Response of peatland water table recession to extreme drought: Experimental results and historical analysis, Marcell Experimental Forest, Northern Minnesota

Peatlands are globally distributed water and carbon reservoirs in terrestrial catchments, and may be threatened due to changes in climate. As water tables are vital to accumulation and preventing decomposition in peatlands, our research is focused on hydrological response to increased warming and/or elevated atmospheric CO2. The Marcell Experimental Forest (MEF) in northern Minnesota includes peatlands with upland forests in catchments that have been studied since the early 1960s, with continuous monitoring of bog water table elevation. In addition, the S1 bog in the MEF is home to SPRUCE, an in-situ experimental peatland study of added warming (+0°C to +9°C) and up to double atmospheric CO2. During the summer of 2021 Minnesota was under severe or extreme drought in >70% of the state. This unique combination of experimental treatment and long-term monitoring in the MEF allow us to compare historic, current, and projected future peatland hydrological response to warming under drought conditions. Here we present an analysis of water table dynamics under drought conditions to provide insight to key peatland hydrological responses. During a statewide drought in 1976 the S1 bog well recorded a median water table recession rate of 10 mm per day, and a maximum of 50 mm per day. This led to a drop in water table elevation to 1000 mm below the long-term median. During our current extreme drought, the S1 bog well recession rates were significantly less, with a median of 1.5 mm and maximum of 4 mm per day (May-July), and water table elevation at least 30 mm below the long-term median. However, inside SPRUCE response appears more drastic; recession rates reached 5-10 mm per day in +0°C plots and 10-40 mm per day in the +9°C plots. Water table elevations within SPRUCE have dropped to below 5-year median in the experimental plots; with +0°C treatments already at 200 mm below median, and in the +9°C, 450-600 mm below median. Compared with the S1 bog well, the +0°C plots are experiencing 2-6 times greater recession while the +9°C rates are at least 10 times, and more similar to those measured during the drought of 1976. Finally, increased CO2 plots had a somewhat more exaggerated responses to drought, indicating plant-hydrological feedbacks are important for understanding water table fluctuations and hydrological dynamics of peatland catchments.