Dead or alive: greenhouse gas fluxes from live and girdled trees in a coastal freshwater wetland

Abstract:

Many studies have focused on greenhouse gases (GHG) emissions, such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) from live trees in various ecosystem types including upland forest and forested wetlands. Fewer studies have focused on GHG production from standing dead trees (i.e. snags), which are becoming a more common occurrence across many freshwater forested wetlands along southeastern US. Trees can act as conduits for CH₄ produced from inundated soils, but little is known about how decay status influences GHG emissions. Methanogenesis in decayed wood has been suggested to be fueled by consumption of labile nonstructural carbohydrates, which is more abundant on newly decayed trees Snags have the potential to remain standing for many years long after tree mortality depending on diameter, species, rate of decay, and environmental factors such as fire and high winds. Organic matter decay produces CO₂ and CH₄ gases as part of the biogeochemical cycling of carbon, therefore, snags of coastal wetland ecosystems have the ability to produce GHGs from heartwood and/or soils.

During the summer of 2019, I conducted a tree stem-girdling experiment to determine the amount of greenhouse gases (CO₂, CH₄, and N₂O) emitted as trees decay. The primary objective of this study was to compare the composition and magnitude of GHGs from live and girdled trees at two heights (base and above girdle) and examine the variation in GHGs over time throughout different stages of decay. Girdling had no immediate impact, meaning trees gradually died over time, allowing us to measure changes in GHGs over different levels of decay. We observed gradual increases in CO₂ fluxes from 52 to 300 mg m^-2 hr^-1 in girdled trees when measuring above the girdle, while live trees remained around 150 mg m^-2 hr^-1 from a similar height. Girdled trees measured from the base also showed significant increases in CH₄ (0.4 to 1.5 mg m^-2 hr^-1) and N₂O (2 to 250 µg m^-2 hr^-1) over time compared to live trees from a similar height. Live tree CH₄ fluxes were around 0.2 - 0.9 mg m^-2 hr^-1 while N₂O fluxes were around 80 µg m^-2 hr^-1. It is important to determine how different stages of decay can effect GHG fluxes because various disturbances, such as saltwater intrusion and drought, are expected to continue causing further and more rapid forested wetland retreat.