尚占環(huán)，董世魁，周華坤，董全民，龍瑞軍

1 蘭州大學(xué)生命科學(xué)學(xué)院，草地農(nóng)業(yè)生態(tài)系統(tǒng)國(guó)家重點(diǎn)實(shí)驗(yàn)室，蘭州 730000 2 北京師范大學(xué)環(huán)境學(xué)院，北京 100875 3 中國(guó)科學(xué)院西北高原生物研究所，青海省寒區(qū)恢復(fù)生態(tài)學(xué)重點(diǎn)實(shí)驗(yàn)室，西寧 810008 4 青海大學(xué)畜牧獸醫(yī)科學(xué)院，西寧 810016

在過去的一個(gè)世紀(jì),全球草地生態(tài)系統(tǒng)經(jīng)歷著有人類以來最為嚴(yán)重的威脅。這種威脅導(dǎo)致了大范圍和多形式的草地退化,其退化的結(jié)果包括生物多樣性喪失、生態(tài)系統(tǒng)功能減弱、當(dāng)?shù)刭囈陨娴木用裆?jì)減少或淪為難民。近50a來,發(fā)展中國(guó)家步入生產(chǎn)高速發(fā)展的階段,草地生態(tài)系統(tǒng)亦趨工業(yè)化毀壞之后塵[1- 3]。大范圍的草地荒漠化、毒雜草化等等,都在更嚴(yán)峻威脅著占全球陸地面積25%的草原。例如,從本世紀(jì)初到現(xiàn)在,中國(guó)媒體一直宣稱中國(guó)有90%的草原處于退化狀態(tài)[4]。因此,全球退化草地生態(tài)恢復(fù)面臨著提高其生計(jì)維持功能、生態(tài)服務(wù)功能的巨大挑戰(zhàn)。

生態(tài)學(xué)家,特別是恢復(fù)生態(tài)學(xué)家長(zhǎng)期以來一直關(guān)注退化草地生態(tài)系統(tǒng)的恢復(fù),并致力于研究和實(shí)踐工作[5- 7]。研究案例的年限與恢復(fù)效果,方法有效性有著緊密的聯(lián)系,對(duì)實(shí)踐的指導(dǎo)價(jià)值也不同。在生態(tài)恢復(fù)案例中,成功的生態(tài)恢復(fù)效果往往都基于長(zhǎng)期的研究經(jīng)驗(yàn)和積累[5]。因?yàn)闇?zhǔn)確和正確的評(píng)估草地植被演變必須基于長(zhǎng)期的監(jiān)測(cè)結(jié)果,這樣才能對(duì)生物多樣性,植被生態(tài)功能給予詳細(xì)評(píng)估,使制定的管理方案更可靠[8]。從歐美開始,較長(zhǎng)期的草地恢復(fù)生態(tài)學(xué)研究工作很早就得到重視和實(shí)施[9-12]。在長(zhǎng)期生態(tài)學(xué)研究中,一般界定長(zhǎng)期的時(shí)間范圍是20—50a間,或者更長(zhǎng)時(shí)間[13-14]。沒有長(zhǎng)期恢復(fù)經(jīng)驗(yàn)積累,不可能獲得很好的恢復(fù)效果,其結(jié)果很可能適得其反。突出考慮生態(tài)恢復(fù)的長(zhǎng)期性特征,更能警示我們政策制定者,慎重考慮政策對(duì)環(huán)境影響。更多的了解不同年限的生態(tài)恢復(fù)研究案例,可為我國(guó)目前大量的生態(tài)恢復(fù)工程提供借鑒,實(shí)現(xiàn)揚(yáng)長(zhǎng)避短、查漏補(bǔ)缺；還可以為客觀評(píng)估我國(guó)生態(tài)恢復(fù)工程提供參考。本文從調(diào)研的國(guó)內(nèi)外149個(gè)退化草地恢復(fù)研究案例出發(fā),對(duì)退化草地生態(tài)恢復(fù)的時(shí)間期限,方法,效果,及研究年限規(guī)律進(jìn)行綜合分析,以期給我們當(dāng)前退化草地生態(tài)恢復(fù)提供借鑒價(jià)值。

1 退化草地生態(tài)恢復(fù)研究的時(shí)間、效果及分布

1.1 時(shí)間范圍

當(dāng)代生態(tài)學(xué)研究已經(jīng)突破傳統(tǒng)生態(tài)學(xué)研究范疇,從時(shí)間跨度之廣就能看出來。Jackson[15]認(rèn)為生態(tài)學(xué)研究中的時(shí)間問題,分real time、Q-time (quaternary)和deep-time；第一個(gè)是一般的生態(tài)學(xué)研究時(shí)間范圍,從幾周到幾十年范圍內(nèi),主要包括競(jìng)爭(zhēng),捕食等；第二個(gè)是從100年到1萬年之間,包括演替、遷移、穩(wěn)定性,保護(hù)和滅絕等；第三個(gè)范圍超過1萬年,是古生物學(xué),進(jìn)化,地理變化范疇的研究。根據(jù)這個(gè)劃分,恢復(fù)生態(tài)學(xué),特別是草地恢復(fù)生態(tài)學(xué)一般屬于real time時(shí)間范疇,即大多數(shù)在幾十年范圍內(nèi),也少有案例超過100a。與森林相比,退化草地得到恢復(fù)的時(shí)間相對(duì)較短,因此大多草地生態(tài)學(xué)研究時(shí)間跨度也較短。

雖然,長(zhǎng)期生態(tài)學(xué)研究的時(shí)間界定一般認(rèn)為是20—50a之間[13-14]。與時(shí)間尺度有關(guān)的退化草地生態(tài)恢復(fù)研究案例主要側(cè)重于對(duì)恢復(fù)技術(shù)實(shí)施后的監(jiān)測(cè),根據(jù)監(jiān)測(cè)結(jié)果來評(píng)判恢復(fù)效果。根據(jù)ISI Web of Science數(shù)據(jù)庫中,隨機(jī)獲取的149個(gè)關(guān)于草地恢復(fù)的研究案例中,大多數(shù)的時(shí)間尺度都小于10a,也就是大多數(shù)案例根據(jù)較短期的時(shí)間研究結(jié)果來評(píng)估退化草地恢復(fù)(圖1)。其次的時(shí)間尺度在10—20a之內(nèi)也較多,這些案例主要集中于較長(zhǎng)期的連續(xù)監(jiān)測(cè)(圖1)。恢復(fù)生態(tài)學(xué)中,長(zhǎng)期性研究手段主要有3個(gè),第一是相同樣地的長(zhǎng)期連續(xù)性監(jiān)測(cè)(long-term)；第二是多年代序列樣地(chronosequence)的比較性分析；第三個(gè)是間斷性研究樣地的重訪研究。在大于20多年的研究案例中,多數(shù)是關(guān)于退化草地恢復(fù)后,通過再訪途徑獲得評(píng)估,一般是根據(jù)案例的文獻(xiàn)或者資料記載來進(jìn)行試驗(yàn)地重訪 (revisiting)(圖1)。

圖1 隨機(jī)調(diào)查的149個(gè)草地恢復(fù)研究案例在不同年限段的數(shù)量分布Fig.1 The numbers pattern in different period (years) of 149 study cases of degraded grassland restoration

圖2 隨機(jī)調(diào)查的149個(gè)退化草地恢復(fù)案例的恢復(fù)效果Fig.2 Evaluation of restoring effect for 149 grassland restoration cases

1.2 恢復(fù)的效果

在對(duì)149個(gè)草地恢復(fù)研究案例的綜合比較中,根據(jù)恢復(fù)目標(biāo)和最終達(dá)到的效果進(jìn)行了人為評(píng)分,評(píng)分從-5到+5,代表恢復(fù)技術(shù)實(shí)施后負(fù)作用和正作用的程度。從統(tǒng)計(jì)結(jié)果看(圖2),恢復(fù)結(jié)果與恢復(fù)年限并沒有關(guān)系,短期和長(zhǎng)期的恢復(fù)結(jié)果都有好有壞。說明了適宜的恢復(fù)技術(shù)與恢復(fù)時(shí)間相適應(yīng)的重要性?？傮w來看,大于5a的生態(tài)恢復(fù)效果都較好,年限越長(zhǎng)草地恢復(fù)越趨于向良性方向演替。對(duì)于恢復(fù)技術(shù)而言,退化草地的恢復(fù)盡量采用排除干擾的方法,一般都能達(dá)到很好的效果,其結(jié)果的顯著性決定于年限長(zhǎng)短[16-19]。這也提示我們,目前在我國(guó)實(shí)施的各種退化草地恢復(fù)技術(shù)短期內(nèi)的效果評(píng)估難以代表對(duì)技術(shù)真實(shí)評(píng)價(jià),需要長(zhǎng)期跟蹤及再評(píng)估,才能獲得對(duì)生態(tài)恢復(fù)作用的深入認(rèn)識(shí)。

不同草地類型、恢復(fù)技術(shù),以及對(duì)不同觀測(cè)對(duì)象產(chǎn)生的結(jié)果差別較大。在產(chǎn)生負(fù)結(jié)果的案例中主要集中于目標(biāo)物種/群落結(jié)構(gòu)的恢復(fù)[20-21]、土壤功能或氮水平的恢復(fù)[10,22],無機(jī)碳[23]。植被覆蓋度降低的案例中集中于8—9a期限中[24]。而較長(zhǎng)期土壤系統(tǒng)恢復(fù)(72a)一般都很難達(dá)到目標(biāo),這與氣候環(huán)境變動(dòng)有關(guān)[10]。相對(duì)而言,在草地生他系統(tǒng)中,針對(duì)生物多樣性和植被覆蓋度,一般適宜的恢復(fù)技術(shù)都能實(shí)現(xiàn)[11, 25-31]。而強(qiáng)干擾的措施一般都或多或少的在某個(gè)方面偏離預(yù)期目標(biāo),如放牧[29-30],火燒[25],施肥[32],耕作[10, 33],刈割[34]。從植被類型來看,沙化草地、干旱草地,在采取恢復(fù)措施后都能體現(xiàn)出正的恢復(fù)效應(yīng)[35-40 ],但是對(duì)于草甸、濕地草地,海邊草地而言,其結(jié)果較不確定[20-21, 34,36, 41-44,]?？傮w而言,物種豐富度較高,土壤水、養(yǎng)分異質(zhì)性較高的草地變化不確定較高,并且難以預(yù)測(cè)。

1.3 研究案例在全球分布及年限格局

圖3 調(diào)研案例全球分布(澳洲,非洲,美洲,歐洲和亞洲)及年限格局Fig.3 All 149 study cases′ pattern in different continents (Australia, Africa, America, Europe and Asia) and periods (years)

退化草地恢復(fù)長(zhǎng)期性研究案例年限與研究地區(qū)、國(guó)家的科研能力有很大關(guān)系(圖3)。歐洲研究基礎(chǔ)設(shè)施較完善,并且擁有較多的長(zhǎng)期性研究定位工作站,因此現(xiàn)有的報(bào)道中歐洲在不同年限都有很多案例,并且伴隨著時(shí)間的推移這些案例都將成為較長(zhǎng)年限案例。美洲主要集中在美國(guó)、加拿大的研究,近些年的長(zhǎng)期性研究快速發(fā)展,并且隨著美國(guó)長(zhǎng)期性生態(tài)學(xué)研究計(jì)劃的實(shí)施,會(huì)更加加強(qiáng)。亞洲關(guān)于草地恢復(fù)的研究主要分布在中國(guó),混合了長(zhǎng)期性定位研究和多次重訪案例,以及多年代比較性工作,近些年隨著中國(guó)科研投入的增加逐漸得到豐富和完善。澳洲和非洲的案例相對(duì)較少,但是澳洲退化草地恢復(fù)長(zhǎng)期性工作堅(jiān)持較好,而且研究?jī)?nèi)容也非常豐富,值得我們借鑒。

2 長(zhǎng)期性退化草地生態(tài)恢復(fù)的目標(biāo)和措施

2.1 恢復(fù)目標(biāo)

退化草地生態(tài)恢復(fù)目標(biāo)與其他生態(tài)系統(tǒng)恢復(fù)目標(biāo)都相似,都集中于兩種功能,即生態(tài)功能、生產(chǎn)功能,但在具體研究案例中,有較詳細(xì)的區(qū)分[5]。在隨機(jī)調(diào)研的149個(gè)案例中,生物多樣性(29.48%)、植被覆蓋度(24.28%)、土壤碳庫(13.87%)3個(gè)恢復(fù)目標(biāo)占大多數(shù),其余恢復(fù)目標(biāo),如生產(chǎn)力,昆蟲群落,土壤微生物,土壤氮庫,目標(biāo)物種,植物群落結(jié)構(gòu),土壤種子庫,土壤濕度,飼用牧草等都相對(duì)較少(圖4)。實(shí)際案例中更多的是多個(gè)目標(biāo)的恢復(fù),例如植被覆蓋度和生產(chǎn)力[45-47],生物多樣性與生產(chǎn)力[48],在案例中很少有單一的恢復(fù)目標(biāo)。很多研究中,恢復(fù)目標(biāo)或多或少與研究對(duì)象生態(tài)生物學(xué)特點(diǎn)和研究者擅長(zhǎng)領(lǐng)域相關(guān),例如土壤種子庫、群落結(jié)構(gòu)穩(wěn)定性等,而植被覆蓋度永遠(yuǎn)是恒定的指標(biāo),在各年限研究案例都存在。

圖4 隨機(jī)調(diào)查的149個(gè)退化草地生態(tài)恢復(fù)不同目標(biāo)占案例總數(shù)的百分比Fig.4 The ratio (%) of each restoring role in all grassland restoration cases (149)

調(diào)查的研究案例中,在恢復(fù)年限與恢復(fù)目標(biāo)中沒有一致的規(guī)律性。在歐美國(guó)家的案例中,更多是關(guān)于生物多樣性,他們認(rèn)為生物多樣性如果能夠恢復(fù)的話,那么其他諸如生產(chǎn)力,目標(biāo)物種,土壤養(yǎng)分等功能都能隨之恢復(fù),這種思想實(shí)際上一直主導(dǎo)著當(dāng)前生態(tài)恢復(fù)理論與實(shí)踐[5, 49-50]。而在生物多樣性恢復(fù)的實(shí)踐中,當(dāng)前更側(cè)重于生態(tài)系統(tǒng)的管理[38,46, 51-52]。調(diào)查的研究案例中,生物多樣性恢復(fù)研究年限在長(zhǎng)期生態(tài)研究中較多,例如在澳大利亞,研究開墾的草地在棄耕后生物多樣性恢復(fù)達(dá)到80a[53]。北美斯泰普草地棄耕監(jiān)測(cè)多樣性達(dá)50多年[54],歐洲控制實(shí)驗(yàn)中N素對(duì)草本植物多樣性恢復(fù)影響能監(jiān)測(cè)70a[21]。事實(shí)上大多數(shù)長(zhǎng)期生態(tài)恢復(fù)案例主要依靠的是定期或者不定期的監(jiān)測(cè)。

比較而言，生產(chǎn)力恢復(fù)的研究案例一般時(shí)間都比較短,這說明長(zhǎng)期生態(tài)學(xué)研究中生態(tài)功能是首要的。生產(chǎn)力功能在一定時(shí)期內(nèi)受人類需求影響較大,暫時(shí)性特點(diǎn)較明顯。調(diào)查的研究案例中,關(guān)于生產(chǎn)力的案例都是少于5a的研究[46- 48,55],只有來自中國(guó)的一個(gè)案例監(jiān)測(cè)了24a后草地恢復(fù)的生產(chǎn)力[33]。事實(shí)上我國(guó)目前正在綜合各種長(zhǎng)期性監(jiān)測(cè)研究對(duì)生態(tài)恢復(fù)的長(zhǎng)期性效果、規(guī)律,進(jìn)行更大范圍的總結(jié),爭(zhēng)取給全球提供更多參考。草地恢復(fù)中將土壤系統(tǒng)作為恢復(fù)考察對(duì)象基本上都在短、中、長(zhǎng)期年限都有,因此生態(tài)恢復(fù)始終將土壤系統(tǒng)視為關(guān)鍵[10, 22, 28,37, 44,56-60]。

在以前的草地生態(tài)恢復(fù)研究案例中土壤微生物關(guān)注較少,隨著土壤微生物研究技術(shù)的發(fā)展,近些年關(guān)注較多[37, 40, 59, 61]。但是土壤微生物系統(tǒng)較為復(fù)雜,而且其恢復(fù)目標(biāo)難以把握,在生態(tài)恢復(fù)中可能作為參考指標(biāo),況且與土壤養(yǎng)分緊密相關(guān),一般與土壤養(yǎng)分一起作為整體考慮[37, 59]。作為近些年發(fā)展起來的綜合分析方法,現(xiàn)代統(tǒng)計(jì)學(xué)(例如多元統(tǒng)計(jì)、數(shù)量分析、結(jié)構(gòu)方程模型等)為植被-土壤養(yǎng)分-土壤微生物的關(guān)聯(lián)分析提供了手段,因此越來越多的生態(tài)恢復(fù)中土壤微生物成為熱點(diǎn)目標(biāo)。

有兩篇研究涉及到了螞蟻[39]、蝴蝶[61]在草地生態(tài)恢復(fù)中的情況,這兩篇報(bào)告針對(duì)不同草地類型,不同恢復(fù)技術(shù),是草地生態(tài)恢復(fù)中考慮動(dòng)物,特別是昆蟲的典型案例。其中Menke在2015年的研究涉及了螞蟻群落在普列里,薩王納草地在封育恢復(fù)的短期(3a),中長(zhǎng)期(5—15a),以及大于15a的變化,很顯然年限長(zhǎng)的恢復(fù)對(duì)螞蟻群落恢復(fù)更有利[39]。關(guān)于蝴蝶的報(bào)告中,Woodrock 等人在2012年的報(bào)道中研究了草地開墾后采用人工播種恢復(fù),以及退化草地(灌叢化)清除灌叢等方法對(duì)蝴蝶的影響,其中棄耕人工播種對(duì)蝴蝶的恢復(fù)效果表現(xiàn)好些,這個(gè)研究年限是10a[61]。還有一個(gè)關(guān)于昆蟲群落的研究案例時(shí)間較長(zhǎng),前后間隔了40a,Schuch等人在2011年報(bào)告了德國(guó)東部干草原在控制木本方式下昆蟲群落恢復(fù)情況,并且效果很好[62]。

2.2 恢復(fù)措施

圖5 隨機(jī)調(diào)查的149個(gè)退化草地生態(tài)恢復(fù)不同恢復(fù)技術(shù)措施占案例總數(shù)的百分比Fig.5 The ratio (%) of each restoring technique in all grassland restoration cases (149)

生態(tài)恢復(fù)技術(shù)一直是恢復(fù)生態(tài)工作者的重要研究方向,然而迄今為止,難以找到很通用的恢復(fù)技術(shù)。對(duì)于退化生態(tài)系統(tǒng)而言,最通用的途徑就是消除干擾,但是有時(shí)候消除干擾后漫長(zhǎng)的恢復(fù)時(shí)間,確實(shí)是人們難以堅(jiān)持,不得不尋求在景觀、生態(tài)功能方面更加快速的恢復(fù)技術(shù)。就退化草地生態(tài)系統(tǒng)而言,更多恢復(fù)技術(shù)集中于土壤-植物界面,因?yàn)椴莸刂脖粚悠鄬?duì)簡(jiǎn)單。從調(diào)查的149個(gè)研究案例可以看出,這些案例基本上涵蓋了地球上關(guān)于退化草地恢復(fù)的主要技術(shù)(圖5)。其中應(yīng)用最多的技術(shù)是人工補(bǔ)播或播種[36,46,55,59,63-69, ],圍欄禁牧(或禁止干擾)[11,18, 28,37,39,66,70-71,],放牧利用[24,29-30,36,42,72-74],刈割[41,75-80],施肥[20，32,77,81- 83 ]等技術(shù)。在澳大利亞火燒是常用的措施[25, 35],其他的技術(shù)如灌叢清除[61, 84],翻耕[33, 38],表土移植[44],木本防除[62],干草覆蓋[85],增加CO2[86],增水[26]等措施帶有很強(qiáng)的區(qū)域性,應(yīng)用不是很廣泛。

調(diào)查的案例中,圍欄封育、人工播種,以及棄耕的恢復(fù)技術(shù),有較多的長(zhǎng)期研究案例[11,28, 53-54,59],其中對(duì)開墾草原的棄耕恢復(fù)研究是當(dāng)前的熱點(diǎn)內(nèi)容,這對(duì)在自然狀態(tài)下草原演替、草原群落建成等生態(tài)學(xué)重要性有關(guān)。調(diào)查的研究案例中火燒措施持續(xù)的時(shí)間都比較長(zhǎng),例如美國(guó)的林地草地火燒恢復(fù)研究持續(xù)了4a[87],美國(guó)高羊茅草地火燒恢復(fù)研究持續(xù)了5a[88],美國(guó)對(duì)林木入侵的草地恢復(fù)使用火燒恢復(fù)的研究持續(xù)了7a[89]；澳大利亞南澳草地火燒技術(shù)恢復(fù)研究則持續(xù)了10a[25],瑞典干草原的火燒恢復(fù)研究持續(xù)了15a[35]。歐洲由于牧業(yè)利用需要,很多草地恢復(fù)技術(shù)集中在如何從草地上清除擴(kuò)張或侵入的灌木、林木,大多時(shí)候這種措施與放牧相結(jié)合[30],或者與人工播種結(jié)合[61]。

在這些草地恢復(fù)技術(shù)中,值得注意的是人工播種(或人工種草)的技術(shù)應(yīng)用。這種技術(shù)一般在歐美國(guó)家是用來較快速的恢復(fù)草地生態(tài)功能,且采用本地的鄉(xiāng)土植物種子。同時(shí)一個(gè)非常重要的問題是,在已經(jīng)報(bào)道的歐美國(guó)家草地恢復(fù)中使用人工播種,一般都是混合播種,并且混播的物種都在10多種,甚至20、30種,很少有像我們國(guó)家采用的1種、2—3種、4—5種這樣少的混播技術(shù)應(yīng)用到草地生態(tài)恢復(fù)中[36，51，58-59，64，66，68，90]。在美國(guó)內(nèi)布拉斯加州一個(gè)草地人工種植恢復(fù)研究中,研究人員認(rèn)為15種的混播是低物種數(shù)的混播,同時(shí)他們采用了95種物種種子的混播作為高豐富度混播材料,這項(xiàng)研究用來分析人工混播群落的穩(wěn)定性和恢復(fù)能力[47]。但是,在我國(guó)人工播種恢復(fù)退化草地,采用混播草地植物物種非常少,甚至有時(shí)只有一種[91]。從生態(tài)學(xué)原理上,幾種草種混播做草地生態(tài)恢復(fù)應(yīng)該禁止,這樣會(huì)導(dǎo)致人工種植草地極其不穩(wěn)定,且造成再次退化,以致恢復(fù)工作投入的人力物力大量被浪費(fèi)。這也是我國(guó)草地人工恢復(fù)又迅速退化的一個(gè)非常重要原因[92]。

3 退化草地生態(tài)恢復(fù)研究獲得長(zhǎng)期數(shù)據(jù)和結(jié)果的途徑

3.1 連續(xù)監(jiān)測(cè)方法

在我們調(diào)查的149個(gè)研究案例中,其中涉及到連續(xù)監(jiān)測(cè)方法的有66個(gè)(表1)。很顯然,連續(xù)監(jiān)測(cè)對(duì)長(zhǎng)期生態(tài)學(xué)研究十分重要,這對(duì)于研究的可信度,和更加清楚的認(rèn)識(shí)生態(tài)系統(tǒng)變化十分有用,但是這依賴于大量的人力、物力投入[33,109]。對(duì)退化草地生態(tài)恢復(fù)而言,連續(xù)的監(jiān)測(cè)有助于展示詳細(xì)恢復(fù)過程和提出更有效的管理措施。連續(xù)監(jiān)測(cè)面臨最大的挑戰(zhàn)是長(zhǎng)期資助項(xiàng)目,可持續(xù)的研究團(tuán)隊(duì)和經(jīng)費(fèi)支撐,這在全球都是十分困難的。隨著自動(dòng)化設(shè)備的發(fā)展,連續(xù)的監(jiān)測(cè)已經(jīng)變得越來越容易實(shí)現(xiàn)了。短期的研究很多能夠?qū)崿F(xiàn)連續(xù)監(jiān)測(cè),特別是每年或者每個(gè)月的觀測(cè)研究[41,55, 84]。越來越多的自動(dòng)化設(shè)備、遙感、氣象監(jiān)測(cè)等方法被應(yīng)用到連續(xù)的監(jiān)測(cè)中。

完全采用連續(xù)監(jiān)測(cè)方法主要集中在短期的研究中,一般小于10a[30,43,47,55,84,94-96]；也有長(zhǎng)于10a的連續(xù)監(jiān)測(cè)研究,這種研究除了依靠較穩(wěn)定的研究隊(duì)伍,經(jīng)費(fèi)資助外[16,26, 110],也依靠國(guó)家的持續(xù)監(jiān)測(cè)工程,例如中國(guó)內(nèi)蒙古一項(xiàng)研究連續(xù)持續(xù)了24a[33]。

連續(xù)監(jiān)測(cè)或者調(diào)查方法也出現(xiàn)在定期、不定期重訪過程中。諸如對(duì)以前研究地點(diǎn),案例隔多少年以后重新再研究,那么可能在此期間開展連續(xù)2、3a或者更長(zhǎng)時(shí)間研究,來驗(yàn)證前期研究產(chǎn)生的結(jié)果(表1)。例如,Fritch等人[18],對(duì)于愛爾蘭集約化農(nóng)業(yè)利用的草地恢復(fù)過程中的研究,7a前恢復(fù)措施,在7a后連續(xù)開展了2a的調(diào)查。Matějková等人[24]對(duì)于希臘山地草地放牧恢復(fù)的8a研究中，其中后4a采用了連續(xù)調(diào)查方法。Maccherini 等人[36]在意大利對(duì)于鈣化草地恢復(fù)研究持續(xù)了9a,但是由于中間某種原因中斷了,其研究在開始和后來都采用了連續(xù)調(diào)查監(jiān)測(cè)的方法。Sykora等人[107]在荷蘭14a間的研究也是中間間隔了一段時(shí)間又重啟研究。Munson 和 Lauenroth[91]在對(duì)美國(guó)半干旱草地研究,持續(xù)了18a,但是采用了不定期重新調(diào)查方法,每次調(diào)查都堅(jiān)持2—3a的連續(xù)研究。Pavlu 等人[80]在德國(guó)草地研究持續(xù)了20a,只有中間一段時(shí)間連續(xù)了4a調(diào)查工作。更長(zhǎng)期的對(duì)比工作,例如Van Eekeren等人[111]在比利時(shí)草田輪作時(shí)間跨度36a,Bakker 等人[108]在北美間隔了50a,他們的工作都是很久以后重新連續(xù)調(diào)查。

表1 研究案例中實(shí)地調(diào)查重復(fù)方法、年限及文獻(xiàn)來源

3.2 定期重訪式方法

實(shí)驗(yàn)案例的重訪研究是獲得生態(tài)恢復(fù)效果可靠性評(píng)估的有效有段。事實(shí)上,就研究的時(shí)間特點(diǎn),連續(xù)調(diào)查也屬于定期重訪,在這里定期重訪,特指一項(xiàng)研究持續(xù)進(jìn)行,但至少間隔時(shí)間以年為單位,進(jìn)行有規(guī)律的重新研究。在我們調(diào)查的149個(gè)研究案例中,這種嚴(yán)格定期重訪的案例不多,僅有7項(xiàng)(表1),可見定期重訪的方法應(yīng)用不是很廣泛。的確這需要很長(zhǎng)遠(yuǎn)研究計(jì)劃,和更為持續(xù)的人力、物力支撐。在我們調(diào)查7個(gè)案例中,共出現(xiàn)了3個(gè)研究報(bào)道,分別是Halpern 等人研究美國(guó)草地清除林木的影響,每隔兩年重復(fù)調(diào)查[87]；在石楠灌叢草地研究工作,每隔3—4a定期重復(fù)調(diào)查,一共持續(xù)了17a[57]；在英國(guó)集約化農(nóng)業(yè)利用草地恢復(fù)研究中每隔2a重復(fù)一次調(diào)查,一共持續(xù)了14a[32]。Schrautzer[79]在德國(guó)湖邊草地研究案例也采用了定期重訪方法,持續(xù)了28a,是在多年后重新在原來一個(gè)實(shí)驗(yàn)地點(diǎn)重新調(diào)查了10a,在這10a內(nèi)每隔2—3a調(diào)查一次。

3.3 不定期重訪方法

在草地生態(tài)恢復(fù)中,大多年限較長(zhǎng)的研究工作更多的依賴于不定期重復(fù)方法[19, 31,35, 37, 61, 83]。我們調(diào)查的案例中除了每年連續(xù)監(jiān)測(cè)案例,和定期重復(fù)案例,其余都與不定期重復(fù)方法有關(guān),更有82個(gè)案例是典型的不定期重復(fù)的研究工作(表1)。很顯然,不定期重復(fù)方法更符合當(dāng)前生態(tài)恢復(fù)研究工作的特點(diǎn)和人類認(rèn)識(shí)、人力/物力支撐現(xiàn)狀,相對(duì)而言信息量也較大。不僅在發(fā)達(dá)國(guó)家,這種方法應(yīng)用廣泛,而且在更多發(fā)展中國(guó)家和地區(qū),隨著社會(huì)穩(wěn)定,科學(xué)研究工作的興起,這種方法也得到了更多推廣和應(yīng)用[28, 37, 59]。在我們調(diào)查的案例中,不定期重訪集中在大于5a的研究案例中,也很多10a以上的研究案例(表1)。而混合了連續(xù)監(jiān)測(cè)的不定期重訪的研究案例更多的是10a以上的案例。

不定期重訪方法在草地恢復(fù)研究中,從低成本、信息量、研究意義、可對(duì)比性等方面來說,非常值得推廣。事實(shí)上在草地恢復(fù)在演替過程中,連續(xù)監(jiān)測(cè)有時(shí)候非屬必要,畢竟自然演替,尤其在植被較穩(wěn)定階段,短期難以反映出變化。更長(zhǎng)年限的研究,如果想得到持續(xù)性結(jié)果,也只能通過不定期重訪方法,畢竟一代人、兩代人獲得可持續(xù)的研究支持十分有限。例如Gardner[11]研究新墨西哥荒漠草地間隔了30a；Kinucan 和Smeins[97]研究德克薩斯草地間隔了36a進(jìn)行重訪；Hejcman 等人[83]在捷克的研究間隔了37a重訪；Bakker等人[108]再次重訪并連續(xù)監(jiān)測(cè)相同樣地間隔了50a；Coffin等人[54]在北美重訪了53a前的斯泰普草地研究樣地；White等人[10]根據(jù)資料重訪了54a前和72a前的濱州草地樣地。這些長(zhǎng)時(shí)間跨度的研究都得到了很有意義的結(jié)果,對(duì)一個(gè)地區(qū)植被、管理都有很好的借鑒意義。較長(zhǎng)時(shí)間跨度的不定期重訪研究有一個(gè)特別要注意的事情,就是以前研究資料的可尋性,也就是必須能夠借助原來的資料能夠準(zhǔn)確的找到原來地點(diǎn),并且較詳細(xì)資料記載,否則其研究對(duì)比性難以把握。在有定位實(shí)驗(yàn)站條件的區(qū)域容易實(shí)現(xiàn),當(dāng)然在現(xiàn)在信息技術(shù)發(fā)展條件下,這樣的研究也比較容易實(shí)現(xiàn)。

3.4 基于大數(shù)據(jù)的時(shí)間序列的建立與分析

大數(shù)據(jù)分析的可靠性在于大量數(shù)據(jù)的統(tǒng)計(jì)趨勢(shì)性分析,基本上在草地恢復(fù)生態(tài)學(xué)研究中,一種是整合分析(Meta-analysis)的大數(shù)據(jù)統(tǒng)計(jì),另外一種是實(shí)地測(cè)量的大數(shù)據(jù)統(tǒng)計(jì)分析。大數(shù)據(jù)的時(shí)間序列分析,也必須建立在某種,或者各種實(shí)驗(yàn)調(diào)查基礎(chǔ)上。特別是對(duì)各種相似、相同方法研究的整合分析?，F(xiàn)在比較流行的meta分析方法,也很多是案例挖掘,采用研究結(jié)果的無量綱化后統(tǒng)一對(duì)比分析。在生態(tài)學(xué)領(lǐng)域,meta技術(shù)招致大量的批評(píng),導(dǎo)致很多單純的數(shù)據(jù)挖掘者,而非實(shí)驗(yàn)主義者[112-113]。在這里我們更推薦實(shí)地的meta技術(shù),也就是親自采用同一種方法調(diào)查,然后綜合對(duì)比分析。加拿大湯普森河大學(xué)大學(xué)(Thompson Rivers University)Fraser Lauchlan教授提出的“分散-協(xié)作-統(tǒng)一”的大數(shù)據(jù)整合的研究途徑(Coordinated distributed experiments)來獲取更多的生態(tài)數(shù)據(jù),這種方法具有方法統(tǒng)一性,跨區(qū)域性等特點(diǎn),是目前生態(tài)學(xué)大數(shù)據(jù)網(wǎng)絡(luò)研究的典范[114]。

長(zhǎng)期性研究時(shí)間序列建立,或者進(jìn)行大數(shù)據(jù)綜合分析實(shí)際上更多依賴于田野的不定期重訪。在案例統(tǒng)計(jì)中,我們根據(jù)實(shí)際情況將其作為不定期重復(fù)部分(表1),但在討論中,我們單獨(dú)分開討論,目的是能夠?qū)⑵涮厥鈨r(jià)值體現(xiàn)出來。更多的綜合對(duì)比工作，借助于對(duì)很多研究樣地重訪后建立長(zhǎng)期年代序列(chronosequence),進(jìn)行對(duì)比分析[21, 40, 53, 56, 59, 60, 99]。Carilla和Grau[101]的研究比較特殊,他們采用第三方年代(樹輪)確定方法來研究草地演變,但其信息不確定性也較大。

4 總結(jié)和展望

通過上述對(duì)文獻(xiàn)報(bào)道的149個(gè)研究案例分析,我們確定長(zhǎng)期性退化草地生態(tài)恢復(fù)研究確實(shí)對(duì)科學(xué)研究和生態(tài)恢復(fù)實(shí)踐具有重要價(jià)值,受限于人力、物力的支持,且依賴于多方面,方法至關(guān)重要。因此,借助于案例重訪方法獲得可比較的長(zhǎng)期性數(shù)據(jù),以及在生態(tài)恢復(fù)中長(zhǎng)期性年代序列數(shù)據(jù)庫方法值得推薦和加強(qiáng),可以很好獲得長(zhǎng)期性研究結(jié)果?；謴?fù)方法與恢復(fù)年限之間沒有必然聯(lián)系,更取決于恢復(fù)方法的適宜性。特別注意,在我國(guó)采用植物種子種植恢復(fù)退化草地的工作,應(yīng)該注重更多的物種數(shù)混合種植技術(shù),而非我們當(dāng)前的僅采用幾種植物物種混合方法。當(dāng)前,我國(guó)已經(jīng)大范圍的開展了生態(tài)恢復(fù)研究工作,并且建立了各種研究基地,以便于開展綜合性,長(zhǎng)期性研究工作,特別需要能夠持續(xù)的研究設(shè)計(jì)和資助體系。因此,當(dāng)前的短期實(shí)地研究工作,盡可能的實(shí)現(xiàn)連續(xù)性的觀測(cè)研究,這樣為后來的長(zhǎng)期性研究能夠提供更多的數(shù)據(jù)參考。從學(xué)科范疇來講,恢復(fù)生態(tài)學(xué)更需要新的范式來引領(lǐng)理論和技術(shù)的發(fā)展[115-116]。

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