Drivers of intra-individual spatial variability in methane emissions from tree trunks in upland forest

CH4 emissions from tree trunks in upland forests should be scaled accurately in order to assess the role of tree trunks in the forest CH4 budget. As the chambers used to measure emissions cover only a small part of the tree trunks, it is necessary to understand the intra-individual spatial variability in trunk CH4 emissions. We measured trunk CH4 flux at nine locations per individual on four trees in a cool-temperate upland forest. To appreciate the origin of this variability and the underlying processes, we also measured the potential rate of CH4 production and CH4 concentration at sapwood and characterized wood and bark. Up to 15-fold spatial variation in CH4 fluxes were observed at the individual level. This variability can be highlighted by the variation in the sapwood CH4 concentration which was further explained by the variation in CH4 production rate. The radial CH4 diffusivity calculated from concentration gradients and emissions was not related to the measured characteristics of either wood or bark, raising the question of the diffusion pathway. We emphasized the importance of sampling trunk CH4 flux at multiple locations on the surface of a tree trunk to capture spatial variability, a prerequisite for estimating tree-level CH4 emissions.

Methods
Trunk CH4 flux was measured using a cavity-enhanced absorption spectroscopy gas analyzer (Li-7810, Licor, Lincoln, USA), from the trunk surface area of 127 cm2 on average, with a volume of a measuring system including a chamber, tubes, and an analyzer of 2509 cm3. Changes in the CH4 and CO2 concentrations in the stem chamber were recorded for 4 min at a frequency of 1 Hz.

After finishing all trunk CH4 flux measurements, chambers were removed, and the wood at these chamber locations was bored with an increment borer (5.15 mm internal diameter, Haglof Sweden) to a depth of 20 cm. After the removal of the wood core, the hole left by the borer was enlarged using a driller to a diameter of 10 mm. A stainless-steel tube (10 mm outer diameter, 12 cm length) was inserted into the hole to a depth of 10 cm. The tube was flushed with pure N2 to expel as much O2 as possible, then the open end of the tube was capped with a rubber septum (Asone, Butyl W Plug, Osaka, Japan). As the inner 10 cm part of the hole was not covered with the stainless tube, gases inside the trunk can diffuse into the tubing system and reach an equilibrium. One week later, 0.1 mL of the gas inside the tube was sampled with a syringe penetrating the septum and immediately measured by gas analyzer (Li-7810) with a small volume sample kit (Li 7810-110).

Wood cores were extracted from the trunk with an increment borer and the innermost 7 cm core was incubated using a mixture (N2, 10% CO2 and 1% H2) at 25 °C in a dark incubator for one week. An air sample (10 μl) was then drawn from each vial through the septum into a syringe and injected through the septum of the small volume sample kit (Li 7800-110) connected to the gas analyser. The ex-situ CH4 production rate was calculated by dividing the increase in CH4 concentration by the duration of the incubation and the dry mass of the wood segment, accounting for the volume of the tube corrected for the volume of the segment.