Southern 

Research Station

Research Paper

SRS–45 

United States

Department of

Agriculture

Forest Service

Leaf Gas Exchange of Mature 

Bottomland Oak Trees

Rico M. Gazal, Mark E. Kubiske, and Kristina F. Connor
July 2009

Southern Research Station

200 W.T. Weaver Blvd.

Asheville, NC 28804

Authors: 

Rico M. Gazal, Assistant Professor, Glenville State College, Department 

of Land Resources, Glenville, WV 26351; Mark E. Kubiske, Plant 

Physiologist, U.S. Forest Service, Forestry Sciences Laboratory, 

Rhinelander, WI 54501-9128; and Kristina F. Connor, Plant Physiologist, 

U.S. Forest Service, G.W. Andrews Forestry Sciences Laboratory, 

Auburn, AL 36849. 

Cover photo: Bottomland hardwoods by Brian Lockhart, U.S. Forest Service, Bugwood.org

DISCLAIMER

The use of trade of firm names in this publication is for reader information and does not imply 

endorsement by the U.S. Department of Agriculture of any product or service.
Abstract

We determined how changes in environmental moisture affected leaf gas 

exchange in Nuttall (Quercus texana Buckley), overcup (Q. lyrata Walt.), 

and dominant and codominant swamp chestnut (Q. michauxii Nutt.) oak 

trees in Mississippi and Louisiana. We used canopy access towers to 

measure leaf level gas exchange rates, e.g., photosynthesis, transpiration, 

stomatal conductance, on the upper crowns. Leaf gas exchange rates of 

the dominant swamp chestnut oak tree increased in response to summer 

precipitation. Nuttall and overcup oak trees exhibited constant gas 

exchange rates throughout the measurement periods. Dominant swamp 

chestnut oak also maintained high water use efficiency during periods of 

low soil moisture. These responses and access to water sources during 

soil drying may buffer extreme fluctuations in leaf gas exchange rates in 

mature canopy oak trees. Seasonal fluxes in water use and transpiration 

among individual oak trees of various sizes and other bottomland hardwood 

species need to be determined to assess their physiological and ecological 

contributions on the water balance of southeastern bottomland hardwood 

forests. 

Keywords: Drought, leaf water potential, photosynthesis, Quercus spp., 

stomatal conductance, water use efficiency.

Introduction

Bottomland hardwood forests in the floodplains of the 

Southeastern United States make up a highly diverse 

and productive ecosystem. This complex ecosystem is 

subjected to extreme fluctuations in soil and climate 

conditions throughout the year (Kellison and others 1998). 

Most of these sites are flooded for several days or weeks 

in winter and spring and have experienced extremely 

dry soil conditions in summer (Hodges 1998). Oak 

(Quercus) species dominate the bottomland forests, and 

their distributions depend on their tolerance to extreme 

fluctuations in edaphic, hydrologic, and climatic conditions 

(Hodges 1998). Oak species of bottomland forests coexist 

by differing means of adaptation, including anatomical and 

morphological structure as well as physiological response to 

the environment (McKevlin and others 1998). 

The physiological responses of several bottomland oak 

species to flooding have been studied under nursery and 

Leaf Gas Exchange of Mature 

Bottomland Oak Trees

Rico M. Gazal, Mark E. Kubiske, and Kristina F. Connor

field conditions (Gardiner 1994, McKevlin and others 

1998, Oren and others 2001, Pereira and Kozlowski 1977, 

Pezeshki and Chambers 1985). These studies revealed both 

morphological and physiological responses of bottomland 

oak seedlings to flooding. However, the responses of 

bottomland oak trees to drought have not been studied, and 

little is known about how they sustain productivity under 

extremely dry conditions in the growing season, and about 

the differential access of trees to water sources during 

drought (Dawson 1996, Teskey and Sheriff 1996). 

Overcup (Quercus lyrata Walt.) and Nuttall (Q. texana

Buckley) oak are considered flood-tolerant species and 

are most commonly found on poorly drained soil in the 

major floodplains of the Mississippi River. Swamp chestnut 

oak (Q. michauxii Nutt.) and other oak species, such as 

cherrybark oak (Q. pagoda Raf.) and water oak (Q. nigra 

L.), are classified as less flood-tolerant species and occur on 

bottomland sites where soil conditions range from poorly 

drained to well drained (Hodges 1998). These species, 

differing in tolerance to soil drainage and flooding, also may 

have unique responses to soil drought. High vapor pressure 

deficit (VPD) and substantial soil moisture reductions may 

greatly affect their water consumption and loss during 

summer drought (Bassow and Bazzaz 1998, Giuliani and 

others 1997, Oren and others 2001, Pataki and others 

1998). This study was conducted to characterize the diurnal 

variations of leaf gas exchange and water relations among 

mature oak trees in response to changes in environmental 

moisture (soil and atmosphere). 

Materials and Methods

Field Site and Plant Material

This research was conducted at two sites: the bottomland 

hardwood forests of the Delta National Forest in Mississippi 

and the Kisatchie National Forest in Louisiana. The Delta 

National Forest site (Delta: 32.81° N, 90.78° E) is a major 
2

floodplain about 2 km from the Mississippi River. The Delta 

site is dominated by sweetgum (Liquidambar styraciflua L.), 

Nuttall oak, and overcup oak. Sharkey (Chromic Epiaquerts) 

is the dominant soil series on the site, characterized by 

very deep, poorly drained, and very slowly permeable soils 

formed in clayey alluvium (Natural Resources Conservation 

Service 2001, Pettry and Switzer 1996). Flooding in major 

bottoms is usually seasonal with long duration (Hodges 

1998). The Kisatchie National Forest site (Iatt: 31.73° N 

92.64° W) is a 345-ha minor alluvial floodplain of Iatt 

Creek dominated by sweetgum, cherrybark oak, and water 

oak. The site was characterized in detail by Meier and 

others (1999). Soils are Typic Glossaqualfs of the Guyton 

soil series, consisting of very deep, poorly drained soils 

formed in thick loamy sediments (Broadfoot 1976, Natural 

Resources Conservation Service 2001). Flooding in minor 

floodplains is frequent and associated with local rainfall 

(Hodges 1998). The average annual precipitation (50-year 

period) is 1300 mm at the Delta site and 1450 mm at the Iatt 

Creek site (Unified Climate Analysis Network 2001). Mean 

maximum and minimum temperature (July and August 

2000) at both sites were similar [Delta site: 31 and 5 °C, 

respectively; and Iatt Creek site: 31 and 3 °C, respectively 

(Unified Climate Analysis Network 2001)].

Each site has a canopy tower that provides access to the 

crowns of mature canopy oak trees. The access canopy 

tower at the Delta site (26 m) is within reach of the crowns 

of a Nuttall oak (height 31 m, d.b.h. 57 cm) and an overcup 

oak (height 28 m, d.b.h. 83 cm). The tower at the Iatt Creek 

site (29 m) is accessible to the crowns of dominant (height 

27 m, d.b.h. 70 cm) and codominant (height 21 m, d.b.h. 31 

cm) swamp chestnut oak trees. 

Leaf Gas Exchange Measurements

Leaf gas exchange and water relations variables were 

measured using a portable leaf gas exchange system (PP 

Systems model, CIRAS-I, Hertfordshire, UK) on July 

20 and August 9, 2000, at the Delta site, and July 12 and 

August 2, 2000, at the Iatt Creek site. Photosynthetic 

carbon assimilation rate (A), stomatal conductance (gs

), 

and transpiration rate (E) were measured on three to five 

fully expanded leaves randomly selected on each of two 

branches per tree. Leaf water potential (Ψ) was determined 

with a pressure chamber (PMS Instrument Company, 

Corvallis, OR). All measurements were conducted during 

relatively cloud-free days from 0700 to 1800 hours under 

ambient light and temperature conditions. Predawn leaf 

water potential (Ψpd) was determined at 0500 hours. On 

each leaf gas exchange measurement day, four soil samples 

were collected at four cardinal directions within the drip 

line of each tree, using 20-cm deep soil cores for water 

content analysis. Soil water content was determined by the 

gravimetric method (oven-dried at 105 °C for 48 hours).

Meteorological Measurements

The topmost portion of each tower site was instrumented 

with an array of meteorological instruments (Delta: 26 

m, Iatt: 29 m). Air temperature and humidity (Vaisala 

model HMP45A, Columbus, OH), photosynthetically 

active radiation (LI-COR model LI-190SZ, Lincoln, NE), 

and wind speed and direction (Windset model MET One 

034A-L Windset, Logan, UT) were continuously monitored 

throughout the study using a data logger (Campbell 

Scientific model CR10x, Logan, UT). Rainfall and other 

meteorological data of each site were taken from the nearest 

weather stations (< 1 km from the project sites) maintained 

by U.S. Department of Agriculture World Agricultural 

Outlook Board and the Forest Service.

Statistical Analysis 

Gas exchange variables and meteorological data were 

compared by one-way analysis of variance using Statistical 

Analysis System (SAS Institute 2001). Within a site, each 

species and sampling date combination served as a separate 

treatment. Fisher’s least significant difference was used 

to compare responses between species within a site and 

measurement periods. Because only one tree per species 

was accessible from the towers, inferences can be drawn 

only for these trees and locations. However, common 

patterns of response may be suggestive and indicative of 

bottomland oak species in general.

Results

Microsite and Soil Moisture Conditions

For much of the Southeastern United States, 2000 was a 

relatively dry year (Drought monitor 2002). In 2000, the 

Delta site’s rainfall accumulation was 1147 mm, constituting 

88 percent of the site’s 50-year mean of 1300 mm. The Iatt 

Creek site’s rainfall total that year was 1194 mm, or 82 

percent of its yearly mean of 1450 mm. 

During the leaf gas exchange measurement period in July 

(July 20) at the Delta site, the highest daily temperature 
3

was 38 °C and the mean daylight VPD was 1.59 (±0.26)

kilopascal (kPa). In August (August 9), the highest daily 

temperature was 36 °C and the mean daylight VPD was 

1.60 (±0.28) kPa. The Iatt Creek site, on the other hand, had 

a maximum temperature of 34 and 35 °C in July (July 12) 

and August (August 2), respectively, and a mean daylight 

VPD of 1.56 (±0.26) kPa in July and 1.63 (±0.29) kPa in 

August. Amount of soil moisture at the Delta site did not 

vary significantly between July and August (28 percent ±0.5 

and 27 percent ±0.5, respectively). Significantly lower soil 

moisture at the Iatt site was observed in July (12 percent 

±0.6) compared to August (17 percent ±0.6). The increase 

in soil moisture in August at the Iatt Creek site was due to 

rainfall 3 days before soil sample collection and leaf gas 

exchange measurements. The soils of the Delta site are rich 

in clay while those of the Iatt Creek site are rich in sand 

(Natural Resources Conservation Service 2001).

Predawn Leaf Water Potential and Diurnal Physiology

At the Delta site, the Ψpd of Nuttall oak and overcup oak 

differed in July but were not significantly different between 

July and August (table 1). The differences between the 

species disappeared in August. The Ψpd of dominant and 

codominant swamp chestnut oak trees at the Iatt Creek 

site significantly increased (became less negative) by 47 

and 50 percent, respectively, from July to August. The 

increase in Ψpd coincided with an increase in soil moisture 

in August. gs

 and E of Nuttall oak and overcup oak at 

the Delta site did not differ significantly in both July and 

August measurement periods (table 1). At the Iatt Creek 

site, gs in July was significantly lower in the dominant 

swamp chestnut oak tree compared to the codominant tree. 

The gs and E of the dominant swamp chestnut oak tree in 

August were significantly higher than in July. Water use 

efficiency (WUE) (A/E) of the dominant swamp chestnut 

oak tree significantly declined in August when precipitation 

created an increase in soil moisture. The codominant swamp 

chestnut oak tree had significantly lower gs and E in August 

than in July. 

In July and August, gas exchange rates of the overcup 

oak tree increased between 0700 and 1200 hours and then 

declined throughout the afternoon (fig. 1). On the other 

hand, the Nuttall oak tree had low but almost constant leaf 

gas exchange rates throughout the day in July. In August, 

however, leaf gas exchange of Nuttall oak increased 

between 0700 and 1200 hours and then gradually declined 

throughout the day. Leaf gas exchange patterns for both 

species were related to photosynthetically active radiation 

(PAR) values that increased between 0700 and 1200 hours 

and then declined the rest of the day. The Ψ of both trees 

declined between 0700 and 1200 hours and then recovered 

in the afternoon (fig. 1I). The codominant swamp chestnut 

oak tree started with a high gs and E during early morning to 

midafternoon of the July and August measurement periods 

(fig. 2). In July, gs and E of the codominant swamp chestnut 

oak tree started to decline at 1200 hours and continued to 

decrease the rest of the afternoon (figs. 2C and 2E). The 

increase in leaf gas exchange rates by the codominant tree 

in the morning to midafternoon was related to an increase 

in mean daylight PAR values from 0700 to 1200 hours in 

July. In dominant swamp chestnut oak, gas exchange rates 

remained the same throughout the day (figs. 2C, 2E, and 

2G). In July and August, the Ψ of dominant and codominant 

swamp chestnut oak declined from early morning to 

midafternoon and then increased the rest of the afternoon 

(figs. 2I and 2J). Gas exchange rates and Ψ of both dominant 

and codominant swamp chestnut oak trees in August were 

similar throughout the day. 

Discussion

Relatively dry conditions in the summer of 2000 allowed 

the investigation of the water relations characteristics of 

four bottomland oak trees in response to soil drying in two 

bottomland forest sites that differ in hydrology and soil 

conditions. Different physiological responses of oak trees to 

summer precipitation were also observed in this study. 

High Ψpd has been linked with the deep root systems of 

several species (Abrams 1990, Abrams and Knapp 1986, 

Hinckley and others 1983, Whitehouse and others 1983). 

However, in bottomland forests where soil oxygen is 

limiting due to regular flooding, roots of most trees do not 

grow beyond a certain soil depth (Hodges 1998). A similar 

study in a mature bottomland deciduous forest revealed that 

average rooting depth was ~40 cm (Pataki and Oren 2003). 

Consequently, the bottomland oak trees appear to have 

access to soil moisture despite their shallow root systems. 

Further, bottomland trees would have the same root depth 

and access to ground water but, depending on the species 

and size, these trees would vary in their root distribution 

and hence water extraction capabilities. The increase in 

Ψpd (became less negative) from July to August in swamp 

chestnut oak trees was in response to precipitation. Deeper 

and more extensive root systems are in contact with moist 

soil layers and can establish water potential equilibrium 

with soil at night (Hinckley and others 1983). Therefore, 
4

Ψpd is considered a good measure of how soil drying affects 

plant water status during the day (Nadezhdina 1999). It is an 

indirect measure of the “effective rooting depth” at which 

trees extract the bulk of their water requirements (Abrams 

and Knapp 1986). 

None of the oak trees in this study exhibited midday 

depressions in leaf gas exchange rates (gs

, E, and A; figs. 

1and 2). The decline in leaf gas exchange rates followed 

the decline in maximum rates at noon and the decline in 

PAR as the day progressed. This result was consistent with 

a study by Bassow and Bazzaz (1998) of four deciduous 

Table 1—Response of leaf gas exchange of mature Nuttall oak (Quercus texana Buckley) and overcup oak 

(Q. lyrata Walt.) trees in the Delta National Forest, Mississippi, and dominant and codominant swamp chestnut oak 

(Q. michauxii Nutt.) trees in the Kisatchie National Forest (Iatt Creek), Louisiana, at different measurement dates and 

daylight mean photosynthetically active radiation in summer 2000 

Parameters

Delta site Iatt Creek site

July 20 August 9 July 12 August 2

Nuttall 

oak

Overcup 

oak

Nuttall 

oak

Overcup 

oak

Dominant 

swamp 

chestnut 

oak

Codominant 

swamp 

chestnut 

oak

Dominant 

swamp 

chestnut 

oak

Codominant 

swamp 

chestnut 

oak

gs 87.4 A 98.7 A 91.3 A 95.4 A 59.5 C 161.9 A 100.7 B 119.2 B

se 5.0 8.1 7.7 9.6 7.0 19.7 5.6 5.3

E 3.2 A 3.7 A 3.4 A 3.6 A 2.2 A 4.5 A 3.2 B 3.6 B

se 0.2 0.3 0.3 0.3 0.2 0.4 0.1 0.1

A 1.4 B 3.0 A 2.4 AB 2.7 A 2.2 A 2.1 A 1.4 1.7 A

se 0.3 0.5 0.4 0.5 0.8 0.7 0.2 0.2

Ψ -1.13 A -1.49 B -1.50 B -1.32 AB -1.15 A -1.34 A -1.35 A -1.23 A

se 0.11 0.15 0.10 0.13 0.22 0.12 0.11 0.09

Ψpd -0.36 B -0.21 A -0.31 AB -0.22 A -0.28 BC -0.33 C -0.19 A -0.22 AB

se 0.08 0.04 0.02 0.02 0.01 0.04 0.04 0.03

WUE 0.57 A 0.71 A 0.47 A 0.49 A 0.79 A 0.41 B 0.41 B 0.46 B

se 0.07 0.09 0.09 0.09 0.29 0.11 0.06 0.05

PAR 84.4 C 377.56 AB 260.8 BC 510.8 A 85.46 A 158.79 A 87.79 A 110.88 A

se 12.5 74.8 40.0 93.0 30.9 63.6 14.9 22.0

gs = stomatal conductance (mmol m-2 s-1); E = transpiration rate (mmol m-2 s-1); A = photosynthesis rate (μmol m-2 s-1)]; Ψ (MPa) = leaf water potential; Ψpd

(MPa) = predawn leaf water potential; WUE = water use effi ciency (A/E); PAR (μmol m-2 s-1) = photosynthetically active radiation. 

In a row (each site), means with the same letter are not signifi cantly different (P > 0.05).
5

Figure 1—Diurnal response of Nuttall oak (NO) and overcup oak (OCO) on Delta National Forest (Delta 

site), Mississippi, on July 20, 2000 [PAR (A), gs (C), E (E), A (G), and Ψ (I)] and on August 9, 2000 [PAR 

(B), gs (D), E (F), A (H), and Ψ (J)].
6

Figure 2—Diurnal response of dominant (DSCO) and codominant swamp chestnut oak (CDSCO) on 

Kisatchie National Forest (Iatt Creek), Louisiana, on July 12, 2000 [PAR (A), gs

 (C), E (E), A (G), and Ψ (I)] 

and on August 2, 2000 [PAR (B), gs (D), E (F), A (H), and Ψ (J)].
7

species that exhibited no midday depressions in diurnal 

patterns of photosynthesis and that showed photosynthesis 

rates affected by canopy light intensity, leaf and air 

temperature, and VPD. Bassow and Bazzaz (1998) argued 

that the drought conditions when measurements were taken 

might not have been extreme enough for the trees to exhibit 

stomatal closure at midday. In this study, the relatively high 

values of Ψpd (–0.19 to –0.36 megapascal [MPa]) of the 

four oak trees suggest the ability of the roots of these trees 

to reach ground water. However, differences in their water 

utilization patterns during daylight hours triggered different 

responses of these trees to summer drought. 

Stomata conductance of several species also has been 

observed to decline as soil dries out (Hogg and Hurdle 

1997, Hogg and others 2000, Martin and others 1997, Pataki 

and others 1998). In this study, gs

 and E of codominant 

swamp chestnut oak declined in response to high VPD in 

August. The high VPD also caused decline in A and gs in 

a number of species such as red maple (Acer rubrum L.), 

trembling aspen (Populus tremuloides Michx.), eucalypts 

(Eucalyptus tetrodonta F. Muell.), and hazelnut (Corylus 

cornuta Marsh.) (Hogg and others 2000, O’Grady and 

others 1999). In contrast, there was no change in gs, E, and 

daily water use for both Nuttall and overcup oak trees from 

July to August. The constant leaf gas exchange rates can be 

attributed to constant soil moisture content and VPD in July 

and August. 

Water use efficiency (A/E) in the dominant swamp chestnut 

oak tree was significantly higher in July than in August. 

This suggests that the dominant tree was conserving more 

water in July, when soil water was limiting. Apparently, the 

dominant swamp chestnut oak tree tended to conserve water 

by restricting water loss during the driest measurement day 

in July. Thus, low gs

 and E and high A were observed in 

July in the dominant tree. In August, however, an increase 

in soil moisture due to precipitation increased gs and E in 

the dominant swamp chestnut oak tree. In another study, the 

increase in WUE during soil drying was attributed to the 

stomatal control of water loss at the beginning of the stress 

(Arndt and others 2000). Swanson (1994) explained that 

high canopy gs on moist soils contributes greatly to total 

stand water loss during soil drying between rain events. 

Thus, dominant oak trees that can consume considerably 

large amounts of water on a daily basis have a great impact 

on the overall water balance in bottomland forests.

In summary, the increase in leaf gas exchange and Ψpd of 

the dominant and codominant swamp chestnut oak trees 

was related to summer precipitation in August. Differential 

access of oak trees to water sources was evident with the 

relatively constant leaf gas exchange despite substantial 

environmental variation between July and August 

measurements. Long-term measurements of sap flow of oaks 

and other bottomland species are necessary to determine 

the seasonal fluxes in water use and their physiological and 

ecological consequences in water balance of bottomland 

forests. 

Acknowledgments

This study was supported by the Forest Service, Southern 

Global Change Program and the Mississippi State 

University, Department of Forestry. Milan Vavrek and 

Diomides Zamora provided valuable reviews of the 

manuscript.

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Department of Agriculture Forest Service, Southern Research Station. 8 p.

We determined how changes in environmental moisture affected leaf gas exchange in Nuttall 

(Quercus texana Buckley), overcup (Q. lyrata Walt.), and dominant and codominant swamp 

chestnut (Q. michauxii Nutt.) oak trees in Mississippi and Louisiana. We used canopy 

access towers to measure leaf level gas exchange rates, e.g., photosynthesis, transpiration, 

stomatal conductance, on the upper crowns. Leaf gas exchange rates of the dominant swamp 

chestnut oak tree increased in response to summer precipitation. Nuttall and overcup oak 

trees exhibited constant gas exchange rates throughout the measurement periods. Dominant 

swamp chestnut oak also maintained high water use efficiency during periods of low soil 

moisture. These responses and access to water sources during soil drying may buffer extreme 

fluctuations in leaf gas exchange rates in mature canopy oak trees. Seasonal fluxes in water use 

and transpiration among individual oak trees of various sizes and other bottomland hardwood 

species need to be determined to assess their physiological and ecological contributions on the 

water balance of southeastern bottomland hardwood forests. 

Keywords: Drought, leaf water potential, photosynthesis, Quercus spp., stomatal 

conductance, water use efficiency.
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