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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. 

Abstract AimsHerbivores create large differences in litter decomposition rates, but identifying how they do this can be difficult because they simultaneously influence both biotic and abiotic factors. In the Yukon-Kuskokwim (Y-K) River Delta in western Alaska, geese are dominant herbivores in wet-sedge meadows, where they create ‘grazing lawns’ that have nutrient-rich litter and an open habitat structure. To understand how geese affect decomposition, we tested the effects of litter quality and habitat type on litter decomposition over one year. MethodsWe performed a litter bag study in which we collected two litter types representing grazed and ungrazed vegetation conditions (high quality litter similar to grazed litter, and lower quality senesced, ungrazed litter), then incubated them in ‘grazing lawn’ and ungrazed meadows. Litter mass loss, carbon, nitrogen, cellulose and lignin content were measured after 3, 6, 9, and 52 weeks. We also monitored abiotic conditions (i.e., soil temperature, UV radiation, throughfall, and soil moisture content) in each habitat type. ResultsHigh-quality litter (lower lignin:N ratios) lost more mass than low-quality ungrazed litter over the whole study. However, at different times during the decomposition process, lower quality litter decomposed faster in grazed habitat, whereas higher quality litter decomposed faster in ungrazed habitat. This occurred despite abiotic conditions in grazed habitat that generally promote faster decomposition. ConclusionResults suggest that herbivore-induced increases in litter quality increase decomposition rates, and that the accumulation of the low-quality litter in ungrazed habitats is partly due to slow decomposition rates. While herbivores influence habitat conditions, the effects of habitat on decomposition differed across litter qualities, which suggests that other variables, such as differing microbial communities, play a role in decomposition processes. Graphical abstract 

This dataset was created to understand plant trait responses to warming, flooding, and herbivory in the Yukon-Kuskokwim (Y-K) Delta (western Alaska, USA). We conducted a one-year field mesocosm experiment in which we passively increased temperatures, simulated periodic tidal flooding at two intensity levels (low and high), and applied three components of goose herbivory (grazing, feces addition, and trampling) during the summer growing season. Our treatments reflect changes expected in the Y-K Delta in the next 10-20 years. We conducted the experiment in three community types: a wet sedge-shrub meadow, a tundra, and a transitional wet community between the meadow and tundra, and only sampled the dominant species in these communities. At the end of the season, we harvested height, leaf area, specific leaf area, and leaf dry matter content from randomly selected individuals. 

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). 

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. 

Abstract Global change drivers that modify the quality and quantity of litter inputs to soil affect greenhouse gas fluxes, and thereby constitute a feedback to climate change. Carbon cycling in the Yukon–Kuskokwim (Y–K) River Delta, a subarctic wetland system, is influenced by landscape variations in litter quality and quantity generated by herbivores (migratory birds) that create ‘grazing lawns’ of short stature, nitrogen-rich vegetation. To identify the mechanisms by which these changes in litter inputs affect soil carbon balance, we independently manipulated qualities and quantities of litter representative of levels found in the Y–K Delta in a fully factorial microcosm experiment. We measured CO₂fluxes from these microcosms weekly. To help us identify how litter inputs influenced greenhouse gas fluxes, we sequenced soil fungal and bacterial communities, and measured soil microbial biomass carbon, dissolved carbon, inorganic nitrogen, and enzyme activity. We found that positive correlations between litter input quantity and CO₂flux were dependent upon litter type, due to differences in litter stoichiometry and changes to the structure of decomposer communities, especially the soil fungi. These community shifts were particularly pronounced when litter was added in the form of herbivore feces, and in litter input treatments that induced nitrogen limitation (i.e., senesced litter). The sensitivity of carbon cycling to litter quality and quantity in this system demonstrates that herbivores can strongly impact greenhouse gas fluxes through their influence on plant growth and tissue chemistry. Graphical abstract 

Global change drivers that modify the quality and quantity of litter inputs to soil affect greenhouse gas fluxes, and thereby constitute a feedback to climate change. Carbon cycling in the Yukon-Kuskokwim (Y-K) River Delta, a subarctic wetland system, is influenced by landscape variations in litter quality and quantity generated by herbivores (migratory birds) that create ‘grazing lawns’ of short stature, nitrogen-rich vegetation. To identify the mechanisms by which these changes in litter inputs affect soil carbon balance, we independently manipulated qualities and quantities of litter representative of levels found in the Y-K Delta in a fully factorial microcosm experiment. We measured carbon dioxide (CO2) fluxes from these microcosms weekly. To help us identify how litter inputs influenced greenhouse gas fluxes, we sequenced soil fungal and bacterial communities, and measured soil microbial biomass carbon, dissolved carbon, inorganic nitrogen, and enzyme activity. We found that positive correlations between litter input quantity and CO2 flux were dependent upon litter type, due to differences in litter stoichiometry and changes to the structure of decomposer communities, especially the soil fungi. These community shifts were particularly pronounced when litter was added in the form of herbivore feces, and in litter input treatments that induced nitrogen limitation (i.e., senesced litter). The sensitivity of carbon cycling to litter quality and quantity in this system demonstrates that herbivores can strongly impact greenhouse gas fluxes through their influence on plant growth and tissue chemistry. 

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. 

Herbivory can have strong impacts on greenhouse gas fluxes in high-latitude ecosystems. For example, in the Yukon-Kuskokwim (Y-K) Delta in western Alaska, migratory goose grazing affects the magnitude of soil carbon dioxide (CO2) and methane (CH4) fluxes. However, the underlying drivers of this relationship are unclear, as few studies systematically tease apart the processes by which herbivores influences soil biogeochemistry. To examine these mechanisms in detail, we conducted a laboratory incubation experiment to quantify changes in greenhouse gas fluxes in response to three parameters altered by herbivores in situ: temperature, soil moisture content, and nutrient inputs. These treatments were applied to soils collected in grazing lawns and nearby ungrazed habitat, allowing us to assess how variation in microbial community structure influenced observed responses. We found pronounced differences in both fungal and prokaryotic community composition between grazed and ungrazed areas. In the laboratory incubation experiment, CO2 and CH4 fluxes increased with temperature, soil moisture, and goose fecal addition, suggesting that grazing-related changes in the soil abiotic environment may enhance soil C losses. Yet, these abiotic drivers were insufficient to explain variation in fluxes between soils with and without prior grazing. Differences in trace gas fluxes between grazed and ungrazed areas may result both from herbivore-induced shifts in abiotic parameters and grazing-related alterations in microbial community structure. Our findings suggest that relationships among herbivores and soil microbial communities could mediate carbon-climate feedbacks in rapidly changing high-latitude ecosystems.