Does precipitation variability increase from mesic to xeric biomes?

Desert precipitation is often characterized as more variable than precipitation in nondesert biomes. However, empirical support for this generalization is lacking. I analysed precipitation variability in five desert and four nondesert biomes that represent a xeric to mesic gradient in the South-western United States, using data from 328 weather stations, for a total of 18 337 station-years. I examined precipitation variability at two temporal scales: among years and among months, within years. Each of these scales was analysed for annual precipitation, summer precipitation and winter precipitation for a total of 18 separate analyses. Overall, precipitation variability does increase as precipitation decreases. However, I found little support for the generalization that desert precipitation is more variable than precipitation in nondeserts. At most time scales, many south-western American desert biomes are no more variable than nondesert biomes. The level of variability is highly dependent on which desert biome is under study, where in the biome a particular site is located, and what time scale is used to measure variability. Certain desert sites are in fact more variable than nondesert sites, in some cases by as much as fivefold. However, and perhaps more importantly, each desert biome includes many sites that are no more variable, across many or all time scales, than sites in the grassland or pine forest biomes. These results emphasize the importance of measuring variability for each study site of interest and caution against extrapolating from a few sites to biomes or gradients in general. Implications for the study of adaptations to environmental variability are discussed.