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magnesium affect spring performance. Controlling nutrients also change with
elevation. In cooler deserts with winter precipitation (Great Basin, Columbia
Plateau) and thus even greater soil moisture availability, B. tectorum populations are
controlled by nitrogen, phosphorus, or potassium. Experimental nitrogen additions
stimulate Bromus performance. The reason for different nutrients limiting in dis-
similar climatic regions is not known, but it is likely that site conditions such as soil
texture (as it affects water and nutrient availability), organic matter, and/or chemis-
try interact in a manner that regulates nutrient availability and limitations. Under
future drier, hotter conditions, Bromus distribution is likely to change due to changes
in the interaction between moisture and nutrient availability.
Keywords Climate • Geomorphology • Nitrogen • Nutrients • Phosphorus • Soils
8.1 Introduction
Despite much research, we still know little about what makes arid and semiarid
(hereafter referred to as dryland ) ecosystems susceptible to invasion by exotic
annual grasses such as Bromus tectorum (L.) and Bromus rubens (L) (Fig. 8.1 ).
Because annual grasses are often associated with soil surface disturbance, this is
Fig. 8.1 Unless the invasion is a result of fi re, annual grass invasions in the drier parts of the
Western USA often occur in distinct patches (as indicated by arrows ) as can be seen in this photo
(taken adjacent to the Great Salt Lake on lacustrine sediments from Lake Bonneville)
J. Belnap et al.