著：（德）西格麗德·赫爾-蘭格 （德）?？ㄌ亍ぬm格 譯：陸熹

1 背景

本文旨在概述影響珠江三角洲（簡(jiǎn)稱珠三角）變化的相關(guān)因素和驅(qū)動(dòng)因素，試圖為解決珠江三角洲地區(qū)所面臨的復(fù)雜問(wèn)題，并為其實(shí)現(xiàn)彈性與可持續(xù)的轉(zhuǎn)型及演進(jìn)提供多尺度、跨學(xué)科協(xié)作的框架支撐研究。

對(duì)于今天走訪珠三角地區(qū)的人來(lái)說(shuō)，或許很難想象，在1700年左右，廣州地區(qū)仍然存在華南虎的蹤跡[1]。然而在近幾十年的短暫時(shí)期里，華南虎在粵北地區(qū)近乎絕跡。人類的狩獵行動(dòng)以及對(duì)虎類棲息地的破壞，不僅導(dǎo)致華南虎瀕臨滅絕，也更使得這里的景觀格局發(fā)生了質(zhì)的改變。過(guò)度伐木致使本地區(qū)原有大量森林覆蓋的格局在20世紀(jì)初消失殆盡。而伴隨著珠三角經(jīng)濟(jì)的迅捷發(fā)展和人口膨脹，該地區(qū)已經(jīng)發(fā)展成容納約1.2億居民的現(xiàn)代都市群[2]（圖1），這個(gè)數(shù)值在未來(lái)將依舊呈現(xiàn)上漲趨勢(shì)。與此同時(shí)，其自然資源也面臨著日益嚴(yán)峻的壓力與挑戰(zhàn)。

城市化進(jìn)程加快的同時(shí)也伴隨著城市基礎(chǔ)設(shè)施領(lǐng)域的建設(shè)需求，新機(jī)場(chǎng)、海港、橋梁、道路、新住宅甚至新城的建設(shè)都需要大量的砂石作為主要建材，而建材市場(chǎng)對(duì)這種砂石剛性需求的解決辦法通常是通過(guò)對(duì)河道進(jìn)行疏浚采砂。此外，珠江中水壩和水庫(kù)的建設(shè)也減少了江水中的泥沙沉積，因此珠三角的沉積物總量整體上已處于負(fù)平衡狀態(tài)。在珠江的部分河段，無(wú)序的采砂行為使得采砂量已是天然河床的可承受負(fù)荷的10倍[3]，這嚴(yán)重破壞了河道的動(dòng)態(tài)平衡。圖2展示了在3個(gè)不同的歷史時(shí)期中，與珠江泥沙沉積有關(guān)的重要因素及其相互作用。圖片中的“+”或“+ +”符號(hào)用來(lái)表示該因子有利于沉淀或是沉積物給該因子帶來(lái)了正面影響；反之，用“－”或“－－”來(lái)表示這些因素間的負(fù)面關(guān)聯(lián)。

2 森林砍伐和植樹(shù)造林

珠三角的植被類型以亞熱帶典型的原生常綠闊葉林為主，然而近幾個(gè)世紀(jì)以來(lái)，這些原生林地遭遇了大量的砍伐。伴隨人口的膨脹，人們對(duì)木柴、木炭、建筑材料以及農(nóng)業(yè)用地的需求日益增加[1]。“刀耕火種”的傳統(tǒng)耕作方法已實(shí)踐百年有余[4]，這使得大量的林地被轉(zhuǎn)換為農(nóng)業(yè)用途，進(jìn)一步導(dǎo)致林地格局的碎片化以及生物多樣性的喪失?，F(xiàn)今，珠三角地區(qū)只剩下不到1.5%的原生常綠闊葉林[5]。這幾個(gè)世紀(jì)以來(lái)發(fā)生的森林砍伐已經(jīng)嚴(yán)重?cái)_亂珠三角的自然沉積體系（圖2）。

在珠三角開(kāi)展貿(mào)易往來(lái)的商人曾繪制過(guò)大量描繪這里的繪畫，畫作表明珠江沿岸的景觀格局在17世紀(jì)前后便已經(jīng)開(kāi)始退化[6-7]。He等[8]指出，近百年來(lái)廣東省的森林覆蓋率呈現(xiàn)穩(wěn)步下降趨勢(shì)，由1700年的44%下降到1949年的22%。在“二戰(zhàn)”結(jié)束后到新中國(guó)成立伊始，為了改善當(dāng)時(shí)荒蕪貧瘠的土地面貌，政府采取了一些短期的植樹(shù)造林措施。如桉樹(shù)、金合歡樹(shù)以及馬尾松等生長(zhǎng)速度快且輪伐期短的樹(shù)種被廣泛種植。然而，由于20世紀(jì)50—60年代大量建設(shè)項(xiàng)目的需要，森林砍伐依然在繼續(xù)，繼而導(dǎo)致大量土壤侵蝕與沉積。因此與30年前相比，1980年時(shí)的珠江通航路線已減少1/2[9]。在改革開(kāi)放以后，為防止局面的惡化，中央政府在1990年實(shí)施了一系列“植樹(shù)造林”和“退耕還林”的項(xiàng)目。這些舉措已初具成效，截至2004年，珠三角森林總面積中約有19%是次生亞熱帶常綠闊葉林（圖3），其中大部分是由幼齡和中齡樹(shù)木組成的森林[10]。

3 農(nóng)業(yè)和水產(chǎn)養(yǎng)殖

水流中泥沙沉積物的沉積，尤其是淤泥和黏土等細(xì)粒沉積物，對(duì)三角洲的農(nóng)業(yè)發(fā)展起著至關(guān)重要的作用。珠江頻繁的洪澇災(zāi)害裹挾了大量的沉積物。因禍得福，珠三角洪泛區(qū)的土質(zhì)格外肥沃，加上溫暖濕潤(rùn)的亞熱帶氣候眷顧，珠三角成為中國(guó)全年農(nóng)產(chǎn)品生產(chǎn)最豐富的地區(qū)之一[11]（圖4），在珠三角的丘陵地區(qū)各種經(jīng)濟(jì)作物和果樹(shù)隨處可見(jiàn)。

在珠三角的許多地區(qū)，傳統(tǒng)農(nóng)業(yè)實(shí)踐只能與水交織而生。例如當(dāng)?shù)剞r(nóng)民開(kāi)發(fā)了一種“高壟低畦”的栽培模式，在田埂上種植蔬菜和水果，在溝渠中則種植蓮花或水稻等親水植物。

1 近年來(lái)珠江沿岸驟增的城市天際線The towering skyline along the Pearl River only exists since a few years

特別是在佛山南部、順德和南海等洼地，這些地區(qū)的高程在海平面以下，很容易受到洪水浸澇的危險(xiǎn)，所以農(nóng)業(yè)生產(chǎn)受到嚴(yán)重局限。因此先人將農(nóng)業(yè)發(fā)展與堤防建筑技術(shù)緊密結(jié)合[12]以避免收益受損。在清代這里便孕育出了中國(guó)特有的?；~(yú)塘系統(tǒng)，依托于物質(zhì)循環(huán)與能量的流動(dòng)，這一系統(tǒng)實(shí)現(xiàn)了系統(tǒng)內(nèi)各生態(tài)因子的良性互動(dòng)[13-14]。在?；~(yú)塘系統(tǒng)中，農(nóng)戶在池塘周圍的堤壩上種植桑樹(shù)、甘蔗、蔬菜和水果，桑葉可作為蠶的養(yǎng)料供給。有時(shí)農(nóng)戶也會(huì)在堤上飼養(yǎng)牲畜（例如雞、豬或鵝），這樣家禽的排泄物和蠶沙又可作為池塘中魚(yú)類的食物來(lái)源，如此循環(huán)，生生不息。得因于桑基魚(yú)塘的系統(tǒng)性運(yùn)用，順德地區(qū)成為中國(guó)三大絲綢產(chǎn)區(qū)之一。盡管自1926年后，國(guó)際絲綢貿(mào)易總量嚴(yán)重下滑，但這樣的堤壩系統(tǒng)并未就此瓦解，只是由甘蔗代替桑樹(shù)占據(jù)了主導(dǎo)格局[15]。20世紀(jì)90年代起，基于技術(shù)的發(fā)展和對(duì)高效率養(yǎng)殖的需求，現(xiàn)代化水產(chǎn)養(yǎng)殖系統(tǒng)受到了當(dāng)?shù)剞r(nóng)戶的推崇（圖5），傳統(tǒng)桑基魚(yú)塘式的生產(chǎn)格局幾近消殆[14]。

如今，人工養(yǎng)殖已主導(dǎo)了水產(chǎn)養(yǎng)殖行業(yè)，并以高度工業(yè)密集化、單一化為特征[16-17]，且依賴于化學(xué)肥料、殺蟲(chóng)劑、抗生素、曝氣器等設(shè)施來(lái)加速魚(yú)類的繁殖[18]。中國(guó)是全球水產(chǎn)品的主要出口國(guó)，其中珠三角便擁有超過(guò)10萬(wàn)hm2的水產(chǎn)養(yǎng)殖面積，占據(jù)全國(guó)出口量的主要份額。當(dāng)?shù)仞B(yǎng)殖的種類主要包括一系列魚(yú)類，例如羅非魚(yú)、鯉魚(yú)、鱖魚(yú)、鰻魚(yú)；甲殼類動(dòng)物，如巨型淡水蝦、小蝦、淡水龜和甲魚(yú)等?，F(xiàn)代的水產(chǎn)養(yǎng)殖方式完全改變了傳統(tǒng)?；到y(tǒng)中土地與水的混合模式，兩者的比例從清末的3∶2演變?yōu)楝F(xiàn)代的1∶4。此外，現(xiàn)在的堤圍上也鮮少有種植果樹(shù)和蔬菜，這樣的現(xiàn)狀已無(wú)法實(shí)現(xiàn)傳統(tǒng)桑基魚(yú)塘模式的可持續(xù)循環(huán)（圖5）[19]。

2 不同歷史時(shí)期內(nèi)影響珠江泥沙沉積的關(guān)鍵因素及其相互作用Main diagrammatic relationships of key factors and their effects over time

4 土壤沉積和采砂工程

珠江是中國(guó)第二大河流，排水量達(dá)260 km3/年。珠江由3條支流匯合而成，分別是西江（全長(zhǎng)2 214 km，集水區(qū)35萬(wàn) km2），北江（全長(zhǎng)468 km，集水區(qū)3.8萬(wàn)km2）和東江（全長(zhǎng)562 km，集水區(qū)2.5萬(wàn)km2）。截至20世紀(jì)90年代，西江承載的沉積物負(fù)荷最高（6 800萬(wàn) t/年），其次是北江（550萬(wàn) t/年）和東江（250萬(wàn) t/年）[20]。自20世紀(jì)50年代起，出于防洪、供水、發(fā)電和農(nóng)田灌溉等不同的用途，人類在珠江流域興建了大量的水庫(kù)和水壩。最初階段由于這些集水區(qū)的森林砍伐也在同時(shí)進(jìn)行，水庫(kù)水壩的興建并沒(méi)有有效緩解珠江流域泥沙淤積的問(wèn)題，嚴(yán)重的土壤侵蝕情況甚至一直持續(xù)到20世紀(jì)90年代[21]。河流沉積物在高負(fù)荷情況下的持續(xù)堆積致使珠三角在這30年間不斷向海洋延伸[22]，形成了新的灘涂濕地。

20世紀(jì)90年代以來(lái)的植樹(shù)造林計(jì)劃以及新建的總庫(kù)容高達(dá)467億 m3的387座大型水庫(kù)[23]導(dǎo)致珠江的沉積物大幅減少。截至2011年，短時(shí)間內(nèi)沉沙量降幅高達(dá)80%，達(dá)到1 270萬(wàn) t/年[24]。

與此同時(shí)，由于改革開(kāi)放以來(lái)的快速工業(yè)化和城市化，建筑和大型填海工程（包括新機(jī)場(chǎng)、海港、道路和橋梁）用砂的需求量巨大（圖6）。20世紀(jì)90年代初，僅在香港境內(nèi)開(kāi)展的采砂工作便動(dòng)員了世界上3/4的疏浚船隊(duì)[25]。據(jù)估計(jì)，1980—2002年間，僅珠江東江干線的采砂總量約為3.32億 m3。這個(gè)數(shù)量與東江末端的博羅站近166年來(lái)的總輸沙量值持平[26]。

近年來(lái)，伴隨著政府對(duì)河道采砂的管控力度不斷提升，目前砂石來(lái)源中約60%為海砂[27]。長(zhǎng)期持續(xù)的采砂工程削弱了三角洲的泥沙沉積，并進(jìn)一步減少了濕地中沉積物的來(lái)源[3]，泥灘濕地范圍也因此日益萎縮。

5 濕地、紅樹(shù)林和潮間帶泥灘

根據(jù)《拉姆薩爾濕地公約》的定義，濕地是“天然或人工、長(zhǎng)久或暫時(shí)的沼澤、濕原、泥炭或水域地帶（淺水湖泊、河流、泛洪區(qū)等），包括靜止或流動(dòng)的淡水、半咸水或咸水水體，以及海洋和低潮時(shí)水深不超過(guò)6 m的淺海水水域”。

珠三角最顯著的濕地資源是潮間帶泥灘和紅樹(shù)林。紅樹(shù)林具有重要的生態(tài)價(jià)值和功能。直到20世紀(jì)50年代，珠江河口的岸線上依舊分布了大量的紅樹(shù)林。當(dāng)時(shí)廣東省沿海地區(qū)約212 km2的紅樹(shù)林資源約占據(jù)全中國(guó)同類資源的一半面積，至1990年，這個(gè)面積驟減到38 km2[28]。

由于城市化進(jìn)程，牡蠣類的養(yǎng)殖和其他活動(dòng)對(duì)濱水資源的侵占，珠江河口的東岸線上僅存有3 km2的紅樹(shù)林[29]，沙井和寶安地段的紅樹(shù)林幾乎全部消失[30-31]。在西岸線，有關(guān)問(wèn)題同樣未能幸免，大量的開(kāi)墾工程和海蝦養(yǎng)殖侵占了這里絕大多數(shù)的紅樹(shù)林資源[29]。

自20世紀(jì)80年代以來(lái)，華南地區(qū)的政府已經(jīng)逐漸意識(shí)到了紅樹(shù)林對(duì)生態(tài)涵養(yǎng)的重要性，為了保護(hù)和恢復(fù)紅樹(shù)林，政府牽頭建立了數(shù)個(gè)自然保護(hù)區(qū)。其中較為出名的保護(hù)區(qū)包括香港米埔自然保護(hù)區(qū)（3.8 km2），深圳福田國(guó)家級(jí)自然保護(hù)區(qū)（3.8 km2）和內(nèi)伶仃島自然保護(hù)區(qū)（3.3 km2）。它們中有些也是世界級(jí)的生態(tài)保護(hù)區(qū)，并且是來(lái)自大洋洲的候鳥(niǎo)在遷徙回歸北亞地區(qū)途中重要的棲息地[3]。盡管相關(guān)部門在珠江河口地段進(jìn)行了大量的海水凈化工程，但并未起到顯著的效果，海水質(zhì)量狀況評(píng)級(jí)較低，沉積物中也檢測(cè)出超標(biāo)超量的鎘、鋅、鎳等重金屬元素[32]。

隨著珠江三角洲經(jīng)濟(jì)的快速增長(zhǎng)和水庫(kù)的建設(shè)，河流中的天然輸沙量減少。同時(shí)，堤防、碼頭和港口的數(shù)量急劇增加，加上建筑行業(yè)對(duì)沙石的大量需求，種種因素導(dǎo)致灘涂濕地的壓力劇增。建設(shè)活動(dòng)侵占了600 km2的天然灘涂濕地[3]，自2005年起，珠江口東西海岸線的潮間帶泥質(zhì)灘涂已然被蠶食殆盡[33]。而在珠江口的西北部，因龍穴島和南沙港的建設(shè)而進(jìn)行的土地復(fù)墾，使得潮間帶泥灘面積從1988年的171 km2下降到2008年的68 km2[34]。

3 廣州白云山坡地上生長(zhǎng)的次生亞熱帶常綠闊葉林Secondary subtropical evergreen broadleaved forests on the slope of Baiyun Mountain, Guangzhou

4 城市中殘留的小片農(nóng)田景觀A remnant of small-scale agriculture in the urban landscape

6 結(jié)論與展望

人為因素對(duì)珠三角的演變起著不容忽視的作用。珠三角區(qū)域見(jiàn)證著人們對(duì)自然資源大量剝奪與使用的現(xiàn)象。雖然森林砍伐的負(fù)面影響已通過(guò)“植樹(shù)造林”工程得到緩和，然而珠三角的生態(tài)系統(tǒng)依舊面臨著其他的威脅，其中既包括高度密集型的農(nóng)業(yè)以及水產(chǎn)養(yǎng)殖業(yè)，也包括對(duì)河岸緩沖區(qū)和濕地的破壞，對(duì)紅樹(shù)林地的蠶食，以及高強(qiáng)度的采砂工程。

展望未來(lái)，很多復(fù)雜問(wèn)題亟待解決。如果珠三角的城市化進(jìn)程繼續(xù)以這種快節(jié)奏發(fā)展下去，會(huì)造成什么影響？是否我們真的需要在珠江上游建造更多、更大規(guī)模的水壩和水庫(kù)？是否需要在珠江下游開(kāi)展更多的采砂和疏浚工程來(lái)滿足日益增長(zhǎng)的建筑材料供應(yīng)需求？面對(duì)著海平面的日益上升，我們?cè)撊绾谓鉀Q嚴(yán)峻的洪水風(fēng)險(xiǎn)以及土壤鹽堿化的問(wèn)題[35]？

解決這些問(wèn)題的一個(gè)關(guān)鍵因素是解決土壤沉積物的供應(yīng)問(wèn)題。珠江上游的沉積物采集嚴(yán)重超量，在下游河段的采砂工程以及海平面的日益上升共同作用下，珠三角面臨著嚴(yán)峻的洪水風(fēng)險(xiǎn)[36]。

解決這些風(fēng)險(xiǎn)的關(guān)鍵核心，是平衡沉積物的供應(yīng)和提取。首先，水庫(kù)清淤是緩解泥沙堆積的有效方法[37-38]。它不僅有助于提高水庫(kù)的運(yùn)行效率，而且能增加下游河段的沉積物總量。盡管這一措施或多或少也會(huì)對(duì)下游生態(tài)系統(tǒng)產(chǎn)生如促使魚(yú)類死亡的負(fù)面影響[39]，但是總體來(lái)說(shuō)因?yàn)檫@些沉積物會(huì)為三角洲和河口的濕地提供養(yǎng)分，以維持其生物多樣性[40]，因此依舊利大于弊。修復(fù)珠三角生態(tài)系統(tǒng)是一項(xiàng)宏大而復(fù)雜的工程，盡管在最初會(huì)需要較大程度的人工干預(yù)，但是三角洲自然沉積和塑造的過(guò)程同樣也起著重要的作用[41]。

世界上的許多三角洲都在遭受著沉降威脅，與之不同，珠三角的地面沉降只發(fā)生在局部地區(qū)[42]。然而由于其整體加積率低、萎縮率加劇以及海平面上升的風(fēng)險(xiǎn)等多種因素，珠三角已成為世界范圍內(nèi)環(huán)境最脆弱的三角洲之一[36]。

在城市化的進(jìn)程中，我們需要保留和擴(kuò)展足夠的生態(tài)空間來(lái)支撐大自然的動(dòng)態(tài)演變。例如，在珠海淇澳島，當(dāng)?shù)氐募t樹(shù)林區(qū)域面積從1957年的200 hm2，銳減為1999年的32 hm2。而得益于2004年建立的省級(jí)自然保護(hù)區(qū)，當(dāng)?shù)氐募t樹(shù)林面積從2004年時(shí)的533 hm2增長(zhǎng)到2019年的700 hm2[43]。

自然濕地除了具有自然生態(tài)價(jià)值，也在預(yù)防洪澇災(zāi)害和減少經(jīng)濟(jì)損失方面扮演著重要的角色。政府不僅將許多現(xiàn)存的濕地資源設(shè)立為自然保護(hù)區(qū)，同時(shí)也在珠江河口（圖7）和城區(qū)環(huán)境中（圖8）新建了許多濕地公園。

毋庸置疑，興建濕地公園這一倡議和舉措值得稱贊。從休閑娛樂(lè)的角度來(lái)看，濕地公園成為居民戶外休閑的絕佳去處。與此同時(shí)，我們也需要對(duì)人造濕地是否能夠提供高質(zhì)量的生態(tài)功能與生態(tài)系統(tǒng)服務(wù)，進(jìn)行建成前的策劃以及建成后的評(píng)估[44]。興建濕地公園前，相關(guān)的區(qū)域曾以濕地、湖泊、魚(yú)塘、農(nóng)田或棕地等多種形態(tài)出現(xiàn)，如今當(dāng)它們被改造為濕地公園后，生態(tài)系統(tǒng)的服務(wù)效益與之前相比又如何呢？

傳統(tǒng)可持續(xù)的?；~(yú)塘數(shù)量已經(jīng)銳減，鑒于珠三角目前普遍存在的高強(qiáng)度人工水產(chǎn)養(yǎng)殖現(xiàn)狀，相關(guān)人士還應(yīng)當(dāng)設(shè)計(jì)和改造一些現(xiàn)代型魚(yú)塘并使其成為人工濕地，以兼具生態(tài)、防洪和娛樂(lè)功能（圖9）。

要使珠三角的發(fā)展更具靈活性、適應(yīng)性和可持續(xù)性并不是一件容易的事情，為了應(yīng)對(duì)諸多問(wèn)題與挑戰(zhàn)，當(dāng)?shù)卣呀?jīng)實(shí)施了一系列環(huán)境整治的項(xiàng)目以及如“海綿城市”這樣雄心勃勃的計(jì)劃。然而，從長(zhǎng)期可持續(xù)發(fā)展的角度以及從景觀設(shè)計(jì)師自身的職業(yè)角度來(lái)看，這些努力和嘗試還遠(yuǎn)遠(yuǎn)不夠。

誠(chéng)如斯坦尼茨[45]所說(shuō)：“景觀設(shè)計(jì)師在定義它們工作尺度的同時(shí)，也定義了他們的未來(lái)?！睆倪@個(gè)意義上說(shuō)，采用局部和小規(guī)模的干預(yù)方法并不能根治珠江三角洲所面臨的問(wèn)題。介于珠三角問(wèn)題的復(fù)雜性和其所具有的巨大維度，我們唯有開(kāi)展多尺度跨學(xué)科的協(xié)作，才能實(shí)現(xiàn)更加可持續(xù)的環(huán)境管理愿景，以求收獲良好的成效。

致謝：

本研究由牛頓基金（Newton Fund）、英國(guó)工程與自然科學(xué)研究理事會(huì)（EPSRC）支持，是可持續(xù)三角洲獎(jiǎng)（Sustainable Deltas award）的一部分（編號(hào) P/R024979/1）。本研究為我方與華南理工大學(xué)、代爾夫特理工大學(xué)（TU Delft）的共同合作項(xiàng)目。

圖片來(lái)源：

圖1～8由作者拍攝或繪制；圖9來(lái)自謝菲爾德大學(xué)景觀學(xué)院John Ho Shun Yuen于2018年的畢設(shè)作品。

1 Introduction

This paper aims to provide an overview of interconnected factors and drivers influencing change in the Pearl River Delta and tries to make a case for multiscale, cross-disciplinary and collaborative approaches that are needed to address the complex issues the Pearl River Delta is facing in order to support a more adaptive and sustainable transformation.

For someone visiting the Pearl River Delta region today, it is hard to imagine that still at around AD 1700 the South China tiger was present near Guangzhou[1]. Up until a few decades ago very few remaining individuals of the South China tiger have been sighted in the Northern Guangdong province.Because of hunting and loss of habitat since this time not only the tiger has disappeared but much more obvious the landscape has changed radically. The once forested landscape was largely deforested in the early 20th century. With the rapid economic growth of the Pearl River Delta in recent decades the pressure on the natural resources overall increased dramatically. Along with a massive population growth the Pearl River Delta region has been turned into a megacity (Fig. 1) of an estimated 120 million inhabitants[2]and is still projected to grow in the future.

This corresponds directly to a rocketing demand in construction material for infrastructure projects such as new airports, seaports, bridges, roads as well as construction of new homes and even whole new cities.This demand is largely satisfied through sand mining and dredging in the delta region. In addition, the construction of dams and reservoirs reduces sediment supply resulting in an overall dramatic negative balance in sedimentation in the delta region. Sand mining exceeded in some parts more than 10 times the annual sediment delivery of the natural bed load[3]. Fig. 2 illustrates the main relationships of the key factors in three different time periods. A positive effect on or of sedimentation is symbolized through “+” or “++”. A negative effect on or of sedimentation is symbolized through “－ ” or“－－”.

2 Deforestation and Afforestation of Woodlands

Like all subtropical areas of South East Asia,originally, most of the Pearl River Delta region, was covered by primary evergreen broad-leaved forests.Over the centuries these native woodlands have been harvested. As population grew, the demand for firewood, charcoal and construction material increased[1]. At the same time the demand for agricultural land increased. Slash-and-burn practice —a shifting cultivation method — has been practiced for hundreds of years[4]transforming the woodlands into agricultural land. As a result, fragmentation of woodlands and deforestation together with a loss of biodiversity can be observed. Nowadays less than 1.5 % of the native evergreen broad-leaved forest is left[5]. The deforestation that took place over several centuries heavily disturbed the natural sediment regime in the Pearl River Delta (Fig. 2).

Paintings and drawings by traders and merchants travelling on the Pearl River to Canton give evidence that the landscape along the Pearl River was already deforested in the 17th century[6-7]. He et al.[8]estimated that the forest coverage in Guangdong Province was on a steady decline from 44 % in 1700 to 22 % in 1949. After WW II, in the People’s Republic of China, initiatives were started to afforest the then often barren landscape with short rotation timber plantations mainly using exotic fast-growing Eucalyptus and Acacia species and native Masson pine(Pinus massoniana). However, to meet the increasing demand for timber, overlogging continued in the 1950s and 60s, still causing enormous soil erosion.By the 1980s the sedimentation in rivers reduced the navigable routes by half compared to the 1950s[9].After the Opening Reforms in China from approx.1990 several rehabilitation and afforestation programs started to stop the erosion and to revegetate the degraded forest land. By 2004 already approximately 19% of the PRD’s total forest area was secondary subtropical evergreen broadleaved forests (Fig. 3) of which most of them are young or middle-aged[10].

3 Agriculture and Aquaculture

The deposition of sediments, especially the finegrained sediments silt and clay, plays an important role for agriculture in the delta. In the past, regular flooding events provided the delta with sediments, helping to develop nutrient-rich soils for agricultural production.Due to its subtropical climate and the fertile soils, the Pearl River Delta is one of the most productive areas in China for agricultural products which grow here throughout the whole year (Fig. 4)[11]. On the hilly areas of the delta dry crops and fruit trees are planted.

In many parts of the Pearl River Delta traditional agriculture practice was only possible in interaction with water. This includes sophisticated paddy field systems temporary submerged for rice production. In the high bed — low ditch system, vegetables and fruits are growing on the high beds and hydrophilic plants such as lotus or rice are growing in the ditches.

Especially in the low-lying area in the Shunde and Nanhai region south of Foshan the landscape is below sea level. The high water table in these wetlands limits agricultural production. Therefore, historically agricultural development was strongly associated with dike building technology[12]. The unique Chinese dikepond system combining aquaculture and sericulture was already developed in the Qing Dynasty. The dikepond system is a sustainable system, built on a natural nutrients cycle[13-14]. On the dikes surrounding the dug out ponds mulberry trees, sugarcane, vegetables and fruit are planted. The leaves of mulberry trees are fed to silkworms used for silk production. Sometimes in addition livestock (e.g. chicken, pigs or geese) are raised on the dikes. The animals’ waste, as well as the silkworm excrements are used in the ponds as nutrients for the different fish species. The Shunde district became one of the three major silk-producing areas in China. Although after 1926 the silk trade internationally suffered a severe decline, the dike-pond system continued to exist. Instead of mulberry dike-fishponds, sugarcane dike-fishponds became more dominant[15]. In the 1990s the traditional mulberry-dike-fish-pond in Pearl River Delta almost disappeared[14]and instead the modern hard dike fish pond farming is pursued (Fig. 5).

This now dominating type of aquaculture[16-17]is a very intensive, industrial-type monoculture system,highly depending on chemical fertilizers, pesticides,antibiotics and aerators[18]. China is the leading exporter of seafood globally and with more than 100,000 hm2of aquaculture the Pearl River Delta contributes a major share. This includes a range of fish species such as tilapia, carp, mandarinfish, eel, and also crustaceans such as giant prawn, shrimp as well as terrapin and also soft-shelled turtle.

At the same time the land to water ratio of the dike-pond systems changed from 3 to 2, as in the late Qing Dynasty, to 1 to 4 and the dikes are now either unplanted or with very little fruit and vegetables (Fig. 5)[19].

4 Sedimentation and Sand Mining

5 現(xiàn)代魚(yú)塘養(yǎng)殖使用增氧曝氣機(jī)Modern fish ponds with aerators

6 珠三角河口的采砂工程Sand mining in the Pearl River estuary

7 南沙濕地公園——開(kāi)墾地上新建的人工濕地公園Nansha Wetland Park, a recently established man-made wetland park on reclaimed land

8 廣州海珠區(qū)濕地公園Haizhu Wetland Park in Guangzhou

9 香港南生圍，魚(yú)塘復(fù)興并建立城市濕地的規(guī)劃設(shè)計(jì)Nam Sang Wai, Hong Kong. Reactivating fishponds as urban wetlands注：左下方的圖片表示現(xiàn)狀和規(guī)劃中的濕地（淺藍(lán)色）與魚(yú)塘（深藍(lán)色），右下方的圖片表示現(xiàn)狀與規(guī)劃中的植被。Note: Bottom far left and left, existing and proposed wetlands (light blue) and fishponds (dark blue). Bottom right and far right, existing and proposed vegetation.

The Pearl River is the second-largest river in China in terms of water discharge, totalling 260 km3/year.It has three main tributaries, the Xijiang River (length 2,214 km, catchment 350,000 km2), the Beijiang River (length 468 km, catchment 38,000 km2) and the Dongjiang River (length 562 km, catchment 25,000 km2).Correspondingly, until approx. 1990 the highest sediment load was carried by the Xijiang River(68 million tons/year), followed by the Beijiang River(5.5 million tons/year) and the Dongjiang River(2.5 million tons/year)[20]. Already since the 1950s reservoirs and dams were built in the Pearl River basin for different purposes such as flood control,water supply, electricity generation and farmland irrigation. Initially, these reservoirs did not reduce the overall sedimentation, as at the same time deforestation in the catchment area continued. This caused severe soil erosion, which resulted in an increase in the sediment load until the 1990s[21]. Because of the high riverine sediment load the Pearl River Delta kept extending towards the sea during the 1950s—1980s[22], continuously shaping new mud flat wetlands.

Afforestation programs since the 1990s and the construction of 387 new and bigger reservoirs with a total storage capacity of 46.7 billion m3[23]led to a decrease of sediments. Within a very short period of time until 2011, the sediment load dramatically decreased by approx. 80% to 12.7 million t/year[24].

In parallel, because of rapid industrialisation and urbanisation since the economic reform in 1979, the demand of sand as construction material for buildings and large-scale reclamation projects including new airports, seaports, roads and bridges was gigantic (Fig. 6).In the early 1990s three-quarters of the world’s dredging fleet was involved in the extraction of sand and gravel deposits only within the territory of Hong Kong[25]. It is estimated that the total of the mined sand between 1980 and 2002 only from the Dongjiang River amounts to approx. 332 million m3. This corresponds to an estimate of the total sediment load within a period of 166 years at Boluo station, situated at the end of the Dongjiang River[26].

As environmental regulations of sand mining in rivers became stricter now about 60% of sand mining comes from marine sand[27]. The large quantity of sandtake over time has reduced the source of sediment for the wetlands and withered mud flat wetlands[3].

5 Wetlands, Mangroves and Mud flats

In article 1.1 ofthe Ramsar Convention wetlandsare defined as “areas of marsh, fen, peatland or water,whether natural or artificial, permanent or temporary,with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres.”

Most significant in the Pearl River delta are intertidal mudflat areas and mangrove wetlands.Mangroves have important protection and ecosystem functions. Until the 1950s they were very common along the coastline of the estuary. In 1950,212 km2of natural mangrove wetlands were fringing Guangdong’s coastland, half of the entire mangrove forests in China.Until 1990 this area was reduced to 38 km2[28].

Along the eastern coast of the Pearl River estuary,as a result of urbanisation, oyster farming and other activities, in the 1990s only 3 km2of mangroves were left[29]. E.g. between Shajing and Bao’an mangrove wetlands almost entirely disappeared. Also, in Shenzhen Bay most of the original mangrove forest was lost, with only a small proportion of the original mangrove forest remaining[30-31]. Similarly, on the western coast of the Pearl River estuary most mangrove forest disappeared, e.g.because of reclamation projects and shrimp farming[29].

Already since the 1980s the importance of mangrove ecosystems has been recognised and South China’s administration began to set up mangrove nature reserves to protect and restore mangrove forests.Widely-known examples include Mai Po nature reserve(Hong Kong, 3.8 km2), Futian national nature reserve(Shenzhen, 3.8 km2) and Neilingding Island with 3.3 km2of mangrove forests. Some of these sites are also protected habitats on an international level and play an important role for migratory birds returning from Oceania to north Asia[3]. Despite huge investments in sanitary and water purification projects in the estuary the seawater quality is rated low, also the pollution of sediments e.g. through Cd, Zn and Ni[32]is problematic.

Along with the rapid economic growth of the Pearl River Delta and the construction of reservoirs the natural sediment delivery in the streams was reduced. At the same time the number of dikes, wharfs and ports at the waterfront increased as well the excavation of sand due to the increased demand in the construction industry led to increased pressure on mud flat wetlands.A total of 600 km2of the natural mud flat wetland were consumed[3]. Since around 2005 on the western and eastern coastline of the Pearl River estuary intertidal mud flats are basically inexistent[33]. In the north-western part of the Pearl River estuary the area of intertidal mudflats dropped from 171 km2in 1988 to 68 km2in 2008[34]due to land reclamation for the creation of Longxue Island and Nansha Port.

6 Discussion and Outlook

The overall level of human influence on the Pearl River Delta is dramatic. The region is characterised by a highly intense use of the natural resources. This includes logging of forests, some of its major effects have been addressed in the past through afforestation.Ongoing issues are high-intensity agriculture and aquaculture, destruction of riparian buffers and wetlands, destruction of mangroves and intensive sand mining for construction purposes.

Looking into the future a number of highly complex questions urgently need addressing. What are the effects if urbanisation continues at this fast pace?Will this require building more and even bigger dams and reservoirs upstream? Will this require more sand mining and dredging downstream to supply the needed construction material? And in combination with sealevel rise how can the problem of increased flood-risk and increased salinity of soils[35]be tackled?

A key factor is the supply of sediments. The combination of capturing sediment upstream, sandmining in downstream reaches and an increasing threat by projected sea-level rise leads to an increased risk of flooding in the Pearl River Delta in the future[36].

While a series of steps can be taken, overall an equilibrium between supply of sediments and extraction of sediments is highly desirable. As a starting point, the flushing of reservoirs is an effective method to mitigate sediment accumulation in reservoirs[37-38].This helps to increase the efficiency of the reservoirs and also increases sediment load again in downstream reaches. There is a risk that it may also negatively impact(e.g. fish mortality) on the downstream ecosystems[39].Generally, a largely positive effect can be expected, as wetlands in deltas and estuaries are reliant on periodic supply of nutrient-rich sediment-recharge to maintain their ecological functions and biological diversity[40].While there is a high level of initial human intervention needed such restoration also relies to a good degree on natural sedimentation and delta-building processes[41].

Unlike many other deltas in the world that are subsiding, this phenomenon seems to apply only to parts of the Pearl River Delta[42]. However, with a very low aggradation rate overall, risk of accelerated compaction and sea-level rise the Pearl River Delta belongs to the most vulnerable deltas on a global scale[36].

In the context of the ongoing urbanisation it is important to preserve and expand sufficient space for supporting natural flood dynamics. E.g. on Qi’ao Island a provincial nature reserve was established in 2004. The mangrove area, which decreased from 200 hm2in 1957 to 32 hm2in 1999 could be enlarged to 533 hm2in 2004, and up to 700 hm2in 2019[43].

Besides their ecological value, natural wetlands are an important element for an effective flooding adaptation policy, helping to reduce financial losses caused by flooding. In addition to government action to protect the remaining wetlands as nature reserves,government also started to build a large number of new wetland parks within the estuary (e.g. Nansha Wetland Park, 2007, Fig. 7) and also in urban environments, e.g.Haizhu Wetland Park in Guangzhou (Fig. 8).

This is a highly commendable initiative, which also needs ex-post research on the qualities, ecological functions[44]and ecosystem services that man-made wetlands are able to deliver. Also, the question is, what is the starting point. Is it an already existing wetland, a lake,a fishpond, agricultural land or brown field and how does this compare in terms of ecosystem services with the new wetland park. Certainly, from a recreational perspective these new wetland parks have been highly successful.

While the sustainable traditional dike-pond systems are on the decline, given the widespread high intensity aquaculture practices in the Pearl River Delta,it should also be considered to re-design and retrofit some of the modern type fishponds to provide artificial wetlands that have the potential to fulfil an ecological, flood protection as well as a recreational function (Fig. 9).

Overcoming the hurdles for developing a more resilient, adaptive and sustainable environment in the Pearl River Delta is highly challenging. A number of projects and ambitious initiatives (e.g. Sponge City programme) are already trying to address some of these issues and challenges. However, from a long-term systems perspective and also from the perspective of landscape architects this is not sufficient. As Steinitz[45]stated, “l(fā)andscape architects define their future when they define the scope of their work”. In this sense,instead of pursuing sectoral approaches and small-scale interventions, the level of complexity and the gigantic dimension of the issues requires multi-scale, crossdisciplinary and collaborative approaches leading to a more sustainable environmental management.

Acknowledgments:

This research is supported by the Newton Fund/Engineering and Physical Sciences Research Council(EPSRC) as part of the Sustainable Deltas award (No. P/R024979/1).The Aaptive Urban Transformation research project is a collaborative project with South China University of Technology and Delft University of Technology (TU Delft).

Sources of Figures:

All figures, except Fig. 9 are by the authors; Fig 9 ? John Ho Shun Yuen, Final Year Project 2018, The University of Sheffield.