Climatic and hydrologic processes leading to wetland losses in Yellowstone National Park, USA

•Recent wetland decline in Yellowstone NP has co-occurred with climate changes.•Different hydrologic regimes cause wetlands to have varying water table changes.•Wetland classes from two years of hydrologic data correlate to long-term processes.•A multi-disciplinary approach enhances understanding of wetland processes.

SummaryWetlands are vital habitats and can be used as landscape indicators because they integrate catchment-scale processes. Wetland drying during the recent decades in Yellowstone National Park's Northern Range has incited concern among National Park managers and the public at large. Our research was focused on developing an understanding of the processes controlling wetland water levels and the changes contributing to wetland decline in the Northern Range. We integrated analyses of hydrology, climate, soils, and vegetation. In 2009, 24 study wetlands were instrumented each with an average of five shallow groundwater monitoring well and piezometer nests. We mapped hydric soils, analyzed aerial photographs, and identified geomorphic indicators of higher water to quantify historic wetland area. The Trumpeter Lake study site was intensively studied to resolve watershed processes driving water table changes through time, and it was used to identify the timescale on which a regionally critical wetland varies. Climate data indicated that warming and drying occurred during the last century, but that this pattern was within the natural range of variation for the study region over the past 800 years, as determined from tree ring data. Hydrologic data revealed that study sites included locations of groundwater discharge, recharge, and flow-through as well as water perched above the regional water table. Hydrologic regimes were classified using a shape-magnitude framework and seven wetland classes were characterized, and the robustness of this classification is assessed using longer-term datasets. Aerial photographs and hydric soil delineation both confirmed formerly greater wetland abundance. Changes varied by wetland class and the presence or absence of surface water outlets. Wetland plant species inhabited distinct habitats of water table depth and variation, and can be used to infer subsurface hydrologic regime in the absence of extensive monitoring well networks. A subset of long-term monitoring wells has been instrumented with groundwater pressure transducers, enabling further understanding of wetland hydrologic processes, promoting additional assessment of the wetland classification, and aiding in identification of wetlands especially vulnerable to climate change.