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地理科学, 2020, 40(5): 823-832 doi: 10.13249/j.cnki.sgs.2020.05.017

1982-2015年全球植被变化及其与温度和降水的关系

李茂华,1, 都金康,1,2, 李皖彤1, 李闰洁1, 吴森垚1, 王姗姗1

1.南京大学地理与海洋科学学院,江苏 南京 210093

2.江苏省地理信息资源开发与利用协同创新中心,江苏 南京 210093

Global Vegetation Change and Its Relationship with Precipitation and Temperature Based on GLASS-LAI in 1982-2015

Li Maohua,1, Du Jinkang,1,2, Li Wantong1, Li Runjie1, Wu Senyao1, Wang Shanshan1

1. School of Geography and Ocean Science, Nanjing University, Nanjing 210093, Jiangsu, China

2. Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing 210093, Jiangsu, China

通讯作者: 都金康,教授。E-mail: njudjk@163.com

收稿日期: 2019-03-12   修回日期: 2019-06-25   网络出版日期: 2020-05-10

基金资助: 国家自然科学基金项目.  41371044
国家自然科学基金项目.  41471343
国家自然科学基金项目.  41771029

Received: 2019-03-12   Revised: 2019-06-25   Online: 2020-05-10

Fund supported: National Natural Science Foundation of China.  41371044
National Natural Science Foundation of China.  41471343
National Natural Science Foundation of China.  41771029

作者简介 About authors

李茂华（1994-）,男,四川德阳人,硕士研究生,主要从事环境遥感与GIS应用方面研究E-mail:maohua.li7@foxmail.com , E-mail：li7@foxmail.com

摘要

采用1982-2015年的GLASS-LAI (Global Land Surface Satellite-Leaf Area Index )遥感数据和CRU(Climatic Research Unit)气象数据,利用Mann-Kendall趋势法分析了过去34 a全球9种植被的叶面积指数(Leaf Area Index,LAI)时空变化特征;使用相关分析和逐步线性回归分别探讨了全球9种植被LAI与降水、温度的年际与月关系。结果表明：① 全球植被总体呈现绿化趋势,其中变化较大的是草原、稀树草原、常绿阔叶林和多树草原;在植被生长的绿化和褐化趋势中,面积占比最大的植被类型均为草原,说明草原生态系统易受环境因素的影响。② 从年际关系看,草原和开放灌丛的LAI与年均降水多呈正相关关系,而温度对不同纬度植被的LAI存在正负2种影响。其原因为温度升高对中低纬度的植被生长有抑制作用,而对高纬度地区植被生长有促进作用。③ 从年内关系看,南半球降水和温度共同作用于植被的生长;而北半球除常绿阔叶林的生长与温度关系更为紧密外,其它类型植被的生长主要受降水影响。④ 逐步线性回归结果表明,当月温度的升高对常绿阔叶林、混交林和农作物的生长具有促进作用,而多树草原和草原2种植被的生长受当月降水的影响最为显著。

关键词： 全球植被 ; LAI ; 温度 ; 降水

Abstract

We analyzed the spatial-temporal changes in global vegetation and their dynamic responses to temperature and precipitation using GLASS-LAI (Global Land Surface Satellite-Leaf Area Index ) and CRU (Climatic Research Unit) meteorological data from 1982 to 2015. The results showed that: 1) Global vegetation showed an overall greening trend, which is paricularly significant over grasslands, savannas, evergreen broadleaf forests and woody savanas. Grasslands have the largest greening and browning area, indicating their vulnerability to climate. 2) In terms of the inter-annual relationships, the LAI of grasslands and open shrublands were overall positively correlated with precipitation, while temperature had different effects on the growth of vegetation at diverse latitudes. This could be attributed to that, the growth of vegetation at middle and low latitudes was inhibited by the increasing temperature, while facilitated by the rising temperature at high latitudes. 3) In terms of the intra-annual relationships, precipitation and temperature together promoted vegetation growth in the southern hemisphere, however, the vegetation in the northern hemisphere was mainly affected by precipitation, except evergreen broadleaf forests. 4) The results of stepwise multiple regression indicated that the rising instant temperature had positive influence on the growth of evergreen broadleaf forests, mixed forests, and croplands, while woody savannas and savannas were significantly impacted by the instant precipitation.

Keywords： global vegetation ; LAI ; temperature ; precipitation

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本文引用格式

李茂华, 都金康, 李皖彤, 李闰洁, 吴森垚, 王姗姗. 1982-2015年全球植被变化及其与温度和降水的关系. 地理科学[J], 2020, 40(5): 823-832 doi:10.13249/j.cnki.sgs.2020.05.017

Li Maohua, Du Jinkang, Li Wantong, Li Runjie, Wu Senyao, Wang Shanshan. Global Vegetation Change and Its Relationship with Precipitation and Temperature Based on GLASS-LAI in 1982-2015. Scientia Geographica Sinica[J], 2020, 40(5): 823-832 doi:10.13249/j.cnki.sgs.2020.05.017

全球植被变化是全球变化研究中非常重要的一个课题,植被作为陆地生态系统的主体,在陆地-大气的交互影响下,对全球物质和能量的循环起着决定性作用[1]。全球植被变化既受限于气候条件,对温度和降水变化敏感响应,又对气候变化起到反馈作用,影响着地表反照率、粗糙度和陆地碳储量等地球理化属性[2]。近年来,在全球、全国或是区域尺度下利用遥感数据对植被动态变化及其与气候关系的研究都取得了很多进展[3]。Nemani等研究了1982-1999年全球陆地净初级生产力(Net Primary Productivity, NPP)与气候的关系,认为全球气候朝着有利于植被生长的方向发展[4]。一些学者利用归一化植被指数(Normalized Difference Vegetation Index, NDVI),发现北半球高纬度地区植被生长季的NDVI与温度显著正相关[5]。温度对于寒冷的草原生态系统作用显著,而增长的降水对除北极区外的草原植被生长起促进作用[6]。在全球半干旱区域尺度上,也有学者提出植被总体呈增长趋势,并讨论了降水与温度对植被变化不同的作用程度[7,8,9]。于德永等研究发现东亚地区1982-1999年NPP在波动中呈现出增加趋势,年均NPP与年均温度呈显著相关关系[10]。朴世龙等分析认为,全国尺度上NDVI的增加源于温度升高,而区域尺度上则与降水更相关[11]。国内区域尺度上,李月臣等分析NDVI数据发现中国北方13个省1982-1999年间植被总体呈现增加趋势,植被变化与气温显著相关而与降水无显著相关,气温升高导致生长期提前和生长季延长是植被增加的一个重要原因[12]。方利等分析黑龙江流域植被NDVI数据,发现植被的NDVI在春季主要受温度影响,夏季受降水影响,秋季林地NDVI与温度正相关、草地NDVI与降水正相关[13]。

前人分析研究NVDI或者其模拟的NPP等分别发现并探讨了各自研究区域下的植被变化,及其与温度、降水的关系,对于理解陆地-大气的交互作用很有意义。遥感技术为全球尺度下长时间序列的植被监测提供了可能性,很多科学工作者致力于研究各种植被指数,使其能够很好地反映植被生产力,但这些植被指数各有优劣,具有不同的应用场景[14]。其中经常使用的NDVI,在植被稀疏时易受土壤背景影响,而植被密集覆盖度较高时则会饱和[15]。叶面积指数作为植被指数之一,不仅能很好地反映植被生产力,而且对NDVI在植被高覆盖区易饱和的缺点进行了有效地改进[16]。梁博毅等人利用GLASS-LAI（Global Land Surface Satellite-Leaf Area Index,全球陆表叶面积指数）数据分析近30 a的亚马逊热带雨林变化,并重点论证了此数据的可用性[17]。有学者通过类比多种LAI数据产品,提出近30 a全球植被呈绿化趋势,其中气候变化起重要作用,但并未就植被对气候响应的关系做深入探讨[18]。

本文利用1982-2015年的GLASS-LAI数据,结合同期的温度和降水数据,在全球尺度下,针对不同的植被生态系统,系统和全面地解读植被生长的时空变化及其与温度和降水的关系,旨在研究全球不同类型植被的时空变化特征及其与温度和降水的关系,以期科学地阐述气候对全球植被的影响,有助于了解不同类型的植被在陆地-大气交互作用下的响应。

1 数据与方法

1.1 数据来源与处理

GLASS-LAI数据产品,来源于国家科技基础条件平台——国家地球系统科学数据共享服务平台(http://www.geodata.cn),时间跨度为1982-2015年,时间分辨率为8 d,空间分辨率为0.05°,集合了AVHRR LAI(1981-2000)和MODIS LAI (2000-2015) 2个数据,使用广义回归神经网络训练法进行数据融合。通过与其他的产品进行对比验证,发现该产品具有很高的可靠性[19,20,21]。全球范围1982-2015年的气象数据来源于英国东安格利亚大学气候研究中心逐月的降水和温度数据集(CRU TS 4.01)[22],空间分辨率为0.5°,该数据已得到广泛应用和评价[23,24,25]。全球土地覆盖类型数据为0.05°空间分辨率的IGBP(International Geosphere-Biosphere Program)的IGBP-DIScover[26] (http://www.igbp.net),该产品将全球覆被划分为17种类型,本研究仅选择了其中9种重要植被类型,包括常绿阔叶林、混交林、郁闭灌丛、开放灌丛、农作物和自然植被混合、农作物、草原、稀树草原和多树草原（图1）。由于各种数据的投影和空间分辨率不一致,故对所有数据定义统一的投影(WGS 84),空间分辨率重采样到0.5°,并将8 d时间分辨率的叶面积指数（Leaf Area Index,LAI）,数据采用均值法获得逐月和逐年的数据。

图1

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图1   基于IGBP分类的全球主要植被类型的空间分布

Fig.1   Spatial distribution of Global main classes of vegetation based on IGBP classification


1.2 研究方法

1.2.1 全球LAI的年际趋势分析

利用MK检验法进行趋势分析,计算出植被不同程度的生长趋势。该方法具有无需样本服从特定分布,也不受极少数极端值干扰,适用于计算年均LAI的变化趋势[27,28]。结果需通过P<0.05的显著性检验,其中0.01<P<0.05为拟合效果显著,P<0.01为拟合效果非常显著。

1.2.2 全球LAI与降水、温度的年际相关分析

使用1982-2015年月LAI、降水、温度数据利用均值法获得逐年数据,并按照9种植被类型对年数据进行分类求平均。基于皮尔逊相关分析法[29],分别对LAI和降水、LAI和温度进行相关分析。其中,相关系数为正,表示2因子存在正相关关系;反之,存在负相关关系。

1.2.3 全球LAI与降水、温度的年内季节性关系与滞后效应

将1982-2015年的月LAI、降水和温度数据按9种植被类型计算得到同一月份的平均值,考虑到南北半球的植被生长季不同,所以分别对南北半球进行LAI与降水和温度的年内季节性讨论。利用逐步线性回归方法,探讨降水和温度的即时和滞后效应对LAI的影响[30]。将当月LAI作为因变量,把当月、前1个月、前2个月、前3个月的降水和温度作为逐步控制的自变量,即：当月降水（P0）、当月温度（T0）、前1个月降水（P1）、前1个月温度（T1）、前2个月降水（P2）、前2个月温度（T2）、前3个月降水（P3）、前3个月温度（T3）。逐步线性回归的公式如下：

yt=a0+∑i=1majxjt+εt

式中, yt为月平均LAI值、 xjt分别用P0、P1、P2、P3、T0、T1、T2、T3代入计算, a0代表回归常数, aj是每个自变量的回归系数, εt表示残差。所有参数的数值都进行了标准化,且结果需通过0.05显著性检验。

2 结果与分析

2.1 全球LAI年际趋势分析

1982-2015年全球LAI总体呈显著增长趋势（图2）,该部分面积占整个研究区有效趋势（通过0.05显著性检验）面积的84.98%（表1）,其中LAI呈非常显著(P<0.01)增长趋势的面积占69.03%,主要位于北美中部和东部、南美中部、非洲中部和南部、萨赫勒、欧亚大陆西部、东南亚、北亚西部和澳大利亚东北部。仅15.02%的面积呈显著下降趋势,显著退化的地区主要位于北美西部、南美南部、非洲萨赫勒以南部分地区、东亚北部以及澳大利亚西南部。表1显示,草原(22.46%)、稀树草原(13.35%)、常绿阔叶林(11.55%)和多树草原(11.51%)对全球植被的绿化趋势贡献最大,而LAI呈下降趋势的植被主要为稀树草原(2.83%)、开放灌丛(3.24%)和草原(4.91%)3种类型。不同区域的草原存在明显的增长或退化趋势,可能是由于草原生态系统脆弱,极易受人为干扰和气候变化的影响[5,31]。

图2

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图2   1982-2015年全球年均LAI变化趋势的空间分布

Fig.2   Spatial distribution of global annual mean LAI trends from 1982 to 2015


表1   1982-2015年全球9种植被类型年均LAI显著变化的占比(%)

Table 1  Proportions of areas with significant changing of annual mean LAI among different classes of global nine vegetation types in 1982-2015(%)

下降
P < 0.01		下降 0.01<
P<0.05		上升
P<0.01		上升0.01<
P<0.05		总计
常绿阔叶林0.450.249.991.5612.24
混交林0.230.145.940.807.11
郁闭灌丛0.080.040.090.040.24
开放灌丛1.821.426.222.2011.67
多树草原1.080.569.911.6013.15
稀树草原1.821.0110.582.7716.19
草原2.981.9317.544.9227.37
农作物0.690.418.151.9111.16
农作物和自然
植被混合		0.060.050.620.140.87
总计9.215.8169.0315.95100.00

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从图3可以看出,除郁闭灌丛和开放灌丛没有通过0.05显著性检验外,其它植被类型的LAI均呈现非常显著（P<0.01）上升趋势。其中,上升速率最高的为常绿阔叶林（0.036/a）,表明其生长变动的幅度较大。其它6种植被类型的年上升速率在0.002/a~0.018/a,速率从高到低依次为：农作物和自然植被混合>混交林>多树草原>稀树草原>农作物>草原。

图3

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图3   1982-2015年全球9种植被类型的年均LAI变化趋势

Fig.3   Temporal evolution of annual mean LAI among different classes of global nine vegetation types during 1982-2015


2.2 全球LAI与降水、温度的年际相关分析

图4a为年均LAI和降水进行相关分析的空间分布结果。非常显著(P<0.01)的正相关关系广泛分布在澳大利亚、非洲南部、萨赫勒、东亚北部、欧亚大陆中部、美国中部和南美东南部区域,占有效面积的57.01%,显著(0.01<P<0.05)正相关占比36.72%,主要分布在上述地区及其周边;LAI与降水呈现负相关的区域只占6.27%,主要分布在加拿大巴芬岛南部。表2显示各类型植被LAI与降水呈负相关的比例非常低,仅占到有效面积的6.27%。草原和开放灌丛与降水呈现非常显著的正相关关系,分别占了研究区有效面积的29.14%和12.69%,与前人研究结论相似,由于这2种植被类型大多处于干旱与半干旱区,降水的增加很大程度上会促进植被生长[11,32];其他呈现正相关关系的依次是农作物(12.34%)、稀树草原(10.65%)、常绿阔叶林(3.51%)、多树草原(3.47%)。

图4

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图4   1982-2015年全球年均LAI和降水(a)、年均LAI和温度(b)的相关性P值的空间分布

Fig.4   Global spatial distribution of the p-value for the correlation between annual mean LAI and precipitation (a), annual mean LAI and temperature (b) from 1982 to 2015


表2   1982-2015年全球9种植被年均LAI和降水显著相关的占比(%)

Table 2  Proportions of areas with significant correlation between annual mean LAI and participation among different classes of global nine vegetation types in 1982-2015 (%)

负相关
P < 0.01		负相关
0.01< P < 0.05		正相关
P < 0.01		正相关
0.01< P < 0.05		总计
常绿阔叶林0.110.321.052.463.95
混交林0.080.370.250.971.67
郁闭灌丛0.000.000.520.230.74
开放灌丛0.270.9112.695.2919.16
多树草原0.410.721.192.284.59
稀树草原0.110.485.215.4411.24
草原0.841.2629.1414.1745.41
农作物0.080.256.785.5612.67
农作物和自然
植被混合		0.010.040.180.330.57
总计1.914.3657.0136.72100.00

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年际LAI和温度的相关关系空间分布见图4b,与降水不同的是,显著的负相关关系出现在澳大利亚中部与北部、非洲东部以及南美中部,占总面积的22.01%;而占比77.99%的显著正相关出现在北半球高纬地区,如欧亚大陆,以及赤道附近降水充沛的区域,如非洲中部、南美与东南亚。从表3可以看出,开放灌丛和草原的LAI与温度明显存在2种相关关系,其中开放灌丛因所处不同地区,温度对其有正负两种影响,澳洲中西部处于高温缺水环境中,升温对其生长起到抑制作用,只有增加降水才能促进其生长[33],而俄罗斯北部和东部处于高纬度地区,温度对开放灌丛的生长则起到促进作用;草原生态系统较为脆弱,不仅受降水影响,温度对其也有较大影响。正相关关系中,除草原(17.29%)外,常绿阔叶林(14.88%)占比最大。因为常绿阔叶林生态系统较为稳定,通常受到较好的水热条件,温度的升高更有助于其进行光合作用,增加植被生产力。温度升高对LAI有促进作用的植被类型依次为：稀树草原(13.54%)、多树草原(10.05%)、农作物(8.37%)、混交林(5.18%)及农作物和自然植被混合(0.82%)。

表3   1982-2015年全球9种植被年均LAI和温度显著相关的占比(%)

Table 3  Proportions of areas with significant correlation between annual mean LAI and temperature among different classes of global nine vegetation types in 1982-2015 (%)

负相关
P< 0.01		负相关
0.01< P < 0.05		正相关
P < 0.01		正相关
0.01< P < 0.05		总计
常绿阔叶林0.670.4610.524.3616.02
混交林0.040.162.502.685.39
郁闭灌丛0.140.150.000.030.32
开放灌丛4.723.284.253.5815.84
多树草原0.310.475.544.5110.83
稀树草原1.201.417.885.6616.14
草原3.663.939.288.0124.88
农作物0.450.884.613.769.69
农作物和自然
植被混合		0.040.050.430.390.90
总计11.2310.7845.0132.98100.00

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2.3 全球 LAI与降水、温度的年内季节性响应与滞后效应

图5是南北半球不同植被34 a月均LAI随降水、温度的季节性波动曲线。北半球除常绿阔叶林生长全年较为稳定外,其它植被的生长季明显开始于3月份,LAI在7月份达到最大值之后,逐月下降直到次年3月（图5a）。月降水量总体上从3月开始增加,到8月达到最大值,随后逐渐下降（图5c）。北半球温度的季节性波动较为复杂,除常绿阔叶林全年波动不明显外,其余可分为2种模式：一种是多峰模式,即全年多个月份出现局部温度峰值,比如：郁闭灌丛、开放灌丛、草原、稀树草原和多树草原等;另一种是单峰模式,全年仅在8月出现一次最高温的模式,比如混交林、农作物、农作物和自然植被混合等（图5e）。综合图5a、c、e可以看出,常绿阔叶林LAI变化趋势与温度变化较为一致,受降水的影响较小;农作物及农作物和自然植被的LAI与降水和温度的变化趋势较为一致;其它类型植被的LAI则与降水的变化趋势较为一致。

图5

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图5   1982-2015年南（b,d,f）北（a,c,e）半球主要植被类型LAI、降水、温度的年内波动

Fig.5   Intra-annual variations of LAI, precipitation and temperature among different classes of global main vegetation in the Northern (a,c,e) and Southern (b,d,f) Hemisphere in 1982-2015


南半球植被的生长季从8月份开始,到次年的2月份达到峰值,且温度的季节性波动曲线与LAI的季节性波动曲线非常吻合（图5b、f）,说明温度的升高对南半球植被生长整体呈现促进作用。月降水量在2月份达到峰值,然后开始下降,到9月份达到最低值,之后再逐渐上升（图5d）,总体上看,降水同样控制着LAI的变化。草原、多树草原、稀树草原的LAI在3月达到最大值,而降水在3月份有小幅下降,说明降水对LAI可能存在滞后效应。

本研究在前人研究的基础上,增加LAI对降水、温度的季节性响应与滞后效应分析,深化了关于全球主要植被类型的LAI与气候因子的年内响应关系的探讨。通过逐步线性回归模型从以下8个气候因子：当月降水(P0)、当月温度(T0)、前1个月降水(P1)、前1个月温度(T1)、前2个月降水(P2)、前2个月温度(T2)、前3个月降水(P3)、前3个月温度(T3)来选择合适的气候因子,并基于拟合结果,阐述滞后和即时的降水和温度对全球不同类型的植被LAI的影响（表4）。

表4   1982-2015年全球9种植被月均LAI和多种气候因子的逐步线性回归拟合

Table 4  Stepwise multivariate regression between monthly mean LAI and climate predictors as identified among different classes of global nine vegetation types from 1982 to 2015

植被类型逐步线性回归R2F
常绿阔叶林LAI=0.575T0+0.328T3+ 0.005P3-19.0770.34371.354
混交林LAI=0.1T0+0.058T3+0.036P0-0.059T1-0.011P2-0.02P3+0.380.878484.502
郁闭灌丛LAI=0.011T3+0.002P0+0.002P1+0.3360.479125.039
开放灌丛LAI=0.016T3+0.006P0-0.034T0-0.02T1-0.011P2-0.006P1+1.360.889542.541
多树草原LAI=0.031P0+0.007P1-0.072T1-0.034T0-0.018P3-0.009P2+2.440.879489.831
稀树草原LAI=0.053T3+0.012P0-0.072T0-0.028T1-0.023T2-0.014P2-0.007P3-0.005P1+3.8450.722131.927
草原LAI=0.008P0-0.06T0-0.011P3-0.007P2-0.007P1+2.3670.671165.877
农作物LAI=0.112T0+0.026T3+0.008P0-0.008P1-0.003P2-1.2510.892669.094
农作物和自然植被混合LAI=0.155T0+0.005P0-0.052T1-0.002P3-0.5310.742291.878

注：T0当月温度,T1前1个月温度,T2前2个月温度,T3前3个月温度,P0当月降水,P1前1个月降水,P2前2个月降水,P3前3个月降水。

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从拟合结果看,只有常绿阔叶林和郁闭灌丛拟合结果的相关系数较低,其他植被拟合结果的相关系数均较高。温度T3对除了草原、多树草原和农作物与自然植被混合外的植被的LAI起到促进作用,其中对开放灌丛,郁闭灌丛和稀树草原起到主导作用。农作物、农作物及自然植被混合主要受T0正影响,即农作物主要受当月温度影响,因为受人类农耕活动影响,农作物有足够的水分条件,当温度升高,且没有达到最适宜温度时,农作物的光合作用增强,生产力增加[34]。多树草原和草原主要表现出受降水P0正影响,说明促进其生长的首要因素是当月降水。

温度和降水对不同植被的LAI还存在抑制作用,与前人研究结论相似的是温度的影响程度普遍大于降水[5,6,12]。温度T1对除了常绿阔叶林、农作物、郁闭灌丛和草原外的其他植被均有不同程度的抑制,其中对混交林、多树草原和农作物与自然植被混合起到了主要抑制作用;降水P1、P2对开放灌丛和农作物,P2、P3对混交林和多树草原,前3个月P1、P2和P3对稀树草原和草原均存在抑制作用,但影响普遍较小。

3 结论与讨论

3.1 结论

基于1982-2015年GLASS-LAI遥感数据,结合同期CRU降水和温度数据,分析全球主要植被类型近34 a的年际生长趋势及其对降水、温度变化的响应,研究总结出以下结论：

1） 全球植被总体呈现绿化趋势,LAI显著增长的面积是显著下降的区域的5倍以上,广泛分布在北美、南美、非洲撒哈拉以南、欧亚大陆以及澳大利亚东北部。草原、稀树草原、常绿阔叶林和多树草原呈现绿化趋势的面积最大,其中常绿阔叶林LAI的增长速率最大。

2） 随着全球气候变化和人类活动的增加,草原生态系统表现出较高的敏感性,既存在显著退化的区域,也存在显著绿化的区域,且影响因素各不相同。在较为干旱的区域生长的植被,尤其是缺水环境下的草原与开放灌丛,降水的增加是利于其生长主要的条件,而温度升高对其生长起到抑制作用。在水分条件充足的区域,以常绿阔叶林为代表的植被LAI往往与温度呈现较好的正相关关系。

3） 从整体上看,南半球降水和温度的升高对植被生长起促进作用,但降水在个别月份的小幅波动对多树草原、稀树草原和混交林的当月LAI值没有影响,却对后一月LAI值有影响,说明降水可能存在一定的滞后效应。北半球只有常绿阔叶林受到温度同步控制并表现正相关关系,其它类型的植被LAI季节性波动主要由降水影响。

4） 从逐步线性回归的结果看,当月降水、温度仍然是最重要的年内气候因子,当月降水的增加促进了混交林、多树草原和草原的生长;而当月温度的升高抑制了开放灌丛和稀树草原的生长,却促进常绿阔叶林、混交林和农作物的生长。

3.2 讨论

在全球变暖的背景下,本文利用34 a的长时序LAI与气象数据,较为完整地分析了全球9种主要植被的变化趋势,系统地阐述了植被与降水、温度的年际和年内关系。本研究有助于进一步了解,在全球陆地-大气交互作用下,不同类型植被与温度、降水的年际与年内关系。

虽然NVDI和NPP等都常用来研究植被动态变化,但NDVI易受云和水汽影响,在植被覆盖度较低或较高的情况下会出现不敏感或饱和的现象[15,35],如在分析茂密的常绿阔叶林或非生长季的草原时会存在一定偏差。NPP有多种估算模型[36],有不少需要借助NDVI来模拟,可能引入后者存在的误差。本研究使用的LAI被认为能够稳健的描述植被动态变化[37],具体使用的GLASS-LAI数据,去除了云和阴影的影响,具有时间跨度长且时间空间连续等特性,弥补了NDVI存在的缺陷,被认为有较高的可靠性[38,39,40]。

需要说明的是,本研究结果存在一定的不确定性,植被绿化的因素除降水和温度外,还受太阳辐射、CO2浓度、人类活动等诸多因素影响;土地覆盖类型仅为一期数据,没有考虑土地覆被类型的变化对实验结果产生的影响;受降水和温度数据的空间分辨率和时间分辨率的限制,LAI数据重采样之后可能存在混合像元的问题,因此后续工作还需结合更多更准确的数据,深入地探讨全球性与区域性植被LAI的时空变化及其原因。

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