Results of the first digital European Photo International Cross-comparison Course 2025 (Photo ICC 2025) as part of the quality assurance of the crown condition assessment

The International Photo Cross-comparison Course (Photo ICC 2025) was conducted to ensure the quality of visual crown condition assessments within the framework of the ICP Forests monitoring program. For the first time, the course was implemented entirely as a web-based application in a regular operational format. As in previous years, it was carried out separately for Northern, Central, and Mediterranean Europe, covering the main tree species in each region. In total, 208 teams from 27 countries participated-the highest number of participating countries to date-resulting in more than 18,000 individual assessments of crown pictures. Overall, the results show that most countries are, on average, close to the regional reference median, and that the mean absolute deviation between countries remains below 5 percentage points for the majority of regions and tree species. Nevertheless, notable differences in assessment results were observed for some countries. Larger deviations occur particularly for Picea abies in Northern and Central Europe and for Pinus sylvestris in the Mediterranean region. The analysis of temporal continuity between the Photo ICC 2021 and 2025 shows a heterogeneous pattern. For most tree species, no significant differences were detected, indicating a generally stable assessment over time. However, significant changes were identified for individual species in Northern and Central Europe. These differences are likely influenced, at least in part, by changes in personnel. This highlights the importance of continuous training and capacity building to maintain consistent and reliable crown condition assessments. In this context, the combination of international and national field and photo-based intercalibration courses represents an effective approach to quality assurance.