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

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 sometimes, or in higher northern latitudes frequently limited, especially in darker seasons and areas with frequent cloud and fog, which questions the robustness of fitted 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 irradiance. 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 torch as a flexible, low-power and low-weight option. We investigated the wavelength spectrum and photosynthetically active radiation (PAR) output under controlled conditions and applied it under light limiting field conditions. Increased PAR from short-term measurements did not increase chamber temperature significantly but improved confidence in fitted light response curves, especially for situations with higher flux variability.