Phloem

Phloem is the food-conducting vascular tissue of trees, responsible for transporting the sugars produced by photosynthesis in the leaves to all other parts of the tree that need energy, including roots, developing buds, fruits, and the cambium. This process, called translocation, moves dissolved organic compounds through specialized cells called sieve tube elements.

In trees, the functional phloem is a thin layer located just inside the bark (and outside the cambium). Unlike xylem, which accumulates as wood over many years, only the most recent phloem tissue is actively conducting. Older phloem is incorporated into the bark as it is pushed outward by new growth. This thin band of active tissue is critically important and surprisingly vulnerable.

Many significant tree pests and diseases target the phloem. Bark beetles excavate galleries in the phloem layer to feed and reproduce. Phytophthora pathogens can kill phloem tissue, causing cankers. Girdling, whether by rodents, string trimmers, or improper staking, disrupts phloem transport and can starve the roots. When the phloem is destroyed in a complete ring around the trunk, the roots lose their energy supply and the tree eventually dies, even though the canopy may appear normal for a period.