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amount of precipitation that falls when air temperatures are below 10 °C (winter)
relative to total precipitation (winter/total precipitation; W/T), as well as the total
amount of cool season precipitation, indicates the frequency with which conditions
occur that allow the conversion of bio-unavailable P into bioavailable P, as dis-
cussed in the P section above. The desert regions discussed occur along a gradient
of a W/T ratio and total winter precipitation. The Chihuahuan Desert occurs at one
end of the gradient, as it has the lowest total winter precipitation, the lowest propor-
tion of winter rain, and the highest winter temperatures of the deserts we sampled.
Under these conditions, it is expected that P is seldom freed from carbonates and
thus available P in the soils remains extremely low most of the year. The Mojave has
the next lowest winter precipitation and W/T ratio and the next highest winter
Fig. 8.3 A hypothesized relationship between climate and soil factors (nutrients and water) limit-
ing Bromus cover in the different deserts of the Western USA. Limiting nutrients are listed in sug-
gested order of importance within regional boundaries. The numbers within the regional boundaries
indicate the average annual amount of precipitation (mm) when mean daily air temperatures are
below 10 °C relative to total precipitation. We suggest that as this ratio and the total amount of cool
season precipitation increase, so do conditions that allow the conversion of bio-unavailable phos-
phorus (P) into bioavailable P. As P becomes less limiting, other nutrients such as N nitrogen (N),
potassium (K), manganese (Mn), and water become more important to Bromus establishment. We
do not have data on the soil factors constraining invasive annual grasses in the Sonoran desert.
Based on this model, P is expected to be most limiting to Bromus in hotter and drier regions
J. Belnap et al.