Relationship Between Stem Respiration and Tree Growth in Tropical Forests

The Science

Current models predict that tree respiration increases with growth rates and temperature. Scientists found that when averaged over the annual timescale, a positive relationship existed between tree stem growth and carbon dioxide (CO₂) emitted from the stem into the atmosphere as a part of growth respiration. However, over a single day, growth and respiration were suppressed during the warmer periods associated with high transpiration and water use.

The Impact

The mechanisms involved in this apparent suppression of respiration are a hot topic of research because the pattern behaves opposite of expectations when considering only temperature. Mechanisms under investigation include: (1) increased CO₂ transport in the transpiration stream, as well as (2) an actual decrease in cellular respiration rates linked to reduced stem water potentials during warmer daytime periods of high transpiration and inhibited growth.

Summary

Tropical forests cycle a large amount of CO₂ between the land and atmosphere, with a substantial portion of the return flux due to tree respiratory processes. However, on-site estimates remain scarce of woody tissue respiratory fluxes and carbon use efficiencies (CUEW) and their dependencies on physiological processes, including stem wood production (Pw) and transpiration in tropical forests. This study synthesized monthly Pw and daytime stem CO₂ efflux (ES) measurements over one year from 80 trees with variable biomass accumulation rates in the central Amazon. On average, carbon flux to woody tissues, expressed in the same stem area normalized units as ES, averaged 0.90 ± 1.2 µmol m^-2 s^-1 for Pw, and 0.55 ± 0.33 µmol m^-2 s^-1 for daytime ES. A positive linear correlation was found between stem growth rates and stem CO₂ efflux, with respiratory carbon loss equivalent to 15 ± 3% of stem carbon accrual. CUEW of stems was non-linearly correlated with growth and was as high as 77 to 87% for a fast-growing tree. Diurnal measurements of stem CO₂ efflux for three individuals showed a daytime reduction of ES by 15 to 50% during periods of high sap flow and transpiration. The results demonstrate that high daytime ES fluxes are associated with high CUEW during fast tree growth, reaching higher values than previously observed in the Amazon Basin (e.g., maximum CUEW up to 77 to 87%, versus 30 to 56%). These observations are consistent with the emerging view that diurnal dynamics of stem water status influences growth processes and associated respiratory metabolism.

Principal Investigator

Kolby Jardine
Lawrence Berkeley National Laboratory
[email protected]

Program Manager

Brian Benscoter
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This research is based upon work supported as part of the Next-Generation Ecosystem Experiments Tropics (NGEE Tropics) as a part of work package 1.4 (Autotrophic respiration) funded by the Biological and Environmental Research (BER) program’s Environmental System Science program, within the U.S. Department of Energy’s (DOE) Office of Science. Additional funding for this research was provided by the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

References