Application of a hand-held supplementary light for extending field-based net ecosystem exchange carbon flux measurements in low light conditions

Abstract Manual chamber-based carbon flux measurements are frequently used to capture terrestrial carbon cycle processes over vegetated areas. Light response curves, achieved by sequential shading, enable obtaining model parameters of light compensation points, maximum photosynthetic rates and dark respiration. However, light conditions in the field are often limited, especially in darker seasons and areas with frequent cloud and fog, which questions the robustness of model parameters. Artificial light therefore offers a crucial way to assess and address these potential limitations, especially recent advances in LED lights with improved wavelength spectra and intensity. However, previous LED lights were fixed on the chamber top, blocking out natural light, heavy and with a high power demand unsuitable for remote field deployment. Here we tested a handheld LED flashlight as a flexible, low power and low weight option. We investigated the wavelength spectrum and photosynthetic active radiation (PAR) output under controlled conditions and applied it under light limiting field conditions. Carbon uptake benefited from the increased PAR range without affecting chamber temperatures, and whilst there was overall good agreement for model parameters between with and without supplementary light, model fit was improved by the wider light range for situations with higher variability.