Abstract
summary Arid and semi-arid ecosystems represent a dynamic but poorly understood component of global carbon, water, and energy cycles. We studied a semi-arid mountain big sagebrush (Artemisia tridentata var. vaseyana; hereafter, ‘‘sagebrush’’) dominated ecosystem to quantify the (1) relative control of surface (0– 15 cm) versus deep (15–45 cm) soil moisture on leaf transpiration (EL) and stomatal conductance (gS); (2) response of EL and gS to light and soil and atmospheric drought; and (3) physiological mechanisms underlying these responses. The physiological mechanisms were tested using a simple plant hydraulic model for gS based on homeostatic regulation of minimum leaf water potential (WLmin) that was originally developed for trees. Our results showed that a combination of atmospheric and surface soil drought controlled EL, whereas gS was mainly driven by atmospheric drought. Sagebrush displayed greater reference conductance [gS@1 kPa vapor pressure deficit (D), gSR] and greater sensitivity (� m )o fgS to D than mesic
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