Large methane emissions from tree stems complicate the wetland methane budget

Abstract:

Our understanding of tree stem methane (CH4) emissions is evolving rapidly. Few studies have combined seasonal measurements of soil, water and tree stem CH4 emissions from forested wetlands, inhibiting our capacity to constrain the tree stem CH4 flux contribution to total wetland CH4 flux. Here we present annual data from a subtropical freshwater Melaleuca quinquenervia wetland forest, spanning an elevational topo-gradient (Lower, Transitional and Upper zones). Eight field-campaigns captured an annual hydrological flood-dry-flood cycle, measuring stem fluxes on 30 trees, from four stem heights, and up to 30 adjacent soil or water CH4 fluxes per campaign. Tree stem CH4 fluxes ranged several orders of magnitude between hydrological seasons and topo-gradient zones, spanning from small CH4 uptake to ~203 mmol
m-2 d-1. Soil CH4 fluxes were similarly dynamic and shifted from maximal CH4 emission (saturated soil) to uptake (dry soil). In Lower and Transitional zones respectively, tree stem CH4 contribution to the net ecosystem flux was greatest during flooded conditions (49.9 and 70.2 %) but less important during dry periods (3.1 and 28.2 %). Minor tree stem emissions from the Upper elevation zone still offset the Upper zone CH4 soil sink capacity by ~51% during dry conditions. Water table height was the strongest driver of tree stem CH4 fluxes, however, tree emissions peaked once the soil was inundated and did not increase with further water depth. This study highlights the importance of quantifying the wetland tree stem CH4 emissions pathway as an important and seasonally oscillating component of wetland CH4 budgets. This data comprises of all the soil, aquatic and tree stem methane fluxes recorded over the annual cycle.

摘要：

我们对树干甲烷（CH4）排放的理解正迅速发展。目前很少有研究结合森林湿地中土壤、水体和树干甲烷（CH4）排放的季节性测量数据，这限制了我们确定树干CH4通量对湿地总CH4通量贡献的能力。本文呈现了亚热带淡水白千层（Melaleuca quinquenervia）湿地森林的年度数据，该森林跨越海拔地形梯度（低区、过渡区和高区）。

八次野外调查捕捉了年度水文的淹水-干旱-淹水周期，每次调查测量30棵树的树干通量（涵盖四个树干高度），以及多达30个相邻的土壤或水体CH4通量。树干CH4通量在不同水文季节和地形梯度区之间跨越多个数量级，范围从微弱的CH4吸收到约203 mmol m⁻² d⁻¹。土壤CH4通量同样具有动态变化，从最大CH4排放（饱和土壤）转变为吸收（干旱土壤）。

在低区和过渡区，树干CH4对生态系统净通量的贡献分别在淹水条件下最大（49.9%和70.2%），而在干旱期则较低（3.1%和28.2%）。在干旱条件下，高海拔区微弱的树干排放仍抵消了该区域土壤CH4汇能力的约51%。地下水位高度是树干CH4通量最主要的驱动因素，然而，树干排放在土壤被淹没时达到峰值，且不会随水深进一步增加而上升。

本研究强调了量化湿地树干CH4排放路径的重要性，它是湿地CH4收支中一个重要且季节性波动的组成部分。本数据集包含全年周期内记录的所有土壤、水体和树干甲烷通量数据。