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：我们对树木茎秆甲烷（CH₄）排放的认知正快速更新。目前鲜有研究同步开展森林湿地的土壤、水体与树木茎秆甲烷排放的季节监测，这制约了我们精准量化树木茎秆甲烷通量在湿地总甲烷通量中占比的能力。本研究针对亚热带淡水白千层（Melaleuca quinquenervia）湿地林展开，覆盖沿海拔梯度划分的低、过渡与高海拔三个区域，报道其年度监测数据。研究共开展8次野外采样，覆盖完整的洪水-干旱-洪水年度水文周期，对30棵树木的4个茎秆高度点位进行茎秆通量测定，且每次采样同步记录最多30组邻近的土壤或水体甲烷通量。树木茎秆甲烷通量在不同水文季节与地形梯度区间内差异可达数个数量级，通量范围从微弱的甲烷吸收最高可达约203 mmol·m⁻²·d⁻¹。土壤甲烷通量同样呈现显著动态变化，从饱和土壤环境下的甲烷排放峰值转变为干旱土壤环境下的甲烷吸收。在低海拔与过渡海拔区域，树木茎秆甲烷通量在生态系统净通量中的占比在淹水期达到峰值（分别为49.9%与70.2%），而在干旱期占比则显著降低（分别为3.1%与28.2%）。高海拔区域的树木茎秆甲烷排放虽微弱，但仍在干旱期抵消了约51%的该区域土壤甲烷汇容量。地下水位高度是调控树木茎秆甲烷通量的最关键驱动因子，但树木甲烷排放仅在土壤被淹时达到峰值，并不会随水深进一步增加而升高。本研究强调了量化湿地树木茎秆甲烷排放路径的重要性：该路径是湿地甲烷收支中重要且随季节波动的组成部分。本数据集包含了年度水文周期内记录的全部土壤、水体与树木茎秆甲烷通量数据。