HYDROLOGICAL PROCESSES Hydrol. Process. 18, 213–227 (2004) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1371 Seasonal transpiration pattern of Phragmites australis in a wetland of semi-arid Spain Mar´ ıa Jos´ e Moro, 1 * Francisco Domingo 2 and Germ´ an L´ opez 1 1 Departamento de Ecolog´ ıa, Universidad de Alicante, 03080 Alicante, Spain 2 Estaci´ on Experimental de Zonas ´ Aridas, CSIC, General Segura 1, 04001 Almer´ ıa, Spain Abstract: Transpiration rates were measured in a flooded population of Phragmites australis ssp. altissima in a wetland located in ‘El Hondo’ Natural Park (southeastern Spain) during the growing season of 2000. The heat balance method for measuring sap flow was used to calculate the rate of water transpiration on a whole-stem basis. Four series of measurements were carried out in selected weeks in May, June, August and October. Structure, biomass and leaf area index of the reed population were simultaneously quantified in order to scale transpiration on a plot-area basis. Overall, transpiration flux was high during the sampling period and showed a typical diurnal pattern with a maximum at about midday. Mean transpiration was highest at the end of June coinciding with the peak of reed growth and with the maximum leaf area both at individual and plot scales. Rates decreased abruptly in October, in parallel with the advanced foliar senescence. The variation of both midday and integrated daily transpiration is significantly related to that of the air temperature on clear days. Cloudy and rainy days exert a pronounced effect on water loss by decreasing transpiration. Our results highlight the potential use of the sap-flow method to measure transpiration in reed ecosystems and the relevance of this flux for the water balance in wetlands in semi-arid environments. Thus, it is suggested that water management in these areas could be favoured by acquiring high-quality experimental data. Copyright 2004 John Wiley & Sons, Ltd. KEY WORDS Phragmites australis ; sap flow; transpiration; stem heat-balance method; wetlands; semi-arid environments; El Hondo; Alicante; southeastern Spain INTRODUCTION Actual evapotranspiration (AET) is regarded as a major component of the surface energy balance in vegetated wetlands because it is frequently quoted as the largest consumer of the incoming energy (Priban and Ondok, 1985). However, few studies have specifically addressed the calculation of the surface energy fluxes from experimental data in reed-dominated wetlands. In these studies, semi-empirical approximations were used to differentiate AET into evaporation E and transpiration T (Burba et al., 1999; Herbst and Kappen, 1999). The fast growth combined with the large leaf area of reed species (Haslam, 1972) suggests a high transpiration rate. Therefore, a more accurate description of the magnitude of transpiration along with its temporal and spatial variation seems crucial for parameterization and validation of climate models, and for management of water resources. In Mediterranean semi-arid areas, meteorological conditions favouring transpiration are more prolonged in time than in temperate areas. Nevertheless, plants reduce transpiration through different mechanisms during water deficit periods in these environments. Plants with access to deep water sources (Domingo et al., 1999) or inhabiting flooded wetlands can transpire continuously, even during extreme dry periods. The Spanish semi- arid wetlands are frequently fringed by monospecific, dense, vigorous formations of common reed (Phragmites australis ssp. altissima), which can likewise grow in shallow water bodies. P. australis ssp. altissima can * Correspondence to: Mar´ ıa Jos´ e Moro, Departamento de Ecolog´ ıa, Universidad de Alicante, 03080 Alicante, Spain. E-mail: [email protected]Received 1 November 2001 Copyright 2004 John Wiley & Sons, Ltd. Accepted 1 November 2002
Seasonal transpiration pattern of Phragmites australisin a wetland of semi-arid Spain
Marıa Jose Moro,1* Francisco Domingo2 and German Lopez1
1 Departamento de Ecologıa, Universidad de Alicante, 03080 Alicante, Spain2 Estacion Experimental de Zonas Aridas, CSIC, General Segura 1, 04001 Almerıa, Spain
Abstract:
Transpiration rates were measured in a flooded population of Phragmites australis ssp. altissima in a wetland locatedin ‘El Hondo’ Natural Park (southeastern Spain) during the growing season of 2000. The heat balance method formeasuring sap flow was used to calculate the rate of water transpiration on a whole-stem basis. Four series ofmeasurements were carried out in selected weeks in May, June, August and October. Structure, biomass and leaf areaindex of the reed population were simultaneously quantified in order to scale transpiration on a plot-area basis.
Overall, transpiration flux was high during the sampling period and showed a typical diurnal pattern with a maximumat about midday. Mean transpiration was highest at the end of June coinciding with the peak of reed growth and withthe maximum leaf area both at individual and plot scales. Rates decreased abruptly in October, in parallel with theadvanced foliar senescence. The variation of both midday and integrated daily transpiration is significantly related tothat of the air temperature on clear days. Cloudy and rainy days exert a pronounced effect on water loss by decreasingtranspiration. Our results highlight the potential use of the sap-flow method to measure transpiration in reed ecosystemsand the relevance of this flux for the water balance in wetlands in semi-arid environments. Thus, it is suggested thatwater management in these areas could be favoured by acquiring high-quality experimental data. Copyright 2004John Wiley & Sons, Ltd.
KEY WORDS Phragmites australis; sap flow; transpiration; stem heat-balance method; wetlands; semi-aridenvironments; El Hondo; Alicante; southeastern Spain
INTRODUCTION
Actual evapotranspiration (AET) is regarded as a major component of the surface energy balance in vegetatedwetlands because it is frequently quoted as the largest consumer of the incoming energy (Priban and Ondok,1985). However, few studies have specifically addressed the calculation of the surface energy fluxes fromexperimental data in reed-dominated wetlands. In these studies, semi-empirical approximations were used todifferentiate AET into evaporation E and transpiration T (Burba et al., 1999; Herbst and Kappen, 1999). Thefast growth combined with the large leaf area of reed species (Haslam, 1972) suggests a high transpirationrate. Therefore, a more accurate description of the magnitude of transpiration along with its temporal andspatial variation seems crucial for parameterization and validation of climate models, and for management ofwater resources.
In Mediterranean semi-arid areas, meteorological conditions favouring transpiration are more prolonged intime than in temperate areas. Nevertheless, plants reduce transpiration through different mechanisms duringwater deficit periods in these environments. Plants with access to deep water sources (Domingo et al., 1999) orinhabiting flooded wetlands can transpire continuously, even during extreme dry periods. The Spanish semi-arid wetlands are frequently fringed by monospecific, dense, vigorous formations of common reed (Phragmitesaustralis ssp. altissima), which can likewise grow in shallow water bodies. P. australis ssp. altissima can
* Correspondence to: Marıa Jose Moro, Departamento de Ecologıa, Universidad de Alicante, 03080 Alicante, Spain. E-mail: [email protected]
Received 1 November 2001Copyright 2004 John Wiley & Sons, Ltd. Accepted 1 November 2002
reach up to 6 m (Mateo Sanz and Crespo Villalba, 1990), and has numerous leaves which are also larger thanthose from other temperate reed ecotypes. Management attempts to avoid an excessive invasion into waterbodies and to reduce water losses by transpiration are currently a controversial subject in some semi-aridwetlands, such as the one under study in this paper. Partly, the controversy arises from the fact that some ofthese wetlands are used as water reservoirs for irrigation agriculture. Simultaneously, they are legally protectedbecause of their conservation value. This singular combination of meteorological conditions, plant attributesand land use provides good reasons to study transpiration flux in these semi-arid wetlands. However, despiteclear theoretical and applied interest, studies in semi-arid environments where reed transpiration is directlymeasured in natural conditions are lacking.
Porometry and sap flow are techniques frequently used to obtain direct measurements of transpirationin plants (Stewart, 1984; Pearcy et al., 1992; Rana and Katerji, 2000). Porometry measures instantaneoustranspiration rates of individual or small groups of leaves. The major advantages of this method are therelative ease of use and the capacity for measuring leaves from many individuals of the population. However,some studies have demonstrated that the scaling up from individual leaves to the whole plant or canopy issubject to several constraints, and tends to overestimate the transpiration flux, mainly at high transpiration rates(Ansley et al., 1994). Moreover, in tall canopies, such as the P. australis ssp. altissima, the inaccessibilityto top-canopy leaves considerably reduces the application of the porometry technique. Sap-flow techniquesmeasure the transpiration with gauges placed around the stem. The mass sap flow is calculated from the heatbalance and can be considered equivalent to transpiration if plant capacitance is ignored and the averagingperiod is sufficiently long (Smith and Allen, 1996). The main advantage of the sap-flow technique is thatit provides an integrated measurement of all the leaves on a stem, thus reducing the effect of variabilitywithin the canopy. Nevertheless, this method requires more complex equipment, which is also more difficultto maintain. Additionally, the sample size is restricted by the number of gauges available.
This study attempts primarily to address the description of the magnitude and seasonal variation oftranspiration in reed stands growing in a protected wetland of semi-arid Spain. To do so, we selected aflooded population growing into a pond and we applied the sap-flow technique to take direct measurementsof transpiration. We are also interested in the practical application of the sap-flow technique in the contextof a difficult environment characterized by saturated air humidity, mainly at night, which might damage theequipment and generate noise in the data. Another difficulty was the presence of gas flow through the hollowstems of P. australis (Armstrong et al., 1992, 1996) that might interfere with sap-flow measurements due tothe heat diffusion. This study, therefore, is designed to evaluate the major patterns in reed transpiration,its relationship with plant structure and meteorological conditions and the potential application of sap-flow techniques in these ecosystems. Further studies in these wetlands will address the development andparameterization of an evapotranspiration model using eddy covariance (evapotranspiration measurements)combined with the stem heat balance method (transpiration measurements) as validation tools.
STUDY AREA
The study area is ‘El Hondo’ Natural Park, a semi-artificial wetland located in the south of the province ofAlicante, southeastern Spain (38°100N, 00°420W, 3 m a.s.l.). The climate is semi-arid, with a mean annualtemperature of 20Ð6 °C. Mean annual precipitation is 233Ð9 mm, with rain falling mainly in winter, whichis followed by a dry period centred on the months of June–September. Originally, “El Hondo” was part ofthe ancient Elche lagoon that was drained during the 18th and 19th centuries. In the 1940s, the area wastransformed into a semi-artificial wetland in order to provide water for agriculture. Its importance as a breedingand resting site for birds, especially during migration, subsequently determined its legal protection.
The area includes two artificial water reservoirs (‘embalses’) and some natural ponds (‘charcas’). Reservoirsreceive eutrophic, poor-quality water conducted through a system of long channels from the mouth of
the Segura River. Water in natural ponds originates from water-table oscillations and runoff from adjacentcultivated lands. This water is less eutrophic and more brackish than that of the reservoirs.
All these wetlands are fringed by vigorous, tall, monospecific stands of P. australis (Cav) Trin. Ex. Steud.ssp. altissima, with an average live stem density of 72 m�2 (Colmenarejo et al., 1999). P. australis formsan extensive belt around water bodies. Moreover, reed stands successfully colonize water bodies with a lowmean depth (maximum 150 cm). Management practices of the common reed include burning and cutting, thelatter applied mainly for waterlogged populations.
This study was carried out in the ‘Charca Sur’, a shallow, natural pond of 200 ha, with eutrophic andrelatively brackish water. The current reed population growing into the water body is the result of thevegetative regeneration after a prescribed burning carried out in 1994. Presently, this population occupiesabout a quarter of the total pond area, forming patches of variable size which are sparsely distributed amongthe free water. Measurements were made in one of these patches, which is orientated towards the southeasternedge of the pond. A floating platform was installed in a small, reed-bare space surrounded by a dense mass ofP. australis (densities of 52 m�2 of live shoots and 190 m�2 of standing-dead shoots) with an average heightof 3 m above the sediment surface. Water depth varied along the 2000 growing season from 1Ð25 m in May,decreasing throughout June �1Ð1 m� and mainly in mid-August �0Ð1 m�, with a moderate recovery �0Ð5 m� inOctober.
MATERIALS AND METHODS
Sap-flow measurements
Theory. The stem heat-balance method (SHB) is used for measuring sap flow from single, woody andherbaceous stems (Smith and Allen, 1996). The SHB requires a steady state and a constant energy input Pin
from the heater inside the gauge body. The energy balance (in watts; Sakuratani, 1981) is expressed as
Pin D Qr C Qv C Qf �1�
where Pin is the power input to the stem from the heater, Qr is the radial heat conducted through the gaugeto the external air, Qv is the vertical heat conduction through the stem and Qf is the heat carried by themoving sap. The value of Qf is obtained by subtracting Qv and Qr from Pin, which can all be calculated.After dividing by the sap heat capacity Cp and the sap temperature increase dTba, Qf is converted directly toa mass flow rate Q (g s�1 of H2O).
Qv is calculated from
Qv D AstKst
(dTa C dTb
dx
)
where Ast is the cross-sectional area of the heated section of the stem, Kst is the thermal conductivity of stem,dTb and dTa are the vertical temperature gradients below and above the heater respectively and dx is thedistance between the two thermocouple junctions on each side of the heater.
The radial component Qr is determined by
Qr D Ksh dTr
where Ksh is the thermal gauge conductance and dTr is the radial temperature gradient. For a detailedinstrument description and operation theory, see Sakuratani (1981) and Smith and Allen (1996).
Field measurements. The heat balance method for measuring sap flow rates was applied to P. australisstems for at least five consecutive days during May, June, August and October 2000. Ten sap-flow gauges(Dynagauge, Dynamax Inc., Houston, USA) were installed in stems of P. australis selected randomly. The
diameter of the stems ranged from 5 to 16 mm). Ast was determined from the stem diameter at the height ofthe gauge installation. Ksh was calculated for each stem/gauge combination during times of zero flow at night(from 3:00 a.m. to 5:00 a.m.). A value of 0Ð28 W m�1 K�1 recommended for hollow stems (Van Bavel, 1994)was used for Kst. Sap-flow velocity measurements (Q, g h�1 of H2O) were taken every 1 min and averagedevery 60 min. After removing the gauges, the leaf area of each stem was measured destructively to relatesap-flow velocity to leaf area units. Owing to the site’s environmental conditions, mainly the high relativehumidity, the habitual maintenance prescribed by the manufacturer for this type of equipment was carriedout every 2–3 days to avoid errors due to water condensation in the gauges. Absolute rates of transpiration(Q, g h�1 of H2O) were normalized by the total leaf area of each stem (Ql, g h�1 m�2 leaf area). Cumulativedaily values from the ten sensors were then averaged (cum Ql, l day�1 m�2 leaf area) and extrapolated tounit of ground area (Qplot, mm day�1� by multiplying by the leaf area index (LAI) of the stand.
Stand structure measurementsStructural measurements of the P. australis population found in the ‘Charca Sur’ were carried out seasonally.
In each sampling, three plots of 0Ð25 m2 were randomly selected near to the site where the sap-flowmeasurements were taken. In each plot, aerial biomass was harvested. The numbers of live, dead and senescentshoots were registered. For each stem, the diameter and length were measured and the numbers of green andsenescent leaves were counted. Standing-dead biomass along with fresh biomass of live shoots, previouslyclassified into stems and leaves, were weighed. A representative sample of the biomass fractions was takento determine the ratio of fresh to dry weight. Three samples with about ten leaves each were taken in eachplot to measure leaf area and calculate leaf area ratio (LAR). Total leaf area of the sample was measured bymeans of a leaf-area meter (Mk 2, Delta T Devices, Burwell, UK) and the corresponding oven-dry weightwas obtained. The LAR, combined with the total dry weight of leaves in each plot, was used to calculate thestand LAI.
Pump experiment
In order to check whether the presence of gas flow in the hollow stem interfered with the sap-flow method(due to internal heat diffusion), a specific experiment was carried out. It was conducted in late afternoonwhen the transpiration from the leaves, i.e. sap flow rate, was low. An aquarium pump was connected to a3 mm hole bored above two gauges (B16 and A13 models), installed around stems of 15 mm and 13 mmrespectively. Air was blown in and sucked out from the inner space of the nearest internode at a constantrate of 3 l min�1. Blown/sucked air was applied for 5 min by the pump alternating with 5 min intervals inwhich the pump was stopped. Sensor records of sap flow velocity Q �g h�1�, as well as of the componentsof the energy balance Qr, Qv and Qf (watts) were compared.
Meteorological measurementsTwo aspirated humidity sensors (MTH-A1, ITC, Almerıa, Spain) were installed on a 6 m mast, thus
measuring air temperature and air humidity at reference height zr and at the mid-canopy height zm. Two cupanemometers (A100, Vector Instruments, Rhyl, UK) measured wind speed at zm and zr. All measurementswere recorded every 1 s by a data logger (CR10, Campbell Scientific Ltd, Logan, UT, USA). Hourly averageswere calculated for all data. Data on rainfall intensity and volume, number of sunshine hours and potentialevapotranspiration (ETP) were obtained from the nearby meteorological station of the ‘Aeropuerto del Altet’,which is located 5 km away from the field site.
RESULTS
MeteorologyThe prevalent meteorological conditions during the middle and peak growing season (May–June) of the reed
stand studied were characterized by high values of average and maximum diurnal air-temperature (Table I).
Relative humidity varied from 65 to 77% on clear days, and average wind speed ranged from 0Ð7 to 2Ð7 m s�1.In August, the air temperature increased, the relative humidity decreased slightly, and the average wind speedremained low. The sunshine duration reflected the generally clear skies on all sampling days in May, June andAugust. In October, meteorological conditions between days were more variable, but average air temperatureand relative humidity were similar to that observed for some days in May and June. Sunshine duration waslower and more variable in October, which is partly due to the cloudy skies that occurred occasionally (Table I).
Reed population structure
Table II shows some structural features of the reed population in our study area. A mean shoot density ofabout 50 m�2 was found throughout the mid and peak growth season. The maximum standing aerial biomasswas attained in June. The seasonal pattern of specific leaf area (SLA) reflected the progressive ageing of theleaves, decreasing continuously from May to October (Table II). The contribution of photosynthetic surface inindividual plants, as expressed by LAR (Table II), was high in May but attained the maximum value in June(27Ð1 m2 leaf area per gram of aerial biomass), then decreased throughout August and October in parallelwith the leaf senescence on individual shoots. On a plot-scale basis, the green LAI showed the same pattern,
Table I. Diurnal average of relative humidity (RH), diurnal average air temperature T, wind speed u at mid-canopy heightzm and at reference height zr, daily maximum and minimum air temperature, sunshine duration (SUN) and rainfall volume
P in the study area. RH, T and daily maximum and minimum air temperatures were measured at zr
Table II. Seasonal changes for some structural attributes at leaf, individual and plot scales. Specific leaf area (SLA), leafarea ratio (LAR), number of green leaves per shoot (Leaves), leaf area index (LAI), green shoot density D and live aerial
reaching a maximum value of 8Ð9 m2 m�2 at the end of June. Nevertheless, relative differences between Juneand August in LAI values were more pronounced than the corresponding LAR values for the same months.This was caused by the increase in shoot mortality as senescence advanced during July and August, thusreducing the mean density in the plots.
Leaf area was more closely related to stem height than to stem diameter in all months. Thus, the regressionequations used to estimate stem leaf area selected only the logarithm of stem height as an independent variable(Table III). The predictive power of these equations was greater in June and August �r > 0Ð95� than in October�r D 0Ð716�, owing to the senescence of leaves in this month.
Pump experiment
The effect of the artificially pumped air experiment on both sap flow rate Q �g h�1� and the components ofthe heat balance is shown in Figure 1 for the B16 gauge. Control flow averaged about 40 g h�1 and increased1Ð3-fold when air was blown, and 1Ð7-fold when the air was sucked (Figure 1a). Qr decreased when the airwas blown, whereas it increased when air was sucked. Nevertheless, the heat conducted vertically �Qv� wasless sensitive to movement of the forced air. In consequence, changes in Qf were strongly coupled to thevariation of Qr (Figure 1b). The observed increase in Q (Figure1a) when the air is sucked was mainly causedby a reduction of dT and not by an increase in Qf (Figure 1b). Results for the A13 gauge were similar (datanot shown).
Transpiration pattern
Anomalous peaks of sap flow were noticeable in some sensors at the pre-dawn period and after sunset.These fluxes were not very high and probably could have been caused by water condensation in the gauges,as the relative humidity during this interval was around 90–95% for the overall sampling period. As shownin Figure 2, the variability of sap-flow measurements throughout the daytime ranged from 9 to 14% of themean, with the exception of 6 a.m., 7 a.m., 7 p.m. and 8 p.m., when the standard errors increased significantly.Based on these results, we eliminated these data from further analysis.
The plants in this study transpired water continuously throughout the daytime. The diurnal transpirationpattern on clear days reached its maximum about midday (12–14 h) (Figure 3a). Nevertheless, the pattern
Table III. Correlation coefficient and regression equations relating ln(leaf area, cm2) (dependent variable) to ln(stem height,cm) as the independent variable in each month
Figure 1. (a) Sap flow Q �g h�1� and (b) the components of the heat balance, Qr, Qf, Qv (W) and variation in sap temperature dT (°C)measured at late afternoon in reed stems after experimentally blowing (clear arrows) and sucking (black arrows) air across a hole bored
above the gauge. Q is calculated from Equation 1 after dividing by the sap heat capacity Cp and dT
and magnitude of diurnal transpiration changed significantly on rainy and cloudy days (Figure 3b). In allmonths, both maximum diurnal �max Ql� and cumulated daily transpiration rates �cum Ql� were significantlycorrelated with the air temperature (Figure 4a and b). Analysis of covariance (ANCOVA), performed usingthe air temperature as a covariate, showed a significant monthly effect on max Ql �F D 26Ð03, P < 0Ð001�,but the slopes of these regressions were not different among months (F D 1Ð09, n.s.; Figure 4a). Aposteriori tests (Newman–Keuls tests) showed that only the June mean maximum rate (298Ð7 g h�1 m�2
leaf area) was significantly higher �p < 0Ð001� than that found in May �188Ð4 g h�1 m�2 leaf area), August(186Ð5 g h�1 m�2 leaf area) and October (159Ð4 g h�1 m�2 leaf area). The relationship between cum Ql andthe average diurnal air temperature showed a similar pattern (ANCOVA. Month effect: F D 23Ð3, P < 0Ð001;comparison of slopes: F D 1Ð40, n.s.) (Figure 4b), but the differences were significant among all the months(a posteriori test, p < 0Ð01). The cum Ql on days with similar temperature was about 1Ð6 and 1Ð3 timesgreater in June than in August and May respectively. Cloudy and rainy days have been shown to exert
Figure 2. Hourly variability of the sap flow measurements (SE/mean) throughout daytime for three sampling periods
a pronounced effect on water losses, causing sharp decreases in the magnitude of midday transpiration(Figure 4a).
The variability of cum Ql (Table IV) ranged between 9 and 15% of the mean in months where ten gaugeswere used, and between 13 to 19% in May, where only four sensors were available for the experiment. Asexpected, rainy days (days of year 142 and 286) showed the highest variability (25%, 63%). The cum Ql
values were highest in June, in parallel with the maximum leaf development (Figure 5a). These rates wereconsiderable, ranging from 2Ð2 to 3Ð2 l day�1 m�2 leaf area. Despite the similar meteorological conditions inJune and August (Table I), daily transpiration rates decreased moderately in August to 1Ð7–1Ð8 l day�1 m�2
leaf area, mainly due to the beginning of leaf senescence. In October, as the days were shorter and senescenceprogressed, daily water losses per unit of leaf area decreased to 0Ð9–1Ð5 l day�1 m�2 leaf area on dry weatherdays (Figure 5a).
A highly significant linear correlation �p < 0Ð001� was found between the leaf area of sampled individualshoots and their absolute daily transpiration rates Q (g day�1) for all sampling days except for the rainyor cloudy days 142, 144 and 286 (data not shown). To test if the size of the stem affected the cum Ql
(g day�1 m�2 leaf area) we performed a repeated measures analysis of variance for each month, where reedstems were placed in two diameter classes (<6Ð5 mm and 9–14 mm). Size effect was detected only in August�F1,2 D 7Ð09, P D 0Ð02� when the transpiration rate of the large stems averaged 2138 g day�1 m�2 leaf area.For small stems the average was 1436 g day�1 m�2 leaf area.
The scaling-up of cum Ql (g day m�2 leaf area) to the plot scale (g day�1 m�2 ground area or mm day�1)was set up taking into account the seasonal variation of the LAI. When transpiration is expressed on a plot scale(Qplot, Figure 5b), relative differences between May (mid-growth), June (peak-growth) and August–October(senescence) became more pronounced than when transpiration was expressed for individual plants (cum Ql,Figure 5a). Leaf senescence and shoot mortality in both months (Table II), along with the cooler weatherconditions in October (Table 1) seem to be the main causes of the observed abrupt decrease in transpirationrates. Monthly daily average transpiration rates for the sampling period were 10Ð5 mm day�1, 23Ð9 mm day�1,7Ð3 mm day�1 and 2Ð2 mm day�1 in May, June, August and October respectively.
May (rainy)October (rainy)May (cloudy)October (cloudy)
Ql (
g w
ater
h-1
m-2
leaf
are
a)
(b)
Figure 3. Diurnal pattern of transpiration Ql on (a) clear and (b) cloudy days for each sampling period
DISCUSSION
Venturi convective throughflow has been described as an efficient mechanism enhancing gas transfer fromthe atmosphere to the rhizomes in P. australis (Armstrong et al., 1992; Allen, 1996). Wind blowing acrosscrashed, top-open dead stems generates a suction pressure, which forces the oxygen into a connected live stem.Our pump experiment showed that sap flow rate increased when air was artificially blown/sucked through, atrates of 3 l min�1. The observed changes were due to the combined effect of the pumped air on the radialheat diffusion Qr and on the sap temperature gradient dT (Figure 1b).
The increase of Qf when the air was blown could be explained because it produced a decrease in the radialheat transport Qr and dT due to the external air probably having a higher temperature than that inside the
Figure 4. Relationships between (a) air temperature at reference height and midday transpiration rates (max Ql) for each sampling periodand (b) cumulative daily average transpiration (cum Ql) and average diurnal air temperature at the reference height. The smaller points
(rainy days) and encircled points (cloudy days) were omitted from the regression analysis
stem. The decrease of Qf when the air was sucked out from inside the stem could be produced because itincreased Qr and sharply reduced dT, as the sucked air could transport heat from inside.
These results suggest that the heat transported by the air actually interfered with the heat balance used tocalculate the sap flow. Nevertheless, Qr was only a significant component of the heat balance in conditions oflow or zero transpiration (e.g. late afternoon and night-time) when it reached its maximum values (Van Bavel,1994). Moreover, the artificially pumped air flow in our experiment was over 1000 times greater than thereal convective flow measured for P. australis in other studies. For instance, Brix et al. (1992) give values
Table IV. Integrated daily average transpiration at individual-scale basis for the dayssampled. Transpiration is expressed as a rate normalized by stem leaf area (g day�1 m�2
leaf area). Standard error and variability of mean are also shown
of about 6 ð 10�3 l min�1 for small stems. Armstrong et al. (1992) experimentally submitted P. australisstems to a range of wind speeds, measuring gas flows of 0Ð0114 cm3 s�1 at 5Ð6 m s�1. Therefore, the sap-flow technique based on the heat-balance approach seems to be a good method to measure transpiration inP. australis in natural conditions.
Some authors have suggested that a sample size of 10–12 gauges should be sufficient to estimate accuratelythe average sap flow in individual plants for even-aged populations (Cermak et al., 1995; Kostner et al., 1996;Loustau et al., 1996; Smith and Allen, 1996). In order to obtain realistic average values, especially when dataare further used to scale up to ground area units, it is recommended to consider the overall range of sizeclasses of the population (Ham et al., 1990). In this study, ten shoots (except on May sampling) varying from5 to 14 mm diameter and from 1 to 4Ð8 m tall were found to be a representative sample of the size range inthe Phragmites population under study. A low, acceptable, inter-individual variability was found when theaveraged daily transpiration rates were normalized by leaf area (Table IV), except for cloudy and rainy days.The small variation among sensors could be explained by the significant linear relationship between shoot leafarea and absolute daily transpiration rates, and by the weak effect of shoot size on normalized transpiration
Figure 5. Cumulative daily average transpiration rate for all sampling dates: (a) cum Ql (l day�1 m�2 leaf area) and (b) Qplot (mm day�1)
rate. Nevertheless, a significant difference on normalized transpiration rates between tall and short shoots wasfound in August. This size effect could be caused by a differential shading upon small and tall stems. Smallstems were partly shaded during a more prolonged period, whereas at least the top-canopy leaves from tallindividuals received almost unobstructed sunlight. No size effects were detected in the other months, probablydue to the greater variability of weather conditions.
The scaling up of transpiration data from individual to plot area was made by multiplying the normalizedtranspiration rates, cum Ql, by the stand LAI. This seems a good approximation to estimate the reedtranspiration in our study site because it has been found to apply to other monospecific, even-aged vegetation
stands (Kostner et al., 1996; Loustau et al., 1996). The use of leaf-area normalized rates reduces the variabilityamong individuals. In addition, frequent, easy and accurate LAI measurements over a large area can be takenwith the light interception-based portable instrumentation (e.g. DEMON, SUNSCAN). Alternatively, if thedistribution of stem heights in the reed population is estimated then the equations of Table III could providea reasonable estimate of LAI, at least in June and August.
Structural features, such as density, height, LAR, and LAI, measured in our study area reached high valuesthat are unusual for most temperate wetlands (e.g. Ekstam, 1995; Cizkova et al., 1996; Herbst and Kappen,1999). Nevertheless, the few published studies about reed wetland structure in Mediterranean and arid areasshowed values in the range found in this study. High density in reed stands seems to be frequent in semi-aridwetlands, especially when water is moderately eutrophic.
Hocking (1989) found 180 live stems per square metre, an average height of 5 m and an average of 30leaves per stem in an Australian reed stand, whereas Ennabili et al. (1998) found a stem density of 100 m�2
in northern Morocco. LAI was not measured in these two studies but, taking into account the structural standattributes, its value could be even higher than that found in our study. LAI, however, is expected to varyowing to, for example, gradients in microclimate across reed patches of variable size.
The meteorological characteristics in the study area were potentially capable of sustaining high transpirationrates under conditions of permanent water availability, such as found in the reed population studied duringthe 2000 growing season. Reeds are regarded as plants with a high transpiration flux (Haslam, 1972), but therates found in this study are higher than those published for transpiration rates in P. australis (Bernatowiczet al., 1976; Burba et al., 1999; Herbst and Kappen, 1999; Lisnner et al., 1999). However, special caution isneeded when quantitative comparisons are made, owing to the fact that the available data were obtained fromseveral different methodological and experimental protocols, although never utilizing the sap-flow method. Forinstance, transpiration has been measured in reed saplings growing in pots by means of porometry (Lisnneret al., 1999) and by estimation of weight losses (Bernatowicz et al., 1976). Under natural conditions, seasonaltranspiration in some reed stands from the USA (Burba et al., 1999) and central Europe (Herbst and Kappen,1999) have been calculated from partitioning evapotranspiration into evaporation and transpiration by meansof empirical approaches based on the Penman–Monteith equation (Monteith, 1965) or the Shuttleworth andWallace (1985) model. Burba et al. (1999) and Herbst and Kappen (1999) described a similar diurnal andseasonal transpiration pattern to that found in our study, where vapour pressure deficit and available radiantenergy, combined with the ageing of leaves, were the major factors explaining the particular pattern of reedwater losses throughout the growing season. In both studies, the maximum seasonal AET and transpiration ata plot-scale basis were reached at the peak of green LAI. This is in agreement with our results.
High maximum daily transpiration rates �16 mm day�1� were obtained in summer during a relatively dryand warm growing season in reed belts in Germany, with maximum average values of about 10 mm day�1 innormal years (Herbst and Kappen, 1999). However, in Nebraska, under a semi-arid climate, the correspondingdaily transpiration peaks reached only 4 mm day�1 (Burba et al., 1999). We found a maximum dailytranspiration rate of about 23 mm day�1 in June under similar meteorological conditions. These results arestrongly influenced by LAI. In fact, the reed population in Nebraska only reached a maximum LAI of2Ð4 m2 m�2, which contrasts with the almost four times higher LAI found in this study.
A gross approximation to the total losses by transpiration during the growing season could be obtained byconsidering sampled days in this study as being representative of meteorological monthly variability in thestudy area. In June, long-term data of daily mean temperature, daily mean relative humidity, rainfall frequencyand sunshine hours reached values of 22Ð1 °C, 69Ð2%, 2 days month�1 and 11Ð5 h day�1 respectively (PerezCueva, 1994). These values are within the range of the average data found in the study area in June.Consequently, it can be suggested that about 600 mm of water were transpired in June. Even though thisvalue is likely to be overestimated, it is probably close to the actual amount transpired by the reed stand.
The potential evapotranspiration achieves maximum values during the summer months in Spanish Mediter-ranean areas, particularly in southeastern Spain. The calculated long-term ET0 for the south of Alicanteprovince by means of the Hargreaves (1956) equation yielded values of 1600 mm year�1, and 150 mm,
175 mm and 165 mm in June, July and August respectively (Perez Cueva, 1994). Our transpiration data fromAugust and June greatly exceed these ETP empirical values. However, there is general agreement about theimprecision of ETP empirical formulas when applied to the calculation of potential evapotranspiration in plantcovers very different with respect to the reference vegetation used. The Hargreaves ET0 formulation assumesa continuous and no water-limiting grass cover. Taking into account the obvious differences between thesetwo covers in relation to some of the most relevant parameters influencing transpiration, such as the roughnesslength coefficient and LAI, it appears that reeds are not at all comparable to the reference grass.
In temperate European environments, Herbst and Kappen (1999) found large maximum daily transpirationrates, ranging from 10 to 16 mm day�1 in July, along with LAI values of 5 m2 m�2. Taking into accountthe warmer climatic conditions and the lower LAI in this stand compared with that found in our field site,it can be concluded that transpiration rates such as those measured in ‘El Hondo’ were easily achievable.Moreover, ‘El Hondo’ is surrounded by an arid landscape consisting of extensive areas of sparsely distributedcover of salt marshes and bare soil, which could act as a significant source of sensible heat. Thus, horizontaladvection from adjacent lands, know as the ‘oasis effect’ (Jones, 1992), could provide additional energy tothe evaporation, mostly in summer. In arid Australia, Lang et al. (1974) found a significant contribution ofexternal sensible heat to adjacent rice fields, with evapotranspiration values accounting for 170% of horizontalnet radiation.
The clumped disposition of reeds, as opposed to making up a continuous cover, can generate a largerroughness length that would facilitate the interchange of heat, water vapour and momentum between thesurface and the turbulent boundary layer, hence favouring evaporation. In our study area, an estimation ofthe available energy per unit of land area (without external inputs) can be roughly obtained from long-termdata on horizontal global radiation. This value is 28 MJ day�1 m�2 in June (Perez Cueva, 1994), which isequivalent to a 10 mm water evaporation. Therefore, in order to evaporate up to 23 mm occasionally, anextra source of energy is needed. We suggest that this energy source could be horizontal advection, which isa relevant component of the energy balance in wetlands. In semi-arid Spain, such wetlands constitute fertilityislands integrated in a dry landscape.
ACKNOWLEDGEMENTS
This work has been funded by the Generalitat Valenciana (project GV99-43-1-03), the CICYT project‘Modelizacion del balance de energıa en areas espacialmente heterogeneas de clima semiarido’ (ref. CLI99-0835-C02-02), by the Spanish National R&D Programme on Climate and by a short-visit grant funded by theGeneralitat Valenciana (INV00-16-8) to the University of Alicante for the second author. We wish to thankthe Conselleria de Medio Ambiente of Alicante for the licensing of our work in ‘El Hondo’ Natural Park.
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