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On the relationship of NDVI with leaf area index in a deciduous forest site

机译：落叶林地NDVI与叶面积指数的关系

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Numerous studies have reported on the relationship between the normalized difference vegetation index (NDVI) and leaf area index (LAI), but the seasonal and annual variability of this relationship has been less explored. This paper reports a study of the NDVI-LAI relationship through the years from 1996 to 2001 at a deciduous forest site. Six years of LAI patterns from the forest were estimated using a radiative transfer model with input of above and below canopy measurements of global radiation, while NDVI data sets were retrieved from composite NDVI time series of various remote sensing sources, namely NOAA Advanced Very High Resolution Radiometer (AVHRR; 1996, 1997, 1998 and 2000), SPOT VEGETATION (1998-2001), and Terra MODIS (2001). Composite NDVI was first used to remove the residual noise based on an adjusted Fourier transform and to obtain the NDVI time-series for each day during each year. The results suggest that the NDVI-LAI relationship can vary both seasonally and inter-annually in tune with the variations in phenological development of the trees and in response to temporal variations of environmental conditions. Strong linear relationships are obtained during the leaf production and leaf senescence periods for all years, but the relationship is poor during periods of maximum LAI, apparently due to the saturation of NDVI at high values of LAI, The NDVI-LAI relationship was found to be poor (R2 varied from 0.39 to 0.46 for different sources of NDVI) when all the data were pooled across the years, apparently due to different leaf area development patterns in the different years. The relationship is also affected by background NDVI, but this could be minimized by applying relative NDVI, Comparisons between AVHRR and VEGETATION NDVI revealed that these two had good linear relationships (R{sup}2=0.74 for 1998 and 0.63 for 2000). However, VEGETATION NDVI data series had some unreasonably high values during beginning and end of each year period, which must be discarded before adjusted Fourier transform processing. MODIS NDVI had values greater than 0.62 through the entire year in 2001, however, MODIS NDVI still showed an "M-shaped" pattern as observed for VEGETATION NDVI in 2001. MODIS enhanced vegetation index (EVI) was the only index that exhibited a poor linear relationship with LAI during the leaf senescence period in year 2001. The results suggest that a relationship established between the LAI and NDVI in a particular year may not be applicable in other years, so attention must be paid to the temporal scale when applying NDVI-LAI relationships.
机译：关于归一化植被指数（NDVI）和叶面积指数（LAI）之间的关系的大量研究已有报道，但是这种关系的季节和年度变化却很少被研究。本文报告了从1996年到2001年之间在落叶林站点上对NDVI-LAI关系的研究。使用辐射转移模型估算了六年的森林LAI模式，其中输入了全球辐射的上下冠层测量值，而NDVI数据集则是从各种遥感源的复合NDVI时间序列中获得的，即NOAA高级超高分辨率辐射计（AVHRR; 1996、1997、1998和2000），SPOT VEGETATION（1998-2001）和Terra MODIS（2001）。复合NDVI首先用于基于调整后的傅立叶变换消除残留噪声，并获取每年每一天的NDVI时间序列。结果表明，NDVI-LAI关系可以随树木物候变化和环境条件的时间变化而随季节和年际变化。在所有年的叶片生产和叶片衰老期间都获得了很强的线性关系，但是在最大LAI期间该关系很差，这显然是由于LDVI值较高时NDVI饱和，发现NDVI-LAI关系为当收集所有数据的年份时，R2差（R2在NDVI的不同来源范围从0.39到0.46），这显然是由于不同年份的叶面积发展模式不同。该关系也受本底NDVI的影响，但是可以通过应用相对NDVI来最小化。AVHRR和VEGETATION NDVI的比较表明，两者具有良好的线性关系（1998年的R {sup} 2 = 0.74和2000年的0.63）。但是，VEGETATION NDVI数据系列在每年开始和结束期间具有一些不合理的高值，必须在调整傅立叶变换处理之前将其丢弃。在2001年全年，MODIS NDVI的值均大于0.62，但是，正如2001年在VEGETATION NDVI上所观察到的那样，MODIS NDVI仍显示出“ M形”模式。MODIS增强植被指数（EVI）是唯一表现较差的指数2001年叶片衰老期间与LAI的线性关系。结果表明，在特定年份中LAI和NDVI之间建立的关系可能在其他年份不适用，因此在应用NDVI-时必须注意时间尺度LAI关系。

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