Technical Note: Semi-rigid chambers for methane gas flux measurements on tree stems

There is increasing interest in the measurement of methane (CH4) emissions from tree stems in a wide range of ecosystems so as to determine how they contribute to the total ecosystem flux. To date, tree CH4 fluxes are commonly measured using rigid closed chambers (static or dynamic), which often pose challenges as these are bulky and limit measurement of CH4 fluxes to only a very narrow range of tree stem sizes and shapes. To overcome these challenges we aimed to design and test new semi-rigid stem-flux chambers (or sleeves). We compared the CH4 permeability of the new semi-rigid chambers with that of the traditional rigid chamber approach, in the laboratory and in the field, with continuous flow or syringe injections. We found that the semi-rigid chambers had reduced gas permeability and optimal stem gas exchange surface to total chamber volume ratio (S-c / V-tot) allowing better headspace mixing, especially when connected in a dynamic mode to a continuous flow gas analyser. Semirigid sleeves can easily be constructed and transported in multiple sizes, are extremely light, cheap to build and fast to deploy. This makes them ideal for use in remote ecosystems where access logistics is complicated.