Las Vegas averages ~4 inches of rain (a little more than 10 cms) per year with less than 2 days of precipitation per month, mostly in very short intense bursts.

Las Vegas averages ~4 inches of rain (a little more than 10 cms) per year with less than 2 days of precipitation per month, mostly in very short intense bursts. In June and July, when a tree needs it most because of the ever intensifying heat waves, the rains may not happen at all. Even the desert trees struggle in such an inhospitable environment and can be drought deciduous to conserve water (a notable desert adaptation among many). Without tons of water, an oak doesn't stand a chance.