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

Abstract

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. 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. Crown condition of Abies mariesii was visually rated using a vitality index, and DSO values of the same trees were calculated. 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. 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.