Mapping forest canopy gaps using air-photo interpretation and ground surveys

Canopy gaps are important structural components of forested habitats for ma ny wildlife species. Recent improvements in the spatial accuracy of geograp hic information system tools facilitate accurate mapping of small canopy fe atures such as gaps. We compared canopy-gap maps generated using ground sur vey methods with those derived from air-photo interpretation. We found that maps created from high-resolution air photos were more accurate than those created from ground surveys. Errors of omission were 25.6% for the ground- survey method and 4.7% for the air-photo method. One variable of inter est in songbird research is the distance from nests to gap edges. Distances fro m real and simulated nests to gap edges were longer using the ground-survey maps versus the air-photo maps, indicating that gap omission could potenti ally bias the assessment of spatial relationships. If research or managemen t goals require location and size of canopy gaps and specific information a bout vegetation structure, we recommend a 2-fold approach. First, canopy ga ps can be located and the perimeters defined using 1:15,000-scale or larger aerial photographs and the methods we describe. Mapped gaps can then be fi eld-surveyed to obtain detailed vegetation data.