How do Tree Growth Regulators (TGRs) primarily influence a tree's growth?

Tree Growth Regulators (TGRs) influence a tree's growth primarily by reducing crown size. These substances are designed to modify the growth patterns of trees, and one of their main functions is to control excessive vertical and lateral growth. By reducing crown size, TGRs can help manage the overall growth of the tree, making it more suitable for urban environments where space is limited. This reduction in crown size can lead to a more dense and manageable canopy, which can also improve the distribution of sunlight and enhance the health of the lower branches.

In contrast, the other options do not accurately represent the primary effects of TGRs. While they may have secondary effects on leaf size, root expansion, or bark thickness, those are not the main mechanism through which TGRs operate. Instead, TGRs are specifically designed to influence how trees grow, primarily affecting their size and shape by regulating growth hormones, thereby achieving a balance that is often needed in landscaping and forestry management.