.
Photosynthesis, climate and
Kozlowski, T. T. 1962.
lil T. T. Kozlowski [ed.]
tree growth, p. 149-164.
Tree growth, Ronald Press Co., New York. 442 p.
Kramer, P. J. 1957. Photosynthesis of trees as affected
111 K. Thimann
by their environment, p. 157-186.
[ed.] Physiology of forest trees, Int. Symp. Harvard
Forest, Ronald Press Co., New York. 678 p.
Kramer, P. J., and W. S. Clark. 1947. A comparison
of photosynthesis in individual pine needles and en­
tire seedlings at various light intensities.
Plant
Physio!. 22: 51-57.
Kramer, P. J., and T. T. KozlowskI.
1960. Physiology
of trees. McGraw Hill Book Co., New York. 642 p.
Krotkov, G., V. C. Runeckles, a , n d K. V. Thimann.
Ecology, V o l . 46, No. S
1 958. Etl'ect of l ight in thc CO.) a h sorption and cvo­
lution by K a l a ncho(:. whea t a l 1 pea kaves. Pl ant
1'IlY
' s iol. 33: 2l) -202.
Kuroiwa, S.
19()(}a.
Ecological and
physiological
s t ndies on the vegdation of l'vlt. Shilllagarc. I V :
Some phy siologieal funl'lions concerning matter pro­
duction in young .'i /Ji,·s trees. Hot. Mag. Tokyo 73
( 862 ) : 133-141.
---. 1960/>. Ewlogical and physiological studics on
the vcgl'tation of Mt. Shimagare. V : I n traspecific com­
petition and pro(luctivity differcncc among tree classes
in the A b ies stand. Bot. Mag. Tokyo 73 (863) : 165174.
M c Gregor, W. H. D" and P. J. Kramer. 1963. Sea­
sonal trcnds in the rates of photosynthesis and respi­
ration of loblol l y pine and white pinc sccdlings.
Amer. J . Bot. 50 : 760-765.
Miller, R. 1959. Assimil ationsuntersuchungen an Tan­
nen und Fichten einer Naturverjiingung im Bayeris­
chen Wald. Forstwiss. Cbl. 78 (0/10) : 297-317.
Myers, J. 1946. Culture conditions and the d evelop­
ment of the photosynthetic measurement. IV : Influ­
ence of light intensity on photosynthetic character is­
tics of Clzlorc/la. ]. Gen. Physiol. 29 : 429-440.
N e gisi, K., and T. Satoo. 1961. Effect of temperature
upon photosynthesis and respiration of Akamatu (Pi/illS
dCIIsijlora ) , Sugi ( Cryp to1llc ria japollica) and Hinoki
( Cha1llaNyjJaris ob tllsa ) . J. Jap. For. Soc. 43 (10) :
336-343.
Neuwirth, G. 1963. Die soziologischen Bedingungen
der Energieverwertung durch Assimilation in Fich­
tenbestanden ( Picea abies L. ) . Arch. Forstw. 12 : 12241239.
Oshima, Y. 1961. Ecological stu d ie s of Sasa commu­
Photosynthesis and respiration of Sa.w
nities III.
/llIri/clIsis. Bot. Mag. Tokyo 74 : 349-356.
Palmer, ]. D., L. Livingston, and Fr. Dennis Zusy.
1904. i\ pl'I'sistent d iurnal rhythm in photosynthetic
capacity. Natme 203: 10K7-I ORK.
Parker, ].
1 % 1 . Sea sonal trcnds in CO2 absorption,
colcl re s i s t ancc and transp iration of some c vergreens.
Ecol ogy 42: 372-3KO.
Pisek, A., and W. Tranquillini. 1954. Assimilation u nd
Koh1enstoffhaushall in der Kronc von Fichten- ( P icea
1'.1'(('/,1'1/ )
unci
1 ()th(!chenbiiu11len ( Jill.r} IIS syl7·atica ) .
Flora ( J ena ) 14 1 : 237-270.
Polster, H. 1 950. Die physiol ogischen G rundlagcn del'
Stofferzcugung im "Valde.
Untersuchungen iiber
Assimilation, Respiration und Transpiration u n5ere
Hauptholzarten. D ayerischer Landwirtschaftsverlag
G.m.b.H. Miinchcn. 96 p.
��-. 1955. Vergleichende Untersuchungen iiber die
Kohlendioxydassimilation und Atmung der Doug­
l asie, Fichte und \Veymouthskiefer. Arch. Forstw.
4: 689-714.
Saeki, T., and N. Nomoto. 1958.
On the season 1
change of photosynthetic activity o f some deciduous
and evergreen broadleaf trees. Bot. :Mag. Tokyo 7 1 :
235-241.
Strugger, S., and W. Baumeister. 1951. Zur Anwen­
d un g d es Ultr arotabsorptionsschreibers flir CO2-As­
similationsmessungen im Laboratorium.
Ber. Deut.
Bot. Ges. 64: 5-22.
Talling, J. F. 1961. Photosynthesis under natural con­
d itions. Ann. Rev. Plant Physiol. 12 : 133-154.
Tranquillini, W. 1954.
D ie Lichtabhiingigkeit del'
Assimilation von Sonnen- und Schattenbliittern einer
Buche unter Cikologischen Bedingungen. 8th Int. Bot.
Congr. Paris Sect. 13 : 100-102.
WilSOIl, C. C. 1948. Fog and atmospheric CO2 as re­
l ated to apparent photosynthetic rates of some broad­
l ea f evergreens. Ecology 29: 507-508.
About this file: This file was created by scanning the printed publication. Some mistakes introduced by scanning may remain.