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).