Aboveground biomass dynamics and growth efficiency of Sequoia sempervirens forests

The tallest species, Sequoia sempervirens, inhabits old-growth forests with global maximum biomass and leaf area. Here we determine if these forests also have maximum productivity. Intensive measurements of 114 trees 18–116 m tall and 115–2340 yr old were used to improve allometric equations for Sequoia. Applying the best available allometry to all vegetation in eleven 1-ha plots re-measured at 5-yr intervals yielded accurate estimates of aboveground biomass and growth increments. Plots initially held 1594–4341 Mg ha−1 of biomass (805–2201 Mg C ha−1), which increased by 14.7–40.7 Mg ha−1 yr−1, including input of dead biomass from outside plots. Live biomass increments ranged from 1.2 to 19.2 Mg ha−1 yr−1 and were dominated by heartwood accumulation in Sequoia (up to 15.6 Mg ha−1 yr−1). Exceptional old-growth Sequoia forests thus produce more aboveground biomass annually—and sequester more carbon in decay-resistant heartwood—than forests dominated by any other species. However, live biomass increments are considerably higher in an exceptional second-growth Sequoia forest and a fully stocked Sequoia plantation. Old-growth forest productivity falls short of the maximum, because growth efficiency declines with increasing tree size such that much of an old forest’s photosynthetic capacity resides in crowns of very large trees producing considerably less aboveground biomass per unit leaf mass than small trees. Decreasing Sequoia growth efficiency is strongly related to heartwood proportions of biomass increments, which increase as trees enlarge with age. With increasing tree size, more productivity may also be directed to roots supporting clonal regeneration, explaining why small Sequoia produce several times more aboveground biomass per unit leaf mass than co-occurring species. Since decay-resistant heartwood promotes tree longevity and supports biodiversity, silviculture that includes indefinite retention of Sequoia trees can sustain high productivity while also improving both the carbon sequestration potential and conservation value of managed forests.