- Two types of temporary poois, known as playas, occur in semiarid southeast Oregon.
Vegetated upland playas are found in the High Lava Plain province; barren alkaline-saline
playas are found in the Basin and Range province. Both types become ponded
in most years, but they often lack redoximorphic features indicative of hydric soils.
We studied the morphology, hydrology, temperature, chemistry, redox potentials, and
vegetation in one pool of each type to elucidate processes of soil genesis, to determine
if the soils are hydric soils, and to evaluate relevant field indicators of hydric soils.
We collected data for 3 years from piezometers, Pt electrodes, and thermocouples
installed at several depths at monitoring sites within and around the pools. Chemical
and physical parameters of soil and water samples were analyzed. Plant species were
identified and their cover estimated. The upland playa was ponded annually but never
exhibited subsurface saturation. Its soils were hydric, meeting the hydric criteria for
hydrology, redox potential, and temperature. Redox potentials indicated an anaerobic
soil environment, but they were never low enough to significantly reduce iron.
Depleted matrices and soft Fe-Mn concentrations were present; soft concentrations
were too small and inconspicuous to meet the criteria of any current hydric indicators.
We propose new indicators based on observed morphological features. Vegetation,
soils, and hydrology at this site indicate great similarity with vernal pools in northern
California. The pool meets all criteria for classification as a jurisdictional wetland.
Effects of current management on the ecology of these pools is not known. The
alkaline-saline playa was ponded annually and also saturated at depth. Its soils were
hydric, but low soil organic carbon content, low soil Fe-Mn content, high pH, and
redox potentials above the thresholds for Fe and Mn reduction suppress formation of
hydric soil morphological indicators. Fe-Mn nodules, found throughout the profile,
are probably not formed by redox processes. We propose that the limit of greasewood
encroachment onto the playa be used as a field indicator of the boundary between
hydric and non-hydric soils.