Detection of the slope of the grain on standing trees [70]. Legend: (a) Slope of the grain at several angles in Abies concolor (http://tchester.org/sj/analysis/pix/Abies_concolor_3_1_crop_40.jpg accessed 17 January 2023); (b) Spiral grain in a log of (Pinus longaeva) ([76], Fig. 1); (c) Elliptical path of ultrasonic wave propagation in a standing tree, determined with three measurements of longitudinal velocities V₁, V₂, and V₃ which can precisely determine the slope of the grain (angle φ), bearing in mind that along the fibres in the L axis, the velocity value is at a maximum; (d) The array of transducers for grain angle measurements on a tree at breast height. ϕ the angle of the slope of grain [70].