利用半球图像法提取植被冠层结构特征参数

植被冠层结构深刻地影响着植物群落与环境的相互作用，对植被冠层结构的研究是深入理解植被生态系统格局、过程及其运作机制的重要基础。冠层结构特征参数的快速测量方法是植被冠层结构研究的前提，目前测量方法主要是基于实际测量的地面法，地面法一般费时费力，受人为因素影响较大，因此本文探索利用半球图像法获取植被冠层结构特征参数。通过对半球图像进行几何纠正并建立参数图层，与分类后的植被冠层图层进行运算提取植被冠层结构特征参数。将该方法应用于祁连山旺腰沟流域青海云杉冠层结构特征参数的提取，包括植被冠幅、冠层面积、冠层周长等，结果显示：半球图像法能够较好的提取植被冠层结构特征参数，该方法具有简单、客观、可重复等优点，也可作为植被冠层结构变化的监测方法。

Extracting the canopy structure parameters using hemispherical photography method

One of the most important processes in global hydrological cycling is evapotranspiration (ET), the loss of water to the atmosphere by evaporation from soil and plant surfaces and by transpiration from plants. ET directly links the land-surface eco-hydrological processes with the atmospheric processes and it can thus effectively modulate climatic and eco-hydrological systems through altering water pathways (e.g., ET vs. runoff) and altering energy balance (e.g., albedo vs. absorption, and latent heat vs. sensible heat). Forest canopy has been abundantly demonstrated to be the most effective regulator of evapotranspiration in the forested ecosystems. However, adequate and quantitative characterization of forest canopy structure remains to be a serious challenge. Traditionally, forest canopy parameters (e.g., crown diameters, crown area and crown perimeter) are obtained by ground survey methods. And, these ground survey methods have been proven to be time-consuming and often subjective. High-resolution remote sensing images were also used to study the forest canopy structure parameters; but the accessibility to those images is not always easy either because of their expensive costs or because of no coverage for the areas of interest. Here, we present an alternative method that was tested to be more efficient and more objective to quantitatively measure forest canopy structure parameters. This method is called hemispherical photography because it uses the digital camera with a fisheye lens to obtain the hemispherical images of forest canopy. Specifically, we applied this method to a Picea crassifolia forest at the Wangyaogou catchments in the Qilian Mountains. Specifically, a 1600 m2 (40m ? 40m) plot of moss-Picea crassifolia forest at the outlet of the catchments was selected study the canopy structure character of Picea crassifolia. Totally, 29 hemispherical images were randomly obtained by a Nikon D80 digital camera with Nikon Fisheye AF DX 10.5mm f/2.8G lens. To quantitatively test the accuracy of this hemispherical photography method, the canopy structure parameters were carefully measured using ground survey methods for those 29 image-covered areas. The ground-survey measurements include the diameter at breast height (DBH), total tree height, two crown diameters (north-south and east-west directions) and height to the lowest branch. The hemispherical images were processed through following procedures. First, the projection function of the fisheye lens was developed to rectify the geometric distortion. Second, the hemispherical images were classified into binary-image data layer including sky (as 0) and Picea crassifolia (as 1). Finally, the canopy structure parameters were obtained based on the projection function (i.e., geometrically rectified) and the binary-image data layer (i.e., the canopy). The results show that canopy structure parameters (i.e., crown diameters, crown area and crown perimeter) extracted from the hemispherical images are highly correlated with our carefully-surveyed ground-truth data. We employed statistical method to test the spatial consistency of the correlation between the image-obtained parameters and the ground-surveyed parameters and the test assured our confidence that the hemispherical photography method is an efficient and objective method to measure forest canopy structure parameters.