Sequoia sempervirens and Sequoiadendron giganteum, the only two species exceeding 90 m in height and 2000 years of age, provide an ideal platform upon which to examine interactions between tree structure, age, and growth. We climbed 140 trees in old-growth forests across California and used a combination of methods to quantify tree structure and annual growth increments through old age. Compared to Sequoia, Sequoiadendron develops thicker bark on lower trunks, provisions leaves with more sapwood, and delays heartwood production throughout the crown. Main trunk wood volume growth, aboveground biomass growth, and aboveground growth efficiency are all higher in Sequoia. Two independent dimensions of structure—size and aboveground vigor—are the strongest predictors of tree-level productivity in both species. Recent growth rates are higher than in the past for the majority of trees, and old trees are just as responsive to environmental changes as young trees. We find little (if any) evidence for senescence in either species. Except for recovery periods following temporary reductions in crown size, annual increments of wood volume and biomass growth increase as redwoods enlarge with age until fire, fungi, or falling cause tree death.
Stephen C. Sillett is the Kenneth L. Fisher Chair of Redwood Forest Ecology at Humboldt State University, where he has taught since 1996. His research focuses on the ecology and physiology of 6 tree species: Sequoia sempervirens, Sequoiadendron giganteum, Pseudotsuga menziesii, Picea sitchensis, Eucalyptus regnans, and Eucalyptus globulus.
His research is featured in Richard Preston’s New York Times bestseller The Wild Trees.
In this National Geographic video clip, Steve climbs a massive redwood, 350 feet in the sky, in order to study the unique environment of the forest canopy.