Effects of woody elements on simulated canopy reflectance: Implications for forest chlorophyll content retrieval
Effects of woody elements on simulated canopy reflectance: Implications for forest chlorophyll content retrieval
复制标题
DOI：
10.1016/j.rse.2009.11.004
复制
发表时间：
2010-03
影响因子：
13.5
通讯作者：
J. Verrelst;M. Schaepman;Z. Malenovský;J. Clevers
中科院分区：
文献类型：
--
作者：
J. Verrelst;M. Schaepman;Z. Malenovský;J. Clevers
叶绿素a和b的含量(Cab)是反映植物实际健康状况的重要生物指标，可以用成像光谱学方法进行估算。然而，对于森林冠层，光谱响应和叶化学之间的关系受到背景(例如林下植物)、树冠结构和非光合作用植被(NPV，例如木质元素)的存在等因素的混淆--特别是在较老的森林中发现的可观数量的直立和倒下的枯木。利用辐射传输模型对木质针叶树冠的叶绿素含量进行了敏感性分析，并利用模拟的冠层顶部反射率数据分析了木质元素、叶面积指数(LAI)和冠层盖度(CC)对反演叶Cab含量的贡献。所使用的辐射传输模型包括两个链接的子模型：一个在叶层(展望)，另一个在冠层(飞行)。这是根据紧凑型高分辨率成像光谱仪(CHRIS)的波段设置生成的双向反射数据，从中计算出了叶绿素指数。大多数叶绿素指数对冠层Cab含量的预测效果优于单一波长，其中Maccioni指数([R780−R710]/[R780−R680])预测效果最好。我们展示了这一指数在三种不同针叶林类型(幼林、早熟林和老林)的结构信息方面的表现。模拟结果表明，冠层密度中等的林分最能反映冠层中叶绿素含量变化引起的光谱变化。然而，随着树冠NPV散射元素的增加，放大的Cab信号的强度减弱，特别是当树冠覆盖率超过30%时。LAI施加的扰动最小。我们的结论是，木质元素的光谱影响是一个重要的变量，当在异质林分，特别是当林分部分落叶或寿命较长时，在辐射转移方法中应该考虑到这一变量。
An important bio-indicator of actual plant health status, the foliar content of chlorophyll a and b (Cab), can be estimated using imaging spectroscopy. For forest canopies, however, the relationship between the spectral response and leaf chemistry is confounded by factors such as background (e.g. understory), canopy structure, and the presence of non-photosynthetic vegetation (NPV, e.g. woody elements)—particularly the appreciable amounts of standing and fallen dead wood found in older forests. We present a sensitivity analysis for the estimation of chlorophyll content in woody coniferous canopies using radiative transfer modeling, and use the modeled top-of-canopy reflectance data to analyze the contribution of woody elements, leaf area index (LAI), and crown cover (CC) to the retrieval of foliar Cab content. The radiative transfer model used comprises two linked submodels: one at leaf level (PROSPECT) and one at canopy level (FLIGHT). This generated bidirectional reflectance data according to the band settings of the Compact High Resolution Imaging Spectrometer (CHRIS) from which chlorophyll indices were calculated. Most of the chlorophyll indices outperformed single wavelengths in predicting Cab content at canopy level, with best results obtained by the Maccioni index ([R780−R710]/[R780−R680]). We demonstrate the performance of this index with respect to structural information on three distinct coniferous forest types (young, early mature and old-growth stands). The modeling results suggest that the spectral variation due to variation in canopy chlorophyll content is best captured for stands with medium dense canopies. However, the strength of the up-scaled Cab signal weakens with increasing crown NPV scattering elements, especially when crown cover exceeds 30%. LAI exerts the least perturbations. We conclude that the spectral influence of woody elements is an important variable that should be considered in radiative transfer approaches when retrieving foliar pigment estimates in heterogeneous stands, particularly if the stands are partly defoliated or long-lived.