Methane emissions from upland trees

Background/Question/Methods

Most work on methane (CH4) emissions from natural ecosystems has focused on wetlands and wetland soils because they are predictable emitters and relatively simple to quantify. Less attention has been directed toward upland ecosystems that cover far larger areas, but are assumed to be too dry to emit CH4. There is abundant evidence that upland ecosystems emit small amounts of CH4 during hot moments that collectively constitute a significant source in the global budget of this potent greenhouse gas. Almost no attention has been given to trees as significant sources of methane in upland forests.  Tree’s root systems can extend to soil depths and moistures where abundant anoxic microsites can develop. These sites can produce methane that could enter woody plant tissue.  To address these facts we asked the following questions:

1. Do upland trees release methane to the atmosphere?
2. What are the controlling factors that determine the size and timing of upland tree methane fluxes?

We have established two transects across natural moisture gradients in two forests near Annapolis, Maryland. Both tree and soil methane fluxes were measured using chamber methods. Each tree chamber was custom fit to the stem near the base. In addition, porewater methane concentrations were collected at multiple depths near trees. Abiotic parameters such as soil temperature, soil moisture, water potential, and depth-to-groundwater were monitored using a wireless sensor network. 

Results/Conclusions

Upland emissions from tree stems were over +200 ug CH4 m-2 hr-1 while the soil uptake was -15 ug CH4 m-2 hr-1. Methane emissions varied greatly over the year with the highest emissions occurring during the summer months.  Factors controlling methane emissions were soil moisture and depth to groundwater.  We observed stem emissions and soil uptake in over 13% of our paired stem-soil measurements.  This demonstrates that trees can be emitting methane when the adjacent soil is consuming it, challenging the assumption that upland forests are only sinks of methane.  Based on our preliminary data, tree mediated methane emissions may be offsetting the soil methane sink of upland forests by 10 to 30%.  Future methane budgets and climate models will need to include tree fluxes and the parameters that control methane emissions for accurate accounting and predictions.