行政院國家科學委員會專題研究計畫 成果報告

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行政院國家科學委員會專題研究計畫 成果報告

模擬水道中水生植物抗流機制之種間差異研究 研究成果報告(精簡版)

計 畫 類 別 : 個別型

計 畫 編 號 : NSC 99-2410-H-216-009-

執 行 期 間 : 99 年 08 月 01 日至 100 年 08 月 31 日 執 行 單 位 : 中華大學景觀建築學系

計 畫 主 持 人 : 陳湘媛

計畫參與人員: 碩士班研究生-兼任助理人員:陳莉茹 大專生-兼任助理人員:蘇皖兒

大專生-兼任助理人員:王張桂 大專生-兼任助理人員:李宛賢 大專生-兼任助理人員:易穗安 大專生-兼任助理人員:彭禹川 大專生-兼任助理人員:佘玟憫 大專生-兼任助理人員:范聿萱

報 告 附 件 : 赴大陸地區研究心得報告

處 理 方 式 : 本計畫可公開查詢

中 華 民 國 100 年 10 月 13 日

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模擬水道中水生植物抗流機制之種間差異研究

陳湘媛1、林鎮洋2

1.中華大學景觀建築學系助理教授 2.國立台北科技大學土木系教授

摘 要

本研究係從水工模型出發,比較不同物種之水生植物在 抗流反應方面的種間差異。在水芹菜的研究中,發現在面對 較高流速沖刷時,其生長速度趨緩,莖芽組織變得矮小且柔 軟,以增加植物的抗流彈性,此外,其平均根長變短,錨定 能力降低,根、莖與芽的生物量也隨之下降。

鑑於適性植物種類可能是多元化的組成,因此本研究另 針對不同的水生植物:台灣原生種—柳葉水蓑衣為材料,探 討其面對不同流速之抗流機制的種間變化。結果顯示流速增 加時,其生長速度、乾鮮重、直徑降低,但是與水芹菜反應 不同的是水蓑衣在面對較高流速的環境,其平均根長變長,

株高也較高。本研究之成果除可進一步確認適性植生種類或 先驅植物種外,亦可瞭解植物在生態工程上可扮演的角色與 極限,未來可做為河道植生工程之設計依據。

關鍵詞:抗流機制、模擬水道、水生植物、生態工程

ABSTRACT

The present study is carried out in a simulated channel for examining how different aquatic macrophytes respond to different channel flow velocities in terms of changes in their flow resistance mechanisms. Preliminary research of the planting material Oenanthe javanica DC. (water celery) showed that when facing higher flow velocity, the growth rate of water celery became slow and plant shoots were shorter and softer to increase plant flexibility. Root length and root anchorage decreased. Root, stem, and shoot were also found to reduce their biomass.

It is significant for the following stage to examine the interspecific differences in flow resistance mechanisms among different planting materials since suitable streambank vegetation might include a variety of plants. The native species Hygrophila

salicifolia (Vahl) Nee. was chosen as the next planting material.

Experimental data show that the growth rate, dry weight, fresh weight, diameter of this planting material decreased as flow velocities increased. However, the average roots length, average height of Hygrophila salicifolia (Vahl) Nee. became higher when facing higher flow velocity. This is different to water

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celery. This research is anticipated to verify the suitable planting materials or precursors for riverbanks and, additionally, to clarify the roles and limitations of applying aquatic macrophytes in ecological engineering.

Keywords : Flow resistance; simulated channel; aquatic macrophytes; ecological engineering.

Ⅰ、研究動機與目的

本研究係延續過去兩年的實驗,從水工模 型出發,比較不同物種之水生植物在抗流反 應方面的種間差異。根據過去兩年的實驗,

發現以水芹菜為材料的實驗中,因應不同流 速,確實在植物生理上產生不同的反應機 制,在面對較高流速沖刷時,單株栽植之水 芹菜之生長速度、莖芽組織、根長、地上莖 葉與地下根系的生物量等均產生不同的變 化。當改變水芹菜的配置方式時,在總株數 不改變的條件下,簇群栽植模式的水芹菜較 單株者有較強的抗流能力,雖然其在株高、

根長與平均綠葉數方面之差異並不如單植者 明顯,但是平均單位面積之維管束數量在高 流速之實驗組仍高於低流速之控制組,呈現 與單植者相同的結果,亦即水芹菜在面臨較 高流速時,確實以減少莖芽之斷面積但是增 加維管束密度的方式作為生理機制的調整。

鑑 於 適 性 植 物 種 類 可 能 是 多 元 化 的 組 成,因此本次研究係針對不同的水生植物,

以台灣原生種—柳葉水蓑衣為實驗材料,探 討其面對不同流速之抗流機制變化,以與水 芹菜比較是否在抗流機制上呈現種間差異。

預期研究成果除可進一步確認適性植生種類 或先驅植物種外,亦可瞭解植物在生態工程 上可扮演的角色與極限,未來可做為河道植 生工程之設計依據。

II、文獻回顧 2.1 國外對於植物抗流之相關研究

研究發現,在河道抗流與河道植生覆蓋比 例間存在非線性的關係。雖然學者很早即提 出抗流平均值包括岩屑碎石與水中植物,認 為 水 生 植 物 主 宰 了 其 盤 據 的 水 道 之 水 力

(hydraulic),但對植被於河道的抗流作用之 相關研究卻很少見。因此,非常缺乏對於植 生河川流速之模式研究,特別是植生自由分 佈的河道(Green, 2004)。許多的實證研究是

在模擬流場中以塑膠葉片或沉水性植物進行 流速測試,少有操作於自然河道或者先行試 種水生植物,再以不同流速之水流測試植物 之 生 理 反 應 機 制 者 ( Green, 2004; Järvelä, 2004)。

雖然對於植被影響河岸之研究已陸續有 文獻發表,例如:Simon and Collison 提出河 岸植被對河岸的穩定性有機械與水力兩種影 響,有的改善河岸穩定性,有的卻相反(Simon and Collison, 2002),Greenway 提出植物將其 根錨定土壤中以支撐植物的地上部,因此對 土壤基質產生強化作用(Greenway, 1987);

一些研究也發現,植被的根系型態也對河道 之沖蝕作用有影響(Anderson et al., 2004)。

但是植物是如何影響河道抗力?其機制為 何?卻仍有很大的研究空間。Lewis 提出河 道抗力係由兩種因素組成:經由摩擦力產生 的能量損失與河道內流速的變化作用(Lewis, 1997),後者在植被河道中特別明顯,植物莖 的尺寸形成的抗力會導致植株內的低速與植 株外的高速這種大幅度的流速變化,不同莖 葉尺寸將形成不同的流速。

歷來的研究已證明植被的存在確實影響 河流的流速,利用植物做為河岸緩衝帶的相 關研究包括 Dabney 等人提出的論點,認為緩 衝帶可降低沖蝕、攔截沉澱物,以及經由緩 慢逕流移除污染物質,即使緩衝帶小於 1m 寬也能攔截許多沉澱物(Dabney, 2006)。

研究同時發現,淡水水生植物在流動水流 中遭遇潛在的阻力時,必須在形態上加以適 應以避免機械性的傷害與連根拔起。部分物 種自短莖上長出小而硬的簇葉狀(rosette),

以便抵抗強的阻力與加強裸露湖岸的力量。

其他物種則遺傳了流線型的長線狀葉與莖。

大部分的物種無關乎生長的形式,會形成很 具彈性的枝芽以讓其順著水流並降低直接暴 露於水流的表面積(Sand-Jensen, 2003)。

從國外相關研究中可以得知,對於植物在 抗流上的研究已開始受到重視,但因為侷限

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於開放河道複雜的不可確定因素,研究仍多 以實驗室操作為主,而且不乏以塑膠葉片為 實驗材料者,實驗的方式則多以模擬河道中 植被如何影響流速為重點,對於從植物生理 例如植物解剖學的角度探討植物因應流速變 化的生理反應機制,如莖葉分歧、維管束之 變化等之研究尚很罕見,也從未就配置模式 差異或不同物種產生的抗流機制做相關研 究,因此可預見此領域的研究價值。

2.2 國內相關研究

2.2.1 植生對邊坡穩定性的相關研究

關於植生對邊坡生態穩定性的研究,國內 拱祥生、林宏達曾利用植生材料的特性,結 合不飽和土壤理論,進行邊坡生態工法穩定 機制的探討,以釐清植生對邊坡穩定性的影 響。其提出植生根系的強度及錨定至岩層中 的厚度,為邊坡植生工程的重點,而草本植 物的高地表覆蓋率是防止邊坡沖蝕的重要因 素;木本植物的高根系強度及土壤含根比則 是抑制邊坡淺層崩塌的有效方法。(拱祥生、

林宏達,2003)。林信輝等(2005)九芎植生 木樁之生長與根系力學之研究,針對九芎植 生木樁之生長特性與根力進行研究,探討不 同生長地點與處理方式之萌芽樁成活率;吳 瑞賢的研究團隊則利用根系力學模式,計算 百喜草的植根對土壤強度之增量,並建立分 析模型(陳秀婷等,2006);另外尚有吳正雄

(1990)針對植生根力與坡面穩定關係之研 究、游新旺等(2006)提出「根力模式對含 根土壤剪力強度評估之影響」,以及朱榮華等 (2005)對於「根系變形模式與含根土壤剪力強 度之研究」等,均是針對植物根系對土壤強 度影響之研究,至於植物如何因應流速變化 而產生抗流反應的相關研究則闕如。

2.2.2 國內關於生態渠道之研究

國內關於生態渠道之研究包括楊紹洋等

(2006)針對植生護岸和粗糙渠床之渠槽試 驗,以人造草皮模擬護岸植生,分析渠道在 不同植物種類和高度時的水理特性;林鎮洋 等(2006)以實驗水槽養殖指標魚種,嘗試 建立本土性的水理參數(如雷諾數與福祿數 等),據以模擬變遷的水域生態環境,以預測 溪流完工後的生態環境變化趨勢。呂珍謀等

(2008)針對河道植生群型態對水流之影響 的研究,嘗試建立一水流通過植生群之水深

平均二維水理模式,並探討植生群型態對水 理特性之影響。但在其研究中植物本身的抗 流機制未被考量,而研究以竹子模擬植株,

亦無法代表一般植物之生理與型態特性。

根據本研究團隊第一階段之研究,水芹菜 在面對較大的流速變化時,其反應機制為降 低株高、根長,採收後計算株高與根長之比 值,實驗組 CD 槽比值要大於控制組 AB,意 謂流速增加會抑制植物根部的生長速度,水 芹菜用以避免機械傷害的反應機制乃是降低 植株的高度與直徑,以便有較彈性柔軟的莖 部對抗拖曳力,至於其根部變短,推測乃是 為使植株降低錨定作用,以便有機會尋找更 適當的生長環境(陳湘媛等,2008)。由於柳葉 水蓑衣與水芹菜同為溼地植物,前者更是台 灣原生物種,針對不同物種面對流速變化的 環境,是否有相同或相異的反應機制?則是 本次研究擬釐清的課題。

III、實驗設備與研究方法 3.1 採土原則

由 於 研 究 係 以 河 川 砂 石 為 植 物 栽 培 介 質,以更能模擬實際河道之環境,因此選定 苗栗縣南庄鄉蓬萊溪中上游段為採土樣區,

實驗土壤與水芹菜實驗之土壤來源相同,至 於土壤採土原則如下:

1、未經過人為整治過之天然河川或經整治但 已植生穩定之河川。

2、人員車輛可及性高的地點。

3、採土周圍有植栽及生物生長,未受污染之 土壤。

4、採取約 10cm 厚的表層土壤。

5、過大的石塊及多餘的水份於採土前事先排 除。

3.2 蓬來溪環境資料

蓬萊溪是中港溪的上游重要支流之一,位 於苗栗縣南庒鄉蓬萊村,因屬上游河段,河 岸植被樹木茂密,河中大石林立,以前蓬萊 溪因清澈的河水、豐富的魚蝦,每至假日吸 引眾多遊客至此戲水烤肉,因此造成河川污 染,加上因網捕、毒魚而破壞河川生態(資 料來源:台灣河川復育網站),因此近年來推 動封溪護漁運動,希望能夠回復當地的生態 資源。

3.3 植物選種

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3.3.1 植物材料選種依據:

1、為配合水槽尺寸,植物材料尺寸需低於 30 cm。

2、植物生長勢強、易於繁殖、多年生草本、

分佈範圍廣。

3、屬本土或馴化種,對本土生態環境無威脅 性。

4、低人工維護,天然之環境可自然生長。

5、根系需有良好之固土定砂能力。

3.3.2 植栽選定

柳 葉 水 蓑 衣 : 爵 床 科 (Acanthaceae) 學 名:Hygrophila salicifolia (Vahl)

1、植物分佈:全台灣溼地均可見。

2、植物生理特性:一年或多年生草本,高可 達 80 cm,莖略為木質化,方形,有稜溝。

葉對生,寬線形或倒披針形,長 3~8cm,寬 0.7~1.5 cm,有柄,,近全緣,雙面略覆軟毛。

花腋生,淡紫色唇型;可以扦插法無性繁殖

(黃增泉等,1978;台北植物園資訊網,

2010)。 3.4 實驗設備

3.4.1 實驗水道模型設備

1、水槽:1cm 厚可調式壓克力水槽兩組,長 200cm,寬 30cm,高 40cm。

2、變頻馬達 2 具。

3、植栽槽木箱,長 90cm,寬 29cm,高 5cm,

板厚 1cm,以樹脂與鐵釘膠著固定。

4、三尺長 40w 雙管植物燈兩盞。

5、定時器 Timer,(設定照光時間 6:30am~

17:30pm)。

6、溫度計。

7、水槽平均坡度設定在 2%以下,屬於緩流 型河岸之坡度。

3.4.2 實驗操作 3.4.2.1 植物栽種計劃

1、於栽種前每株水蓑衣扦插芽先於電子秤秤 得其鮮重,再測量個別水蓑衣之株高、直 徑、與綠葉數。

2、每組植栽槽種植 3 株一簇共 48 株水蓑衣,

採品字型種植,初期兩槽固定相同流速,

讓水蓑衣生長勢穩定後調整實驗組流速。

3、植栽槽覆土深度 4cm。

3.4.2.2 實驗採收

1、採收後測量個別水蓑衣之高度及根長、最 高綠芽之直徑、全株鮮重。

2、於攝氏 105℃、24 小時烘乾後測其實驗後 總乾重。

3、烘乾後將地上部與根部分開,量測地上部 乾重與根部之乾重。

IV、結果與討論

前期研究中水芹菜之實驗從 2006 年 9 月 進行至 2008 年 8 月,歷時兩年,有三次實驗 成功紀錄,至於本次研究的水蓑衣實驗期間 則是從 2009 年 8 月至 2010 年 6 月,於實驗 後第 43 周實驗組對照組之成活率相差超過 10%以上時結束並採收,實驗期間曾感染介 殼蟲害,以無毒農藥「葉潔 Globrite(Potassium salts of fatty acids 49%)」1/50 濃度噴灑控制。

4.1 不同流速下水生植物之生長速度比較 在水芹菜實驗中,三次實驗在實驗前平均 綠葉數的差距均在 3.0%以內,而採收後均呈 現實驗組平均綠葉數低於控制組平均綠葉數 的結果,說明流速會抑制水芹菜綠芽之生長

(表 1)。

至於水蓑衣的實驗,因採取扦插方式栽 植,所有的扦插芽均無根,並將其大部分的 葉片剪除以降低蒸散。起初兩組之平均綠葉 數均在 1.0 以下,在實驗組平均流速低於 16.0 cm/s 時,其平均綠葉數略高於控制組,當實 驗組平均流速提高至 16.0 cm/s 以上後,其綠 葉數量急遽降低,成活率也快速下降,顯示 逐漸接近水蓑衣的流速耐受極限(圖 1),同 時也說明水蓑衣與水芹菜的生長均會受到流 速之影響。

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0 5 10 15 20 25

0 5 10 15 20 25 30

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43

Average green leaves

Flow velocities of planters AB and CD (cms-1)

Weeks after planting

圖1 水蓑衣於不同流速下之平均綠葉數變化

Flow velocities of planter AB (cms-1) Flow velocities of planter CD (cms-1) Control group planter A Control group planter B Experimental group planter C Experimental group planter D

4.2 不同流速下水生植物生物量之變化 在水芹菜的實驗中,雖然在初始實驗時將鮮 重差距控制在 10%以下,但在採收後控制 組之總鮮重均高於實驗組(圖 2),實驗一 之流速差異性大,反應在乾鮮重上的差異性 尤其明顯。至於在水蓑衣的實驗最初,控制 組 AB 與實驗組 CD 總鮮重相差在 1.0%以

下 , 實 驗 結 束 後 總 鮮 重 之 差 距 卻 達 到 56.0%,也是呈現實驗組低於控制組之現 象,當計算平均鮮重時,控制組亦高於實驗 組 9.0%(表 2),與水芹菜實驗的結果一致,

說明流速對水生植物生物量之影響。

表 1. 水芹菜三組實驗前後平均綠葉數之變化比較

(資料來源:陳湘媛,2010)

 

Control group Ex perimentag roup

Chang es in qua ntity

of g reen leaves before exp. a fter ex p. befo re exp . a fter e

Exp eriment I 2.9 2 4.86 3. 00 3.33

Exp eriment II 4.6 2 6.48 4 .67 5.68

Exp eriment III-1 3.60 6.61 3 .85 6.76

Exp eriment III-2 1.6 3 8.73 1 .61 7.0 9

 

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00

Fresh wt.

before exp. I (gm)

Fresh wt.

after exp. I (gm) (20 wks)

Fresh wt.

before exp. II (gm)

Fresh wt.

after exp. II (gm) (18 wks)

Fresh wt.

before exp. III (gm)

Fresh wt.

after exp. III-1 (gm) (13 wks)

Fresh wt.

after exp. III-2 (gm) (40 wks)

Weight (gm)

Control group planter A Control group planter B Experimental group planter C Experimental group planter D

圖 2. 三組水芹菜實驗前後總鮮重比較表(資料來源:陳湘媛,2010)

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6 0

1 2 3 4 5 6 7 8

Dry wt. of exp. I (gm)

Dry wt. of exp. II (gm)

Dry wt. of exp. III-1 (gm)

Dry wt. of exp. III-2 (gm)

Weight (gm)

Control group planter A Control group planter B Experimental group planter C Experimental group planter D

表 2.實驗前後水蓑衣之鮮重變化

Fresh wt.

before Exp.

Fresh wt.

after Exp.

(gm)

Average fresh wt.

of Exp.

Control group

237.51 348.92 3.88 Experimental

group 236.00 223.63 3.56

當以攝氏 105℃,24 小時烘乾後,水芹菜 控制組的總乾重均高於實驗組(圖 3),與總 鮮重之變化趨勢相同。而在水蓑衣的實驗 中,採收後測得控制組 AB 總乾重為 58.73 gm,實驗組總乾重為 35.23 gm,控制組之總 乾重約高於實驗組 66.7%,當比較平均乾重

時 , 控 制 組 的 平 均 乾 重 僅 較 實 驗 組 高 16.4%。如將乾重分為地上部的芽重與地下部 的根重時,控制組 AB 之平均芽重較實驗組 高 18.4%,但平均根重卻是實驗組的 95%,

顯示水蓑衣在面對較高流速環境時,其地上 部莖葉生長速度會降低,但是地下根部的生 長速度與生物量卻會增加,與水芹菜面對高 流速環境時同時降低地上部與地下部之生物 量的反應不同(表 3、圖 3)。

4.3 不同流速下水生植物之型態變化比較 4.3.1 不同流速下之株高變化

圖 3. 水芹菜三組實驗總乾重比較圖

在水芹菜實驗中,實 驗組在平均株高上要 較 控 制 組 低 矮 ( 圖 4),實驗 III 中雖然實 驗 組 略 高 於 控 制 組 7.0%,但是株高平均 值均低於實驗之初始 值,表示流速對水芹 菜之株高確有影響。

而 在 水 蓑 衣 的 實 驗 中,栽植之初水蓑衣 係以扦插法種植,實 驗前控制組 AB 平均 株高較實驗組 CD 的 平 均 株 高 要 高 8.8%;實驗 43 週後控 制組 AB 平均株高卻 是 實 驗 組 CD 的 99.0%,實驗後在 株高的增加率方 面也是實驗組較 控 制 組 高 ( 表 4),與水芹菜實 驗之結果不同。

表 3. 實驗前後水蓑衣之乾重變化

Dry wt. after Exp.

Dry wt. of shoots after

Exp.

Dry wt. of roots after Exp.

Average dry wt. after Exp.

Average dry wt. of shoots after Exp.

Average dry wt. of roots after Exp.

Control group

58.73 54.62 4.11 0.65 0.61 0.05

Experimental

group 35.23 32.25 2.97 0.56 0.51 0.05

(資料來源:陳湘媛,2010)

0.0 50.0 100.0 150.0 200.0 250.0 300.0

Height after exp. I (mm)

Height before exp. II (mm)

Height after exp. II (mm)

Height before exp III. (mm)

Height after exp. III-1

(mm)

Height after exp. III-2

(mm)

Average height (mm) Control group

planter A

Control group planter B

Experimental group planter C Experimental group planter D

圖 4. 水芹菜三組實驗前後平均株高比較(資料來源:陳湘媛,2010)

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7 0

0.5 1 1.5 2 2.5 3 3.5

Dia meter a fter exp. I (mm)

Dia meter before exp II.

(mm)

Dia meter a fter exp II. (mm)

Dia meter before exp. III (mm)

Dia meter a fter exp. III-1

(mm)

Dia meter a fter exp. III-2

(mm)

Diameter (mm)

Control group planter A Control group planter B Experimental group planter C Experimental group planter D

4.3.2 不同流速下之直徑變化

在水芹菜實驗中,無論單植或簇群栽植 方式,實驗組之平均直徑均低於控制組

(圖 5),顯示水芹菜以降低直徑的方 式,讓植株本身與水流接觸的面積減 少,以避免受到高流速的機械性傷害。

而在水蓑衣的實驗中,實驗前控制組的 平均直徑為實驗組之 96.4%,控制組低 於實驗組 3.6%,實驗後控制組之平均直 徑則較實驗組高 1.7%,雖然變化趨勢與 水芹菜相似,但不如水芹菜明顯(表 5)。

圖 5. 水芹菜三組實驗前後平均直徑比較

4.3.3 不同流速下根長之變化

在水芹菜實驗中,實驗組之平均根長均低 於控制組,顯示水芹菜面臨高流速之惡劣環 境,以降低根長的方式離開原來的生長地 區,讓自己有機會尋求更佳的生存環境(表 6),但是在水蓑衣的實驗中,因採扦插法栽 植,實驗前均無根,實驗後則發現實驗組的 平均根長大於控制組,與水芹菜的反應不 同,卻與先前乾鮮重量測時實驗組平均根重 大於控制組的結果相符合(表 7)。

表 6.水芹菜實驗前後平均根長比較

表 7. 水蓑衣實驗前後平均根長比較

Average root length after exp. II planter A 252.84 planter B 191.27 planter C 274.57 planter D 272.18

4.3.4 不同流速下水平莖之數量變化

在水芹菜實驗中,部分水芹菜之葉片會 由挺立生長變為水平(水平葉之認定以葉柄 斜角大於 45˚者為限),經由統計分析,水芹 菜控制組的平均水平葉數量低於實驗組;而 在水蓑衣的實驗中,實驗組的植栽形成水平 莖型態的平均數量亦高於控制組(表 8),而 且呈現流速越高傾向水平生長的趨勢越明 表 4 水蓑衣於不同流速下之株高變化

Height before exp. (mm)

Height after exp. (mm)

Percentage of increasing (%)

planter A 138.8 306.8 121.0

planter B 137.0 289.3 111.2

planter C 130.6 321.7 146.3

planter D 122.9 280.2 128.0

表 5.水蓑衣實驗前後平均直徑比較

  Di ame ter

before e xp. I I

D ia m et er afte r exp . II plant er A 4 .4 8 1 .8 4 plant er B 4 .3 8 1 .7 4 plant er C 4 .4 8 1 .6 4 plant er D 4 .7 1 1 .8 8

  Root length

before exp. II (mm)

Root length after exp. II (mm)

Root length before exp. III (mm)

Root length after exp. III-2 (mm) Control group

planter A 26.4 30.4 42.0 45.4

Control group

planter B 28.7 22.5 39.8 49.0

Experimental group

planter C 24.6 19.1 45.1 41.7

Experimental group

planter D 23.3 20.0 40.5 44.6

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8

顯,由於水平葉數量增加可以使植株更具流 線型,以降低被水流沖斷的危險,因此從水 芹菜與水蓑衣的實驗中證實植物確實會以改 變自己的型態例如以水平化的方式,來增加 植株對較高流速的耐受度。

表 8. 水蓑衣水平莖之數量變化

parallel stems after exp.

Average parallel stems after exp.

planter A 46 1.02

planter B 36 0.80

planter C 45 1.50

planter D 30 0.91

V、結論與建議

本研究主要在了解不同水生植物在面對 流速變化時的各種生理反應,以確定適生物 種。在前期水芹菜的研究中,發現在面對較 高流速沖刷時,水芹菜的生長速度趨緩,莖 芽組織變得矮小且柔軟,以增加植物的抗流 彈性,此外,其平均根長變短,錨定能力降 低,流速越高平均乾鮮重越低,而為了提高 對流速的耐受度,水芹菜也透過水平莖葉的 形成,讓植株本身具有流線型,可降低植株 被水流沖斷的危險。

在水蓑衣的實驗中,水蓑衣在面對流速變 化時的部分反應與水芹菜相同,例如:生長 速度受到抑制、平均乾鮮重降低、直徑較小、

水平莖增加等,其中在水平莖增加方面,根 據 Manz 與 Westhoff 之研究,植物可能透過 增加芽的長度以增加本身的彈性,或者因增 加芽的厚度而降低其個體之彈性(Manz and Westhoff, 1988),然而在本研究中卻發現,水 生植物也可能透過水平莖葉之形成,讓其植 株具有流線形狀,在流速增加時可以有較大 的抗流能力,同時避免植株被水流沖斷。

除了以上相同的反應外,水蓑衣也有部分 生理反應與水芹菜不同,例如:實驗組之平 均株高大於控制組、根長較長、根部之乾鮮 重較大等,顯示水芹菜以降低根長的方式讓 植株有機會在錨定降低的情形下,較容易離 開高流速的不良環境而尋得更適合繁殖的環 境。此一現象與 Puijalon 在 2005 年的研究相 符,Puijalon 發現有些水生植物在面臨較高流 速時會在型態上改變,例如降低錨定的強 度,以便增加其散佈的能力(Puijalon et al., 2005)。然而在水蓑衣的實驗中,其面對較高

流速的反應卻是加強根長的錨定作用,所以 實驗採收後量測到的根部乾鮮重均是實驗組 高於控制組。至於實驗後平均株高大於控制 組的原因,初步推斷由於水蓑衣莖部有木質 化現象,莖部必須長高變細才能水平化與呈 現流線型狀。相較於水芹菜而言,水蓑衣在 固土作用下似乎要較水芹菜表現更佳。

本研究因以模擬水道實驗,未來將以初步 結果應用於實際河川環境中,用以確定水生 植物的抗流模式,並推廣於生態工程的應用 中。

VI、誌謝

本研究獲得國科會專題研究補助,研究編 號 98-2410-H-216-019;99-2410-H-216-009,謹 此誌謝。

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ORIGINAL ARTICLE

Flow Resistance Adaptation of Aquatic Macrophytes Under Different Flow Velocities

Chen Shiang-Yuarn1,* and Jen-Yang Lin2

1Department of Landscape Architecture, Chung Hua University, Hsinchu, Taiwan.

2Department of Civil Engineering and Water Environment Research Center, National Taipei University of Technology, Taipei, Taiwan.

Received: September 7, 2010 Accepted in revised form: January 18, 2011

Abstract

Many studies have demonstrated that plants have the ability to affect flow velocity, and plant materials have been investigated for their potential to be used as a buffer zone to prevent riverbank erosion. However, relatively few studies have investigated the effects of plant characteristics on flow conditions. In this study, an artificial channel was constructed to (1) investigate the nature of the morphological changes undergone by aquatic Oenanthe javanica DC (water celery) macrophytes in response to different channel flow velocities and (2) identify the tolerance limit of aquatic macrophytes under different flow velocity conditions. Results show that the morphology of Oenanthe javanica DC exhibits the following variations under different flow velocities: as flow velocities increase, growth rate slows and plant shoots become shorter and softer, thereby increasing plant flexibility. These variations were accompanied by a decrease in root length and root anchorage capacity. In response to different flow velocities, a nonlinear relationship in growth rate between total new green leaves and yellow leaves was also observed. The number of vascular bundles in new shoots was found to decrease in a flowing water environment, compared with the number of vascular bundles in terrestrial environments. The average density of vascular bundles, however, was found to increase as flow velocity increased, most likely to provide a compensatory structural support mechanism. Results of this research identified a suitable range of flow velocity for water celery as 0.05–0.30 m s1, which is approximately equal to the average flow velocity of dredged rivers in Taiwan. Because of its abundant growth in Taiwan and its ability to adapt to the range of velocity conditions found in Taiwan’s dredged rivers, water celery was found to be an appropriate planting material for intertidal zones and reservoir bank protection.

Key words: flow resistance adaptation; flow velocity; aquatic macrophyte; artificial channel; ecological engineering

Introduction

R

ecent studies in Taiwanhave investigated the use of ecological engineering methods to mitigate the impact of disasters, such as landslides and floods (Kuo, 2006; Wu and Feng, 2006). These studies, which have demonstrated that plants can affect flow velocities and prevent riverbank erosion (Greenway, 1987; Simon and Collison, 2002; Simon et al., 2006;

Wynn and Mostaghimi, 2006), show that aquatic macro- phytes are frequently the dominant factor influencing flow conditions within the channels they occupy. Flow velocity has been demonstrated to affect plant stem/leaf scales and veg- etation density as well as plant length, stiffness, and diameter (Manz and Westhoff, 1988; Green, 2005). However, the effects

of vegetation on flow resistance are still not fully understood ( Ja¨rvela¨, 2004; Green, 2005). Most empirical studies of the effects of vegetation have been conducted in artificial chan- nels using plastic leaves or submerged vegetation, and they have focused on hydraulic effects, such as drag and vegeta- tion configurations (Sand-Jensen, 2003; Ja¨rvela¨, 2004). The remaining studies have focused on types of riverbank, mate- rials, and construction methods used for riverbank protection, slope stabilization, erosion control, or development of meth- ods to increase the survival rate of selected plant species (El- liott, 1998, 2004; Anderson et al., 2004).

Submerged plants have also been found to affect flow ve- locities. Plants can be utilized as buffers to reduce erosion, trap sediment, and remove contaminants by slowing runoff, increasing infiltration, and facilitating contaminant uptake and subsequent transformation (Dabney et al., 2006; Wynn and Mostaghimi, 2006). Riparian vegetation has both me- chanical and hydrological effects on streambank stability;

some of its effects improve bank stability and others reduce

*Corresponding author: Department of Landscape Architecture, Chung Hua University, No. 707, Sec. 2, WuFu Road, Hsinchu, Tai- wan 300, R.O.C. Phone: þ886-3-5186676; Fax: þ886-3-5186670; E-mail:

sharon@chu.edu.tw

ENVIRONMENTAL ENGINEERING SCIENCE Volume 28, Number 5, 2011

ª Mary Ann Liebert, Inc.

DOI: 10.1089/ees.2010.0291

1 EES-2010-0291-Shiang-Yuarn_2P.3d 03/07/11 6:13pm Page 1

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bank stability; the latter results in an increase in the flood stage (Simon and Collison, 2002; Rhee et al., 2008). In open- channel hydraulics, aquatic plants typically cause changes in flow resistance as well as changes in the retardance coefficient (Rhee et al., 2008; Chen et al., 2009) by anchoring their roots into channel soil to support the aboveground portion of a plant (Greenway, 1987). Variation in root type (root depth and density) can also affect channel erosion (Anderson et al., 2004).

Relatively few field-monitoring studies have verified the effects of vegetation on channel and flow velocities or inves- tigated the morphological adaptation of plants to flow con- ditions (Watson, 1987; Asaeda et al., 2005; Green, 2005).

Moreover, few studies have tested the tolerance limit of aquatic macrophytes under different flow velocities, exam- ined aquatic macrophytes’ modification of their physical characteristics to adapt to flow velocities, or investigated the effects of those changes on channel flows.

The present study utilizes an artificial channel to assess the ability of aquatic macrophytes to resist various flow velocities.

The primary aim of this study was to determine whether local aquatic macrophytes are suitable for riverbank protection projects by examining how they respond to different channel flow velocities, which is measured by observed changes in their form and structure. Additional goals were to determine the growth rate and biomass of the macrophytes, morpho- logical variations in their stems and roots, their tolerance limits, and their erosion resistance response under various flow velocities. The final purpose of this study was to deter- mine aquatic macrophytes’ range of application in ecological engineering and design.

Materials and Methods

Simulated plant environment selection

Natural riverbanks can be classified by environment, to- pography, and slope, all of which affect flow rate. Taiwan has three different flow environments for aquatic macrophytes:

rapid flow, moderate flow, and slow flow.

1. Rapid flow: Slope exceeds 4%; only a few emerging plants can survive in this flow, which is always found in upstream sections.

2. Moderate flow: Slope is 2%–4%; this environment, which is suitable for emerging plants, can be seen in all midstream sections and some upstream sections of Taiwan’s western rivers.

3. Slow flow: Slope is <2%; this environment, which is suitable for most aquatic macrophytes, can be found in all downstream sections of Taiwan’s western rivers (EPA, 1995; WRA, 2008).

Tatun Creek was chosen as the channel for this experi- ment, because it was dredged using ecological engineering methods and was well populated with plants at 2 years after its construction. The average slope of Tatun Creek is

<2%; thus, it represents a typical slow flow environment suitable for most aquatic macrophytes. Culture media used in this experiment were also extracted from Tatun Creek.

The site chosen for soil collection was covered with native plants such as Miscanthus floridulus (Labill.) Warb. ex Schum ( Japanese silvergrass) and invasive species such as Bidens pilosa. Dragonfly nymphs were also found under gravel in the river.

The results of grain size analysis indicate that 90% of par- ticles were >0.15 mm in diameter and their silt content was less than 15%. According to the soil texture classification method of the United States Department of Agriculture, the texture of culture media from Tatun Creek was of the ‘‘sandy type,’’ which has an exceptionally low water-holding capacity and also the same texture as the soil typically found in Tai- wan’s rivers.

Artificial channel construction

Factors affecting flow resistance include the size and structural characteristics of plants, their location in a channel, and local flow conditions (Green, 2005). Channel structure and hydrology also contribute to setting flow velocity. For the purpose of this study, an artificial channel was designed to incorporate all of the factors listed above. Artificial double channels (200 cm long, 30 cm wide, and 40 cm deep) were constructed of 1-cm-thick transparent acrylic panels. The channels’ other components consisted of two adjustable water pumps, four planters, and two 200-L water tanks. The plant- ers were made of 1-cm-thick wooden panels (90 cm long, 29 cm wide, and 5 cm deep). Lighting was supplied by four 40-watt plant lights, each 100 cm long, which were illumi- nated from 06:30 to 17:30, with an average luminance of 843 Lux. A control group and experimental group were subjected to the same environmental conditions with variation in flow rates (Fig. 1). All experiments were conducted at room temperature.

The main factors contributing to total channel resistance, according to the Cowan equation, are channel materials, surface irregularities, variations in the channel cross-section, obstructions, vegetation, and channel meandering (Green, 2005). The Manning’s roughness coefficient in this experiment is between 0.025 and 0.054.

Choice of plant species

Differences in plant structure, such as stem and leaf mor- phology, have been demonstrated to affect flow rate (Scul- thorpe, 1967; Sand-Jensen, 2003). Plant materials chosen for this study were required to meet the following criteria:

1. Plant species must be native aquatic macrophytes or domestic species that pose no threat to native species.

2. Plants must be shorter than 30 cm in height (the acrylic channel was 30 cm in height).

3. Plants must be easy to cultivate.

4. Plants must be perennial herbs with thread-like rootlets, whose growth rate is easy to compare and whose soil stability is easy to assess.

5. Plants must exhibit widespread growth in Taiwan.

6. Plants must have a short lifecycle.

Oenanthe javanica (Blume) DC (water celery) was selected because it grows in ditches, ponds, paddy fields, and other wet locations at low-to-medium altitudes all over Taiwan. It also fulfills all of the other conditions for species choice (Huang et al., 1998).

Experimental procedure

Three experimental trials were completed between No- vember 2006 and August 2008. A total of 48 water celery

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plants were planted 6 cm apart in each of the four planters. All flow rates were controlled via the rotational speed of adjust- able water pumps. At the beginning of the experimental trial period, the flow rates of both groups were nearly the same.

Subsequently, the rotational speed of the water pump in the control group was kept constant, but the rotational speed of the water pump in the experimental group was increased by increments of 0.2–2.0 Hz once every 4 weeks (Table 1). Var- iation of rotational speeds was introduced to investigate the flow resistance adaptation of water celery over a range of flow rates. All three experiments were conducted over an 18-week period and were terminated when the difference in survival rate between the two groups exceeded 10%.

An initial experiment was conducted to identify the flow velocity tolerance limit of water celery to assess its compati- bility with average flow velocities of dredged rivers in Tai- wan, which are 0.02–0.60 m s1 (Dago Stream, 0.05–0.13 ms1; Fungaue River, 0.02–0.52 m s1) (Lin, 2003; Lin et al., 2005),

Step I. For the control group and the experimental group, the following measurements were obtained: culture media properties, plant weights, root lengths, channel slope, water depths, flow velocities, water qualities, pH values, and lighting duration. Water celery plants took 3–4 weeks to

establish stability in the planters, so channel flow velocities were kept constant during the first 4 weeks for both the con- trol and experimental groups and increased thereafter for the experimental group once every 4 weeks.

Step II. The number of green leaves, yellow leaves, hori- zontal leaves, epicormic shoots, and stolon shoots was re- corded every week. Additionally, height, root length, diameter, fresh weight, and total dry weight measurements were recorded for each plant after harvesting.

Step III. Original stems and green leaves were harvested from the plants in Experiment III-1, and only new shoots were left uncut at week 13. The purpose of Step III was to observe the difference in morphology between new shoots and origi- nal terrestrial shoots.

Plant tissue sectioning

Paraffin method. After harvesting, the plant material was analyzed for vascular bundles using the paraffin method, a slicing technique for preserving fresh plant tissue. Plant sec- tions were obtained to compare physical anatomical changes in the structure of water celery plants under different flow velocity conditions. The paraffin method procedure is out- lined below:

FIG. 1. Layout of an artificial channel.

Table1. Different Experimental Conditions

Experiment I Experiment II Experiment III-1 Experiment III-2 Flow velocities in the control

group (m s1)

0.06–0.08 0.05–0.06 0.10–0.11 0.10–0.13

Flow velocities in the

experimental group (m s1)

0.05–0.35 0.08–0.10 0.10–0.17 0.17–0.24

Flow velocity adjusting range Rising the rotational speed by 1.0 Hz every 4 weeks (equivalent to a velocity increase of 0.04–0.05 m s1).

Rising the rotational speed by 0.20 Hz every 4 weeks (approximately equivalent to a velocity increase of 0.01 m s1).

Rising the rotational speed by 1.0 Hz every 4 weeks (equivalent to a velocity increase of 0.01–0.04 m s1).

Rising the rotational speed by 2.0 Hz every 4 weeks (equivalent to a velocity increase of 0.01–0.05 m s1).

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Fixation ? dehydration ? infiltration of paraffin ? em- bedding ? slicing with rotary microtome ? adhesion on slide ? drying out ? staining ? mounting with Entellen (Tsai, 2000).

Freehand sectioning

Freehand sectioning, the simplest slicing technique for observing fresh plant tissue, was employed as the second analytical method. The thickest stem was chosen for each water celery plant and then fresh stems were sectioned and temporarily fixed with 5 mL formalin, 5 mL glacial acetic acid, and 90 mL of 50%–70% alcohol (FAA) (Tsai, 2000). The re- sulting sections were analyzed, and the number of vascular bundles in each section was recorded.

Results and Discussion

Each experiment was conducted over a period of 18–40 weeks. In Experiment I, 20 weeks after planting, the survival rate of the experimental group declined to 75%, whereas the survival rate of the control group was 95%. Suitable flow velocities were estimated to be 0.05–0.30 m s1.

By the second week of Experiment II, >80% of shoots had been eaten by Spodoptera litura Fabicius. After application of a pesticide, new shoots sprouted during week 5.

Growth rate variation under different flow velocity conditions

At the start of Experiment I, the total number of green leaves in the control group (planters A and B) was only 2.8%

less than the number of green leaves in the experimental group (planters C and D). When flow velocity was increased to 0.30 m s1, the total number of leaves in the experimental group increased continuously, and the plant growth rate was higher than that of the control group. However, when flow velocity was increased to 0.35 m s1 in the experimental group, the total number of new green leaves that sprouted after planting began to decline, whereas the number of yellow leaves increased and remained higher than the number of yellow leaves in the control group. After harvesting, the total number of yellow leaves in the experimental group was 35.1%

higher than in the control group (Table 2). When flow velocity was fixed for the control group, the difference between the total number of green and yellow leaves in planters A and B continued to increase (Fig. 2). When flow velocity was in- creased once every 4 weeks in the experimental group, the

difference between the total number of green and yellow leaves in planters C and D began to decrease at velocities exceeding 0.30 m s1(Fig. 3). These results indicate that suit- able flow velocities for water celery are in the range of 0.05–

0.30 m s1.

In Experiments II and III, the difference between the total number of new green leaves and yellow leaves not only in planters C and D but also in A and B continued to increase, because flow velocities in the experimental groups were kept under the endurable limit. Growth rates in planters C and D remained lower than those in the control groups, and the total number of new green leaves in the control groups was higher than in the experimental groups. All three results indicate that flow velocities affect the growth rate of water celery.

Biomass variation under different flow velocity conditions

At the start of Experiment I, the total fresh weight of plants in the control and experimental groups was 120.26 and 131.36 g, respectively. At week 20, the total fresh weight of plants in the control group was 148.70 and 80.03 g in the ex- perimental group; thus, the total fresh weight of plants in the experimental group comprised only 53.8% of the control group weight. After harvesting and drying for 26 h at 1008C, the dry weight of plants in the control group was 13.42 and 5.82 g in the experimental group (Table 3).

In Experiments II and III, no distinct differences were found in biomass between the two groups at flow velocities of

<0.17 m s1. After harvesting, both the fresh and dry weights of plants in the experimental group were lower than those in the control group (Table 3). All three experiments demon- strate variation of biomass under different flow velocities.

Growth rate and biomass reduction results are consistent with the results obtained in previous studies of plants ex- posed to increasing flow or waves; the plants generally present growth modifications and morphological changes such as height and density reduction as well as a decrease in biomass production (Idestam-Almquist and Kautsky, 1995;

Coops and Van der Velde, 1996; Puijalon and Bornette, 2004;

Puijalon et al., 2005).

Morphological variation under different flow velocity conditions

Mechanical constraints limit plant survival and growth in environments with flowing water, because hydraulic force

Table2. Changes in Number of Green and Yellow Leaves

Control group Experimental group

Changes in green/yellow leaves

Total green leaves before

experiment

Total new green leaves after experiment

Total yellow leaves after experiment

Total green leaves before

experiment

Total new green leaves after experiment

Total yellow leaves after experiment

Experiment I 280 720 541 288 689 731

Experiment II 443 702 625 448 577 574

Experiment III-1 345 629 340 369 630 350

Experiment III-2 — 1,010 657 — 862 623

Experiment I: Flow velocities were 0.06–0.08 m s1in planter AB and 0.05–0.35 m s1in planter CD. Experiment II: Flow velocities were 0.05–0.06 m s1in planter AB and 0.08–0.10 m s1in planter CD. Experiment III-1: Flow velocities were 0.10–0.11 m s1in planter AB and 0.10–0.17 m s1in planter CD. Experiment III-2: Flow velocities were 0.10–0.13 m s1in planter AB and 0.17–0.24 m s1in planter CD.

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generally dislodges or breaks plants (Schutten and Davy, 2000). A flowing water environment can also trigger mor- phological adaptations; for example, some species, such as Eichhornia crassipes and Pistia stratiotes, develop a rosette of small, stiff leaves from a short stem, which can resist strong drag and accelerational forces on wave-exposed lakeshores.

Other species, such as Vallisneria natans and Sparganium, de- velop a streamlined morphology of long linear leaves or stems in response to higher flow velocities (Sculthorpe, 1967; Sand- Jensen, 2003). Most species develop very flexible shoots, which allows them to bend and twist in flowing water, and reduces the surface area directly exposed to current flow (Koehl, 1984; Sand-Jensen, 2003).

In Experiments I and II, average plant height after har- vesting in the experimental groups was less than in the control groups. For the fast flow velocity condition in Experiment I, average plant height in the experimental group was 54.3% of the control group plant height. The average diameter of stems in the experimental group was 84% of the control group di- ameter (Table 3).

Experiment II results show the plant height to be inversely related to flow velocity. The difference between the two groups was only 11% in Experiment II (not as large as in Experiment I), and the average length of roots in the experi- mental group was 73.8% of the average control group length (Table 3).

The results of Experiment III were similar to those of Ex- periment I: as flow velocities for the experimental group had been increased gradually to just under 0.24 m s1, a rate that does not exceed the endurable limit, no distinct differences were found between the two groups with respect to epicormic shoots or dwarfish shoots. However, the average plant height in the control group was 6.5% lower than in the experimental group, a finding that is not consistent with the results of Ex- periments I and II (Table 3). Comparison of the final average plant height with the average plant height at the start of the experiments shows that average heights were lower after the experiment for both groups. However, average root lengths in the experimental groups were shorter than root lengths in the control groups, which is consistent with the results of Ex- periment II (Table 3).

All three experiments show that water celery plants de- crease their height and root length under high flow velocity conditions. After harvesting, the ratio of plant height to root length in the experimental group was higher than in the control group (Table 4). Flow velocity hindered the overall growth rate of the water celery plants, which affected the plant root length considerably more than the plant height. It was also found that a decrease in the plant root length re- duces the use of root anchorage as a mechanism for creating a more favorable propagation environment. Similar results were obtained by Puijalon et al. (2005); aquatic plant species

FIG. 2. Difference between total new green leaves and yellow leaves in the control group of planters A and B for Experiment I: When flow velocity was controlled under 0.08 m s1, difference between total green/yellow leaves of planters A and B continued increasing. Total number of new green leaves was the total new leaves that sprouted after planting.

FIG. 3. Difference between total new green leaves and yellow leaves in experimental group of planters C and D for Experiment I: When flow velocity was adjusted higher every 4 weeks in experimental group, difference between the total green/

yellow leaves of planters C and D decreased when the velocity exceeded 0.30 m s1.

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Table3.MorphologicalVariationsBeforeandAfterExperiments BeforeexperimentAfterexperiment ControlgroupExperimentalgroupControlgroupExperimentalgroup PlanterAPlanterBPlanterCPlanterDPlanterAPlanterBPlanterCPlanterD ExperimentIPlantheight(mm)————195.558.9183.063.0111.164.494.561.7 Diameter(mm)————1.330.271.230.301.110.321.030.26 Averageleafnumber2.690.803.150.973.041.052.960.974.891.524.831.483.471.073.191.10 Totalfreshweight(g)59.2760.9960.9270.4479.5269.1841.4538.58 Averagefreshweight(g)1.140.121.170.281.170.201.350.251.730.071.500.070.940.061.070.05 Totaldrymass(g)————7.555.872.862.96 ExperimentIIPlantheight(mm)175.333.3169.332.8168.532.0173.730.280.435.770.032.667.625.567.335.1 Rootlength(mm)26.410.228.716.924.612.223.39.630.416.322.510.119.110.720.08.3 Diameter(mm)2.590.362.600.492.650.472.980.600.970.391.140.620.970.440.900.44 Averageleafnumber4.771.424.461.474.191.574.691.487.364.185.603.345.103.046.274.21 Totalfreshweight(g)76.4476.2172.4186.6125.6921.1214.1820.32 Averagefreshweight(g)1.590.201.590.101.510.421.800.190.570.120.530.210.350.070.480.15 Totaldrymass(g)————3.342.751.482.65 ExperimentIII-1Plantheight(mm)131.936.0123.331.6123.032.8127.539.4255.2116.0251.5135.4253.2135.0253.4135.5 Rootlength(mm)42.015.139.814.645.119.040.516.3———— Diameter(mm)2.100.272.090.322.080.402.220.421.800.361.800.451.790.331.860.45 Averageleafnumber3.651.063.540.943.750.933.940.986.753.286.462.486.792.346.732.44 Totalfreshweight(g)74.1878.3976.3479.3363.1366.1162.8665.41 Averagefreshweight(g)1.550.611.630.661.590.731.650.761.320.781.380.951.340.761.390.94 Totaldrymass(g)————5.565.034.765.25 ExperimentIII-2Plantheight(mm)————106.565.1107.649.6108.745.9120.352.3 Rootlength(mm)————45.429.549.025.641.730.444.625.2 Diameter(mm)————1.150.331.100.211.170.221.100.27 Averageleafnumber1.692.681.562.111.582.221.631.939.715.357.754.496.453.317.734.08 Totalfreshweight(g)————39.9131.6525.0131.20 Averagefreshweight(g)————1.141.020.990.730.860.631.040.72 Totaldrymass(g)————5.464.242.993.88 EES-2010-0291-Shiang-Yuarn_2P.3d 03/07/11 6:13pm Page 6

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were found to undergo morphological changes, which de- creased their anchorage strength, thereby increasing their spreading ability in high flow velocity conditions (Puijalon et al., 2005).

The results presented above demonstrate variation in the morphological characteristics of aquatic macrophytes, such as plant height and root length, under different flow velocity conditions. Water celery avoids mechanical stresses under drag forces at high flow velocities by reducing height and diameter, thereby forming relatively softer and more flexible shoots. This is consistent with the results of previous studies of macrophytic freshwater plants, in which they were found to morphologically adapt to prevent mechanical damage and uprooting when exposed to substantial drag forces in flowing water (Coops and Van der Velde, 1996; Sand-Jensen, 2003). In contrast, differences in the number of epicormic and dwarfish shoots between the control and experimental groups were not as obvious, because flow velocities had been increased incrementally.

Change in plant tissue sections under different flow velocity conditions

Section analyses show that the average height of new shoots in experimental specimens was lower than that of terrestrial plants and that experimental specimens contained fewer vascular bundles than terrestrial water celery. Terres- trial water celery also presented larger stem diameters and more vascular bundles than water celery that had been

planted in water (terrestrial specimens, >5 vascular bundles;

water specimens, <5 vascular bundles) (Fig. 4).

Greater stem thickness in the control groups than in the experimental groups was also observed (Fig. 5). However, data analysis shows that, although average diameter and stem thickness decreased as flow velocity increased, the av- erage ratio of vascular bundles to square millimeter of stem sectional area increased as flow velocity increased (Fig. 6), most likely as a compensatory structural support mechanism.

The water celery plants adapted to changing conditions in the flowing water environments via alterations in stem thickness and vascular bundle density.

Formation of algal mats to protect topsoil

During the three experiments, algal mats, including Ana- baena azollae, Oscillatoria Formosa Bory, and Navicula sp., began to form at week 6 after planting. In Experiment III, Chroococcus sp. grew in the control group planters A and B at week 24 after planting, in which the flow rate had been controlled at 0.10–

0.13 m s1. This kind of algal mat forms in high-temperature environments with low oxygen levels. All of the algal mats mentioned protect topsoil, especially from silt erosion. The water celery in planters provided algal mats with an oppor- tunity to attach without being flushed away by flowing water.

In this study, algal mats formed more slowly in the experi- mental group than in the control group. When plants died or were exposed to high flow velocities, the attached algal mats broke away, reducing topsoil protection.

Table4. Ratio of Average Plant Height to Root Length Height/root length

before experiment II

Height/root length after experiment II

Height/root length before experiment III

Height/root length after experiment III

Control group 6.3 2.9 3.1 2.3

Experimental group 7.2 3.5 2.9 2.7

FIG. 4. Comparison of plant tissue sections from terrestrial and water- planted water celery: Terrestrial water celery had larger diameter and more vascular bundles than that of water-planted water celery.

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Tolerance limit of water celery to flow velocities

The three experimental trials also served to determine the flow velocity tolerance limit of water celery. In Experiment I, the number of green leaves in the experimental group in- creased at flow velocities of <0.30 m s1, and growth rate declined after flow rate had been increased to >0.30 m s1. Survival rate decreased to <75% at a flow velocity of 0.35 m s1. Calculation of the total number of green and yellow leaves in planters C and D at various flow velocities showed that the difference between the number of green and yellow leaves decreased when velocity exceeded 0.30 m s1. The total number of new green leaves and yellow leaves in the exper- imental group also displayed a nonlinear relationship over a range of flow velocities (Fig. 7).

In Experiment II, <80% of green shoots had been eaten by Spodoptera litura Fabicius, so the total number of yellow leaves initially exceeded the number of new green leaves (Fig. 8).

However, the number of green leaves in the experimental group continued to increase until harvesting at week 18 after planting, because flow velocities had been consistently maintained at a level below the tolerance limit of water celery.

In Experiment III, the number of green leaves in the ex- perimental group continuously increased during the two stages. The difference between the total number of new green leaves and yellow leaves in planters C and D increased at velocities of <0.30 m s1. Nonlinear regression relationships were found between flow velocities and the number of new green leaves and yellow leaves, which is consistent with the results of Experiments I and II (Figs. 9 and 10).

Conclusion

This research demonstrates that the way in which water celery avoids mechanical stresses when encountering drag forces at higher flow velocities is to reduce both plant height and stem diameter, thereby forming softer and more flexible shoots. Such morphological adaptations to reduce root length also serve to reduce root anchorage strength, which increases plants’ spreading ability at high flow velocities.

The plant section data from Experiments II and III show that the number of vascular bundles in new shoots was lower in flowing water environments than in terrestrial environ- ments. However, subsequent data analysis of Experiments I–

III revealed that although the total number of vascular bun- dles per stem decreased under high flow velocity conditions, the density of vessels per unit area increased. Plants undergo this kind of adaptation to avoid breakage or mechanical in- jury. At high flow velocities, aquatic macrophytes, such as Oenanthe javanica (Blume) DC, adapt to produce fewer vas- cular bundles per stem and a higher vascular bundle density per unit area to compensate for the structure lost in reduction of stem diameter, thereby reducing the likelihood of damage due to breakage. It must be noted here that morphological adaptation triggers may include factors other than flow ve- locity. To investigate this possibility, future experiments will vary other environmental conditions or planting methods, such as hydrology and cluster patterns.

The suitable flow rate range for water celery was deter- mined to be 0.05–0.30 m s1, which is approximately equal to most average flow velocities of dredged rivers in Taiwan.

FIG. 5. Plant section variations in the three experiments: Stem thickness was thicker for the control groups than for the experimental groups.

FIG. 6. Ratio of vascular bundles to stem section area: Average ratio of vascular bundles to square millimeter of stem section area increased as flow velocity increased.

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Water celery, a wet land plant with the advantages of easy cultivation and the ability to protect topsoil, has been found to be an appropriate planting material for intertidal zones and reservoir bank protection. As suitable streambank vegetation may include a variety of plants, future studies can examine the flow resistance mechanisms of clustered water celery and other plants, to further investigate the

feasibility of applying aquatic macrophytes to ecological engineering.

Acknowledgments

This work was funded in part by the Taiwan National Science Council (Research Grant No. 96-2221-E-216-022;

FIG. 7. Relationship between total new green leaves and yellow leaves and flow velocities in experimental groups of Experiment I: Difference between total new green leaves and yellow leaves in planters C and D decreased as velocity exceeded 0.30 m s1.

FIG. 8. Relationship between total new green leaves/yellow leaves and flow velocities in experimental groups of Experiment II (first 2 weeks were not included):

Flow velocities were under tolerance limit of water celery in this trial. Green leaves in the experimental group kept increasing until harvested at 18 weeks after planting.

FIG. 9. Relationship between total new green leaves/yellow leaves and flow velocities in experimental groups of Experiment III-1: In Experiment III-1, the green leaves in experimental group increased during the first stage with flow velocities below than 0.20 m s1.

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