Print PageSince trees are sessile organisms, crown plasticity plays an important role in maximizing resource capture especially when canopy gaps are created in forests. The determinants of crown plasticity are, however, largely unexplored. Although the slow branch growth in mature trees suggests that they are less plastic compared to smaller trees, it is unknown whether declining growth is primarily a consequence of growing larger (e.g. owing to increasing support costs either at branch or tree level), or getting older (e.g. owing to senescence or increased allocation to reproduction or defense).
In this study, I examined the relationships between tree diameter at breast height and age and lateral branch growth (estimated retrospectively using a canopy lift) in three tolerant tree species (e.g. sugar maple, yellow birch, and beech) into canopy gaps. I also examined the relationships between shade tolerance, tree height, branch length, light availability and lateral branch growth.
Results/Conclusions
I found that branch growth into gaps declined significantly (p<0.05) with DBH for sugar maple and yellow birch, independently of increases in height, branch length and gap light. This result suggests that the decline is not caused by increased support costs or senescence; rather it might be driven by ontogenetic processes, such as allocation to reproduction or defense, both of which could increase with size, but not necessarily with age. I also observed that branch growth into gaps varied significantly (p<0.01) between species with varying shade tolerance: the highest magnitude of branch growth was observed in mid-tolerant yellow birch (11 cm year-1) followed by tolerant beech and sugar maple (7 and 5 cm year-1). The branch growth information obtained in this study should assist foresters to choose environmentally benign harvesting practices that might be conducive to the sustenance of both tolerant and less-tolerant species in forest stands.
