4.3 Change of branch scaling relationship across growth levels
and layers
Contrary to one of our hypotheses, the scaling exponent governing the
relationship between branch length vs. diameter of inferior trees
numerically exceeded that of superior or moderate trees,
which
indicates that inferior trees appear to invest more growth in branch
length compared to girth (Fig 3B, Table 2).
This phenomenology may confer an
advantage with regard to canopy spread and light interception (Newtonet al. 2012; MacFarlane & Kane 2017; Van de Peer et al.2017). Longer branches (i.e., wider crown) allow plants to capture more
light resource (Iida et al. 2014; Loubota Panzou et al.2018). However, in our study, individual trees that tended to increase
branch length compared to growth in thickness also tended to fall into
the inferior category of tree performance. One possible explanation for
the trends observed in our study is that the leaf biomass per branch of
inferior trees is significantly smaller compared to trees assigned to
the other categories of tree performance (Fig 5). The total tree leaf
area is a decisive factor in the utilization of light energy (Shiet al. 2015), and the leaf area is proportional to leaf biomass.
Thicker branches provide greater mechanical rigidity and thus tend to
carry more leaves, which has long been summarized by Corner’s Rule
(Corner 1949). A recent study demonstrated that the stem diameter limits
leaf biomass at the twig level (Sun et al. 2019), and the sum of
the cross-sections of twigs is the same as that of the first-order
branch (Chiba 1998), which supports our results at the branch level.
Another possible reason for the variation in the scaling of branch
length vs. thickness is the “compensatory effect”, i.e., inferior
trees extend their branches allowing leaves to be illuminated
sufficiently to compensate for their lack of leaves. Our study has shown
that different strategies at the individual level are adopted by trees
to cope with light competitors. At
the individual level, trees with greater height have a competitive
advantage for light, while at the branch level, a strategy of increasing
branch thickness to sustain the mechanical loads of more photosynthetic
organs improves energy acquisition and transformation. These results are
in line with the trade-off between tree height gain and crown expansion
(Osunkoya et al. 2007).
Another of our hypotheses was confirmed, i.e., the branch length vs.
diameter (L -d ) scaling exponent decreases as the location
of branches deepens within the crown and differs across tree performance
in the intermediate layer. The variant L-d scaling exponent among
branches in different canopy layers indicates that the scaling of
branches depends on location and perhaps age (because branches closer to
the ground level tend to be branches produced during the earlier growth
of trees).
Previous studies indicate that branch traits such as diameter, length,
and death vary as a function of the relative position of branches within
a canopy (Umeki & Seino 2003; Chen & Sumida 2017; Lemay,
Pamerleau-Couture & Krause 2019). Our study highlights the co-variance
of branch traits along crown-depth based on scaling relationships. The
competition for light and space maybe the main driver of this pattern.
The space within a canopy tends to become more and more crowded
basipetally from the top to the bottom of a crown such that the
horizontal growth of upper branches is not as limited for space. Thus,
the scaling exponent for the branches in the upper part of the canopy
tends to be numerically larger than that for branches lower within the
canopy. Interestingly, branches in intermediate layer tend to intercept
more light than those of upper and lower branches because the upper
branches are illuminated fully but hold low biomass (hence less leaf)
whereas the bottom branches are light deprived because of shelf-shading
within the canopy (Osada & Takeda 2003). Consequently, the fiercest
competition for light and space within canopies tends to be in the
intermediate layer, which leads to a significant difference of branchL -d scaling exponent in this layer among tree performance
levels. Meanwhile, this research can provide references for pruning in
management. In pruning, branch diameter rather than length should be
considered preferentially, and more attention should be paid to the
branches in intermediate layer.
Finally, it is worth noting that the statistically significant
dissimilarities in the scaling exponents among tree conspecifics
assigned to different growth vigour draws focus on the importance of
intraspecific variance (Bolnick et al. 2011). The scaling
exponent of inferior and moderate trees significantly differs from that
of superior trees at the individual level, indicating that, in addition
to studying trees growing under optimal growth conditions (Poorteret al. 2018), averaging all the data drawn from all the
individuals within a study site can bias and obscure our understanding
of community scaling relationships.