3.2 Photosynthesis, Respiration, and Energy Allocation

Energy production controls growth and recovery

A tree must capture energy before it can grow, defend, compartmentalize, store, flower, fruit, or replace lost parts. Photosynthesis uses light energy, carbon dioxide from the air, and water from the soil to produce sugars in green tissues. Oxygen is released as a byproduct. Leaves are the main photosynthetic organs on most trees, so leaf area and leaf health directly influence the tree energy budget.

Respiration is different from photosynthesis. Respiration breaks down sugars to release usable energy for living cells. It occurs in leaves, roots, cambium, buds, and other living tissues during day and night. A tree can photosynthesize only when conditions allow, but it continues to respire as long as cells are alive. High temperatures, stress, or large living tissue demands can raise respiration costs.

The exam often tests this distinction through real arboricultural choices. Removing too much live crown reduces sugar production. Severing roots reduces water uptake and also removes storage tissue. Repeated defoliation forces the tree to spend stored carbohydrates on refoliation. Poor planting depth and compacted soil reduce root function, then the crown has less water and fewer resources for photosynthesis.

Allocation is the key exam word even when it is not named. A tree cannot invest heavily in every function at once. Under stress, it may reduce shoot extension, produce smaller leaves, shed leaves or branches, increase root growth in favorable zones, or prioritize defense and compartmentalization. These responses are normal biology, but they also show limits.

Pruning decisions should respect this budget. Removing dead, diseased, broken, or conflicting parts may support objectives, but excessive live crown reduction can remove the very tissues needed for recovery. A young tree can usually replace small, well-timed cuts more easily than a mature stressed tree can replace large live branches. The question is not only whether the cut can be made. It is whether the remaining tree has the resources to respond.

Energy also links biology to diagnosis. A declining tree may have enough stored energy for one flush of shoots after stress, then decline later because roots or leaves cannot rebuild reserves. Epicormic shoots can indicate stress, sudden light exposure, or loss of normal crown function. Fruit production may increase in some stressed trees, but reproduction does not prove health.

Use this practical energy checklist:

• Preserve enough healthy leaf area to support photosynthesis.
• Protect roots because they absorb water and store reserves.
• Avoid assuming a quick flush means long-term recovery.
• Expect stress factors to compound when production falls and demand rises.
• Link pruning, watering, soil management, and diagnosis back to allocation.