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Field measurements were carried out to assess the feasibility of ultrasound velocity and damping for the non-invasive testing of standing trees. A total of 87 trees of European beech (Fagus sylvatica L.) and 68 trees of Norway spruce (Picea abies Karst.) were measured in the field, felled and assessed individually for the presence of red heartwood or butt rot. The field assessment of these internal structural defects at the stump level (SL) of the trees was compared with the ultrasound measurements recorded at two tree heights [(i.e. at the SL and 0.5 m above the stump level (ASL)] and in several directions, all perpendicular (PP) to the tree stem. Lower ultrasound velocity and higher damping were found in both species with the presence of internal defects in both the radial and tangential directions of the tree stem. The diameter at breast height (DBH) had a varying effect on both ultrasound velocity and damping. A binary logistic regression was used to test the potential of ultrasound velocity and damping to predict the presence of internal defects. Both the approaches offer similar levels of prediction accuracy (0.72 and 0.76 in beech, and 0.83 and 0.82 in spruce). Due to the significant reduction in measuring time when using ultrasound damping only, this principle is recommended for the detection of red heartwood in beech trees and butt rot in spruce trees. [ABSTRACT FROM AUTHOR]