Visual assessments of establishment success in urban Prunus avium (L.) and Quercus rubra (L.) in relation to water status and crown morphological characteristics

Assessing the establishment of newly planted urban trees is important to ensure that establishment management is maintained as long as needed. Such assessment is often based on visual appraisal of the post-transplant vitality two or three years after planting. How visual assessments relate to water status in the trees, or which morphological characteristics that are driving forces in such assessments have received limited attention in past research. In this interdisciplinary study, several leaf characteristics, such as leaf shape, size and colour were determined on 50 trees each of sweet cherry (Prunus avium L.) and red oak (Quercus rubra L.), transplanted to two Swedish sites three seasons before the study. In addition, shoot length was measured, and crown density, crown shape and die-back were estimated. Visual establishment assessments were performed by professional evaluators, who also completed a questionnaire on their background knowledge and priorities during assessments. The study investigated which of these leaf and crown characteristics had the greatest impact on the visual assessments. In addition, it compared the characteristics and evaluator assessments against the tree water status, determined by pre-dawn shoot water potential measurements. Visual assessments proved to be unrelated to shoot water potential for both species but, for red oak the assessments were strongly related to leaf colour and leaf shape. Shoot water potential was related to leaf size but not to leaf colour, indicating that one of the most influential parameters for visual assessments of red oak were not well-suited for determining tree establishment success, if defined by water status. The assessments for sweet cherry were best explained by leaf area. Our findings suggest that leaf size is a good indicator of tree establishment and that leaf shape, the character considered least important by tree evaluators, is influential on tree assessments. These findings are important for the development of more accurate tree expert assessments.