Evaluation of estimates of crown condition in forest monitoring: Comparison between visual estimation and automated crown image analysis

Context: In long-term forest monitoring, tree crown condition has been visually rated to diagnose tree vigor and forest condition. However, visual estimates are subjective. A semiautomatic image analysis system, called CROCO, was developed to estimate crown condition quantitatively. CROCO calculates a DSO value which decrease with increasing crown transparency. Aims: This study aims to verify visual estimates objectively using CROCO and to assess characteristics of visual estimates and DSO values by comparing the effectiveness as indicators of radial growth. Methods:Crown condition of Abies mariesii was visually rated using a vitality index, and DSO values of the same trees were calculated.Results: When the top of the tree was intact, the trees with a higher vitality index showed a significantly higher DSO. Vitality index showed the strongest relationship with DBH increment for 8 years. DSO had a significant relationship with DBH increment by adding information of the crown top condition. Conclusion: Analysis of tree crown images is effective to verify visual estimates. Vitality index is a synthetic index involving factors affecting radial growth. DSO could be utilized similarly to vitality index, as an indicator of radial growth, by addition of information on crown size and/or shape.