Proc. 3rd IUFR0 Workshop P 1.07-00, 1984. Eidg. Anst. forstl. Versuchswes., Ber. 270 (1985): 53-60.
TOPOGRAPHIC AND MICROCLIMATE CONTROL OF SIMULATED PHOTOSYNTHESIS AND TRANSPIRATION IN CONIFEROUS TREES
St.W. Running and R. Nemani School of Forestry University of Montana Missoula, Montana, USA
ABSTRACT
A computer simulation model, DAYTRANS/PSN, has been used to assess microenvironmental control of tree photo- synthesis and. transpiration as related to topography. Meteorological data were recorded on three paired north-south slopes in western Montana for the growing season of 1983 and used to drive the model.
Although south slopes were only 0.7 to 1.7° warmer with 6.1 to 9.7% lower rela- tive humidity than the opposing north slope, seasonal potential evapotranspira- tion was 17% to 37% higher. Because of earlier snowmelt and higher springtime transpiration, the south slopes consumed their available soil water more quickly and endured almost 1 month more water stress than the north slopes. Conse- quently, the north slopes had 33 to 45%
higher seasonal photosynthesis than the south slopes. This model prediction matches observed patterns of forest pro- ductivity in the mountains of western Montana, USA.
INTRODUCTION
Forest productivity in the northern Rocky Mountains of the USA is markedly controlled by microclimatic factors.
Visible differences in species composi- tion and growth occur over small areas in a fairly predictable response to topography (Daubenmire 1956,1976,1980).
Figure 1 illustrates the general pattern of forest composition in western Montana and gives approximate ranges of elevation, precipitation and summer temperature found in these forest types. Low eleva- tion valleys with precipitation less than 35 cm support grasslands and sparse for- ests of Juniperus. Low hills and south- facing slopes support Pinus ponderosa and Pseudotsuga menziesii. At mid elevations of 1300-1700 ma variety of conifers can be found, Abies grandis, Larix occiden- talis, Tsuga heterophylla, Pinus monti- coZa, Thuja plicata. At elevations above about 2000 moron north-facing slopes Pinus contorta, Abies Zasiocarpa, Picea engeZmannii predominate. Timberline
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P INUS PON DEROSA SERIES
L - - - 1 1 0 5 0 grassland
Figure 1. The general elevational distribu- tion of forest trees in western Montana, with approximate ranges of temperature and precipitation.
vertical arrows give the elevational range of each species.
occurs near 2500 mat 46 degrees north latitude, with Pinus aZbicauZis, Tsuga mertensiana and Larix ZyaZZii the common coniferous species.
General patterns of forest produc- tivity can also be related to elevation and topographic position. Fifty year
site indices for the dry, low elevation P. ponderosa stands may be only 15 m.
Mid elevation stands benefit from both cooler temperatures and higher precipi- tation. Site indices of 22 m/50 yr are found in A. grandis forest types (Pfister and Arno 1980). Aspect can be an equally strong determinate of site productivity.
Tesch (1981) studied the structure of two old growth stands on opposing slopes of a mountain valley at 1500 m in western Montana. He found a north-facing 50%
slope to carry 34% higher basal area than the opposing south slope. At elevations below 1200 m south slopes may only sup- port grass communities while north slopes at the same elevation have closed canopy forest.
While description of these forest patterns is logical and rather straight- forward, explaining the mechanisms that control forest development has been more difficult. While plant ecologists have agreed in principle that moisture and temperature gradients produce this for- est mosaic, a quantitative analysis of differences in both abiotic site factors and responses of specific physiological processes to mountainous topography has not been developed. My research of the past five years has attempted to analyze by computer modeling this observed pattern of productivity as influenced by elevational and aspect driven differences in site factors important to tree growth
(Running 1981). I have analyzed the
problem in two steps. First, what observable and predictable relationships can be found of microclimate with topog- raphy? Can climatological principles predict accurately the air temperature decrease with increasing elevation, or the precipitation increase at higher elevation? How much warmer is a south- facing slope than the analagous north slope?
Second, what physiological processes of tree growth and development are di- rectly influenced by the topographically induced microclimatic differences? I have chosen to explore the transpiration/
net photosynthesis balance or water use efficiency of coniferous trees in a rela- tively cold, dry environment where the interaction of temperature and water stress physiology is critical. This paper will report on a computer simula- tion of annual transpiration and photo- synthesis for three pairs of north-south slope study sites where meteorological data were recorded for the entire growing season of summer 1983. Does the observed difference in microclimate found between north and south facing slopes produce measurable differences in seasonal trans- piration and photosynthesis? Is seasonal photosynthesis correlated with observed patterns of forest productivity?
MODEL DEVELOPMENT
The DAYTRANS/PSN model is a daily resolution model of a tree water balance developed over the last ten years (Running et al 1975, Waring and Running 1976, Running 1984a), coupled with the photo- synthesis equations in FAST-P, a model of conifer gas exchange developed by the Swedish Coniferous Forest Project
DAYTRANS/PSN
r---,
Foliar ~ NET :
~
~,_ L~':?2°~~2~~::>_J
-
TRANSPIRATION RH -Tair ,...RAD Tree ht.
X { 4)
Leof area
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I I I I I I I I
I
SAPWOOD X (3)
- , -
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PRECIPITATION
Canopy int
' ' '
' ) - - - - ' - - " - - , . . . . ~ ROOT ZONESOIL X { 2)
GROUNDWATER X{6)
SNOW PACK X {I)
EVAPORATION X(7)
RUNOFF X (5)
Figure 2. A compartment flow diagram of the DAYTRANS/PSN model. Boxes represent com- partments or storages of water. Heavy lines represent water flow within the system.
Dotted lines transfer information that control flow rates.
(Lohammar et al 1980). A compartment flow diagram is shown in Figure 2.
DAYTRANS/PSN first calculates a hydro- logic mass balance for a stand or single tree, including precipitation and snow- pack inputs and surface runoff, evapora- tion, transpiration and groundwater seepage outputs. From this soil water balance a measure of leaf water potential is derived. The average leaf conductance
(k 1 ) of the canopy is calculated with controls by ~l, incoming shortwave radia- tion attenuated through the canopy, humidity, and air temperature, including a special frost reduction. Transpiration is calculated using the Penman-Monteith equation with the aerodynamic resistance fixed at ra
=
5.0 sec/m and the net radiation component divided by projected leaf area index to approximate radiation absorption through a multilayered canopy.The photosynthesis routine multi- plies the CO2 diffusion gradient by a radiation and temperature controlled mesophyll conductance and the stomatal conductance generated by DAYTRANS.
Net daily photosynthesis is arrived at by subtracting a temperature controlled night respiration component from the daylight net photosynthesis (Emmingham and Waring 1977). The net photosynthesis box is shown in dashed lines in Figure 2 because i t represents a calculation of CO2 fixation, not a compartment for water that the other boxes represent. Further documentation is available in Running
(1984a) and Lohammar et al (1980).
Table 1 lists the primary inputs, outputs, site and stand variables needed to run DAYTRANS/PSN. Because the purpose of this simulation is to compare micro- climate effects on gas exchange processes a "standard tree" was programed that was identical for all sites. This tree was ten meters tall with a LAI of 6.0 and 28.6 cm of available water in the rooting zone. Physiological parameters were dev- eloped primarily from work on Pseudotsuga menziesii in Oregon (Running 1976), and Pinus contorta in Colorado (Running 1980).
To illustrate the capabilities of DAYTRANS/PSN to accurately predict the seasonal physiology of a tree, a compari- son of model estimates with field measure- ments was conducted for a previous paper
(Running 1984b). Leaf conductance was measured with a null-balance diffusion porometer and xylem pressure potential measured.with apressure chamber on three sapling size Pinus contorta for the entire growing season of 1982 at the Lubrecht Experimental Forest in western Montana. Predicted and observed results of predawn leaf water potential and daily average canopy leaf conductance are shown inFigure 3. Both the timing and overa~l magnitude of predawn leaf water potential was predicted closely. The seasonal_
dynamics of leaf conductance were quite evident and fairly well simulated by the model. The erratic leaf conductances illustrated between JD 65 and 120 were caused by sporadic night frosts reducing
k1 despite optimum water status (Fahey 1979 Graham and Running 1984). From JD 120 to 19 8 small scale k 1 reductions were due to humidity response during variable spring weather conditions. Soil moisture depletion and decline of predawn le~f water potential caused the progressive
Table 1, --Inputs, outputs, and parameters needed to run the DAYTRANS/PSN model
Driving Variables l. Julian date
2. Precipitation 3. Air temperature 4. Relative humidity
5. Soil temperature (20 co depth) 6. Incoming shortwave radiation 7. Day length
8. Night minimum air temperature
Stand and Site Parameters 1. Ground surface area
2. Rooting zone soil water storage 3. Leaf area (tree or stand) 4. Tree height (midcrown)
5. Maximum soil surface infiltration rate 6. Haximuo canopy leaf conductance 7. Stomata! closure limit
8, Spring minimum leaf water potential
Primary Output Variables l. Transpiration
2. Evaporation
3. Soil moisture depletion 4. Tree Yater stress 5. Subsurface outflov 6. Net photosynthesis
Units
Day cm/day
·c
Percent
•c Ly /day
•c
3.SE + 4 cm2 LOE + 6 cci3 2.1E+5cm2 10 m 50 cm/day 0.16 cm/sec 1.65 MPa 0, 5 HPa
cm3/day cm3 /day cm3 /day -HPa cm3 /day mgC02/day
reduction of k1 through the rest of the season with a small recovery from fall rains.
FIELD SITES
Three sites were chosen from aerial photographs of western Montana that exhib- ited matching north and south facing slopes of greater than 30% slope gradient each extending from a common ridgeline.
Each site was a 5 to 10-year-old clearcut with sufficient regrowth that the energy exchange surface was green vegetation but avoiding the logistical problem of erecting towers above a forest canopy.
Table 2 provides some general character- istics of the six instrumented study sites. Note that the aspects are very close to true north and south and each slope pair is almost exactly opposite its counterpart. On each site a standard vented weather station box was mounted 1.4 m above the soil surface approximate- ly 100-200 m below the ridgeline. All six sites were instrumented with elec- tronic air temperature and humidity
Table 2. General Characteristics of the Study Sites.
Aspect Slope Elevation
Name (0) ___ill_ (ml
Ambrose N 350 40 1830
Ambrose
s
195 50 1830Ninemile N 330 40 1700
Ninemile
s
170 50 1700Schwartz N 40 40 1525
Schwartz
s
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50 100 150 200 250 300 Julian date
0.20
Julian date
Figure 3. A seasonal comparison of aver- age canopy leaf conductance and predawn leaf water potential values predicted bv the DAYTRANS/PSN model (solid lines) - against measured data (dotted lines) from three Pinua co~torta saplings in western Montana.
sensors recording hourly on digital data- loggers. On either the north or south site of each pair a pyranometer and rain gauge was also established recording hourly to the digital datalogger. No wind or soil temperature data was taken due to lack of instrumentation and data- logger capacity. Data collection com- menced on March 20 at Schwartz Cr., Aprill at Ambrose, and May 25 at Nine- mile because snow blocked access to the sites. We were attempting to initiate data collection before significant poten- tial gas exchange commenced, and were successful at two of the three sites.
Meteorological data collection continued until October 25 on all sites. At this point night temperatures were regularly below freezing and the public deer hunt- ing season jeapordized the safety of our equipment.
To prepare the meteorological data for driving the DAYTRANS/PSN model (see Table 1) recorded air temperature and relative humidity was arithmetically averaged for every hour showing incoming radiation above 100W/m2 to provide day- light average conditions. The level of
l00W/m" was chosen to represent the canopy average threshold of stomatal opening and photosynthetic compensation point. As such, we define daylength from the func- tional point of view of the tree rather than a strict meteorological basis.
Night average air temperature, for com- puting night respiration, was estimated by the arithmetic mean of daylight aver- age air temperature and night minimum temperature. Night minimum air tempera- ture was also used directly as a control of leaf conductance. Incoming shortwave radiation and precipitation were entered as daily totals. Because radiation was only measured on one of each site pairs, radiation for the opposing slope was calculated using a slope-aspect corrected potential radiation model to correct the data from the measured slope (Swift 1976).
Precipitation was assumed equal for the opposing slope of each pair because in all cases the two meteorological stations were less than 500 m apart. Additionally, daylength (and nightlength) in seconds for a flat surface was calculated using a sine function adjusted for 46 degrees north latitude.
Potential evapotranspiration was calculated seasonally for all six sites using a Penman-Monteith equation with the canopy resistance set to r0
=
0. This provided a quantitative comparison of the evaporative demand and relative stress conditions for each site.RESULTS Site Microclimates
On first inspection the microclimate differences between the north-south slope pairs seemed surprisingly modest. The daylight average air temperature differ- ence for the Ninemile site was south + l. 7 degrees ( compared to the north site) for the entire season (Table 3). At Ambrose the difference was even less, south +0.7 degrees. Maximum afternoon air temperatures were often 5 to even 10 degrees higher on the south slopes, but this extreme difference would occur for only a few hours on the brightest days.
Night minimum air temperatures averaged -0.5 degrees south on both sites, the south slopes almost always wer:e cooler at night than the north slopes. Daytine average relative humidity ,-,as -9. 7'/i south at Ninemile and -6.1% south at Ambrose
(Table 3).
Seasonal differences in potential E'r proved to be more substantial (Figure 4). The south slope at Ambrose had +17%
potential ET relative to the north slope.
Despite the later startino point of the Ninemile data, the south ~lope had +37~
potential ET compared to the north slope.
Differences between the slopes developed steadily throughout the summer, whereas based on potential radiation models one would expect the periods of relatively
low sun angle in the spring and fall to generate the largest aspect related dif- ferences in microclimate. Although dif- ferences in potential ET did not develop with the timing expected, seasonally the relative magnitude of difference in po- tential evaporation was proportional to radiation received. The ratio south/
north of potential ET at Ambr:ose was 341 mm/291 mm 1.17, the ratio of inci-
de::it radiation was 517vJm2 /403Wm2 = I. 28.
Similarly at Ninemile the south/north rcltio of potential ET was 230 m.rn/170 mm=
1.35, the ratio of incident radiation was 502Wm2 /338Wrn2 = l. 48.
Table 3. Measured growing season differ- ences in the microclimate of two paired sites on north-south slcpes.
Daylight Average Radiation (W/m2)
Air Temperature c0c) Relative Hu!;lidity (%) Seasonal Potential E'l' (mr:i)
Radiation (\'l/m2)
Air Temperature (~C) Relative Humidity (%) Seasonal Potential ET(mw)
NINEMILE North South 338
11. 9 67.5 190
502 13.6 57.8 261 AMBROSE North South 403
10.6 63.2 291
517 11. 3 57.1 341
Lest we become overconfident in our ability to predict mountain microclimates the Schwartz site produced completely different patterns. Almost six weeks of data were lost due to, you guessed it, instrument failure, but our basic con- clusions £or this site are not compro- mised by this loss. 'rhe daylight average air temperature was -0.3 degrees south, the south slope was actually cooler throughout the daylight period than the north slope.. r-~ighttime average air tem- peratures were +1.2 degrees on the south slope, again contradicting the patterns of the other two sites. Seasonal poten- tial ET was equal on the two slopes. The vegetation on these two slopes at Schwartz appears similar to the other two sites.
The south slope is sparsely vegetated with evergreen shrubs, and conifer regen- eration (10 years after clearcutting) is only slowly being reestablished. Con-
Seesona! Potential :=T Seasonal Potential ET
350 - - - - ~ - - - -- 350 -,---~---~
§ 2i0 E i75
.,
o.. ":40
7C 35 j ;::./
0 , :J
80 132
,...-~-:~~...,...-...---.-...-i
174 216 258 300 Julfan day
a.
3151 2801 245'
Julian day
b.
~igure 4. The seasonal trend of poten- cial evapotranspiration simulated by D17'Y"'RANS/PSN for the north and south s-1opes of (a) 1'.:mbrose and (b) Ninemile sites~
..J i
versely, the north slope has abundant regeneration of conifers and other decid- uous shrubs. We would expect soil surface temperatures of these two slopes to be very different, while our data was taken at 1.4 m above the surface. Also, the north slope actually faced 40 degrees east of north, and in midsummer at 46 degrees latitude the sun rises at 55 degrees east of north. Consequently, morning radiation loads on the north slope may have provided early heating of the site relative to the south slope.
However, even in early April and October, when sun anoles would favor the south slope, there s t i l l was no regular temper- ature difference between the two slopes.
The only explanation remaining is that significant atmospheric turbulent mixing must occur that homogenizes sensible heat throughout the local area. Without wind data we have no way of analyzing this possibility. It does remind us that one dimensional energy budget analyses of rnicroclimates are not always accurate.
Unfortunately, we see no way of predict- ing r::here these anomalous sites may occur based on this study, which is bad news for those of us attempting to model mountain microclimates from general prin- ciples. We are exploring the use of high spatial resolution digital satellite imagery taken four times per day to assist in this problem.
Transpiration/Photosynthesis Model The north facing slopes produced substantially more seasonal photosynthe- sis than the south slopes on both the Ambrose and Ninemile sites (Figure 5).
Schwartz was not computed because of the long period of missing data. The over- riding factor producing these patterns of photosynthesis was late season plant moisture stress resulting from soil water depletion. South slopes had higher photo- synthetic and transpiration rates through- out much of the growing season, but came under progressive plant moisture stress earlier than the north slopes, 21 days earlier at Ninemile and 2 7 days at Ambrose
(Figure 6,7). Differences in hydrologic timing were in part caused by the early
Seasonal Photosy:ithes:s
; : ; - 1 0 0 ~ , - - -
't 90 i
E ao:
o·
7o 1u
< ~ 60
i
50>
C 40 -§. c 30 1
; North I
~ 1
; . / I
/'\...
:
/ ~ i// Sci'..!tn !
1
a.
100 Seasonal Photosynthesis 90
20
Jt..kmc!ay
b.
Figure 5. The seasonal trend of tree
!)notosynthesis simulated by DAYTRANS/PSN for the north and south slopes of (a) Ambrose and (b) Ninemile sites.
Seasonal Transpiration Seasonal Transpiration 800 . , - - - ~ 700
~ 600 0 500
-:£
.g
400·i
C: 300~ 200 100
North
South
132 174 216 258 300 Julian day
a.
700
600 500 400
300 200 100
North
172 204 236 268 300 Julian day
b.
Figure 6. The seasonal trend of tree transpiration simulated by DAYTRANS/PSN for the north and south slopes of (a) Ambrose and (b) Ninemile sites.
spring rates of snowmelt. The snowpack at the beginning of the simulation at Ambrose (JD 91) was 29 cm water on the north, 11 cm on the south. As a conse- quence, snowmelt was complete 27 days earlier on the south slope, and soil moisture depletion in this snow dominated hydrologic system started that much sooner. This implies that the origins of differences in seasonal photosynthesis on water limited sites are very early in the spring when snowmelt first begins differ- entially on south slopes.
As expected, higher evapotranspira- tion on the south slopes contributed to more rapid soil moisture depletion (Fig- ure 6,7). For example, at Ninemile up to the day {JD 195) of first water stress, the south slope had 38% higher transpiration and 7% higher photosynthesis than the north slope. At Ambrose on JD 176, the south slope had 16% higher cumulative transpiration and 6% higher photosynthe- sis. The higher temperatures and higher irradiance on the south slopes produced higher photosynthesis rates when water was not limiting. However, the concur- rently higher evaporative demands only modestly affected photosynthesis rates through the leaf conductance response, while directly increasing transpiration
rates. In other words, humidity appears to exert strong control over water use efficiency as defined by CO2/H2O exchange.
Optimum water use efficiency is attained with high radiation loads and low evapo- rative demands, conditions more nearly met on the north facing slopes. Since the south facing slopes had less snow- pack and "wasted" their water early in the summer, a long period, almost three months, of progressive water stress had to be endured.
DISCUSSION
Our simulation of seasonal photo- synthesis correlates quite well with the observed productivity differences found on north and south slopes in western Montana. At this point some critical questions must be addressed. Were our predictions merely good luck? How depend-
"'
0. ::e ..!..
]
C: 2
s 0
0.
Seasonal Water Stress
Ambrose o+.-~~~~~~~--r--..-1
80 132 174 216 258 300 Julian day
a.
Seasonal Water Stress 3 . , - - - ~
2
Nine Mile
0 +.-~~~~~~~r-J
140 172 204 236 268 300 Julian day
b.
Figure 7. The seasonal trend of tree water stress, as predicted by the pre- dawn leaf water potential, simulated for the north and south slopes of (a) Ambrose and (b) Ninemile sites.
able is this transpiration/photosynthesis model? Is there any way of verifying or validating these model estimates?
The slope microclimates were directly measured, so we have high confidence in the north-south differences in tempera- ture, humidity, radiation and potential evapotranspiration (Figure 4 and Table 3).
It is somewhat surprising that such seemingly small differences in tempera- ture and humidity, on the order of l degree and 7%, produce the 20-30% slope differences in potential evapotranspira- tion calculated. H~sler (1982) found a north aspect mountain site in the Swiss Alps to be 1.8 degrees cooler and 25%
lower vapor pressure deficit than an adjacent east facing slope, similar mag- nitudes to our results. Parker (1952) working in the mountains of Idaho, USA, came to the same conclusion that measur- able screen height temperature differ- ences were very modest between north- south slope pairs. However, on three mountains, the north slopes averaged 38%
higher soil moisture content when mea- sured on May 25-29. Likewise, our simu- lations estimated much more rapid soil moisture depletion on the south slopes.
Cantlon (1953) studied the microclimate and vegetation of a small mountain in New Jersey, USA. He found forest cover on the north slope to have 29% higher basal area than the south slope despite an air temperature at 2 m only 0.5 degree cooler than the south slope. As with other results, evaporative demand on the north slope was 22% less than the south slope. Cooper (1961) related 32%
lower evaporation and 2 degrees cooler temperatures on north slopes to soil profile development on a mountain in Michigan, USA, finding deeper, finer soils with more water holding capacity on the north slopes. And, of course, Geiger (1961) illustrated numerous data showing south slopes to be warmer than north slopes. Although the south slope at Schwartz was not measurably warmer than the north slope, the snowpack melted six weeks earlier, which suggests that the surface microclimate is warmer, soil moisture depletion would occur sooner, and the· seasonal photosynthesis s t i l l
would be markedly lower than the north slope.
Running (1984b) recently completed more extensive testing of the sensitivity of the DAYTRANS/PSN model. Using esti- mated rather than measured meteorological data, he predicted 16% higher seasonal photosynthesis on a north slope than south slope in a water limited climate.
However, when water limitations were removed by artificially adding precipita- tion to the meteorological data set, 23%
higher seasonal photosynthesis was calcu- lated on the south slope. With water not limiting, the higher radiation and air temperatures on the south slope increased photosynthesis rates as would seem reason- able. Energy limited environments like Alaska, USA, support exactly that kind of forest pattern, where the south facing warmer slopes are more productive than the cold north slopes (Van Cleve et al.
1983).
Generalized use of a gas exchange physiology model like this ignores the species and seasonal differences in plant response to environment that are known to exist. We consider these models to rep- resent more "microclimate from the trees perspective," or an advanced form of operational environment analysis, rather than an accurate assessment of gas ex- change rates by trees. We do feel that basic responses of leaf CO2 and water vapor conductance to microenvironment in trees to be fairly uniform, although absolute magnitudes of gas exchange rates vary greatly (Reed et al. 1976, Sinclair et al. 1976, Tenhunen et al. 1980).
However, even for use in a relative sense as we have done here, the ratio of stoma- tal/mesophyll conductance is quite impor- tant. If stomatal conductance is large relative to mesophyll conductance, high evaporative demand will decrease photo- synthesis through stomatal closure. As the proportion of mesophyll conductance to overall CO2 conductance increases the higher temperatures and radiation associ- ated with high evaporative demand will increase mesophyll conductance faster than i t decreases leaf conductance. The result to photosynthesis rates will be the opposite of when stomatal conductance dominates. our model results here again point out that photosynthesis and trans- piration are only partially related and we find this stomatal/mesophyll conduc- tance to summarize the relative balance of the two variables. Unfortunately, a wide range of stomatal/mesophyll conduc- tance values are found for coniferous trees in the research literature (Bennett and Rook 1978, Beadle et al. 1981, Troeng and Linder 1982).
Seemingly, the logical way to vali- date this model would be to measure trans- piration and photosynthesis of trees on these sites. We predict that would be only partially successful because more vegetative competition developes during early succession on the north slopes consuming any "surplus" water. An indi- vidual tree on a north slope may then have higher plant moisture stress than an equivalent tree on a south slope.
This simulation artificially restricts the rooting zone of the standard tree on each slope, whereas in reality south slope trees probably allocate more carbon below ground to develop more extensive
root systems. Once established, indiv- idual trees can grow just as fast on a south as a north slope in this region, but the density of trees on the south slope is much less (Tesch 1981). The hypothesis by Grier and Running (1977) that stand LAI is controlled by site water balance would predict north slopes
to have higher total stand LAI and pro- ductivity. Consequently, our future research will attempt to empirically validate these estimates of seasonal photosynthesis by measuring productivity rates of stands in variable topography.
We see the greatest utility of this modeling as a "Biophysical evaluation of forest site potential" as suggested by Lee and Sypolt (1974) or an "ecological tree growth model," proposed by Reed
(1980).
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Bennet, K.J., and D.A. Rook. 1978.
Stomatal and mesophyll resistances in two clones of Pinus radiata D.
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Cooper, A.W. 1961. An example of the role of microclimate in soil gene- sis. Soil Sci. 90:109-120.
Daubenmire, R. 1956. Climate as a determinate of vegetation distribu- tion in eastern Washington and northern Idaho. Ecological Mono- graphs. 26(2):131-154.
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Emmingham, W.H., and R.H. Waring. 1977.
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483-490.
Geiger, R. 1950. The climate near the ground. Cambridge, Mass., Harvard Univ. Press.
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Relative control of air temperature and water status on seasonal gas exchange of Pinus contorta. Canad- ian Journal of Forest Research, (in press).
Grier, C.C., and S.W. Running. 1977.
Leaf area of mature northwestern coniferous forests: relation to site water balance. Ecology, 58:
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Hasler, R. 1982. Net photosynthesis and transpiration of Pinus montana on east and north facing slopes at alpine timberline. Oecologia.
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Lee, R., and C.R. Sypolt. 1974. Toward a biophysical evaluation of forest site quality. Forest Science, 20
(2) :145-154.
Loharnmar, T., S. Larsson, S. Linder and S.O. Falk. 1980. FAST simulation models of gaseous exchange in Scots pine. In: T. Persson (Editor), Structure and function of northern coniferous forests. Ecological Bulletins (Stockholm), 32:505-524.
Parker, J. 1952. Environment and for- est distribution of the Palouse Range in northern Idaho. Ecology, 33 (4): 451-461.
Pfister, R.D., and S.F. Arno. 1980.
Classifying forest habitat types based on potential climax vegeta- tion. Forest Science 26(1)52-70.
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