A Comparative Analysis in Methodologies Used to Measure Forest Canopy Gaps in the Root River Floodplain Forest

Canopy Gaps in an Upper Mississippi River floodplain plot were measured as part of a songbird nest-site selectivity study. Two methods of measuring floodplain forest canopy gaps were compared. One method used a ground crew to sweep the plot and record spatial and botanical information of canopy gaps > 10 meters in diameter. The second method used 1:15,000 scale color infrared stereoscopic aerial photographs, a high resolution scanned image and a Geographic Information System (GIS) to interpret canopy gaps > 10 meters. Botanical characteristics of the gap’s interior were not collected with the second method. A nearest feature distance analysis was performed on both the ground sweep and air photo data sets. One hundred random points were selected from within the plot and the distances from the random points to the nearest gap feature were calculated. A paired two tailed t-test of nearest feature distance showed a significant difference between data sets (P < 0.001). When a nearest feature analysis was performed from songbird nest sites to the nearest gap feature a paired two tailed t-test showed no significant difference between the air photo derived and ground sweep methods (0.05 < P < 0.10). The air photo/GIS method performed a more complete survey of the canopy gaps then the ground sweep. The air photo/GIS method omitted fewer gaps in its survey then did the ground sweep. The ground sweep method and the air photo/GIS method had comparable rates of commission error. The air photo/GIS method recorded more accurate and detailed spatial data of the canopy gaps then the ground sweep.