Methane emissions from temperate forest tree stems across an upland to wetland gradient

Background/Question/Methods

Forests are active sites of methane (CH4 ) uptake and emission through various biotic and abiotic pathways. While CH4 uptake by soils in upland temperate forest systems is considered a significant sink for atmospheric CH4, tree stems have recently been identified as important sites of CH4 efflux across diverse ecosystem types, leading to concerns that tree emissions may offset or diminish net CH4 uptake in upland systems. Recent investigations across ecosystems have found the presence of CH4 release from plant surfaces to be widespread, and this growing recognition of widespread CH4 efflux from plant surfaces suggests that even small fluxes per unit area could be relevant for the purposes of global greenhouse gas accounting in many ecosystems. Despite this, tree methane emissions from the upland forests of the northeast United States have been relatively understudied. Further, there remain important unresolved questions about the origins (soil vs. plant tissue) of plant-emitted CH4 and the variability between species and across environmental gradients. Here, we measured stem CH4 emissions from 160 individual tree stems across 17 species spanning an upland-to-wetland gradient in the Yale-Myers Forest (Union, Connecticut).

Results/Conclusions

Using static chambers connected to portable greenhouse gas analyzers, we made 480 flux measurements during the peak growing season of 2021. We found that methane emissions were pervasive among tree species, with all 17 species measured containing one or more individuals that emitted CH4 to the atmosphere. Species ranged in the magnitude of their CH4 flux rates, but also exhibited high within-species variability. Fluxes were generally higher in more saturated soils, but high fluxes were also detected even on upland ridge sites. In upland sites, stem CH4 emissions did not decrease with stem height (measured at 50, 125, and 200cm above the soil surface), suggesting likely within-tree origin of tree-emitted CH4. Overall, our results indicate that CH4 release through trees comprise a significant gross flux and may potentially offset net ecosystem fluxes in upland temperate forests.