Arizona has 3 distinct zones:
Low desert extends all the way from the Mexican border to north of Phoenix. Low desert is very heavily vegitated. Mesquite, Saguaro, and Palo Verde are some of the more familar plants of the low desert area. These larger plants are interspersed with ground cover like grass. The mountain region includes scrub pine, and ponderosa pine forrests. The fire is burning in this region. Whats burning is Dense Ponderosa forrest, with a heavy ground cover of scrub pines and fallen pine needles. We are in the midst of a major drought, which means that all of this stuff is very very dry. The mountain region is band shaped, and extends the entire width of Arizona in a band a 100 or so miles wide (north to south). North of this is the High Desert region. This high desert is the same terrain as Las Vegas (if you've ever been there. Lots of open dirt and sand, lots of small ground cover.
There is your answer, along with a small geography lesson.