Soil Moisture and Biogeochemical Factors Infl uence the Distribution of Annual Bromus Species

Abiotic factors have a strong infl uence on where annual Bromus species are found. At the large regional scale, temperature and precipitation extremes determine the boundaries of Bromus occurrence. At the more local scale, soil characteristics and climate infl uence distribution, cover, and performance. In hot, dry, summerrainfall-dominated deserts (Sonoran, Chihuahuan), little or no Bromus is found, likely due to timing or amount of soil moisture relative to Bromus phenology. In hot, winter-rainfall-dominated deserts (parts of the Mojave Desert), Bromus rubens is widespread and correlated with high phosphorus availability. It also responds positively to additions of nitrogen alone or with phosphorus. On the Colorado Plateau, with higher soil moisture availability, factors limiting Bromus tectorum populations vary with life stage: phosphorus and water limit germination, potassium and the potassium/magnesium ratio affect winter performance, and water and potassium/ J. Belnap (*) US Geological Survey , Southwest Biological Science Center , Moab , UT 84532 , USA e-mail: [email protected] J. M. Stark Department of Biology, and the Ecology Center , Utah State University , Logan , UT 84322-5305 , USA e-mail: [email protected] B. M. Rau US Department of Agriculture, Forest Service , Southern Research Station , Aiken , SC 29803 , USA e-mail: [email protected] E. B. Allen Department of Botany and Plant Sciences , University of California, Riverside , Riverside , CA , USA e-mail: [email protected] S. Phillips US Geological Survey , Forest and Rangeland Ecosystem Center , Corvallis , OR 97330-6169 , USA e-mail: [email protected]