王迎新,陳先江,婁珊寧,胡安,任勁飛,胡俊奇,張靜,侯扶江
(草地農(nóng)業(yè)生態(tài)系統(tǒng)國家重點實驗室,農(nóng)業(yè)部草牧業(yè)創(chuàng)新重點實驗室,蘭州大學(xué)草地農(nóng)業(yè)科技學(xué)院,甘肅 蘭州730020)
草原灌叢化(woody-plant encroachment in grassland)是全球性問題。過去150多年全球草原最主要的變化之一,就是木本植物擴張[1]。全球草原區(qū)域約占陸地總面積的41%,其中有10%~20%的地區(qū)發(fā)生了灌叢化,且大多以放牧業(yè)為主[2]。在北美,灌叢化的非林地面積約占3.3×108hm2,每年以0.2%~0.5%的速度擴張[3]。在南非,約有1.3×107hm2稀樹草原發(fā)生了灌叢化[4]。在澳洲,Mesquite植物以大約每年0.4%~1.2%的速度入侵[5]。在中國,草原灌叢化現(xiàn)象也有大量報道[6-7],其中,內(nèi)蒙古草原的小葉錦雞兒(Caraganamicrophylla)灌叢化現(xiàn)象最為典型,約有5.1×106hm2的草原出現(xiàn)了灌叢化[6]。青藏高原東南麓,從1990年到2009年至少有39%的高山草甸已被灌叢草地取代[8]。
草原灌叢化一直受到各國科學(xué)家的高度關(guān)注,在國際生態(tài)學(xué)頂級期刊闡述了其重要研究進展(圖1),其中,Sala等[1]組織發(fā)表在《Journal of Ecology》上的灌木入侵特刊格外引人注目。研究認(rèn)為干旱以及降水格局改變[12,14,19]、放牧[17,20]、火燒[9,13,21]、溫室氣體增加[10,18]等均可導(dǎo)致灌木入侵。灌叢化打破了草原生態(tài)系統(tǒng)的穩(wěn)定,影響生態(tài)系統(tǒng)的功能和服務(wù)[11,15-16,22-23]。本研究緊扣草原灌木化的過程、機制、效應(yīng)及其防控等關(guān)鍵問題,總結(jié)了國內(nèi)外學(xué)者對這一問題的最新研究動態(tài),旨在闡明草原群落演替機制,為我國的草原灌木入侵研究和防控提供理論借鑒。
圖1 灌木入侵的重要研究-問題及分布Fig.1 Key questions and distribution of selected important studies related to woody-plant encroachment
灌木入侵途徑包括有意識引進入侵和自然入侵2類。其中,有意引進入侵是指人類出于食用、觀賞、飼料、引種、環(huán)境治理等目的在草原上種植灌木種, 該灌木在新的生態(tài)環(huán)境內(nèi)生長,定殖并迅速擴散而成為優(yōu)勢種群,對該生物境內(nèi)的其他種群構(gòu)成生存威脅。自然入侵指生物靠自身的擴散傳播能力或借助于自然力(如鳥類、哺乳動物、螞蟻和風(fēng))入侵[24]。
草原灌木入侵過程分為傳播到達(dispersal)、定居建群(colonizing and establishing)和擴散入侵(diffusing)(圖2):Ⅰ) 傳播到達階段:外來灌木在自然因子和人類活動的影響下到達其自然演化區(qū)域以外的地區(qū),并成活。Ⅱ) 定居建群階段:外來灌木在自然狀態(tài)下通過與當(dāng)?shù)厣鷳B(tài)因子相互作用實現(xiàn)定居和建群。Ⅲ) 擴散入侵階段:外來灌木通過自身繁殖和當(dāng)?shù)匚锓N的競爭等,不斷擴大分布區(qū)[25-26]。
圖2 草原木本植物入侵過程Fig.2 Stages of woody-plant encroachment
木本植物在新的環(huán)境中入侵成功,機制非常復(fù)雜,與植物自身屬性、功能(圖3)和外界環(huán)境(降水格局改變、溫室氣體增加、氣溫升高、放牧以及火燒等)息息相關(guān)。
圖3 木本植物入侵機制Fig.3 Mechanisms of woody-plant encroachment
木本較草本植物根深,耐旱且水分利用方式及對水分的需求程度不同。Darrouzet-Nardi等[27]在Sierra Nevada山地草甸研究發(fā)現(xiàn)大部分的灌木更多利用深層土壤水分,但也會利用 10%~30%的淺層土壤水分(<30 cm),而依賴淺層土壤水分的草本則因降水量減少而逐漸衰落。降水強度的增加可以促進深層次土壤水分的儲存,促進木本植物的生長和分布[28-30]。相反,研究發(fā)現(xiàn)山地草甸在干旱脅迫作用下,草本蓋度因灌木去除顯著增加,隨著土壤水分增加呈線性增加趨勢[31]。另外,在極端降水天氣下,地表徑流常常會導(dǎo)致土壤表層N和其他營養(yǎng)元素的淋洗,從而更有利于深根性的木本植物生長[32]。降水波動(precipitation variability)可以提高木本植物種間競爭、定居能力及豐富度,引發(fā)入侵[26]。Holmgren等[33]分析遙感數(shù)據(jù)發(fā)現(xiàn),在全球尺度下,降水波動對木本蓋度的影響因區(qū)域各異,南美顯著增加,澳洲顯著降低。Heisler等[11]通過北美6年的模擬實驗研究得出,降水波動顯著影響生態(tài)系統(tǒng)初級生產(chǎn)力,多年生草本下降了81%,木本增加了67%。究其原因,草本植物的根系分布和吸水能力顯著低于木本植物,而木本植物根系在濕季能儲存足夠的水分以備旱季所需。
灌木的生長受到低溫的限制。氣候變化導(dǎo)致低溫升高、凍害減少,降低灌木的死亡率,為灌木的擴張和定居創(chuàng)造了條件。北美洲西南部荒漠的典型植物L(fēng)arreatridentata能耐受的極端低溫為-20~-18 ℃左右[34];南非稀樹草原也發(fā)現(xiàn)氣候變暖降低了對極端低溫敏感的灌木死亡率[35]。氣候變暖會加速極地土壤C、N循環(huán)以及凋落物分解, 進而刺激微生物活動并促進苔原灌叢化的發(fā)生[36]。與此類似,氣候變暖也會導(dǎo)致高寒地區(qū)灌木向高海拔入侵。如西班牙中部山脈受氣候變暖的影響(最低溫和最高溫均升高),原本生長在低海拔地區(qū)的刺柏屬(Juniperus)灌木植物逐漸取代以羊茅屬(Festuca)植物占優(yōu)勢的草本植物群落[37-38]。
另外,CO2濃度增加也可促進木本入侵。木本是C3植物,CO2可以通過改變水分利用效率、光合速率以及光合養(yǎng)分的利用率來影響植被生長[39]。大氣中CO2濃度升高會增加灌木胞間CO2濃度與大氣中CO2濃度梯度,增強灌木的水分利用效率,使得灌木比草本的競爭力更強[40]。
放牧家畜選擇性采食削減了草本植物的競爭力[41],踐踏和排泄物改變了土壤碳庫、氮庫及分配[42]。過度放牧,尤其是長期過度放牧后,減低牧壓甚至禁牧的情況更利于木本植物的建立[43]。Christensen等[44]模擬了14種不同放牧率下植被動態(tài),結(jié)果表明過度放牧增加木本植物的分布,取代草本植物成為優(yōu)勢種,而適度放牧可以促進草本生長和營養(yǎng)物質(zhì)循環(huán)。Allred等[45]也發(fā)現(xiàn)放牧降低了北美草原多年生禾草的組分,重牧下的木本植物的蓋度高于輕牧和對照。牲畜可以作為木本種子擴散的載體。在澳洲,綿羊和有袋動物是桉樹(Eucalyptusspp.)最主要的種子擴散載體[46];在北美,家畜傳播扁擔(dān)桿屬(Grewia)灌木種子的距離遠遠超過自然因素的限制[47];在非洲,捻角羚(Tragelaphusstrepciseros)以采食-排泄的方式來擴散濱藜屬(Atriplex)植物的種子[48]。
火燒(fire regimes)包括火燒頻率、強度、大小及其時空組合,對維持群落 “草本-木本”的平衡起到重要的作用[49]。木本植物耐火性好,木質(zhì)素豐富且分蘗點較高[50],而草本植物短期恢復(fù)能力強。輕度、低頻火燒對木本植物損傷較小,卻降低了作為燃料的草本植物及其枯落物的生物量,有利于木本的生長[51];而重度、高頻的火燒對木本和草本都會造成損害,木本植物生長周期長,恢復(fù)速率低,在一定程度上限制了木本的擴張[52]。另外,火燒產(chǎn)生的次生物質(zhì)(如氰化物)促進灌木種子的萌發(fā)[53],過火后土壤微生物的群落結(jié)構(gòu)和營養(yǎng)食物網(wǎng)的改變,有利于入侵植物競爭資源[54]。
各因素與植被具有復(fù)雜的相互作用,降水量決定植被類型和土壤屬性,土壤屬性可以被火燒改變,而火燒與放牧的相互作用可以決定植被的變化,植被的變化轉(zhuǎn)而會影響火燒和放牧機制[55-57]。通過利用Lefkovitch階段分類矩陣模型研究美國San Carlos東部灌木石南茄(Fabianaimbricata)在火燒和降水共同作用下的種群動態(tài)表明,夏季火燒和春季豐富的降水最有利于石南茄種群擴展,并通過模型預(yù)測,石南茄種群在火燒頻率為每4年1次到每100年1次的范圍內(nèi)都能增大,在火燒為每6~10年1次的頻率下,石南茄種群的增長率最大[58]。Ratajczak等[59]在美國Kansas東北部生態(tài)試驗站,利用當(dāng)?shù)夭煌攴莸姆拍谅室约盎馃l率數(shù)據(jù)進行研究分析。研究表明,在每年1次的火燒頻率下,放牧地灌木蓋度有小幅度增加;在火燒頻率為每年1~3次的放牧地,在前17年(1980-1997年)中,灌木蓋度以每年小于0.5%的幅度緩慢增加,1998-2001年灌木蓋度以每年5%~10%的幅度增加,2001-2012年灌木蓋度以每年1.4%~2.7%的幅度增加。
灌木通過改變土壤結(jié)構(gòu)、微生物的生物量、土壤濕度和小氣候,并將有機物質(zhì)集中于其樹冠層之下,從而對養(yǎng)分的空間分布和循環(huán)產(chǎn)生影響,這就是所謂的“沃島”效應(yīng)[60-61](圖4)。首先,氣候變化、過度放牧等會破壞草原植被的均一性,致使部分地表裸露。裸露的地表經(jīng)受高溫和高蒸發(fā)的影響,阻止或延遲了有機氮的吸收、反硝化以及氨化等一系列過程,導(dǎo)致養(yǎng)分流失[62]。風(fēng)和水?dāng)y帶的富含養(yǎng)分的土壤、碎屑以及種子會聚集在灌木植物冠層下,提高冠層下方土壤的滲透能力和肥力[63]。這些效應(yīng)綜合在一起,進一步加強土壤資源在灌木植物冠層下的聚集,使灌叢對環(huán)境擾動具有更強的抵抗力[64-65]。其次,灌木植物從周圍吸收養(yǎng)分,通過自身凋落物分解,構(gòu)成一個養(yǎng)分庫,增加了灌叢的自持能力[66]。最后,土壤微生物和土壤動物在灌叢下的活動較為強烈,進一步促進了灌叢下養(yǎng)分循環(huán)。而草本植物生物量的降低,進一步加強了灌木植物的定居及穩(wěn)定發(fā)展[67]。Myers-Smith等[68]在澳大利亞灌叢化草地中的觀測發(fā)現(xiàn),與草地斑塊相比,灌叢斑塊灌木冠層下方養(yǎng)分循環(huán)和水分入滲率指數(shù)都較大。Parizek等[69]得到巴塔哥尼亞東北部的灌叢化草地中灌叢斑塊土壤容重低于草地斑塊,土壤水分入滲速率和土壤有機質(zhì)含量高于草地斑塊,灌叢化草原中草地斑塊的土壤侵蝕程度增加。Lett等[70]對北美普列里高草草原(tallgrass prairie)群落進行研究指出灌木入侵后草原的碳庫和氮庫儲量高于入侵之前。Li等[71]比較了中國北方科爾沁地區(qū)小葉錦雞兒灌叢斑塊及其周圍草地斑塊土壤理化性質(zhì),認(rèn)為灌叢斑塊冠層下方土壤容重較低,土壤有機碳和全氮含量比草地斑塊分別高23%~32%和14%~27%,灌叢斑塊有更強的土壤持水能力。
圖4 沃島效應(yīng)Fig.4 The capture of water and nutrients by fertile bush mounds
斑塊尺度上,灌叢自身以及冠層下草本的生產(chǎn)力增加,物種組成改變。灌木入侵帶來了新的生命形式,新的物候類型(如侵入美國西部灌叢大草原的Bromustectorum),新的方式攝取資源(如夏威夷島的固氮植物Myricafaya),新的演替生態(tài)位。同時,木本植物的適口性、植株生理特征等因素增加了動物的采食難度,為其冠層下方的其他植物提供一定的庇護。加利福尼亞草原灌叢化過程中,地上、地下生物量呈增加趨勢,灌叢斑塊更容易維持較高的生物量[72]。內(nèi)蒙古典型草原在小葉錦雞兒入侵后,多年生根莖禾草、多年生叢生禾草與多年生雜類草有所增加,而其他一年生的植物則逐漸減少,改變了草地的物種更替[7]。
群落尺度上,在多因素(自然,人為因素)的干擾下,木本入侵后,草原生產(chǎn)力和多樣性的變化具有不確定性。美國弗吉尼亞的灌木林區(qū)葉面積指數(shù)達到峰值時,灌木下層接受的光照僅為上層的0.7%,草本生產(chǎn)力下降[73]。Bekele等[74]得出在木本植物密度(583.2±79.3) 樹·hm-2下草本植物生物量為(2625±300) kg·hm-2,而當(dāng)木本植物密度為(2827±276) 樹·hm-2時,草本植物的生物量只有(1176±110) kg·hm-2。近100年來,美國新墨西哥州的草原灌叢化過程中,物種豐富度呈下降趨勢;在短時間尺度內(nèi),與草本植物占優(yōu)勢的群落相比,灌叢化區(qū)域的群落穩(wěn)定性較低[3]。
木本化的出現(xiàn),改變動物賴以生存的生境和資源,影響動物的生產(chǎn)力和習(xí)性。高山苔原灌木入侵后,北極地松鼠(Spermophilusparryii)覓食策略發(fā)生改變,覓食成本增加。非洲喀拉哈里牧場南部熱帶稀樹草原灌叢化過程中,哺乳動物的數(shù)量隨灌木蓋度的不同而不同,灌叢化對非洲野貓(Felissilvestrislybica)、條紋臭鼬(Mephitismephitis)、好望角狐(Otocyonmegalotis)、灰沼貍(Suricatasuricatta)產(chǎn)生負影響,對黃色貓鼬的影響呈駝峰形曲線狀態(tài)[75]。木本蓋度的增加可以顯著增加山羊(Capraaegagrushircus)的采食量和生產(chǎn)性能[76]。Anadón等[19]系統(tǒng)研究了南北美洲草原木本植物入侵對家畜生產(chǎn)力的影響,發(fā)現(xiàn)在高生產(chǎn)力草原,增加1%的木本植物蓋度會降低2.5%的家畜生產(chǎn)力,而在低生產(chǎn)力草原,木本植物蓋度增加反而對家畜生產(chǎn)力有積極作用。
人類是生態(tài)系統(tǒng)的設(shè)計者,管理者和受益者。木本植物入侵三階段中,Ⅰ→Ⅱ演替過程可以逆轉(zhuǎn),Ⅱ→Ⅲ過程難以逆轉(zhuǎn)。人類需重新思考草原“草本-木本”轉(zhuǎn)化的邏輯,變輸血為造血,變對抗為兼容。
合理放牧和火燒可以驅(qū)動Ⅱ→Ⅰ過程的發(fā)生。一些家畜(如山羊偏好采食灌木的葉,花或種子直接降低了灌木的生物量和種子擴散的機會?;馃芍苯託⑺乐参铮荼局参镌诙唐趦?nèi)恢復(fù)能力更強,一定程度上抑制了木本的擴張。Roques等[9]在非洲南部的Swaziland地區(qū),分別于1947,1971,1979,1990年,利用航空照片對該地薩王納草原103個固定地點進行分層抽樣,結(jié)合當(dāng)?shù)鼗馂?zāi)頻率、放牧強度、降水量等數(shù)據(jù)進行研究分析后發(fā)現(xiàn),頻繁火災(zāi)、低強度的放牧以及干旱,可以減少高大灌木的密度。在澳州北領(lǐng)地,土著居民用火燒管理、更新灌木草原的辦法經(jīng)久不衰[77]。在北美高原草原,大約每3~5年的火燒間隔有利于生態(tài)系統(tǒng)健康發(fā)展[78]。在我國內(nèi)蒙古錫林河流域,1~2年的火燒頻次明顯抑制了小葉錦雞兒灌叢的發(fā)展[6]。
一旦Ⅱ→Ⅲ過程已發(fā)生,根除和控制灌木發(fā)展就會非常困難。根除和控制已入侵灌木的方法主要有機械法(適用于種群數(shù)量小的灌木,包括拔除、砍倒、火燒、水淹、光照和遮陰等);化學(xué)法(如使用專一性除草劑);生物防治法(利用入侵灌木的天敵控制其種群密度和擴展速度);綜合管理系統(tǒng)(integrated brush management)[25]。
草原灌叢化已經(jīng)成為全球性熱點問題。灌木入侵打破了草原生態(tài)系統(tǒng)的穩(wěn)定,影響生態(tài)系統(tǒng)的功能和服務(wù)。因此, 研究木本植物的入侵格局、機制、規(guī)律和趨勢, 并探索防治、管理的方法是面臨的長期而艱巨的任務(wù)。首先,盡管已從水分、溫度、土壤、生物以及人為活動等自然與人為因素綜合作用角度對草原灌叢化的成因進行了深入分析,對草地灌叢化的過程和機制有了較為深入的認(rèn)識,但這些研究結(jié)果多基于對自然過程的觀察和分析,缺乏基于這些過程的大尺度、長時間野外控制實驗證據(jù)的支持。其次,由于對草地灌叢化過程和機制的理解仍不完善,導(dǎo)致灌叢化草地能否恢復(fù),在哪種條件下可以恢復(fù)的全球性認(rèn)識不完全、不一致。第三,灌叢化是否會造成生態(tài)系統(tǒng)的退化仍然沒有定論。Eldridge等[4]基于全球尺度的分析,認(rèn)為灌叢化并未在全球尺度上造成普遍性的生態(tài)環(huán)境退化。最后,自然和人工方法及其組合,哪種控制草地灌叢化的方法更為高效仍無定論。因此,未來灌木入侵研究應(yīng)在幾個方面推進:1)全球尺度長期控制實驗的實施與數(shù)據(jù)整合分析;2)入侵過程和機制需進一步明確。例如,如何綜合多重因素建立木本植物入侵的生態(tài)學(xué)框架等;3)控制管理措施科學(xué)有效,更系統(tǒng)化。尤其是灌木入侵后,應(yīng)該以自下而上的視角,復(fù)雜的思維方式,生態(tài)文明的視野,復(fù)合生態(tài)倫理機制,尋找木本植物與草原生態(tài)系統(tǒng)各組分的共存之道;4)灌木入侵與動物生產(chǎn)、人類健康的關(guān)系。
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