PHOTOSYNTHESIS IN SEEDLINGS OF SIX CONIFER SPECIES UNDER NATURAL ENVIRONMENTAL CONDITIONS J OHN D. HODGES! AND DAVID R. M. SCOTT
Ultiversity of Washingtolt, Seattle, Washington
(Accepted for pUblic ation June 30,
1968)
Absl>ract. Highest aver age d aily r ates of net photosynthesis in hemlock, gr and fir, Sitk a
spruce, and Douglas-fir occurred at the outer m argin of a forest st and. For Scots pine and
noble fir highest r ates occurred in an open, fully exposed are a. The environment at the
st and border app arently resulted in more f avor able intern al water rel ations.
Species which
assimilated best under exposed conditions app arently were better able to control moisture
loss th an were the other species. Species differed in photosynthetic efficiency. Sh ade-grown
seedlings app arently are c ap able of higher r ates of net photosynthesis than sun-grown plants.
'
of the s ame species.
INTRODUCTION
The purpose of this investigation was to study
photosynthesis and photosynthetic efficiency in
seedlings of several species of conifers in a range
of natural environments. By measuring various
plant and environ1?ental factors we hoped to shed
some light on the apparent differences between
species in photosynthetic efficiency and on the
mechanisms of the plant-environment interaction
which result in markedly higher rates of net photo­
synthesis in some environments than in others.
MATERIALS AND METHODS
Plant 11Ulterial a,nd environment ,
Seedlings (2-0 stock) of six species-Douglas­
fir (Pseudotsuga menziesii (Mirb.) Franco),
grand fir (Abies grandis (Lindley», western
hemlock (Tsuga heterophyila. (Rafinesque) Sar­
gent), Sitka spruce (Picea sitchensis (Bongard»,
lOble fir (Abies procera (Rehd.», and Scots
pine (Pinus silvestris (L.))-were out-planted in
plots along north-south transects extending from
deep within a 35- to 40-year-old Douglas-fir stand
into an adjacent open area cleared of all vegeta­
tion. In addition, seedlings of each species were
potted in 5-gal cans and placed along the transect.
Seedlings were thus subjected to a wide range of
environments, and by manipulating the cans the
environment of a seedling could be changed at
will.
The following four distinctly different environ­
ments were recognized:
1) Open, little influence of adjacent stand­
Maximum light intensities in excess of 11,000 ft-c.
2) O,uY;ide stand border--Diffuse light except
,
. " late afternoon; intensities of 2,000 ft-c and less.
1 Present address: Southern
tion, Alex andria, Louisi ana.
Forest
Experiment
St a­
3) Inside stand border-Diffuse light, intensi­
ties up to 1,000 ft-c.
4) Inside stand-Light intensity normally less
than 100 ft-c and seldom over 200 except during
occasional "sun flecks."
The location designated as "outside stand bor­
der" was immediately adjacent to the otiter mar­
gin of the Douglas-fir stand, while "inside stand
border" was approximately 6 m inside the stand.
Thus the environment a1 both locations was in­
fluenced by the Douglas-fir trees, but the effect
of the stand was much greater just inside the
stand border. The location deep inside the stand
was approximately 38 m from the margin of the
stand. In addition, seedlings were planted at
locations intermediate ,in position between the
open area and the. stand border ( outside) and
between the location inside the stand border and
deep shade. These seedlings were not studied
intensively, however, since their environment and
rates and patterns of net photosynthesis were al­
most identical either to' seedlings in the open area
'
or in deep shade.
Plantings were made in January 1%2 and Jan­
uary 1%3. Two transects of plots were estab­
lished in 1962 and one transect in 1%3. The 1962
plots contained six seedlings of each species, while
those established in 1%3 contained 20 seedlings
of each species. Thus 32 seedlings of each species
were out-planted at e<!,ch experimental location.
In addition, two potted seedlings of each species
were placed at each location. The study area was
located on Charles Lathrop Pack Demonstration
Forest of the College of Forestry, University of
Washington, located near LaGrande, Washington.
Measurement
of pla,nt factors and proce'sses
An infrared gas analyzer (Hartman-Braun,
Vvest Germany) was used for the measurement of
JOHN D. HODGES AND DAVID R. M. SCOTT
974
net photosynthesis during the daylight hours and
of respiration at night. Gas samples were collected from a small, (24.0 qy 7.5 by 5.0 cm) plastic
(polypropylene) cu.:v,ette which surrounded the
foliage sample. Th/' !6tal needle mass sampled
was kept at about 0.5 g ovendry weight. This
combination of cuvette size, cuvette material, small
foliage sample, and an air flow of 60 liters/hr
combined to prevent an excessive difference between leaf temperature inside and outside the
cuvette. Even on the ,brightest days the difference
was 3-4°C or,less.
For all samples a single branch from the uppermost whorl of branches which developed during
the second growing season in the field was used,
except where comparisons 'were made of needle
age. Thus, seedlings used from June 1963 to
about June 1964 were planted in January 1%2,
and those used from June 1964 until the end of
the study were planted in January 1963. The
nee'dles were fully developed and, depending on
the species, 2-4 inches of. the branch tip were
placed in the cuvette.
,
An automatic switching mechanism at the CO2
analyzer pex;mitted cydic sampling at six separate
points. Since at least ohe point (line) was always
used to monitor ambient C O2, concentration, up
to five, ,seedlings could be sampled at any time.
The sampling procequre was varied so that at
times alCsample lines were placed in one location
in order' to compare photosynthesis in different
species under the same environment. At other
times the sample lines were placed along the en­
vironmel'ltal trans'eet, and rates and patterns of
assimilation were compared for the same species
over a range of environments. Net photosynthe­
sis was normally determined for a period of 3
days on ,each sample seedling, after which the
foliage sample was removed and needle dry weight
determined. Seedlings were used only once except
at the location deep inside the stand where sur­
vival was poor.. The investigation extended over
a 2 year period beginning in June 1%3. Thus
measurements were made in all seasons of the
year. A total of 390 seedlings was used in the
investigation.
In t e spring, sutpmer, and fall of 1964 mea­
surements were made of leaf water potential, leaf
temperature, and relative stomatal aperture. A
thermocouple psychrometer, similar to the one
described by Spanner (1951) , was used for water
potential measurements,. Needles were collected
at 1 to 3' hr intervals from a branch on the same
whorl as the branch used for measurement of pho­
'
tosynthesis. The needles were collected in air­
tight vials but were ttimsferred within minutes
to the thermocouple units of the psychrometer.
'
Ecology, Vol. 49, No.5
Leaf temperature was measured by a thermo
couple attached to the under surface of the needle.
On two occasions field measurements of relative
stomatal aperture were made throughout the day
and into the night. In addition, stomatal measure­
ments were made in the laboratory under con­
trolled conditions of light, temperature, and hu­
midity. All stomatal measurements were made by
an infiltration technique described by Fry and
Walker (1%7) . Measurements listed above as
well as the measurement of environmental factors
were made concomitantly with measurements of
C02 assimilation.
Measurement of environmental factors
Measurements of light intensity, air temperature
inside and outside the cuvette, and relative hu­
midity inside and outside cuvettes were made for
the duration of the study. Light intensity was
measured by means of selenium photocells and/or
radiometers. Temperature and humidity were
measured with wet-dry bulb thermocouples and
with hygrothermographs. All light intensity and
thermocouple temperature measurements were re
corded on a 24-point millivolt recorder. Soil mois­
ture was determined by gravimetric sampling.
RESULTS AND DISCUSSION
Comparison of environments
It has been shown that under exposed condi­
tions the basic pattern of photosynthesis is con­
trolled by the general weather conditions and that
the highest average daily rates of net photosynthe­
sis occur on overcast days (Hodges 1967) . For
the environmental situations examined, the in­
fluence' of the Douglas-fir stand was exerted not
so much in changing the basic pattern but in modi­
fying the rates of photosynthesis, the extent of
the modification depending on the extent to which
the environment was modified.
The highest average daily rates of net photo­
synthesis for all species except Scots pine and
noble fir normally occurred at the location just
outside the stand (Table 1). This was true at
all seasons of the year as well as under all weather
conditions except very dark, rainy days, but the
difference between this location and the open area
was most pronounced on clear days in the growing
season. For Scots pine and noble fir the highest
daily average rates of assimilation sometimes oc­
curred at this location also, but more frequently
occurred in the open, especially for Scots pine.
Rates of net photosynthesis for all species were
normally less inside the stand border than outside.
However, for the more tolerant species (grand fir,
hemlock, and Sitka spruce) rates on clear days
975
PHOTOSYNTHESIS IN CONIFER S EEDLINGS
L ate Summer 1968
. TABLE 1. Aver age r ates of net photosynthesis during d aylight hours at different loc ations on. clear and ·overc ast
d ays (mg CO2/g per hour)
Grand fir
Sitka spruce
Hemlook
Douglas-fir
Noble fir
Scots pine
--- ---
. Location
Clear"
Overcast
Clear
Overcast
Clear
Overcast
Clear
Overcast
Clear
Overcast
Clear
1.19b
2.52
0.91
2.84
0.96
2.46
1.02
2.37·
1.60
2.25
2.95
1.88
3.81
1.87
3.62
1.06
3.03
1.50
2.89
1.56
2.05
1.71
1.76
1.65
2.09
1.44
2.05
1.22
1.54
1.02
1.31
0.88
0.78
0.60
0.45
0.68
0.50
0.63
0.39
0.57
0.30
0.45
0.24
0.30
0.17
0.17
0.07
Overcast
----
Open
Outside
border
Inside
border
Beneath
stand
i
2.28
-Days were srstematically picked io represent average conditions.
bEach value 18 an average of three ..observations.
were higher than in the open area. On very dark
days, with low light intensity, highest rates for all
species often occurred in the open, and in this
case relative rates for the various species were
quite different th<:l.n for clear days, i.e. rates for
grand fir, hemlock, and Sitka spruce were higher
than for Scots pine. Reasons for this will be
discussed later. Deep inside the stand, net photo­
synthesis was primarily dependent on occasional
"sun flecks" passing through the overhead canopy.
Differences in photosynthesis at the different
locations must be ex:plained on the basis of modifi­
cation of the environment by the Douglas-fir stand.
Table 2 gives an indication of the degree to which
certain factors were modified. On most occasions,
and especially 'clear days, there was little or no
difference in CO2 concentration in the different
environmental situations, but on occasion the con­
centration was higher under the stand than in the
open. On these occasions the difference was
usually less than 2.5 ppm but at times as high as
10 ppm. There appeared to be no real difference
in CO2 concentration at the stand border and in
the open.
Temperature differences between the locations
were sometimes pronounced, and it is probable
that higher rates of respiration due to higher tem­
peratures in the open account for some of the dif­
ference in net assimilation between the open and
the stand border. However, close examination of
the data indicates that temperature is not the only
factor responsible for differences in net photo­
synthesis. On clear days in the winter months,
photosynthesis was often greatly reduced as com­
pared to preceding or succeeding .overcast days
even though temperature was more favorable for
, phptosynthesis on the clear days. In addition, in
t, the sUfi-uner photosynthesis was normally lower on
.
.Æclt';ar'\days than on overcast days inside the stand
border; this was not likely a temperature effect,
since temperatures were not excessive and the dif­
ference between clear and overcast days was oftell
less than 30 C.
The factor which seemed most closely related
to daily photosynthate production was vapor pres­
sure deficit (VPD) of the atmosphere. VPD
was almost always lower beneath the stand and
at the border than' in the open, and the difference
was very pronounced on clear days (Table 2).
That VPD exerted a tremendous influence on
photosynthesis is shown by the fact that on nu­
merous occasions photosynthesis was markedly
reduced on clear days compared to overcast day&
even inside the stand border. On such occasions
light intensity just inside the stand was about the
same as on overcast days, and temperature was
favorable and often averaged no more than 30 C
higher than on overcast days. The only factor
greatly affected seemed to be VPD. In addition,
in laboratory tests it was shown that when relative
humidity was decreased from 96% to 75%, pho­
tosynthesis in grand fir began to decrease after
about i hr, and when relative humidity was low­
ered to 45 %, net photosynthesis decreased to zero.
Lowering of the relative humidity to 75% did not
seem to affect photosynthesis in noble fir, at least
for 3 hr, but when lowered to 45% the rate de­
creased abruptly. This has been discussed in de­
tail elsewhere (Hodges 1967), but it appears that
for noble ·fir the decrease in photosynthesis was
related to an. increase in stomatal resistance to
CO2, whereas in grand fir some other factor,
probably mesophyll resistance, was
more impor­
.
tant.
Bierhuizen and Slatyer (1%5) state that the
CO2 gradient from air to photosynthetic sites, and
the associated diffusive resistances (which are
markedly affected by the vapor pressure gradient),
effectively determine photosynthesis. They showed
that the ratio of. water transpired to carbohydrate
produced increased linearly with leaf-air vapor
pressure difference. The authors state that the
.
9 76
JOHN D. HODGES AND DAVID R.
M.
SCOTT
Ecology, Vol.
49, No.5
TABLE 2. Modification of environmental factors by the Douglas-fir stand and influence on photosynthesis of Douglas­
fir seedlings
-
-
Locntion
-
Photosynthesis
Cuvette
temperature
(OC)
Light
(ft-c)
Relative
humidity
VPD
(mm/Hg)
(%)
-------- ------
Open
Clear (July 13) ................
Cloudy (July 15) .... ..........
100"
228
Outside b ord er
Clear (July 13) ................
Cloudy (July 15)...............
251
388
Inside border
Clear (July 13) ................
Cloudy (July 15)...............
111
166
.
25b
16
65b
97
8.2b
0.4
760
'743
18
15
83
98
2.6
0.3
297
440
16
14
85
98
2.1
0.2
5,886b
1,2'16
·Percentage of average daily rate in open on clear day.
bAverage for daylight hours.
/
main source of variation in the leaf-air vapor pres­
sure difference will be due to the humidity of the
ambient air. However, in the present investiga­
tion leaf temperature at the exposed location was
sometimes 3-4°C higher than ambient tempera­
ture. Thus, the vapor pressure gradient would
be much greater in the open for this reason as
well as higher ambient temperature and lower
moisture content of the air. This in turn results
in higher rates of transpiration and, if excessive,
less favorable water relations in the plants.
It has been shown (Boyer 1964, Hodges 1967)
that rates of photosynthesis are strongly related to
plant water potential. In the present investiga­
tion leaf water potential for' all species averaged
3-10 bars higher at the stand border than in the
open. The least difference between locations
(about 3 bius) occurred with Scots pine. In this
species water potential under exposed conditions
was normally higher, by as much as 10 bars, than
for the other species. Differences between the
open area and the two locations beneath the stand
were even more marked. For example, on Au­
gust 20, 1964, potted grand fir seedlings grown
in the open and deep inside the stand were com­
pared at the location just inside the stand border.
Leaf water potential did not change much through­
out the day. However, water potential of the
shade-grown seedling averaged about 13 bars
higher (-15 compared to -28) than for the sun­
grown seedling. Both seedlings were well watered
the day before the test and neither had been sub­
jected to severe stress, but soil-moisture fluctua­
tions within the available range were greater for
the open-grown seegling. Greater net assimila­
tion for most species at the stand border may thus
'be related to improved plant-water relations re··
suiting from a more favorable environment. As
will be shown later, differences in rates of photo
TABLE 3. Average rate of nighttime respiration during
growing season under different ecologiCal situations
(m
CO2/g per hour)
Hemlock
Location
Grand
fir
Sitka
spruce
Douglastir
Noble
tir
Scots
pine
-- --- --- -- -- --
Open
0.54"
0.40
0.32
0.27
0.24
Outside border ..
0.77
0.49
0.39
0.38
0.39
0.80
Inside border ........
0.03
0.34
0.40
0.35
0.32
0.23
. ,.
0.21
0.18
0.11
0.17
0.08
0.07
B >
a h Bt ld.. ,
_
.
O .1
'Each value is 'an average of six observations, three cloudy and three clear nights
synthesis between species under the various en­
vironmental conditions probably result in part
fr0111 differences in control of internal moisture
relations, i.e., some species apparently are better
able to control loss of moisture than other species.
The possibility exists that histological differ­
ences between plants grown under the different
ecological situations contributed to the difference
in photosynthesis. For example, there is a dis­
tinct possibility of differences in membrane per­
meability to CO2, but this was not definitely estab­
lished in this study. In any case, the histological
differences are probably controlled by, or strongly
related to, differences hI. plant-water relations.
Nighttime respiration was also higher at the
stand border (inside and outside) than in the open
(Table 3), thus decreasing to some extent the
advantages gained by higher rates of photosynthe­
sis. Average rate of respiration was lower for all
species deep inside the stand. However, the per­
centage of daily photosynthate production used in
nighttime respiration was least in the open and
increased along the transect t6 the location deep­
est inside the stand. For grand fir the percentages
in the open, outside stand border, inside border,
and beneath the stand were 22, 37, 43, and 69 re­
spectively. These figures represent an average
for six nights, three overcast and three clear.
Based on the few determinations made of dry
weight production in potted material, it appears
that highest rates of net photosynthesis and dry
weight production for each species occurred in the
same environment. Thus, dry weight production
for Scots pine and noble fir was highest in the
open area. For the other species it was as high
or much higher outside the stand border.
Sun
977
PHOTOSYNTHESIS IN CONIFER SEEDLINGS
L ate Summer 1968
VS.
TABLE 4. Aver age daily r ate of photosynthesis (rug
CO2/g per hour) in sun- and sh ade-grown seedlings
re ared in pots which were moved from one loc ation to
another
Environment in which tested
Species
Grand fir
Open
Outside Border
Inside bor"er
sh4de foliage
Most investigators have concluded that shade­
grown foliage is 1110re efficient at photosynthesis
at low light intensities than sun-grown foliage.
There are exceptions, however, and major differ­
ences between species in this respect (Bourdeau
and Laverick 1958). There is also some indica­
tion that when shade-grown foliage is exposed to
high light intensity, rates of assimilation are lower
than for foliage produced under high light (Koz­
lowski 1949, Kramer and Kozlowski 1960).
In a prelimimiry investigation it appeared that
foliage from seedlings produced in shade was more
efficient than foliage from sun-grown plants. To
test this possibility further and to compare sun and
shade foliage under high light intensities, potted
seedlings were used to observe, under any ecologi­
cal situation, rates of photosynthesis for seedlings
. produced in any other environment.
For all species, foliage produced under shade
was found to be more efficient at low light intensi­
ties than foliage from sun-grown plants (Table 4).
Except for Douglas-fir, seedlings grown deep in­
side the stand showed higher average rates of
assimilation than seedlings produced in the open
or at the stand' border (outside) when all were
compared at the same location inside the stand.
The reason for the seemingly lower efficiency of
Donglas-fir seedlings from deep inside the stand
'is not clearly understood but was probably re­
lated to the poor condition of the seedlings.
Due to scarcity of potted material no compari­
sons were made between potted seedlings from
deep inside the stand and those only a few meters
inside. On the basis of comparisons with out­
planted seedlings, however, it appears that the
seedlings from deep inside the stand were more
efficient.
Potted seedlings from a few feet inside the
stand were transported to the open area for com­
parison with sun-grown seedlings. After a period
of 2-3 weeks rates of net photosynthesis were still
higher £.or shade plants than for the sun plants,
esp tially on cl ar days. Thus, the observation
that 5un-groWlJ.foliage is able to utilize light at a
higher intensify than shade foliage was not sub­
stantiated. It may be, however, that the apparent
Preconditoning
environment
Beneath Stand
Hemlock
Open
Outside border
Inside bor"er
Beneath st nd
Sitka spruce
Open
Outside border
Inside bor"er
Beneath stand
Douglas-fir
Open
Outside border
Inside bor"er
Beneath stand
Noble fir
Open
Outside border
Inside border
Beneath stand
Scot. pine
Open
Outside border
Inside border
Beneath stand
Outside
InBide
Btand border Btand border
0.52"
0.91
0.04
0.05
-.
-
1.50
1.14
0.31
2.37
0.79
1.47
-
-
2.03
1. 74
1.58
2.00
0.64
0.95
-
-
2.25
1.18
1.05
3.10
0.79
1.36
-
-
1.99
1.21
1.69
2.84
0.34
0.68
-
-
2.59
0.85
1.13
1.76
0.28
0.58
-
-
2.46
0.62
Open
1.03
-
1.80
-
1.47
-
2.70
-
1.08
-
3.22
-
1.07
-
1.80
-
1.22
-
1.84
-
2.78
-
6.04
-
-Values valid only for a comparison of rates for a given species at a given
ocation since rates for species were determined at different times.
advantage of shade foliage will diminish with time
if allowed to remain in a high light environment.
The greater efficiency of shade foliage has been
attributed to anatomical differences (Bormann
1958) and to differences in chlorophyll content
(Tranquillini 1954, Wassink, Richardson, and
Peters 1956, Bormann 1958). In the present
study, a marked difference in the morphology of
sun and shade plants was noted. On the fully
exposed seedlings the needles were shorter, there
were more needles per unit of branch length, the
needles were thicker, and there were more branches
on the seedlings. For example, in Douglas-fir the
average needle length on fully exposed seedlings
was 2.4 cm while at the stand border the average
length was 4.0 cm "'and seedlings at the border
averaged five to six fewer needles per inch of'
branch length. As suggested by Bormann (1958),
these differences may lead to differences in assimi­
lation because of more mutual shading in sun
plants. No determinations were made of chloro­
phyll content, but seedlings receiving some shade
were obviously greener than those from the open
area.
In the present investigation, another reason for
978
JOHN D. HODGES AND DAVID R. M. SCOTT
the apparent difference in efficiency of sun and
shade foliage was suggested. Shade foliage showed
consistently higher water potentials than. sungrown foliage. On August 20, 1%4, grand fir
seedlings from the open area and from deep in­
side the stand were compared at the stand border.
At 8 AM the water potential for sun-grown foliage
was -26.6 bars and that for shade-grown was
-13.8. Corresponding figures for hemlock seed­
lings on September 14, 1964, were -25.7 and
-14.0 bars. The other four species showed simi­
lar results.
A strong correlation between rates of photo­
synthesis and leaf water potential has been dem­
onstrated (Brix 1%2, Hodges 1967). Thus, it
seems likely that in many cases reported in the
literature differences in photosynthesis between
sun and shade foliage were related, at least in part,
to differences in plant-water relations.
As noted earlier, differences between sun and
shade foliage may be related to histological differ­
ences. Thus lower rates of photosynthesis for
sun-grown seedlings may result in part from an
increase in resistance to CO2 diffusion due, for
example, to a decrease in membrane permeability.
Although other factors may be involved, the re­
sistance to CO2 diffusion, is directly related to
water potential. Thus, resistance to CO2 diffusion
is a likely mechanism whereby rates of photosyn­
thesis are controlled by variation in leaf water
potential.
Age of foliage
In the two years of this investigation several
interesting observations were made concerning
foliage age and photosynthesis. In general, the
pattern of photosynthesis with increasing age fol­
lowed that reported for other conifers (Freeland
1952, Clark 1%1), but of special interest was the
rapidity with which the new foliage of the native
species developed to a stage at which it exceeded
old foliage in net photosynthesis. For example,
on May 20, 1964, grand fir needles were just
emerging on plants just inside the stand, but by
May 28, 1964, the rate of photosynthesis was
higher for the new foliage than for the old (2.1 mg
CO2/g foliage per hour, average for daylight
hours, as compared to 0.8 mg CO2!g per hour).
Respiration, however, was still much higher in
the new foliage (1.1 mg C02/g per hour, average
for night hours, as compared to 0.3 mg C02/g
per hour). At the same location on June 2, 1%4,
the avera%e rate of net assimilation for new growth
of both.granq fir and Douglas-fir was 2.7 mg CO2/
g per hour while that for old foliage was about 1.9
mg CO2/g per hour. Again, respiration was much
higher (more than twice as much) for new foliage.
Ecology, Vol. 49, No.5
In agreement with the work of Neuwirth (1959)
it was found that rate of net assimilation for new
foliage was highly erratic as compared to older
foliage.
Photosynthetic efficiency of the different species
Photosynthetic .efficiency of the six species was
compared in two ways: by observing in the open,
unshaded area rates of photosynthesis at low light
intensities and by comparing rates of photosyn­
thesis beneath the Douglas-fir stand.
Under natural conditions there are obvious diffi­
culties in trying to compare the efficiency of spe­
cies in the utilization of light for photosynthesis.
No control of the environment is possible and, as
has been shown (Hodges 1967), environmental
factors other than light intensity often control
rate of photosynthesis even when light is well
below saturating intensity. For this reason all
comparisons were made in the morning hours on
days with a light overcast. Experience has shown
that highest rates for most species normally occur
under these conditions and that rates, especially
in early morning, are closely related to light in­
tensity. At any intensity below saturation, rates
of photosynthesis were always lower in the late
afternoon hours than in the early morning hours.
This may .also have been related to lower water
potential in the afternoon hours.
Below about 1,400 ft-c rates of photosynthesis
for grand fir and hemlock were generally above
that for the other species. Below 1,400 ft-c the
normal order was: grand fir ::::,. hemlock > Sitka
spruce ::::,. Douglas-fir > noble fir > Scots pine.
After extended periods of light intensities above
about 2,000 ft-c, the photosynthetic rate for noble
fir and especially Scots pine was almost.. always
higher than for the other species.
When rates of photosynthesis as a percentage
of maximum were compared, there was a wide
L-���
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LIGHT INTENSITY (HUNDREDS OF FT. c.)
FIG. 1. Efficiency of different conifer species in.the
use of light at low intensity.
Late Summer
1968
979
PHOTOSYNTHESIS IN CONIFER SEEDLINGS
spread between species in their efficiency in the
use of light at low intensities (Fig. 1). Grand
fir and hemlock were about equal and appeared
to be the most efficient of the six species. Noble
fir and especially Scots pine seemed much less
. efficient than the other species. Douglas-fir was
in an intermediate position between the tolerant
grand fir and hemlock and the intolerant· Scots
pine.
Under shaded and partly shaded conditions,
grand fir showed the highest rate of net assimila­
tion per gram of dry foliage. Grand fir was fol­
lowed in order by hemlock, Sitka spruce, Douglas­
fir, noble fir, and Scots pine (Table 1).
. The above evidence supports the statement'made
long ago by Baker (1934) concerning the strong
relationship between "degree of tolerance" of a
species and its photosynthetic efficiency. Species
used in the present study have been given empiri­
cal tolerance ratings from very tolerant for hem­
lock, grand fir, and Sitka spruce, to intermediate
for Douglas-fir, and to intolerant and very in­
tolerant for noble fir and Scots pine.
The assumption that respiration is lower in the
more tolerant species was not substantiated. In general, the more tolerant species showed higher respiratory rates than did the intolerant ones (Table 3). Also, it was demonstrated that, ex­
cept under dense stands, the more tolerant species used a slightly higher percentage of their net
photosynthesis in nighttime respiration. An ex­
ception to this was noble fir, in which respira­
tion rates were sometimes as high or higher than
for the more tolerant species. This could be re­
lated to the rearing of these seedlings at an eleva­
tion well below that of their natural occurrence.
Again, the number of determinations of dry
weight increase was small, but there appears to be
a good relationship between species performance,
as indicated by increase in dry matter, and effi­
ciency ratings based on assimilation rates per
gram of foliage.
In addition to indicating differences in photo­
synthetic efficiency, comparisons of net photosyn­
thesis under various intensities of shading indi­
cated a strong interaction between species and
environment (Table 1). In the shaded environ­
ments the tolerant grand fir and hemlock had the
highest average rates of assimilation, but in the
open on clear days the highest average rates for
the daylight hours were recorded for the intolerant
Scots pine. There was also an interaction between
species and local weather conditions, with species
other than Scots pine showing greater assimilation
on days with a d.ark overcast.
The interaction between species and environ­
ment, in terms of net photosynthesis, was appar­
ently the result of difference in efficiency of the
photochemical process and differences in the regu­
lation of internal water relations. On dark, over­
cast days and under deep shade, photosynthesis
was apparently determined by the ability of the
species to utilize light of low intensity. Under
such conditions net rate of photosynthesis was
highest for grand fir and hemlock and lowest for
Scots pine. For all species, rate of net photo­
synthesis increased with increase in light inten­
sity. On other occasions, e.g. bright clear days,
it was obvious that the efficiency of the photo­
chemical process alone did not control net photo­
synthesis. Rates of photosynthesis for species
other than Scots pine, and to a lesser extent noble
fir, were lower than on overcast days especially
in the open, but often beneath the stand, also.
With a progression of clear sunny days the differ­
ence between species became more pronounced
(Table 5) . This may be related to differences
between species in ability to control internal mois­
TABLE 5. Effect of successive days of clear weather 0:1
average rate of CO2 assimilation in summer, 1964
(same seedlings used throughout)
Number of
days
with BUn
1.............
2.............
3.............
4 .............
Average
temp€'rature
Average daily rate of IIBSimilation
(mg/g per hour)
('C)
Average
V D
(mm Hg)
Douglas-fir
Noble fir
Scot. pine
23 .
26
28
26
12.8
15.9
20.1
15.6
1.4
0.8
0.6
0.5
1.9
1.1
1.0
1.1
2.3
2.4
2.6
2.8
P
ture relations. It has already been shown that
water potential in all species .is greatly reduced
under exposed conditions as compared to partial
shade but that the reduction is much less for Scots
pine than for species such as grand fir, hemlock,
and Douglas-fir. In addition, with the progres­
sion of the dry summer season, water potential in
out-planted seedlings decreased less in Scots pine
than in the other species. For example, in Doug­
las-fir water potential at sunrise decreased from
about -9 bars to -28 bars during dry weather
from July 23 to August 12, 1964. The minimum
potential recorded for Douglas-fir was -33 bars.
In Scots pine the miti,J.imum early morning reading
was about -14 bars and the lowest recorded
value was about -19 bars.
As previously suggested (Hodges 1967), the
differences between species may be the result of
differences in degree of stomatal control of tran­
spiration. The stomata of Scots pine, and to a
lesser degree noble fir, seem to open and close
more rapidly than those of the other species per­
haps resulting in less water loss and higher daily
rates of photosynthate production. The degree
980
JOHN D. HODGES AND DAVID R. M. SCOTT
of stomatal resistance to t ansfer of water vapor
and carbon dioxide varies widely in different spe­
cies (Holmgren, Jarvis, and Jarvis 1965), and
cyclic opening and closing can occur over short
time intervals (StiUfelt 1956, Ehrler, Nakayama,
and van Bavel 1965), resulting in less water loss.
Furthermore, there is evidence to indicate that
'
photosynthesis is reduced less by stomatal move­
ments than is transpiration (Shimshi 1963, Bier­
huizen and Slatyer 1%4).
Ecological a.nd silvicultural implications
The species used in this investigation show
adaptations to widely different environments, and
the results offer some evidence as to the mecha­
nisms involved in this adaptati?n. The efficiency
of the photochemical process in grand fir, hemlock,
and Sitka spruce is such that they are able to
photosynthesize and grow well under moderate
shade. Under fully exposed conditions these spe­
cies may not do as well seemingly because of ad­
verse internal moisture relations. Even under
shade, moisture stress and not light may often be
the controlling factor in survival and growth of
these species. In noble fir and especially Scots
pine the photochemical process apparently is less
efficient than in the other species, and light inten­
sity under shade is probably a more important
factor in controlling photosynthesis and growth.
Scots pine and noble fir, however, seem to control
internal moisture better than the other species and
are thus able, in exposed areas, to take advantage
of high light intensities for high rates of net photo­
synthesis.
The silvicultural· implications of this study are
obvious. With the exception of Scots pine and
noble fir, rates of net photosynthesis and prob­
ably dry matter production were highest just out­
side the stand border. Therefore, in the reproduc­
tion of these species some influence from residual
trees, probably in the form of a shelterwood, seems
desirable. Early growth of Scots pine and pos­
sibly noble fir reproduction would probaly be best
in the absence of influence from a residual stand
of trees.
The investigation emphasizes the need, when
comparing rates of photosynthesis, for considera­
tion of the history of the plant samples. Thus, a
test may or may not indicate greater photosyn­
thetic efficiency of shade-grown seedlings over
sun-grown plants and the results could depend on
the water relations of the plants or, more specifi­
cally, the mo)sture stress to which they have been
subjected. (
ACKNOWLEDGMENTS
The authors wish to express appreci ation to the U.S.
Forest Service and to the N ational Science Foundation
Ecology, Vol. 49, No.5
for providing p artial support for this investig ation. The
work was accomplished while the senior author was a
c andid ate for the Ph.D. degree at the ,University of
W ashington.
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COMPETITION BETWEEN BROMUS AND ERODIUM
Tranquillini, W.
1954.
Die Licht abhangigkeit der
Assimil ation yon Sonnen und Sch attenblattern einer
Buche unter okologischen Bedingungen.
8th Int.
Bot. Congr, P aris, Sec. 13: 100-.\02.
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-l
PHOTOSYNTHESIS IN SEEDLINGS OF SIX CONIFER SPECIES
UNDER NATURAL ENVIRONMENTAL CONDITIONS
By
JOHN D. HODGES AND DAVID R. M. SCOTT
Reprinted from ECOLOGY, Vol. 49, No.5, Late Summer, 1968
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