Permafrost thaw is transforming boreal forests into mosaics of wetlands and drier uplands. Topographic controls on hydrological and ecological conditions impact methane (CH4) fluxes, contributing to uncertainty in local and regional CH4 budgets and underlying drivers. The objective of this study was to explore CH4 fluxes and their drivers in a transitioning boreal forest-fen ecosystem (Goldstream Valley, Alaska, USA). This landscape is characterized by thawing discontinuous permafrost and heterogeneous mosaics of fens, collapse-scar channels, and small mounds of permafrost soils. From a survey in July 2021, observed chamber CH4 fluxes included fen areas with intermediate to very high emissions (29.8-635.3 mg CH4 m(-2) d(-1)), clustered locations with CH4 uptake (-2.11 to -0.7 mg CH4 m(-2) d(-1)), and three anomalous emission hotspots (342.4-772.4 mg CH4 m(-2) d(-1)) that were located near samples with lower emissions. Some surface and near-surface variables partially explained the spatial variation in CH4 flux. Log-transformed CH4 flux had a positive linear relationship with soil moisture at 20 cm depth (R-2 = 0.31, p-value < 1e-5) and negative linear relationships with microtopography (R-2 = 0.13, p-value < 0.006) and slope (R-2 = 0.28, p-value < 2e-5). Methane emissions generally occurred in flat, wet, graminoid-dominated fens, whereas CH4 uptake occurred on permafrost mounds dominated by feather mosses and woody vegetation. However, the CH4 hotspots occurred on drier, slightly sloped locations with low or undetectable near-surface methanogen abundance, suggesting that CH4 was produced in deeper soils. When the hotspot samples were omitted, log-transformed CH4 flux had a positive linear relationship with near-surface methanogen abundance (R-2 = 0.29, p-value = 0.0023), and stronger linear relationships with soil moisture, slope, and soil macronutrient concentrations. Our findings suggest that some CH4 emission hotspots could arise from CH4 in deep taliks. The inference that methanogenesis occurs in deep taliks was strengthened by the identification of intrapermafrost taliks across the study area using low-frequency geophysical induction. This study assesses surface spatial heterogeneity in the context of subsurface permafrost conditions and highlights the complexity of CH4 flux patterns in transitioning forest-wetland ecosystems. To better inform regional CH4 budgets, further research is needed to understand the spatial distribution of terrestrial CH4 hotspots and to resolve their surface, near-surface, and subsurface drivers.

Increasing greenhouse gas levels drive extensive changes in Arctic and cold-dominated environments, leading to a warmer, more humid, and variable climate. Associated permafrost thaw creates new groundwater flow paths in cold regions that are causing unprecedented environmental changes. This review of recent advances in groundwater research in cold environments has revealed that a new paradigm is emerging where groundwater is at the center of these changes. Groundwater flow and associated heat and solute transport are now used as a basis to understand hydrological changes, permafrost dynamics, water quality, integrity of infrastructure along with ecological impacts. Although major advances have been achieved in cold regions' cryohydrogeological research, the remaining knowledge gaps are numerous. For example, groundwater as a drinking water source is poorly documented despite its social importance. Lateral transport processes for carbon and contaminants are still inadequately understood. Numerical models are improving, but the highly complex physical-ecological changes occurring in the arctic involve coupled thermal, hydrological, hydrogeological, mechanical, and geochemical processes that are difficult to represent and hamper quantitative analysis and limit predictive capacity. Systematic long-term observatories where measurements involving groundwater are considered central are needed to help resolve these research gaps. Innovative transdisciplinary research will be critical to comprehend and predict these complex transformations.