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1. Precipitation‐drainage cycles lead to hot moments in soil carbon dioxide dynamics in a Neotropical wet forest

   https://doi.org/10.1111/gcb.15194  

   Fernandez‐Bou, Angel Santiago ; Dierick, Diego ; Allen, Michael F. ; Harmon, Thomas C. ( July 2020 , Global Change Biology)

   Soil CO₂concentrations and emissions from tropical forests are modulated seasonally by precipitation. However, subseasonal responses to meteorological events (e.g., storms, drought) are less well known. Here, we present the effects of meteorological variability on short‐term (hours to months) dynamics of soil CO₂concentrations and emissions in a Neotropical wet forest. We continuously monitored soil temperature, moisture, and CO₂for a three‐year period (2015–2017), encompassing normal conditions, floods, a dry El Niño period, and a hurricane. We used a coupled model (Hydrus‐1D) for soil water propagation, heat transfer, and diffusive gas transport to explain observed soil moisture, soil temperature, and soil CO₂concentration responses to meteorology, and we estimated soil CO₂efflux with a gradient‐flux model. Then, we predicted changes in soil CO₂concentrations and emissions under different warming climate change scenarios. Observed short‐term (hourly to daily) soil CO₂concentration responded more to precipitation than to other meteorological variables (including lower pressure during the hurricane). Observed soil CO₂failed to exhibit diel patterns (associated with diel temperature fluctuations in drier climates), except during the drier El Niño period. Climate change scenarios showed enhanced soil CO₂due to warmer conditions, while precipitation played a critical role in moderating the balance between concentrations and emissions. The scenario with increased precipitationmore »(based on a regional model projection) led to increases of +11% in soil CO₂concentrations and +4% in soil CO₂emissions. The scenario with decreased precipitation (based on global circulation model projections) resulted in increases of +4% in soil CO₂concentrations and +18% in soil CO₂emissions, and presented more prominent hot moments in soil CO₂outgassing. These findings suggest that soil CO₂will increase under warmer climate in tropical wet forests, and precipitation patterns will define the intensity of CO₂outgassing hot moments.

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2. 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 datamore »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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3. FLUXNET-CH₄: a global, multi-ecosystem dataset and analysis of methane seasonality from freshwater wetlands

   https://doi.org/10.5194/essd-13-3607-2021  

   Delwiche, Kyle B. ; Knox, Sara Helen ; Malhotra, Avni ; Fluet-Chouinard, Etienne ; McNicol, Gavin ; Feron, Sarah ; Ouyang, Zutao ; Papale, Dario ; Trotta, Carlo ; Canfora, Eleonora ; et al ( January 2021 , Earth System Science Data)

   null (Ed.)

   Abstract. Methane (CH4) emissions from natural landscapes constituteroughly half of global CH4 contributions to the atmosphere, yet largeuncertainties remain in the absolute magnitude and the seasonality ofemission quantities and drivers. Eddy covariance (EC) measurements ofCH4 flux are ideal for constraining ecosystem-scale CH4emissions due to quasi-continuous and high-temporal-resolution CH4flux measurements, coincident carbon dioxide, water, and energy fluxmeasurements, lack of ecosystem disturbance, and increased availability ofdatasets over the last decade. Here, we (1) describe the newly publisheddataset, FLUXNET-CH4 Version 1.0, the first open-source global dataset ofCH4 EC measurements (available athttps://fluxnet.org/data/fluxnet-ch4-community-product/, last access: 7 April 2021). FLUXNET-CH4includes half-hourly and daily gap-filled and non-gap-filled aggregatedCH4 fluxes and meteorological data from 79 sites globally: 42freshwater wetlands, 6 brackish and saline wetlands, 7 formerly drainedecosystems, 7 rice paddy sites, 2 lakes, and 15 uplands. Then, we (2) evaluate FLUXNET-CH4 representativeness for freshwater wetland coverageglobally because the majority of sites in FLUXNET-CH4 Version 1.0 arefreshwater wetlands which are a substantial source of total atmosphericCH4 emissions; and (3) we provide the first global estimates of theseasonal variability and seasonality predictors of freshwater wetlandCH4 fluxes. Our representativeness analysis suggests that thefreshwater wetland sites in the dataset cover global wetland bioclimaticattributes (encompassing energy, moisture, and vegetation-relatedparameters) in arctic, boreal, and temperate regions but only sparselycovermore »humid tropical regions. Seasonality metrics of wetland CH4emissions vary considerably across latitudinal bands. In freshwater wetlands(except those between 20∘ S to 20∘ N) the spring onsetof elevated CH4 emissions starts 3 d earlier, and the CH4emission season lasts 4 d longer, for each degree Celsius increase in meanannual air temperature. On average, the spring onset of increasing CH4emissions lags behind soil warming by 1 month, with very few sites experiencingincreased CH4 emissions prior to the onset of soil warming. Incontrast, roughly half of these sites experience the spring onset of risingCH4 emissions prior to the spring increase in gross primaryproductivity (GPP). The timing of peak summer CH4 emissions does notcorrelate with the timing for either peak summer temperature or peak GPP.Our results provide seasonality parameters for CH4 modeling andhighlight seasonality metrics that cannot be predicted by temperature or GPP(i.e., seasonality of CH4 peak). FLUXNET-CH4 is a powerful new resourcefor diagnosing and understanding the role of terrestrial ecosystems andclimate drivers in the global CH4 cycle, and future additions of sitesin tropical ecosystems and site years of data collection will provide addedvalue to this database. All seasonality parameters are available athttps://doi.org/10.5281/zenodo.4672601 (Delwiche et al., 2021).Additionally, raw FLUXNET-CH4 data used to extract seasonality parameterscan be downloaded from https://fluxnet.org/data/fluxnet-ch4-community-product/ (last access: 7 April 2021), and a completelist of the 79 individual site data DOIs is provided in Table 2 of this paper.« less
4. Experimental Soil Warming and Permafrost Thaw Increase CH ₄ Emissions in an Upland Tundra Ecosystem

   https://doi.org/10.1029/2021JG006376  

   Taylor, M. A. ; Celis, G. ; Ledman, J. D. ; Mauritz, M. ; Natali, S. M. ; Pegoraro, E. ‐F. ; Schädel, C. ; Schuur, E. A. G. ( November 2021 , Journal of Geophysical Research: Biogeosciences)

   Rapid Arctic warming is causing permafrost to thaw and exposing large quantities of soil organic carbon (C) to potential decomposition. In dry upland tundra systems, subsidence from thawing permafrost can increase surface soil moisture resulting in higher methane (CH₄) emissions from newly waterlogged soils. The proportion of C released as carbon dioxide (CO₂) and CH₄remains uncertain as previously dry landscapes transition to a thawed state, resulting in both wetter and drier microsites. To address how thaw and moisture interact to affect total C emissions, we measured CH₄and CO₂emissions from paired chambers across thaw and moisture gradients created by nine years of experimental soil warming in interior Alaska. Cumulative growing season (May–September) CH₄emissions were elevated at both wetter (216.1–1,099.4 mg CH₄‐C m⁻²) and drier (129.7–392.3 mg CH₄‐C m⁻²) deeply thawed microsites relative to shallow thaw (55.6–215.7 mg CH₄‐C m⁻²) and increased with higher deep soil temperatures and permafrost thaw depth. Interannual variability in CH₄emissions was driven by wet conditions in graminoid‐dominated plots that generated >70% of emissions in a wet year. Shoulder season emissions were equivalent to growing season CH₄emissions rates in the deeply thawed, warmed soils, highlighting the importance of non‐growing season CH₄emissions. Net C sink potential was reduced in deeply thawedmore »wet plots by 4%–42%, and by 3.5%–8% in deeply thawed drier plots due to anaerobic respiration, suggesting that some dry upland tundra landscapes may transition into stronger CH₄sources in a warming Arctic.

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5. Effects of topography on tropical forest structure depend on climate context

   https://doi.org/10.1111/1365-2745.13261  

   Muscarella, Robert ; Kolyaie, Samira ; Morton, Douglas C. ; Zimmerman, Jess K. ; Uriarte, María ; Jucker, ed., Tommaso ( August 2019 , Journal of Ecology)

   Topography affects abiotic conditions which can influence the structure, function and dynamics of ecological communities. An increasing number of studies have demonstrated biological consequences of fine‐scale topographic heterogeneity but we have a limited understanding of how these effects depend on the climate context.

   We merged high‐resolution (1 m²) data on topography and canopy height derived from airborne lidar with ground‐based data from 15 forest plots in Puerto Rico distributed along a precipitation gradient spanningc. 800–3,500 mm/year. Ground‐based data included species composition, estimated above‐ground biomass (AGB), and two key functional traits (wood density and leaf mass per area, LMA) that reflect resource‐use strategies and a trade‐off between hydraulic safety and hydraulic efficiency. We used hierarchical Bayesian models to evaluate how the interaction between topography × climate is related to metrics of forest structure (i.e. canopy height and AGB), as well as taxonomic and functional alpha‐ and beta‐diversity.

   Fine‐scale topography (characterized with the topographic wetness index, TWI) significantly affected forest structure and the strength (and in some cases direction) of these effects varied across the precipitation gradient. In all plots, canopy height increased with topographic wetness but the effect was much stronger in dry compared to wet forest plots. In dry forest plots, topographically wetter microsites alsomore »had higher levels of AGB but in wet forest plots, topographically drier microsites had higher AGB.

   Fine‐scale topography influenced functional composition but had only weak or non‐significant effects on taxonomic and functional alpha‐ and beta‐diversity. For instance, community‐weighted wood density followed a similar pattern to AGB across plots. We also found a marginally significant association between variation of wood density and topographic heterogeneity that depended on climate context.

   Synthesis. The effects of fine‐scale topographic heterogeneity on tropical forest structure and composition depend on the climate context. Our study demonstrates how a stronger integration of topographic heterogeneity across precipitation gradients could improve estimates of forest structure and biomass, and may provide insight to the ways that topography might mediate species responses to drought and climate change.

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