**4. Discussion**

## **4.1 Wood traits demonstrate significant variations among genotypes**

Although there is a clear association between climate and wood properties, genetic factors influence their expression [5, 29] and shape the adaptive responses [30, 31]. This explains why the best performing *Eucalyptus* clones (elite clones) selected for this study are genetic materials which have relative drought tolerance [4, 31]. Thus, a selection metric of *Eucalyptus* clones based on traits for drought tolerance and high productivity combined with versatile wood properties for pulp and paper offers a good perspective for the current world pulp and paper industry.

Similar fiber dimensions were reported in the literature for young *Eucalyptus* trees with fiber length between 670 and 1040 μm, fiber diameter between 16 and 19 μm, fiber wall thickness between 3 and 5 μm and lumen width between 7 and 9.5 μm [16, 32, 33]. Wood-fiber characteristics are among the first indicators to be evaluated for screening potential fibrous raw materials for paper production. Fiber dimensions are generally related to collapsibility and flexibility properties, which are strongly associated with paper strength and surface properties [34, 35].

The lowest values presented by clone B for RR, WP and LSF is related to the fact that this genotype has comparatively thin walled fibers. It is assumed that an RR value less than 1.0 is favorable to produce an inter-fiber contact in manufacturing paper using hardwoods, and greater than 1.5 is not recommended. Percentages below 60% are indicated for the wall proportion, while Luce's factor is a ratio related to the final paper sheet density and can be used for specific selection [22, 23]. In this sense, all clones have adequate wall proportion, and clones B, C and E presented better RR values. According to [36], flexibility coefficient values between 0.50–0.75, as verified in this study (except for clone D), classify the fibers as flexible, which tend to form a highly resistant paper when intertwined. Finally, the slenderness ratio is a quality directly related to paper tear index, for which values above 50 are preferable. None of the clones showed a higher value as indicated, with clone D being the highest.

The values presented for basic wood density must be classified as average, meaning 450–540 kg m−3, considered acceptable for commercial pulpwood and are similar to previous reports for young *Eucalyptus* trees between 450 and 500 kg m−3 [37, 38]. According to Gomide et al. [39], the projects to increase production capacity and to deploy new factories have prioritized the use of wood with a density close to 500 kg m−3, as verified for the study clones, with an emphasis on clones D (*E. grandis* x *E. urophylla* x *Eucalyptus tereticornis*) and E (*E. grandis* x *E. urophylla*).

Significant correlations between fiber dimensions and density reflect the intrinsic.

association between these properties. Barroto et al. [11] suggest a mediated effect of fiber wall fraction on the functionality of *Eucalyptus* trees through vessel dimensions and basic density. It is possible to note that clone D displayed higher fiber wall proportion and wood density, however it also had the lowest vessel area (see **Table 1**). Thus, a disadvantage is verified for this clone in terms of fibrous raw materials (quality *Wood Quality and Pulping Process Efficiency of Elite* Eucalyptus *spp. Clones Field-Grown… DOI: http://dx.doi.org/10.5772/intechopen.106341*

indices), although the higher average density results in lower wood specific consumption. Similar behavior can be observed for clone A (*E. urophylla*).
