Measuring and Modeling Photosynthesis in a Species-Rich Rainforest

The Science

A team of researchers measured the photosynthetic properties of leaves from multiple species inside a complex tropical forest canopy in Panama using traditional approaches. Researchers combined that with rapid extensive measurements of leaf reflectance that are a reliable proxy for traditional measurements.

The combination of these two approaches enabled the team to determine data-rich vertical gradients in key photosynthetic parameters. The observed gradients were compared to the hypothesized gradients that are used by climate models. This study evaluated the impact of observed and hypothesized gradients on modeled photosynthesis and transpiration.

The Impact

Comparison between observed gradients’ photosynthetic traits differed from those hypothesized by the models. These differences affected simulations of photosynthesis and transpiration. Most notably, the ratio of dark respiration (carbon dioxide [CO₂] loss) and carboxylation capacity (a key parameter that largely determines CO₂ uptake) was hypothesized to be constant through a vertical profile. However, observations showed that the ratio decreased with canopy depth. If implemented in climate models, these observed gradients would likely increase the carbon gain by understory vegetation.

Summary

Terrestrial biosphere models (TBMs) include the representation of vertical gradients in leaf traits associated with modeling photosynthesis, respiration, and stomatal conductance. However, model assumptions associated with these gradients have not been tested in complex tropical forest canopies.

A team of researchers compared TBM representation of key leaf traits’ vertical gradients with measurements made in a tropical forest in Panama. They then quantified the impact of the observed gradients on simulated canopy-scale CO₂ and water fluxes.

Comparison between observed and TBM trait gradients showed divergence that impacted canopy-scale simulations of water vapor and CO₂ exchange. Notably, the ratio between the dark respiration rate and the maximum carboxylation rate was lower near the ground than at the canopy top. Leaf-level water-use efficiency was markedly higher at the canopy top, and the decrease in maximum carboxylation rate from the canopy top to the ground was less than TBM assumptions.

The representation of leaf trait gradients in TBMs is typically derived from measurements made within individual plants or, for some traits, assumed constant due to a lack of experimental data. This research shows that these assumptions are not representative of the trait gradients observed in species-rich, complex tropical forests.

Principal Investigator

Alistair Rogers
Brookhaven 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 work was supported by the Next-Generation Ecosystem Experiments–Tropics project, which is supported by the Biological and Environmental Research (BER) program in the U.S. Department of Energy’s (DOE) Office of Science.

References