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The sagebiush-steppe ecosystem coveis much of western North Amer tea, and its pioductivity is sensitive to wanning and increasingly variable piecipitation. Intel annual variation in piecipitation has been shown to be the most significant factor controlling biogeochemical cycling while both soil and atmospher ic drought are dominant factor s of ecosystem fluxes We show that plant canopies can also act to limit water losses through stomatal and aerodynamic control We use 4 data-yr horn 2 sites (2 069 and 2 469 m above sea level elevation, respectively Lo evaluate control of carbon and water fluxes and to calculate the degree to which the ecosystem canopy and atmosphere are decoupled Environmental conditions were similar between the two sites, although the lower elevation site was slightly warmer (1.8 degrees C higher temperature) and drier (0.2 kPa higher vapor pressure deficit) Ecosystem responses of net ecosystem exchange (NEE) and evapotranspiration (ET) to environmental drivers were similar between sites and years, with the wet site-yr 2009 having the largest ET and NEE fluxes. Canopy leaf area led to divergent behavior of the canopy-atmosphere decoupling parameter under high (>11% by volume) soil moisture conditions During low (< 11%) soil moisture periods, both sites had tight ecosystem stomatal contr ol on ET with little NEE activity. This study highlights how semi arid ecosystems can alter then canopy leaf area in oeder to control how decoupled semi-ai id canopies are to the atmosphere, potentially moderating impacts of climate change. (C) 2018 The Society for Range Management. Published by Elsevier Inc. All rights reserved.