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.