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2Department of Agronomy, University of Nebraska, Lincoln, Nebraska 68583-0817 USA; 3Department of Oceanography, Florida State University, Tallahassee, Florida 32306-3048 USA; 4School of Natural Resource Science, University of Nebraska, Lincoln, Nebraska 68583-0728 USA; and 5Department of Atmospheric Sciences, Yonsei University, Seoul 120-749, Korea
Received for publication March 7, 2000. Accepted for publication June 20, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUDING REMARKSLITERATURE CITED |
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Key Words: convective throughflow • methane • Phragmites • Poaceae • prairie wetland
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUDING REMARKSLITERATURE CITED |
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; Morrissey, Zobel, and Livingston, 1993
). Transport can occur through aerenchymatous tissue via simple diffusion, as in Peltandra virginica and Sagittaria lancifolia (Sebacher, Harriss, and Bartlett, 1985
). Recently, the importance of convective throughflow (i.e., mass transport of gases through plants driven by pressure gradients) in enhancing gas transport has been demonstrated in diverse genera including Nuphar, Nymphaea, Nelumbo, Typha, and Phragmites (Armstrong and Armstrong, 1990
; Grosse, Büchel, and Tiebel, 1991
; Brix, Sorrell, and Ott, 1992
). The pathway for convective throughflow in Phragmites australis is from live present-year culms, through the rhizomes located in marsh sediments, and out standing-dead culms from previous years (Armstrong and Armstrong, 1990, 1991
). Convective throughflow enhances oxygen availability to rhizomes and roots while at the same time increasing methane transport from anaerobic sediments, where CH4 concentration is relatively high, to the atmosphere, which typically has a much lower CH4 concentration. This pathway thus bypasses the water column where CH4 oxidation can occur.
We have previously conducted integrated research programs to study surface–atmosphere exchange of carbon and energy in wetlands in northern Minnesota (Shurpali et al., 1995
) and central Saskatchewan (Suyker et al., 1996
). Our specific goals included quantification of diel and seasonal emissions of methane with emphasis on understanding the role of environmental factors controlling methane emissions. We quantified spatially averaged methane emissions using the eddy covariance technique. Results from diel measurements indicated small differences between daytime and nighttime methane emissions in these two ecosystems. From late July through September 1993, we conducted a pilot study at a wetland in north–central Nebraska (Ballards Marsh) as part of an integrated research program similar to the Minnesota and Saskatchewan investigations. In contrast to our previous results, we observed pronounced diel variations in methane emissions that were most highly correlated with incoming photosynthetically active radiation (Kim et al., 1998
). Moreover, we subsequently observed a distinct seasonal variation in the magnitude of the day/night differences (Kim, Verma, and Billesbach, 1998
).
In keeping with our overall goal of elucidating relevant factors controlling CH4 emissions at Ballards Marsh, we initiated investigations into the role of plant-mediated transport of CH4 in this ecosystem. Our objective was to investigate the governing transport mechanisms, and their diel variation, in Phragmites australis (Cav.) Trin. ex Steudel. We used a two-pronged approach: (1) direct measurements of pressure inside Phragmites culms and (2) measurements of CH4 concentrations and carbon isotopic composition. This paper presents results from the pressure measurements. Measurements of CH4 concentrations and carbon isotopic composition will be reported elsewhere.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUDING REMARKSLITERATURE CITED |
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2.5 km2 and is typical of many of the interdunal areas in the Sandhills where the water table is near, or above, the ground surface. The marsh is characterized by three major plant communities. The most common consists of 2–3 m tall Phragmites australis. A second plant community is dominated by 1–2 m tall Scirpus acutus Muhl. Both of these communities also contain scattered individuals of Typha latifolia L. and Sagittaria latifolia Willd.; however, the total biomass represented by species other than Phragmites and Scirpus is very small. The third community occurs in areas of relatively deep water (>1 m) where Zizania aquatica L. is the major emergent species and a submerged Potomogeton species is also common. Within 500 m of the primary measurement location, 70% of the surface is covered by Phragmites, 25% by Scirpus, and 5% by deep water. In the Phragmites- and Scirpus-dominated areas the water depth was 0.4–0.5 m during the entire measurement period. The water table depth in the marsh does not vary much during the year, as the marsh is fed primarily from ground water. In late April 1993, the water had a pH of 7.2 and a conductivity of 46 mS/m. Phragmites and Scirpus are mostly rooted in a 0.1–0.3 m thick layer of recent plant litter; this layer is underlain by sediments consisting primarily of highly decomposed litter, organic materials, and sand.
Field measurements
Healthy green leafy culms of Phragmites were selected randomly from an area near the location of the main micrometeorological instrumentation setup (see Kim et al., 1998
; Kim, Verma, and Billesbach, 1998
) 65 m south of the north edge of the marsh. Static pressures (
Ps, i.e., the pressure differential between the inside of the culm and the ambient air outside the culm) were measured in the following manner. The culm was taped in two or three places to surrounding culms and then severed with a razor blade 3–5 cm above the water surface. The taping was done to maintain the culm in the same position in the canopy after it was severed. The cut end was then connected to a pressure transducer (model CPFM, Furness Controls, Bexhill, Sussex, UK) with a short length of plastic tubing. The tubing contained a three-way valve that was vented to the atmosphere while the culm was being connected. The valve was then closed to the atmosphere, and the resulting pressure differential was recorded. Typically, pressures were recorded within 30 sec of severing the culm.
In 1994, measurements of Ps were made on individual culms throughout the day. The total number of samples per day varied due to periods of inclement weather and the ease with which suitable culms could be located and manipulated. Number of culms sampled during midday (1130–1330 central daylight time [CDT]) periods ranged from 5 to 29. In 1996, four culms were measured at each sampling time. Since measurements involved severing the culms from the rest of the plant, new plants were selected for each sampling time.
Supporting meteorological measurements included solar radiation (R) incident on a horizontal surface above the canopy and air temperature (T) and relative humidity (RH) at 2 m above the water surface. Wind speed (above the canopy) was measured with a cup anemometer 3 m above the water surface. Data were read at 1-min intervals, averaged over 30-min periods, and recorded at an automated weather station located 10 m from the measurement area.
In 1994, biomass and green foliage area index in Phragmites were determined from four 0.25 m2 quadrats located randomly in Phragmites-dominated areas. Culms were severed from the rhizomes and dried to constant weight before mass determinations. A subsample of 10 culms was selected from each quadrat and individual leaves were removed. The area of all green leaves was measured using a leaf area meter (model LI-3100, Li-Cor, Lincoln, Nebraska, USA). Projected area of the stem was calculated from measurements of culm length and diameter (Vanyarkho, 1996
).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUDING REMARKSLITERATURE CITED |
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Ps) were routinely greater than zero in live, green Phragmites culms. Similarly, we were able to measure Ps > 0 in leaves of a few scattered specimens of Typha latifolia. However, despite trying on several occasions, we never measured Ps > 0 in stems of Scirpus acutus. Internal pressurization has been previously reported for both Phragmites australis (e.g., Armstrong and Armstrong, 1990
) and Typha latifolia (e.g., Bendix, Tornbjerg, and Brix, 1994
), but we are not aware of any previous reports for Scirpus acutus.
Static pressure differentials measured in Phragmites culms were, temporally, quite variable. For example, Fig. 1 shows results obtained from a single leafy culm in the early morning of 30 June 1994 (day of year [DOY]: 181). This culm was 173 cm tall (above the water surface) and had a diameter of 1.0 cm at the cut end. The culm was representative of many of the culms in the marsh and similar behavior of Ps was observed in other culms on several occasions. During the measurement period (0800–0830 CDT) mean R was 260 W/m2, T was 18°C, and RH was 74%. Initially, at 0804 CDT, Ps was 93 Pa. As a cloud obscured the sun, Ps decreased to 67 Pa (0808 CDT). Cloudiness then decreased and by 0813 CDT the culm was in full sunlight. During this period Ps increased to 100 Pa. Thereafter, Ps continued to increase and reached a maximum of 136 Pa at 0816 CDT. Subsequently, beginning at 0823 CDT, clouds again obscured the sun, and Ps decreased. At midday on DOY 181 Ps measured in seven nearby leafy culms was 365 Pa. During the midday measurement period R was 905 W/m2, T was 29°C and RH was 40%.
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have reported Ps increasing with increasing light intensity in Phragmites in the laboratory. Increasing stomatal conductance with increasing light intensity may explain the correlation of Ps with R seen in the field. In a related study (Vanyarkho, 1996
) we measured stomatal conductance in Phragmites leaf blades using gas exchange techniques. We observed changes in stomatal conductance in response to changes in environmental conditions within 1 min. Also, Morrissey, Zobel, and Livingston (1993)
showed the importance of stomatal control in regulating CH4 emissions from Carex-dominated wetlands. However, as an alternate explanation, increased R is likely accompanied by an increase in net radiation absorbed by the plant. This increased energy load may lead to increased sensible heat flux, by increasing plant temperatures, which would tend to increase the plant interior to boundary layer vapor pressure deficit. The resulting increased gradient in relative humidity between the inside of the plant and the ambient air, in particular in the leaf sheaths, may lead to higher Ps. Armstrong and Armstrong (1991)
noted this possibility and discussed the likelihood of "thermally enhanced Knudsen diffusion" as accompanying bright light-induced increases in Ps. Armstrong and Armstrong (1991)
also reported fairly rapid (1–2 min) changes in Ps when Phragmites leaf sheaths were cut or resealed. Whatever operant mechanism is regulating Ps, it is clear that in the field culm pressures are dynamic; they respond to environmental conditions, and they can change dramatically over the course of a few minutes.
Seasonal behavior
The seasonal pattern of mean midday Ps is shown in Fig. 2. All measurements were taken between 1130 and 1530 CDT. During the summer at Ballards Marsh solar noon occurs at 1340 CDT. Early in the measurement period (DOY 154–173) mean Ps were below 200 Pa. During this time, the Phragmites green biomass (above the water surface) increased to 500 g/m2 and the foliage area index (FAI, the sum of the projected area of the leaf blades and the leaf-sheath-encircled culm) reached 1.0. By DOY 210 biomass reached its seasonal peak of 920 g/m2 and green FAI increased to 2.8. Mean Ps also increased to values >500 Pa. From DOY 210 to DOY 260 mean Ps were often >500 Pa; biomass and FAI remained high. After DOY 260, both green biomass and FAI in Phragmites decreased as the plants senesced; both were close to zero on DOY 300. Midday Ps during this period appeared to decrease slightly; however, no measurements were made after DOY 273. Over the course of the growing season, variability about the mean appeared to be related to the magnitude of the pressures. High Ps values had relatively high standard errors, while lower Ps values were associated with lower standard errors, even when these occured at midseason (e.g., DOY 222).
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Ps was high (Fig. 2). This variation appeared to be related to prevailing weather conditions (Table 1). For example, conditions were moderate during the measurement period on DOY 215 (R was 671 W/m2, T was 25°C, and RH was 62%) and mean Ps (±1 SE) was 443 ± 81 Pa. One week later (DOY 222), following a rain the previous night, R was 267 W/m2, T was 19°C, RH was 90%, and mean Ps only 29 ± 27 Pa. During the following week (see DOY 223, 228, 229) R and T had increased, RH had decreased and mean Ps increased to values >300 Pa. Again, during a period of low R, low T, and high RH (DOY 243 and 244) mean Ps were also very low (-13 ± 13 Pa and 14 ± 8 Pa, respectively). Typical of the continental climate prevailing in the semiarid Nebraska Sandhills, there were few days in midsummer that were sunny, cool, and dry; nor were many days overcast, hot, and humid during the study period. Overall, high R, high T, and low RH coincided with relatively high mean midday Ps while low R, low T, and high RH were associated with low midday Ps. Again, these results are supported by laboratory measurements reported by Armstrong and Armstrong (1990, 1991)
where high Ps was associated with high light intensity and low RH.
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Ps. This is particularly noteworthy when R, T, and RH were otherwise favorable for high Ps. For example, DOY 228 and 229, DOY 238 and 239, and DOY 259 and 260 (Table 1) are three cases where on consecutive days environmental conditions, except for wind speed, were similar but Ps, in all three cases, was smaller on the days with higher wind speed. High wind speeds decrease the leaf (and leaf sheath) boundary layer and tend to make environmental conditions near the guard cells more similar to those in the bulk atmosphere. This would tend to increase the leaf to boundary layer gradient in relative humidity, which, all else being equal, would increase Ps (Armstrong and Armstrong 1994
). However, if the decreased boundary layer thickness had the effect of reducing stomatal conductance through the complete closure of stomatal pores, Ps may well be reduced; in effect, the surface area of the leaf sheath contributing to effusive gas transport is reduced. Since detailed interpretation of limited field data, and in particular assignation of cause and effect, is difficult, further investigations of this phenomenon appear warranted. We are not aware of previous reports of the relationship between wind speed and internal pressurization in the literature. Furthermore, unless specifically included in the experimental design, investigations conducted in greenhouses and controlled environments may not highlight the importance of this phenomenon.
Diel behavior
The diel behavior of Ps for a typical period in mid-July (15–17 July 1996; DOY 197–199) is shown in Fig. 3. Near dawn (0600 CDT) on DOY 197, Ps was zero, as was typical of dawn Ps throughout the growing season. During the morning of DOY 197, Ps climbed rapidly to 954 ± 35 Pa by 1100 CDT. This rapid rise coincided with an increase in R that peaked near 900 W/m2. During midday, R remained near 800 W/m2 and Ps rose slightly to 1232 ± 55 Pa; T was 26°–30°C and canopy RH was 35–40%. By 1700 CDT solar radiation decreased dramatically due, in part, to increasing cloudiness (R dropped below 350 W/m2), and Ps decreased to 442 ± 18 Pa. After sunset, at 2130 CDT, R was near zero while Ps remained at 141 ± 3 Pa. The following day (16 July 1996; DOY 198) was partly cloudy. R rose during the morning and peaked at 700 W/m2 at 1600 CDT. Ps also rose during the day, and by 1800 CDT it had reached 505 ± 59 Pa. At 1800 CDT, T was near 28°C, and canopy RH was 71%. Humidity was higher throughout DOY 198 than DOY 197 or 199; the minimum RH recorded on DOY 198 was 64% at 1900 CDT. Afternoon and evening values of both R and Ps declined with the decrease in Ps lagging that in R. Again, after sunset, R was zero while Ps remained slightly positive (66 ± 11 Pa). The morning of 17 July 1996 (DOY 199) was very clear and hot and at 1300 CDT R was 936 W/m2, T was 34°C, RH was 38%, and Ps was very high at 1651 ± 57 Pa. These Ps values were amongst the highest we observed at Ballards Marsh.
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Ps for this period in July reflected the midseason values obtained at midday in 1994. The highest Ps values were obtained during sunny, hot days. Furthermore, times of low RH corresponded with periods of high Ps.
Relations to isotope data and methane emissions
In Phragmites we repeatedly observed large Ps values in leafy green culms during the daytime, small Ps values just after sunset, and Ps essentially zero by predawn. This is consistent with a diel shift in dominant gas transport mechanisms from convective throughflow during the day to diffusion at night. Our measurements of methane concentrations and stable carbon isotopes in methane samples from live and dead Phragmites culms also lead to the conclusion that the dominant gas transport mechanism is convective throughflow during the day and diffusion at night. Furthermore, during the day, the proportion of 12C and 13C in methane sampled from dead culms was similar to that found in sediments; at night, methane in dead culms was enriched in 13C relative to 12C.
Results obtained from diel and seasonal analysis of eddy covariance measurements of CH4 fluxes from Ballards Marsh (Kim et al., 1998
; Kim, Verma, and Billesbach, 1998
) are also consistent with the overall conclusion that temporal patterns in CH4 emissions are controlled largely by convective throughflow in Phragmites. Morrisey, Zobel, and Livingston (1993) reported that emergent plants in Carex-dominated wetlands have significant control over CH4 emissions and that stomatal conductance plays an important role in regulating CH4 release. Armstrong and Armstrong (1990)
have demonstrated enhanced oxygen transport from the atmosphere to rhizomes of Phragmites under convective throughflow conditions (i.e., high light, low RH) compared with diffusive (i.e., darkness, high RH) conditions. Brix (1988)
has shown light-dependent variations in the composition of air in the culms of Phragmites, with oxygen being depleted and CO2 enriched at night (diffusive conditions) vs. day (convective conditions). We have also measured diel changes in CH4 and CO2 concentrations consistent with these earlier results. Moreover, we have directly measured Ps in the field during periods of enhanced gas transport by convective throughflow.
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CONCLUDING REMARKS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUDING REMARKSLITERATURE CITED |
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; Kim, Verma, and Billesbach, 1998
). Taken together, results from all three approaches highlight the importance of convective throughflow in Phragmites in determining magnitudes and temporal patterns of methane emissions we observed at this wetland.
At the Ballards Marsh site we measured a net ecosystem exchange of carbon dioxide of 330 g CO2-C/m2 for the late April through late October growing season. During the same period, methane emissions were 48 g CH4-C/m2 (Kim, Verma, and Billesbach, 1998
). Thus, within a single growing season, 15% of the total amount of carbon taken up as carbon dioxide was returned to the atmosphere as methane. Due to the enhanced transport of gases via convective throughflow as opposed to diffusion (e.g., Joabsson, Christensen, and Wallén, 1999
), internal pressurization in emergent aquatic plants is an important component of surface-to-atmosphere gas exchange and the resulting carbon sequestration in wetlands.
In summary, we found internal pressures in Phragmites culms to be significantly higher than atmospheric pressure during the daytime. At night, internal pressures dropped and approached atmospheric pressure levels by dawn. Internal pressures in Scirpus were always equal to atmospheric pressure. The magnitude of the internal pressures appeared to be correlated with ambient environmental conditions. High temperatures, high solar radiation, and low relative humidity coincided with periods of high pressurization. Pressurization is a dynamic process, and large changes can occur over short time periods (several minutes).
Phragmites australis is a cosmopolitan species. We have noted it growing in such diverse habitats as ditches in central Saskatchewan and marshes in central Florida. In addition to North America, it is also found in Central and South America, Eurasia, Africa, and Australia (Hitchcock, 1971
). Moreover, several other widespread plant species are also known to transport gases, including methane, via convective throughflow (e.g., Typha latifolia and T. angustifolia—Bendix, Tornbjerg, and Brix, 1994
; Tornbjerg, Bendix, and Brix, 1994
; Yavitt and Knapp, 1998
; Nuphar luteum and Nymphaea odorata—Sebacher, Harriss, and Bartlett, 1985
; Spartina alterniflora—Hwang and Morris, 1991
; see also Chanton and Dacey, 1991
).
In wetlands, as well as in many other ecosystems, researchers have often used chamber techniques to quantify surface–atmosphere methane and carbon dioxide exchange (e.g., Whiting and Chanton, 1992
). In order to ensure accurate interpretations of data from such experiments, special care must be taken during the measurement process (e.g., climate control, light transmission). In our view, chamber measurements over plants that transport gases via convective throughflow are especially complicated. Accurate measurements are desirable given the enhanced transport of methane via convection as opposed to diffusion. However, reliable quantification of spatially averaged methane emissions remains difficult in ecosystems where the dominant species pressurize.
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FOOTNOTES |
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6 Author for reprint requests (e-mail: tja{at}unl.edu
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUDING REMARKSLITERATURE CITED |
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———, and ———. 1991 A convective through-flow of gases in Phragmites australis (Cav.) Trin. ex Steud. Aquatic Botany 39: 75–88
———, and ———. 1994 A physical model involving nuclepore membranes to investigate the mechanism of humidity-induced convection in Phragmites australis. Proceedings of the Royal Society of Edinburgh 103: 529–539
Bendix, M., T. Tornbjerg, and H. Brix. 1994 Internal gas transport in Typha latifolia L. and Typha angustifolia L. 1. Humidity-induced pressurization and convective throughflow. Aquatic Botany 49: 75–89[CrossRef]
Brix, H. 1988 Light-dependent variations in the composition of the internal atmosphere of Phragmites australis (Cav.) Trin. ex Steudel. Aquatic Botany 30: 319–329[CrossRef]
———, B. K. Sorrell, and P. T. Ott. 1992 Internal pressurization and convective gas flow in some emergent freshwater macrophytes. Limnology and Oceanography 37: 1420–1433[ISI]
Chanton, J. P., and J. W. H. Dacey. 1991 Effects of vegetation on methane flux, reservoirs, and carbon isotopic composition. In T. D. Sharkey, E. A. Holland, and H. A. Mooney [eds.], Trace gas emissions by plants, 65–92. Academic Press, San Diego, California, USA
Grosse, W., H. B. Büchel, and H. Tiebel. 1991 Pressurized ventilation in wetland plants. Aquatic Botany 39: 89–98
Hitchcock, A. S. 1971 Manual of grasses of the United States, 2nd revised ed. Dover, New York, New York, USA
Hwang, Y., and J. T. Morris. 1991 Evidence for hygrometric pressurization in the internal gas space of Spartina alterniflora. Plant Physiology 96: 166–171
Joabsson, A., T. R. Christensen, and B. Wallén. 1999 Vascular plant controls on methane emissions from northern peatforming wetlands. Trends in Ecology and Evolution 14: 385–388
Kim, J., S. B. Verma, and D. P. Billesbach. 1998 Seasonal variation in methane emission from a temperate Phragmites-dominated marsh: effect of growth stage and plant-mediated transport. Global Change Biology 5: 433–440
———, ———, ———, and R. J. Clement. 1998 Diel variation in methane emission from a midlatitude prairie wetland: significance of convective throughflow in Phragmites australis. Journal of Geophysical Research 103: 28 029–28 039
Morrissey, L. A., D. B. Zobel, and G. P. Livingston. 1993 Significance of stomatal control on methane release from Carex-dominated wetlands. Chemosphere 26: 339–355[CrossRef][ISI]
Sebacher, D. I., R. C. Harriss, and K. B. Bartlett. 1985 Methane emissions to the atmosphere through aquatic plants. Journal of Environmental Quality 14: 40–46
Shurpali, N. J., S. B. Verma, J. Kim, and T. J. Arkebauer. 1995 Carbon dioxide exchange in a peatland ecosystem. Journal of Geophysical Research 100: 14 319–14 326
Suyker, A. E., S. B. Verma, R. J. Clement, and D. P. Billesbach. 1996 Methane flux in a boreal fen: season-long measurement by eddy correlation. Journal of Geophysical Research 101: 28 637–28 647
Tornbjerg, T., M. Bendix, and H. Brix. 1994 Internal gas transport in Typha latifolia L. and Typha angustifolia L. 2. Convective throughflow pathways and ecological significance. Aquatic Botany 49: 91–105[CrossRef]
Vanyarkho, O. 1996 Seasonal changes in vegetative characteristics and gas exchange of Phragmites australis and Scirpus acutus in a Sandhills wetland ecosystem. Master's thesis, University of Nebraska-Lincoln, Lincoln, Nebraska, USA
Whiting, G. J., and J. P. Chanton. 1992 Plant-dependent CH4 emission in a subarctic Canadian fen. Global Biogeochemical Cycles 6: 225–231[CrossRef]
Yavitt, J. B., and A. K. Knapp. 1998 Aspects of methane flow from sediment through emergent cattail (Typha latifolia) plants. New Phytologist 139: 495–503[CrossRef][ISI]
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