How do tree structure and old age affect growth potential of California redwoods?

Abstract

As the only species exceeding 90 m in height and 2000 years of age, Sequoia sempervirens and Sequoiadendron giganteum provide the optimal platform upon which to examine interactions among tree structure, age, and growth. We climbed 140 trees in old‐growth redwood forests across California, USA, spanning a broad range of sizes and including the tallest, largest, and oldest known living individuals (i.e., 115.86 vs. 96.29 m tall, 424 vs. 582 Mg aboveground dry mass, and 2510 vs. 3240 years old for Sequoia and Sequoiadendron, respectively). We used a combination of direct measurements, hierarchical sampling, and dendrochronology to quantify tree structure and annual growth increments through old age. We also developed equations to predict aboveground attributes of standing redwoods via ground‐based measurements. 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 (up to 1.6 vs. 0.9 m3/yr), aboveground biomass growth (up to 0.77 vs. 0.45 Mg/yr), and aboveground growth efficiency (0.55 ± 0.04 vs. 0.22 ± 0.01 kg annual growth per kg leaves, mean ± SE) 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. A third dimension, relative trunk size, is a significant predictor of growth in Sequoia such that trees with relatively large main trunks compared to their crowns produce more wood annually. Similar‐size trees grow at similar rates regardless of latitude or elevation in tall forests of each species. Recent annual growth increments are higher than in the past for the majority of trees, and old trees are just as responsive to environmental changes as young trees. Negative growth–age relationships in previous centuries and positive growth–age relationships in recent decades reflect sampling bias and shifting disturbance regimes. Overall, we find little (if any) evidence for negative effects of old age on tree‐level productivity 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 extrinsic forces cause tree death.