Background/Question/Methods Shrubland ecosystems play an important role in shaping climatic conditions and hydrology of the Inter-mountain basins of the United Sates, and represent a potentially dynamic but poorly understood component of the global water cycle. Our objective was to quantify the controls on ecosystem water loss in a sagebrush steppe by using a model of canopy stomatal conductance (GC) that incorporates plant hydraulics. The Sap Flow Meter-T4.1 (EMS Brno, Czech Republic) system was used to measure the sap flow of Artemisia tridentata var vaseyana (mountain big sagebrush). This system works on a constant heating approach which is also known as tissue heat balance (THB) method. We calculated GC from, sapflux and branch leaf area measurements during the summer of 2005 in a 38 year old sagebrush steppe ecosystem near the Sierra Madre Mountains, Wyoming, USA. We hypothesized that sagebrush will regulate minimum leaf water potential in response to both soil and atmospheric drought regardless of variability in leaf (EL), and canopy transpiration (EC).
Results/Conclusions Our results show diurnal and monthly variability in transpiration which we explained using vapor pressure deficit (D), soil moisture (q), light (I), and air temperature (T). The exponential saturation between daily EC and D verified the regulation of minimum leaf water potential in sagebrush using a simple plant hydraulic model. Sagebrush shows high reference GC (GCref; measured at 1 kPa D; 450 mmol m^-2s^-1) as compared to tree GCref from literature (50-200 mmol m^-2 s^-1). The tradeoff between GCref and the sensitivity of GC to D suggests that sagebrush must decrease GC in response to atmospheric drought faster than trees with lower GCref. We investigated the mechanism behind sagebrush response to drought using vulnerability to cavitation measurements. As water potential gradient increased, we observed an initial sharp decline in hydraulic conductance (50% loss of conductance at -0.5 MPa xylem pressure) followed by a saturating response. Our results support the idea that hydraulic conductance decreases in response to soil or atmospheric drought which signals stomatal conductance to drop in order to avoid runaway cavitation. Quantification of stomatal control over ecosystem water loss and mechanisms behind it provide valuable information about plant response to drought and will improve future predictions of water cycling in the drought prone western US.