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Not AvailabMethane (CH4) is the second-largest contributor to human-induced climate change, with significant uncertainties in its terrestrial sources and sinks. Tree stems play crucial roles in forest ecosystem CH4 flux dynamics, yet much remains unknown regarding the environmental drivers of fluxes. We measured CH4 flux from three tree species (Picea rubens, Tsuga canadensis, Acer rubrum) along an upland-to-wetland gradient at Howland Research Forest, a net annual sink of CH4, in Maine USA. We measured fluxes every two weeks and at three heights from April to November 2024 to capture a range of environmental conditions. Tree species influenced CH4 flux more than any of the environmental variables considered. Among environmental variables, soil moisture was the most important driver of CH4 flux, and our models suggested a significant interaction between soil moisture and soil temperature, such that the effect of higher soil moisture was greater at warmer soil temperatures. We determined a “breakpoint” in soil moisture along the upland-to-wetland gradient at ~ 60% volumetric water content, above which CH4 flux rates increased dramatically. All stems measured were net CH4 sources throughout the sampling period, with rare, isolate measurements of minimal uptake. The magnitude of flux varied by species: red maple stems were the largest emitters (1.946 ± 5.917 nmol m−2 s−1, mean ± SD), followed by red spruce (0.031 ± 0.065) and eastern hemlock (0.016 ± 0.027). This study highlights the contribution of these species to ecosystem CH4 fluxes. Our results establish the sensitivity of stem flux rates to projected increases in regional precipitation and temperature, potentially shifting the site from a net CH4 sink to a source.le 

Northern forest soils are vital for climate change mitigation since upland sandy soils favor the net consumption/oxidation of atmospheric methane (CH4). We are studying biogeochemical CH4 cycle processes in a Northern Forest (Howland Research Forest, Maine), where upland soils are interspersed with wetland (Sphagnum bog), and upland-wetland transition soils along with hummock-hollow microtopography. This complex mosaic of microsites with sources and sinks of CH4 is subjected to change under future wet climates projected for this region, with a potential for these forests to shift from a net CH4 sink to a net CH4 source. Net CH4 emissions in a wet climate can increase either by inhibiting methanotrophs or favoring methanogens, or both. Thus, quantifying underlying processes of gross CH4 production and consumption can reduce the uncertainty of CH4 sink/source estimation in this critical ecosystem. We have collected baseline soil data across the forest's landscape including Total Carbon and Total Nitrogen with the Elemental Analyzer, Gravimetric Soil Moisture, and pH. Furthermore, stable isotope dilution method will serve as a proxy for methanogenic and methanotrophic activities to quantify gross rates of CH4 production and consumption from a flooding (wet-up) experiment in Howland Forest. We will differentiate between CH4 consumption and production by measuring both the change in the amount of CH4 and the ratio between labeled and unlabeled CH4 in a closed system. We will analyze the stable C isotope in 13CH4 to determine gross rates of CH4 production and oxidation in situ and within laboratory incubations. The in situ stable isotope dilution technique will be compared with the gas push-pull method, to test the suitability of a simple, low cost method to quantify gross CH4 oxidation rates. Novel data obtained in this study will constrain CH4 cycle processes in a biogeochemical model to quantify CH4 source-sink potential in Northern Forests under current and future climatic conditions. 

This data set documents the temporal and spatial variation of soil and deadwood moisture, and nearby microclimate, for the a four-month period from June to October 2018. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and main tained by the USDA Forest Service, Northern Research Station.