Developing an ecosystem perspective from experimental monitoring programs: II. Ecophysiological responses of a rare geothermal grass to soil water.

Measurements of xylem water potential, leaf conductance, and leaf pressure-volume characteristics on the geothermal endemic Dichanthelium lanuginosum var. thermale (DILA) were used to delineate operational ranges during wet and dry years and among several microsites at Little Geysers, Sonoma County, California, USA. Plants seldom experienced water potentials more negative that -1.5 MPa. Other nongeothermal, widespread species experienced the lower water potentials typical of chaparral and woodland plants. DILA was able to effectively utilize geothermal water while the widespread species could not and was able to keep stomata open during most of the year. There was evidence to suggest that DILA had some ability to acclimate with significant shifts in Pio and psio during the dry 1994 summer, especially in the upland microhabitat. Nevertheless, minimum leaf turgor values in the upland came very close to, or dropped below, the 0.2-0.3 MPa threshold thought necessary to maintain stomatal opening and photosynthesis. DILA thus depends upon the unique water status of fumarole soils in the vicinity of the Little Geysers to persist in an otherwise lethal regional mosaic of climate, soil, and vegetation. The physiological data were used to derive reference ranges for subsequent monitoring of DILA at Little Geysers. Such ranges are required to determine the future impact, if any, of geothermal development on the persistence of this rare grass and its complex ecosystem.