高山林線變化的更新受限機制研究進展

沈 維,張 林,羅天祥

中國科學院青藏高原研究所,高寒生態與生物多樣性重點實驗室, 北京 100101

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高山林線變化的更新受限機制研究進展

沈 維*,張 林,羅天祥

中國科學院青藏高原研究所,高寒生態與生物多樣性重點實驗室, 北京 100101

全球林線位置對氣候變暖的響應表現為上升、無變化或下降等截然不同趨勢,表明影響林線位置及動態的因子十分復雜,除了較普遍認為的低溫調控機制外,還存在其它控制林線位置變化的機制。林線向上遷移開始于種子向林線以上的傳播及幼苗在林線以上的定居,這些過程中的限制因子均會影響林線的位移,因此研究更新過程及其限制因子對理解高山林線對氣候變化的響應具有重要的科學意義。主要從種子和幼苗兩個關鍵階段綜述高山林線森林更新的研究進展。在種子階段,夏季積溫不足導致種子產量和活力下降,風速過低和濃密灌叢限制種子向林線以上傳播,近地表的霜凍/水分脅迫和灌木釋放的化感物質會阻礙種子在林線以上萌發。在幼苗階段,除冬季低溫外,生長季內較大的溫度日振幅和偶然出現的凍害事件也是導致幼苗死亡的重要原因,而低溫環境下的強烈光照引起的低溫光抑制會顯著降低生長季的光合作用；土壤低溫、由土壤溫度晝夜變化引起的凍舉事件、夏季土壤干旱可能會導致幼苗光合作用下降和死亡率上升；積雪太淺會導致生長季早期幼苗水分供應的嚴重缺乏,但積雪太深會導致幼苗感染真菌的可能性增加；濃密的灌木和草本植物以及植食動物的啃食也會降低林線以上的幼苗存活率。氣候變暖對林線幼苗定居的影響復雜且具有很大不確定性,需要進一步研究氣候變暖導致的環境因子變化對林線更新各關鍵階段的影響。未來氣候變暖無疑會導致生長季起始日提前,結束日推遲,這很可能會增加生長季期間尤其是早期的低溫凍害事件,對高山林線樹種幼苗的存活具有重要影響。在未來研究中,需要找出定義生長季凍害事件的溫度閾值,利用長期氣象觀測數據分析增溫背景下生長季早期凍害事件特征的變化趨勢,并進一步開展野外模擬增溫實驗以深刻理解林線樹種的種子萌發和幼苗定居與生長季凍害事件的關系,加強對不同地區林線樹種的繁殖策略研究,這將有助于人們進一步理解不同區域林線的形成機制并預測未來氣候變化條件下林線的動態變化趨勢。

氣候變化；生長季凍害事件；種子萌發；幼苗定居；林線動態

高山林線過渡帶(alpine treeline ecotone)是指從山地郁閉森林到樹種線之間的生態過渡帶[1]。通常認為低溫是高海拔地區樹木生長的普遍限制因子,因此理論上高山林線應隨氣候變暖而上升[1-2]。然而,全球林線位置對氣候變暖的響應表現為上升、無變化或下降等截然不同的趨勢[3-8],表明影響林線位置及動態的因子十分復雜,除較普遍認為的低溫調控機制外,還存在其它控制林線位置變化的機制。過去對高山林線形成機理的研究多集中在低溫與樹木生長限制之間的關系,提出了霜凍脅迫、風雪機械干擾、碳受限、生長受限等眾多假說[2,9],而繁殖更新障礙作為林線形成機制的假說之一并未得到足夠重視[10- 11]。高山林線嚴酷的環境可能并不足以導致樹木死亡或生長受阻,卻能強烈地限制種子萌發和幼苗定居等森林更新過程[12- 14]。2003年,Smith等以“Another perspective on altitudinal limits of alpine timberlines”為題對高山林線幼苗定居的生態促進機制進行了綜述,提出林線是否向高海拔和高緯度遷移主要取決于幼苗的成功定居和后續的存活[15]。此后,越來越多的研究開始關注森林更新對林線位置及其動態的影響[16- 22]。林線向上遷移開始于種子向林線以上的傳播及幼苗在林線以上的定居,這些過程中的限制因子均會影響林線的位移。氣候變暖對林線環境因子的影響十分復雜,既可能緩解也可能加劇對林線更新的限制。升溫對更新的有利影響(如提高種子產量和活力,促進幼苗光合生長等)可能被其它環境限制因子所掩蓋。因此,研究森林更新過程及限制因子對理解高山林線的形成及其對氣候變化的響應具有重要意義。本文主要從種子和幼苗兩個關鍵階段綜述高山林線森林更新的研究進展,并在此基礎上對未來研究方向進行展望。

1 高山林線地區種子的形成、傳播和萌發

種子形成是森林更新過程的開始,對更新能否成功具有極其重要的意義[23]。通常認為,高山林線地區的樹木產生種子的間隔較長,如在高海拔地區恩格曼云杉(Piceaengelmannii)和毛果冷杉(Abieslasiocarpa)每3—6a、瑞士石松(Pinuscembra)每7—10a才出現1個種子豐年[24]；且林線的種子產量和質量均低于低海拔地區,如Sveinbj?rnsson等和Cuevas對瑞典和智利的高山林線種子雨調查發現,種子數量、質量和活力都隨海拔升高而降低[25- 26]。這主要是由于種子成熟與夏季積溫密切相關,林線地區較短的生長季和較低的夏季溫度導致產生有效種子的積溫不足[27- 28]。因此,種子的形成可能是高山林線森林更新的限制因子之一。

種子傳播距離決定了該物種能否維持或擴大其分布范圍,進而達到延續物種的根本目的,因此種子的傳播距離也可能是影響林線森林更新的重要因子。種子傳播距離很大程度上取決于傳播方式和環境條件。風在林線種子傳播過程中具有重要作用,因為大多數林線針葉樹的種子以風媒傳播為主,其種子傳播距離主要取決于風速、風向、種子質量、種翅大小及種子散布時所在高度等[29]。其中,外在的環境條件相對更重要,如與種子重量相比,風速對種子傳播距離的影響更大[30]。大多數種子的傳播距離只有樹高的幾倍,而強風和暴風雨可能將種子帶到幾十公里以外[23,30- 31]。此外,高山地區的地形和植被可能會阻止種子向林線以上傳播。如澳大利亞Snowy Mountains林線位置穩定的原因可能是種子難以克服重力向陡峭的山坡上傳播[32]。在厄瓜多爾北部的高山林線,幼苗密度隨距林緣的距離增加而顯著下降,而移植幼苗的存活率并沒有隨距離增加而下降,其原因可能主要是濃密的灌叢限制了種子向林線以上的傳播[33]。

傳播到林線以上的種子必須在適宜的生境中才能萌發[17]。林線以上的嚴酷環境如近地表的霜凍事件及水分脅迫[34],較厚的凋落物層[35- 36]以及灌木釋放的化感物質[37- 41]等,均有可能阻礙林線樹種的種子萌發。此外,種子萌發還與其自身特性有關,如冷杉屬植物的種子通常處于不同程度的生理性休眠狀態,需要一定時間的低溫層積處理才能解除休眠[42- 44]。光照對林線種子萌發也有一定的促進作用。Li 等的研究發現,光照顯著提高了未經層積處理的巨冷杉(Abiesgrandis)種子的萌發率,但對經過低溫層積的種子萌發率沒有影響,因此郁閉林冠下極低的光照水平可能會推遲種子的萌發,但未必會降低最終萌發率[45]。

2 影響高山林線樹種幼苗定居的主要因子

幼苗定居是林線向上遷移和亞高山森林擴展的必經階段[15]。然而,在樹木生活史中,幼苗生長的早期階段死亡率最高[46- 47]。自然定居的幼苗,第1年死亡率就大于60%,特別是在開闊生境中死亡率可高達90%以上[48]。在林線進行的人工播種實驗中,也只有不到20%的幼苗能存活到第2年[49]。因此,幼苗的低存活率成為林線以上森林更新受限的主要原因之一。在高山林線地區,影響幼苗定居的非生物因子主要包括溫度、光照、土壤和積雪等,生物因子包括植被、菌根和植食動物等。

2.1 影響高山林線樹種幼苗定居的非生物因子

2.1.1 溫度

低溫是林線樹種幼苗存活和生長的重要限制因子[9]。冬季頻繁的凍融事件引起的木質部栓塞和霜凍干旱是導致幼苗死亡的主要原因之一[40,50]。除導致幼苗死亡外,低溫對幼苗的生理過程及生長也有著極其不利的影響,包括導致幼苗針葉發育受阻并出現物理性損傷,限制幼苗的光合作用,從而影響其碳獲取能力及高生長,并提高幼苗對其它環境限制因子的敏感性[47,51- 53]。Tranquillini發現歐洲云杉(Piceaabies)的針葉需要至少50d連續無霜凍來避免生長過程中的損傷,至少90d大于-3℃發育足夠的表皮來抵抗冬季干旱[54]。

此外,生長季內較大的晝夜溫差和偶然出現的極端溫度事件也會導致林線樹種幼苗的死亡。在溫室實驗及林線野外觀測中均發現,云杉和冷杉的幼苗在溫度日振幅最大時死亡率最高[49,55-56]。與較低的日平均氣溫相比,極端低溫事件對林線幼苗的傷害可能更大。低于0 ℃的凍害事件通常隨海拔升高而增加[57- 59]。高海拔地區的空氣更加潔凈且密度較小,從而導致在靜風、晴朗且干燥的夜間輻射冷卻效應更強[58],因此生長季各階段均有可能出現凍害事件[60- 61]。特別是在生長季早期,凍害事件對高山林線植物存活的影響更大,因為在該時期內大部分植物在-1.8℃以下就會發生凍結[62]。而與成年樹木相比,幼苗受凍害事件的影響更大,這主要是由于夜間風速的下降使得冷空氣在地表附近聚集,導致生長在地表的低矮幼苗遭遇更多的凍害事件[63],以及林下幼苗的發芽及展葉時間都要早于成年樹木,也會導致其暴露在生長季早期低溫環境下的可能性更高[64]。因此,生長季早期不可預測的凍害事件對林線樹種幼苗的存活有著極其重要的影響[65- 67]。藏東南色季拉山的對坡移植實驗顯示,生長季早期的凍害事件是導致林線急尖長苞冷杉(Abiesgeorgeivar.smithii)幼苗死亡的主要原因[68]。

2.1.2 光照

生長在高海拔地區的植物通常會在形態和生物化學等方面對強光產生一定的適應特征,如通過增加葉傾角和葉厚度以及葉片中的花青素含量來適應強光環境[69- 72]。然而,與多年生草本相比,林線針葉樹種的幼苗通常缺乏躲避強光的結構[73]。在熱帶地區的高山林線,過強的太陽輻射對幼苗定居的影響可能比低溫更重要[33]。因此,強光也是林線樹種幼苗定居的重要限制因子之一[48,53,74- 76]。強光會加劇低溫和水分脅迫對幼苗的影響[49],低溫環境下的強光引起的低溫光抑制會顯著降低生長季的光合作用[72,77-78]。例如Germino 和 Smith發現夜間霜凍導致冷杉幼苗的同化速率下降40%,強光導致下降22%,而這兩者聯合導致下降90%[47]。而不同物種對低溫光抑制的敏感性存在差異,如與恩格曼云杉(Piceaengelmannii)相比,毛果冷杉(Abieslasiocarpa)幼苗的光合作用受低溫光抑制的影響更大[47,73]。

2.1.3 土壤

土壤溫度、含水量和質地也會影響林線樹種幼苗的存活。有研究顯示,根生長所需的最低溫度為4—6℃,土壤溫度低于6℃會強烈抑制根的生長[79],而幼苗對土壤低溫更加敏感[80]。Karlsson 和Nordell發現,樺樹幼苗的氮攝取主要取決于土壤溫度[81],而當年幼苗的氮攝取與其冬季存活率密切相關[82]。Karlsson和Weih進一步發現林線地區的土壤低溫導致當年幼苗難以渡過首個冬季[83]。除土壤低溫的不利影響外,林線幼苗還可能受到由土壤溫度晝夜變化引起的凍舉事件的威脅。在高海拔地區的無積雪季節內,表層土壤通常會隨晝夜節律出現凍結和解凍現象,由此導致的凍舉事件能將表層土壤抬高數厘米,將幼苗連根拔起,且這一現象在生長季早期和晚期最為頻繁,更易對幼苗造成致命傷害[84]。適宜的土壤含水量對幼苗存活也十分重要[85],夏季土壤干旱可能會導致幼苗光合作用下降和死亡率上升[26,48,53],但過度潮濕的土壤也不利于幼苗存活[86]。質地細膩的土壤通過減緩根生長和降低水分有效性來阻止幼苗定居[87]。此外,質地細膩、潮濕且營養豐富的土壤能促進草本植物的生長,從而加劇其與林線樹種幼苗的競爭[88- 89]。

2.1.4 積雪

適宜的積雪深度對林線樹種幼苗定居非常重要。H?ttenschwiler 和 Smith認為積雪深度與幼苗密度密切相關,積雪太淺或太深幼苗均無法存活,主要表現在樹島向風側由于積雪過少幾乎沒有幼苗存在,而背風側積雪深度為0.5—1.5 m之間的位置則幼苗密度最大[90]。雪被可以有效防止幼苗暴露在極端低溫條件下[91],防止冬季干旱及其造成的養分缺乏,也可以避免強光對植物休眠組織的傷害[92]。Hu等在北美西部山區的研究發現,持續的冬季變暖導致積雪減少,進而引起生長季早期植物水分供應的嚴重缺乏[93]。然而,積雪太深也不利于幼苗存活,除導致生長季縮短外,長期積雪覆蓋為真菌提供了適宜的生存條件,從而導致的雪霉病也可能會引起幼苗死亡[94]。

2.2 影響高山林線樹種幼苗定居的生物因子

2.2.1 植被

樹島能夠改善高山地區惡劣的氣候條件(如霜凍、干旱、強光和強風等),提供適宜的微環境,從而有利于幼苗在林線以上的存活[15,47,49]。此外,樹島也能改變土壤的理化特征,并為幼苗提供菌根來源[95- 97]。因此,在樹島周圍幼苗和幼樹的出現頻率最高,距樹島近的幼苗存活率較高[49,90]。與樹島相似,高山矮曲林也能夠改變微環境條件,促進林線幼苗定居[40,98- 99]。高山矮曲林能有效地降低風速,且能在其順風側積累更厚的雪蓋,因此在高山矮曲林順風側幼苗存活率較高[49,90,100]。

灌叢對林線幼苗存活的影響在不同研究中存在差異。有些研究發現,灌叢下的凋落物層和化感物質等不利于幼苗存活[40,101],如Liang等對青藏高原東部14個云、冷杉林線的研究發現,林線以上的濃密灌叢抑制了幼苗定居,從而減緩了氣候變暖條件下林線的上升[102]。而另一些研究則發現,灌叢降低了光照強度并提高土壤濕度,從而有利于幼苗定居[71,103- 106]。灌叢對幼苗定居的影響在一定程度上取決于太陽輻射強度和林線樹種的耐蔭性。在太陽輻射強度較高的熱帶林線,林線以上的灌叢能為幼苗提供遮蔭,從而提高幼苗存活率[33]；而對于光合能力較高的光皮樺(Betulalitwinowii),灌叢的遮蔭反而不利于其幼苗的生長和存活[103]。

草本植物對林線樹種幼苗定居的影響主要取決于土壤水分條件。在土壤水分比較充足的林線,草本植物提供的遮蔭能夠提高幼苗的光合速率和存活率[107]；而在相對干旱的林線,草本植物降低了幼苗的光合速率,其對土壤水分的競爭可能比遮蔭的保護效應更重要[108]。Germino等對草本植物和幼苗存活率關系的研究發現,幼苗存活率在有草本覆蓋時為90%,沒有地被物時為44%,被草環繞卻不被覆蓋時為19%,因此對當年幼苗來說,溫度和水分脅迫導致了10%的死亡率,強光導致了56%的死亡率,而這兩者的聯合導致了81%的死亡率[49]。

2.2.2 菌根和植食動物

高海拔地區的針葉樹種幼苗存在碳同化限制,因此其受菌根的影響可能比其它地區的植物更大[47,73,53]。菌根從寄主植物獲取碳,同時改善寄主植物的養分和水分關系,并能阻止根部病原體,其對針葉樹種幼苗的存活具有潛在的生態重要性[109- 110]。Hasselquist等對恩格曼云杉(Piceaengelmannii)和毛果冷杉(Abieslasiocarpa)幼苗的研究發現,外生菌根(Cenococcumgeophilum)能夠顯著提高幼苗水勢,但對光合作用的影響不大[111]。

植食動物的取食和踩踏也會影響高山林線的幼苗存活。馴鹿、 駝鹿、松鼠、囊鼠等的啃食會降低林線幼苗的存活率[108,112- 115]。然而,中、低強度的家畜踩踏會導致幼苗密度的顯著增加[76,116],這主要是由于動物的踩踏減少凋落物層的厚度,從而促進了幼苗定居[35- 36]。

3 問題與展望

綜上所述,林線以上嚴酷的環境條件限制了種子萌發及幼苗存活等更新關鍵過程。氣候變暖可以在一定程度上緩解低溫造成的限制,但同時也可能會通過改變其它環境特征而加劇對更新的限制。如增溫可能會提高種子的產量和活力[23,117],但冬季溫度升高也可能導致種子因不能經歷足夠的低溫而萌發率降低[42- 44]；增溫能促進幼苗光合生長[118- 120],但增溫也可能會引起生長季提前和積雪減少,導致幼苗在生長季早期遭受凍害和干旱的可能性增加[93,121- 122],從而在一定程度上削減增溫對幼苗生長的正效應。因此,氣候變暖對林線幼苗定居的影響復雜且具有很大不確定性,需要進一步明確氣候變暖導致的環境因子變化對林線更新各關鍵階段的影響。

3.1 氣候變暖條件下生長季凍害事件對林線位置及動態的影響

與平均溫度的升高相比,凍害事件的頻率、強度和持續時間的增加對高山林線幼苗定居的影響可能更大[65,123- 124]。在全球范圍內,長期器測資料及大氣環流模型顯示未來氣候變暖條件下極端溫度事件(包括極端高溫和凍害事件)具有增加趨勢[125- 127]。但就高山林線而言,相關數據十分缺乏,且為數不多的幾個研究結果并不一致,如,我國川西以及智利安第斯山中部高山林線的模擬增溫實驗表明,增溫使得生長季開始時間提前從而導致生長季早期凍害事件增加[121- 122],而瑞士阿爾卑斯山林線近35年的氣象數據分析結果表明,盡管溫度呈持續增加趨勢,生長季期間凍害事件頻率并沒有顯著變化規律[66]。對藏東南色季拉山不同坡向氣象數據的分析顯示,在更加溫暖的陽坡生長季凍害事件更加頻繁而劇烈,且生長季早期凍害事件的數量隨年平均氣溫的升高而增加[68]。因此,未來氣候變暖雖然能使生長季提前,但也會導致生長季早期凍害事件的顯著增加。

在過去100年里,全球47%的林線沒有隨氣候變暖而上升[8]。Harsch 和 Bader提出,幼苗死亡是決定林線位置和動態的重要機制之一[128]。相對于成年樹木,幼苗對環境因子的變化更加敏感,尤其是基于種子繁殖的幼苗具有更高的脆弱性[129- 131]。氣候變暖可能并沒有改善林線以上的嚴酷環境條件(如強光和極端低溫等),甚至還可能加劇其對林線樹種幼苗定居的限制,從而導致全球變暖后林線位置相對穩定。已有實驗證據顯示,在較溫暖的氣候環境下,基于種子繁殖的冷杉幼苗對生長季早期凍害事件更加敏感和脆弱,從而為冷杉林線位置沒有隨過去200年的氣候變暖而上升提供了一種機制上的解釋,表明極端氣候事件和幼苗死亡在控制林線位置變化方面發揮著更重要的作用[68]。然而,只有通過對長期氣象數據的統計分析才能明確凍害事件特征(如頻率、強度和持續時間等)的變化規律,而高山林線地區微環境的長期持續觀測記錄十分缺乏[58,62],對高山林線樹種繁殖方式的研究也較少,因此目前對于這一機制是否具有普遍意義還不清楚,而這對理解高山林線形成機理及其對氣候變化的響應具有重要意義。

此外,前期研究中通常采用0℃作為定義生長季凍害事件的溫度閾值[68,121],但這一溫度閾值是否適用于高山地區,本身存在不確定性。K?rner認為,在生長季期間大部分高山植物在低于-1.8℃時發生凍結[62]。Taschler和Neuner的研究表明奧地利4個主要林線樹種葉片出現凍害特征時的溫度在-8.0 — -4.1℃之間[60]。Rehm和Feeley基于室內模擬凍害實驗,發現同一物種在不同生活史階段出現凍害的溫度也存在很大差異,如Gynoxysnitida幼苗出現凍害時的溫度為-6.0℃,而其成年植株則低至-10.3℃[67]。由此可見,不同物種、不同生長階段植物葉片遭受凍害脅迫時的溫度閾值存在很大差異,明確這一溫度閾值是量化生長季凍害事件特征的關鍵。因此,要研究增溫背景下生長季凍害事件對林線冷杉幼苗定居的影響,找出定義生長季凍害事件的溫度閾值非常關鍵。通過對林線樹種幼苗進行自動、連續拍照觀測,確定其開始生長和出現凍害的溫度,能夠為進一步利用長期氣象觀測數據分析生長季早期凍害事件特征提供更加合理的溫度閾值,從而有助于我們從森林更新的角度理解林線的形成機制,為氣候變暖下全球林線位置的相對穩定性提供新解釋。

3.2 基于野外環境的種子萌發實驗

從種子的形成到萌發是森林更新的關鍵過程。已有研究顯示,種子的形成、傳播和萌發均有可能是高山林線森林更新的限制因子[26,32,34]。然而,目前對高山林線森林更新過程中種子階段的認識還遠遠不如幼苗階段,關于林線樹種的種子產量和壽命、土壤種子庫等方面的研究還相對缺乏,特別是對種子在林線及林線以上的萌發特征的了解還十分有限。目前已有的對種子萌發的認識則大多是來自于不同實驗室條件下(如溫度、光照、濕度等)的萌發特征[44],而野外條件下的種子萌發實驗十分缺乏[49]。在自然條件下,種子的萌發率遠遠小于其在實驗室理想條件下的萌發率[132],因此基于室內控制條件下的種子萌發實驗結果可能會高估林線樹木的更新潛力。目前,由于實驗條件的限制,在實驗室內通常只能對單一環境因子進行控制,無法模擬氣溫日振幅或極端溫度等因子對種子萌發的影響,并不能反映出林線復雜的氣候條件對種子萌發的真實影響,也不利于研究氣候變化對種子萌發的影響。因此,未來基于野外模擬增溫的種子萌發實驗將有助于人們理解種子萌發在高山林線森林更新中的重要作用及未來氣候變化對林線種子萌發的影響。

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Advances in the study of the limitations of seedling recruitment for alpine timberline forests

SHEN Wei*, ZHANG Lin, LUO Tianxiang

KeyLaboratoryofAlpineEcologyandBiodiversity,InstituteofTibetanPlateauResearch,ChineseAcademyofSciences,Beijing100101,China

Advances of alpine timberline forests during last century are not ubiquitous worldwide, suggesting additional factors and mechanisms likely affect the response of alpine timberline forests to climate warming. Upward shifts of treelines begin with seed dispersal and germination, and seedling establishment above the treeline and any limiting factors during these processes may affect treeline migration. Therefore, investigation of mechanisms controlling seedling recruitment at alpine treeline will be helpful to elucidate treeline formation and its response to future climate change. We reviewed recent advances in tree seedling recruitment at alpine treelines from the key seed and seedling stages. For the seed stage, the seed quantity and quality generally decreased with the sum temperature during summer; the seed dispersal to elevations above treeline was impeded by low wind speed, dense dwarf shrub and grass cover; the ability of seed germination above the treeline was impaired by frost and water stresses near the ground. Also, the allelochemical properties of shrubs had negative effects on seed germination. For the seedling stage, large temperature amplitudes and freezing events during the growing season, as well as the extremely low temperature during winter, were important factors affect seedling mortality. Also the low-temperature photoinhibition resulted from the combination of low temperature and high sunlight significantly decreased seedling photosynthesis during the growing season. Besides, frost-heave activity induced by large soil temperature amplitude and soil water deficits during summer impeded seedling establishment at and above the treeline. Snowpack could keep the seedlings away from the extremely low air temperature during the winter and supply snowmelt water in the early growing season. However, too long duration of the snowpack might increase the possibility of fungal infection that promote seedling mortality. Dense shrub and grass cover above the treeline and the presence of herbivores might decrease seedling survival. In all, the influence of climate warming on seedling establishment across the timberline ecotone is complex and uncertain. Further research is needed to explore the exact effects of warmth-induced environmental changes to seedling recruitment at the alpine treeline. Since the beginning of the growing season might advance under scenarios of climate warming, which in turn led to more early-season freezing events at and above the treeline, it is important to define the temperature threshold of freezing events to analyze the relationship between growing-season freezing events and increasing temperature in the future. Based on this threshold, we can further disclose the effects of growing-season freezing events on seedling establishment at alpine treeline, which will be helpful to elucidate treeline formation and predict treeline dynamics under future climate change.

climate change; growing-season freezing events; seed germination; seedling establishment; treeline dynamics

中國科學院戰略性先導科技專項(XDB03030402)；國家973計劃項目課題(2010CB951301)

2016- 01- 23; 網絡出版日期:2016- 12- 19

10.5846/stxb201601230158

*通訊作者Corresponding author.E-mail: shenwei1984@itpcas.ac.cn

沈維,張林,羅天祥.高山林線變化的更新受限機制研究進展.生態學報,2017,37(9):2858- 2868.

Shen W, Zhang L, Luo T X.Advances in the study of the limitations of seedling recruitment for alpine timberline forests.Acta Ecologica Sinica,2017,37(9):2858- 2868.