宋述靈 易伶俐 張李龍 陳琳 余澤平 宋慶妮 欒豐剛 方熊 劉駿
摘 要: 木質(zhì)藤本是維持森林物種多樣性的組成部分。近年來(lái),熱帶、亞熱帶森林尤其是次生林內(nèi)的木質(zhì)藤本數(shù)量的過(guò)度增長(zhǎng)給森林的恢復(fù)和健康發(fā)展造成了威脅。至今,國(guó)內(nèi)尚少有研究對(duì)森林木質(zhì)藤本過(guò)多的現(xiàn)象、增長(zhǎng)機(jī)制與生態(tài)效應(yīng)進(jìn)行綜合認(rèn)識(shí)。該文梳理國(guó)內(nèi)外相關(guān)文獻(xiàn),從木質(zhì)藤本數(shù)量增長(zhǎng)的機(jī)制與生態(tài)效應(yīng)進(jìn)行分析和總結(jié),綜合相關(guān)研究認(rèn)為:(1)木質(zhì)藤本數(shù)量的增長(zhǎng)與干旱化加劇、大氣CO2濃度上升、自然干擾和森林破碎化有關(guān),在環(huán)境變化的情況下,木質(zhì)藤本在形態(tài)、行為、生理等方面比樹(shù)木更具優(yōu)勢(shì),表現(xiàn)為更快的生長(zhǎng)速率、更強(qiáng)的繁殖力和可塑性以及資源利用效率。(2)木質(zhì)藤本主要通過(guò)遮蔭脅迫、資源競(jìng)爭(zhēng)和機(jī)械壓力與損傷等方式對(duì)樹(shù)木造成影響。(3)木質(zhì)藤本過(guò)度增長(zhǎng)在個(gè)體水平上會(huì)阻礙樹(shù)木生長(zhǎng)、生殖并引起樹(shù)木死亡,在群落水平上會(huì)改變物種組成、降低多樣性,在生態(tài)系統(tǒng)水平上會(huì)降低森林碳儲(chǔ)量,改變碳、礦質(zhì)養(yǎng)分和水分循環(huán)過(guò)程等。因此,建議結(jié)合野外長(zhǎng)期監(jiān)測(cè)與控制實(shí)驗(yàn)開(kāi)展木質(zhì)藤本數(shù)量動(dòng)態(tài)與環(huán)境變化關(guān)系、森林干擾對(duì)木質(zhì)藤本生長(zhǎng)的影響、木質(zhì)藤本對(duì)環(huán)境變化的響應(yīng)及適應(yīng)機(jī)理、木質(zhì)藤本數(shù)量過(guò)度增長(zhǎng)的生態(tài)效應(yīng)評(píng)價(jià)研究。同時(shí),應(yīng)積極探索合理的森林木質(zhì)藤本管控方法,為森林管理和恢復(fù)等方面的宏觀決策提供科學(xué)依據(jù)。
關(guān)鍵詞: 木質(zhì)藤本, 干擾, 全球變化, 森林健康, 森林管理
中圖分類號(hào): Q948? 文獻(xiàn)標(biāo)識(shí)碼: A? 文章編號(hào): 1000-3142(2023)05-0980-11
Abstract: Lianas are one of the components in maintaining forest diversity. The over-increase of lianas in tropical and subtropical forests, especially in secondary forests is threatening the restoration and healthy development of forests. Few domestic researches make a comprehensive evaluation on the mechanism of the over-increase of lianas and negative effects on forests. In this paper, we reviewed and summarized the related researches on mechanisms and ecological consequences of the over-increase of lianas both at home and abroad. Based on the relevant studies we hold the opinions: (1) The increase of lianas can be corelated with drought severity, elevated atmospheric CO2, increased natural disturbance (gaps) and forest fragments. Lianas possess the advantages of rapid growth, strong reproductive capacity and phenotypic plasticity, and high resource use efficiency under the altered environment. Thus, the favorable environment and inherent advantages can contribute to the increase of lianas. (2) Lianas compete with trees by means of shading stress, resource competition and mechanical loading, abrasion, and strangulation, which all may increase the mortality of trees. (3) The over-increase of lianas can negatively affect the tree growth, reproduction, and survival; at community level, lianas can alter tree community composition and decrease community diversity; at ecosystem level, lianas can ramify forest ecosystem function by decreasing carbon storage and altering carbon, nutrient, and water cycling. Therefore, we recommend that future studies should be aimed at the correlations between population dynamic changes of liana species and environmental changes, the effect of forest disturbance on the growth of lianas, responses of lianas to the environmental changes and the adaption mechanisms, comprehensive evaluation on the ecological consequences of the increase of lianas by means of long-term dynamic monitoring plots and controlling experiments. It is also essential to find appropriate management and control strategies of the over-increase of lianas so as to provide reference for forest management and forest ecological restoration.
Key words: liana, disturbance, global change, forest health, forest management
木質(zhì)藤本(lianas)是不能單獨(dú)直立、通過(guò)主莖或借助攀援器官進(jìn)行生長(zhǎng)的植物類群,與樹(shù)木相比具有較大的長(zhǎng)度和胸徑比(Schnitzer & Bongers, 2002)。木質(zhì)藤本是森林群落的常見(jiàn)組分,對(duì)群落結(jié)構(gòu)和功能的維持具有一定的作用(Schnitzer & Bongers, 2002)。然而,當(dāng)森林受到嚴(yán)重干擾時(shí),木質(zhì)藤本數(shù)量的過(guò)度增長(zhǎng)給森林健康造成威脅,樹(shù)木生長(zhǎng)受抑制、群落結(jié)構(gòu)單一化、物種多樣性下降,嚴(yán)重影響森林生態(tài)系統(tǒng)功能(Schnitzer & Bongers, 2011)。這一問(wèn)題已引起了國(guó)外學(xué)者的普遍關(guān)注。
國(guó)外學(xué)者較早關(guān)注到木質(zhì)藤本數(shù)量的增長(zhǎng)及其負(fù)面影響,在木質(zhì)藤本的分布格局(Schnitzer, 2005)、數(shù)量動(dòng)態(tài)(Addo-Fordjour et al., 2021)、監(jiān)測(cè)方法(van der Heijden et al., 2022)、藤與樹(shù)的關(guān)系(Leonor et al., 2015)、木質(zhì)藤本增長(zhǎng)的機(jī)制及生態(tài)影響(Schnitzer & Bongers, 2011; de Azevedo Amorim et al., 2018; Marshall et al., 2020; Reis et al., 2020; Meunier et al., 2021a)等方面做了較多的研究。國(guó)內(nèi)學(xué)者多關(guān)注森林木質(zhì)藤本的多樣性(Hu et al., 2010;王業(yè)社等,2014)、經(jīng)濟(jì)價(jià)值(季夢(mèng)成等,2008)和生態(tài)功能(張朝陽(yáng)等,2007)等積極作用。由于野外辨識(shí)和調(diào)查的困難,因此在很多調(diào)查與研究中往往忽略這一類特殊生活型植物。實(shí)際上,木質(zhì)藤本數(shù)量過(guò)度增長(zhǎng)的現(xiàn)象在我國(guó)南方很多森林,尤其是在次生林內(nèi)較為普遍。例如,在一些自然保護(hù)區(qū)次生林內(nèi),木質(zhì)藤本甚至形成地毯式覆蓋,嚴(yán)重阻礙了森林恢復(fù),使保護(hù)區(qū)實(shí)行的封山育林政策變成了“封山育藤”(圖1)。
目前,國(guó)內(nèi)關(guān)于木質(zhì)藤本對(duì)森林影響的研究還較少。關(guān)列(1980)較早摘譯報(bào)道了人工林受藤本植物的危害及其防治,但當(dāng)時(shí)并未得到較大響應(yīng);陳亞軍等(2007)綜述了木質(zhì)藤本在熱帶森林的生態(tài)學(xué)功能;王伯蓀等(2009)對(duì)某種木質(zhì)藤本的影響進(jìn)行了具體的評(píng)述。隨后,諸多學(xué)者陸續(xù)在木質(zhì)藤本對(duì)森林物種多樣性、群落結(jié)構(gòu)及更新的影響(郝建輝等,2011;王忠偉等,2020)、木質(zhì)藤本與樹(shù)木的關(guān)系(陸芳等,2021)、藤樹(shù)性狀比較(Chen et al., 2021)等方面進(jìn)行了研究。然而,至今國(guó)內(nèi)尚未見(jiàn)有研究對(duì)森林木質(zhì)藤本過(guò)多的現(xiàn)象、增長(zhǎng)機(jī)制與生態(tài)效應(yīng)進(jìn)行綜合認(rèn)識(shí)。
為引起國(guó)內(nèi)學(xué)者對(duì)森林木質(zhì)藤本過(guò)度增長(zhǎng)現(xiàn)象及其影響的關(guān)注,本文綜合國(guó)內(nèi)外研究進(jìn)展,重點(diǎn)分析木質(zhì)藤本過(guò)度增長(zhǎng)機(jī)制,詳細(xì)闡述木質(zhì)藤本增長(zhǎng)的潛在生態(tài)效應(yīng),并提出新的研究方向,以期為木質(zhì)藤本過(guò)度增長(zhǎng)的防控及森林管理政策的制定提供參考。
1 木質(zhì)藤本數(shù)量過(guò)度增長(zhǎng)
木質(zhì)藤本具有維持森林物種多樣性(Schnitzer & Bongers, 2002)、經(jīng)濟(jì)、藥用和生態(tài)功能(張朝陽(yáng)等,2009;劉揚(yáng)等,2021)。同時(shí),作為層間植物,木質(zhì)藤本既是很多動(dòng)物的生境和通道,又是重要的食物來(lái)源(Adams et al., 2016, 2019; Odell et al., 2019)。然而,當(dāng)木質(zhì)藤本和樹(shù)木比例超出臨界閾值,木質(zhì)藤本在森林中的積極作用將轉(zhuǎn)變?yōu)樨?fù)面作用,尤其在次生林或破碎化森林中,木質(zhì)藤本數(shù)量的過(guò)度增長(zhǎng)將阻礙森林演替與更新(Marshall et al., 2020)。由于外界環(huán)境的變化導(dǎo)致木質(zhì)藤本相對(duì)或絕對(duì)數(shù)量的增長(zhǎng),并給當(dāng)?shù)厣稚鷳B(tài)系統(tǒng)造成負(fù)面影響,本文稱為木質(zhì)藤本數(shù)量過(guò)度增長(zhǎng),簡(jiǎn)稱為木質(zhì)藤本增長(zhǎng)。
木質(zhì)藤本增長(zhǎng)的表現(xiàn)形式主要為多度(密度)、斷面積、生物量、豐度和攀爬覆蓋樹(shù)木的程度等的增加(Wright et al., 2004; Ingwell et al., 2010; Laurance et al., 2014; Wright et al., 2015)。Phillips等(2002a)首次提出了森林木質(zhì)藤本增加的現(xiàn)象,通過(guò)綜合分析亞馬遜4個(gè)區(qū)域的47個(gè)非破碎化熱帶雨林木質(zhì)藤本的數(shù)量變化,發(fā)現(xiàn)20年間大型木質(zhì)藤本(DBH ≥10 cm)相對(duì)斷面積、相對(duì)多度和樹(shù)木死亡風(fēng)險(xiǎn)顯著增加。此后,眾多研究均得出相似的結(jié)果。例如,在尼日利亞一個(gè)經(jīng)歷過(guò)火燒的低地?zé)釒в炅郑?005—2014年間攀爬植物的種數(shù)由49種增至61種,密度由448~1 152 hm2增至1 712~4 492 hm2(Uwalaka & Muoghalu, 2017);溫帶森林木質(zhì)藤本也有所增加,在1967—2007年間歐洲溫帶森林林下層木質(zhì)藤本出現(xiàn)的頻率增加了近39%(Perring et al., 2020)。雖然也有研究結(jié)果與此不同(Smith et al., 2017; Bongers et al., 2020; Umaa et al., 2020),但多數(shù)研究認(rèn)為木質(zhì)藤本的數(shù)量呈遞增趨勢(shì)。在中國(guó)熱帶及亞熱帶區(qū)域的很多次生林內(nèi)木質(zhì)藤本長(zhǎng)勢(shì)旺盛、分布廣泛,如熱帶森林的金鐘藤(Merremia boisiana)(王伯蓀等,2009)、亞熱帶森林的鉤藤(Uncaria rhynchophylla)、葛(Pueraria lobata)等在林內(nèi)形成大面積覆蓋,嚴(yán)重影響了當(dāng)?shù)厣值慕】蛋l(fā)展(圖1)。
2 木質(zhì)藤本過(guò)度增長(zhǎng)的機(jī)制
諸多研究認(rèn)為木質(zhì)藤本在森林群落中優(yōu)勢(shì)度的增加與外界環(huán)境變化有關(guān),主要因素為全球氣候變化、自然干擾(如林窗形成)和森林砍伐等人為干擾導(dǎo)致的森林破碎化、次生化(圖2)。木質(zhì)藤本能因環(huán)境變化做出調(diào)整和適應(yīng),其生理、結(jié)構(gòu)及行為等特征比樹(shù)木更具優(yōu)勢(shì)。
2.1 干旱化加劇
有研究表明木質(zhì)藤本數(shù)量動(dòng)態(tài)與大氣降水量和蒸發(fā)量密切相關(guān)。木質(zhì)藤本的多度、豐度和生物量均隨大氣降雨量和土壤濕度的增加而減少,隨干旱程度的加劇和干旱周期的延長(zhǎng)而增加(Schnitzer, 2005; Swaine & Grace, 2007)。例如,在巴拿馬干旱森林內(nèi),木質(zhì)藤本密度和豐度的絕對(duì)、相對(duì)值均高于濕潤(rùn)森林(Parolari et al., 2020)。
在干旱的條件下,木質(zhì)藤本比樹(shù)木在生長(zhǎng)和生理等方面更具優(yōu)勢(shì)。研究發(fā)現(xiàn)木質(zhì)藤本在雨季的生長(zhǎng)量比樹(shù)木高出2倍,而在旱季高達(dá)7倍(Schnitzer & van der Heijden, 2019),木質(zhì)藤本碳累積量在旱季大于濕季(van der Heijden et al., 2019)。當(dāng)降雨量減少時(shí),木質(zhì)藤本幼苗的存活量也高于樹(shù)木(Umaa et al., 2019)。此外,在干旱條件下,木質(zhì)藤本水分利用效率、導(dǎo)水效率和資源獲取策略等均優(yōu)于樹(shù)木(Cai et al., 2009; Zhu & Cao, 2009; Medina-Vega et al., 2021a, b)。例如,熱帶濕潤(rùn)雨林樹(shù)木在水力安全和導(dǎo)水效率之間具有權(quán)衡性,而木質(zhì)藤本卻沒(méi)有,較強(qiáng)的導(dǎo)水效率和抗栓塞能力表明木質(zhì)藤本比樹(shù)木更耐旱(van der Sande et al., 2019)。
在干旱條件下,木質(zhì)藤本各構(gòu)件還表現(xiàn)出較強(qiáng)的可塑性和適應(yīng)性。在旱季,木質(zhì)藤本比樹(shù)木具有更強(qiáng)的葉片滲透調(diào)節(jié)能力,這使其在干旱加劇的環(huán)境下更具生長(zhǎng)優(yōu)勢(shì)(Maréchaux et al., 2017)。Smith-Martin等(2019)發(fā)現(xiàn)在旱季進(jìn)行灌溉能使樹(shù)木的胸徑和生物量顯著增加,而木質(zhì)藤本并未發(fā)生變化,這表明在干旱條件下木質(zhì)藤本具有更強(qiáng)的適應(yīng)力。此外,木質(zhì)藤本具有發(fā)達(dá)的根系維管系統(tǒng),可進(jìn)行營(yíng)養(yǎng)物質(zhì)的高效運(yùn)輸與分配。
木質(zhì)藤本根系長(zhǎng)而錯(cuò)綜復(fù)雜且具有較強(qiáng)的耐脅迫能力,有些甚至能達(dá)10 m深(Restom & Nepstad, 2004),可從四周或深處利用不同來(lái)源的水分,有效減緩干旱脅迫(Chen et al., 2015)。因此,在全球干旱加劇下,木質(zhì)藤本的形態(tài)、生理和行為特征的優(yōu)勢(shì)可導(dǎo)致其相對(duì)和絕對(duì)數(shù)量的增加。
2.2 大氣CO2濃度增加
持續(xù)上升的大氣CO2濃度是木質(zhì)藤本增長(zhǎng)的潛在因素。大氣CO2濃度的增加有利于木質(zhì)藤本的生長(zhǎng)(Granados & Korner, 2002)。當(dāng)CO2濃度增加時(shí),木質(zhì)藤本洋常春藤(Hedera helix)新梢長(zhǎng)度和生物量的增長(zhǎng)量提高了近60%(Zotz et al., 2006)。CO2增加控制實(shí)驗(yàn)也發(fā)現(xiàn)溫帶森林木質(zhì)藤本毒漆藤(Toxicodendron radicans)的生長(zhǎng)速率、光合作用、水分利用效率顯著高于正常大氣CO2條件下的狀態(tài)(Mohan et al., 2006)。并且,木質(zhì)藤本對(duì)CO2增加的響應(yīng)比樹(shù)木更加迅速(Belote et al., 2004)。木質(zhì)藤本具有較高的葉莖比和較低的葉片單位面積構(gòu)建成本,這使其對(duì)CO2吸收和固定比樹(shù)木更具優(yōu)勢(shì)(Zhu & Cao, 2010)。
2.3 自然干擾
冰雪、火災(zāi)和樹(shù)木死亡等自然(林窗)干擾易促使林內(nèi)木質(zhì)藤本的增長(zhǎng)。通常情況下,林窗內(nèi)木質(zhì)藤本多度和豐度均顯著高于非林窗地帶(Schnitzer & Carson, 2001)。一方面,林窗地帶土壤肥沃、光照較強(qiáng)和空余生態(tài)位較多,為木質(zhì)藤本生長(zhǎng)提供了良好的環(huán)境。另一方面,很多木質(zhì)藤本在林窗內(nèi)具有生長(zhǎng)快速和繁殖力強(qiáng)的特征。由于減少了對(duì)自身支持結(jié)構(gòu)的投資,木質(zhì)藤本可將大部分營(yíng)養(yǎng)資源用于莖的快速生長(zhǎng)和葉的累積(Putz, 1984)。除通過(guò)種子萌發(fā)來(lái)進(jìn)行植株定居外,木質(zhì)藤本在林窗內(nèi)也可進(jìn)行無(wú)性克隆繁殖,依靠地下莖、匍匐莖不斷地萌發(fā)新枝來(lái)向新地段蔓延擴(kuò)散從而促使植株數(shù)量的增加(Schnitzer et al., 2021)。此外,即便是寄主樹(shù)木倒塌死亡,很多木質(zhì)藤本也能夠通過(guò)萌發(fā)新枝重新攀爬到林冠層,使得木質(zhì)藤本絕對(duì)或相對(duì)數(shù)量有所增長(zhǎng)(Rocha et al., 2020)。
2.4 森林破碎化
森林砍伐、捕獵等人為干擾使森林破碎化、次生化,會(huì)導(dǎo)致林緣地帶或次生林內(nèi)木質(zhì)藤本增長(zhǎng)。森林破碎化后,森林環(huán)境變得干燥,土壤肥沃,光照增加,植株矮小,這些條件都有利于木質(zhì)藤本的攀爬和快速生長(zhǎng)。因此,在林緣處和次生林木質(zhì)藤本的生長(zhǎng)速率更快、植株數(shù)量更多(Ladwig & Meiners, 2010; Roeder et al., 2019),并且林緣地木質(zhì)藤本的豐度和碳儲(chǔ)量也高于林內(nèi)(Londré & Schnitzer, 2006, Magnago et al., 2017; Campbell et al., 2018)。森林破碎化也為木質(zhì)藤本克隆生長(zhǎng)創(chuàng)造了條件,如在拉塞爾瓦擇伐的熱帶雨林,木質(zhì)藤本長(zhǎng)距離克隆生殖對(duì)多度增加的貢獻(xiàn)率達(dá)到7.5%,斷面積增量高達(dá)60%(Yorke et al., 2013)。
3 木質(zhì)藤本與樹(shù)木的競(jìng)爭(zhēng)方式
3.1 遮蔭脅迫
木質(zhì)藤本借助莖、枝、葉等器官攀爬到樹(shù)木的樹(shù)冠,大面積遮蔭導(dǎo)致樹(shù)木可獲得光源減少,光合作用受阻,進(jìn)而影響樹(shù)木的生長(zhǎng)和發(fā)育,嚴(yán)重時(shí)可致樹(shù)木死亡(圖3)。木質(zhì)藤本在林冠層形成的覆蓋會(huì)增加林分郁閉度,從而阻礙林下幼苗更新。在巴拿馬熱帶雨林,木質(zhì)藤本對(duì)樹(shù)冠的嚴(yán)重覆蓋使樹(shù)木死亡率增加了100%(Ingwell et al., 2010);木質(zhì)藤本使林窗內(nèi)樹(shù)木更新量下降了46%(Schnitzer & Carson, 2010)。木質(zhì)藤本葉的物候期更長(zhǎng),很多種甚至全年常綠(Putz & Windsor, 1987),常年的遮蔭易使林下幼苗得不到充足的光照而死亡。
3.2 資源競(jìng)爭(zhēng)
資源的爭(zhēng)奪是木質(zhì)藤本與樹(shù)木的競(jìng)爭(zhēng)方式,二者對(duì)光照、水分和養(yǎng)分等資源存在激烈的競(jìng)爭(zhēng)(Meunier et al., 2021b)。在清除木質(zhì)藤本后,樹(shù)木的樹(shù)干液流速率增加了60%(Leonor et al., 2015)。在礦質(zhì)養(yǎng)分與水分競(jìng)爭(zhēng)中,藤與樹(shù)的競(jìng)爭(zhēng)甚至比樹(shù)與樹(shù)的競(jìng)爭(zhēng)更加強(qiáng)烈。Tobin等(2012)研究發(fā)現(xiàn)樹(shù)木的樹(shù)干液流速率在清除藤本后增加了約8%,而對(duì)樹(shù)木的清除并未做出響應(yīng)。此外,木質(zhì)藤本具有更高的資源吸收、運(yùn)輸和利用效率,對(duì)資源的爭(zhēng)奪更具競(jìng)爭(zhēng)力,木質(zhì)藤本葉單位質(zhì)量碳同化速率、光能吸收和利用效果均高于樹(shù)木(Cai et al., 2009; Asner & Martin, 2012),且木質(zhì)藤本根系發(fā)達(dá)(Collins et al., 2016)。例如,當(dāng)土壤養(yǎng)分增加時(shí),木質(zhì)藤本葉面積比和光合能力顯著高于樹(shù)木(Cai et al., 2008; Pasquini et al., 2015)。
3.3 機(jī)械壓力與損傷
木質(zhì)藤本攀爬會(huì)給樹(shù)木帶來(lái)機(jī)械壓力與損傷(Vleut & Pérez-Salicrup, 2005)。木質(zhì)藤本在樹(shù)冠形成大面積覆蓋,給樹(shù)冠帶來(lái)巨大的載重,使樹(shù)梢或樹(shù)枝極易折斷,新梢不易萌生。一些木質(zhì)藤本對(duì)樹(shù)干的纏繞可導(dǎo)致樹(shù)干扭曲、凹陷,增加樹(shù)木折斷和病蟲(chóng)害風(fēng)險(xiǎn),影響樹(shù)木水分和營(yíng)養(yǎng)物質(zhì)運(yùn)輸,最終導(dǎo)致樹(shù)木死亡(圖3)。
4 木質(zhì)藤本過(guò)度增長(zhǎng)的生態(tài)效應(yīng)
4.1 阻礙樹(shù)木生長(zhǎng)發(fā)育
從個(gè)體水平來(lái)看,木質(zhì)藤本增長(zhǎng)會(huì)從以下三個(gè)方面阻礙樹(shù)木生長(zhǎng)發(fā)育。(1)阻礙樹(shù)木的正常生長(zhǎng)。Finlayson等(2022)研究表明木質(zhì)藤本會(huì)使樹(shù)木的生長(zhǎng)量降低156%;宋述靈(2019)發(fā)現(xiàn)杉木被木質(zhì)藤本攀爬后連年徑向生長(zhǎng)和胸?cái)嗝娣e生長(zhǎng)分別降低29%和40%。(2)增加樹(shù)木的死亡率。在亞馬遜低地雨林,大樹(shù)被木質(zhì)藤本攀爬后死亡率增加了2倍多(Phillips et al., 2005)。(3)降低樹(shù)木的結(jié)實(shí)率和結(jié)實(shí)量。在熱帶雨林,樹(shù)木結(jié)實(shí)率與木質(zhì)藤本的蓋度成顯著負(fù)相關(guān)(Nabe-Nielsen et al., 2009),木質(zhì)藤本使堅(jiān)果類樹(shù)種果實(shí)產(chǎn)量下降了200%(Kainer et al., 2014)。
木質(zhì)藤本對(duì)幼苗和幼樹(shù)的生長(zhǎng)也會(huì)有負(fù)面影響。例如,木質(zhì)藤本會(huì)使樹(shù)木幼苗的葉面積減少5倍(Toledo-Aceves & Swaine, 2008);Martínez-Izquierdo等(2016)將14種樹(shù)木幼苗分別種于清除和未清除木質(zhì)藤本的樣地,發(fā)現(xiàn)木質(zhì)藤本使幼苗存活率和株高生長(zhǎng)量分別降低了75%和300%;Schnitzer等(2005)將3個(gè)不同耐蔭程度的幼樹(shù)種植于木質(zhì)藤本密集的低地雨林,發(fā)現(xiàn)2 a后木質(zhì)藤本使樹(shù)木生物量分配格局和樹(shù)形均發(fā)生變化。由此可見(jiàn),木質(zhì)藤本對(duì)樹(shù)木的影響貫穿了樹(shù)木整個(gè)生活史。
4.2 改變?nèi)郝浣M成與結(jié)構(gòu)
從群落水平來(lái)看,木質(zhì)藤本過(guò)度增長(zhǎng)會(huì)從以下兩個(gè)方面改變?nèi)郝浣M成與結(jié)構(gòu)。(1)改變?nèi)郝湮锓N組成。木質(zhì)藤本對(duì)不同樹(shù)種影響的差異性易使群落樹(shù)種組成發(fā)生改變(Visser et al., 2018; Reis et al., 2020)。例如,在加納熱帶常綠落葉混交林內(nèi)木質(zhì)藤本使先鋒樹(shù)種Nauclea diderichii和Khaya anthotheca的生物量分別下降了32%和50%,而對(duì)非先鋒耐蔭樹(shù)種Garcinia kola并無(wú)影響(Toledo-Aceves & Swaine, 2008)。(2)阻礙群落更新、降低物種多樣性和改變?nèi)郝浣Y(jié)構(gòu)。木質(zhì)藤本會(huì)降低群落樹(shù)種的生殖力,使樹(shù)木開(kāi)花結(jié)實(shí)的植株數(shù)、開(kāi)花量和結(jié)果量顯著下降(García León et al., 2018);Schnitzer和Carson(2010)發(fā)現(xiàn)木質(zhì)藤本使林窗內(nèi)樹(shù)木的更新和多樣性分別下降了46%和65%。木質(zhì)藤本對(duì)不同胸徑樹(shù)木影響的差異性也會(huì)導(dǎo)致群落結(jié)構(gòu)發(fā)生變化(Estrada-Villegas et al., 2020)。
4.3 影響生態(tài)系統(tǒng)功能
從生態(tài)系統(tǒng)層面來(lái)看,木質(zhì)藤本過(guò)度增長(zhǎng)會(huì)影響森林生態(tài)系統(tǒng)碳匯功能。在巴拿馬熱帶雨林,由于受木質(zhì)藤本的干擾,5 a間樹(shù)木生物量的增長(zhǎng)量降低22%,據(jù)模型預(yù)測(cè),30 a后樹(shù)木生物量可降低32%,60 a后將達(dá)到47%(Lai et al., 2017)。藤本清除實(shí)驗(yàn)驗(yàn)證了上述模型的合理性,在巴拿馬60 a的熱帶雨林內(nèi),木質(zhì)藤本使地上生物量的年凈累積量下降了76%(van der Heijden et al., 2015)。其他連續(xù)監(jiān)測(cè)、模型預(yù)測(cè)輔助研究和木質(zhì)藤本清除方式實(shí)驗(yàn)均得出相似結(jié)果(di Porcia e Brugnera et al., 2019; van der Heijden et al., 2019; Estrada-Villegas et al., 2020; Meunier et al., 2021a)。雖然木質(zhì)藤本生物量有所增長(zhǎng),但其增量遠(yuǎn)無(wú)法補(bǔ)償樹(shù)木生物量的損失量,對(duì)樹(shù)木生物量損失量的彌補(bǔ)不足30%(Schnitzer et al., 2016; Lai et al., 2017)。
木質(zhì)藤本過(guò)度增長(zhǎng)可改變森林生態(tài)系統(tǒng)水分動(dòng)態(tài)。雖然木質(zhì)藤本的胸?cái)嗝娣e僅占熱帶森林的5%,但葉蒸騰量占森林總量的12%(Restom & Nepstad, 2001)。此外,木質(zhì)藤本在水分吸收、運(yùn)輸和利用等過(guò)程比樹(shù)木具有優(yōu)勢(shì)(Andrade et al., 2005; Cai et al., 2009; Zhu & Cao, 2010; Chen et al., 2015)。木質(zhì)藤本也會(huì)影響樹(shù)木的水分平衡。例如,木質(zhì)藤本使樹(shù)木樹(shù)干液流速率下降近一半(Campanello et al., 2016)。
木質(zhì)藤本過(guò)度增長(zhǎng)亦能改變森林生態(tài)系統(tǒng)的礦質(zhì)養(yǎng)分動(dòng)態(tài)。較高的葉產(chǎn)量占比(Schnitzer & Bongers, 2002; Kusumoto & Enoki, 2008)和葉片性質(zhì)與樹(shù)木的差異(Cai & Bongers, 2007)能改變森林養(yǎng)分動(dòng)態(tài)和土壤理化性質(zhì)。熱帶森林木質(zhì)藤本葉生物量占地上葉總生物量比值高達(dá)36%(Tang et al.,2012),凋落物產(chǎn)量和葉面積占比均達(dá)到40%(Schnitzer & Bongers, 2002; Tang et al., 2012)。木質(zhì)藤本的養(yǎng)分含量(如氮、磷)顯著高于樹(shù)葉(Asner & Martin, 2012)。因此,木質(zhì)藤本的過(guò)度增長(zhǎng)將可能加速森林葉凋落物的分解和養(yǎng)分釋放、改變土壤的養(yǎng)分狀態(tài)與呼吸速率、增加土壤養(yǎng)分異質(zhì)性和養(yǎng)分流失的風(fēng)險(xiǎn)(Putz, 1984; Kusumoto & Enoki, 2008; Liu et al., 2017)。
5 研究展望
雖然越來(lái)越多的學(xué)者參與到木質(zhì)藤本對(duì)森林影響的研究,但是現(xiàn)階段對(duì)木質(zhì)藤本增長(zhǎng)的機(jī)制和評(píng)價(jià)認(rèn)識(shí)還有諸多不足,很多問(wèn)題尚未解決。結(jié)合上述內(nèi)容,本文認(rèn)為今后研究可從以下方面開(kāi)展。
5.1 重視森林動(dòng)態(tài)監(jiān)測(cè)樣地木質(zhì)藤本定位研究
長(zhǎng)期定位監(jiān)測(cè)研究是深入認(rèn)識(shí)種群與森林群落特征變化的重要手段??傮w上,木質(zhì)藤本的定位監(jiān)測(cè)研究起步較晚、研究區(qū)域有限、研究尺度較小。在森林動(dòng)態(tài)監(jiān)測(cè)樣地網(wǎng)絡(luò)中很多研究單位都未將木質(zhì)藤本納入監(jiān)測(cè)體系(Schnitzer & Bongers, 2011),現(xiàn)有主要研究地為中美洲和南美洲的巴西、巴拿馬和墨西哥等地的熱帶雨林,非洲少量分布,其他洲較少參與,亞熱帶、溫帶森林關(guān)于木質(zhì)藤本的研究十分缺乏(Schnitzer et al., 2015)。因此,今后森林動(dòng)態(tài)監(jiān)測(cè)樣地的相關(guān)研究應(yīng)將木質(zhì)藤本納入其中,尤其是要增加亞洲、非洲的研究點(diǎn),從全球尺度上認(rèn)識(shí)和解析木質(zhì)藤本的分布格局、動(dòng)態(tài)變化等問(wèn)題。另外,鑒于木質(zhì)藤本野外識(shí)別和數(shù)據(jù)采集難度較大,建議采用遙感信息技術(shù)開(kāi)展相關(guān)研究(Waite et al., 2019; Chandler et al., 2021)。
5.2 加強(qiáng)環(huán)境變化與木質(zhì)藤本數(shù)量動(dòng)態(tài)關(guān)系研究
現(xiàn)有研究從氣候變化和干擾的角度對(duì)木質(zhì)藤本的增長(zhǎng)原因進(jìn)行了初步探討,但研究都較為分散、獨(dú)立,環(huán)境變化導(dǎo)致木質(zhì)藤本增長(zhǎng)的發(fā)生機(jī)制尚不清楚。建議開(kāi)展以下研究:(1)各因子的單獨(dú)和協(xié)同作用對(duì)木質(zhì)藤本動(dòng)態(tài)變化的影響研究,尤其是大時(shí)空尺度的大氣CO2變化與木質(zhì)藤本數(shù)量變化的關(guān)系研究(Schnitzer & Bongers, 2011);(2)森林干擾使木質(zhì)藤本增長(zhǎng)的發(fā)生機(jī)制研究,應(yīng)將干擾狀況、生態(tài)因子與木質(zhì)藤本生物學(xué)、生理特性相結(jié)合分析木質(zhì)藤本更新和多樣性維持機(jī)制。同時(shí),應(yīng)加強(qiáng)不同森林干擾類型對(duì)木質(zhì)藤本數(shù)量變化影響的比較研究;(3)木質(zhì)藤本對(duì)環(huán)境變化的響應(yīng)研究,對(duì)變化環(huán)境的適應(yīng)和調(diào)整是木質(zhì)藤本是否具有競(jìng)爭(zhēng)優(yōu)勢(shì)的關(guān)鍵,應(yīng)從生理、行為、功能性狀等方面探討木質(zhì)藤本對(duì)環(huán)境變化適應(yīng)機(jī)理(Liu et al., 2021),結(jié)合控制實(shí)驗(yàn)?zāi)M多種環(huán)境因子(如光照、CO2、水分)對(duì)木質(zhì)藤本生長(zhǎng)、生理及行為等特征的影響。
5.3 構(gòu)建木質(zhì)藤本過(guò)度增長(zhǎng)的生態(tài)效應(yīng)評(píng)價(jià)體系
評(píng)價(jià)木質(zhì)藤本對(duì)森林的影響是制定管理政策的基礎(chǔ)。木質(zhì)藤本過(guò)度增長(zhǎng)在個(gè)體、種群、群落和生態(tài)系統(tǒng)層面均具有一定的負(fù)面影響,而相關(guān)研究仍然存在不足,對(duì)一些領(lǐng)域的深入認(rèn)識(shí)較為缺乏。(1)大時(shí)空尺度研究較少。木質(zhì)藤本的影響與森林類型、年齡等密切相關(guān)(Lai et al., 2017; Estrada-Villegas et al., 2021),現(xiàn)有研究多位于熱帶森林,其他區(qū)域開(kāi)展的相關(guān)研究不足且長(zhǎng)周期的研究較為缺乏。今后應(yīng)加強(qiáng)多區(qū)域、多種森林類型木質(zhì)藤本的影響評(píng)價(jià)研究,從大時(shí)空尺度認(rèn)識(shí)木質(zhì)藤本如何影響森林恢復(fù)。(2)對(duì)木質(zhì)藤本增加的生態(tài)效應(yīng)評(píng)價(jià)不夠全面,主要集中在木質(zhì)藤本對(duì)樹(shù)木個(gè)體的影響,評(píng)價(jià)指標(biāo)較為單一,多為樹(shù)木胸徑生長(zhǎng),而木質(zhì)藤本對(duì)森林生物量、生態(tài)系統(tǒng)物質(zhì)循環(huán)和生產(chǎn)力等方面的影響研究十分缺乏,建議借助模型構(gòu)建木質(zhì)藤本對(duì)生態(tài)系統(tǒng)的影響的全面認(rèn)識(shí)(di Porcia e Brugnera et al., 2019; Meunier et al., 2022)。在我國(guó)南方諸多自然保護(hù)區(qū)次生林內(nèi),木質(zhì)藤本過(guò)度增長(zhǎng)嚴(yán)重阻礙著森林的恢復(fù),建議開(kāi)展木質(zhì)藤本過(guò)度增長(zhǎng)對(duì)自然保護(hù)區(qū)生態(tài)系統(tǒng)功能的潛在影響研究。(3)木質(zhì)藤本與樹(shù)木的關(guān)系研究通常將木質(zhì)藤本和樹(shù)木作為統(tǒng)一的整體,然而,不同樹(shù)種受木質(zhì)藤本的影響(Visser et al., 2018)、不同木質(zhì)藤本對(duì)樹(shù)木施加的壓力都具有種間差異(Muller-Landau et al., 2018),因此難以準(zhǔn)確量化出木質(zhì)藤本對(duì)森林的影響。此外,樹(shù)木被木質(zhì)藤本干擾后的響應(yīng)、環(huán)境變化如何影響木質(zhì)藤本與樹(shù)木的關(guān)系等方面的研究仍較為缺乏。(4)木質(zhì)藤本與森林動(dòng)物的相互關(guān)系缺乏關(guān)注。動(dòng)物是森林生態(tài)系統(tǒng)的重要組成部分,雖然木質(zhì)藤本是森林動(dòng)物的重要食物來(lái)源(Odell et al., 2019),可增加動(dòng)物的多樣性(Schnitzer et al., 2020),但它對(duì)樹(shù)木的負(fù)面影響會(huì)降低動(dòng)物的食物來(lái)源從而影響動(dòng)物群落(Adams et al., 2016; García León et al., 2018)。因此,木質(zhì)藤本對(duì)動(dòng)物凈影響力有多大;木質(zhì)藤本的增加分別對(duì)哪些種、哪些類型的動(dòng)物有何影響;動(dòng)物對(duì)木質(zhì)藤本的取食又如何影響木質(zhì)藤本數(shù)量變化,以及二者間的相互作用關(guān)系是否因森林類型、所處區(qū)域而具有差異。今后研究都需對(duì)這些問(wèn)題進(jìn)行系統(tǒng)、定量的評(píng)價(jià)(Schnitzer, 2018; Coverdale et al., 2021)。
5.4 綜合木質(zhì)藤本的保護(hù)和藤本過(guò)度增長(zhǎng)的管控
木質(zhì)藤本過(guò)度增長(zhǎng)會(huì)造成諸多負(fù)面影響,而它們亦是森林物種多樣性組成部分,并且部分還屬于珍稀瀕危物種,一些還具有一定的經(jīng)濟(jì)和觀賞價(jià)值。由于有的木質(zhì)藤本與樹(shù)木具有互利共生的關(guān)系,因而在某些情況下這些木質(zhì)藤本有助于森林的恢復(fù)。大型木質(zhì)藤本的數(shù)量也是評(píng)價(jià)原始森林健康的指標(biāo)。因此,今后應(yīng)立足森林恢復(fù)和保護(hù),開(kāi)展木質(zhì)藤本保護(hù)與管控權(quán)衡的研究。在生態(tài)環(huán)境良好的區(qū)域應(yīng)對(duì)木質(zhì)藤本加以保護(hù),而受影響嚴(yán)重的森林應(yīng)對(duì)木質(zhì)藤本進(jìn)行適當(dāng)清除。然而,木質(zhì)藤本管控的新技術(shù)探索將是一大挑戰(zhàn),當(dāng)前對(duì)木質(zhì)藤本管理方式呈一刀切的特點(diǎn),費(fèi)力耗時(shí)、成本高且成效低(Pérez-Salicrup et al., 2001)。在哪里清除、如何清除、如何評(píng)價(jià)清除效應(yīng)等問(wèn)題都需深入研究,從而探索出合理的木質(zhì)藤本管控方法,為森林管理和生態(tài)恢復(fù)等方面的宏觀決策提供科學(xué)依據(jù)。
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(責(zé)任編輯 李 莉 王登惠)