Soil–plant water relations of two subalpine herbs from Mount St. Helens

The subalpine environment of Mount St. Helens and other southern Cascade volcanoes is characterized by porous, pyroclastic soils and summer droughts. To evaluate plant drought stress in this environment, we examined plant water relations of Eriogonum pyrolifolium, a wintergreen, shallow-rooted, rosette perennial, and Polygonum newberryi, a deciduous, deep-rooted semierect perennial (both in Polygonaceae), at a subalpine site (elevation 1575 m) on Mount St. Helens. In a very dry summer, soil moisture below 20 cm remained above −0.1 MPa, but surface tephra deposits developed soil water potentials below −4.0 MPa. Surface tephra deposits had a mulching effect on underlying pre-eruption soils. Predawn xylem pressure potentials for adults of each species were never below −0.8 MPa, but midday xylem pressure potentials were often measured near or below the estimated turgor-loss point when vapor pressure deficits were high (maximum 3.1 kPa). Compared with Polygonum, Eriogonum had lower xylem pressure potentials, a lower turgor-loss point (mean −1.00 and −1.42 MPa, respectively), and higher conductance. In both species there was no midday depression in leaf conductance and little photosynthetic response to high evaporative demand. Thus, these species are not particularly conservative in water use and appear to rely on abundant soil moisture throughout the short growing season.