Growth of Phragmites australis and Phalaris arundinacea in constructed wetlands for wastewater treatment in the Czech Republic

Ecological Engineering 25 (2005) 606–621

Growth of Phragmites australis and Phalaris arundinacea in constructed wetlands for wastewater treatment in the Czech Republic Jan Vymazal a,c,∗ , Lenka Kr˝opfelov´a b ˇ ıcˇ anova 40, 169 00 Praha 6, Czech Republic ENKI, o.p.s, R´ ENKI, o.p.s., Dukelsk´a 145, 379 01 Tˇreboˇn, Czech Republic Duke University Wetland Center, Nicholas School of the Environment and Earth Sciences, Durham, NC 27708, USA a

b

c

Accepted 11 July 2005

Abstract Common reed (Phragmites australis) and reed canarygrass (Phalaris arundinacea) are two most commonly used plant species in constructed wetlands for wastewater treatment in the Czech Republic. Growth characteristics of both plants (biomass, stem count, and length) have been measured in 13 horizontal sub-surface flow constructed wetlands since 1992. The results revealed that while Phalaris usually reaches its maximum biomass as early as during the second growing season, Phragmites usually reaches its maximum only after three to four growing seasons. The maximum biomass of both species varies widely among systems and the highest measured values (5070 g m−2 for Phragmites and 1900 g m−2 for Phalaris) are similar to those found in eutrophic natural stands. The shoot count of Phragmites decreases after the second growing season while length and weight of individual shoots increases over time due to self-thinning process. Number of Phalaris shoots is the highest during the second season and then the shoot count remains about the same. Also the shoot length remains steady over years of constructed wetland operation. © 2005 Elsevier B.V. All rights reserved. Keywords: Aboveground biomass; Constructed wetlands; Phalaris arundinacea; Phragmites australis; Shoot density

1. Introduction The emergent macrophytes growing in constructed wetlands designed for wastewater treatment have several properties in relation to the treatment processes that make them an essential component of the design. The most important effects of the emergent macro∗

Corresponding author. E-mail address: [email protected] (J. Vymazal).

0925-8574/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2005.07.005

phytes in relation to the wastewater treatment processes in constructed wetlands with horizontal subsurface flow (HF CWs) are the physical effects such as erosion control, provision of surface area for attached microorganisms and insulation of the bed surface during winter (Brix, 1997; Vymazal et al., 1998a). The metabolism of the macrophytes (e.g., plant nutrient uptake, oxygen release) affects the treatment processes to different extents depending on design but in HF CWs those processes play less important

J. Vymazal, L. Kr˝opfelov´a / Ecological Engineering 25 (2005) 606–621

role than the physical processes (Brix, 1993, 1997, 1998; Brix and Schierup, 1990; Vymazal, 1999, 2001, 2004a,b). It has been suggested that in temperate and cold climates, insulation of the bed during cold periods is probably the most important role of vegetation in HF CWs (Mander and Jenssen, 2003) and therefore it is desirable to use plants which have high aboveground biomass and grow fast and create a dense cover soon after planting. The plants used in constructed wetlands designed for wastewater treatment should also (1) be tolerant to high organic and nutrient loadings, (2) have rich belowground organs (i.e., roots and rhizomes) even under certain level of anoxia and/or anaerobiosis in the rhizosphere in order to provide substrate for attached bacteria and oxygenation (even very limited) of areas ˇ ızˇ kov´a-Konˇcalov´a adjacent to roots and rhizomes (C´ et al., 1996; Kvˇet et al., 1999). In general, there is a broad group of plants that could possibly be used in constructed wetlands. However, the field experience has proven that only few plants are commonly used. By far the most frequently used plant in HF CWs around the world is Phragmites australis (common reed) (Cooper et al., 1996; Vymazal et al., 1998b; Kadlec et al., 2000). The use of Phragmites is limited only in North America because this plant is not considered to be native there (Kadlec and Knight, 1996). Other species frequently used are Phalaris arundinacea (reed canarygrass) and Glyceria maxima (sweet managrass) in Europe, Typha spp. (cattails) and Scirpus spp. (bulrush) mostly in North America.

607

In the Czech Republic, P. australis (Cav.) Trin. ex Steudel and P. arundinacea L. have widely been used in HF CWs. These plants are used as monostands but very often they are planted in bands perpendicular to water flow (Vymazal, 1998, 2002). The objectives of the paper were to evaluate and compare the establishment after planting, seasonal/long-term growth, and growth characteristics (biomass, stem count, stem length) for P. australis and P. arundinacea planted in constructed wetlands for wastewater treatment in the Czech Republic.

2. Materials and methods Since 1991, the results on Phragmites and Phalaris aboveground biomass have been recorded. At present we have results from 14 constructed wetlands available (Table 1). The biomass was sampled during the period of peak standing crop—in July for Phalaris and in the period end of August/beginning of September for Phragmites. In constructed wetland at Morina, a seasonal variation in aboveground biomass of both Phragmites and Phalaris was monitored by harvesting the plants in 2002 (April, May, June, July, September, October, and December) and 2003 (March, April, May, June, July, August, and December). Three replicate 0.25 m2 quadrants were collected during the harvest. Stems were cut at the ground level, counted and length of stems was measured. The biomass was divided into stems, leaves including leaf sheaths and flowers and dried at 70 ◦ C until constant

Table 1 Constructed wetlands with subsurface horizontal flow included in the study Locality

Start of operation

PE

Size of the beds (m2 )

Vegetation

Results from the period

Onrejov Spalene Porici Vesely Zdar Kotencice Studenka Kamen Nucice Cista Zbenice Nezdice Pribraz Trhove Dusniky Morina

1991 1992 1994 1994 1995 1995 1995 1995 1996 1998 1999 1999 2000

356 700 75 326 56 340 650 800 200 450 300 500 700

806 2500 288 1800 280 1740 3224 3040 1000 2100 1512 1800 3520

Phragmites Phragmites + Phalaris Phragmites + Typha Phragmites Phragmites + Phalaris Phragmites + Phalaris Phragmites + Phalaris Phragmites + Phalaris Phragmites + Phalaris Phragmites + Phalaris Phragmites + Phalaris Phragmites + Phalaris Phragmites + Phalaris

1992–2003 2003 1996–2003 2003 2002–2004 1996–2003 2003 2003 1997–2003 2003 1999–2003 2003 2000–2004

608

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Fig. 1. Phragmites australis aboveground biomass from 13 constructed wetlands treating municipal sewage in the Czech Republic in 2003. Different letters indicate significant difference at α = 0.05 between the means.

weight. The results were expressed in g dry mass m−2 . For statistical analyses Turkey HSD test was used.

3. Results and discussion 3.1. Standing crop The survey carried out during 2003 revealed that there is a great difference among aboveground biomass of both Phragmites (Fig. 1) and Phalaris (Fig. 2) grow-

ing in constructed wetlands for wastewater treatment. The survey included only systems which had been at least four years in operation at the time of harvesting so both Phalaris and Phragmites could have reached the maximum biomass (see further text). Phragmites aboveground biomass (Fig. 1) varied widely between 1652 and 5070 g m−2 with an average value of 3266 g m−2 (±1050 g m−2 ). In Table 2, examples of Phragmites aboveground biomass from both natural and constructed wetlands are presented. In the literature, there is considerably more informa-

Fig. 2. Phalaris arundinacea aboveground biomass from seven constructed wetlands treating municipal sewage in the Czech Republic in 2003. Different letters indicate significant difference at α = 0.05 between the means.

J. Vymazal, L. Kr˝opfelov´a / Ecological Engineering 25 (2005) 606–621 Table 2 Examples of aboveground biomass (maximum values) of Phragmites australis and Phalaris arundinacea growing in natural stands and constructed wetlands Plant species Reference Phragmites australis Larsen and Schierup (1981) Ho (1979a)

Biomass

413 669

Kansanen et al. (1974)

1000

Dykyjov´a (1989)

2050

Bj¨ork (1967)

2400

Dykyjov´a and Kvˇet (1982)

3000

Ho (1979a)

3975

Boar et al. (1989)

4424

Nov´akov´a (1989)

4933

Hocking (1989)

9890

Adcock and Ganf (1994) Gries and Garbe (1989) Vymazal et al. (1999)

788 1360 2088

Duˇsek and Kvˇet (1996)

2172

Obarska-Pempkowiak and Ozimek (2003) Toet et al. (2005)

2353

Haberl and Perfler (1990) Behrends et al. (1994)

3100 4046

Phalaris arundinacea Ho (1979b) Lawrence and Ashford (1969) Kline and Broersma (1983)

2850

440 742 817

Ho (1979a)

1151

Lukavsk´a (1989)

1408

Hl´avkov´a-Kumnack´a (1980)

2304

Vymazal et al. (1999)

731

Table 2 (Continued ) Plant species Reference Behrends et al. (1994)

Locality

Denmark, oligotrophic lake Scotland, mesotrophic lake Finland, oligotrophic lake Czech Republic, meso-eutrophic pond Sweden, eutrophic lake Czech Republic, eutrophic pond Scotland, hypertrophic lake England, polluted lakes Czech Republic, eutrophic ponds Australia, nutrientenriched swamp Australia, HSF CW Germany, HSF CW Czech Republic, 5 HSF CWs (mean) Czech Republic, 2 HSF CWs (mean) Poland, FWS CW, stormwater runoff The Netherlands, FWS CW Austria, HSF CW Alabama, USA, mesocosm HSF CW Scotland, meso-eutrophic lakes Canada, dump soils Canada, fertilized meadow Scotland, polytrophic lake Czech Republic, wet meadows Czech Republic, wet meadows Czech Republic, 2 HSF CWs (mean)

609

a

Biomass 831

Marten et al. (1979)

1226

Bernard and Lauve (1995)

1713

Hurry and Bellinger (1990)

2458

Locality Alabama, USA, mesocosm HSF CW Minnesota, USA, stands treated with wastewatera New York, USA, HSF CW, landfill leachate England, overland flow wetlanda

Multiple harvest.

tion on Phragmites biomass in natural stands as compared to constructed wetlands. The available results indicate that Phragmites aboveground biomass is comparable with biomass found in highly eutrohic natural stands. However, there are data of Phragmites biomass from natural stands which are much higher than those recorded in constructed wetlands. Standing crop up to 9300 g m−2 was reported by Gopal and Sharma (1982) from nutrient-enriched wetlands in India, Korelyakova (1971) reported a peak standing crop of live shoots of 9340 g m−2 from stands of P. australis fringing reservoirs on the Dnieper River in Ukraine and Wallentinus et al. (1973) reported a standing crop of shoots of 5620 g m−2 in Sweden. Such high aboveground biomasses have not been reported from constructed wetlands so far but it seems that the reason for lack of high biomass values is the fact that biomass is not so often measured in constructed wetlands. However, it seems that very high standing crop could occur in constructed wetlands as well. In Studenka, standing crop as high as 11 280 g m−2 was measured in the inlow part of filtration beds in 2004. The aboveground biomass of Phalaris varies between 345 g m−2 at Cista and 1902 g m−2 at Zbenice (Fig. 2) with an average value of 1286 g m−2 (±477 g m−2 ). Comparison with data on Phalaris biomass (Table 2) revealed that values found in the survey were within the range found in both natural stands and constructed wetlands or natural stands treated with wastewater. However, field observations indicate that Phalaris biomass is also affected by water level in the bed. The higher the water is kept the higher biomass is achieved.

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Fig. 3. Aboveground biomas of Phalaris arundinacea in constructed wetlands Morina (top left), Zbenice (top right), Pribraz (bottom left) and Kamen (bottom right). Different letters indicate significant difference at α = 0.05 between the means.

The highest value for Phalaris in Table 2 reported by Hurry and Bellinger (1990) was obtained when Phalaris was mowed twice a year. However, it seems that further mowing may not increase the standing crop. Marten et al. (1979) reported the total annual standing crop of Phalaris treated with municipal wastewater of 1226, 1096 and 859 g m−2 for stands which were cut two, three and four times a year, respectively. 3.2. Establishment of plants after planting In the Czech Republic, seedlings are used for plantation of both Phalaris and Phragmites. The seedlings are planted bare-root and after planting the aboveground parts usually die back but shortly after that re-sprouting occurs. The results obtained from the Czech constructed wetlands revealed that Phalaris grows much faster than Phragmites and reaches the maximum biomass within the second growing sea-

son (Fig. 3). For Phragmites, it usually takes three to four growing seasons to reach the maximum standing crop but in some systems it may take even longer (Fig. 4). Under the climatic conditions of the Czech Republic, Phalaris starts to sprout, depending on the weather, in the period end of March/beginning of April while Phragmites starts to sprout in May. Aboveground biomass of Phragmites achieves higher values than that of Phalaris usually in early July. Phalaris usually starts to senesce during late summer while Phragmites during late fall. Comparison of aboveground biomass development in CW at Morina is presented in Fig. 5. While Phragmites stalks stay usually upright after senescence, Phalaris shoots bend down and form kind of thick “blanket” on the surface of the filtration bed. As a consequence, Phalaris does not lose as much dry mass because it is sheltered from wind. On the other hand, dry leaves of Phragmites are often blown out of the

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Fig. 4. Aboveground biomas of Phragmites australis in constructed wetlands Kamen (top left), Morina (top right), Vesely Zdar (bottom left) and Zbenice (bottom right). Different letters indicate significant difference at α = 0.05 between the means.

Fig. 5. Abovegound biomass of Phalaris and Phragmites during the period April 2002–December 2003 in the constructed wetland Morina, Czech Republic.

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Fig. 6. Average shoot counts of Phragmites australis in 13 constructed wetlands surveyed in 2003. For the length of operation see Table 1.

system. From that point of view, it seems that Phalaris may provide better insulation than Phragmites. If not harvested, Phalaris litter is very often colonized by stinging nettle (Urtica dioica) which within two years can entirely outcompete Phalaris. In most systems Phalaris and Phragmites are planted in bands perpendicular to the flow. In many systems encroachments of Phragmites into Phalaris bands have been observed. In Spalene Porici which was planted in 1992, Phragmites outcompeted and eliminated Phalaris completely in 2000, eight years after planting. On the other hand, in some systems Phragmites encroachments are limited and in some systems there is no visible encroachment at all. At present, we do not have the answer for different patterns in Phalaris–Phragmites interactions. 3.3. Shoot count Both Phragmites and Phalaris are usually planted with the density of about 5–8 seedlings m−2 . In Fig. 6, average stem counts of Phragmites in 13 Czech constructed wetlands is shown. The average numbers vary widely between 114 and 332. The numbers indicate dense stands as compared to natural stands but comparable with numbers reported from other constructed wetlands or nutrient enriched wetlands (Table 3). How-

ever, it seems that the stem count decreases over the period of operation of constructed wetlands. The number of stems is very high during the second growing season and then decreases gradually (Fig. 7). Similar results were reported by Duˇsek and Kvˇet (1996) for constructed wetlands at Kolodeje and Cicenice in the Czech Republic. At Kolodeje, number of stems per square meter decreased from 141 during the second growing season to only 86 during the fourth growing season. At Cicenice, number of stems dropped from 202 to 94 m−2 between third and fourth growing seasons. This is an example of the self-thinning process which has been observed in many plant monocultures, particularly those used for agriculture and forestry (Parr, 1990). Self-thinning is observed in dense populations where as individuals become larger with age, the total density decreases due to mortality, usually cause by light deficiency. Despite the decrease in plant density total biomass continue to increase because mean plant weight increases (see Figs. 4 and 7, CWs Kamen and Vesely Zdar) faster than density decreases (Parr, 1990). At Fig. 8, increase in individual Phragmites shoot weight is documented. Results presented in Fig. 9 also indicate a good agreement with the self-thinning theory because despite decrease in stem numbers the biomass either increases or is stable (see

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613

Table 3 Examples of stem counts of Phragmites australis growing in natural stands and constructed wetlands Locality

No. of stems

Remark

Reference

Lake Parstreiner See, Germany Lake Templiner See, Germany Three marshes, Connecticut, USA Trebon Biosphere Reserve, Czech Rep. Loch of the Lowes, Scotland Lake T˚akern, Sweden Lake St. Francis, Qu´ebec, Canada Balgavies Loch, Scotland Trebon Biosphere Reserve, Czech Rep. Danube Delta Reserve, Romania Trebon Biosphere Reserve, Czech Rep. Constructed wetland, Queensland, AUS Forfar Loch, Scotland Constructed wetland, Poland Constructed wetland, Germany Norfolk Broads, England Mirooll Creek, NSW, Australia

11 28 26–46 49 49 61 80 84 91 105 72–142 106–146 173 216 150–220 220 320

mesotrophic eutrophic

Lippert et al. (1999) Lippert et al. (1999) Mayerson et al. (2000) Rolletschek et al. (2000) Ho (1979a) Gran´eli (1989) Auclair (1979) Ho (1979a) Rolletschek et al. (2000) Rolletschek et al. (2000) Dykyjov´a and Hradeck´a (1976) Greenway (2002) Ho (1979a) Obarska-Pempkowiak and Ozimek (2003) Gries and Garbe (1989) Boar et al. (1989) Hocking (1989)

a b

eutrophic mesotrophic

eutrophic mesotrophic eutrophic eutrophic tertiary polytrophic sewage stormwater a b

Affected by wastewater treatment plants discharges. Nutrient enriched swamp.

Fig. 4). System at Spalene Porici has never developed high aboveground biomass (Fig. 1) and therefore the weight of individual stems have never been high. In Ondrejov, weight of individual stems was low in 2003, i.e. during 13th growing season, but during the seventh growing season in 1998, the average stem weight was nearly 19 g. After that the stand underwent a substantial growth changes and the weight of individual shoots dropped down to about 10 g and since 2001 the weight has increased again reaching the value of 14.7 g per shoot. Weight of individual Phragmites shoots vary widely in natural stands according to the literature data (Table 4). Comparing the data presented in Fig. 9 it seems that Phragmites shoot weight in “younger” constructed wetlands are within the lower range found in natural wetland but shoot weight in “older” systems is comparable with higher values found in natural wetlands. The highest value of 24.45 g per shoot recorded in Vesely Zdar was lower as compared to the highest values shown in Table 4. However, there is considerably less information about shoot counts in constructed wetlands as compared to natural stands and therefore it is not possible to make a sound conclusion that Phragmites shoot counts in constructed wetlands do not achieve the maximum values found in natural wetlands.

Table 4 Examples of average individual Phragmites shoot weight in natural stands Locality

Stem weight (g)

Reference

Eleven UK sites

3.2–21.7

Norfolk Broads Lake Parsteiner See, Germany St. Francis Lake, Qu´ebec, Canada Lake Mikolajskie, Poland

6.6 7.2

Gorham and Pearsall (1956) Boar et al. (1989) Lippert et al. (1999) Auclair (1979)

Trebon Biosphere Reserve, Czech Rep. Loch of the Lowes, Scotland Balgavies Loch, Scotland Forfar Loch, Scotland Mirooll Creek, NSW, Australia Danube Delta Reserve, Romania Lake Templiner See, Germany

10.7–23.4

8.3 9–22

12.2

MochnackaLawacz (1974) Dykyjov´a and Hradeck´a (1976) Ho (1979a)

15.7 21 30.9

Ho (1979a) Ho (1979a) Hocking (1989)

33

Rolletschek et al. (2000) Lippert et al. (1999)

33

For further information on particular locations see Table 2.

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Fig. 7. Phragmites stem count at constructed wetlands Vesely Zdar (top) and K´amen (bottom) over the whole period of operation.

Fig. 8. Phragmites shoot weight at constructed wetland Vesely Zdar.

For example, in constructed wetland at Studenka, average Phragmites shoot weight of 38.6 g was recorded in 2004 in an inflow part of the system. There is only limited information on individual shoots of Phragmites in constructed wetlands. Greenway (2002) reported the range of 8.5–22.2 g per shoot from experimental constructed wetlands in Australia. Duˇsek and Kvˇet (1966) reported average Phragmites shoot weight of 9.4 g during the third year of operation of constructed wetland at Cicenice. During the next growing season, the average shoot weight increased to 19.8 g. Phalaris shoot counts exhibit a different pattern as compared to Phragmites. Similarly to Phragmites, the

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615

Fig. 9. Individual shoot weight of Phragmites australis based on the survey of 13 Czech constructed wetlands carried out in 2003. The numbers indicate the length of operation.

stem count increases substantially during the second growing season but then the stem count decreases only slightly or remains more or less constant (Fig. 10). The stem count varies between 353 and 579 m−2 during the 2003 survey (Fig. 11). The Phalaris density is much higher then that of Phragmites but comparable with numbers reported by Duˇsek and Kvˇet (1996) of 470–482 shoots m−2 at constructed wetland Chmeln´a. Natural stands seem to be less dense, for example, Ho (1979b) reported stem count between 50 and 300 for three Scottish lochs. Phalaris shoot weight varied between 1.75 and 4.21 g in seven surveyed constructed wetlands. Ho (1979b) reported the maximum average dry weight per shoot attained by plants growing in Scottish lakes between 3.98 and 4.58 g, Conchou and Fustec (1988) reported a maximum shoot weight of 2.2 ± 0.2 g for Phalaris growing in meanders of the Garrone River in France and Duˇsek and Kvˇet (1996) reported an average shoot weight of 1.31 g at constructed wetland Chmelna in the Czech Republic. 3.4. Shoot length The average Phragmites shoot length varied during 2003 survey between 185.2 and 281.5 cm (Fig. 12.). The literature survey (Table 5) indicates that the length of Phragmites shoots found in the Czech constructed wetlands was within the length range found in natural

stands. Gries and Garbe (1989) reported an average shoot length of 180 cm in a constructed wetland in North Germany. Duˇsek and Kvˇet (1996) reported average shoot lengths of 196 and 223 cm from constructed wetlands at Kolodeje and Cicenice in the Czech Republic. Greenway (2002) reported average shoot length of

Table 5 Examples of average length (in cm) of Phragmites shoots reported in the literature from natural stands Locality

Shoot length Reference

Norfolk Broads, UK Lake Parsteiner See, Germany Trebon Biosphere Reserve, Czech Rep. Danube Delta Reserve, Romania Three marshes in Connecticut Lake T˚akern, Sweden Loch of the Lowes, Scotland Balgavies Loch, Scotland Trebon Biosphere Reserve, Czech Rep. Lake St. Francis, Qu´ebec, Canada Forfar Loch, Scotland Lake Templiner See, Germany

115–271 157

Boar et al. (1989) Lippert et al. (1999)

171

Rolletschek et al. (2000)

194

Rolletschek et al. (2000)

208–380

Mayerson et al. (2000)

215 219

Gran´eli (1989) Ho (1979a)

239 244

Ho (1979a) Rolletschek et al. (2000)

246

Auclair (1979)

269 315

Ho (1979a) Lippert et al. (1999)

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Fig. 10. Phalaris stem count at constructed wetlands K´amen (top) and Zbenice (bottom) over the whole period of operation.

204 cm in an experimental wetland in Australia. However, it seems that the length of Phragmites shoots strongly increases during the years of operation (see Section 3.2 on self-thinning). This phenomenon is documented in Fig. 13. Average Phalaris shoot length varied between 58 and 135 cm (Fig. 14). The development of shoot length is different as compared to Phragmites. The shoots increase their length substantially between the first and second growing seasons and then the shoot length does not change too much during the constructed wet-

land operation (Fig. 15). The steep increase in length between the first and second growing seasons is mainly due to occurrence of flowers which may extend the shoot length by about 30–50 cm as compared to nonflowering shoots. Duˇsek and Kvˇet (1996) reported average length of flowering and non-flowering shoots at constructed wetland Chmeln´a 151 and 109 cm, respectively, with an average shoot length of 121 cm. It also seems that Phalaris shoot length depends on the water level because root system does not penetrate as deep as in the case of Phragmites. While Phragmites root-

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617

Fig. 11. Average stem count of Phalaris arundinacea in seven constructed wetlands surveyed in 2003. For the length of operation see Table 1.

ing system usually penetrates to a depth of 60–70 cm, Phalaris roots penetrate usually to a depth of 20–40 cm. It has been observed that when water level is kept too low in the filtration bed, shoots are shorter and usually less flowers occurs. This was the case at constructed wetland Cista, where average shoot length was only 58 cm with only occasional flowering. On the other hand, at newly built system at Brehov, where the water was kept above or at the surface of the bed during

the second growing season, the average shoot length amounted to 181cm with individual stems up to 224 cm. There is considerably less information on morphometric data for Phalaris as compared to Phragmites, which is one of the best studied wetland plants ever. It seems, however, that the length of shoots in constructed wetlands is comparable with that in natural stands. Ho (1979b) reported and average Phalaris shoot length between 161 and 164 cm for three Scottish lakes.

Fig. 12. Average length of Phragmites shoots at 13 constructed wetlands surveyed in 2003.

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Fig. 13. Phragmites shoot length at constructed wetlands Vesely Zdar (top) and Kamen (bottom) over years of operation.

Fig. 14. Average length of Phalaris shoots at seven constructed wetlands surveyed in 2003.

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619

Fig. 15. Phalaris shoot length at constructed wetlands Zbenice over years of operation.

4. Conclusions P. australis aboveground biomass is comparable with biomass found in eutrophic natural stands and it takes three to five years to achieve its maximum. P. arundinacea aboveground biomass is comparable with natural stands and the maximum aboveground biomass usually occurs as early as during the second growing season. Both Phragmites and Phalaris stands are very dense. For both plants the number of shoots increases steeply between the first and the second growing seasons. The stem count then steadily decreases for Phragmites as a consequence of self-thinning process. As a result, the shoots become taller and heavier so despite decreasing shoot count the aboveground biomass is either stable or increases. Phalaris shoot count also increases steeply during the second growing season but then remains about the same in the next growing seasons. Also, the shoot length remains about the same. Both plants grow very well in constructed wetlands for wastewater treatment. When planted together, Phragmites tends to outcompete Phalaris, but this phenomenon does not occur in all systems where both plants are present.

Acknowledgements During years 2001–2004, the study was supported by grant MSM 000020001 “Solar Energetics of Natural

and Technological Systems” from the Ministry of Education and Youth of the Czech Republic and by grant No. 206/02/1036 “Processes Determining Mass Balance in Overloaded Wetlands” from the Grant Agency of the Czech Republic. During years 1992-2001 the study was supported by the first author’s family budget.

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