Exceedance of Historic Limit of Dryness Triggers Hydrological Tipping Point in High Elevation Peatlands.

While snowpack provides the dominant water storage reservoir in mountain regions, the soil plays a fundamental role in secondary water storage and is a critical and often overlooked component of mountain hydrology. With their large quantity of stored carbon, high elevation peatland soils provide essential ecosystem services related to water storage, filtration and slow release to downstream communities; and carbon sequestration. Extreme dry years can cause the water table in these systems to drop below historic levels. This drying can induce changes in the structure of the soil through capillary consolidation, coupled to a simultaneous change in the rate of decomposition of soil organic matter. Using a multiple method approach, we investigate the historic limit of dryness that high elevation peatland soils in the Central Sierra Nevada have experienced in order to determine if future drying can trigger a hydrological tipping point resulting in an irreversible loss of ecosystem services. We found that within the historic limit of dryness (up to 0.04 bar suction), high elevation peatlands are resilient and accumulating carbon. After exceeding the 0.04 bar dry limit, however, the peatlands begin to consolidate leading to loss of porosity and permeability, and loss of soil carbon through decomposition. In addition we show that the structural changes in the soil are rapid, have immediate consequences for high elevation peatland resilience and have a disproportionately large impact on hydrology in comparison to decomposition. This research highlights how small changes in climate can trigger local hydrologic tipping points in mountainous regions with cascading regional scale impacts.