Emergent crowns and light-use complementarity lead to global maximum biomass and leaf area in Sequoia sempervirens forests

Abstract

Forests >80m tall have the highest biomass, and individual trees in these forests are Earth’s largest with deep crowns emerging above neighboring vegetation, but it is unclear to what degree these maxima depend on the emergent trees themselves or a broader-scale forest structure. Here we advance the concept of emergent facilitation, whereby emergent trees benefit co-occurring species. Trees reorganize foliage within crowns to optimize available light and, if long-lived, can reiterate after crown damage to become emergent. The height, depth, and spacing of emergent trees in turn allows for abundant light to pass through the canopy, leading to light-use complementarity as well as elevated biomass, leaf area, and species diversity of the forest as a whole. We chose Sequoia sempervirens to develop this concept and installed eleven 1-ha plots in old-growth forests spanning nearly six degrees of latitude in California. Each plot was based off a 316-m-long centerline where biomass and leaf area of all vegetation were quantified. We employed hierarchical measurements and stratified random sampling spanning the full size range of individuals to generate 180 equations for determining biomass and leaf area of all dominant plant species in these forests. Biomass (5190Mgha−1), leaf area (LAI=19.4), and aboveground carbon (2600Mgha−1) are global maxima, occurring in plots with the highest proportion of emergent trees. Decay-resistant Sequoia heartwood contributes the bulk of this mass, ranging from 61.5 to 76.7% of plot totals. Heartwood is a key contributor to the development of trees with emergent crowns, since its durability enables trees to recover leaf area and to re-grow crowns after damage so that they can continue expanding for millennia. By distributing leaf area among fewer trees with deeper crowns, Sequoia maintains very high leaf area itself (LAI up to 14.5) while simultaneously allowing other species to flourish underneath (non-Sequoia LAI up to 8.0). Because Sequoia is not replaced by other species, aboveground biomass, leaf area, and carbon content of these forests are essentially asymptotic over time.