Comparative development of the four tallest conifer species

Abstract

The tallest conifers—Picea sitchensis, Pseudotsuga menziesii, Sequoia sempervirens, Sequoiadendron giganteum—are widely distributed in western North America, forming forests > 90 m tall with aboveground biomass ≥ 2000 Mg ha−1. Here we combine intensive measurements of 169 trees with dendrochronology and allometry to examine tree and stand development. The species investing least in bark protection and heartwood defense—P. sitchensis—has more leaves, denser wood, larger appendages, and produces more aboveground biomass during its relatively brief lifespan than other conifers at equivalent ages. The species investing most in bark protection and heartwood defense—S. giganteum—has the least dense wood, largest appendages, and greatest longevity. Evidence for senescence diminishes with longevity; only P. sitchensis exhibits a post-maturity decline in tree productivity after accounting for leaf mass. Growth efficiency declines with age in all species, falling most rapidly in P. sitchensis followed by P. menziesii, S. sempervirens, and S. giganteum in the same sequence as longevity. Centuries-long time series of age, size, and growth increments identify years when trees first reach a given height as well as biomass and growth rates at that height, providing snapshots of performance useful for simulating development. Stands dominated by P. sitchensis and P. menziesii gain height at similar rates, but P. sitchensis accumulates biomass more rapidly until senescence curtails tree productivity, which takes centuries longer in P. menziesii. Whereas S. sempervirens in primary forest grows more slowly than P. sitchensis and P. menziesii until ~70 m tall, S. sempervirens in secondary forest outpaces other conifers with biomass increments approaching global maxima within a few centuries. Beyond ~70 m, S. giganteum gains height more slowly than other conifers, but it sustains relatively high biomass increments for millennia. Both within and beyond their native ranges, the four tallest conifers have unrealized potential to provide ecosystem services.