Evaluating ginkgo leaf necrosis and asymmetric crown discoloration induced by typhoon 0613 with RGB image analysis

As one kind of disaster, typhoons can cause serious damage to landscape trees both mechanically and physiologically. The Typhoon 0613 was characterized by strong wind associated with less rainfall when it passed through Yamaguchi City, Japan. After hit by it, the symptoms of leaf necrosis appeared on many ginkgo trees from coast to inland and the crowns of them became asymmetrically discolored. Due to the big body of ginkgo trees, this kind of phenomenon was often described by visual scale method characterized by significant deviation and observer specific. In order to quantitatively estimate the symptoms of damage, the leaf necrotic area percentage (LNAP), crown discoloration area percentage (CDAP) and inflection point (IP) of the threshold response function for asymmetric discolored crowns were determined by image pixel method. The green/luminance (G/L) value was measured by using the RGB images respectively scanned by a flat bed scanner from individual leaves and taken with a CCD digital camera from crowns. Significant relationship between G/Lsub(leaf) and LNAP, and between G/Lsub(crown) and CDAP were obtained with the correlation coefficient of R**2=0.941 and R**2=0.826 separately. This suggested that both leaf necrosis and asymmetric crown discoloration of ginkgo trees induced by Typhoon 0613 can be quantitatively estimated by measuring the G/L value from both leaves and crowns. The significant relation between CDAP and IP (R**2=0.811) reveals that both statistically calculated IP value and visually divided CDAP value can be used in estimating the asymmetric crown characters. The estimation of sampled leaves showed that it was the difference of necrotic leaves between windward and leeward made the crown of damaged ginkgo trees appear asymmetrical discoloration. Image threshold responsive analysis of ginkgo crowns hit by Typhoon 0613 indicated that the injury of ginkgo crowns was more serious on windward than leeward. The relationship between the distance from coastline (DC) and G/Lsub(crown) value presented that the farther from the coastline, the bigger the G/Lsub(crown) value of ginkgo crowns. Comparing to the sampling method, image analysis can be more effective being applied in field measurement of damaged status of ginkgo trees hit by severe typhoons for the reason of less labor and less time requirement. It may be an alternative tool to be used in estimating the degree of damage to ginkgo trees by typhoons like Typhoon 0613.