Proc. 3rd IUFRO Workshop P 1.07-00, 1984. Eidg. Anst. forstl. Versuchswes., Ber. 270 (1985): 95-101.
ENVIRONMENTAL LIMITATIONS ON LEAF CONDUCTANCE IN CENTRAL ROCKY MOUNTAIN CONIFERS, USA
W.K. Smith
Department of Botany, University of Wyoming Laramie, Wyoming 82071, U.S.A.
ABSTRACT
Patterns in leaf conductance to water vapor diffusion (gl) in conifers of the Central Rocky Mountains, USA, appear influenced by a number of environmental factors which are of primary importance at different times during the summer growth period. An interactive model is proposed which identifies specific re- lationships between environmental and plant water parameters that may act to limit gl. In early summer, minimum air and soil temperatures appeared to have both a direct and indirect inhibitory influence on the maximum gl the following day. Indirect inhibition of stornatal opening may occur via reductions in sap- wood water recharge or stern flow which result in lower xylem pressure potentials
('l'p) measured before dawn. As temp- eratures approach maximum values during midsummer, only daily stornatal responses to LAVD may be limiting while influences of soil drying on gl may also occur only at low elevation sites or for particu- larly dry years. The return of near- freezing air temperatures at night corresponded to a rapid reduction in gl, lower predawn 'l'p, and a gradual rise in afternoon minimum 'l'p to springtime values. Environmental factors influ- encing gl during spring stornatal opening or fall closure may be operative during a major portion of the growth period for higher elevation conifers.
INTRODUCTION
Numerous studies have been concerned with the influence of specific environ- mental variables on leaf conductance to water vapor and the resulting effects on transpiration and photosynthesis in conifers (see Jarvis 1980 for recent re- view) as well as a variety of other plant types (see Schulze and Hall 1982). Re- cent investigations that have focused on predicting summer leaf conductance patterns in coniferous species of the Central Rocky Mountains, USA, have in- cluded stornatal responses to solar
irradiance, air and soil temperature, the leaf-to-air vapor deficit, and plant water stress (Fetcher 1976; Running 1980;
Kaufmann 1982a,b; Murphy and Ferrel 1982;
Smith 1984). Kaufmann (1982a,b) used a rnultivariable regression analysis to con- clude that the primary factors influ- encing leaf conductance in lodgepole pine
(Pinus contorta), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelrnannii) were photon flux density
(PPFD) and the leaf-to-air vapor deficit (LAVD). Detailed experiments on the in- fluence of light and vapor pressure on stornatal conductance in Douglas fir
(Pseudotsuqa rnenziesii) have also been reported (Meinzer 1982a,b). In addition, near-freezing air temperatures at night were important in reducing leaf con- ductance during part of the growth seasor
(Tranquillini 1957; Fahey 1979; Kaufmann 1982b) and a strong influence of near- freezing nights on seasonal stornatal closure has been observed recently in six species of Central Rocky Mountain
conifers (Smith et al. 1984). Smith et al. (1984) proposed an interactive model for environmental influences on seasonal stornatal closure that could influence leaf conductance patterns during spring stornatal opening as well as stomatal closure in fall. Moreover, the environ- mental factors that may act to limit stomatal opening in spring and lead to stomatal closure in fall may be operative for a major portion of the summer growth period in higher elevation forests.
The purpose of the present investi- gation was to evaluate the temporal im- portance of specific environmental con- straints on summer leaf conductance patterns of Central Rocky Mountain conifers. Leaf conductance to water vapor and water status measurements for trees growing at higher and lower
elevations were used to evaluate seasonal differences in environment and to test hypotheses regarding specific responses in leaf conductance to a given environ- mental parameter. The specific objective of this study was to identify the primary environmental factors limiting leaf con- ductance at various times during the summer growth period. A comprehensive model of environmental influences on leaf conductance during the entire summer growth period is proposed which empha- sizes the interaction of air and soil temperature, the predawn maximums and afternoon minimum values of xylem
pressure potential, and leaf conductance
in subalpine conifers.
MATERIAL AND METHODS
Data presented here are from water relation measurements taken at various elevations in the Medicine Bow Mountains of southeastern Wyoming, USA, (41° 15' N, 106° 15' W) over the past three summers
(1982-84). All measurements are for mature trees (8-15 m in height and> 20 cm dbh) from two high elevation (3220 m and 3144 m), bispecific stands of
Engelmann spruce (Picea engelmannii Parry ex. Engelm.) and subalpine fir (Abies lasiocarpa [Hook) Nutt.); a mid-elevation
(approx. 2860 m) stand of spruce, fir, and lodqepole pine (Pinus contorta Dougl.
ex Loud: spp. latifoII'af; or a lower elevation site (2572 m) where all six of the common Central Rocky Mountain
conifers were found growing together naturally (Smith et al. 1984). Data on limber pine (Pinus flexilus James) were also collected at low elevation sites
(1500 m) in southwestern Nebraska, USA (41' 10° N, 104° W), and at a high elevation site (3100 m) in the Medicine Bow Mountains. Other species included from the six-conifer site were ponderosa pine (Pinus ponderosa Dougl.) and
Douglas fir (Pseudotsuga menziesii ssp.
glauca Franco). In general, the higher elevation forests (> 3200 m) are occupied by~- engelmannii and~- lasiocarpa, mid- elevation (2500-3200 m) by P. contorta, and lower elevations by~- ponderosa and P. menziesii (Whipple and Dix 1979).
However, P. flexilus can be found over this entire elevational range and P.
engelmannii and~- lasiocarpa may be found at quite low elevations (< 2500 m) associated with riparian systems.
Air temperatures were taken within 5 cm of the same shoots usen for leaf conductance measurements using
ventilated, copper-constantan thermo- couples (36 ga.) shielded from direct sunlight. Soil temperatures (Tsoil) be- neath a shaded soil surface were measured between 0600 and 0800 hr using buried thermocouples. Leaf temperatures were taken a few seconds prior to the leaf conductance measurements using thermo- couples wrapped tightly around the mid- points of individual needles located near the center of an individual shoot.
Photosynthetically active solar
irradiance (0.4-0.7 µm) was measured as photon flux density (PPFD) using LI-COR 190S quantum sensors. Long term records of air temperature and relative humidity at individual research sites were also taken using hygrothermographs located in protected shelters within 30 m of the research trees. Ambient humidities at the time of leaf conductance measurements were recorded using a Vaisala humidity sensor. All temperature and humidity instruments were calibrated periodically using either a self-constructed, ven- tilated thermocouple psychrometer or a dew-point hygrometer (Cambridge
Instruments model 910). Minimum air temperatures at night (TWi¥l were de- termined from the hygrothermograph measurements.
Plant water status was evaluated using xylem pressure potentials (wp) measured before sunrise and approximately every 2 hrs throughout the day using a PMS model 2000 pressure bomb. Leaf con- ductances to water vapor (g1) and the maximum daily leaf conductance (g~ax) were determined for needles from shoots sampled from mid-canopy heights using a ventilated transient porometer (Kaufmann and Eckard 1977). For each gl measure- ment, adjacent fasicles having similar orientation were excised, their bases coated with petroleum jelly, and immedi- ately sealed into the porometer chamber.
The procedure required less than 30s and 3-5 repetitions were taken during each
~ampling usinq 2-4 trees at each site.
Total needle surface area inside the porometer (approx. 30-35 cm2) was com- puted by converting leaf dry weight to total leaf surface area determined by coating single needles with microscopic glass beads (Thompson and Leyton 1971).
Leaf conductance values were calculated on a total leaf area basis, and correc- ted for the effects of temperature and water vapor diffusivity due to elevation and ambient pressure. Also, corrections for the absorbtion and release of water from the chamber walls were determined over the range of temperatures and humid- ities encountered on the sampling days.
Leaf conductance values for detached needles showed little change after ex- cision until after approximately 4-9 min and then slowly declined: Statistical differences in leaf conductance measure- ments for attached and excised needles have been insignificant (a= 0.95) for numerous comparisons in spruce, fir, lodgepole, and limber pine during summer using the method described above.
RESULTS AND DISCUSSION
Most studies of environmental influ- ences on leaf conductance patterns have focused on specific effects of a single environmental or plant parameter without much interpretation of seasonal trends and interactions. In the Central Rocky Mountains, USA, both cold temperatures and water stress may be interactin~ to significantly limit leaf conductance, especially at higher elevations. Also, with greater elevation, significant in- creases in transpiration rates at a given stomatal opening are anticipated due to the increase in the diffusion co- efficient for H2o vapor, increased solar irradiance, and lower air temperatures.
These biophysical factors result in a greater tendency for leaf temperatures to rise above air temperature, leading to an increase in the LAVD (Smith and Geller 1980, 1982). However, several other en- vironmental variables other than daily elevational effects, such as seasonal precipitation increases with elevation, could also result in major influences on water status and leaf conductance pat- terns during summer. In general, the comprehensive influence of elevation on summer plant water relations of montane
species remains unknown. A preliminary schematic model of environmental vari- ables that interact to influence leaf conductance has been provided by Smith et al., 1984 (Fig. 1).
Fig. 1. Interactive relationships be- -tween environmental variables, plant
water status, and leaf conductance to water vapor diffusion in conifers:
Photo flux density (PPFD); vapor deficit of the air (VPD) and from the leaf-to-air
(LAVD); leaf temperature and the minimum air temperature at night (Tfil~gt); soil temperature (Tsoill and water potential
(~soil); x~lem pressure potential in the morning (~i) and minimum afternoon ~P
(~t); sapwood water storage; nocturnal recharge mechanisms for water uptake and transport (taken from Smith et al. 1984)
Air and Soil Temperature
There are no studies which have attempted to interpret the influence of the seasonal interaction of air and soil temperature on leaf conductance in high- er elevation forests. However, there have been reports of major decreases in stomatal opening effect following night- time air temperatures that were near freezing (Walker and Zelitch 1963, Kauf- mann 1976, Fahey 1979, Drew and Bazzaz 1979). Leaf conductance may also be inhibited by low air temperatures due to a reduction in stem sap flow. Water up- take was restricted in P. contorta at soil temperatures as high at 7° C (Run- ning and Reid 1980; Running and Reid 1980) and P. engelmannii (Kaufmann 1975) as well as-in other conifers (Dalton and Gardener 1978; Teskey et al. 1983, 1984).
Thus, during spring when soil tempera- tures at root depths (5-30 cm) may lag behind a warming in minimum air temper- atures, the primary temperature inhibi- tion on stomatal opening might be due to low Tair· Because Tair warms more quickly than Tsoil, there could be a somewhat more prolonged limitation on leaf conductance due to soil temperature.
During fall, air temperatures would again appear to have the initial, primary influence on seasonal stomatal closure
(Smith et al. 1984) due to the lag in Tsoil· Of course, variations in this scenario would depend on specific site conditions such as exposure, snow accum- ulation, etc. Windblown areas of the timberline ecotone and ribbon forests
will most likely have considerably dif- ferent patterns of Tsoil and Tair rela- tive to the intact subalpine forest.
The results presented here for six conifer species of the Central Rocky Mountains indicate that both cold air and soil temperatures may be interacting to limit stomatal opening (Fig. 2 and 3). Stomatal opening appeared to be prolonged in both spring and fall due to temperature limitations, but spring sto- matal opening occurred more rapidly over a narrow temperature range than fi;>-11 stomatal closure (Fig. 2) . As T\llrP rose above about o° C in spring, a sharp in- crease occurred in gTax the following day. A sharp increase also occurred when the 15 cm soil temperature ap- proached very near 7° C, in close agree- ment with the experimental findings des- cribed above. When this 15 cm depth approached 7° C in fall, no abrupt de- crease in gmax was noted, although values were generally less than 2.0 mm s-1 following a prior decrease in gmax that coincided with the initial occur- rence of near-freezing Tair values at night.
5r---r----.--.--.----.---.---.---~
4
'(I) 3
-5
Ei
CJ) 2·4
►
11!1
t,
~~
·2 0
r:;; (
0c)
I 1~
o
I I I I
2 4
[>
T
6
Fig. 2. Maximum daily leaf conductance (gmax) versus the minimum air temperature on preceeding nights in spruce (6), fir
(0), limber pine(>) and average values for six common conifer species of the Central Rocky Mountains growing natur- ally at the same location (~, see text for elevations and site descriptions).
Solid symbols are for fall (Aug. 26- Sept. 30); open symbols are for early summer (June 5-July 15). Vertical bars represent greatest range of values for each data set.
6 C
"
.,:- C
·~ 4
.§ 0
g "
c,>
1 "
2
"
0t •
i"
0 A"
"
0 8"
00 0
0 2 4 6 8 10
T soll (15 cm)
Fig. 3. Maximum daily leaf conductance (g
1
ax) and soil temperatures at 15 cm measured between 0600 and 0800 hr during the summers of 1982-84. See Fig. 2 for explanation of symbols and measurement variation.More data on Tair and Tsoil are needed over continuous measurement days during both spring and fall before the environmental temperature limitations on stomatal behavior will be fully under- stood. The specific influences of cold temperatures may include direct as well a~ ind~rect effects on g
1
ax via altera- tions in water status.water Status
The interaction of leaf conductance and plant water status may involve both cause and effect relationships where the status of either variable can lead to an effect on the other (Fig. 1). There appear to be no studies which have com- pared predawn
*o
(which is usually con- sidered to be the maximum*o
for theday) and afternoon
*o
values in conifers.This comparison of predawn versus after- noon minimum
*o
can provide insight into the dynamics of water loss via transpir- ation, recharge from the soil, and pos- sible influences of water storage in the sapwood.The data presented here show that in both spring and fall, predawn
*o
canbe considerably less than afternoon min- imums in
*o
on t.he same day (Fig. 4).During these periods (early summer and fall) there were also major reductions in gTax to below about 2 mm s-1 during the day and predawn
*o
were often below -1.0 MPa. Predawn*o
appeared to be more similar to afternoon minimum*o
during fall stomatal closure than spring opening (Fig. 4).
The direct influence of cold air temperatures at night on stomatal open- ing the following day could be separate from other limitation that may be indir- ect, such as a possible inhibition of water uptake from soil or water movement in the xylem. During spring when near- freezing nights may alternate with above
JUNE JULY AUG SEPT OCT NOV
Fig. 4. Seasonal variation in predawn and minimum xylem pressure potentials during the afternoon (1200 to 1600 hr) along with maximum daily leaf conduc- tance (gTax). 6tp is the predawn tp minus the afternoon minimum t0• See Fig. 2 for explanation of symbols.
freezing nights, these indirect effects could be operating to lower predawn
*o
following a day when some stomatal open- ing did occur. However, if, for example, predawn t0 were -1.2 MPa and increased to -0.9 MPa by afternoon (as a minimum value), then a greater predawn
*o
of atleast near -0.9 MPa would be expected for the following predawn t0 value. If the ensuing predawn tp is much lower than -0.9 MPa, then the possibility exists that some osmotic adjustments or a disturbance in xylem water movement, such as cavitation, must have occurred.
More field data on consecutive days when minimum air temperatures are fluxuating near
o°
C and/or controlled experiments are needed before a mechanistic explana- tion of the lower predawn*o
versushigher afternoon minimum
*o
will be pos- sible. Also, other possible mechanisms such as stomatal responses to photo- period may be operating and not neces- sarily related to cold temperature re- sponses. For example, McNaughton and Smith (unpublished) have found that fall stomatal closure in P. flexilus began at the lower elevation site without any apparent correspondence with lowTJRlQ
or Tsoil' and along with a relatively high water status to needle desiccation and survival during winter may also be an important factor in the temporal aspects of seasonal stomatal closure in fall. Winter needle desiccation has recently been shown to be an important factor contributing to needle death in timberline P. engelmannii and A. lasio- carpa (Hadley and Smith 1983, 1 9 8 ~
LAVD
In general, there was considerable variation between maximum leaf conduc- tance (gfax) on a given sampling day and
the corresponding atmospheric vapor deficit (VPD) measured at the time of the gTax measurement (Fig. 5). Higher ele- vations were characterized by relatively low VPD (< 2 kPa) compared to lower sites
(2-5 kPa). The relatively dry air tem- perature lapse rates of these mountains during summer (approx. 7-8° C km-1; Smith and Geller 1980) result in a significant, expected decrease in VPD with elevation.
Under low wind and high solar irradiance, needle temperatures may be expected to be cooler than, but further above, air tem- perature than at lower elevations (Smith and Geller 1980). In the case of P.
flexilus, leaf temperatures were commonly above air temperature at both the high and low sites, especially early in the morning (McNaughton and Smith, unpublish- ed). Thus, the greater SVD at the lower site was due solely to less air humidity with little influence due to leaf-to-air temperature differences (i.e. LAVD).
5
•
A
I
4 A
1
- 3
1
'
II) El
..§ ><
!
A~ 2 Ill Ill
CJ)
•
r¾ l .
80
t
Ill
A
g t
ll>•
0 ►Ill ► ll>
► ll>
0 2 3 4
LAVD (kPo)
Fig. 5. Maximum daily leaf conductance and the leaf-to-air water vapor deficit
(LAVD) measured simultaneously. See Fig. 2 for explanation of symbols. Ver- tical bars indicate maximum ranges of variation for each data set.
Very few data exist which describe needle temperature variation in conifers at different elevations and the strong potential influence of these differences on LAVD and transpiration. Recent re- search from this laboratory show that considerable elevation in needle temper- atures can occur naturally in virtually all species of Central Rocky Mountain conifers and is dependent on variations in shoot structure such as needle clum- ping and orientation (W. K. Smith,
unpublished data). However, more in- formation is needed using simultaneous comparisons of needle temperatures in the field before the influence of eleva- tion on LAVD and the leaf conductance response to LAVD are fully understood.
CONCLUSIONS
According to the preliminary model presented here (Fig. 1), maximum daily leaf conductance in higher elevation forests appear to be limited signifi- cantly by cool air and soil temperatures throughout a relatively large portion of their summer growth period. Although minimum air temperatures that are near and below freezing appear to have severe effects on predawn
•P
and gfax the fol- lowing day, soil temperatures well above freezing may also act to inhibit stomata! opening. During spring, an abrupt increase in gfax occurs almost immediately following a rise inTJlii¥
above freezing. A much less abrupt, negative influence on stomata! opening appears to occur during fall, but also corresponds to a substantial lowering of predawn
•P
to below the afternoon mini- mum in•o·
During summer there may exist a relatively short period when temperature and water limitations are small and gmax is at a maximum for the year, especially for lower elevation species. Two im- portant environmental factors influen- ci~g the duration of this period are probably snow accumulation and exposure of the soil surface to sunlight pene- trating through the overstory canopy.
For example, P. engelmannii and A.
lasiocarpa krummholz had greater-gmax and Tsoil in late June compared to ad- jacent forest trees due to a much earlier snowmelt in and around the krum- mholz mat (Hadley and Smith, unpublish- ed). Regardless of microhabitat dif- ferences, when air and soil temperatures increase and spring runoff begins to subside the only limitation to leaf con- ductance may be a response to daily var- iations in LAVD. At higher elevations SVD appears considerably lower in the Medicine Bow Mountains due primarily to a relatively dry air temperature lapse rate. Increases in seasonal precipita- tion do occur with elevation, with the exception of wind effects on snow accum- ulation in the alpine and timberline ecotone. However, no apparent patterns occur in SVD with elevation. Also, the data reported here for P. flexilus show that, although needle temperatures may be considerably elevated above Tair, this difference was similar over a wide elevational range.
As summer progresses and soil moisture depletes, there may be an eventual soil moisture limitation on stomata! opening. However, at high enough elevations, strong air and soil temperature limitations may occur late enough into summer and by early fall so
that very little, if any, water stress occurs due to soil depletion. Soil water potentials in the Medicine Bow Mountains have been shown to remain high during certain years, even above -0.4 MPa from
(5 to 40 cm), during most of the entire summer at 3220 m (Smith 1981, Smith et al. 1984, Carter, Hadley, Smith, unpub- lished data), although Knapp and Smith
(1981) measured soil water potentials at 15 cm depths that were as low as -3.5 MPa
(3,000 m elevation) in an open clear cut area and even lower values -6.0 MPa in- side an adjacent, mixed stand of pine, spruce, and fir (2650 m elevation).
Future work is also needed to clarify the temporal importance of air and soil temperature limitations versus potential soil moisture restrictions on grax during the summer growth period.
Figure 6 is a schematic illustra- tion that provides a summary of the hypo- thetical, temporal relationships between environment and seasonal leaf conduc- tance patterns for conifers of the Cen- tral Rocky Mountains, as discussed above.
Specific dates and quantative comparisons should be regarded as variable according to species, elevation, and, of course, are dependent upon differences in en- vironment between years. Table 1 is a summary of the relative importance of each environmental and plant parameter depicted in Fig. 1 during a certain por- tion of the growth period. These hypo- thetical scenarios (Fig. 6 and Table 1) are preliminary attempts to develop a comprehensive framework for evaluating the environmental limitations on leaf conductance patterns during the summer growth period.
a:
::iE---
---
o•c W T,lj~NI-0 , 1 - - - I
+
~ a. 0
<l -
)(
0 E O>
TAIR
JUNE
~SOIL
JULY AUG SEPT OCT
Fig. 6. Idealized relationships between air and soil temperatures, predawn and afternoon minimum ~P' and maximum leaf conductance to water vapor during the day. Tair, Tsoil, ~so'l indicate limi- tations on maximum leaf conductance due to cold air and soil temperatures and soil water potential. See Table 1 for more detailed description of primary and secondary environmental and plant para- meters limiting leaf conductance.
Table 1. Hypothetical interaction between plant and environmental water parameters within the spruce- fir zone during the growth season. Double asterisks indicate a primary limiting effect,
~ither directly on stomatal opening (stomatal} or indirectly through limitations on water uptake from soil and/or movement to the leaves (recharge). Single asterisks denote important, but less significant effects. Based on the interactive model proposed by Smith et. al. (1984).
Late spring Early Summer Midsummer Late Summer Fall
stomatal recharge stomatal recharge stomatal recharge stomatal recharge stomatal recharge
•
**•
•• . •
. •
• ..
~soil
•• •
LAVD
• . •
Tair - air temperature; Tsoil - soil temperature at root depths; ~: - morning xylem pressure potential,
•: - afternoon xylem pressure potential; ~soil - soil water potential at root depths; LAVD - Leaf-to- air saturation_ vapor deficit.
ACKNOWLEDGMENTS
The author thanks Gregory Carter, Julian Hadley, Dr. Don Young, and Geoff
McNaughton for assistance in the field work. This work has been supported in part by a National Science Foundation grant (BSR-8200742) and the Wyoming Water Research Center, University of Wyoming.
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