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

Abstract

Abstract 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 CH 4 , CO 2 , and N 2 O 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.