Vertical Scaling of Leaf Maintenance Respiration Through the Canopy Influences Individual Tree Carbon Budgets with Consequences for Forest Leaf Area and Biomass

Authors

Jessica Needham¹* ([email protected]), Charles Koven¹, Sharmila Dey², Rosie Fisher³, Ryan Knox¹, Julien Lamour⁴, Gregory Lemieux¹, Marcos Longo¹, Alistair Rodgers¹, Jennifer Holm¹, Jeffrey Chambers^1,5

Institutions

¹Lawrence Berkeley National Laboratory, Berkeley, CA; ²Harvard University, Cambridge, MA; ³CICERO Centre forInternational Climate Research, Oslo, Norway; ⁴Centre National de la Recherche Scientifique, Paris, France; ⁵University of California–Berkeley, CA

URLs

https://ngee-tropics.lbl.gov

Abstract

The balance between respiration and photosynthesis influences the carbon budget of individual plants, with consequences for ecosystem biomass and leaf area. Photosynthesis decreases with light attenuation through the canopy, but it is less well understood how leaf maintenance respiration (R_dark) scales through the canopy. Recent observations from a tropical forest in Panama suggest that decreases in R_dark may be greater than decreases in photosynthesis. Here, the project uses the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) with the E3SM Land Model (ELM) to test the sensitivity of ecosystem dynamics to the vertical scaling of R_dark. In pantropical simulations, researchers alter the parameter k which controls the shape of the negative exponential describing the decrease in R_dark with canopy depth. The team compares simulations to a control simulation where photosynthesis and respiration are proportional through the canopy. Simulations with k decreased by 0.75, corresponding to R_dark approximately 40% lower at the bottom of the canopy, have annual understory mortality rates approximately 1.5% lower than the control. In FATES, plants dynamically adjust their target allometric leaf biomass to prevent leaf layers with a negative carbon budget, and this trimming is decreased with decreased R_dark. As a result of both increased understory survival and reductions in trimming, decreased R_dark results in a 21% increase in leaf area and a 12% increase in total vegetation biomass. Next-generation vegetation demographic models with multiple canopy layers must correctly represent the vertical scaling of photosynthesis andrespiration through the canopy. Further observations from a diversity of sites and plant functional types could help to constrain this important process.