Diurnal Tree Stem CH4 and N2O Flux Dynamics from a Riparian Alder Forest

Thomas Schindler,Katerina Machacova,Ülo Mander,Jordi Escuer-Gatius,Kaido Soosaar

doi:10.3390/f12070863

Abstract

Tree stems play an important role in forest methane (CH4) and nitrous oxide (N2O) flux dynamics. Our paper aimed to determine the unknown diurnal variability of CH4 and N2O exchange in grey alder tree stems. The gas fluxes in tree stems and adjacent soil were measured using manual static and dynamic chamber systems with gas chromatographic and laser-spectroscopic analysis, respectively. The alder trees were predominant emitters of CH4 and N2O; however, N2O emission from stems was negligible. The soil mainly emitted N2O into the atmosphere and was both a source and sink of CH4, depending on environmental conditions. Neither the tree stems nor the riparian forest soil showed significant differences in their CH4 and N2O fluxes between the daytime and nighttime, independently of the exchange rates. In contrast to several previous studies revealing a diurnal variability of greenhouse gas fluxes from tree stems, our investigation did not show any clear daytime–nighttime differences. On the other hand, we found quite clear seasonal dynamics initiated by changing environmental conditions, such as temperature and soil water conditions and tree physiological activity. Our results imply a transport role of tree stems for soil-produced CH4 and N2O rather than the production of these gases in tree tissues, even though this cannot be excluded.

Highlights

Recent studies on methane (CH4) and nitrous oxide (N2O) balance have highlighted the importance of trees, which may act either as sources or sinks for both gases [1,2,3]
The majority of studies on tree stem CH4 and N2O fluxes have focused on different tree species growing in various forest ecosystems, seasonal flux dynamics, and the effects of environmental conditions on the gas exchange [17,25,26]; investigations on diurnal flux dynamics, and on measurements during the nighttime, are still scarce [27,28,29,30] but are necessary in order to understand the temporal patterns, better explain the origin of the exchanged gases, and especially to correctly estimate the daily fluxes of CH4 and N2O from tree stems and forest ecosystems, which are the basis for correct estimation of the monthly and annual greenhouse gas (GHG) fluxes
There were no significant differences in volumetric soil water content (SWC) between daytime and nighttime

Summary

Introduction

Recent studies on methane (CH4) and nitrous oxide (N2O) balance have highlighted the importance of trees, which may act either as sources or sinks for both gases [1,2,3]. Grey alder (Alnus incana (L.) Moench) is a fast-growing, common pioneer tree species in the Northern hemisphere, often found in riparian zones [4,5,6] and with excellent potential for short-rotation forestry [7,8,9]. Wetland plants such as alder adapt to wet conditions by possessing air-filled aerenchyma to supply tissues with oxygen [10] and constitute a pathway for general gas exchange through plants [11,12]. The majority of studies on tree stem CH4 and N2O fluxes have focused on different tree species growing in various forest ecosystems, seasonal flux dynamics, and the effects of environmental conditions on the gas exchange [17,25,26]; investigations on diurnal flux dynamics, and on measurements during the nighttime, are still scarce [27,28,29,30] but are necessary in order to understand the temporal patterns, better explain the origin of the exchanged gases, and especially to correctly estimate the daily fluxes of CH4 and N2O from tree stems and forest ecosystems, which are the basis for correct estimation of the monthly and annual GHG fluxes

Objectives

Methods

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Discussion

Conclusion