Results of the European Photo International Cross-comparison Course as part of the quality assurance of the crown condition assessment 2015 (Photo ICC 2015)

Photo ICCs represent one of different measures for quality assurance of the crown condition survey, which is part of the European forest monitoring. The European Photo ICC 2015 took place between June 23rd and September 29th and was carried out for the three regions Northern, Central and Mediterranean Europe. In total 123 teams participated. The defoliation and assessable crown were assessed on the basis of photos for all main tree species of one region. Half of the photos had already been used at the Photo ICC 2010. The mean deviation of the countries from the weighted median of all countries of one region was evaluated as well as the occurrence of outliers and the specification of the assessable crown. The repeatedly used photos further enabled the evaluation of the assessment continuity. The results reveal a good homogeneity of the defoliation assessment within Europe. The mean deviation of the countries usually lay within a ±10%-interval and most countries only showed few outliers. For some regions and tree species, however, systematic differences in the defoliation assessment among countries were observed. Countries participating in the Photo ICC for two regions, in general displayed the same trend for both regions. Differences in the definition of the assessable crown did not result in larger deviations regarding the defoliation assessment. In Central Europe, the defoliation of beech (Fagus sylvatica) and oak (Quercus robur und Q. petraea) was significantly higher for the same photos in 2015 than in 2010. This observation existed for almost all participating countries. Several reasons for this increase are possible like differences in the quality of the printed photos or varying experience of teams in the defoliation assessment. No significant differences between 2010 and 2015 were found for the investigated tree species of Northern and Mediterranean Europe as well as for the coniferous tree species of Central Europe. The results of the Photo ICC 2015 underline the need to carry out photo courses as regular quality assurance in the crown condition survey. The advantages include the reproducibility of results, the option of testing the continuity of assessment, participation of a larger number of teams and saving of costs and time. Thus, Photo ICCs represent a useful addition to nation-wide and European-wide Field ICCs to ensure data quality in the crown condition survey.