Woody vegetation expansion in a desert grassland: Prehistoric human impact?

Woody plant encroachment into grasslands and savannas is a global phenomenon with undisputed environmental and economic consequences. In central Arizona, the location of our study, it is well known that mesquite, juniper, and cacti account for the majority of the woody plant expansion into arid grasslands. Using aerial photographs (1940 and 2001), we quantified an increase in woody vegetation in this area. We estimated that from 1940 to 2001, the amount of woody vegetation at our study site increased from 559.7 ha (6.1% of the area) to 1326.6 ha (14.4%); an increase of 766.9 ha (8.3%). A GIS model which included two soil types (Rock Land and Springerville (fine montmorillonitic, thermic typic chromusterts)) with an elevation range from 1142 to 1183 m and slopes from 0° to 6° is able to account for 30.3% (234 ha) of the increase in woody vegetation at our study site between 1940 to 2001. We also examined the locations of woody vegetation in relation to archaeological sites (pueblos with over 40 rooms) and determined that human activities roughly 600 year ago continue to impact the distribution of woody plants on the modern day landscape.