DIFFERENCES IN DROUGHT ADAPTATION BETWEEN SUBSPECIES OF SAGEBRUSH (ARTEMISIA TRIDENTATA)

Three subspecies of Artemisia tridentataoccupy distinct habitats in the Great Basin of North America: ssp. wyomingensis in low, arid elevations; ssp. vaseyana in high, mesic elevations; and ssp. tridentata in intermediate zones. We evaluated differences in the drought experienced and drought tolerance among the subspecies. Drought tolerance was measured by two traits: the xylem pressure (Cx) causing xylem cavitation and Cx causing loss of leaf turgor (Ctlp). As expected from habitat, ssp. wyomingensisexperienced a more severe summer drought than ssp. vaseyana (minimum Cx 52 7.5 MPa and 23.8 MPa, respectively). Despite the large difference in drought exposure, the subspecies ex- hibited similar drought responses, including a reduction in transpiration ( E) below 0.5 mmol·s 21 ·m 22 , a shedding of 60-65% of foliage, and .95% decline in soil-to-leaf hydraulic conductance. The similarity in the drought response was consistent with pronounced dif- ferences in Cx, causing 50% loss in xylem conductivity by cavitation (C50). The C50 was 24.9 MPa in ssp. wyomingensis vs. 23.0 MPa in ssp. vaseyana; ssp. tridentata was in- termediate (C50 52 3.9 MPa). Differences in cavitation resistance were preserved in a common garden, suggesting that they arose by genetic differentiation. A water transport model indicated that the greater cavitation resistance in ssp. wyomingensiswas a necessary adaptation for its more arid habitat, and that the similar restrictions of E among subspecies were required to avoid hydraulic failure. The Ctlp was also lower in ssp. wyomingensisthan in ssp. vaseyana. Although Ctlp decreased in both subspecies during drought, the adjustment was not sufficient to maintain turgor. Turgor loss may have been adaptive in minimizing shoot growth and stomatal conductance under hydraulically limiting circumstances.