植被生态与粮食安全

20世纪以来中国北方干旱半干旱区针叶树种径向生长动态变化

  • 陈峰 , 1, 2, 3 ,
  • 王世杰 1 ,
  • 赵晓恩 1, 2 ,
  • 陈友平 1 ,
  • 胡茂 1, 2 ,
  • 徐阳 1 ,
  • 唐雯卉 1, 2 ,
  • 张合理 1, 3 ,
  • 陈巧湄 1 ,
  • 岳伟鹏 1 ,
  • 侯体源 1 ,
  • 曹红华 1, 2 ,
  • 刘雨欣 1 ,
  • 谌卓岚 1 ,
  • 武心凤 1 ,
  • 魏家昌 1, 2 ,
  • 王赫川 1
展开
  • 1.云南大学国际河流与生态安全研究院国际河流与跨境生态安全重点实验室,昆明 650504
  • 2.西南联合研究生院,昆明 650092
  • 3.中国气象局乌鲁木齐沙漠气象研究所树木年轮理化重点开放实验室/新疆树木年轮生态实验室,乌鲁木齐 830002

陈峰(1982-), 男, 福建沙县人, 研究员, 博士生导师, 主要从事树轮与水文气候研究。E-mail:

收稿日期: 2024-01-09

  修回日期: 2024-09-10

  网络出版日期: 2024-09-27

基金资助

国家自然科学基金项目(32061123008)

Radial growth dynamics of coniferous species in arid and semi-arid areas of northern China since the 20th century

  • CHEN Feng , 1, 2, 3 ,
  • WANG Shijie 1 ,
  • ZHAO Xiaoen 1, 2 ,
  • CHEN Youping 1 ,
  • HU Mao 1, 2 ,
  • XU Yang 1 ,
  • TANG Wenhui 1, 2 ,
  • ZHANG Heli 1, 3 ,
  • CHEN Qiaomei 1 ,
  • YUE Weipeng 1 ,
  • HOU Tiyuan 1 ,
  • CAO Honghua 1, 2 ,
  • LIU Yuxin 1 ,
  • SHEN Zhuolan 1 ,
  • WU Xinfeng 1 ,
  • WEI Jiachang 1, 2 ,
  • WANG Hechuan 1
Expand
  • 1. Yunnan Key Laboratory of International Rivers and Transboundary Eco-Security, Institute of International Rivers and Eco-Security, Yunnan University, Kunming 650504, China
  • 2. Southwest United Graduate School, Kunming 650092, China
  • 3. Key Laboratory of Tree-ring Physical and Chemical Research/Key Laboratory of Tree-ring Ecology of Xinjiang Uygur Autonomous Region, Institute of Desert Meteorology, China Meteorological Administration, Urumqi 830002, China

Received date: 2024-01-09

  Revised date: 2024-09-10

  Online published: 2024-09-27

Supported by

National Natural Science Foundation of China(32061123008)

摘要

全球变暖不断影响着森林生态系统的结构和功能,其中森林生产力受到大范围极端干旱的限制更为强烈,并表现出显著的空间异质性。本文通过对中国北方干旱半干旱区257个针叶树种年轮样点进行生长趋势检测和气候驱动因子评估,确定了自20世纪以来针叶树种径向生长动态变化及其对气候因子响应模式。滑动平均分析结果表明中国新疆和西北地区针叶树种径向生长呈持续增加趋势,华北地区呈波动式增加,东北地区则在20世纪中叶后呈下降趋势。空间响应模式表明水分对所有区域的针叶树种径向生长存在积极作用,气温上升引起的蒸散发加剧则会抑制径向生长,然而西北地区生长季变化对径向生长的影响受益于暖湿化现象产生的热效应。气候学分析表明大气环流因子从年际到年代际尺度上通过调节全球海温和气压强度变化,对亚洲季风区降水产生显著作用,在一定程度上影响了中国北方针叶树种径向生长动态。CMIP5数据和VS-Lite模型揭示在未来气候变化情景下受全球变暖影响,延长的生长季会促进区域针叶树种径向生长,而低降水量引起的干旱胁迫仍然对径向生长存在限制作用,特别是在水分缺乏的中国西部地区。

本文引用格式

陈峰 , 王世杰 , 赵晓恩 , 陈友平 , 胡茂 , 徐阳 , 唐雯卉 , 张合理 , 陈巧湄 , 岳伟鹏 , 侯体源 , 曹红华 , 刘雨欣 , 谌卓岚 , 武心凤 , 魏家昌 , 王赫川 . 20世纪以来中国北方干旱半干旱区针叶树种径向生长动态变化[J]. 地理学报, 2024 , 79(9) : 2341 -2355 . DOI: 10.11821/dlxb202409012

Abstract

Continuous global warming has been affecting the structure and function of forest ecosystems, and forest productivity is strongly restricted by large-scale extreme drought and shows significant spatial heterogeneity. Patterns of changes in radial growth dynamics of coniferous species and responses to climatic factors since the beginning of the 20th century were identified by examining growth trends and assessing climatic drivers based on the tree-ring data from the 257 sampling sites in arid and semi-arid areas of northern China. The running average results indicated that radial growth of coniferous species in Xinjiang and northwest China showed a continuous increase, and radial growth of coniferous species showed a fluctuating increase in northern China, and a decline in northeast China was detected after the mid-20th century. Moisture has a positive effect on radial growth of coniferous species in all regions, and increased evapotranspiration induced by warmer temperatures will inhibit radial growth to some extent. The effect of growing season changes on radial growth in northwest China benefits from the thermal effects of warming and humidification phenomena. Climatological analyses showed that atmospheric circulation factors have some vital influences on regional precipitation generated by the Asian monsoon system by regulating global SST and baroclinic intensity changes on interannual to interdecadal scales, and potentially affecting radial growth of coniferous species in arid and semi-arid areas of northern China. CMIP5 data and VS-Lite modeling revealed impacts under future climate change scenarios, the extended growing season due to global warming promotes regional radial growth of coniferous species, and however, drought stress due to less precipitation still have some limiting effects on radial growth, especially in western China.

1 引言

气候变化对森林生态系统可持续发展带来了风险和挑战[1-2]。作为陆地碳汇和生物资源的载体,森林在维持全球碳平衡和减缓升温方面具有重要作用[3]。在水分缺失的干旱半干旱区,森林正在经历强烈的外部环境变化[4],特别是全球变暖引起的干旱加剧被认为是森林衰退和树木死亡率增加的关键因素[5-6]。然而,森林结构复杂性和气候变化的空间异质性增加了检测当前树木径向生长模式的不确定性[7]。因此,了解树木径向生长对气候变化的响应模式有助于维系森林生态系统安全和预估森林未来生长动态变化。
中国北方干旱半干旱区受西风和亚洲季风的共同作用,树木径向生长模式受到气候变化的强烈影响[8-9]。非稳态区域气候变化与树木径向生长在空间上表现为复杂的交互作用,林木内部生理结构变化同样受诸多因素综合影响[10-11]。树木年轮是记录树木径向生长动态和环境变化的自然档案,已经成为评估森林生态系统对气候变化响应的关键指标[12-15]。已有研究表明,亚洲内陆干旱区欧洲赤松(Pinus sylvestris)生长遭受干旱的严重限制,而天山山区树木径向生长受暖湿化影响检测到增加趋势[16-17]。受全球变暖影响,东北亚半干旱半湿润地区气候变化会影响树木开始活动时间,进而改变生长季长度促进早材的形成[18-19]。森林生态的异质性导致树木对气候的响应出现分化现象,综合识别气候要素影响成为了解树木径向生长驱动机制的重要途径[20]。此外,大气环流因子已经被证实对湿润区树木径向生长具有动态影响,全球海温(SST)及海平面气压(SLP)在年际和年代际尺度上通过调控区域降水影响中国东部地区阔叶和针叶林生长变化[21]。与此同时,现有研究对大尺度范围的中国北方干旱半干旱区树木径向生长和海气驱动机制上仍存在认识不足,特别是未来极端干旱影响的森林动态变化尚不清楚。
本文选取中国北方干旱半干旱区天然针叶林作为研究对象,旨在提供当前气候变化背景下树木在空间尺度上生长趋势和对气候的响应表现。为此,利用来自中国北方干旱半干旱区257个地点的树木年轮宽度数据,量化分析自20世纪以来径向生长动态变化。基于通用地球系统模式过去千年集合(CESM-LME)和耦合模式比对计划(CMIP)数据分析气候系统内部变率对树木径向生长动态的重要性以及测试未来不同浓度排放路径下低降水引起的极端干旱对树木径向生长的抑制作用。研究结果可为评估中国北方地区针叶树种生长动态和未来森林生态系统应对全球变暖持续影响提供数据支持。

2 研究方法与数据来源

2.1 树轮宽度数据

本文所采用的257个采样点主要分布于中国北方干旱半干旱区,选择以自然生长的云杉(雪岭云杉和青海云杉)和松树(油松、白皮松和樟子松)为主,以减少人为干扰对树木径向生长的影响(图1a~1c)。以树木的胸高为标准,使用生长锥从两个不同方向获取树轮样芯(9623根)并带回实验室。经过自然风干后,将样芯用白乳胶固定在特制的木板中,用砂纸(320~600目)对样芯进行打磨,直至在显微镜下可以清晰看到树轮边界。使用EPSON扫描仪获取了所有样芯的图像,CDendro 9.4 软件(https://www.cybis.se)用来测量每个样芯的树轮宽度(精度为0.01 mm或0.001 mm),并使用COFECHA软件对所有树轮原始宽度数据进行质量控制[22]。由于跨经度范围较大且生态气候环境具有差异性,将中国北方干旱半干旱区所有树轮采样点自西向东划分为4个子区域,即新疆(70)、西北(43)、华北(91)和东北(53)地区 。这里,考虑到新疆西北部与甘肃等地虽在行政区划、区域地理等划分中同处西北地区,但空间位置仍然距离遥远,因此将新疆划分为单独的子区域。
图1 研究区示意图及20世纪以来中国北方干旱半干旱区针叶树种径向生长情况

注:图b~c为各子区域树轮样本和采样点数量分布及总和;图d~k为断面积生长增量(BAI)以及树轮宽度指数(PC1)变化,其中指示各子区域内断面积生长增量均值15 a滑动平均变化,阴影指示各区域内所有样本断面积生长增量的25%~75%范围变化;底图来源于Natural Earth地图数据集(https://www.naturalearthdata.com/features/)。

Fig. 1 Schematic map of the study area and radial growth of coniferous species in arid and semi-arid areas of northern China since the 20th century

2.2 气候水文数据

(1)英国东安格利亚大学气候研究所的全球逐月地表降水、气温和潜在蒸散发网格数据集(CRU TS4.07)[23],水平分辨率为0.5°×0.5°。由于20世纪初期的中国北方气候观测站点稀疏,1940年之前的气候数据时空覆盖度较差[24-25],因此选取1940—2022年数据。
(2)英国东英吉利大学气候研究所的全球逐月地表帕尔默干旱严重程度指数(PDSI)网格数据集(CRU scPDSI 4.05early)[26],水平分辨率为0.5°×0.5°,选取年份为1940—2020年。
(3)西班牙高等学术研究委员会的全球逐月地表标准化降水蒸散指数(SPEI)网格数据集(CSIC SPEI)[27],水平分辨率为0.5°×0.5°,选取年份为1940—2018年。
(4)苏黎世联邦理工学院大气与气候科学研究所的全球逐月地表径流网格数据集(G-RUN)[28],水平分辨率为0.5°×0.5°,选取年份为1940—2019年。
(5)英国气象局哈德利中心的全球逐月SST网格数据集(HadISST1)[29],水平分辨率为1°×1°。由于海温数据自1880年信号较强[30-31],可以涵盖本文时间段,因此选取年份为1901—2022年。
(6)英国气象局哈德利中心的全球逐月SLP网格数据集(HadSLP2r)[32],水平分辨率为5°×5°,选取年份为1901—2019年。

2.3 气候模式数据

CESM-LME是上个千年古气候模拟的最大集合,由CESM1.1版本运行获得[33-34]。CESM-LME的全强迫试验综合考虑了气候系统内部变率影响和公认的外强迫作用(例如太阳活动、火山作用、温室气体等),共运行13个样本,其中水平分辨率在大气和陆地模块约为2°×2°,在海洋和海冰模块约为1°×1°。已有研究证明CESM-LME适用于研究过去千年东亚、中亚等地区气候变化与机理[35-37]。本文使用13个样本的集合平均值进行分析(数据包括SST、纬向风、径向风、比湿及降水),选取年份为1901—2005年,更多有关试验的详细信息请参阅相关文献[33]
21世纪的未来逐月降水和气温来自耦合模式比对计划第五阶段(CMIP5)的降尺度数据集(http://gdo-dcp.ucllnl.org/downscaled_cmip_projections/),水平分辨率为0.5°×0.5°[38-39]。该套数据已被用于树木径向生长的未来预估研究中[14,40],本文对31个大气环流模型(GCMs)数据进行多模型集合平均(MME)用以获取更可靠的公共信号,包括ACCESS1-0、ACCESS1-3、BCC-CSM1-1-M、BNU-ESM、CanESM2、CCSM4、CESM1-CAM5、CESM1-BGC、CMCC-CM、CNRM-CM5、CSIRO-MK3-6-0、FGOALS-g2、FIO-SEM、GFDL-CM3、GFDL-ESM2G、GFDL-ESM2M、GISS-E2-H-CC、GISS-E2-R-CC、GISS-E2-R、INMCM4、IPSL-CM5A-LR、IPSL-CM5A-MR、IPSL-CM5B-LR、MIROC-ESM-CHEM、MIROC4h、MIROC5、MPI-ESM-LR、MPI-ESM-MR、MRI-CGCM3、NorESM-ME、NorESM1-M。RCP 4.5和RCP 8.5是本文关注的两种排放情景,前者是2100年辐射强迫水平达到2.6 W m-2的温室气体中等排放情景,后者则是2100年辐射强迫水平达到8.5 W m-2的温室气体高等排放情景[38,41]

2.4 径向生长趋势检测和公共信号提取

交叉定年完成后,运行ARSTAN程序,选择负指数函数对每根样芯进行去趋势,对个别不满足生长曲线的样芯进行费曼超级平滑处理[42]。使用双权重平均法生成每个子区域内采样点的年表,并进行归一化处理,以减少极值带来的影响[43]。使用样本总体代表性(EPS)对所有年表进行了质量评估,可靠阈值被确定在EPS大于0.85区间内。由于所有年表的可靠时期不一致,在每个子区域内进行嵌套主成分提取公共信号(PC1)用于后续分析。负指数函数去趋势方法容易捕捉气候信号,特别是在强调年际变化的同时保留了年代际信号,然而树木径向生长趋势还受树木年龄和胸径大小的协同影响[44-45]。因此,本文还使用断面积生长增量(BAI)检测径向生长动态变化[46]。断面积生长增量检测方法通过将所有原始树轮宽度数据与生长年份对齐,在每个子区域内平均所有样芯的BAI,计算公式如下:
B A I = π × ( R n 2 - R n - 1 2 )
式中:R是树木半径;n是树轮形成年份。如果没有取得髓心的树芯,通过将具有已知同心圆半径的树芯作为基础,拟合到最内层年轮的曲线上来估计到髓心的长度[44-46]。缺失半径的树芯年轮数量是通过将未知半径的样芯估计长度除以从最内层的20个树木年轮获得的平均年轮宽度。如果最内层年轮离髓心的距离不够近,不用于BAI的计算中。

2.5 指数定义

大西洋多年代际变率(Atlantic Multidecadal Variability, AMV)被定义为北大西洋(0°~70°N, 80°W~0°)区域的平均海温,相较于大西洋多年代际振荡(Atlantic Multidecadal Oscillation, AMO),AMV更为贴合北大西洋海温的长期波动,而不仅仅是由内部气候动力学过程所驱动的海温模式[47];韦伯斯特—杨指数(Webster-Yang Index, WYI)被定义为北印度洋(0°~20°N, 40°E~110°E)的850hPa与200hPa的纬向风垂直切变[48];西风漂流(West Wind Drift, WWD)缺乏公认的指数定义,因此本文将南印度洋及南太平洋(60°S~30°S, 0°~200°)区域的平均海温指示西风漂流;年代际太平洋振荡(Interdecadal Pacific Oscillation, IPO)定义为赤道太平洋中部(10°S~10°N, 170°E~90°W)区域的平均海温与西北(30°N~45°N, 150°E~160°W)和西南太平洋(25°N~45°N, 140°E~145°W)两个区域的平均海温的差值[49];太平洋10 a振荡(Pacific Decadal Oscillation, PDO)被定义为北太平洋(20°N~60°N, 110°E~110°W)海温的第一模态[50];尼诺3.4区(5°S~5°N, 170°W~120°W)的平均海温被用于表征厄尔尼诺—南方涛动(El Niño-Southern Oscillation, ENSO)的年际变化。

2.6 VS-Lite模型

VS-Lite模型被用来模拟未来不同情景下各区域树木径向生长情况(https://www.ncdc.noaa.gov/data-access/paleoclimatology-data[51]。该模型采用“漏桶模型”,根据气温和降水量数据估算土壤湿度[52]。日长由地点的纬度决定,在给定日期内无年际变化。对于每一年,该模型从每月生长对气温和土壤湿度的最小响应开始,由日照调节,模拟树木径向生长情况。其中,气温和湿度响应参数由贝叶斯参数估计方法确定,其他参数设置为默认值[51]

2.7 其他方法

多年滑动平均方法用于研究针叶树种径向生长的长期趋势[53];空间相关性用于确定径向生长与气候水文因子之间的响应模式[54];CESM-LME中计算的大气环流指数被回归到降水和整层水汽输送场上用于揭示径向生长与大尺度环流的联系[36-37];一阶差分箱线图用于指示未来极端干旱事件对于径向生长的影响[55],即所有数据均进行一阶差分处理,用于指示二者的协同变化并去除线性趋势,同时使用一阶差分标准差(SD)的倍数来展示生长情况的升降程度。

3 结果和讨论

3.1 中国北方干旱半干旱区针叶树种径向生长趋势变化和气候响应模式

中国北方干旱半干旱区针叶树种径向生长趋势表现出一定的空间差异性(图1)。与新疆地区径向生长持续增加相比,东北地区自20世纪中叶以来径向生长呈显著下降趋势,西北地区在20世纪80年代后径向生长同样呈下降趋势,而华北地区则呈现波动式增长(图1d~1g)。所有子区域检测到21世纪初以来的径向生长上升趋势,在西北地区最为显著。尽管研究树种存在差异性,但东北高海拔半干旱区树木年代学研究同样发现20世纪50年代以来径向生长下降的趋势。受暖湿化影响的中国西部地区生长季延长加速了径向生长,特别是春季气温上升导致树木早材活动时间发生变化[56-57]。此外,受气候和生态环境等多种因素的调节,径向生长趋势检测方法的不同会影响结果的一致性[43,58]。中国北方针叶树种径向生长变化的空间模式指示出不同气候类型控制下的差异性。尽管检测结果受到采样策略和样本数量的限制,但在大尺度范围内提供了可靠的径向生长变化趋势。
为揭示中国北方针叶树种径向生长对于气候水文的响应,本文将树轮宽度指数(图1h~1k)与主要的气候水文因子进行空间相关分析,包括降水、气温、潜在蒸散发、PDSI、SPEI和径流(图2)。所有数据月份组合均选取完整水文年(上年8月至当年7月),这与径向生长的生理基础相一致[59-62]。结果表明4个子区域径向生长与区域气候水文存在密切联系,特别是与降水存在显著正相关性(图2a~2d)。4个子区域径向生长与蕴含干湿信号的径流均存在显著正相关,意味着水分可能是主导中国北方针叶树种径向生长增加的主要因子(图2m~2x)。在中国北方地区,随着积雪融化和冻土消融,土壤水分的增加促进了树根吸收水分的活性,丰富的细胞产量成为树木早材形成的关键因素[63-64]。同时,夏季降水的增加缓解了由高温带来的土壤水分胁迫,充足的水分可以促进树木形成层细胞的扩张和叶片气孔的打开,提高了树木冠层蒸腾和叶片气体交换,进而提高光合作用速率促进径向生长[65-66]。然而,在湿润半湿润区高降水量可能减少土壤氧气供应从而抑制树根呼吸来限制径向生长[67]。在中国干旱半干旱区,复杂的地质地貌和植被类型影响了径向生长空间格局,但受水分影响的径向生长对气候响应存在广泛一致性。此外,生长期改变对径向生长的积极影响主要受益于气温上升产生的热效应[68]。新疆地区与气温呈正相关,而在其他子区域均显示出负相关。阿尔泰山地区树木年代学研究与本文结果相一致,温暖的春季促进了西北地区针叶树种径向生长[69]。然而,在西北、华北和东北地区,气温升高对径向生长的积极影响未能检测到。高温引起的蒸散发加剧降低了土壤水分含量,严重限制径向生长[70]。特别是对于土层较浅,保持水分能力较弱的贫瘠地区,水分不足引起的干旱胁迫对径向生长限制作用可能会被放大。PDSI和SPEI与径向生长的空间相关性证实了中国北方干旱半干旱区针叶树种径向生长受干湿变化的显著影响。
图2 20世纪以来中国北方干旱半干旱区针叶树种径向生长与水文气候因子的空间响应模式

注:水文气候因子的月份为上年8月至当年7月,点状区域表示5%的显著性水平,三角形表示采样点位置。

Fig. 2 Spatial response patterns of radial growth of coniferous species to hydroclimatic factors in arid and semi-arid areas of northern China since 1940

3.2 大气环流因子是中国北方针叶树种径向生长变化的驱动力

中国北方针叶树种径向生长受区域干湿变化的强烈影响(图3),而大量研究结果表明大气环流因子对中国北方水汽输送的潜在影响[71-74]。为揭示径向生长背后的大尺度环流驱动机制,本文将树轮宽度指数(PC1)与上年8月至当年7月的全球SST及SLP进行空间相关分析(图3a~3h)。空间相关性分析结果表明,新疆地区针叶树种径向生长与热带中东太平洋、北印度洋以及北大西洋的海表温度存在显著的正相关关系(p<0.05),而与南北太平洋SST表现出负相关性。此外,径向生长与印度—西太平洋暖池的海平面气压也呈现出显著的正相关(p<0.05),相对地,与东太平洋SLP的关系则为显著的负相关(p<0.05)(图3a3e)。在西北地区,针叶树种径向生长与南半球西风带中的特定区域SST和SLP显示出显著的正相关性(p<0.05),如图3b3f所示。华北地区针叶树种径向生长同样与南半球西风带的部分区域以及北太平洋的SST和SLP呈现了显著的正相关性(p< 0.05)(图3c3g)。对于东北地区,针叶树种径向生长则与赤道中东太平洋呈现出负相关性,与北太平洋的关系则为正相关(图3d3h)。总体而言,这些发现强调了中国北方针叶树种径向生长与广泛的海洋、陆地和大气交互作用之间的紧密联系,尤其是与北大西洋的AMV指数、北印度洋的WYI指数、西风漂流WWD指数、太平洋的IPO、PDO和ENSO指数密切相关。这些相互作用显著影响着该地区针叶树种的生长模式,反映了径向生长对区域及全球气候变化的响应性。
图3 20世纪以来中国北方干旱半干旱区针叶树种径向生长的大尺度环流驱动机制

注:图a~h中为观测数据中的空间响应模式,SST和SLP的月份为上年8月至当年7月,点状区域表示5%的显著性水平;图i~n中为CESM-LME模拟数据中的标准化大气环流指数回归的降水(阴影,mm/d)和整层水汽通量(kg/(m/s))的空间模式,点状区域代表超过10%显著性水平,方框表示4个子区域。

Fig. 3 Large-scale circulation-driven mechanisms of radial growth of coniferous species in arid and semi-arid areas of northern China since the 20th century

综合考虑空间响应模态特征,本文分别计算了CESM-LME中20世纪以来的6个关键环流指数,均进行标准化并回归至整层水汽输送及降水场(图3i~3n),以揭示响应背后的物理机制。由图3i可知,当AMV正位相时,源自北大西洋、地中海和欧洲的水汽沿增强的北半球西风急流东移,为新疆地区带来更多的降水。这一现象与Ding等[73]和Zhao等[75]的研究结果相一致,进一步强调了北大西洋SST对于新疆降水变化的指示性作用。由图3j可知,当WYI正位相时,北印度洋的异常增暖增强了南亚及南海夏季风,暖湿气流伴随强季风到达中国北方,导致降水增多,尤其是西北、中西和华北地区。Webster等[48]和Chen等[76]通过观测数据也揭示了北印度洋SST对于亚洲夏季风系统调节作用的证据,与我们的发现相吻合。由图3k可知,南半球中高纬度SST的异常增暖加速了海洋的蒸发作用,使更多的水汽沿南半球西风漂流输送至太平洋,随后北流向热带地区的太平洋沃克环流,为中国北方带来丰沛的降水,这与Gillett等的研究结果相符[77]。IPO、PDO及ENSO正位相均指示热带中东太平洋暖SST异常[49,78],如图3l~3n所示,伴随着SST和SLP纬向梯度的下降,太平洋沃克环流发生显著减弱,进而导致西太平洋副热带高压和中国东部雨带南移,南涝北旱空间格局在中国东部形成。此外,印度—西太平洋暖池的异常高SLP使得较强的西南风将印度洋水汽沿索马里半岛、阿拉伯半岛和伊朗高原输入中亚等地区,导致新疆地区降水增多。IPO和PDO正位相同时蕴含北太平洋冷SST异常信号,增强的北太平洋SLP抑制了东亚夏季风向大陆输送水汽,进而减少华北等地的降水。早期研究同样表明,太平洋作为距离中国最近的大洋,其SST及SLP变化对于降水的形成具有显著影响[36,71,75,79 -84]。尽管大气环流因子对径向生长的直接影响可能有限,其对降水模式的影响却在生长趋势分析中扮演着关键角色。各种大气环流因子(如AMV、WYI、WWD、IPO、PDO和ENSO)通过位相调整改变水汽传输的路径和强度,间接调节着区域降水量,进而影响植被的水分供给。因此,在研究针叶树种径向生长时,考虑这些大气环流因子仍然是至关重要的。综上所述,大气环流通过降水间接作用于生态系统,为生长趋势的长期分析提供了重要的环境背景信息。

3.3 全球变暖背景下未来径向生长变化

基于VS-Lite模式和31个CMIP5模式的降尺度降水及气温数据,对21世纪全球变暖背景下中国北方针叶树种径向生长情况进行预估。CMIP5多模式集合的两种排放情景(RCP 4.5和RCP 8.5)均去除随机信号而保留公共信号,存在显著的全球变暖趋势。VS-Lite模型首先基于CRU数据模拟了1940—2014年期间4个区域树轮宽度年表的PC1,结果具有良好的精度,实际值与模拟值具有显著相关性(r=0.511~0.709, n=75, p<0.01)(图4a4c4e4g)。新疆地区针叶树种径向生长情况在RCP 4.5情景下呈现上升趋势(p<0.01),在RCP 8.5情景下呈现不显著下降趋势,且在21世纪最后10年出现严重的低生长(图4b)。西北、华北和东北地区针叶树种径向生长在两种情景下均呈现显著上升趋势(p<0.01),且RCP 8.5情景强于RCP 4.5情景(图4d4f4h)。与已有研究结果相一致,未来全球变暖导致生长季开始和结束时的气温升高,延长的生长季促进了径向生长[40,85 -86]。然而,气温的生长促进作用需要足够的水分支撑[87],显然西北地区干旱的自然条件无法满足促进作用,甚至可能导致径向生长的急剧下降。
图4 基于VS-Lite模型的中国北方干旱半干旱区针叶树种径向生长未来预估

注:图a~h中左侧为年表PC1实际值与VS-Lite模拟值对比,阴影表示±1倍均方根误差(RMSE)范围,右侧为21世纪不同排放情景(RCP 4.5和RCP 8.5)下模拟未来树木径向生长情况;阴影表示±2倍标准误差(SE)范围;图i~j为RCP 4.5和RCP 8.5情景下极端干旱(最低降水10年)对树木径向生长的影响(包括当年和次年),箱线图的方框表示25%~75%范围,须线表示±2倍标准差(SD)范围,中线表示平均值,右侧曲线表示正态分布。

Fig. 4 Future projections of radial growth of coniferous species in arid and semi-arid areas of northern China based on the VS-Lite model

考虑到干旱胁迫作用,本文进一步检验未来极端干旱(最低降水10年)背景下针叶树种径向生长(图4i~4j)。降水及树轮宽度指数均进行一阶差分处理,用于指示二者的协同变化并去除线性趋势,同时使用一阶差分标准差的倍数来展示径向生长的升降程度。结果表明未来四个区域仍有遭遇干旱胁迫的可能,在两种情景的极端干旱当年及次年,中国北方针叶树种径向生长均展现出明显的下降趋势,并且自东向西随降水减少而呈现增强特征,尤其是在高排放情景(RCP 8.5)下。虽然气温可以影响径向生长的长期趋势,但主要限制因素仍然是区域的降水和土壤湿度[40]。全球变暖带来的生长促进并不是无限制的,当超过与有利的生长条件相关的气候变化阈值时,将出现更加严重的负面效应[85,88],例如水分稀缺的西北地区。因此,在全球变暖背景下,中国北方大部分区域针叶树种径向生长将持续增加,但干旱胁迫所带来的挑战和风险不可忽视,尤其是频繁发生的极端气候事件潜在的威胁。
不可忽视的是,VS-Lite模型存在其局限性[40,87,89 -90]。首先,该模型缺乏气候条件对树木径向生长存在“滞后作用”的考虑[40],特别是前一年降水的积累作用会影响当年的生长情况[63]。因此,深入了解树木径向生长和细胞形成机理等,有助于精准研究树木径向生长复杂生理和生态过程。其次,已有研究指出在未来增暖背景下,该模型通过气温估算蒸散发的方法精准度可能下降,导致模型中土壤湿度的计算存在偏差[87,91]。考虑如何避免近似相关的误差,将有助于提高模型性能。一些关键环境因素并没有被考虑在该模型中,例如冬季积雪、CO2、树木死亡率以及未来可能触发或加剧的生理反应[40,87,92]。然而,模型的改进将增加数据和计算的需求,例如MAIDEN模型考虑了水分和碳循环等,但缺乏必要数据而无法进行交叉验证[93]。因此,VS-Lite模型的相对简单性是优势之一,尤其在数据可用性受限的情况下。

4 结论

通过对中国北方干旱半干旱区树木年轮宽度数据分析,发现了20世纪以来针叶树种径向生长在空间模式上表现具有差异性。中国新疆和西北地区针叶树种径向生长呈增加趋势,而东北地区自20世纪中叶以来针叶树种径向生长检测出下降趋势。针叶树种径向生长的气候响应模式在所有区域显示出强烈显著性,特别是水分变化对生长的正响应在空间上具有代表性,而高温引起的蒸散发加剧抑制了径向生长。来自大西洋、印度洋和太平洋的多类别大气环流在年际和年代际尺度上影响区域降水间接作用于中国北方针叶树种径向生长。未来全球变暖气候情景下,新疆地区表现出较为平稳的生长变化,而在西北、华北和东北地区针叶树种径向生长呈显著增加趋势。然而,低降水引起的极端干旱对中国北方干旱半干旱区针叶树种径向生长仍具有强烈限制作用。
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