Longitudinal Gradients in Tree Stem Greenhouse Gas Concentrations Across Six Pacific Northwest Coastal Forests

The aim of this study was to examine the magnitude of greenhouse gas (GHG) concentrations in tree stems of Pacific Northwest, USA coastal forests and evaluate various tree and site characteristics along river-to-sea gradients as possible drivers of tree stem GHG variation. We measured the concentration of CH4, CO2, and N2O during summer and winter in live and dead tree stems of five species from six coastal watersheds and related this to soil porewater GHG concentrations, porewater salinity, and tree characteristics. Overall, average pCO(2) and pCH(4) were elevated above atmospheric concentration, and average pN(2)O was slightly below atmospheric concentration. Stem pCO(2) was higher in the summer than the winter and was higher in angiosperm trees compared to gymnosperm trees, whereas pCH(4) was significantly higher in fresh upstream compared to salt-influenced reaches. Stem pCH(4) was also positively correlated with porewater pCH(4) in contrast to other GHGs. The above results suggest that tree stem pCH(4) in these coastal settings was primarily controlled by soil linkages, pCO(2) was primarily regulated by tree physiology, and factors controlling pN(2)O remain unclear. Plain Language Summary Greenhouse gases undergo continuous cycling between the atmosphere and ecosystems due to a wide variety of biological and nonbiological processes. While terrestrial plants are well known for their ability to remove carbon dioxide from the atmosphere to perform photosynthesis, they can also play a role in moving other gases from soils to the atmosphere. This study investigates the abundance of the major greenhouse gases-carbon dioxide, methane, and nitrous oxide-within the trunks of live and dead trees in coastal forests around the Pacific Northwest. We found that carbon dioxide was in high abundance in the tree trunks due to tree physiology, while methane was almost always above atmospheric levels within the tree trunks likely as a result of transport from soil porewaters. Tree trunk methane levels decreased toward the river mouth-where it meets the ocean-as water in the soil had higher salt content that limits methane production. Patterns in nitrous oxide levels were less clear than the other greenhouse gases. Results from this study build upon a recently growing body of work examining how trees influence greenhouse gas cycles beyond carbon dioxide.