Determination of Fire Impacts on Vegetative Communities of the Upper Sonoran Desert Using Terrestrial Ecological Unit Inventory Data.

The upland subdivision of the Sonoran desert comprises nearly 30% of the total land area of the Tonto National Forest in central Arizona. Studies have shown a sharp increase in the annual number of fires on the forest from the 1950s to present day. Since the 1970s forest staff estimate about 205,000 acres of Sonoran Desert has burned. The Sonoran Desert is not a fire adapted ecosystem and little is understood about how these communities respond to increased fire disturbance. The vegetative communities of the upland division of the Sonoran desert are commonly composed of the Mojave-Sonoran Desert Scrub ecological unit, typically dominated by saguaro cactus (Carnegia gigantean) and palo verde (Parkinsonia microphylla) in the company of creosote and bursage shrubs.

Terrestrial Ecological Unit Inventory data was used to evaluate recovery and the pattern of succession in desert communities. Complete vegetative composition was collected on tenth acre plots in areas within historic burn perimeters and compared with unburned vegetative communities in plots located in the same ecologic units with no recorded fire activity. Ecologic types are mapped based on a distinct combinations of soil, climatic, topographic, and vegetation and each unit has an established ecologic site description as a baseline for potential natural vegetation. Indicator Species Analysis using PC-ORD software was used to determine that saguaro was more frequent and abundant in unburned sites based on indicator values. Non-metric multidimensional scaling (NMS with Sorenson distance) with PC-ORD showed that differences in community composition between the burned and unburned sites based on TSF (time since fire) that vegetative community composition is closer to site potential vegetation in the oldest burned areas (25-35yrs TSF).