DiscussionTop

Mean wood density values of E. grandis reported in this study are in the range of values informed by other authors. Arango & Tamayo (2008) reported mean wood density values ranging from 0.38 g cm-3 to 0.55 g cm-3 in clones of E. grandis (8 year-old), similar to those indicated by Castro da Silva (2002) (0.31 g cm-3 – 0.59 g cm-3). In both cases, the studies were carried out in Brazil. Values reported for different genetic trials and commercial plantations of this species in Argentina were in the range of 0.39 to 0.49 g cm-3 (Harrand & López, 2007; López & López, 2011; Alarcón et al., 2018). However, Monteoliva et al. (2017) obtained mean density values higher than those reported here (0.52 g cm-3) in clones of E. grandis planted in Entre Ríos, Argentina (southwards from CONC). The estimated values are in the range considered suitable for pulp and paper production (0.40 g cm-3 to 0.60 g cm-3, Downes et al., 1997), wood density values that can be obtained from juvenile wood in E. grandis (Núñez, 2011).

In E. viminalis, average wood density was 0.55 g cm-3 with no significant differences between sites. These values are similar than the higher values reported for E. viminalis by Otegbeye & Kellison (1980), Pathauer (2005) (0.40 g cm-3 – 0. 57 g cm-3) and Alarcón et al. (2018) for the basic wood density (0.48 g cm-3 – 0.52 g cm-3). However, our values are lower than those reported by Iwakiri et al. (2013) of 0.61 g cm-3 and Monteoliva et al. (2017) of 0.67 g cm-3 for the basic wood density, in the same plantations of this study. These differences highlight the importance of the methodological approaches to determine the basic wood density, which can lead to different results on the same materials (Alarcón et al., 2018).

Sources of phenotypic variation of wood density

Significant differences among sites, provenances, clones or families have been widely reported for wood characters both in coniferous species (Zobel & Sprague, 1998; Larson et al., 2001; Rigling et al. 2002; Martínez-Meier et al., 2011; Salaya-Domínguez et al., 2012; George et al., 2015; Klisz et al., 2016), and in broad-leaves (Arango & Tamayo, 2008; Harrand et al., 2009; Cappa et al., 2010; López & López, 2011; Moreno & Igartua, 2015). In E. grandis, López & López (2011) showed significant differences involving 12 sites for both mean wood density and radial wood density variation. In the present study, the fixed effect of site showed significant differences in E. grandis for mean, maximum and standard deviation of wood density. Higher values were found in CONC, representing a two-year older plantation than CAAC. CONC is a site with higher average annual precipitation and lower average annual temperature (i.e. a more favourable water balance) (Instituto de Clima y Agua, Instituto Nacional de Tecnologia Agropecuaria, Argentina) than CAAC. In E. viminalis, the site level was also statistically significant for maximum, minimum and standard deviation wood density, but not for mean wood density, showing GUAM the higher values of maximum wood density but the lower minimum wood density, and a larger standard deviation than in VALL.

Significant differences among sites could be attributed partially to slight differences in age, since wood density normally increases with age (Bermúdez Alvite et al., 2002; Resquin et al., 2012; Moreno & Igartúa, 2015), but also to differences in environmental conditions, not only local but also as evolutionary legacies of the climate of the origin of the plant material (Hevia et al., 2020). Dendroecology studies have shown the sensibility of wood density and growth to climatic conditions at inter-annual level (Schweingruber FH, 1996; Rozas et al., 2016) as well as intra-annual level (e.g. Rozenberg et al., 2001; Rozenberg & Paques, 2004; Martinez-Meier et al., 2015). High rainfall, which ensures high soil water availability, stimulates the biomass production (e.g. Le Quéré, 2015), whereas soil water deficit would stimulate the production of high wood density (Bouriaud et al., 2005). In our study, however, higher wood density was produced in the site with higher precipitation and lower mean temperature (e.g. better water balance, CONC site) in E. grandis. In the case of E. viminalis the differences in rainfall between sites are somewhat compensated by differences in temperature and soil conditions making difficult to know a priori which site has a better water balance for the plants (see Table 1). In this regard, the mean wood density did not differ between sites in this species, and maximum and minimum densities did differ in opposite direction.

Regarding the effect of mean temperature, Thomas et al. (2004) found a plastic response of wood density in E. camaldulensis, both variables being positively related. This contrasts to our results in E. grandis, which presented higher wood density in the site with lower temperature (CONC). In the case of E. viminalis, both sites largely differed in mean temperature (around 3°C), but that did not result in significant differences in mean wood density. The different responses to mean climatic conditions of the studied species -among them and compared to other studies- could be associated to the seasonal variability of climatic conditions and to the particular strategies of response to abiotic stressor agents at species level (Olivar et al., 2003; Wimmer & Downes, 2003; Bouriaud et al., 2005). In this sense, it has been proposed that the studied species respond differently to water deficit because of different hydraulic architectures (Barotto et al., 2017).

Another source of phenotypic variation in wood density in our studied species was the genetic provenance. According to Nabais et al. (2018), species with a large geographic range may present high variability between provenances for traits such as wood density. In the case of E. viminalis, although the provenances studied in this work represent only a small portion of the natural range of the species in Australia/Oceania (a range of 15 degrees of latitude, and from the sea level to 1,500 m altitude), they correspond to a second introduction of plant material selected for their growing performance (Cappa et al., 2010). This may imply that they would maintain also significant differences for wood characteristics that can be explored further for the genetic improvement of the species. It is relevant to indicate that in the present study, a trend towards higher wood microdensity values in lower altitude provenances were observed, coincident with the plant material indicated by Cappa et al. (2010) as with the best growth performance in Argentina. No differences were found between provenances in E. grandis, probably because the genetic material was pooled at this level. However, the genetic effect at clone level showed a significant component of variance for all wood density variables, except for Stdr.

Variation in wood density was observed among individuals (in E. viminalis), provenances and sites within each species, but also along the radial section of the stems. The segmentation of the radial profile describing radial wood density variation, both in E. grandis and in E. viminalis, resulted from the impossibility of delimiting annual growth rings in the studied species. The 10 established sections allowed describing their variation by means of a mixed linear model. This variation was significant for the mean and minimum wood density and its standard deviation in E. grandis and for all the variables studied in E. viminalis. These results lead to conceptual patterns of radial variation showing marked differences inherent to the species. It can be attributable to the presence of only juvenile wood in E. grandis and juvenile-mature wood in E. viminalis considering the age of the sampled trees and references of wood stage for the same species (Núñez, 2011; López & López, 2011; Iwakiri et al., 2013). For E. grandis, the pattern of variation was coincident with the type 3 modelled by Panshin & Zeeuw (1980), described for species with circular porosity of vessels, but also for E. grandis of diffuse porosity (Fukazawa, 1984; Arango & Tamayo, 2008; Núñez, 2011). In E. viminalis the significant increase in the mean, maximum and minimum wood density along the radius, from pith to bark, was coincident with the pattern 1 described by the same authors (Panshin & Zeeuw, 1980), which is described for species with diffuse porosity (Fukasawa, 1984). These changes in wood density along the radial direction, from pith to bark, are influenced by the cambial age as well as by the annual growth (Cobas, 2012), and the shape of the curves describing the radial pattern may differ when are analysed based on the growth-ring widths than with fixed sections. The type 1 form of radial variation is described by a positive exponential form with a rapid growth in the juvenile portion (near to the pith) followed by a stabilization of density values towards the bark. That general form was observed for the pooled data of E. viminalis. In this sense, the linear model here proposed implies a simplification of the observed pattern. However, even with this simplification, the model represents a good tool to describe the general trends and their sources of variation, capturing satisfactorily most of the radial variability pattern (Fig. 1). Within-tree source of variation is indicated as the major relevance source of wood density variation for broad-leaf species with diffuse porosity (Downes et al., 1997). On the other hand, although the authors do not propose it as a causal effect, the radial variation in wood density is related to the fibers-length variation in Eucalyptus, which increases from pith to bark (Igartúa & Monteoliva, 2010).

Given that the presence of growth stresses is a phenomenon in Eucalyptus genus, varying at intra and interspecific level, with several implications from the economic point of view (López et al., 2018), identifying traits that can be measured by non-destructive methods renders benefits for both genetic programs and wood industry, optimizing tree selection and industrial processes. In this regard, significant relations between growth stresses and basic wood density variation from pith to bark has been described for several species, including E. dunni (Hernández et al., 2014), E. grandis and interspecific hybrids (López et al., 2018). In our study, we described the form of that variation in the studied two species, as well as how it may change due to site and genetic effects (which must be explored more in detail). The implication of these changes on the technological characteristics of the wood needs still to be explored.

Phenotypic correlations between wood density and mean stem growth

Zobel & Jett (1995) argued that a possible genetic gain in growth could have an indirect effect on wood properties due to a possible unfavourable genetic correlation between growth and wood density. Even when in this study we only explored phenotypic relationships, they were highly complex in E. grandis. The negative relationship between MAG and wood density (measured by x-ray densitometry) was described by Aparicio et al. (2011). However, the study of this relationship at the site level showed that there exists a negative correlation between MAG and density in CAAC, the site with higher mean growth, but a non-significant correlation in CONC, where growth was more limited. It is important to note that although CAAC was the site with the highest growth of both studied, it is not the best site for the species in the studied region.

In the case of E. viminalis, the correlations MAG-Medr and MAG-Maxr although close to zero, were positive in both sites evaluated. This type of positive relationship between growth and wood density has also been described for E. urophylla (Kien et al., 2008) and E. globulus (Igartua & Monteoliva, 2010). However, for this last species, a negative relationship has also been described for Chilean sites (Lizana, 2006), which demonstrates that there are not unique relationships between growth and wood density even at a species level.