Caracterização macro e microestrutural do lenho de troncos de árvores eretas e inclinadas de Corymbia citriodora (Hook.) K.D. Hill & L.A.S. Johnson

Abstract

Reaction wood has different properties when compared to normal wood. However, studies regarding its formation and distribution along the trunk are still scarce, especially for forest species prone to the formation of tension wood. Studies on biomechanics and characterization of leaned trees can contribute to the understanding of the strategies used by trees during their development to reorient their trunks to the vertical position. This study aimed to investigate the dynamics of the development of structural elements in trunks prone to the formation of tension wood. It was collected six Corymbia citriodora trunks at 60-years-old, being three straight and three leaned trees, two scales were defined, one macro: in which the longitudinal residual strain (LRS) was measured in four positions around the trunk at 1.30 m height, while the out-ofroundness index (OOR) and pith eccentricity of the disc were calculated for different heights of the trunks: stump, three and six meters. The orientation of the pith deviation along the trunks was also investigated, as well as the analysis of the basic density. For the microscale: scanning electron microscopy (SEM) was used to check the presence of a gelatinous layer and conformation of the fibers in the different wood; the microfibril angles (MFA) was also measured using the polarized light microscopy technique, and X-ray micro-computed tomography was used to measure the frequency, area, and architecture of vessel elements. Heterogeneous distribution of LRSs was found around the trunks, even in the straight trunks, but with a lower magnitude. Higher OORs were observed for all trees sampled at the stump due to basal thickening. There was a significant difference in the pith eccentricity between the two types of trunks, while the patterns of change in the orientation of the pith deviation were influenced by the shape and type of trunk. Regarding the basic density, there was a significant difference between the radial regions analyzed, being in general denser near sapwood and less dense, in the juvenile wood, close to pith, regardless of being classified as straight or leaned trunks. SEM showed the presence of fibers with a gelatinous layer that was found for some regions of the tension wood, and similar shapes of the fibers in the normal and opposite wood. The MFA presented the lowest value of 8.5o and the highest value of 16,5o between the analyzed regions, being the lowest one in the tension wood. The frequency of the vessel elements increased in the tension wood but its total area was smaller compared to the opposite wood. The architecture of the vessel elements was variable for each type of trunk in the radial and longitudinal directions, generally presenting a sinuous structure, typical of intercrossed grain, or spiraled in leaned trunks, and slightly inclined vessels in straight trunks. Considering the results obtained, the species Corymbia citriodora had relevant features that enable to understand the strategies and trends used in the development of tension wood at mature trees.