Structure and dynamics of forests dominated by Sequoiadendron giganteum.

• New allometric equations for Sequoiadendron based on 60 trees of all sizes. • Old-growth forests have aboveground biomass up to 2683 Mg ha⁻¹ and LAI up to 14.5. • Some forests lost more biomass to mortality than gained by growth during drought. • Distance to water and neighbor competition affect Sequoiadendron during drought. • Long-term analysis reveals relatively small effect of recent drought on Sequoiadendron. The largest tree species, Sequoiadendron giganteum has a small native range restricted to California's Sierra Nevada. Awe-inspiring stature contributed to its protection from logging, but anthropogenic climate change—particularly hotter drought—and over a century of fire suppression are possible threats. We measured 60 trees in seven forests to improve allometric equations for the species and installed five 1-ha plots to quantify biomass of Sequoiadendron and associated vegetation. Plots were re-measured after 5 yr to compute biomass increments and examine effects of management history as well as impacts of recent drought. Forests held up to 2683 Mg ha⁻¹ aboveground biomass (1373 Mg C ha⁻¹) and had leaf area index (LAI) up to 14.5 with Sequoiadendron accounting for the bulk of biomass but not LAI. Live trees of other species contributed up to 10.6 LAI and had biomass increments up to 6.1 Mg ha⁻¹ yr⁻¹, but drought contributed to tree mortality in three plots, where live biomass declined by 6.2–10.3 Mg ha⁻¹ yr⁻¹. Two plots had very little tree mortality and gained 5.5–7.9 Mg ha⁻¹ yr⁻¹. Sequoiadendron productivity was strongly correlated with tree-level photosynthetic capacity, but at the height of the drought, relative growth—expressed as the ratio of biomass produced during the driest and wettest recent years—correlated positively with site productivity and negatively with both vertical distance to water and presumed competition from neighboring trees. Prescribed fire in one plot caused a growth release in trees whose lower trunks were burnt and small neighboring Abies lowiana were killed. Effects of 21st century drought on Sequoiadendron productivity were greater in northern than southern locations but of lesser magnitude than many past events. No Sequoiadendron recruitment was observed. Planting can overcome the species' dependence on fire for regeneration, and Sequoiadendron has great potential for carbon sequestration in a variety of settings, including commercial forestry. [ABSTRACT FROM AUTHOR]