Combined drought and episodic freezing effects on seedlings of low- and high-elevation subspecies of sagebrush (Artemisia tridentata)

doi: 10.1111/j.1399-3054.2007.00904.x Big sagebrush (Artemisia tridentata) is a dominant shrub throughout much of the arid western United States. Several recognized subspecies differ in physiology, morphology and in their distribution in relation to soil water availability. While several studies have compared mature individuals of these subspecies, there is little information on seedling physiological tolerance to physical stresses. Understanding seedling physiology is essential for predicting how speciesmay respond to changes in temperature and precipitation regimes. Our objective was to examine the drought and freezing tolerance of seedlings of twoA. tridentata subspecies: ssp. tridentata, which is found in low-elevation dry sites, and ssp. vaseyana, found in higher, moister sites. We examined growth, gas exchange and quantum yield of chlorophyll a fluorescence from photosystem II (PSII) for seedlings grown in a greenhouse and exposed to two different levels of moisture availability in combination with a simulated growing season freezing event.We found that ssp. tridentata possessed several drought-tolerant characteristics, such as rapid growth rates, thick leaves and low stomatal conductance. Both subspecies shared similar physiological tolerance to the low-moisture treatment but experienced relatively more stress under the freezing treatment. Subspecies vaseyana was more sensitive to freezing when grown with low soil moisture, resulting in reduced stomatal conductance and PSII quantum yield. In contrast, the low-soil moisture treatment did not increase the susceptibility of ssp. tridentata to freezing. These results demonstrate that drought tolerance may be an important trait for seedlings of A. tridentata, but stress because of freezing damage of seedlings may limit the distribution of the species.