林窗内光照强度的测量方法

林窗内光强存在复杂的时空变化,对植物更新和生长有着重要影响,因此,林窗光照强度的快速测量方法是生态学家十分关注的问题.目前,测量林窗光强的方法可分为3类:(1)直接测量法采用光量子探头等仪器直接测量光强,但测量林窗光强异质性时十分费时费力.(2)模型估测法通过几何计算可快速估测林窗任意位置光强,但模型估测法将林窗简化为圆柱体或椭圆柱体,并忽略了许多林窗光强的影响因素,这极大影响了它的测量精度.(3)相片法采用半球面影像等相片间接计算相片拍摄点的光强,但测量林窗光强异质性时需要在林窗内拍摄大量相片;相片法具有较高精度,可区分直射光和散射光,其中,基于半球面影像的林窗光指数(gap light index)精度最高,使用广泛;基于几何计算的林窗光指数不仅具有较高精度,且可以快速测量林窗任意位置光强,该方法适用于林窗光强水平分布格局、垂直结构以及光组成成分(直射光和散射光)特征研究.

Reviews on methods of measuring light intensities within forest gaps

Understory light is essential to the establishment and growth of understory plants and varies temporally and spatially within gaps, therefore, quickly measuring light intensities at different positions within gaps is important. Hitherto, there are three types of methods for measuring light intensities within canopy gaps. (1) Direct measurement methods use light sensors to measure light intensities within canopy gaps. It is expensive for measuring light heterogeneity in gaps because direct measurement methods are only possible over a limited time span and number of points and the measurements are laborious to take. (2) Model methods can quickly assess light intensities at any particular position within canopy gaps and, thus, is suitable for studies on light heterogeneity within canopy gaps. However, these models have low accuracy in assess light intensities because they simplify canopy gaps into cylindrical or ellipsoid shaped gaps. (3) Photographic methods are based hemispherical photographs taken with a skyward-facing fish-eye lens fixed on a camera within a canopy gap, and then calculate light intensity at photo point according to one of three indices: the gap fraction, weighted canopy openness or gap light index (GLI). Of them, GLI has the largest accuracy and has two kinds of methods: hemispherical photograph-based and geometric calculation-based. Hemispherical photograph-based GLI are only available for single-point assessments based on one hemispherical photograph and are therefore also time- and labor-consuming for measuring light intensities for numerous points in a gap, particularly in a large gap. Geometric calculation-based GLI includes four processes: the measurement of tridimensional shape of canopy gap and the aspect and slope of ground, the calculation of gap coordinates based on these three parameters, the transformation from gap coordinates to hemispherical photograph, and the computation of GLI, direct light and diffuse light. The approach can quickly measure light intensities at any position within canopy gaps. Due to these advantages, the geometric calculation-based GLI is a powerful tool for studies on the leveled spatial pattern, the vertical structure and the components (direct and diffuse lights) of light intensities within canopy gaps.