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Abstract Methane (CH₄) dynamics in wetlands are spatially variable and difficult to estimate at ecosystem scales. Patches with different plant functional types (PFT) represent discrete units within wetlands that may help characterize patterns in CH₄variability. We investigate dissolved porewater CH₄concentrations, a representation of net CH₄production and potential source of atmospheric flux, in five wetland patches characterized by a dominant PFT or lack of plants. Using soil, porewater, and plant variables we hypothesized to influence CH₄, we used three modeling approaches—Classification and regression tree, AIC model selection, and Structural Equation Modeling—to identify direct and indirect influences on porewater CH₄dynamics. Across all three models, dissolved porewater CO₂concentration was the dominant driver of CH₄concentrations, partly through the influence of PFT patches. Plants in each patch type likely had variable influence on CH₄via root exudates (a substrate for methanogens), capacity to transport gas (both O₂from and CH₄to the atmosphere), and plant litter quality which impacted soil respiration and production of CO₂in the porewater. We attribute the importance of CO₂to the dominant methanogenic pathway we identified, which uses CO₂as a terminal electron acceptor. We propose a mechanistic relationship between PFT patches and porewater CH₄dynamics which, when combined with sources of CH₄loss including methanotrophy, oxidation, or plant‐mediated transport, can provide patch‐scale estimates of CH₄flux. Combining these estimates with the distribution of PFTs can improve ecosystem CH₄flux estimates in heterogenous wetlands and improve global CH₄budgets. 

Factors driving freshwater salinization syndrome (FSS) influence the severity of impacts and chances for recovery. We hypothesize that spread of FSS across ecosystems is a function of interactions among five state factors: human activities, geology, flowpaths, climate, and time. (1) Human activities drive pulsed or chronic inputs of salt ions and mobilization of chemical contaminants. (2) Geology drives rates of erosion, weathering, ion exchange, and acidification-alkalinization. (3) Flowpaths drive salinization and contaminant mobilization along hydrologic cycles. (4) Climate drives rising water temperatures, salt stress, and evaporative concentration of ions and saltwater intrusion. (5) Time influences consequences, thresholds, and potentials for ecosystem recovery. We hypothesize that state factors advance FSS in distinct stages, which eventually contribute to failures in systems-level functions (supporting drinking water, crops, biodiversity, infrastructure, etc.). We present future research directions for protecting freshwaters at risk based on five state factors and stages from diagnosis to prognosis to cure.