Sap flux-scaled transpiration and stomatal conductance response to soil and atmospheric drought in a semi-arid sagebrush ecosystem

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 (E L) and stomatal conductance (g S); (2) response of E L and g S 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 g S based on homeostatic regulation of minimum leaf water potential (Ψ Lmin) that was originally developed for trees. Our results showed that a combination of atmospheric and surface soil drought controlled E L, whereas g S was mainly driven by atmospheric drought. Sagebrush displayed greater reference conductance [g S@1. kPa vapor pressure deficit (D), g SR] and greater sensitivity (-m) of g S to D than mesic trees, reflecting the high average light intensity within the shrub canopy. The slope of -m/ g SR was similar to mesic trees (∼0.6), indicating an isohydric regulation of Ψ Lmin, but different than previously published values for semi-arid shrubs (∼0.4). Isohydric behavior of sagebrush indicates that well-known forest ecosystem models with greater g SR and -m can be used for modeling water, energy and carbon cycles from sagebrush and similar ecosystems.