Spatial and Temporal Hotspots of Black Alder Stem CH4 Emissions Controlled by Topography and Physiological Activity of the Tree

A growing number of studies from temperate and tropical settings have found that both in dry and wet settings a range of tree species act as an active surface for CH₄-exchange. Stem emissions, as has been shown are very erratic in time and space, depending on various biotic and abiotic factors. To assess spatial variability and potential controls on stem CH₄ emissions in a temperate black alder stand we conducted three measurement campaigns during the period of leaf out in spring 2019. CH₄ exchange was measured on 30 trees. Amongst other factors we focused on the vertical topographic location of the trees, topographic indices and measured sap flow of adjacent black alder trees and respiration rates as an indicator for physiological activity.

In the course of the three campaigns the abiotic and biotic conditions changed drastically. Sap flow variability increased with the emergence of the leaves, while soil and water temperatures increased and the overall water level decreased drastically. The site went from inundation to dry soil in 6 weeks. Stem CH₄ emissions were very variable in time and space, however fell into the range of values previously reported (20-451 μg m^-2 h^-1) with an average of 31.6 μg m^-2 h^-1. Stem CH₄ fluxes peaked during intermediate water levels and increasing sap flow variability. During the last dry and warm campaign stem CH₄ emissions had ceased. Topography as indirect and physiological activity as a direct factor seemed to play the most important role in controlling stem CH₄ emissions.

The study finally looks on opposing these abiotic and biotic factors that influence stem CH₄ emissions and tries to evaluate their importance. Thereby, temporal and spatial hotspots of stem CH₄ emissions may be identified more easily.