Soil Moisture and Biogeochemical Factors Influence the Distribution of Annual Bromus Species

Abstract

Abiotic factors have a strong influence on where annual Bromus speciesare found. At the large regional scale, temperature and precipitationextremes determine the boundaries of Bromus occurrence. At the morelocal scale, soil characteristics and climate influence distribution,cover, and performance. In hot, dry, summerrainfall-dominated deserts(Sonoran, Chihuahuan), little or no Bromus is found, likely due totiming or amount of soil moisture relative to Bromus phenology. In hot,winter-rainfall-dominated deserts (parts of the Mojave Desert), Bromusrubens is widespread and correlated with high phosphorus availability.It also responds positively to additions of nitrogen alone or withphosphorus. On the Colorado Plateau, with higher soil moistureavailability, factors limiting Bromus tectorum populations vary withlife stage: phosphorus and water limit germination, potassium and thepotassium/magnesium ratio affect winter performance, and water andpotassium/magnesium affect spring performance. Controlling nutrientsalso change with elevation. In cooler deserts with winter precipitation(Great Basin, Columbia Plateau) and thus even greater soil moistureavailability, B. tectorum populations are controlled by nitrogen,phosphorus, or potassium. Experimental nitrogen additions stimulateBromus performance. The reason for different nutrients limiting indissimilar climatic regions is not known, but it is likely that siteconditions such as soil texture (as it affects water and nutrientavailability), organic matter, and/or chemistry interact in a mannerthat regulates nutrient availability and limitations. Under futuredrier, hotter conditions, Bromus distribution is likely to change due tochanges in the interaction between moisture and nutrient availability.