林窗几何特征的测定方法

林窗面积、形状及边界木高是决定林窗环境异质性的3个林窗几何特征,影响林窗内植物更新。林窗几何特征的快速测量方法是林窗研究的基础,测量方法可分为2类:基于地面实际测量的地面法和基于林窗林冠照片的相片法。地面法费时费力,受人为因素影响大,可测量林冠林窗和扩展林窗的面积,但不能测量林窗形状和边界木高。相片法具有简单、客观、可重复的优点,但仅适用于林冠林窗。相片法共有5种:"平面相片法"、"航片法"、"半球面影像法"、"双半球面影像法"和"改进的半球面影像法"。前3种测量方法只能测量林冠林窗面积;"改进的半球面影像法"可测量林冠林窗面积和形状,且精度高于前3种相片法,但所需参数最多;"双半球面影像法"可测量林窗面积、形状及边界木高这3个林窗几何特征,且精度较高,但拍摄要求较高。

Rewiews on methods of measuring geometric chracteristics of forest gaps involving gap size, gap shape, and the height of canopy trees surrounding the gap

Geometric characteristics of forest gaps involving gap size, gap shape and canopy height are major determinants of environmental heterogeneity within gap and then affect plant regeneration and succession. Therefore, quickly measuring methods of the three gap properties lay the foundations for studies of forest gaps. The methods for measuring gap geometric properties can be classified into two kinds: ground\|based and photographic. Ground\|based methods are based on actual measurement on the ground, while photographic methods are based on the photographs of canopies of canopy trees surrounding the gap. The ground\|based methods are viable for both canopy gap size and expanded gap size but are time\|consuming and arbitrary, especially for canopy gap size. Moreover, none of the ground\|based methods is viable for measuring gap shape or the height of canopy trees surrounding the gap. Compared with the ground\|based methods, the photographic methods are more objective and less time\|consuming, but all are only suitable for canopy gaps. Of five present photographic methods, the earlier three ones are only viable for canopy gap size. The improved hemispherical photograph method can measure canopy gap size and shape at the same time. Compared with the earlier three ones, the improved hemispherical photograph method is more accurate but needs more parameters. Among all methods for measuring gap geometric characteristics, the two hemispherical photographs (THP) method is the only method that is suitable for measuring gap size, shape and canopy heights at any azimuth (i.e., tridimensional shape of canopy gap) and has good accuracy. In conclusion, THP is a rapid, simple, objective and accurate approach to the determination of tridimensional shape of canopy gap. These advantages make THP a powerful tool for repeatable, comparative, long\|term studies of canopy gaps. It is essential to study environmental heterogeneity and plant regeneration within gap taking gap size, shape and canopy heights into consideration at the same time.