What is considered the main protective barrier of a tree against external threats like insects and diseases?

The bark of a tree serves as the main protective barrier against external threats such as insects, diseases, and physical damage. It is composed of multiple layers, including the outer protective layer that guards against environmental factors, pests, and pathogens. The bark's structure varies from tree to tree but generally functions to shield the inner tissues of the tree, such as the phloem and xylem, which are critical for nutrient and water transport.

In contrast, xylem is primarily involved in transporting water and nutrients from the roots to the leaves, while phloem is responsible for distributing the sugars produced through photosynthesis. Heartwood, which is the dense inner part of a tree, does provide some structural support but does not play a direct role in protecting the tree from external threats. Therefore, while all these components are essential to the tree’s overall health and function, the bark is specifically designed to act as the first line of defense against potential harm from outside sources.