3.2 Photosynthesis, Respiration, and Energy Allocation

Energy production controls growth and recovery

A tree must capture energy before it can grow, defend itself, compartmentalize, store reserves, flower, fruit, or replace lost parts. Photosynthesis uses light energy, carbon dioxide from the air, and water from the soil to build sugars in chlorophyll-bearing tissue, releasing oxygen as a byproduct. The simplified equation worth remembering for the exam is: carbon dioxide + water + light energy yields glucose + oxygen. Leaves are the main photosynthetic organs on most trees, so total leaf area and leaf health directly set the size of the energy budget.

Respiration is the reverse logic and a frequent exam contrast. Respiration breaks down stored sugars to release usable energy for living cells, consuming oxygen and producing carbon dioxide. It runs in leaves, roots, cambium, and buds around the clock. A tree photosynthesizes only when light, temperature, and water allow, but it respires continuously as long as cells are alive. High temperatures and stress raise respiration costs at the same time they may lower photosynthetic income.

Sources, sinks, and the budget

The exam tests this through real choices. Removing too much live crown cuts sugar production. Severing roots removes both uptake and stored reserves. Repeated defoliation by insects forces the tree to spend stored carbohydrates refoliating in midseason. Planting too deep and compacted soil throttle root function, leaving the crown short of water and resources.

Allocation is the key exam word even when it goes unnamed. A tree cannot fully fund every function at once. Under stress it may shorten shoot extension, produce smaller leaves, shed leaves or branches, or prioritize defense and compartmentalization. These are normal responses that also reveal limits.

Applying the budget

Pruning must respect this budget. Removing dead, diseased, broken, or rubbing parts supports objectives, but excessive live crown reduction strips the very tissue needed for recovery. The Pruning standard (ANSI A300 Part 1) and the matching exam logic discourage removing more than about 25% of a mature tree's live foliage in one year for exactly this reason. The question is never only whether a cut can be made; it is whether the remaining tree has resources to respond.

Energy also links biology to diagnosis. A declining tree may hold enough reserves for one spring flush after an injury, then collapse later because roots and leaves never rebuilt the stored carbohydrate. Epicormic shoots (water sprouts) can signal stress, sudden light exposure, or loss of normal crown function rather than vigor. Heavy fruiting often increases on stressed trees, so a big crop is not evidence of health.

Common traps: assuming a vigorous post-topping flush proves recovery; believing fertilizer adds energy (it supplies nutrients, not carbohydrates); forgetting that respiration continues in winter and at night. Use this checklist: preserve enough healthy leaf area to fund photosynthesis; protect roots because they both absorb and store; avoid mistaking a quick flush for long-term recovery; and tie every pruning, watering, and soil decision back to allocation.

The seasonal carbohydrate cycle

The energy budget runs on a yearly rhythm that the exam expects you to understand. In late winter and early spring, a temperate tree pays for its first flush of leaves entirely from stored carbohydrate, because there are no functional leaves yet to earn income. This is the most vulnerable point in the year: a tree defoliated late the previous summer, or one that lost roots to construction, may not have banked enough reserves to fund a full flush and will leaf out weakly or partially.

Once the new leaves mature and turn from sinks into sources, the tree begins running a surplus. Through summer it pays current bills, funds defense and reproduction, and refills storage. By late summer and autumn the priority shifts to storage, packing sugars and starch into roots, stems, and living parenchyma cells to survive winter and prime next spring's flush. This is why late-season heavy pruning or stress is risky: it interrupts the refilling phase and leaves the tree short going into the dormant season.

Why stress compounds

Stress agents rarely act alone, and the exam loves layered scenarios. Picture a tree that loses 30% of its roots to a trench, then suffers a drought, then is defoliated by an insect outbreak. Each event independently lowers photosynthetic income or raises demand, and together they can push the tree past its ability to fund defense. This downward spiral is the basis of the classic decline model: predisposing factors (poor site, compaction) weaken the tree, inciting factors (drought, defoliation) trigger sharp decline, and contributing factors (opportunistic borers, canker fungi) finish a tree that could have resisted them when healthy.

Energy-budget red flags to recognize:

• A tree that flushes normally one spring, then dies back the next, usually exhausted stored reserves and could not rebuild them.
• Progressively smaller leaves and shorter twig growth year over year signal a shrinking energy budget, not a single event.
• Profuse seed or fruit set on an otherwise struggling tree is often a stress-driven reproductive push, not a sign of health.