Stem CH4 and N2O fluxes of Fraxinus excelsior and Populus alba trees along a flooding gradient

Abstract

Methane (CH4) and nitrous oxide (N2O) fluxes from tree stems are still poorly quantified in temperate floodplain forests. Methane and N2O fluxes were repeatedly measured at 0.3, 1.6 and 3.6 m stem height at three sites along a landscape gradient ranging from non-flooded to frequently flooded forest sites. The non-flooded forest was dominated by Fraxinus excelsior and the infrequently and frequently flooded sites by Populus alba. Stem surfaces were net CH4 and N2O sources at all sites. The CH4 source strength increased towards the wetter sites (non-flooded 2.51±12.71, infrequently-flooded 5.2±17.26, and frequently-flooded 11.15±24.04 μg-C-m−2 h−1), but flooding had no immanent effects on CH4 and N2O fluxes. Methane fluxes from poplar stems were highest at the stem base (0.3 m) and decreased with increasing measurement height. Methane fluxes from ash stems were lowest at the stem base and gradually increased until 3.6 m height. Nitrous oxide fluxes were low and did not show clear spatial patterns. The presence of mosses had no significant effects on CH4 and N2O fluxes. Stem fluxes were small when compared to the corresponding soil fluxes at the non-flooded and infrequently flooded site, but significantly reduced the soil CH4 sink capacity at the frequently-flooded site. Methane flux strongly varied between 0.3 and 3.6 m stem height and showed distinctive tree species specific patterns. Our results therefore suggest that measuring at more than a single location near the stem base is inevitable to obtain any reliable CH4 or N2O flux estimate of a whole tree stem.