2019, Vol.39
在气候变暖的背景下，全球生态环境发生着巨大的变化。如两极及高山地区冰川融化，海平面上升、林火增加及极端气候事件增多等，中-高纬度和内陆干旱区气候响应全球变暖可能更加明显[1~5]，包括升温引起植被生态的变化[6]。然而，在气候变暖的大背景下，假若发生重大降温事件，生态环境尤其是植被如何响应值得探讨。进入全新世以来，全球温度迅速回升，在温度回升的过程中伴随着一系列快速而广泛分布的气候突变事件，如格陵兰冰芯提供了清晰的气候记录[7]；北大西洋的海洋记录也显示，全新世共出现8次冰川漂移碎屑事件，显示全新世全球出现多次降温事件[8~9]。研究全新世气候温暖背景下的降温事件，可增加对未来气候预测的准确性和应对策略的可靠性。近几十年对全新世以来最显著的两次降温事件(8.2 ka事件和4.2 ka事件)，不同的学者已经做了大量的研究[10~13]，并认为两次降温事件的驱动因素有所差异。8.2 ka事件可能与北美大湖溃决而导致的温盐环流减弱、北大西洋变冷有关[11~12]；4.2 ka事件与夏季太阳辐射减弱以及低纬海洋环流驱动导致的降温和季风衰退有关[10, 13]。而在传统认为的全新世大暖期内部，7 ka前后发生的较大范围降温事件及其气候联动效应的驱动因素、生态响应却很少报道。最近的一些研究表明，在北半球中高纬度地区，7 ka前后发生明显的气候突变，导致陆地植被生态系统有一个明显的转变[14~16]，进而可能导致人类文明的嬗变与演进[17]。
植被通过影响地面反照率[18]、大气气溶胶[19]、温室气体组分和全球碳循环[20]，在地球系统中起着至关重要的作用[21~22]。适宜的气候环境能促进植物的生长，温度和降水(湿度)是其重要的生态因子；重大降温事件或干旱事件可能会导致植被生态转变甚至崩溃[23]。早期研究表明，中全新世为气候适宜期，被称为中全新世大暖期[24]，也有研究发现早、中全新世为气候最适宜期[25~27]。最近的研究认为，中国北方直到全新世中期约8 ka东亚夏季风才开始加强[28]；东亚夏季风的演化主要受控于北半球夏季太阳辐射变化，但高纬冰量导致的AMOC(大西洋经向翻转环流，Atlantic Meridional Overturning Circulation)变化对冰消期和早全新世的季风增强有明显的抑制作用，认为全新世中期为季风最强盛期[29~31]；中全新世较好的“水热组合”(全新世适宜期)促进了植被的发展[28~29, 32~33]；但在气候总体适宜的全新世中期，也有许多的气候记录显示有不同程度的降温事件[24, 34~36]，伴随着山地冰川冰进事件[37~38]、季风衰退[39~40]、湖泊水位降低[16, 41]等，可能导致植被生态的转型或退化。因此，研究全新世温暖期突变降温事件对植被的影响，是理解和预测全球变暖下植被发展趋势的重要参考。
中国北方地区从东至西北包括季风区、季风边缘区以及内陆干旱区，降水梯度明显，植被差异大；内陆干旱区典型的山地和盆地由于地形差异导致湿度和温度的垂直分异，植被大多显示明显的垂直地带性[42]。进入全新世后全球气候变暖，不同区域的干湿变化不同、植被响应也不同；在中全新世夏季太阳辐射相对较强、水热配置较好的背景下，不同区域植被生态对气候突然变冷的响应过程与方式值得关注。因此，本文从中国北方及毗邻区域选取湖泊、泥炭等沉积物中的孢粉记录来探讨中全新世降温事件如何影响区域的植被生态。
1 研究区概况本文探讨的中国北方地区可以大致分为三大区域。一是以新疆为主体的干旱区与半干旱区，一些低海拔盆地植被类型为荒漠-荒漠草原，山地中部因水汽抬升有林带出现，高山森林上线以上地区为草甸-冻土苔原。其气候受到西风系统影响较大，全新世湿度变化与季风区存在错相位关系[43]。二是半干旱-半湿润的季风边缘区，东起大兴安岭，经阴山、贺兰山、巴颜喀拉山、冈底斯山脉一线，相当于我国的400 mm等降水量线两侧(图 1)，区域主要植被类型为森林、森林草原、草原、荒漠草原、高寒草原草甸，中国北部季风边缘区受东亚季风影响明显，青藏高原东南部季风边缘区受东亚季风和印度季风的双重影响。三是中国季风边缘区以东以南的广大地区受东亚季风影响明显，可以分为温带针叶林、针阔混交林为主，到落叶阔叶林为主的区域，但其高山地区以针叶林、针阔混交林为主。
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图 1 本文引用的中国北方及邻近区域孢粉研究点 红线为季风区与非季风区分界线；圆形为湖泊点，三角形为泥炭点(数据点顺序与表 1一致) Fig. 1 Reviewed sites of pollen records in Northern China and adjacent areas. The red line represents the proximate limitation of the East Asia summer monsoon; The circle dots and triangles represent lake sites and peat sites respectively(numbers are referenced in the Table 1) |
本文研究资料和数据来源包括：1)已发表的中国北方地区的孢粉资料[14~16, 28, 35~36, 44~68](表 1)；2)已发表的部分区域气候(温度/湿度)变化记录；3)年代数据统一用日历年矫正后的数据。
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表 1 主要研究点信息 Table 1 Information of study sites |
用孢粉数据指示、评价过去植被生态的变化，需要用到一些常用的孢粉相关的指标，比如孢粉总浓度值、乔木+灌木花粉百分比、禾本科花粉百分比、喜暖树种花粉浓度或比例、莎草科花粉浓度或比例、NAP/AP(非乔木与乔木比)、A/C比(蒿属和藜科花粉比值)、P/C比(禾本科与藜科的孢粉比值)等等。中国北方现代典型草原和荒漠草原植被及表土花粉调查结果表明[69~70]：蒿属、藜科是草原-荒漠草原-荒漠区表土样品花粉组合中最主要的成分，两者之和在花粉组合中所占比例多大于50 %；典型草原区藜科花粉百分比一般低于25 %，而荒漠草原区藜科花粉含量多高于25 %；蒿属和藜科花粉比值(A/C)可用在降水小于500~450 mm的地区来区分典型草原、荒漠草原和荒漠植被类型及指示相对湿度的变化[71~72]；麻黄属和白刺属均为典型的荒漠植物，它们在孢粉组合中的含量增加代表气候干旱[46, 70]。在森林草原植被中的乔木和灌木花粉含量等指标指示湿润的环境，蒿属、藜科、白刺属等花粉含量指示较干燥的环境，草原森林指数[SFI=(蒿属+藜科+麻黄+石竹科)/木本]可以反映湿度变化，SFI指数偏高表示草原植被扩张，气候较为干旱[73]。在极度干旱的区域，蒿属和麻黄属的比值(A/E)可以作为干旱度的指数[74]。
在一些禾本科比例较高的地区，禾本科与藜科的孢粉比值(P/C)较蒿/藜花粉比值(A/C)更能反映区域有效湿度的变化[45]。莎草科一般生长在湖泊周边、河谷或林下等一些低洼潮湿的地方，为湿生型植物，因此莎草科花粉含量的增加可以指示湿地的发展；而在高山草甸地区，莎草花粉含量的增加可能指示温度的降低[36]。
在季风区，由于跨纬度范围比较大，同一植被类型或树种在不同区域，有着不同的生态指示意义。比如针叶林树木在寒温带、部分中温带山地为建群种，而在低海拔中温带、部分暖温带山地针阔混交林树木为建群种，暖温带的建群树种大部分为落叶阔叶林和针阔混交林[75]。总体来说，阔叶树种相对针叶树更喜温暖湿润的气候。
3 讨论 3.1 全新世中期7 ka前后降温事件及气候变化近年来，大量的古气候记录显示，北半球中全新世7 ka前后发生过大范围的降温。集成北半球的若干古温度记录显示，在中高纬度(30°~90°N)地区从约7 ka左右开始持续降温，温度曲线有一个明显的转折[76](图 2b)，7 ka前后降温事件也记录在格陵兰冰芯中[34](图 2a)。Kaplan和Wolfe[77]利用主成分分析(PCA)综合北大西洋多条气候代用指标记录表明，PCA1显示7 ka前后呈现一个气候转型。北大西洋V28-298孔沉积物中的染色赤铁矿碎屑颗粒含量在7.4 ka处为高值[9]。在亚洲高纬度和高原地区于约7 ka发生了冰川前进：如蒙古阿尔泰山西部冰川在6.8~6.3 ka发生冰进事件[78]，蒙古西部Tsambagarav山脉冰川底部的14 C年代约为6 ka前[79]；此外，青藏高原北部和中部在7 ka左右形成了冰原和冰帽，表明气候变冷变湿[80~81]。冰川底部的14 C测年表明，青藏高原最大的冰原普若岗日冰原在约7 ka开始形成，敦德冰帽也在稍后的时间形成[80]。普若岗日冰芯氧同位素还记录了6.8~5.7 ka期间氧同位素偏负事件，指示气候变冷，可能是青藏高原中全新世降温幅度最大的一次冷事件[81](图 2e)。青藏高原西部古里雅冰芯的氧同位素记录显示，在7 ka前后氧同位突然显著偏负，也指示了强降温事件[82]。在欧洲阿尔卑斯山，10 Be也测到了7.4 ka左右的冰进事件[83]，而在北美加拿大的西部，从6.9 ka开始至5.6 ka期间冰川前进的证据确凿[37~38]。
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图 2 全新世气候变化的指标及集成记录(阴影部分为7±0.5 ka) (a)格陵兰冰芯氧同位素记录[34]；(b)北半球30°~90°N温度变化[76]；(c)阿勒泰ATM10-C4泥炭GDGT重建年平均温度[87]；(d)阿尔泰南坡泥炭δ13C记录[86]；(e)普若岗日冰芯δ18O记录[81]；(f)乌伦古湖水位变化[48]；(g)董哥洞δ18O变化[40]；(h)大九湖泥炭重建的温度变化[89]；(i)北极涛动(AO)变化，改自Rimbu等[96]；(j)60°N 6月太阳辐射量[90] Fig. 2 Holocene climatic records(shaded area is 7±0.5 ka). (a)The North Greenland Ice Core Project(NGRIP)ice core oxygen isotope record[34]; (b)Temperature anomalies of the Northern Hemisphere for 30°~90°N[76]; (c)Temperature anomalies of Altai peat GDGTs record from core ATM10-C4[87]; (d)Altai peat α-cellulose δ13C record from core ATM10-C7[86]; (e)11-point moving average of the δ18O record from the Puruogangri ice core[81]; (f)Lake level variations of Wulungu Lake[48]; (g)Record of Holocene Asian monsoon intensity from Dongge Cave[40]; (h)Temperature anomalies of Dajiuhu peat reconstructed by microbial lipid biomarker[89]; (h)Variations in the Arctic Oscillation(normalized standard deviation scale)after Rimbu et al.[96]; (i)June insolation record for 60° in the Northern Hemisphere[90] |
部分湖泊和泥炭沉积记录了这一次降温事件，如7.4 ka左右，新疆乌伦古湖多指标显示，沉积物中粗颗粒(> 64 μm)分数达到最高值，碳酸盐含量和有机物δ13C达到最低值，多指标重建的乌伦古湖全新世水位达到最低，但在约7 ka水位开始上升[48](图 2f)；很有可能是降温导致的低海拔地区湿度/降水增加。又如新疆巴里坤湖也出现相对低的碳酸盐含量[84]，天山美天鹅湖的孢粉记录也重建了天山中部地区在6.5 ka前后的低温事件[36]；位于蒙古西北部的库苏古尔湖及其子湖Borsog在7.2~7.0 ka出现水位下降，湖泊面积缩小的事件[85]。然而，部分同位素数据并不支持该时段的低温气候，如在阿尔泰山南坡，泥炭植物的纤维素碳同位素重建的夏季温度[86]与GDGTs重建的年平均温度[87]显示在7 ka前后的降温并不明显或者相对温暖的状态，可能与该重建结果为夏季的温度有关或与指标的解译有关(图 2c和2d)。
在季风区与季风边缘区，降温伴随着大陆内部温度降低，蒙古西伯利亚高压增强，导致寒冷干燥的冬季风增强而夏季风相对减弱，内陆季风边缘区气候干旱。如位于季风边缘区湖泊如潴野泽等记录显示在中全新世7 ka左右发生干旱事件或极度干旱事件[16, 35, 59]，岱海温度指标TIC(全无机碳)显示7.1~6.5 ka为降温趋势[88]，达里湖粒度组分分析结果表明自7.1 ka后水位迅速下降[41]。东北哈尼泥炭记录显示，其腐殖化程度在8.2~6.5 ka期间的剧烈变化，表明从温暖到寒冷或从潮湿到干燥的气候变化[67]；大九湖泥炭生物标志化合物指示了7 ka前后显著降温(图 2h)[89]；兴凯湖多指标表明，约7 ka前后，粒度显示含较多砂的粗颗粒增加，指示湖面迅速下降的冷干时期[65]。位于低纬季风区董哥洞石笋氧同位素在7 ka前后明显偏正，表明可能亚洲夏季风偏弱[40](图 2g)。
综上多种记录表明，7 ka降温是一次大范围、持续性的降温，特别在中高纬度地区表现明显，降温引起冰川形成或冰川前进，亚洲夏季风可能衰退，季风边缘区部分地区记录气候变干。7 ka气候突变事件主要发生在中高纬度地区，必然与区域夏季太阳辐射的持续减少有关(图 2j)[90]，赤道辐合带(ITCZ)南移[91]、太阳活动的变化[92]，以及北大西洋的冰筏事件[8~9]导致AMOC的减弱也可能有贡献。此外，很多记录显示中亚山地冰川、高原冰帽等形成，而干旱区的冬季降水(雪)的增加、冰川的扩张导致该区域的地表反照率的增加，会进一步导致区域变冷[93~94]。中全新世大规模的火山活动[95]可能导致气候变冷。受北极涛动(AO)的影响(图 2i)，则会导致西风影响区降水量的增加[96]。太阳辐射减少进而引起的ITCZ南移是气候变化的渐变过程，地表反照率增加、冰筏事件以及其他气候系统内部的反馈会放大这一过程，而气候系统的外部的随机强迫(如太阳活动的变化、火山喷发等)可能会加速这一过程。值得注意的是，不同的代用指标解释的差异和区域的差异，重建的温度结果有不一致的地方；受到沉积记录测年精度的影响，这一事件的发生的起止时间也有一定的不确定性[97~98]，但年代相对可靠的格陵兰冰芯[34]和董哥洞石笋[40]记录显示这一事件可能始于7.3 ka，持续了300~500年时间。
3.2 7 ka前后降温事件对季风边缘区植被生态的影响植被对气候变化比较敏感，特别是在区域气候环境恶劣的条件下，温度、降水的变化对于植被生态的影响很大；而在相对温湿条件下的植被，由于植被发育完善，对气候变化的抗性较强[23]。在季风影响区，7 ka降温事件对山地以及中高纬度地区表现为建群种的变化，如阔叶树种的减少以及针叶树种的增加[14, 65~66, 68]。如位于我国中部湿润高山区的大九湖孢粉记录显示，降温事件对植被的影响表现为树种的变化，青冈属(Cyclobalanopsis)等常绿阔叶乔木比例下降，松属(Pinus)、桦属(Betula)等针叶树和落叶乔木比例上升[14, 68]。中高纬度黑龙江镜泊湖孢粉数据显示，在7~6 ka暖树种栎属(Quercus)、鹅耳枥属(Carpinus)、榛属(Corylus)、桤木属(Alnus)、胡桃属(Juglans)、榆属(Ulmus)、椴树属(Tilia)和湿热种梣属(Fraxinus)花粉浓度下降，孢粉总浓度下降和草本植物玄参科(Scrophulariaceae)花粉增加，表明这时期有气候变冷事件的发生，可能与东亚季风衰退事件有关[66]；同一时段，兴凯湖花粉记录也有相似的变化过程[65]。
7 ka前后的降温事件对地处中国北方季风边缘区的植被也有较大影响，总体表现为干旱气候下植被盖度的降低、草原性植被扩张。如内蒙古巴彦察干湖在7 ka孢粉浓度值迅速下降，表明一个稀疏的植被环境趋势，植被盖度降低，出现快速的植被转型[35](图 3a)；达里湖在8~6 ka期间孢粉浓度偏低，可能由于草原植被退化，麻黄属花粉的含量上升，且乔木树种相对百分含量增加[28](图 3b)；岱海大约在7.9~4.5 ka之间的时期，针阔混交林的大规模覆盖，标志着温暖潮湿的气候，但在7.4 ka时有一个明显的转折，乔木花粉百分比迅速下降，草本花粉上升，可能表明降温事件导致森林植被的衰退和草原的扩张[61](图 3c和3d)；纬度偏高的呼伦湖在7 ka左右乔木花粉有所下降，特别是桦木属花粉下降，而草本植物花粉略有上升，松属花粉零星出现并有增加的趋势，反映山地松林开始逐渐增加，气候渐渐转冷[64](图 3e)。季风边缘区潴野泽全新世孢粉数据表明，7 ka后孢粉总浓度急剧下降，高海拔森林花粉浓度降低，干旱性荒漠草原花粉成分占据主导地位[60](图 3f和3g)；东居延海孢粉数据表明，7~8 ka孢粉浓度较低，A/C比指示在7.5~7.0 ka气候为较干旱，6.5 ka之后花粉组合中森林成分降低[59]。在气候较寒冷湿润的大兴安岭中段阿尔山-柴河火山区，有良好测年数据的月亮湖孢粉记录表明，在7 ka前后发生了较多的种属变化：如栎属增加，榛属减少，麻黄属减少，7~6 ka之后松属花粉增加；7.2 ka前后花粉浓度较低[63](图 3h)。Liu等[99]基于中国北方15个高质量的湖泊沉积物剖面及其现代遥感植被指数，利用植被-气候反馈验证模型重建过去植被覆盖变化表明，华北地区的植被覆盖率在6.5 ka达到最高后显着下降，且自6.5 ka以来植被减少可能引起区域反照率变化和气溶胶增加。这个情况有几个可能的因素：1)该综合重建的结果确实和我们讨论的7 ka事件没有对应关系；2)7 ka事件对区域的影响可能使得湿度的增加、植被盖度增加；3)记录的差异和年代的误差，该文主要讨论的是千年尺度上盖度的演化，而不是气候事件，因而对年代的约束较差；4)气候变化时空差异。
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图 3 季风边缘区孢粉记录的全新世植被变化 (a)和(b)分别为巴彦察干湖[35]和达里湖[28]孢粉总浓度；(c)和(d)分别为岱海乔木花粉百分比和孢粉总浓度[61]；(e)呼伦湖孢粉总浓度[64]；(f)和(g)分别为潴野泽记录的孢粉总浓度和高海拔孢粉浓度[60]；(h)月亮湖乔木+灌木花粉百分比[63] Fig. 3 Pollen records indicated Holocene vegetation dynamics in the marginal areas of the Asian summer monsoon. (a, b)Total concentrations of pollen in Bayanchagan Lake[35] and Dali Lake[28], respectively; (c, d)Tree pollen content and total concentration of pollen at Daihai Lake, respectively[61]; (e)Total concentration of pollen at Hulun Lake[64]; (f, g)Total pollen and upland concentration at Zhuyeze Lake[60]; (h)Tree and shrub pollen content at Moon Lake[63] |
青藏高原东北部湖泊和泥炭花粉主要记录森林-草甸生态类型对温度变化的响应。青海湖地区，在全新世早期乔木花粉含量逐渐增加，全新世中期6.5 ka时达到峰值，可能暗示流域山地森林植被的下移，且孢粉浓度在7 ka时达到峰值之后快速降低，指示区域植被的快速衰退[58]；高原东部若尔盖地区的泥炭孢粉记录显示，乔木花粉在7.3 ka时有个快速降低事件，且部分种属丰度发生转变[26]；位于高原北部祁连山内部的门源盆地乱海子湖在约7 ka时的云杉属、桦属等乔木花粉百分比显著下降，指示森林植被衰退、气候变冷[55](图 4a和4b)；青海湖孢粉记录的青藏高原东北部全新世植被信息表明，在7 ka孢粉总浓度发生了一个明显的植被转变[58]，表明气候变冷导致植被生产量下降，植被衰退(图 4c和4d)；达连海乔木花粉含量在约7 ka前后波动较大[100](图 4e和4f)。更尕海的孢粉也显示，10.2~7.0 ka乔木花粉含量为高值，对应西南季风强盛期，区域降水增加，气候暖湿；而7 ka后孢粉浓度降低、乔木花粉含量波动下降[56~57](图 4g和4h)。
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图 4 全新世青藏高原东北缘孢粉记录的植被变化 (a)和(b)分别为乱海子Betula和Picea花粉百分比[55]；(c)和(d)分别为青海湖乔木花粉百分比和孢粉总浓度[58]；(e)和(f)分别为达连海孢粉总浓度和乔木花粉百分比[100]；(g)和(h)分别为更尕海孢粉总浓度和乔木花粉百分比[56~57] Fig. 4 Pollen records indicated vegetation dynamics on the northeastern Qinghai-Tibetan Plateau during the Holocene. (a, b)Betula and Picea pollen content at Luanhaizi Lake[55]; (c, d)Tree pollen content and total pollen concentration at Qinghai Lake[58]; (e, f)Total pollen concentration and tree pollen content at Dalianhai Lake[100]; (g, h)Total pollen concentration and tree pollen content at Genggahai Lake[56~57] |
7 ka前后降温事件对西风主导区气候影响的表现为温度下降伴随着湿度的增加。在西北内陆山地高山区域因气温下降表现为林线的降低或树种的变化；低山地区森林下线区域因湿度的增加表现为森林成分的增加以及耐旱树种的减少，在山前草原和盆地荒漠草原区域，植被的响应表现为植被盖度的增加和喜湿成分的增加[15]。贝加尔湖地区Kotokel湖的孢粉记录显示，在7.3 ka后，偏暖的阔叶树桦属(Betula)逐渐被针叶树松属(Pinus)所取代，同时木本覆盖的比重下降，重建的约7 ka最暖月与最热月温度降低，反映出7 ka前后的气候变冷和生态变化[54]；中亚高山湖泊Son Kol湖(图 5a，海拔3010 m)和赛里木湖(图 5b，海拔2072 m)孢粉记录显示约7 ka前后孢粉总浓度下降，乔灌木比例减少，荒漠植被成分藜科含量增加，草原植被成分蒿属含量下降[44~45]；天山中段高山湖泊美天鹅湖(海拔2541 m)的孢粉记录表明，6.9 ka气候发生转变，气温下降导致喜冷的莎草花粉增加而偏暖的禾本科花粉相对减少，Cy/Po比值(莎草与禾本花粉含量比值)相对增加(图 5c)[36]；阿尔泰山是温带草原与泰加林的过渡带，植被对气候变化的响应比较敏感，蒙古阿尔泰Hoton-Nur湖(图 5d，海拔2083 m)孢粉记录显示，云杉为主的泰加林从10 ka发育，在约7 ka时云杉、冷杉开始波动下降，植被可能从开放森林逐渐往草原转变，表明降温影响高山地区森林生态系统，引发植被转型[50]。阿尔泰山山地多个泥炭记录(海拔2500~2700 m)显示在7 ka前后乔木花粉显著降低[101~102]。以上这些海拔较高或纬度较高的地区，植被对于温度降低事件响应迅速且显著。海拔略低的娜仁夏泥炭(海拔1760 m)孢粉重建的暖季节温度、冷季节温度与年平均温在7 ka前后出现剧烈下降，伴随着偏低的木本花粉和偏高的NAP/AP和SFI值[49]；Telman湖(海拔1789 m)孢粉资料显示7~4.5 ka为干旱[53]；而森林下线湖泊Achit Nuur(海拔1444 m)在7 ka前后较高的木本孢粉含量表明此阶段森林扩张，较低的NAP/AP值、SFI值和(A+C)/P值表明湿度增加[51]；荒漠草原湖泊巴彦淖尔湖(海拔932 m)孢粉显示7~4 ka时期A/C比与A/E表现出逐渐上升的趋势，表明气候逐渐变湿[52]。新疆阿尔泰山南坡森林下线湖泊喀纳斯湖(图 5e，海拔1365 m)沉积物中的乔木和灌木花粉显示在约7 ka达到最高比例[15]，显示此阶段为森林生长的最适应期，表明气候变冷导致湿度增加，泰加林高度发育，且山地森林往湖区压缩；低山地区因降温湿度增加在中全新世森林达到最佳，此后乔木花粉含量持续降低；阿尔泰山周围山前低地和草原地区表现为典型草原或森林植被的发展[15]。又如位于盆地的乌伦古湖(图 2f，海拔478 m)约7 ka降温使得有效湿度增加，湖泊水位开始上升，盆地植被由荒漠向荒漠草原植被发育[48]；位于南疆的博斯腾湖(图 5f，海拔1048 m)乔木和灌木花粉在约7 ka有较高的浓度，在6 ka之后显著减少，而A/C比指示的区域有效湿度则相对于6 ka之前增加[46~47]。
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图 5 全新世西风主导区孢粉记录的植被变化 (a)Son Kol湖(海拔3010 m)孢粉浓度[44]；(b)赛里木湖(海拔2072 m)孢粉浓度[45]；(c)天山美天鹅湖(海拔2541 m)莎草与禾本花粉比值(Cyperaeae/Poaceae)[36]；(d)Hoton-Nur湖(海拔2083 m)乔木+灌木百分比[50]；(e)北疆阿尔泰山喀纳斯湖(海拔1365 m)乔木+灌木百分比[15]；(f)南疆博斯腾湖(海拔1048 m)乔木+灌木百分比[46~47] Fig. 5 Pollen recorded vegetation dynamics in arid Central Asia during the Holocene. (a)Total pollen concentration at Son Kol Lake(3010 m)[44]; (b)Total pollen concentration at Sayram Lake[45]; (c)Pollen ratio of Cyperaeae to Poaceae(Cy/Po)at Swan Lake(2541 m)in the Tienshan Mountains[36]; (d)Tree and shrub pollen content at Hoton-Nur Lake(2072 m)[50]; (e)Tree and shrub pollen content at Kanas Lake(1365 m)in the Altai Mountains of northern Xinjiang[15]; (f)Tree and shrub pollen content at Bosten Lake(1048 m) in the southern Xinjiang[46~47] |
中国北方地区受不同气候系统的影响，表现为西北干旱区主要受西风环流的影响；北部季风边缘区和季风区受东亚季风控制，青藏高原东部处于季风边缘地带，受控于东亚季风与印度夏季风的进退变化[29, 43]。大陆温度的异常变化引起大气环流的变动，如温度降低则导致西伯利亚高压的增强，西风带的南压，夏季风系统的南撤等[103]。
在中国西北部地区、青藏高原东北部、中国北方季风边缘区的植被生态均对7 ka降温事件都有响应，但响应方式存在区域的差异。在位于季风边缘区的中国东北、内蒙古、青藏高原东北部夏季风衰退[35, 58, 66]，特别是内蒙古中西部、青藏高原东北部区域气候发生了千-百年尺度的干旱事件[16, 60]，伴随着气候干旱，植被发生了不同的变化，如森林的消退[55]、植被的盖度减少且乔木从增加到逐渐减少(青海湖地区)[56~58]、植被生态转换[33]。而内蒙古东南部湖泊Anguli Nuur的孢粉显示7 ka前后，蒿属花粉比例高达75 %，白刺属、麻黄属等指示干旱性气候环境的植被急剧增长，草原植被扩张[16]；但是，同一时期偏北的达里湖乔木花粉增加，榆属花粉含量最高，麻黄花粉含量高，可能指示温暖偏干的气候，与其他记录不太一致[28]。由此认为，降温引起的干旱有区域的差异，可能对低海拔湖盆区乔木植被的影响不是很明显，而对草原植被影响明显，如草原退化、旱生组分增加。
季风区高山或者高纬度植被响应温度变化敏感，或者说温度变化是区域植被发展或衰退最重要的生态限制因子之一，例如大九湖泥炭和镜泊湖[14, 66]；又如位于季风边缘区的山西宁武公海孢粉在7 ka前后，云杉、榆属花粉减少，而栎属花粉含量在7 ka之后逐渐增加达到峰值，表明气候突变可能导致森林生态的演替而不是森林的衰退[33]。
在西风主导区，7 ka降温总体导致了区域气候变得湿润[43, 104]，区域降水可能增加[15, 105]，区域内部山地和盆地的植被响应有一定的差异性[15]。在干旱和半干旱生态系统中，低海拔盆地与林带下线地区植被变化主要受降水量以及有效湿度控制，降温则通过抑制蒸发、增加有效湿度影响植被盖度和组成[15]。区域的荒漠植被变成荒漠草原(如乌伦古湖[48]、Achit Nuur[51]、巴彦淖尔湖[52]等)，中亚的草原逐渐变成森林草原[106]，来自低海拔的湖泊孢粉资料也表明7 ka后区域有效湿度的增加、湖泊水位上升[48]。天山黄土地层中古土壤发育，指示降水的增加[105]；而降温则对高山湿度较大的植被直接作用，低温抑制土壤营养元素的释放，经常的冰雪天气可能通过破坏幼苗的存活率，进而导致林线下降[107]。因此，高海拔与林带上线地区植被变化主要受温度影响，如Hoton-Nur湖[50]，又如天山山地地区早全新世植被生产量大，于7 ka前后乔木也开始衰退[45]。综上表明，北方地区不同区域的植被生态对中全新世的降温事件响应具有明显的区域差异。
从图 6可以看出，气候变化的时间与部分地区生态转变的时间几乎一致，其驱动因素可能因太阳辐射减少进而引起地球气候系统内部的反馈(冰川前进、干旱事件等)会放大这一过程，而气候系统的外部的随机强迫(如太阳活动的变化、大型火山喷发等)可能会加速植被生态变化过程(图 6)。
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图 6 本文讨论的气候变化及植被生态转换的时间节点 (a)北纬60度夏季太阳辐射及气候环境驱动因素；(b)气候变化与生态转变时间节点：干旱事件(Anguli Nuur，董哥洞)、冰川前进(普若岗日冰芯、古里雅冰芯、北美加拿大西部冰川、阿尔卑斯冰川、蒙古阿尔泰西部冰川)、火山爆发[95]；气候变化时间节点来源于文中提及的非孢粉重建的气候数据；生态转变时间节点为文中提及的孢粉数据 Fig. 6 Timings of climate change and vegetation ecological transitions discussed in this study. (a)Solar insolation of the NH and timing of environmental trend, driving forces, and (b) timings of climate change and ecological change in this study. Drought events(Anguli Nuur and Dongge Cave), glacial advance events(Puruogangri ice core, Guliya ice core, Northwestern Canada Glaciers, the Alps Glacier and western Mongolian Altai Glacier)and volcanic eruptions[95]; Timings of ecological transitions and climate change are based on pollen data and other climate proxies mentioned in the paper, respectively |
利用近年来发表的全新世高分辨率古气候记录以及孢粉记录的植被变化，对全新世中期7 ka前后的降温事件及其对植被生态的影响进行了综合分析。7 ka降温事件在北半球有广泛记录，特别是中高纬度表现的尤为明显。但由于不同钻孔年代的不确定性、不同植被类型对气候变化的敏感性不同等因素，不同区域发生植被转型的时间不尽一致。此外，温度的变化在某些区域通过影响气候的降水、干湿再作用于植被生态，而降水的变化具有较大的空间差异性，因此区域植被生态的响应比较复杂。不同区域受气候突变影响植被变化所呈现出来的表现不一样，同一片区域不同高度记录的植被信息可能会相互矛盾或不一致，原因在于不同植被类型的限制因子不一样。在季风边缘区伴随着气候的干旱，而在西风主导的区域总体表现为气候湿润。中国北方季风区山地以及高纬度地区表现为建群种的变化，如阔叶树种的减少以及针叶树种的增加；在季风边缘区总体表现为干旱气候下植被盖度的降低；而在西北内陆山地高山区域表现为林线的降低或树种的变化，低山地区森林下限表现为森林成分的增加以及耐旱树种的减少；在山前草原和盆地荒漠草原区域，植被的响应表现为植被盖度的增加和喜湿成分的增加。降温直接影响湿度较大的高山地区的森林成分和林线高度，而在低山干旱半干旱地区，降温则通过抑制蒸发、增加有效湿度影响植被盖度和组成。一般而言，降温会导致植被生产力下降，植被带南移、山地林线下移，乔木和灌木比重下降，草本植物扩张，但在部分地区降温还伴随着西风增强、降水增加，有效湿度增加，植被盖度增加。全新世中期7 ka降温事件的触发机制有待深入研究，可能与大型火山喷发以及太阳活动变化有关，夏季太阳辐射的持续降低以及地球系统内部反馈也是共同的原因。本研究结果有助于详细评估未来气候变化情景下，不同区域植被响应的方式、幅度、速率等，为探讨植被-气候的反馈机制提供重要参考。
致谢: 感谢审稿专家和编辑部杨美芳老师提供宝贵的修改意见，在此一并致谢!
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Abstract
A number of studies have shown that the global ecological environment has experienced tremendous changes with the global warming. However, in relatively warming climatic conditions, it is still poorly understood that the vegetation ecological responses to major/abrupt cooling events.The Northern China can be roughly divided into three regions based on vegetation and climate, including the climatically westerlies dominated arid and semi-arid areas in Central Asia, the marginal areas of the Asian summer monsoon with highly variable climate and vegetation, and the vast monsoon area in Northern China with relatively high precipitation and better vegetation. We combined published pollen data and climate change records spanning the Holocene in Northern China and nearby areas to explore the major vegetation ecological responses to abrupt climate change. Based on the Holocene high-resolution paleoclimate records and pollen records published in recent years, the cooling events around 7 ka(1 ka=1000 cal.a B. P.) in the Middle Holocene and their impacts on vegetation ecology were reviewed. This review shows that the cooling events at around 7 ka are recorded in a wide area of the Northern Hemisphere, accompanied by climate drought in the edge of the Asian summer monsoon area, while, in the arid area dominated by the westerlies, the climate was generally humid. In different areas of the Northern China, the vegetation feedbacks were quite different. The major plant species changed in the marginal areas of the summer monsoon system under drier climate, which is shown as the decrease of broad-leaved tree species, increase of coniferous tree species, and/or the decrease of vegetation coverage. In the high mountains in Northwestern China, the vegetation feedback was shown as the decrease of tree-line or the change of tree species. While, in the lowland areas surrounding the mountains in Northwestern China, the forest composition increased in the steppe-forest vegetation areas, and vegetation coverage increased in the desert-steppe area with more steppe species. Low temperature directly modified forest compositions and tree-line in the relatively wet high mountains, while, it affected vegetation coverage and composition by increasing humidity/precipitation in arid and semi-arid areas via depressing evaporation. Due to both the uncertainties of chronological data of different studies and the sensitivity of different vegetation types, the timings of vegetation transformations were not consistent. The driving forces of the 7 ka cooling event in the Middle Holocene needs to be further studied, which might be related to the strongest eruptions of large volcanoes, solar activity variabilities. The continuous decrease of summer solar insolation in the Northern Hemisphere and inner feedbacks of the earth system also contributed to this cold period.