1364
Gas exchange characteristics and water relations of Larix occidentalis
STEWARTS. HIGGINS, 1 R. ALAN BLACK, GARY
K.
RADAMAKER, AND WILLIAM R. BIDLAKE 2
Department of Forestry and Range Management, Washington State University, Pullman, WA, U.S.A. 99164
Received December 23, 1986
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Accepted June 26, 1987
HIGGINS, S. S., BLACK, R. A., RADAMAKER, G. K., and BIDLAKE, W. R. 1987. Gas exchange characteristics and water
relations of Larix occidentalis. Can. J. For. Res. 17: 1364-1370.
Photosynthetic, transpirational, and stomatal responses to light, temperature, humidity, and plant moisture stress were
measured for greenhouse-grown seedlings of Larix occidentalis Nutt. Light saturation was 550 JJ.mol photons· m - 2 • s -J; light
and C02 compensation points were 26 JJ.mol photons·m- 2 •
and 59 JJ.L C0 2 ·L- 1 , respectively. Light-saturated photosynthetic rate was over 7 JJ.mol C0 2 • m - 2 • s- 1 with a temperature optimum between 18 and 23°C. Photosynthesis, transpiration,
and stomatal conductance to water vapor declined as xylem pressure potential decreased from -1.5 to less than -2.5 MPa; above
-1.5 MPa no effect was observed. Stomatal conductance declined with increasing leaf to air vapor density difference. Stomatal
conductance increased with increasing irradiance. Nighttime stomatal conductance was about 50% of the daytime maximum
conductance regardless of xylem pressure potential. When plants were well watered, the ratio of xylem pressure potential to
transpiration (XPP/E) decreased by 1.5 x 10- 3 MPa ·(JJ.g H20·cm - 2 ·s -J)- 1 with each mg H20·cm - 2 that had been transpired.
After 7 days of drought, however, XPP/E decreased at 7.9 X 10- 3 MPa·(JJ.g H2 0·cm- 2 ·s- 1)- 1 per mg H 20·cm- 2 • These
characteristics are compared with other conifers and are used to suggest differences between growth patterns of L. occidentalis
and sympatric species.
HIGGINS, S. S., BLACK, R. A., RA·DAMAKER, G. K., et BIDLAKE, W. R. 1987. Gas exchange characteristics and water
relations of Larix occidentalis. Can. J. For. Res. 17: 1364-1370.
On a mesure Ia response photosynthetique, transpirationnelle et stomatale par rapport aIa lumiere, Ia temperature, l'humidite
et le stress relatif al'humidite de Ia plante pour des semis de Larix occidentalis Nutt. eleves en serre. La saturation lumineuse a ete
de 550 JJ.mol de photons· m -z · s- \ les points de compensation de Ia lumiere et du C0 2 ont ete de 26 JJ.mol de photons· m -z · s -I et
de 59 JJ.L C0 2 • L - 1, respectivement. Le taux photosynthetique asaturation lumineuse a depasse 7 JJ.mol de C02 • m -z · s -l avec
une temperature optimale variant entre 18 et 23°C. La photosynthese, Ia transpiration et Ia conductance stomatale a Ia vapeur
d'eau ont toutes diminue amesure que le potentiel de pression du xyleme diminuait de -1.5 amoins de -2.5 MPa; aucun effet
n'aete observe au-dessus de -1.5 MPa. La conductance stomatale a diminue amesure qu'augmentait Ia difference de densite de
vapeur des feuilles a !'air. La conductance stomatale a augmente avec une augmentation de l'irradiance. La conductance
stomatale nocturne etait environ Ia moitie de celle du maximum diume, que! que fut le potentiel de pression du xyleme. Lorsque
les semis etaient bien irrir,ues, le ratio du potentiel de pression du xyleme a Ia transpiration (XPP /E) diminuait de 1.5 X 1o- 3
MPa · (JJ.g H20·cm - 2 ·s- )- 1 pour chaque mg H20·cm -z qui avait ete transpire. Apres 7 jours de secheresse, cependant, XPP /E
a diminue a7.9 x 10- 3 MPa · (JJ.g H2 0·cm - 2 • s- 1)- 1 par mg H20·cm - 2 • Ces caracteristiques sont comparees acelles d'autres
conireres et sont utilisees pour suggerer des differences dans I' allure de Ia croissance entre L. occidentalis etdes especes voisines.
[Traduit par Ia revue)
Introduction
Western larch (Larix occidentalis), a deciduous conifer, is
restricted to moist, well-drained sites of the evergreendominated forests in the Pacific Northwest (Schmidt et al.
1976). Within these forests western larch growth rates exceed
growth rates of sympatric conifers (Shearer 1982). Other larch
species exhibit high photosynthetic rates (Havranek and Benecke 1978; Benecke et al. 1981; Richards 1981; Crane et al.
1983) and water use efficiencies (Benecke et al. 1981) that
suggest high growth rates. Research on western larch has
covered topics from ecology and silviculture (Larsen 1929,
1930; Schmidt et al. 1976) to anatomy and wood quality
(Owens and Molder 1979a, 1979b; Lowery and Schmidt 1967).
However, comparative physiological research has been limited
to measurements of freezing tolerance (Sakai and Okada 1971)
and seedling water relations (Vance and Running 1985).
The response of gas exchange to environmental parameters
may determine in part the growth characteristics of western
larch. The autecology of western larch suggests that its growth
1
Present address and address for correspondence: Department of
Horticulture and Landscape Architecture, Washington State University, Pullman, WA, U.S.A. 99164-6414.
2
Present address: Department of Agronomy and Soils, Washington
State University, Pullman, WA, U.S.A. 99164.
Printed in Canada /lmprime au Canada
and physiology are limited by water. In a water-limited
environment, species may tolerate low moisture stress via
acclimation or they may avoid the stress temporally or spatially
(Hinckley et al. 1978). Stomatal closure in response to low soil
water may occur at a threshold of water stress or as a gradual
decline associated with decreasing availability of soil water.
Similarly, stomata are generally responsive to atmospheric
humidity restricting water loss at the expense of carbon uptake,
thereby modifying responses of photosynthesis to changing
light and temperature.
Characterization of the ecophysiological response of western
larch to soil water, atmospheric humidity, light, and temperature may explain the restriction of this species to particular
moist, upland sites in the Pacific Northwest. The objectives of
this study therefore were to measure the responses of photosynthesis, transpiration, and stomata to these environmental
factors.
Materials and methods
Seedlings of western larch used in this research were either
transplanted from the Coram Experimental Forest near Hungry Horse,
Montana or grown from seed collected on the Experimental Forest. In
1982, transplanted seedlings, <10 em tall, were grown in Tinus
Rootrainers (Spencer-Lemaire Ltd.) filled with an artificial rooting
medium of peat, vermiculite, and perlite (2:1: 1 by volume). In 1984,
1365
HIGGINS ET AL.
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these trees were potted into 18 X 40 em molded fiber pots (Western
Pulp Products) filled with the same rooting medium. All trees were
maintained during the growing seasons in the glasshouse at 25 and 20°C
for day and night temperatures. Metal halide lights extended the
photoperiod to 14 h with a minimum photosynthetically active photon
flux density (PPFD) of 300-400 fLmOI photons· m -z ·s-t. Trees were
fertilized biweekly, using a water injection system, with Peter's
15-30-15 (NPK) at 75 ppm N, MgS0 4 ·7H 20 at 48 ppm Mg,
sequestrene 330 Fe at 2 ppm Fe (Ciba-Geigy Co.), and Peter's soluble
trace element mix at 2 mg ·L-t.
Experiment 1
Most gas exchange measurements were conducted on the seedlings
transplanted from the Experimental Forest that had grown to 30-50 em
tall in the glasshouse. Measurements of stomatal response to leaf to air
vapor density difference (VDD) were conducted on seedlings similarly
grown in the glasshouse, but from seed. All gas exchange measurements of western larch were conducted on long shoots with needles
fully expanded but before overwintering buds had developed. Trees
were selected the day before measurement and watered and needles
were trimmed from the base of the twig to allow for closure of the
gas-exchange cuvette. Each response curve was generated from at least
four trees except the light response at high VDD (n 3).
The gas exchange system used for all measurements has been
described by Bingham et al. (1980) and was connected to an infrared
gas analyzer (ADC 225MK3). The carbon dioxide concentration in the
cuvette was maintained at approximate! y 340 fLL C0 2 · L -I and, to the
extent possible, a VDD of< 10 g H20 · m- 3 was maintained. Irradiance
for response measurements was supplied by a metal halide lamp and
was controlled by varying the distance between the light and cuvette
and by shading with cheesecloth. Leaf areas (one sided) were measured
with a leaf area meter (Delta T Devices). Leaf dry weights were
measured after oven-drying for 24 hat l00°C.
PPFD was maintained at 550 fLmol photons· m - 2 • s -I except for the
light response curves. Leaf temperature was held between 20 and 22°C
except for the temperature response studies, during which gas
exchange was evaluated between 3 and 40°C. Responses to VDD were
measured for differences ranging from 2 to 18 g H 20·m- 3 • Light
response was evaluated from 0 to 900 J.Lmol photons· m - 2 ·s-t. The
quantum requirement for photosynthesis, i.e., the moles of quanta
required for the net uptake of 1 mol C0 2 , based on incident light, was
computed as the reciprocal of the initial slope of the photosynthetic
light response (a in the light response model below). The light
compensation point was estimated from the linear regression of net
photosynthetic rate on PPFD less than 150 1-lmol photons· m - 2
The response of gas exchange to xylem pressure potential was
measured by withholding water from initially well-watered seedlings,
which caused the development of water stress. During this drying
period, gas exchange was measured daily. VDD was maintained at
<10 g·m- 3 until xylem pressure potential dropped below -2.0 MPa,
after which transpiration was too low to maintain a low VDD. Xylem
pressure potential (XPP) was measured with a pressure chamber.
Experiment 2
Approximately 300 seedlings 1 year old and 5 em tall, grown from
seed, were used for measurements of diurnal trends of water relations in
response to drought. Seedlings were well watered until the initiation of
the experiment and were not watered again. Four days after irrigation
ceased, measurements began at 22:00 PST and continued at 2-h
intervals for 24 h. On subsequent days, measurements began before
dawn and continued at 2-h intervals through the afternoon. For each
sample, stomatal conductance was measured with a null balance
porometer (Campbell 1975) in two groups of three excised seedlings.
Leaf area (one sided) was measured with a LiCor Li-3000 area meter.
Xylem pressure potential was measured with a pressure chamber;
ambient vapor density was measured with a sling psychrometer.
Cumulative transpiration for a given day was calculated as the product
of the instantaneous transpiration rate and the amount of time since the
last reading was taken, summed over all readings taken up to that time.
Data analysis for most responses was conducted using linear
VDD<10g-m-3
0
0
0
0
VDD>10g·m-3
300
600
900
600
900
0.40
B
0.10
.. .
.....
0.00
300
0
PPFD ()JmOI·m 2·s- 1}
FIG. 1. Response of net photosynthesis (A) and stomatal conductance (B) to increasing irradiance (PPFD) for young western larch
(30-50 em tall). Net photosynthesis (P n) was measured under
conditions oflow (o) and high (e) leaf to air vapor density difference.
(ForVDD <10 g·m- 3 , Pn = 8.41(1 exp( -0.0246 x PPFD/8.41)),
? = 0.99; for VDD >10 g·m- 3 , Pn = 3.78(1 - exp(-0.0208 X
PPFD/3.78)), ?
0.98.) Stomatal conductance (k 1) was measured
under conditions of low VDD. (k 1 = 0.11 + 0.0004PPFD
0.0000002PPFD2 , ? = 0.73.)
regression techniques on raw or transformed data as required. The
photosynthetic light response data were subjected to nonlinear least
squares analysis (SAS Institute Inc. 1982) using the model
Pn
Pmax [1
e-(a·PPFD/Pm,.l]
where Pn is the net photosynthetic rate, Pmax is the maximum net
photosynthetic rate, a is the initial slope of the light response curve,
and PPFD is the photosynthetically active photon flux density
(Thomley 1976).
Results
Experiment 1
Maximum net photosynthetic rate for western larch seedlings
grown under glasshouse conditions was 8.4 J-Lmol
C0 2 ·m- 2 ·s- 1 when the VDD was less than 10 g H2 0·m- 3
(Fig. lA). Increasing VDD by 36% reduced light-saturated net
photosynthesis by 55%. This decrease in net photosynthesis was
associated with a 62% reduction in stomatal conductance. The
quantum requirement for photosynthesis, based on incident
radiation, was41 mol quanta·(mol C0 2 )- 1 at low VDD (<10 g
H20·m- 3 ) and48 molquanta·(mol C0 2)- 1 at high VDD (> 10 g
CAN. J. FOR. RES. VOL. 17, 1987
1366
100
A
"'
:5"'c:
,.,
80
'iii
"'
60
0
15
.c
0..
-.;
z
40
if'
20
0,0
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For personal use only.
0.5
10
15
20
25
30
35
40
1.5
0.7
Leaf Temperature (•C)
2. Response of net photosynthesis (P n)to leaftemperature (T1)
for young western larch (30-50 em tall). (Pn
0.0468T12.45 X
0.879T1 , r2 = 0.62.)
FIG.
H20 · m- 3). The light and C0 2 compensation points at low VDD
were 26 j.Lmol photons· m - 2 • s -t and 59 fLL C0 2 • L -I, respectively. Stomatal conductance, similar to net photosynthesis,
increased with increasing irradiance (Fig. lB).
The optimum temperature for photosynthesis was between 18
and 23°C (Fig. 2). Photosynthetic compensation at low temperature occurred at 2 to 3°C; net photosynthesis was still positive
(>2 ~J-mol C0 2 ·m- 2 ·s- 1) at 40°C, our upper limit of measurement. Although variability among trees was relatively high, the
pattern of photosynthetic response to temperature was similar
for each tree sampled.
Net photosynthesis was independent of xylem pressure
potential above -1.5 MPa. Below -1.5 MPa net photosynthesis decreased and was less than 15% of the maximum rates at
-2.5 to -3.0 MPa (Fig. 3A). Net photosynthesis appeared
more variable at low xylem pressure potentials.
Like net photosynthesis, stomatal conductance and transpiration rate were not responsive to xylem pressure potential above
1.5 MPa; conductance and transpiration decreased to a
minimum around -2.5 to -3.0 MPa (Fig. 3B, transpiration
data not shown). Stomatal conductance to water vapor
decreased with increasing VDD (Fig. 4). To facilitate comparisons with other species, stomatal conductance (k 1) was also
regressed on the leaf to air vapor pressure difference (VPD,
where k 1
0.56
0.142VPD, r 2 = 0.93). The stomatal
response to irradiance was more variable than its response to
VDD (r 2
0.73 and 0.92, respectively). Transpiration
increased with increasing VDD until12 g H20·m- 3 • At higher
VDD, lowered stomatal conductance apparently restricted
further increases in transpiration. (Fig. 4)
Experiment 2
Diurnal trends revealed that maximum stomatal conductance
occurred early in the morning followed by a midday decrease
(Fig. 5). Nighttime stomatal conductances for well-watered
seedlings were at least 50% of the daily maximum. Daily
predawn and maximum conductances decreased with decreasing xylem pressure potentials over the 9 days of observations
(from 0.2 and 0.5 cm·s- 1 to 0.06 and 0.2cm·s- 1 , respectively). Stomatal conductance was not correlated with xylem
pressure potential until the 7th day of observation (data not
shown); the correlation remained significant for the remaining 2
days of the experiment. Xylem pressure potential decreased
significantly between days 7 and 9. Predawn and noon xylem
8
0.6
~
'
"'
E
s.,
(,)
c:
0.5
04
.l!!
g
'0
c:
0
0.3
{.)
tii
iii
0.2
E
0
iii
0,1
·.
0.0
0.5
1.0
15
2.0
2.5
3.0
3.5
Xylem Pressure Potential (-MPal
FIG. 3. Response of percent normalized net photosynthesis (A)
and stomatal conductance (B) to xylem pressure potential (XPP) for
young western larch (30-50 em tall). Measurements of percent net
photosynthesis (%Pn) and stomatal conductance (k 1) were made daily
at 550 JLmol·m- 2 ·s- 1 and 22°C. (%Pn = 0.002 + 1.911XPP
l.l01XPP 2 + O.l60XPP3 ,?
0.86; k 1 = -0.142 + l.llXPP
0.68XPP2 + 0.11XPP 3 , ?
0.63.)
pressure potentials were lower on day 9 than on previous days
(P < 0.05).
The ratio of xylem pressure potential to transpiration
(XPP /E) varied both diurnally and over the days of the drying
experiment. Diurnally, this ratio became more negative as the
amount of water transpired increased (Fig. 6). These diurnal
patterns also changed markedly as the soil dried. Early in the
experiment, XPP /E decreased gradually with cumulative transpiration. As drought increased, however, the ratio decreased
more abruptly with cumulative transpiration (Fig. 6). By day 9,
XPP/E was extremely low (Table 1) with a cumulative
transpiration (not shown) of 101 mg cm- 2 •
XPP/E at minimum xylem pressure potential (equivalent to
Running's ( 1980) Rspac) decreased eightfold between days 2 and
9 (Table 1) and was positively correlated with base (predawn)
xylem pressure potential (BXPP) over the measurement period
(XPP/E =
1.89 + 3.02BXPP, r 2 = 0.96).
Discussion
Differences in growth rates between western larch and other
conifers (Sweet and Wareing 1968; Gowin et al. 1980; Shearer
1982; Matyssek 1985) may be explained by the net photosynthetic, respiratory, and stomatal activity of the larch (Vance and
Running 1985; this study). The threshold for stomatal closure in
western larch, -1.5 MPa, was equal to or higher than reports
1367
HIGGINS ET AL.
o
0
o oo m
Cumulative Transplrollon CmQ·cm·2 l
o ooo::o
0
0.55
.,
e
OA5
3
00
w
g
..
160
200
240
0
-~ -0.4.
0.35
"'c
c:
..
0
0
;;;
.:"
0
'
~.,
E
0
120
c
u
iii
eo
;
w
.,g
40
0.25
-0.6
0
• ••
Can. J. For. Res. Downloaded from www.nrcresearchpress.com by USDANALBF on 10/31/11
For personal use only.
"~---7----~--~--~.o~--~
..~--~,4----~
••--~,.~
Vapor Density Dilferenca (g.m·3)
FIG. 4. Response of stomatal conductance (e) and transpiration (o)
to leaf to air vapor density difference (VDD) for young western larch
(30-50 em tall). Measurements of stomatal conductance (k 1) and
transpiration (E) were made at 550 j.Lmol·m- 2 ·s- 1 and 22°C. (k 1 =
0.55- 0.02VDD, .,:Z. 0.92;E 0.124 + 0.525VDD- 0.018VDD2 ,
.,:z. = 0.94.)
0
• 1.0;
•
<ii
.E
.,
r-5~
::>
"'
:-2.0 ~
..
X
·I_Q
0
L.J
"-
<ii
.,
E
-,:..
FIG. 6. Relationship between cumulative transpiration (Ecum) during the day and the ratio of minimum xylem pressure potential to
transpiration flux density (XPP/E, MPa·(!J.g H 20·cm- 2 ·s- 1)- 1) for
western larch seedlings (<5 em tall) under greenhouse conditions and
subjected to 2 days of drought (0), 3-5 days of drought (e), and 7 days
of drought (X). (For day 2, XPP/E -0.20 0.0015Ecum•? = 0.60.
For the intermediate days (3 and 5), XPP/E -0.28 0.003Ecum•
? = 0.61. For day 7, XPP/E -0.66 0.079Ecum•? = 0.77.)
E
E
0
Ci) 2000
2000
"'
>.
X
Hour
FIG. 5. Diurnal trends in stomatal conductance (solid symbols) and
xylem pressure potential (open symbols) for well-watered (e, o) and
desiccated (•,
western larch seedlings (<5 em tall) under
greenhouse conditions.
for many other Pacific Northwest conifers (Lopushinsky 1969;
Running 1976, 1980; Higgins 1984). Stomatal conductance of
western larch is relatively insensitive to VDD, decreasing by
only 0.02 cm·s- 1 per g H 2 0·m- 3 • This response to VDD is
similar to field observations of 30-year-old western larch
growing in Montana, U.S.A. (Shaw 1984), Pseudotsuga
menziesii saplings (Meinzer 1982), and tropical conifers (Meinzer et al. 1984). However, the stomatal response to VDD in
western larch is less sensitive than stomatal responses for
field-grown P. menziesii (Tan et al. 1977) and for field-grown
Picea engelmannii, Pinus contorta, and Abies lasiocarpa at
relatively low VDD and at high irradiance (Kaufman 1982). It is
also less sensitive than Picea sitchensis (Watts et al. 1976;
Watts and Neilson 1978). (These comparisons are made only
after adjusting our data to reflect total rather than projected leaf
area.) Direct comparisons with field-collected data are hampered, however, by the interaction between predawn xylem
pressure potential and VDD (Kaufman 1979; Graham and
Running 1984; Running 1984).
The high threshold of XPP leading to stomatal closure
indicates stomatal sensitivity to foliar water potential. However, field measurements showed that noon XPP did not fall
below the threshold value, -1.5 MPa, until late July (Shaw
1984). Sensitivity to XPP may interact with the stomatal
response to VDD in western larch to explain its success in moist,
1. Average base xylem pressure potential and
the ratio of xylem pressure potential to transpiration
(XPP/E) at minimum xylem pressure potential over
9 days of progressive drought
TABLE
Day
2
3
5
7
9
Base xylem pressure
XPP/E,
potential, MPa
MPa·(!J.g·cm- 2 -s- 1)- 1
-0.92
-0.74
-0.82
1.00
1.80
-0.43
-0.59
-0.63
1.25
-3.59
well-drained sites (Boe 1958). High stomatal conductance over
a range of environmental conditions in these habitats results in a
lower quantum requirement for photosynthesis, thereby maximizing carbon assimilation and growth compared with sympatric
conifers. Zobel (197 4) similarly predicted a constant diurnal
conductance of east-slope grand fir in the Oregon Cascade
Mountains would increase photosynthesis compared with more
sensitive west-slope populations.
Western larch leaves have a specific leaf area two to three
times that for Picea sitchensis and Abies grandis (Table 2).
Only Tsuga heterophylla has specific leaf areas similar to larch.
Although specific leaf areas for Larix and Tsuga are higher than
for some conifers, the photosynthetic rate when expressed per
unit leaf area is similar, suggesting that they assimilate more
carbon per unit leaf mass. When photosynthesis is expressed on
a dry weight basis, higher assimilation rates by Larix and T.
heterophylla are readily apparent compared with P. sitchensis
and A. grandis. The higher respiration (expressed by dry
1368
CAN. J. FOR. RES. VOL. 17, 1987
TABLE 2. Maximum net photosynthesis (P 0 ) and dark respiration (Rd), expressed on both leaf area and dry-
weight bases, and specific leaf area (SLA, calculated from the Pn data) for selected conifers
Can. J. For. Res. Downloaded from www.nrcresearchpress.com by USDANALBF on 10/31/11
For personal use only.
Pn,
mg C02 ·h- 1:
Rd,
mgC~·h- 1 :
Species
SLA,
cm2 ·gdry wt- 1
perdm2
per g
drywC 1
perdm2
per g
drywC 1
Source
Western larch
Japanese larch
Japanese larch
Sitka spruce
Grand fir
Western hemlock
144
nd*
141
49
50
130
11.7
nd
9.2
14.0
11.3
10.0
16.8
14.4
13.0
6.9
5.7
13.0
0.8
nd
0.8
1.3
1.5
0.7
1.2
0.9-1.8
1.1
0.6
0.7
0.8
Present study
Crane et al. (1983)
Fry and Phillips (1977)
Fry and Phillips ( 1977)
Fry and Phillips (1977)
Fry and Phillips ( 1977)
*nd, no data.
weight) found in larch compared with other conifers suggests
high metabolic activity. Larch is apparently capable of high
photosynthetic rates and effectively converts its assimilates into
growth, as has been demonstrated for European larch (Gowin et
al. 1980).
The photosynthetic response at low temperatures for western
larch is similar to that of Douglas-fir, but at high leaf temperatures
larch maintains higher photosynthetic rates. The upper limit
of net photosynthesis is 41 °C for seedlings of Douglas-fir
(Krueger and Ferrell 1965), whereas western larch (this study)
and Japanese larch (Crane et al. 1983) have net photosynthetic
rates of2.3 J..Lmol C0 2 ·m- 2 ·s- 1 at 40°C and 1.8 J..Lg C02 ·gdry
we 1 •
at 35°C, respectively. These high photosynthetic
rates for western larch even at high temperatures may allow a
greater carbon uptake during the hot summer days. Larix
leptolepis, when compared with Pinus contorta and P. radiata,
had higher net assimilation rate and relative growth rate than the
pines during the summer, which led to a greater cumulative
height increment for the larch (Sweet and Wareing 1968).
Seasonal carbon uptake is closely correlated with growth (Ledig
and Botkin 1974; Gowin et al. 1980), so we feel that the
photosynthetic characteristics of western larch contribute to its
higher relative growth compared with other conifers.
The XPP threshold for photosynthetic reduction for many
western conifers occurs at higher potentials than the -1.5 MPa
measured for western larch. Abies grandis (Puritch 1973), P.
menziesii, and Tsuga heterophylla (Brix 1979) exhibit higher
thresholds for photosynthetic reductions compared with western
larch, but like the larch, photosynthesis gradually declines with
increasingly negative XPP. Picea glauca, Pinus contorta (Brix
1979), Abies balsamea, A. lasiocarpa, and A. amabilis (Puritch
1973) show more rapid declines in net photosynthesis with
increasing xylem tension. Since western larch is insensitive to
XPP above - 1.5 MPa and its noon XPP does not fall below
-1.5 MPa until late July in the field (Shaw 1984), the species
may exhibit high photosynthesis over much of the growing
season.
Western larch, unlike P. menziesii (Running 1976), showed
no significant relationship between maximum stomatal conductance and the predawn XPP because predawn XPP did not
decrease significantly until late in the experiment. However,
like P. menziesii, the larch seedlings regulated water loss as the
rooting medium dried, as evidenced by the stronger relationship between stomatal conductance and XPP with increasing
moisture stress (e.g., Fig. 3). Though both P. menziesii
(Hodges 1966) and western larch show high stomatal conductance at night, the restriction of western larch to moister sites
than Douglas-fir may be a consequence of its relative insensitivity to predawn XPP.
The response of western larch seedlings to progressive soil
drought can be explained in terms of the ratio of the minimum
XPP to transpiration. In moist soil, the small change in XPP /E
with a large amount of transpired water (Fig. 6, upper curve)
results from adequate water storage in both the soil and the
plant; pathway resistance to the flow of water remains low.
Stomatal conductance remains relatively high and is poorly
correlated to XPP apparently because leaf water potential does
not control conductance at XPP near zero (Fig. 3, Helkvist et al.
1974; Running 1976). Conversely, in a dry soil, soil and tissue
water stores are reduced and pathway resistance to water flow
increases, leading to the abrupt decrease in XPP /E with a small
amount of transpired water (Fig. 6, lower curve) and the
attendant high correlation between stomatal conductance and
XPP.
XPP /E has been taken as the resistance to water flow in the
soil-plant-atmosphere continuum (Rspac) (Running 1980; Pallardy and Kozlowski 1981). To the extent that this computation
of Rspac is appropriate, a significant correlation between Rspac
and the ability of plants to recharge depleted water stores has
been established for five coniferous species, Pinus contorta
(Running 1980), Pseudotsuga menziesii, Tsuga heterophylla,
T. mertensiana (Ballard and Dosskey 1985), and Larix occidentalis. Knowledge of this relationship may provide useful
comparisons among species. For example, the instantaneous
rate of increase in Rspac with a unit increase in predawn XPP for
lodgepole pine is greater than for western larch throughout the
growing season, suggesting that larch, relative to the pine,
remains a more open conduit for water flow. However, the
validity of this calculation of Rspac is open to debate since these
data are rarely collected under steady-state conditions; thus
impedance affects the plant water status (Jarvis 1975; Davies
and Lakso 1979). Also, the resistance of conductive tissues is
linked to the relative water content of the tissues (Edwards and
Jarvis 1982).
The results presented here help to explain the distribution and
growth patterns of western larch. The relative insensitivity of
stomata in western larch to predawn XPP and VDD restricts it to
moister habitats. High metabolic rates and a slow decline in net
photosynthesis with increasing XPP contribute to the rapid
growth of western larch. Knowledge of the physiological
BIGGINS ET AL.
activity of this northwestern conifer under field conditions
remains essential for a complete understanding of how it
uniquely fills its niche.
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For personal use only.
Acknowledgements
We thank T. Hinckley, S. Running, J. Bassman, G.
Campbell, F. Larsen, and C. Rom for criticism of earlier drafts
of this paper. This is scientific paper No. 7637 of the
Agricultural Research Center, College of Agriculture and Home
Economics, Washington State University, Pullman, WA. This
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