Hydro-Ecological Implications of Tephra Layers in the High-Elevation Meadows of the Southern Sierra Nevada.

The most recent volcanic eruptions in the Mono-Inyo chain of craters in the Sierra Nevada, which occurred ~760 and ~1200 years ago, commenced with violent pyroclastic eruptions of pumice that was later carried by wind and deposited over large areas. Today, layers of pumiceous fragments (tephra) are visible in meadow topsoils of southern Sierra Nevada from Yosemite National Park south to the Kern River drainage. In contrast to the overlying (younger) and underlying (older) meadow soils, the tephra layer has very low organic matter content,  and is made up of coarser particles. Consequently, the ability of the tephra to retain moisture against desiccation (by evaporation, drainage, and/or plant uptake) is significantly lower. Moreover, the permeability of a partially-desaturated tephra layer to water is much lower than that of the overlying and underlying layers. Because of these hydrologic differences, a drained tephra layer can behave as a semi-pervious barrier that markedly restricts hydraulic connectivity between its neighboring soil layers. These hydrodynamic considerations and preliminary modeling studies lead us to the following important hydro-ecological implications of the existence of tephra layers in meadows:

1. Moisture available in the top layer is preserved from loss by deep drainage. This effect is pronounced during summer precipitation events that occur after the water table has dropped below the tephra layer. This phenomenon provides competitive advantage to plants with shallow roots and soil microorganisms that inhabit the top layer.
2. Moisture stored below the tephra layer is preserved from evaporative loss when the hydraulic continuity at the tephra layer is broken. This “mulching” effect helps plants with deeper roots to utilize the deeper soil moisture reserve for much longer period.