Growth-strain had a heritability of 0.23, which was significantly lower than the only (known) previously
reported value for the species, 0.63
\cite{Davies2017}. But is similar to the 0.32 value for Eucalyptus dunnii presented by \citet{Murphy2005}. The discrepancy with \cite{Davies2017}
is the individuals used in that trial were from only two isolated sub
populations, and there may have been significantly more genetic
relatedness than assumed. As both were single site trials the
environmental effects cannot be distinguished between studies, however
it could also be possible the \citet{Davies2017} site caused a much stronger expression of the genes
responsible for growth-strain than this studies trial site. It should be
noted that the credible intervals from \citet{Davies2017}
were large (0.28 - 0.98µƐ from a sample size of 423 individuals) and overlap with this
study. The modeling procedure made a number of assumptions with respect
to the relatedness of families and individuals which were likely
inflating the true heritabilities of the traits. Fathers were unknown
and assumed to be unrelated to each other, or to the mothers. As a
result there was likely less genetic variation within families than the
model assumed. Because the fathers were unknown, there was an implicit
assumption within the numerator relationship matrix that all individuals
in a ‘family’ shared a common mother, but none shared a common father
(half-sibling families). Further because the genetics of the mothers was
unknown, they may have been related to each other, the unknown father
trees or self-pollinated. Because of these factors, some (or all)
families were probably less genetically diverse than was assumed,
resulting in over estimation of heritability. It should be noted \citet{Griffin1988} warned breeders about the dangers of open pollenated trials inflating genetic
parameters, which occurs because the variation from inbreeding is
completely confounded with family effects, inflating estimates of
additive genetic variance.
It should also be kept in mind that this experiment was conducted on a
single irrigated nursery site which was not growth restricted. It is
commonly believed favorable environments allow higher expressions of
genetic signals related to growth \cite{Ceccarelli1994},
although no substantial studies have been undertaken to determine if
this holds for wood properties. Growth-strain may not be affected to the
same degree as growth by site uniformity, it may be the case that
environmental effects which influence growth-strain are more site
independent, such as branching asymmetry and stem wobble from
micro-environmental effects. For example a lopsided crown (and hence
uneven stress field within the stem to keep the stem upright), light
competition or wind damage may have a much more significant effect on
growth-strain than, say a temperature difference, nutrient or water
deficiency across sites, as these examples require a mechanical input to
restore gravitropy or heliotropy \cite{Saranpāā2014}. Therefore it should not necessarily be assumed that
growth-strain heritabilities will reduce as drastically as growth traits
when GxE experiments are implemented.
Few studies have been undertaken specifically investigating genetic
parameters of E. bosistoana . Perhaps the most relevant,
\cite{Apiolaza2011}, used a subset of this population and investigated the heritability and
across site genetic correlations of height at age two. They found total
height to have a heritability of 0.10 - 0.14 and across site genetic
correlations to be between 0.74 and 0.99. Two of the three sites had
significant mortality rates, indicating they were probably growth
limited. Two of these three sites (one with high and one with low
mortality) were revisited at age five and comparable height
heritabilities were found (0.09 - 0.17), over-bark DBH heritabilities
were found to range from 0.11 to 0.18 \cite{Burgess2015}. These sites display
considerable heterogeneity. E. bosistoana has been investigated
at a young age once before by \citet{Davies2017} who found hertiablities of 0.63, 0.54, 0.76 0.29 0.97 and
0.82 for growth-strain, density, diameter, volumetric shrinkage,
acoustic velocity and stiffness respectively, although the study was
limited by the narrow genetic base (41 families). There is overlap with
all of the credible intervals between the Davies study and results
reported here. The only other known studies to report heritability of
growth-strain (or stress) were \cite{Henson2004} who found heritability of 0.52 in a breeding population of
E. dunnii and \cite{Murphy2005} who reported heritabilities of between 0.3 and 0.5 for E. dunnii
both with population sizes below 200 individuals.
\citet{Naranjo2012} reported no genetic control (heritability of 0.02) of growth-strain in 4-year-old Tectona grandis.
Previously reported heritabilities in eucalypts exist for a number of
the wood and growth properties reported here. Table \ref{957829} reports relevant
studies of heritability in various eucalypt species for the
properties
investigated here.