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Abstract Climate change is exposing coastal landscapes to more flooding, in addition to rapidly rising temperatures. These changes are critical in the Arctic where the effects of sea level rise are exacerbated by the loss of sea ice protecting coasts, subsidence as permafrost thaws, and a projected increase in storms. Such changes will likely alter the land-atmosphere gas exchange of high-latitude coastal ecosystems, but the effects of flooding with warming remain unexplored. In this work we use a field experiment to examine the interacting effects of increased tidal flooding and warming on land-atmosphere CO₂and CH₄exchange in the coastal Yukon–Kuskokwim Delta, a large sub-Arctic wetland and tundra complex in western Alaska. We inundated dammed plots to simulate two levels of future flooding: low-intensity flooding represented by one day of flooding per summer-month (June, July and August), and high-intensity flooding represented by three-consecutive days of flooding per summer-month, crossed with a warming treatment of 1.4 °C. We found that both flooding and warming influenced greenhouse gas (GHG) exchange. Low-intensity flooding reduced net CO₂uptake by 20% (0.78µmol m⁻²s⁻¹) regardless of temperature, and marginally increased CH₄emissions 0.83 nmol m⁻²s⁻¹(33%) under ambient temperature, while decreasing CH₄emissions by −1.96 nmol m⁻²s⁻¹(40%) under warming. In contrast, high-intensity flooding restored net CO₂uptake to control levels due to enhanced primary productivity under both temperature treatments. High-intensity flooding decreased CH₄emissions under ambient temperature by 0.76 nmol m⁻²s⁻¹(30%), but greatly increased emissions under warming by 4.68 nmol m⁻²s⁻¹(265%), presumably driven by increased plant-mediated CH₄transport. These findings reveal that GHG exchange responds rapidly and non-linearly to intensifying flooding, and highlight the importance of short-term flooding dynamics and warming in shaping future carbon cycling in this Arctic coastal wetland. 

This dataset was used to answer the question: how do flooding and warming alter carbon dioxide and methane flux from coastal wetlands of the Yukon-Kuskokwim (Y-K) Delta (Western Alaska, USA)? Over two years, we simulated periodic summer tidal flood events at two severity levels and passively increased summer temperatures in a full-factorial field experiment, and the response of gas measured the response of carbon dioxide and methane fluxes. We simulated low-severity and high-severity flooding to represent near-future flooding regimes for the Y-K Delta, projected respectively in the next ~5 and ~10 years. The experiment was established in a wet sedge-shrub meadow, an ecotype covering greater than 10% of the vegetated area of the central coast of the Y-K Delta. We measured gas fluxes approximately twice per week using static chambers during the summer of 2023. 

While the Arctic warms rapidly, several coastal tundra regions face increasing threats from altered flooding regimes. Yet, how flooding shapes coastal tundra ecosystems remains largely unknown. We experimentally examined how increased tidal flooding, under both ambient and elevated temperatures, influences key drivers of ecosystem functioning: micro-environment, vegetation, and organic matter decomposition. Data were collected across three summers (2022-2024) in a low-Arctic coastal tundra heath of the Yukon-Kuskokwim Delta (Alaska) – one of the largest high-latitude riverine deltas in North America. In May 2022, soon after snowmelt, we selected seven blocks within the focal tundra heath. Each block contained six plots, for a total of 42 plots. Plots within blocks were randomly assigned to a factorial combination of experimental monthly tidal floods (three levels: no-flooding, low-severity flooding, and high-severity flooding) and experimental warming (two levels: ambient and higher temperatures). We focused on three response categories: (1) micro-environmental changes, including air and soil temperatures, soil active layer thickness, redox potential, salinity, potential of hydrogen (pH), and chemistry; (2) vegetation responses, such as aboveground community composition and biomass, plant height, and root production; and (3) responses of organic matter decomposition (mass loss, decomposition rate, and stabilization factor). 

The large carbon (C) stock of wetlands is vulnerable to climate change, especially in high latitudes that are warming at a disproportional rate. Likewise, low-lying Arctic areas will experience increased coastal flooding under climate change and sea-level rise, which may alter goose herbivory and fecal deposition patterns if geese are pushed inland. While temperature, flooding, and feces impact soil C emissions, their interactive effects have been rarely studied. Here, we explore the impact of these interactions on carbon dioxide (CO2) and methane (CH4) emissions and nitrogen (N) mineralization (ammonification) in soils collected from four plant communities in the Yukon-Kuskokwim (Y-K) Delta, a high latitude coastal wetland in western Alaska. Communities included a Grazing Lawn, which is intensely grazed and susceptible to flooding, a Lowland Wetland and an Upland Wetland that experience moderate grazing and frequent (Lowland) and less frequent (Upland) flooding, and a rarely grazed and flooded Tundra community, located at the highest elevation. Soils were incubated for 16 weeks at 8 degrees Celsius (°C) or 18°C in microcosms and subjected to flooding and feces addition treatments with no-flood and no-feces controls. We quantified C emissions weekly and ammonification over the course of the experiment. While warming increased ammonification and C demand in the Lowland Wetland and always increased CO2 and CH4 emissions, interactions with flooding complicated warming impacts on C emissions in the Grazing Lawn and Tundra. In the Grazing Lawn, flooding increased CH4 emissions at 8°C and 18 °C, but in the Tundra, flooding suppressed CH4 emissions at 18°C. Flooding alone reduced CO2 emissions in the Upland Wetland. Feces addition increased CO2 emissions in all communities, but feces impacts on CH4 emissions and ammonification were minimal. When feces and flooding occurred together in the Lowland Wetland, CH4 emissions decreased compared to when feces was added without concomitant flood. Feces decreased the immobilization of ammonium and N demand in the Tundra only. Our results suggest that flooding could partially offset C loss from warming in less frequently flooded, higher elevation communities, but this offset could be negligible if flooding and warming drastically increase C loss in more flooded lowland areas. 

This dataset was used to answer the question: to what extent do flooding and warming alter plant-community structure in the high-latitude coastal wetlands of the Yukon-Kuskokwim (Y-K) Delta (Western Alaska, USA)? Over two years, we simulated periodic summer tidal flood events at two severity levels and passively increased summer temperatures in a full-factorial field experiment, and measured alterations in aboveground plant functional group (PFG) biomass and composition. We simulated low-severity and high-severity flooding to represent near-future flooding regimes for the Y-K Delta, projected respectively in the next ~5 and ~10 years. The experiment was established in a wet sedge-shrub meadow, an ecotype covering greater than 10% of the vegetated area of the central coast of the Y-K Delta. We characterized aboveground plant-community structure using the point intercept frequency (PIM) methodology. We clumped vascular plant species into five broad PFGs: graminoids, deciduous and evergreen shrubs, forbs, and standing-dead graminoids. 

Vertebrate herbivore excrement is thought to influence nutrient cycling, plant nutrition, and growth; however, its importance is rarely isolated from other aspects of herbivory, such as trampling and leaf removal, leaving questions about the extent to which herbivore effects are due to feces. We hypothesized that as a source of additional nutrients, feces would directly increase soil N concentrations and N2O emission, alleviate plant, and microbial nutrient limitations, resulting in increased plant growth and foliar quality, and increase CH4 emissions. We tested these hypotheses using a field experiment in coastal western Alaska,USA, where we manipulated goose feces such that naturally grazed areas received three treatments:feces removal, ambient amounts of feces, or double ambient amounts of feces. Doubling feces marginally increased NH4 +-N in soil water, whereas both doubled feces and feces removal significantly increased NO3--N; N2O flux was also higher in removal plots. Feces removal marginally reduced root biomass and significantly reduced productivity (that is, GPP) in the second year, measured as greater CO2 emissions. Doubling feces marginally increased foliar chemical quality by increasing %N and decreasing C:N. Treatments did not influence CH4 flux. In short, feces removal created sites poorer in nutrients, with reduced root growth, graminoid nutrient uptake, and productivity. While goose feces alone did not create dramatic changes in nutrient cycling in western Alaska, they do appear to be an important source of nutrients for grazed areas and to contribute to greenhouse gas exchange as their removal increased emissions of CO2 and N2O to the atmosphere. 

Increasing drought pressure under anthropogenic climate change may jeopardize the potential of tropical forests to capture carbon in woody biomass and act as a long-term carbon dioxide sink. To evaluate this risk, we assessed drought impacts in 483 tree-ring chronologies from across the tropics and found an overall modest stem growth decline (2.5% with a 95% confidence interval of 2.2 to 2.7%) during the 10% driest years since 1930. Stem growth declines exceeded 10% in 25% of cases and were larger at hotter and drier sites and for gymnosperms compared with angiosperms. Growth declines generally did not outlast drought years and were partially mitigated by growth stimulation in wet years. Thus, pantropical forest carbon sequestration through stem growth has hitherto shown drought resilience that may, however, diminish under future climate change.