Large Methane Emissions From Tree Stems Complicate the Wetland Methane Budget
Luke C. Jeffrey, Charly A. Moras, Douglas R. Tait, Scott G. Johnston, Mitchell Call, James Z. Sippo, Niemitz Jeffrey, Dylan Laicher-Edwards, Damien T. Maher
Abstract
Abstract Our understanding of tree stem methane (CH 4 ) emissions is evolving rapidly. Few studies have combined seasonal measurements of soil, water and tree stem CH 4 emissions from forested wetlands, inhibiting our capacity to constrain the tree stem CH 4 flux contribution to the total wetland CH 4 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 CH 4 fluxes per campaign. Tree stem CH 4 fluxes ranged several orders of magnitude between hydrological seasons and topo‐gradient zones, spanning from small CH 4 uptake to an emission of ∼203 mmol m −2 d −1 . Soil CH 4 fluxes were similarly dynamic and shifted from CH 4 emission (saturated soil) to uptake (dry soil). In Lower and Transitional zones respectively, tree stem CH 4 contribution to the net CH 4 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 CH 4 soil sink capacity by ∼51% during dry conditions. Water table height was the strongest driver of tree stem CH 4 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 CH 4 emissions pathway as an important and seasonally oscillating component of wetland CH 4 budgets.