首页 >  2014, Vol. 18, Issue (4) : 752-759

摘要

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引用本文:

DOI:

10.11834/jrs.20143205

收稿日期:

2013-08-26

修改日期:

2014-02-21

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树冠形状对孔隙率及叶面积指数估算的影响分析
1.中国科学院 遥感与数字地球研究所 遥感科学国家重点实验室, 北京 100101;2.中国科学院大学, 北京 100049
摘要:

叶片在树冠尺度的聚集是森林场景中的重要聚集形式,模型中常假设树冠为规则的几何形体(椭球、圆锥、圆锥+圆柱等)。对树冠形状归属进行判断时界限并不明显,从而具有很强的主观性。本文首先扩展了Nilson的森林孔隙率模型,使其适用于椭球、圆锥、圆锥+圆柱等3种常见形状的树冠,并基于该模型分析了孔隙率、聚集指数对树冠形状的敏感性。同时,本文还分析了树冠形状对叶面积指数(LAI)地面间接测量精度的影响。基于不同形状树冠的模拟数据分析发现,树冠的体积、投影面积是树冠形状产生作用的主要因子,在冠层底部椭球形树冠和圆锥+圆柱形树冠的平均孔隙率、聚集指数都非常接近,而圆锥形树冠与两者存在较大差异。树冠形状的错误设置在极端情况下可导致估算的真实LAI误差超过25%。

Effect of crown shape on the estimation of gap probability and leaf area index
Abstract:

Tree crowns are usually simulated as basic geometrical shapes in geometric-optical models. However, the selection of specific shape often based on visual judgment and may induce uncertainties, and the uncertainties would propagate to the practical application of the model, retrieving Leaf Area Index (LAI), for example. This study aimed to address the potential effect of crown shape by conducting a simulation experiment. In this study the Nilson's gap probability model was enhanced to ensure compatibility with commonly encountered crown shapes, such as ellipsoid, cone and cone + cylinder. The enhanced Nilson's gap probability model can be used to calculate volumes and projected area profiles of the three shapes. This enhanced model was then compared with a gap probability model derived from Beer's Law. Clumping index was then formulated. This can be described as a function of canopy structure parameters, such as crown volume, projected area, and stem density. On the basis of the enhanced Nilson's gap probability model and formulated clumping index, we analyzed the sensitivity of gap probability, clumping index, and LAI retrieval to crown shape. In this study, the gap probability was found sensitive to crown shape. This result could be attributed to the volumes and projected areas of different crown shapes. A high difference in gap probabilities was observed between various crown shapes in the middle of the canopy. Below the canopy, crowns with ellipsoid and cone + cylinder shapes exhibited very similar (relative error < 9.7%) gap probabilities. By contrast, these gap probabilities were very different from those of cone-shaped crown (largest relative error > 36.7%). Our results further showed that clumping index is a function of crown shape. Similar to gap probability, the clumping indexes of crowns with ellipsoid and cone + cylinder shapes were very similar below the canopy. By contrast, these indexes differed from those of cone-shaped crown. Hence, crown shape should be considered when LAI is retrieved using ground measurements. Relative error likely reached >25% if crowns were set in wrong shapes. Gap probability and clumping index underneath the canopy were very similar when crowns were modeled in ellipsoid or cone + cylinder shapes; however, a considerable difference was observed if crowns were modeled in a cone shape. The volumes and projected areas of different crown shapes could be accounted for similarities or differences in results. Based on the retrieved LAI, crown shapes could account for 25% of the relative retrieval error. Therefore, crown shapes should be carefully specified to obtain a satisfactory result. Our analysis results were derived from simulation experiments; in our future work, ground experiments would be performed to verify our conclusions.

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