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1. ENSO‐Influenced Drought Drives Methane Flux Dynamics in a Tropical Wet Forest Soil

   https://doi.org/10.1029/2018JG004832  

   Aronson, E. L. ; Dierick, D. ; Botthoff, J. K. ; Oberbauer, S. ; Zelikova, T. J. ; Harmon, T. C. ; Rundel, P. ; Johnson, R. F. ; Swanson, A. C. ; Pinto‐Tomás, A. A. ; et al ( July 2019 , Journal of Geophysical Research: Biogeosciences)

   Global atmospheric methane growth rates have wildly fluctuated over the past three decades, which may be driven by the proportion of tropical land surface saturated by water. The El Niño/Southern Oscillation Event (ENSO) cycle drives large‐scale climatic trends globally, with El Niño events typically bringing drier weather than La Niña. In a lowland tropical wet forest in Costa Rica, we measured methane flux bimonthly from March 2016 to June 2017 and using an automated chamber system. We observed a strong drying trend for several weeks during the El Niño in 2016, reducing soil moisture below normal levels. In contrast, soil conditions had high water content prior to the drought and during the moderate La Niña that followed. Soil moisture varied across the period studied and significantly impacted methane flux. Methane consumption was greater during the driest part of the El Niño period, while during La Niña and other time periods, soils had lower methane consumption. The mean methane flux observed was −0.022 mg CH₄‐C m⁻²hr⁻¹, and methane was consumed at all timepoints, with lower consumption in saturated soils. Our data show that month studied, and the correlation between soil type and month significantly drove methane flux trends. Our data indicate that ENSO cycles may impact biogenic methane fluxes, mediated by soil moisture conditions. Climate projections for Central America show dryer conditions and increased El Niño frequency, further exacerbating predicted drought. These trends may lead to negative climate feedbacks, with drier conditions increasing soil methane consumption from the atmosphere.

    

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2. Soil Respiration Responses to Throughfall Exclusion Are Decoupled From Changes in Soil Moisture for Four Tropical Forests, Suggesting Processes for Ecosystem Models

   https://doi.org/10.1029/2022GB007473  

   Cusack, Daniela F. ; Dietterich, Lee H. ; Sulman, Benjamin N. ( April 2023 , Global Biogeochemical Cycles)

   Climatic drying is predicted for many tropical forests yet models remain poorly parameterized for these ecosystems, hampering predictions of forest‐climate interactions. We applied an integrated model–experiment approach, parameterizing an ecosystem model with tropical forest observational data and comparing model predictions to a field drying manipulation. We hypothesized that drying suppresses soil CO₂fluxes (i.e., respiration) in relatively dry tropical forests but increases CO₂fluxes in wetter tropical forests by alleviating anaerobiosis. We measured soil CO₂fluxes during wet‐dry cycles from 2015 to 2022 in four Panamanian forests that vary in rainfall and soil fertility. Measured soil CO₂fluxes declined in the dry season and peaked in the early wet season ahead of peak soil moisture, resulting in lower soil moisture optima for respiration than previously modeled. We then parameterized the model using field data and the new moisture‐respiration response functions. The updated model predicted increased soil CO₂fluxes with drying in wetter and fertile forests and suppressed fluxes in drier, infertile forests. In contrast to model predictions, a chronic throughfall exclusion experiment initially suppressed soil respiration across forests, with sustained suppression for four years in the wettest forest only (−28% ± 4% during the dry season). In the fertile forest, drying eventually elevated CO₂fluxes over this period (+75% ± 28% during the late wet season). The unexpected negative drying effect in the wettest, infertile forest could have resulted from reduced vertical flushing of nutrients into soils. Including hydro‐nutrient interactions in ecosystem models could improve predictions of tropical forest‐climate feedbacks.

    

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3. The Role of the Ecosystem Engineer, the Leaf‐Cutter Ant Atta cephalotes , on Soil CO 2 Dynamics in a Wet Tropical Rainforest

   https://doi.org/10.1029/2018JG004723  

   Fernandez‐Bou, A. S. ; Dierick, D. ; Swanson, A. C. ; Allen, M. F. ; Alvarado, A. G. F. ; Artavia‐León, A. ; Carrasquillo‐Quintana, O. ; Lachman, D. A. ; Oberbauer, S. ; Pinto‐Tomás, A. A. ; et al ( February 2019 , Journal of Geophysical Research: Biogeosciences)

   Leaf‐cutter ants are dominant herbivores that disturb the soil and create biogeochemical hot spots. We studied how leaf‐cutter antAtta cephalotesimpacts soil CO₂dynamics in a wet Neotropical forest. We measured soil CO₂concentration monthly over 2.5 years at multiple depths in nonnest and nest soils (some of which were abandoned during the study) and assessed CO₂production. We also measured nest and nonnest soil efflux, nest vent efflux, and vent concentration. Nest soils exhibited lower CO₂accumulation than nonnest soils for the same precipitation amounts. During wet periods, soil CO₂concentrations increased across all depths, but were significantly less in nest than in nonnest soils. Differences were nonsignificant during drier periods. Surface efflux was equal across nest and nonnest plots (5 μmol CO₂m⁻²s⁻¹), while vent efflux was substantially (10³to 10⁵times) greater, a finding attributed to free convection and sporadic forced convection. Vent CO₂concentrations were less than in soil, suggesting CO₂efflux from the soil matrix into the nest. Legacy effects in abandoned nests were still observable after more than two years. These findings indicate that leaf‐cutter ant nests provide alternative transport pathways to soil CO₂that increase total emissions and decrease soil CO₂concentrations, and have a lasting impact. Estimated total nest‐soil CO₂emissions were 15 to 60% more than in nonnest soils, contributing 0.2 to 0.7% to ecosystem‐scale soil emissions. The observed CO₂dynamics illuminate the significant carbon footprint of ecosystem engineerAtta cephalotesand have biogeochemical implications for rainforest ecosystems.

    

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4. Lagged Wetland CH 4 Flux Response in a Historically Wet Year

   https://doi.org/10.1029/2021JG006458  

   Turner, J. ; Desai, A. R. ; Thom, J. ; Wickland, K. P. ( November 2021 , Journal of Geophysical Research: Biogeosciences)

   While a stimulating effect of plant primary productivity on soil carbon dioxide (CO₂) emissions has been well documented, links between gross primary productivity (GPP) and wetland methane (CH₄) emissions are less well investigated. Determination of the influence of primary productivity on wetland CH₄emissions (FCH₄) is complicated by confounding influences of water table level and temperature on CH₄production, which also vary seasonally. Here, we evaluate the link between preceding GPP and subsequent FCH₄at two fens in Wisconsin using eddy covariance flux towers, Lost Creek (US‐Los) and Allequash Creek (US‐ALQ). Both wetlands are mosaics of forested and shrub wetlands, with US‐Los being larger in scale and having a more open canopy. Co‐located sites with multi‐year observations of flux, hydrology, and meteorology provide an opportunity to measure and compare lag effects on FCH₄without interference due to differing climate. Daily average FCH₄from US‐Los reached a maximum of 47.7 ηmol CH₄m⁻² s⁻¹during the study period, while US‐ALQ was more than double at 117.9 ηmol CH₄ m⁻² s⁻¹. The lagged influence of GPP on temperature‐normalized FCH₄(T_air‐FCH₄) was weaker and more delayed in a year with anomalously high precipitation than a following drier year at both sites. FCH₄at US‐ALQ was lower coincident with higher stream discharge in the wet year (2019), potentially due to soil gas flushing during high precipitation events and lower water temperatures. Better understanding of the lagged influence of GPP on FCH₄due to this study has implications for climate modeling and more accurate carbon budgeting.

    

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5. Assessing mechanisms of climate change impact on the upland forest water balance of the Willamette River Basin, Oregon

   https://doi.org/10.1002/eco.1776  

   Turner, David P. ; Conklin, David R. ; Vache, Kellie B. ; Schwartz, Cynthia ; Nolin, Anne W. ; Chang, Heejun ; Watson, Eric ; Bolte, John P. ( September 2016 , Ecohydrology)

   Projected changes in air temperature, precipitation, and vapor pressure for the Willamette River Basin (Oregon, USA) over the next century will have significant impacts on the river basin water balance, notably on the amount of evapotranspiration (ET). Mechanisms of impact on ET will be both direct and indirect, but there is limited understanding of their absolute and relative magnitudes. Here, we developed a spatially explicit, daily time‐step, modeling infrastructure to simulate the basin‐wide water balance that accounts for meteorological influences, as well as effects mediated by changing vegetation cover type, leaf area, and ecophysiology. Three CMIP5 climate scenarios (Lowclim, Reference, and HighClim) were run for the 2010–2100 period. Besides warmer temperatures, the climate scenarios were characterized by wetter winters and increasing vapor pressure deficits. In the mid‐range Reference scenario, our landscape simulation model (Envision) projected a continuation of forest cover on the uplands but a threefold increase in area burned per year. A decline (12–30%) in basin‐wide mean leaf area index (LAI) in forests was projected in all scenarios. The lower LAIs drove a corresponding decline in ET. In a sensitivity test, the effect of increasing CO₂on stomatal conductance induced a further substantial decrease (11–18%) in basin‐wide mean ET. The net effect of decreases in ET and increases in winter precipitation was an increase in annual streamflow. These results support the inclusion of changes in land cover, land use, LAI, and ecophysiology in efforts to anticipate impacts of climate change on basin‐scale water balances.

    

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