4.1 Differences in the H-D scaling exponents among
different growth vigour
Consistent with our first hypothesis, the H -D scaling
exponent significantly differs among tree performance (growth vigour)
levels. Specifically, the exponent numerically decreases as tree growth
vigour deteriorated (Fig 2B, Table 2). The numerical similarity between
the scaling exponents of living and dead trees is attributable to the
significant variation in the scaling exponents observed for living
individuals (Fig 2A, Table 2). The significantly higher scaling exponent
of superior trees indicates that superior trees grow taller with respect
to diameter increments. The statistically insignificant difference in
the scaling exponents of dead trees and inferior trees is consistent
with the observation that inferior trees are likely to die very soon.
The study indicate that in addition to competition (Trouve et al.2015; Qiu et al. 2021), climate (Hulshof, Swenson & Weiser 2015;
Fortin et al. 2018), forest structure (Feldpausch et al.2011), and species composition (Mensah et al. 2018), the
numerical values of the H -D scaling exponent differ as a
function of tree performance (growth vigour) within species. In
addition, our results did not support the 2/3-scaling (elastic
self-similarity) “law” between tree height and trunk diameter at the
intraspecific level coinciding with previous research (Russo, Wiser &
Coomes 2007; Mensah et al. 2018).
It has been shown that competition stimulate vertical growth of tree
(Wright et al. 2004; Sun et al. 2019), and the growth in
height of suppressed trees exceeds that of their dominant counterparts
with equivalent diameters (Sumida, Miyaura & Torii 2013; Trouveet al. 2015), which appears to contradict the results reported
here. This inconsistency may be attributed to the fact that the trees in
our study were aerially sowed such that the trees examined are almost
even-aged. Consequently, the disparities in tree height are insufficient
to have provided an obvious advantage. In communities composed of
uneven-aged trees, young trees established under the shade of taller
trees typically grow rapidly in height compared to their growth in girth
resulting in large slenderness ratios (i.e., H /D ) even at
the expense of reducing the ability to resist bending forces (Henry &
Aarssen 1999). Plant height is a crucial component of light interception
(Wright et al. 2004; Liu et al. 2019),
and
trees are known to adopt a growth strategy favoring growth to height to
establish their canopies in ways that maximize light interception (Baninet al. 2012; Hulshof, Swenson & Weiser 2015). The plasticity of
the slenderness ratio reflects the ability of trees to self-adjust their
growth responses to stressful environments (Bourque, Bayat & Zhang
2019). In general, cadres of individual trees with numerically large
height vs. trunk diameter scaling exponents manifest a competitive
advantage over trees that grow more slowly in height (Ford 2014).