Manipulating tree crown structure to promote old-growth characteristics in second-growth redwood forest canopies

In old-growth Sequoia sempervirens forests, reiterated trunks and limbs provide required habitat elements for specialized arboreal species, including an endangered seabird, Brachyramphus marmoratus. The oldest second growth redwood forests established after 19th century logging lack species dependent on complex structure, presumably because redwoods maintain simple, model-conforming crowns for centuries unless damaged by wind or fire. We imposed a factorial experiment on 24 redwoods 59-75 m tall in six second-growth forests to determine if trunk reiteration and limb formation can be induced by removing treetops (topping) and branch tips (tipping) to disrupt apical control. We also increased light availability in the upper crown by pruning branches. After intensively mapping trunks and branches and imposing treatments, we re-mapped trees three years later to quantify growth increments. Topping stimulated trunk reiteration from the cut, and reiterated tops gained height more rapidly than controls. Tipping also stimulated trunk reiteration from branches, especially when combined with topping, resulting in formation of limbs (i.e., branches giving rise to reiterated trunks). Pruning had consistently negative effects on trunk and branch growth increments even after accounting for post-treatment variation in leaf area and light availability, suggesting that photosynthate was diverted to compartmentalization of wounds. Strategic injury of trees may have long-term conservation value in second-growth redwood forests if limbs can be initiated high enough in the crown to persist as trees approach maximum height. Topping and tipping treatments should be combined with silvicultural thinning of neighboring trees to increase light availability far more than can be achieved by pruning.