Hardwood Rangeland Monitoring with Aerial Photographs

---

Hardwood rangelands cover 10% of California, and are composed of an overstory of various hardwood species, predominantly oaks (Quercus spp.), and an understory mainly composed of introduced annuals (Gong et al., 2000).

Tree crown closure (also referred to as crown cover, canopy cover, and canopy closure) is defined as the percent of forest area occupied by the vertical projection of tree crowns. It is commonly used as a measure of stand density and an indicator of wildlife habitat.

Aerial photo interpretation techniques have been used in classifying hardwood types and tree crown closure estimation. However, they are dependent on the experience of photo interpreters.

In recent years, we have developed digital image analysis techniques to extract information about hardwood rangelands. Black and white aerial photographs can be scanned at a ground resolution as fine as one foot. Scanned digital aerial photographs can be orthorectified to remove the perspective effects and change of scale caused by surface relief (Figure 1). Such data can then be analyzed to extract crown closure (Figure 2). Finally, digital surface models (DSM) can be built to allow change analysis in 3D; i.e., growth in the vertical direction can be obtained (Figure 3).

Figure 1. Orthophoto of a oak woodland forest in Marine County, California. Black and white aerial photograph taken in the summer of 1995 with a scale of 1:12,000 was scanned, processed through digital photogrammetry, and orthorectified.

Figure 2. This zoomed in image shows crown closure extracted from the orthorectified image.

Figure 3. Digital surface model extracted from stereopairs of scanned aerial photographs. The brightness shows heights of crown and terrain surface. Such data obtained from different times can be subtracted from each other to derive changes in tree heights and terrain deformation.

Reference

Gong, P., G. Biging, and R. Standiford, 2000. The potential of digital surface model for hardwood rangeland monitoring. Journal of Range Management 53:622-626.