树冠尺寸对冠层间隙率和聚集度指数的影响

Influences of crown size on the estimation of gap fraction and clumping index

冠层间隙率和叶片聚集度指数对植被拦截光和冠层辐射传输过程有重要影响。叶片往往以树冠的形式聚集在森林中,而树冠在诸多森林冠层几何光学模型中有重要作用。之前的研究主要集中于树冠的形状(例如圆柱、圆锥、椭球、圆锥+圆柱等)特征对冠层间隙率和叶片聚集度的影响。然而,树冠的结构特征除了包括形状特征外,还包括尺寸特征。事实上,树冠的尺寸特征是植被在长期进化过程中与自然环境相互作用的结果,且比树冠形状特征具有更明显的地理空间特征。本文首先修改了前人常用的用于描述树冠分布的泊松分布模型,其次,利用超几何模型更为真实地、定量地描述树冠之间的排斥效应和树冠的分布情况;最后,以椭球树冠为例,设定了从"瘦高"型到"矮胖"型等7种尺寸的树冠,分别通过固定树冠半径和树冠体积两种途径探讨了树冠尺寸特征对冠层间隙率和叶片聚集度的影响。结果表明:(1)无论固定树冠半径,还是固定树冠体积,树冠尺寸特征对冠层间隙率和叶片聚集度指数的影响,均比树冠形状特征对间隙率和叶片聚集度指数的影响明显;(2)树冠的尺寸对冠层间隙率和叶片聚集度的影响非常显著,且具有明显的规律性。树冠在观测方向的投影面积越大,冠层间隙率越小,叶片聚集指数度越大,冠层内叶片越趋于泊松分布。

Gap fraction and foliage clumping index play key roles in plant light interception; therefore, they have strong influences on plant growth and canopy radiative transfer processes. Leaves always aggregate in crowns, which are important objects innumerous geometric-optical models of forests. Researchers have mainly focused on the influences of crown shape （e.g., cylinder, cone, ellipsoid, and cone ＋ cylinder） on gap fraction and foliage clumping index. However, size is also an important characteristic of tree crowns. Crown sizes result from the interactions between plants and environments during the long-term evolution process of the plants. In fact, crown size characteristics have more obvious geographical spatial features than crown shape. Therefore, crown size should be given considerable attention when studying plant light interception and canopy radiative transfer processes in the fields of phytogeography, remote sensing and global change. The main objective of this study is to exhibit how crown size characteristics influence the gap fraction and foliage clumping index of forest canopies. First, a simple and general distance factor, which is defined as the relative allowable shortest distance between centers of any two crowns divided by the mean diameter of tree crowns, is proposed to quantitatively describe the degree of repulsion effect among trees in forest canopies. Second, the Poisson distribution model of trees is completely replaced by the hypergeometric model, which is more suitable for quantitatively describing the repulsion effect and spatial relationship among trees in forest stands. Finally, seven sizes of ellipsoids （from prolate ellipsoids to oblate ellipsoids） are selected for the tree crowns, and the influences of crown size on gap fraction and foliage clumping index are exhibited by means of fixing the radius and the volume of the tree crowns, respectively. Results show that the following： （1） the influences of crown size characteristics are significantly greater than the effects of crown shape on gap fraction and foliage clumping index, regardless if the radius or volume of tree crowns are fixed. （2） From prolate ellipsoids to oblate ellipsoids, crown size characteristics show significant and regular effects on both gap fraction and foliage clumping index. In extreme cases, when θ=-60°, the canopy gap fraction when ellipsoid height （Hb） is 16 m is 92.8% lower than that when Hb=0.25 m, and the foliage clumping index when Hb=16 m is 261.8% greater than that when Hb=0.25 m when the crown radius is fixed. At θ=0°, the canopy gap fraction when Hb= 16 m is 414.1% greater than that when Hb=0.25 m, and the foliage clumping index when Hb=0.25 m is 11397.2% greater than that when Hb=16 m when the volume of crown is fixed. The larger the projected area of the crown in the observed direction, the lower value of the gap fraction and the higher value of the foliage clumping index, indicating that leaves tend to distribute randomly in forest canopies.