Apical control is the inhibition of a lateral branch growth by shoots above it (distal shoots). If the distal shoots are cut off to remove apical control, the lateral branch can grow larger and may bend upwards. Apical control starts when new lateral buds grow after passing through a period of dormancy. Buds initially break and produce leaves, then apical control is exerted and the lower (proximal) laterals stop growing. Apical control also inhibits growth of large, old branches. Gravimorphism and restricted water and nutrient transport can inhibit branch growth, but they are not primary mechanisms of apical control. Apical control may reduce branch photosynthesis. Under apical control allocation of branch-produced assimilate to the stem is relatively high, so low assimilates in the branch may limit branch growth even though hormone levels are adequate for growth. Hormones appear to be involved in apical control, but it is not known how. One role of hormones may be to maintain the strength of the stem sink for branch-produced assimilate. Upward bending of a woody branch after release from apical control requires both new wood production and production of wood cells that can generate an upward bending moment. Apical control inhibits radial growth of branches and, in some species, may regulate the production of wood with an upward bending moment.

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Natural stands and a 3-year-old plantation of red alder (Alnusrubra Bong.) trees were used to study the incidence of leaning stems, the level of growth stresses and tension wood formation, and the ability of the stems to right themselves to vertical. Overall, 10% of the 512 trees in 10 natural stands leaned >22°. The largest diameter trees on the steepest slopes leaned most. Most (61%) of the trees curved upward, showing a righting response. For samples without tension wood, growth stress levels on the upper side of leaning stems, but not on the lateral or lower sides, were positively correlated with lean angles above 6°. These leaning stems had a significant righting response without tension wood. Tension wood formation was variable at leans from 9° to 26° both within and among trees, but was correlated with eccentric growth rings. We measured stem recovery in the year-old stem of 3-year-old trees bent to angles of 0–37.5°. During the 5-month experiment all stems righted to near vertical. Tension wood formed on the upper side in stems bent >6°, but reversed to the lower side before reaching vertical in 22 of 30 trees.