434

TRANSPIRATIONAL RELATIONSHIPS OF TULIP POPLARI

Ro Ko McConathy and S. B. McLaughlin

Environmental Sciences DivisionOak Ridge National Laboratory

Oak Ridge_ Tennessee 37830

ABSTRACT

Leaf xylem water potential, stomatal diffusion resistance,and calculated transpiration of a mature in situ tulip poplar(Liriodendron _ifera L.) canopy were examined and related tonaturally occurring variations of the surrounding forestenvironment to determine their relative importance in affectingcalculated transpiration rates. Data were collected at threecrown levels during July and August. Transpiration valuesdecreased from 32.0 +_ 15.3 x I0 -s g cm-2 min -I in the uppercrown to 5.5 ± 4.2 x I0 -s g cm-2 min "I in the lower crown inresponse to gradients in vapor pressure deficit, wind speed, andthe difference between leaf and air temperature. Leaf water lossand resulting water deficits were not limited by stomatal controlduring the day. Stomatal diffusion resistance was controlledmainly by light intensity and did not increase as leaf xylemwater potentials approached -22 bars. Transpiration rateswere closely related to energy input and were influenced bymorning dew deposition on leaves. These data indicate tulip ,_poplar is a species in which photosynthesis proceeds at theexpense of large transpirational losses and associated tissuewater deficits.

IResearch supported by the Eastern Deciduous Forest Biome,US/IBP, funded by the National Science Foundation underInteragency Agreement AG-199, DEB76-O0761 with the U.S.Department of Energy under contract W-7405-eng-26 with UnionCarbide Corporation, Oak Ridge National Laboratory.Publication No. 1253, Environmental Sciences Division, ORNL.

435

INTRODUCTION

Tulip poplar (Liriodendron tulipifera L.) is an importantsecond growth species in eastern deciduous forests with a fastgrowth rate that is maximized on mesic sites° Maximum growth offorest trees normally requires a rapid uptake of atmosphericcarbon dioxide with concomitant high rates of transpirationalwater loss, Resulting tissue water deficits may affect sensitivecellular growth processes (Hsaio 1973). Therefore, the balanceof water and carbon dioxide exchange by leaves is crucial tomaintenance of growth processes. This study examines therelationships between selected physiological parameters of insitu tulip poplar foliage and the environment. Calculatedtranspiration rates are compared with stomatal resistances, leafwater potentials, and various environmental parameters to ,elucidate factors influencing transpiration water losses.

METHODS ,

Study Si te

The study area is in the Cesium 137 Forest Research Area,Oak Ridge National Laboratory, Oak Ridge, Tennessee. The forestis a second growth, bottomland, mesic hardwood stand dominatedby tulip poplar (76% of the above ground biomass) (Sollins, etal. 1976). It is located in a karst depression in which analluvial silt loam of the Emory series overlies dolomite bedrock.High humidity and dew deposition in early morning with dewevaporation from leaf surfaces completed by late morning istypical at this site. The site is described in detail bySollins et al. (1976).

A co-dominant tulip poplar (DBH 24.6 cm, height 25.5 m, age53) was selected for this study. A 30.5 meter walk-up towernext to the tree provided access to the eastern half of the treecrown.

S_am.Ele Procedure

Data were collected on seven days in the period July 9 toAugust 27, 1973 at three crown levels (top, middle, and bottom).

i

436

Individual measurements were made on leaves in situ. Datarecorded for each leaf included ° (I) time of day _eastern day-

light saving time), (2) height of leaf above the ground, (3)relative humidity, (4) air temperature_ (5) leaf temperature,(6) light intensity, (7) leaf xylem water potential, (8) leafdiffusion resistance, (9) leaf dimensions, and (10) wind speed.

Air temperature (°C) and relative humidity were measuredusing a Bendix Psychron ventilated wet-dry bulb psychrometerequipped with mercury thermometers. The instrument was shadedand held at the same height as the leaves being sampled.

Leaf temperature (°C) was measured using a 0°2 mmcopper-constantan thermocouple held in contact with the lower leafsurface by a spring clip (Gale et al. 1970). Once in place,temperature measurement required less than 30 seconds. Betweenmeasurements, the sensor was shaded to prevent heating.Thermocouple output was measured with a Wescor Model MJ-55Psychrometri c Mi crovol timeter.

A Weston Illumination Meter was used to measure illuminance. :The meter was oriented with and held next to the leaf duringmeasurements. Foot candle values obtained were divided by 7000to estimate the solar radiation flux density in cal cm-2 min-1 ,_(Reifsnyder and Lull 1965). Solar radiation flux densityestimated in this way had a correlation coefficient of 0.99 withvalues measured by a Kipp and Zonen solarimeter (0.3 to 3.0wave-lengths)(McConathyet al. 1976).

Stomatal diffusion resistance (sec cm-I) was measured onthe underside of leaves using a Lambda Diffusion ResistanceMeter and Diffusion Resistance Porometer (Kanemasu et al. 1969,Lambda 1971).

Leaf diemensions were measured along the midvein (length)and at right angles to the midvein to the side lobe tips(width). Leaf area was estimated from the product of length andwidth using regression analysis (McConathy et al. 1976).

Wind speed (cm sec -I) was measured with a cup anemometerinstalled at the top of the 30.5 meter tower. The anemometerwas about 3 meters above the mean canopy height. Mean wind

437

speed was determined for each sampling period; thus_ short-termvariations are not reflected in this value. The wind speedprofile through the plant canopy was calculated from wind speedat the top of the canopy and an extinction coefficient using theequation (Murphy and Knoerr 1972):

V = Vt exp [k2 (Ht -H)] (1)

where V is the wind speed at heightH in cm sec-l, Vt is thewind speed above the canopy (cm sec-I) at height Ht (3048 cm),k2 is the extinctioncoefficientfor wind speed [equalto 0.0017(Shinn 1969)], and H is the height (cm) for which V is determined.

A Scholander-type pressure bomb was used to measure leafxylem water potential (-bars) (Scholander et al. 1965). Afterall other measurements were completed xylem water potential wasdeterminedon each leaf within two minutesof excision. In thisstudy, leaf xylem water potentialbecomingmore negative isdefinedas "decreasing"and when becomingmore positive isdefinedas "increasing"water potential.

Transpiration Calculations

Transpirationwas calculatedfrommeasured parametersusingthe equation of Gates (1965):

SVD1 - RH (SVDa)E = (2)

R

where E is the transpirationrate in g cm-2 min-I, RH is therelative humidity of air, SVDa is the saturated water vapor

density of free air at air temperature (Ta) in g cm_3, SVD_ isthe saturated water vapor density of air in the sub_tomatalcavity at leaf temperature (TI) in g cm-3 (relative humidity ofthe substomatal cavity is assumed to be 1.00), and R is thetotal resistance of the diffusion pathway in min cm-I. R iscomposed of mesophyll, stomatal, and boundary layer resistances.

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Previous attempts to correlate physiological parameterswith particular environmental variables in the field have notbeen entirely successful because physiological parameters aresimultaneously affected by more than one environmental variable(Jarvis 1976). Some parameters may show a diurnal hysteresis

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when correlated, leading to increased scatter in the plotteddata.

In this study 167 complete leaf-environment measurementswere collected, The lack of independence of someof thevariables measured did not permit the use of multiple regressionanalysis procedures to isolate influences of individual •parameters. Thus, linear regression equations are presentedonly to indicate the general trends of the data while thecorresponding regression coefficients (r 2) indicate dispersionof data aboutthe regressionline.

RESULTS AND DISCUSSION

Calculated transpiration rates (equation 2) during thestudy ranged from 1.7 to 67.7 x I0 -s g cm-2 min -I with the meanequal to 20 +_16 x 10-s g cm-2 min -I. This compares favorably

with published daily average values for tulip poplar rangingfrom 16 to 20 x !0 -s g cm-_ min -z (Kramer and Kozlowski 1960),These tulip poplar transpiration rates are equivalent to orslightly higher than those for red oak (13 g cm-2 rain -I x I0-5),buckeye (12 g cm-2 min-I x lO-S), and boxelder (10 g cm"2 min"Ix 10-5 ) reported by Salisbury and Ross (1969). Thus, tulippoplar appears to have a rather high transpiration rate comparedto thesespecies.

Studies have shown correlations between transpiration andthe environmental variables leaf temperature, vapor pressuredeficit (VPD), and light intensity (Kramer and Kozlowski 1960,Lange et al. 1971). Plant variables, such as stomatal resistanceand water stress, have also been _orrelated with transpiration i(Jarvis 1976, Meldner and Mansfield !968).

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EnvironmentalVari ables

Air and leaf temperature affected transpiration ratethrough the establishment of vapor-pressure gradients betweenthe leaf and atmosphere. During this study, VPDvalues weresignificantly correlated with leaf temperature (TLEAF):

TLEAF = 22,,21+ 0.58 (VPD)

r2=O.74 (5)

where TLEAFis in oC and VPDis in millibars. As air temperature

increased, the difference between leaf and air temperature _(TDIFF) increased (Figure I). Air and leaf temperatures weregreatest in the afternoon in the upper crown, the time and crown .

, '

position where insolation was greatest. In the upper crownTDIFF increased with increasing air temperature. Lower crownleaves were shaded and TDIFF decreased with increasing airtemperature, possibly in response to increased transpiration andconvective cooling during the warmest period of the day. As airtemperature increased above 22oc, the difference between leaftemperature in the upper and lower crown increased. Thisresulted in higher VPD's in the upper crown.

The l inearity of a relationship between two variables overtheir naturally fluctuating range measures the degree of controlone variable has over another. Of all the variables we examined,transpiration rate was most linearly related to VPD (over therange 0 to 24 millibars):

E = 0,000021 (VPD) - 0.000016

r2:0.48 (6)

where E is in g cm-2 min -I and VPD is in millibars. Anynonlinearity in this relationship would be due to changes inplant resistances to transpiration (O'Leary 1975) or temporalchanges in other controlling variables.

441

ORNL--DWG 78-46948

36#

- . - ALL DATA /o

_i 34 TOP CROWN / ;;! ..... MIDDLE CROWN

........ BOTTOM CROWN/

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22 •

ALL DATA: Y = 0.32 + 4.05 (X) r2 = O. 86

!!i TOP' Y = 4.40(X)-- 0.53 r2 = 0.8420 MIDDLE :Y==3.44 +0.93(X) r2 ==0.90

BOTTOM :Y=3.40 +0.92(X) r2 - 0.93

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; 20 22 24 26 28 30 32 34 36AiR TEMPERATURE (°C)

(x) iFigureI. The relationshipbetweenair temperatureand leaf

temperaturefor tulip poplar leaves at three crownlevels.

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442

The relationship between total radiation absorbed by theleaf and TDIFF was slightly non-linear with TDIFF (± SD) valuesof 0°9 _+_0o9OC and 3.8 ± Io8OC associated with values of 0.55and 1o05 cal cm-2 mln -I total radiation absorbed, respectively.Between 1.05 ca! cm"2 min "! and a full sunlight value of I.2 calcm°_2 min _I, TDIFF (_+ SD) values increased nonlinearly to5.8 ± 1°2oco The increase in TDIFF with higher radiation inputincreased the vapor-pressure gradients driving transpiration.

Plant Variables

Hinckley et a]. (1978b) reported stomata react strongly totemporal radiation patterns, apparently a universal pattern inforest trees, with minimum leaf resistance reached at radiationlevels exceeding 10% of full sunlight (e.g., about 0.15 ca]cm-2 min-1)o In tulip poplar the minimum radiant flux densitythat initiated stomatal opening (rs = 4.0 sec cm-I) was about0.04 cal cm-2 min -I, Diffusion resistance reached a minimum

(r s = 1.0 sec cm-1) at about 0.6 cal cm-2 min -I and did not i!i_idecrease significantly at higher radiation flux densities.These light values correspond to those found by Shimshi(unpublished) where photosynthesis increased rapidly below 4.0sec cm-I diffusion resistance (stomatal openlng) and peaked at ,_light levels of 0.7 cal cm-2 min -z (minimum diffusion resistance).Other plant species may attain a minimum diffusion resistance atlower radiation flux densities (e.g., 0.15 to 0.3 cal cm-2 min -1)(Hinckley ez al. 1978, Slatyer and Bierhuizen 1964) than tulippoplar. Thus, the high light threshold for maximum stomatalopening is a partial explanation for the shade intolerance oftulip poplar.

Leaf xylem potential values decreased as stomatal resistancedecreased and transpiration increased. A rapid drop in xylempotential occurred at a stomatal resistance of 3.5 sec cm-I, thepoint where stomata opened in response to increasing radiation.Thus, the point at which transpiration losses exceeded wateruptake was reflected in a decreasing leaf xylem potential.Decreases in leaf xylem potential values from -3 to -18 barsdid not cause stomatal closure, i.e., increased stomatalresistance. A few observations with xylem potentials above -22bars, however, indicated a possible reduction in transpirationrates.

443

Threshold xylem potential values causing stomatal closurein trees have been found over a range from -15 to -24.5 bars(HInckley et al. 1978a, Federer 1977). This threshold value isinfluenced by the growing environment and stage of maturity andgrowth (Hinckley et al, !978b, Sullivan 1974, Watts 1977).Higher threshold water potentials have been found in Douglas fir !seedlings and greenhouse grown trees (®16 bars) than in foresttrees (-22 bars) of the same species (Waring and Running 1978).Hinckley et al. (1978b) and Hsiao (1973) report similar findingsfor other species. Richardson et al. (1973) found the stomatalclosing threshold for tulip poplar seedling was -18 bars.Seedlings in their study were allowed to experience cumulative

increases in water stress, i.e., morning xylem potentials of-15 ibars. The absence of a cumulative water stress in our study _itree may explain the apparent stomatal closure at a lowerpotential of around _22 bars. This tree occupied a moist sitethat rarely developed soll water potentials below -5 bars. _Leaves routinely exhibited early morning leaf xylem potentialsbetween -4 and-7 bars indicating almost complete recovery fromdaily moisture deficits,

Tulip poplar has been shown to exhibit a seasonal osmotic •' adjustment of-1.7 bars thereby maintaining favorable cell water

relations for stomatal opening and cell growth (Federer and Gee1976, Roberts 1977). In the forest, tulip poplar is able to !!_maintain internal water potentials below threshold values forstomatalclosure,thus maintainingphotosynthesisat a highrate.

Crown Height Variations !

,_ Vertical changes in stomatal diffusion resistance, leafxylem potential, and transpiration rate are shown on Figure 2_The crown was sampled as three zones (top, middle, bottom) based

. on leaf morphology and exposure. Fully exposed upper crown !ileaves were the smallest leaves on the tree and, in addition,were more vertically oriented than other leaves. This leaforientation suggested the possibility of a photomorphogeneticresponse. Phototropism has been observed in tulip poplar, butthe small angular change noted was not associated with anydiurnal pattern nor was it synchronized among leaves (Hutchison,personalcommunication). The vertical leaf orientation is

444

445

probably a mechanism that reduces the leaf angle to incomingsolar radiation, thus minimizing heat build_up and admittinglight to the lower canopy that is usually under less stress(Horn 1971). This would then increase photosynthesis in thelower layers. Middle zone leaves were slightly larger than topzone leaves and experienced more shading than the fully exposedupper crown leaves. Crown closure with adjacent trees occurredat about 18 to 19 meters (8 meters below the crown tip). Thebottom zone was almost in complete shade, below the level ofcanopy closure and large shade leaves predominated.

Leaves in the top and middle zone had about the samestomatal diffusion resistance values (Figure 2). Diffusionresistance began to change at the height of crown closure withneighboring tree crowns. This supports the hypothesis thattulip poplar stomata react strongly to light and remain openabove a threshold intensity. Woods and Turner (1971) havereported a gradual stomatal closing in tulip poplar as lightdecreases, with rapid closure beginning at around 0.14 calcm-2 rain -I on the adaxial surface. Hutchison (1977) reportedrapid light extinction through the zone of crown closure from450 to 250 ly day -I, or close to 50%. Thus, rapidly increasingstomatal resistances at around 18 meters resulted from a 50%,or greater, decrease in light intensity through the zone ofcrown closure. High resistance values in the lower crownprobably resulted from low light levels, increased humidity, anda greater sensitivity to water loss than in sunleaves (Hall1975, Kozlowski 1976, Zobel 1978).

The mean stomatal resistance in the lower crown was 3.75sac cm-I, very near the 4.0 sec cmmz value associated with theC02 compensation point for tulip poplar. Hutchison (1977)estimated lower crown light levels to be about 0.16 cal cm-2min -I, or near the light level reported by Woods and Turner(1971) where rapid stomatal closu_e begins. This suggests thatleaves at the base of the crown may be photosynthesizing neartheir compensation point and thus, may be parasitic on the uppercrown at times of low light intensity.

Leaf xylem water potentlal ra!)idly decreased with height Inthe upper two zones (Figure 2). Potentials were uniform in thebottom crown. Xylem potentials in the top and middle zones

446

fo71owed transpiration indicating that the water supply to upperleaves was unable to replace water losses. Diffusion resistancevalues were unaffected even though there was a decrease in xylempotential from -6 to _13 bars in the upper two zones. Thisindicated that at this site normal water potentials did notcause stomata] closure in the upper crown and therefore stomatadid not control transpiration. This agrees with other resultsthat show stomata] control of transpiration occurs in themorning and evening when stomata are opening and closing, butduring the day when stomata remain open VPD determinestranspiration rates (Kozlowski 1976, Landsberg 1975, Watts1977). Good correlation between water potential and VPD hasbeen reported, with the relationship usually forming a hysteresisloop when plotted by time of day (Hinckley 1974, Jarvis 1976,Landsberg 1975). The water potential versus VPD hysteresisrelationship observed during this study agrees with thosereported by Jarvis (1976) who attributed the hysteresis effectto dlurna] changes in VPDlagging behind changes in availableenergy input.

Diurnal Variations

Generalized observations can be made about diurnal trendsin the data. Diurnal trends were driven by the total radiation '_absorbed by the leaves with peak radiation occurring at solarnoon (around I:00 PM EDT). Leaf temperature closely followedsolar radiation values as TDIFF values increased with increasingleaf temperature thereby increasing the moisture gradientbetween the leaf and atmosphere. VPD increased with a slightlag behind solar radiation input and, combined with largerTDIFF, resulted in maximum transpiration in the period around r;

solar noon.

Other factors also influenced these diurnal cycles. Thestudy tree was located in a valley where direct sun did notstrike the upper crown until midmorning, around 9:30 to I0:00AM. Heavydewwascommonon leaves in the morning. Dewevaporation would tend to form a sink for heat absorbed early inthe morning. This could explain the gradual changes in variablesduring the early morning. In the early afternoon the easternport_on of the tree where sampling was done became shaded. This

447

resulted in the steep decline of environmental and plant valuesafter solar noon.

Diurnal trends are represented by the means for morning(0900 to 1200 hrs.) and afternoon (1330 to 1600 hrs,) data inTable 1. These data represent all the data collected andstatistically there appear to be significant changes due to"date," thus the standard deviations may be inflated by thiseffect. Also, the response of some variables to crown positiondoes not appear to be the same with date, indicating aninteraction between data and crown position_

Temperature values between AM and PMshowed a generalincrease throughout the crown. TDIFF values showed an increasefrom AM to PM in the upper crown only, with no change in themiddle crown and a decrease in the lower crown, These resultsindicate the effect of direct solar heating in the upper crownand the relatively constant sun and shade conditions in themiddle canopy. Increased transpiration rates and wind speedsin the lower canopy reduced TDIFF through increased evaporativeand convective heat loss. VPD showed a large percentage changefrom AM to PM and maintained a gradient that decreased withheight down through the crowna Solar radiation showed a similartrend.

Transpiration rates increased from AM to PM with thepercent increase in the upper and lower crown being the same.The PM stomatal resistance increased slightly in the upper andmiddle crown and, whlle not significant, may have been aresponse to decreased water potentials. These results aresimilar to those of Landsberg (!975) who found that littlechange in stomatal resistance occurred from 11.3Q till 4:00.Lower crown stomatal resistance decreased from an AM value closeto 3.75 sec cm-I, an apparent threshold value for physiologicalfunctioning, to a lower PM resistance that resulted in increasedtranspiration. Change in stomatal resistance with height wasprobably a response to changing light values (Pereira andKozlowski 1974, Turner 1969, Watts 1977) with stomatal openingoccurring last in the lower crown. This was reflected in thelarge percentage decrease in PM water potential in the lowercrown that resulted from increased transpirational water loss.

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448

Table I. Environmental and plant variable means (± SD) by crown positionduring two periods of the day (AM = 0900 to 1200 and PM = 1330 to1600)

Crown* PercenttVariable zone AM PM change

Time of day(hr) 1014± 56 1455-+ 48

Air temperature T 24.3 -+!.6 29.1 ± 2.0 19o8(°C) M 24.1 _+2.0 28.5 _+2_4 18.3

B 23.1 +-2.6 27.5 +_2.7 19,0

Leaf temperature T 26.0 + 1.7 31.3 +_2.6 20°4(°C) M 25.2 _+1.7 29.7 ± 2.4 17.9

B 24.3 _+2.4 28.3 ± 2.7 16o5

TDIFF T 1.7 _+1.0 2.2 _+1.5 29.4(°C) M 1.1+-0.8 1.1± 1.1 0o0

B 1.2 + 1.0 0.8 ± 0.8 -50.0

Calculatedwindspeed T 58.5 + 6.3 48.7 +_29.6 -20.1(cmsec-I) M 24.0 _+1.7 19.8 +_17.1 -21.2

B 6.5 +_4.1 10.8 ± 9.7 66.2

Radiation input T 0.75 +-0.05 0.89± 0.13 18.7(cal cm-2 min-I) M 0.69 +-0.03 0.77 +- 0.07 11.6

B 0.65 _+0.04 0.71-+ 0.07 9.2

Vapor pressure deficit T 6.4 ± 2.8 14.9 ± 2.9 132.6(millibars) M 6.3 +-3.4 13.4 +-4.2 114.4 "_

B 5.3 ± 3.3 10.2 +-5.1 92.5

Transpiration rate T 1.67 +-0.68 3.20_+1.53 91.6(g cm-2 m_n-I x 10-") M 1.06 -+ 0.40 1.69_+ 0.83 59.4

B 0.55 _+0.42 1.06+_ 0.74 92.7

Stomatal resistance T 1.73 +-0.57 1.97 +_0.76 13.9(seccm-_) M 1,93+_0.52 2.22+ 0.55 15.0

B 3.44-+1.23 2.77+ 1.13 -24.2

Xylemwaterpotential T 8.58± 2.61 13.27_+2.37 54.7(-bars) M 6.88 ± 2.37 9.93-+2.68 44.3

B 4.36± 1.87 7.43+ 3.02 70.4

*T=top;M=middle;B =bottom. _

tpositive = increase from AM to PM; negative = decrease from AM to PM.

449

The tulip poplar canopy microclimate is essentially "lightdependent." Changes in temperature, relative humidity_ andambient C02 occur in direct proportions to fluctuation inradiant flux density (Burgess and O'Neill 1975)o Stomata openin response to increasing light levels. VPD increases as aresult of temperature rises induced by solar heating. Thispromotes increased transpiration resulting in water potentialdeficits in transpiring leaves. Temporal water potential valuesduring the study were not of sufficient magnitude to causestomatal closure.

These interpretations agree with those drawn from studieswith other species, where water potentials and transpirationrates responded primarily to changing VPD, temperature, andsolar radiation values (Federer and Gee !976o Jarvis 1976,Llepper 1968, Landsberg 1975, Pereira and Kozlowski 1977).Jarvis (1976) found linear regressions of air temperature, VPD,and light in combination accounted for 70 to 95% of waterpotential variations over short-time intervals. Stomata havebeen shown to respond primarily to light with little change abovethreshold values, and to exhibit responses to environmentalconditions which vary with crown positions (Keller and Tregunna1976, Kozlowski 1972, Landsberg 1975, Pereira and Kozlowski1974, Watts 1977, Woods and Turner 1971).

CONCLUSIONS

This study describes responses of tulip poplar foliage to !i_spatialand temporalvariationsof canopy environmentalvariables. The data indicate that tulip poplar tolerates highmoisture stress with a high threshold for stomatal closure andwith overnight recovery from daily water losses. Transpiration !in the upper and middle crown is most closely related to VPDvalues, which is the controlling variable during the middayperiod when stomatal resistance is minimum. Stomata onlycontrol transpiration during opening and closing in the earlymorning and late afternoon. A comparatively high lightintensity required for minimum diffusion resistance relates tothe shade intolerance of tulip poplar. Changes in leaf xylemwater potential follow the temporal trend of transpiration,reflecting water losses in excess of leaf water supply.

450

Stomatal diffusion resistance in the lower crown is higher thanin the upper crown in response to light attenuation below thelevel of crown closure. Early-to-mid morning and mid-to-lateafternoon diffusion resistance values in the lower crownapproach a value corresponding to the carbon dioxide compensationpoint. This implies that photosynthetic production in the lowercrown at these times may just be sufficient to provide for itsohm maintenance.

Early successional species, such as tulip poplar, enhancephotosynthesis and growth by keeping stomata open longer thanlater successional species under conditions of decreasing waterpotential (Phelps et al. 1977). Data from this study indicatethat tulip poplar follows a behavior pattern in which openstomata maximize photosynthesis at the expense of largetranspirational losses and resulting tissue water deficits.Thus, the poor performance of yellow poplar on drier sites maybe a function of high transpirational losses leading to tissuemoisture deficits that limit growth.

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