蒸腾是植物重要的生理活动之一, 受到多个环境要素的综合影响。该文利用热平衡包裹式茎干液流仪Flow32监测系统, 对河西走廊中段绿洲—荒漠过渡带主要固沙植物梭梭(Haloxylon ammodendron)生长季的蒸腾过程进行连续监测, 并分析了其对环境要素的响应。结果表明: (1)梭梭茎干液流速率随枝直径的增加而增加, 液流密度随枝直径的增加而降低, 梭梭液流密度日变化呈多峰特征, 整个生长季液流密度变化比较稳定。(2)主成分分析表明第1、2、3主成分能够分别解释环境信息49%、15%、12%的变化, 第一主成分中水汽压亏缺、光合有效辐射及气温反映了大气的蒸腾需求。(3) S型模型对液流密度日变化模拟精度达到0.86, 考虑时滞效应后模拟精度提高至0.9, 降水天气条件下模拟精度降低至0.65。(4)液流密度对主要环境要素存在非对称响应, 同光合有效辐射呈逆时针环状, 同水汽压亏缺、蒸腾需求指数呈顺时针环状。
Aims Transpiration is one of important physiological activities for plants, which is regulated by many environmental variables. Our objectives were to understand the responses of sap flow density of Haloxylon ammodendron to environmental variables and simulate its diurnal change under different micrometeorological conditions.
Methods Sap flow in stems of H. ammodendron was measured with stem heat balance method using commercial sap-flow gauges from May to October, 2014, in the oasis-desert ecotone, located in the middle range of Hexi Corridor, Northwestern China.
Important findings Sap flow velocity of H. ammodendron exhibited a positive relationship with stem diameter, but sap flow density (Js) decreased with stem diameter. The first three axes of principal component analysis (PCA) explained 49%, 15%, 12% of variances in the environmental datasets, respectively, and vapour pressure deficit (VPD), photosynthetically active radiation (PAR), temperature in the first axes indicated the atmospheric evaporative demand. A sigmoid function could explain 86% of the variation in Js in typical sunny days, while only 65% on rainy days. It was worth noting the simulated Js using the established sigmoid function agreed well with the measurements (R2 = 0.90) if the time lags of Js to principle environmental variables were taken into consideration. Plots of 30-min Js against PAR, VPD, and evaporative demand index (EDI) revealed a counter-clockwise hysteresis for PAR, but a clockwise hysteresis for VPD and EDI, it was possibly affected by water stress and time lags of sap flow density to principle environmental variables.
全 文 :植物生态学报 2015, 39 (9): 890–900 doi: 10.17521/cjpe.2015.0085
Chinese Journal of Plant Ecology http://www.plant-ecology.com
——————————————————
收稿日期Received: 2015-01-06 接受日期Accepted: 2015-05-28
* 通讯作者Author for correspondence (E-mail: xuanzhij@ns.lzb.ac.cn)
典型固沙植物梭梭生长季蒸腾变化及其对环境因
子的响应
徐世琴1,2 吉喜斌1* 金博文1
1中国科学院寒区旱区环境与工程研究所, 兰州 730000; 2中国科学院大学, 北京 100049
摘 要 蒸腾是植物重要的生理活动之一, 受到多个环境要素的综合影响。该文利用热平衡包裹式茎干液流仪Flow32监测系
统, 对河西走廊中段绿洲—荒漠过渡带主要固沙植物梭梭(Haloxylon ammodendron)生长季的蒸腾过程进行连续监测, 并分析
了其对环境要素的响应。结果表明: (1)梭梭茎干液流速率随枝直径的增加而增加, 液流密度随枝直径的增加而降低, 梭梭液
流密度日变化呈多峰特征, 整个生长季液流密度变化比较稳定。(2)主成分分析表明第1、2、3主成分能够分别解释环境信息
49%、15%、12%的变化, 第一主成分中水汽压亏缺、光合有效辐射及气温反映了大气的蒸腾需求。(3) S型模型对液流密度
日变化模拟精度达到0.86, 考虑时滞效应后模拟精度提高至0.9, 降水天气条件下模拟精度降低至0.65。(4)液流密度对主要环
境要素存在非对称响应, 同光合有效辐射呈逆时针环状, 同水汽压亏缺、蒸腾需求指数呈顺时针环状。
关键词 热平衡方法, 非对称响应, 主成分分析, 模拟, 茎干液流
引用格式: 徐世琴 , 吉喜斌 , 金博文 (2015). 典型固沙植物梭梭生长季蒸腾变化及其对环境因子的响应 . 植物生态学报 , 39, 890–900. doi:
10.17521/cjpe.2015.0085
Dynamics and responses of sap flow of typical sand binding plants Haloxylon ammodendron to
environmental variables
XU Shi-Qin1,2, JI Xi-Bin1*, and JIN Bo-Wen1
1Cold and Arid Region Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China; and 2University of Chinese
Academy of Sciences, Beijing 100049, China
Abstract
Aims Transpiration is one of important physiological activities for plants, which is regulated by many environmental
variables. Our objectives were to understand the responses of sap flow density of Haloxylon ammodendron to environ-
mental variables and simulate its diurnal change under different micrometeorological conditions.
Methods Sap flow in stems of H. ammodendron was measured with stem heat balance method using commercial
sap-flow gauges from May to October, 2014, in the oasis-desert ecotone, located in the middle range of Hexi Corridor,
Northwestern China.
Important findings Sap flow velocity of H. ammodendron exhibited a positive relationship with stem diameter, but
sap flow density (Js) decreased with stem diameter. The first three axes of principal component analysis (PCA) ex-
plained 49%, 15%, 12% of variances in the environmental datasets, respectively, and vapour pressure deficit (VPD),
photosynthetically active radiation (PAR), temperature in the first axes indicated the atmospheric evaporative demand. A
sigmoid function could explain 86% of the variation in Js in typical sunny days, while only 65% on rainy days. It was
worth noting the simulated Js using the established sigmoid function agreed well with the measurements (R2 = 0.90) if
the time lags of Js to principle environmental variables were taken into consideration. Plots of 30-min Js against PAR,
VPD, and evaporative demand index (EDI) revealed a counter-clockwise hysteresis for PAR, but a clockwise hysteresis
for VPD and EDI, it was possibly affected by water stress and time lags of sap flow density to principle environmental
variables.
Key words heat balance method, hysteresis, principle component analysis, simulation, stem sap flow
Citation: Xu SQ, Ji XB, Jin BW (2015). Dynamics and responses of sap flow of typical sand binding plants Haloxylon ammoden-
dron to environmental variables. Chinese Journal of Plant Ecology, 39, 890–900. doi: 10.17521/cjpe. 2015.0085
徐世琴等: 典型固沙植物梭梭生长季蒸腾变化及其对环境因子的响应 891
doi: 10.17521/cjpe.2015.0085
蒸腾是植物重要的生理活动之一, 该过程一方
面促进植物体内水分和营养元素的运输, 另一方面
通过气孔控制间接影响光合作用(Jones, 1983)。植物
茎干液流(stem sap flow)的90%以上通过叶片蒸腾
散失到大气中(蒋高明, 2007), 因此能够将植物茎干
液流量用于估算单株或群落蒸腾并获得植物重要的
生理生态信息。茎干液流受到植物生理生化特征、
环境要素及树形因子的综合影响(Andrade et al.,
1998; Meinzer et al., 2001), 其中环境要素主要包括
光合有效辐射(PAR)、水汽压亏缺(VPD)和气温等,
而长时间尺度液流变化则受区域气候的制约
(Granier et al., 1992; Vertessy et al., 1995; McDowell
et al., 2008)。主成分分析(PCA)能将互有联系的环境
要素分为不同的复合分量, 在保留原始数据主要信
息的基础上实现数据降维, 该方法对于分析植物液
流对环境因子的响应十分有效 (O’Brien et al.,
2004)。
20世纪90年代以来, 茎干液流测定技术被广泛
应用在植物单株及群落蒸腾研究上。目前常用的测
量方法包括热脉冲 (heat pulse method)(Granier,
1985, 1987), 热扩散 (thermal dissipation method)
(Smith & Allen, 1996)和热平衡(stem heat balance)
(Baker & van Bavel, 1987), 它们均能获得植物茎干
液流连续变化信息(Čermák et al., 1973; Edwards et
al., 1996; Hall et al., 1998)。但前两种方法需要计算
树干边材面积, 耗时较多, 且不同方位、不同深度边
材上植物液流速率存在的差异也会使测定结果的不
确定性增大(Wullschleger & King, 2000; 徐飞等,
2012), 此外, 梭梭(Haloxylon ammodendron)茎干较
细, 前两种测定技术在操作上并不可行。热平衡方
法直接将加热器包裹在茎干外部对植物液流进行连
续监测, 能有效地避免上述问题(Kigalu, 2007)。
梭梭为藜科梭梭属超旱生小乔木 (戴岳等 ,
2014), 是河西走廊中段绿洲—荒漠过渡带的主要
固沙植物, 对于维持该区生态系统的稳定性及荒漠
化防治发挥着极其重要的作用(李向义等, 2004)。国
内学者对该植物的研究涉及叶片光合、蒸腾及水分
利用效率(苏培玺等, 2005; 苏培玺和严巧娣, 2006;
常学向等, 2007), 抗旱性能(郭泉水等, 2004), 根系
形态特征(弋良朋等, 2007)等方面。目前, 关于植物
茎干液流的研究主要集中于对温带、热带地区高大
乔木的研究, 关于极端干旱环境条件下荒漠植物长
期茎干液流变化特征及其对环境要素的响应并不多
见, 对茎干液流变化的模拟也少有报道。本文对梭
梭生长季茎干液流及主要环境要素进行了监测, 目
的在于: (1)研究梭梭液流变化规律; (2)分析液流对
主要环境因子的响应; (3)构建模型模拟液流日变化,
以期为研究荒漠植物耗水和固沙植物品种选育提供
科学依据。
1 材料和方法
1.1 研究区概况
野外观测实验场位于河西走廊中段临泽绿洲—
荒漠过渡带(39.37° N, 100.14° E, 海拔1 386 m), 为
典型的大陆性干旱气候, 干旱高温和多风是其气候
的主要特点。2005–2014年平均年降水量约123 mm,
近80%降水集中于6–9月, 年日照时数和辐射总量
分别为3 018 h和6 254 MJ·m–2; 年平均气温约8.9
℃, 最高气温38.6 ℃, 最低气温–26.2 ℃, ≥10 ℃的
年积温为3 088 ℃。此地主风向为西北风, 风沙活动
集中在3–5月, 年均风速2.7 m·s–1, 土壤为砂土。
1.2 仪器的安装和测定
2014年5–10月选取相同生境条件下生长状况良
好的梭梭样枝 , 利用热平衡包裹茎干液流仪
(Flow32, Dynamax, Houston, USA)对所选样枝液流
进行测定, 共包裹了9个探头。测定前, 将小的枝条
和萌芽去除, 用砂纸轻轻将茎干打磨光滑, 然后用
游标卡尺测量样枝直径后, 在打磨好的安装区涂抹
油脂, 仔细将加热片安装于被测区, 用铝箔包裹,
最后用胶带密封, 防止雨水进入。将连接探头的电
缆线与数据采集器(CR1000)相应接口进行连接, 通
过计算机分别将被测样枝的茎干类型、横截面积、
探头电压、起始时间、数据记录间隔等参数输入到
数据采集器并定期采集数据, 测定期间每两周更换
一次探头。本研究的数据采集间隔为60 s, 每30 min
计算一次平均值并储存, 由于仪器故障等不可抗因
素最终采用探头型号为SGA9、SGA13、SGA13、
SGA19、SGA25、SGB25数据作为模型输入数据, 探
头型号SGA5数据作为模型验证数据(表1)。
1.3 环境因子测定
试验地建有8 m微气象观测塔, 植被冠层顶部
安装有降水量传感器(TE525MM tipping bucket rain
gauges, Texas Electronics, Texas, USA), 冠层顶部2
m高度安装有净辐射传感器(CNR4, Kipp & Zonen,
892 植物生态学报 Chinese Journal of Plant Ecology 2015, 39 (9): 890–900
www.plant-ecology.com
表1 被测样枝参数
Tab1e 1 Basic parameters of measured stems
编号
Numbers
探头型号
Type of probe
枝直径
Stem diameter (cm)
1 SGA5 0.27
2 SGA9 0.47
3 SGA13 0.75
4 SGA13 0.66
5 SGA19 1.02
6 SGA25 1.32
7 SGB25 1.24
Delft, Netherlands)、光合有效辐射传感器(LI-190SB,
LI-COR, Lincoln, USA), 大气温度和湿度传感器
(HMP155A, Vaisala, Helsinki, Finlnd), 风速风向传
感器(1405-PK-052 WindSonic anemometer, Gill In-
struments Ltd., Lymington, UK)。土壤温度探头
(109-L, Campbell Scientific, North Logan, USA)及水
分探头(CS616, Campbell Scientific, North Logan,
USA), 设置深度均为10、20、40、60、80、100 cm
(本文采用10 cm处的土壤温度和水分数据)。所有数
据通过数据采集器(CR1000-XT, Campbell Scientific,
North Logan, USA)自动记录, 每30 min计算平均值
并进行存储。
1.4 模型
由于植物茎干液流与环境要素存在较高统计相
关性, 本文基于PCA构建梭梭液流密度日变化的经
验模型, 选择晴天条件下梭梭液流密度观测数据在
Origin 9.0中对观测值进行模拟, 选择模拟精度最高
的一个4参数的S型函数, 模型表达式如下:
式中, SSF为标准化液流密度, x为自变量, 即下文提
出的蒸腾需求指数(EDI), 模型参数a为方程的低值
(方程下渐近线趋近的值), b为方程的高值(方程上渐
近线趋近的值), x0为曲点, c为过渡斜率。
2 结果
2.1 茎干液流变化
不同直径梭梭样枝液流速率变化存在显著差异
(图1), 平均液流速率随着被测样枝直径的增大而增
大(图2A), 变化范围为1.9–59.0 g·h–1, 其斜率反映
了梭梭茎干生长对水分传输的需求(Köstner et al.,
1998)。将平均液流速率转化为液流密度之后发现:
不同枝直径平均液流密度最低值(10.3 g·h–1·cm–2)与
最高值(18.7 g·h–1·cm–2)相差不到2倍, 且随着枝直径
的增加而降低(图2B), 这主要是由于随着茎干变粗,
心材所占比例变大, 导致单位横截面积导水率下降。
梭梭液流密度日变化过程表现为多峰特征, 最
高值(34.7 g·h–1·cm–2)出现在10:30左右, 随后, 由于
辐射持续增强, 植物遭受光抑制, 气孔暂时关闭,
液流密度迅速降低, 约在12:00之后缓慢上升, 之后
由于温度和VPD不断升高造成水分胁迫, 气孔再次
关闭, 液流密度也再次降低(图3)。梭梭在夜间依然
保持较低液流密度, 主要用于茎干储水组织补水
(Goldstein et al., 1998)。
2.2 液流密度对环境因子的响应
相关分析表明各环境因子之间均存在不同程度
的相关性(表2)。土壤温度和湿度主要受气温影响,
降水与其他环境要素的相关系数普遍较低。采用
PCA对8个环境因子进行分析, 发现前3个主分量能
够解释全部数据76%的变化(表3), 其中第一主成分
能够解释49%的变化, 与VPD、PAR、气温、风速正
相关, 与空气相对湿度负相关; 第二主成分能够解
释15%的变化, 与土壤湿度、土壤温度、空气相对
湿度正相关; 第三主成分能够解释12%的变化, 与
降水呈正相关(表4)。茎干液流密度同PAR、VPD的
变化存在明显阈值, 当PAR低于1 100 μmol·m–2·
s–1, VPD低于2 kPa时, 液流密度几乎同二者呈线性
关系, 并随着气温的增加而增加(图4)。
表2 基于30 min时间尺度的环境变量相关分析
Table 2 Correlations among the 30-min averages of weather variables
measured during the study period
Hs, 土壤相对含水量(%); P, 降水; PAR, 光合有效辐射(μmol·m–2·s–1);
RH, 相对湿度(%); Ta, 气温(℃); Ts, 土壤温度 (℃); VPD, 水汽压亏缺
(kPa); V, 风速(m·h–1)。**, 显著相关(p < 0.01)。
Hs, soil moisture (%); P, precipitation; PAR, photosynthetically active radiation
(μmol·m–2·s–1); RH, relative humidity (%); Ta, air temperature (℃); Ts, soil
temperature (℃); VPD, vapour pressure deficit (kPa); V, wind speed (m·h–1).
**, significant correlation (p < 0.01).
环境变量
Environmental
variables
RH VPD Ta Ts Hs V P
PAR 0.45** 0.57** 0.54** 0.21** 0.08** 0.36** –0.08**
RH –0.84** –0.65** –0.50** 0.05** –0.34** 0.22**
VPD 0.89** 0.72** 0.19** 0.42** –0.14**
Ta 0.85** 0.36** 0.42** –0.08**
Ts 0.36** 0.30** –0.02
Hs 0.04** –0.01
V –0.02
徐世琴等: 典型固沙植物梭梭生长季蒸腾变化及其对环境因子的响应 893
doi: 10.17521/cjpe.2015.0085
表3 基于主成分分析的前3个主成分特征值及其解释方差
Table 3 Eigenvalues and the variance explained by the first three axes of
principle component analysis on the weather data
主成分
Principle
component
特征值
Eigenvalue
解释方差
Total variance
explained (%)
累积解释方差
Cumulative variance
explained (%)
1 3.9 0.49 0.49
2 1.2 0.15 0.64
3 1.0 0.12 0.76
表4 环境要素在前三个主成分轴上的载荷
Table 4 Factor loadings of the environmental variables on the first three
axes of principle component analysis
环境变量
Environmental variables
主成分1
Factor 1
主成分2
Factor 2
主成分3
Factor 3
PAR 0.72 –0.02 –0.01
VPD 0.87 0.37 –0.16
Ts 0.54 0.70 –0.07
Hs –0.11 0.88 0.02
V 0.66 –0.03 0.20
P –0.05 0.01 0.96
Ta 0.77 0.58 –0.06
RH –0.82 –0.08 0.29
环境变量同表2。
Environmental variables see Table 2.
2.3 液流密度模拟
观测期间, EDI总体表现为先增加后降低的趋
势, 且在7月份最大, 发生降水时EDI明显降低(图5)。
基于上述分析, 对连续晴朗天气条件下梭梭液流密
度进行模拟, 结果表明4参数的S型模型能够解释液
流密度85%的变化, 之后对模型进行残差分析发现
残差同土壤水分的相关系数大于同降水的相关系
数, 因此将土壤水分的PCA得分加入模型对模型进
行线性修正并重新进行参数估计, 模型模拟精度提
高到86% (图6), 模型最终表达式为:
式中: x为蒸腾需求指数EDI, 计算EDI时: (1)通过
PCA分析从全部环境要素中遴选出前3个主成分;
(2)遴选出第一主成分中载荷值最大的前3个环境变
量(本文中为VPD、气温、PAR)并计算所选变量的主
成分得分; (3)将3个环境变量主成分得分相加并进
行标准化即为EDI的值, Z为土壤水分主成分得分。
降水当天梭梭茎干液流密度迅速降低, 降水过
后液流活动增强。对降水天气条件下梭梭液流密度
进行模拟发现液流密度模拟精度降低(图7), 但仍能
反映液流密度的变化, 表明降水对日尺度液流密度
模拟有一定影响, 主要由于降水条件下土壤水分对
液流变化的模拟起着决定作用(Zeppel et al., 2008),
而文中则以蒸腾需求作为模型决定项。
3 讨论
3.1 液流密度时滞效应
液流密度的时滞效应是指当前树干液流密度的
值不是由同期环境因子决定的, 很可能受到之前环
境要素影响或者是植物生理特征影响使得液流峰值
提前或者滞后于环境要素峰值的现象(王慧梅等,
2011)。梭梭液流密度在6:30之前略微滞后于PAR,
之后迅速增加, 总体提前于PAR最大值约60 min,
提前于VPD最大值约240 min, 提前于EDI最大值约
60 min。对EDI数据进行错位后重新对7月10–14日的
液流密度进行模拟发现, 考虑EDI时滞效应的模型
模拟精度显著升高至90% (图8), 表明时滞效应对模
型模拟精度有一定影响。
黄德卫等(2012)发现, 鼎湖山针阔混交林优势
种液流峰值出现的时间晚于PAR峰值出现的时间,
与VPD的时滞效应在不同季节存在明显差异, 王华
等(2008)发现马占相思(Acacia mangium)树干液流
滞后于PAR, 而提前于VPD, 王慧梅等(2011)发现兴
安落叶松(Larix dahurica)树干液流峰值滞后于PAR
约0.5 h, 提前于气温和空气相对湿度0–2 h。有研究
认为植物茎干夜间水分的补充是导致时滞效应的重
要原因(Phillips et al., 2003; Goldstein et al., 1998;
Kumagai, 2009)。此外植物茎干液流与环境要素时滞
效应的差异还与植物木质部结构、树干储水性能及
树形因子等因素有关(Granier et al., 2000; Ford et
al., 2004; Chuang et al., 2006)。有研究指出, 时滞现
象通常在小时间尺度发生, 当上推至日或者更大时
间尺度时, 这一现象不再明显(Oren et al., 1999;
O’Brien et al., 2004)。
3.2 液流密度的非对称性响应
梭梭液流密度与VPD、EDI的日变化响应呈顺
时针环状(图9A、9B), 相同VPD或EDI条件下上午液
流密度更大, 这主要是因为虽然VPD和EDI在下午
降低, 但是叶片气孔导度并没有恢复至原来水平
(Arneth et al., 1996)。液流密度与PAR响应的非对称
性减弱且呈逆时针环状(图9C), 主要因为二者的日
变化过程更为一致。
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图1 不同基茎梭梭液流速率变化。A–G测量枝条的直径分别为1.32、1.24、1.02、0.75、0.66、0.47、0.27 cm。
Fig. 1 Variation of sap flow velocity of Haloxylon ammodendron with different diameters. A–G, diameters of 1.32, 1.24, 1.02, 0.75,
0.66, 0.47, 0.27 cm.
图2 枝直径同平均液流速率(A)、密度(B)的关系。
Fig. 2 The relationship between stem diameter and average sap flow velocity (A) and density (B).
图3 观测期液流密度(平均值±标准偏差)及主要环境要素
日变化。
Fig. 3 The diurnal change of sap flow density (mean ± SD)
and principle environmental variables during study period. PAR,
photosynthetic active radiation; VPD, vapour pressure deficit.
O’Brien等(2004)研究了10种热带雨林植物树干
液流与环境因子的关系发现, 上述非对称性在液流
密度和PAR的变化中最大, 这主要是因为热带雨林
的垂直结构较深, 林内光环境差别很大, 而荒漠环
境中空气水分条件相比于光环境对梭梭茎干水分传
输过程的影响更大(图9)。Stöhr和Lösch (2004)发现
当土壤干旱程度加重时, 欧梣(Fraxinus excelsior)树干
液流对环境要素响应的非对称性增加, 环的开度更
大, 这与Oren等(1999)对落羽杉(Taxodium distichum)
树干液流与辐射、VPD关系的研究结果一致。Motzer
等(2005)认为这一现象是植物气孔对蒸腾调节的结
果, Meinzer等(1997)认为非对称性的程度取决于辐
射和VPD协同变化, 它同植物茎干水分更新密切相
关(Jarvis, 1976; Schulze et al., 1985; Wullschleger et
al., 1998)。
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图4 液流密度对光合有效辐射(PAR)(A)、气温(B)和水汽压
亏缺(VPD)(C)的响应。
Fig. 4 Response of sap flow density photosynthetically active
radiation (PAR) (A), air temperature (B), and vapour pressure
deficit (VPD) (C).
3.3 大气环境蒸发力对液流的影响
辐射和VPD是表征空气蒸腾需求的主要环境因
子。梭梭液流密度在夜间、低辐射或者比较潮湿的
环境条件下较低, 随着空气变干、气温升高、风速
增大, 液流密度几乎随EDI线性增加至EDI对其变
图5 蒸腾需求指数(EDI)动态变化。
Fig. 5 Daily variation of evaporative demand index (EDI)
during the study period.
图6 典型晴天液流密度模拟。SSF, 标准化液流密度。
Fig. 6 Simulation of sap flow density in typical sunny days.
SSF, standard sap flow density.
图7 降水天气液流密度模拟。SSF, 标准化液流密度。
Fig. 7 Simulation of sap flow density in typical rainy days.
SSF, standard sap flow density.
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图8 考虑时滞效应的液流密度模拟。SSF, 标准化液流密度。
Fig. 8 Simulation of sap flow density including sap flow lags.
SSF, standard sap flow density.
化不再起作用后趋于稳定(图10), 这与空气蒸腾需
求对柠檬树蒸腾的研究结果一致 (Ortuño et al.,
2006), 在典型降水天气, 梭梭液流活动明显变弱也
证实了这一结论(图7)。
光是影响植物蒸腾的重要环境驱动因子, 但是
对气孔导度变化具有消减作用(Meinzer et al., 1995),
从而影响茎干液流变化(Motzer et al., 2005)。当VPD
除以PAR以消除光的影响后发现, 液流密度对较低
的VPD十分敏感, 且随着VPD的增加线性增加至其
最大值后缓慢降低(图11), 而O’Brien等(2004)发现
多种热带雨林植物茎干液流密度随VPD增加的速率
同降低的速率基本一致, 这一差异表明在空气极端
干旱时梭梭通过气孔调节尽可能减少水分损失(谭
永芹等, 2011)。Verbeeck等(2007)发现VPD是欧洲赤
松(Pinus svlestris)树干液流的主要驱动因子, 当空
气十分干燥时植物蒸腾受到抑制。大气环境蒸发力
对植物液流的影响还受土壤水分状况以及植物导水
率等因素的综合影响(Schulze et al., 1985; Meinzer
et al., 1995; Motzer et al., 2005)。Oguntunde (2005)
发现当土壤严重干旱时, 空气蒸腾需求仅能解释木
薯(Manihot esculenta)茎干液流24%的变化。
4 结论
本文采用基于热平衡原理的包裹式茎干液流仪
flow32对梭梭液流进行监测, 并结合气象数据通过
上述分析得出如下4点结论:
(1)梭梭液流速率、液流密度同茎径大小密切相
关, 液流密度日变化过程呈多峰特征。
(2)环境要素PCA分析的前3个主成分分别能够
解释梭梭生长环境49%、15%和12%的变化, S型模
型能够较好地模拟日尺度梭梭液流密度的变化, 考
虑液流密度对EDI的时滞效应能够显著提高模型模
拟精度。
图9 液流密度对光合有效辐射 (PAR)(A)、水汽压亏缺
(VPD)(B)和蒸腾需求指数(EDI)(C)的非对称响应。
Fig. 9 Hysteresis between sap flow density and photosyn-
thetically active radiation (PAR)(A), vapour pressure deficit
(VPD)(B), evaporative demand index (EDI)(C) .
898 植物生态学报 Chinese Journal of Plant Ecology 2015, 39 (9): 890–900
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图10 液流密度观测值和模拟值与蒸腾需求指数(EDI)的关
系。
Fig. 10 Plots of sap flow density observed and simulated
against evaporative demand index (EDI).
图11 液流密度同标准化水汽压亏缺(VPD/PAR)的关系。
Fig. 11 Sap flow density in relation to vapour pressure deficit
(VPD) normalized by photosynthetically active radiation (PAR).
(3)液流密度与PAR、VPD、EDI之间存在非对称
响应, 与PAR呈逆时针环状, 与VPD和蒸腾需求指
数呈顺时针环状。
(4)梭梭液流密度起初随EDI线性增加至EDI对
液流变化不再起作用后趋于稳定, 且对VPD变化十
分敏感。
基金项目 国家重点基础研究发展计划(973)课题
(2013CB429902)和国家自然科学基金(41271036)。
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