Appendix B: Determining forest patch size

Determining the sizes of the forest patches in which your study sites are located is critical to BFL. Here’s how to do it. First, identify and outline the forest patch on your aerial photo or map. You can use tracing paper or clear acetate if you don’t want to write directly on the map or photo. If the edge of your forest patch is not distinct, consider areas with less than 15 percent tree canopy cover as nonforest. For this study, a change in tree species is not considered an edge. However, any gap in the forest patch more than 300 feet (90 meters) wide is considered an edge. If your forest patch is connected to other forested areas by a forested corridor, then for the purpose of this project your patch is part of the larger, interconnected forest; measure the total area.

Once your patch is outlined, you can measure patch size in several ways:

• Grids—If you are using maps or aerial photographs with a 1:24,000 scale (such as 7.5-min. USGS topographic maps), you can use the transparent grid overlay provided in this binder. Lay the grid over the outline of the patch. Count the number of squares in the grid that are at least half filled by the forest patch, then multiply this number by a conversion factor of 2.5 to estimate the area in acres or 1.0 to estimate the area in hectares. (Each square in the grid provided in this binder represents 2.5 acres or 1 hectare.) If you are using a different scale, refer to Table 2 on page 2.13 of the Study Site Instructions section.

• Pacing—If the patch is small, you can pace its dimensions on the ground, then calculate the area from your estimates of distance. Refer to Appendix A for information on measuring your pace.

• Assistance of land managers—If you have teamed with professional land managers, you may be able to enlist their assistance in measuring patch size. They may have planimeters or computer mapping systems that allow easy and accurate measurements.

NOTE: If your study site is in a forest patch larger than 500 acres (200 hectares), you may estimate the size to the nearest 100 acres (40 hectares). If the patch is larger than 5,000 acres (2,000 hectares), estimate to the nearest 1,000 acres (400 hectares).