Prospects for high-throughput estimates of photosynthetic parameters in tropical trees using the Dynamic Assimilation™ Technique.

Abstract

Amazônia is a species-rich region of immense importance to Earth's water and carbon cycling. Photosynthesis drives the global carbon cycle, so understanding photosynthetic differences across diverse landscapes is a key task of ecophysiology and ecosystem science. Unfortunately, due to physiological and logistical constraints, ground-based photosynthesis data in Amazônia remains scarce, and the 'traditional' steady-state method (SS) of gas exchange is slow and inefficient. The Dynamic Assimilation™ Technique (DAT; Saathoff and Welles 2021) promises a new way to perform A/Ci curves rapidly without requiring steady-state conditions. Thus far, this technique has only been validated in greenhouse or agricultural-field-grown species and has yet to be tested in forest trees of diverse physiology, morphology, and environmental adaptation. To test the utility of the DAT in a complex tropical forest ecosystem, we compared the DAT to the SS method in 13 Amazonian trees in situ. We found strong agreement between Vcmax from DAT curves and SS curves, while Jmax was underestimated in DAT curves. We conclude that the DAT provides a robust and rapid estimation of Vcmax. We also identified diverse and unexpected DAT curve shapes among some trees, including the presence of an 'overshoot' in assimilation beyond model-derived RuBP regeneration limitations. The presence of overshoot may elucidate microclimate and species differences in RuBP regeneration rates and emphasizes the considerable importance of DAT curve protocol specifications, such as the effect of ramp rate and direction on Jmax and TPU. Overall, the DAT saved time relative to the SS method and proved to be an effective and rapid method for quantifying Vcmax in tropical trees.