Gas exchange systems exhibit large measurement error driven by internal thermal gradients

Portable gas exchange systems provide critical data describing plant carbon and water fluxes, and for parameterizing Earth system models (ESMs) that forecast climate change effects and feedbacks. In this talk, we characterize errors in leaf temperature, air temperature, stomatal conductance (gS), and intercellular CO2 concentration (Ci) data reported by two widely used instruments: the LI-COR LI-6400XT and LI-6800. In the LI-6400XT, internal air temperature gradients caused errors as large as 7.7 °C in reported temperatures, and median relative errors in gS and Ci of 24% and 17%, respectively. These errors in turn produced large errors in leaf-to-air temperature relationships, assimilation-temperature response curves, and A-Ci curves. Estimated temperature-dependencies of maximum Rubisco carboxylation rates (Vcmax) and maximum RuBP regeneration rates (Jmax) also exhibited large errors of 49% and 23%, respectively. The LI-6800 exhibited much smaller errors under all conditions. Much of the data currently used to parameterize ESMs were collected with LI-6400XT, suggesting these errors may propagate through to ESM predictions of climate change impacts and feedbacks.