Seasonality and Ecosystem Response in Two Prehistoric Agricultural Regions of Central Arizona

Around the globe, prehistoric agriculture has impacted the environment in ways
that are observable today. Prehistoric farmers in the Southwestern US modified the landscape with rock alignments to support rain fed agriculture in this semi-arid region. Numerous studies have shown that former agricultural fields are ecologically different than areas that have not been farmed. This thesis explores the independent effects of the manipulation of rocks into alignments, prehistoric farming, and season on soil properties in two areas with a history of prehistoric agriculture in central Arizona, Pueblo la Plata within the Agua Fria National Monument (AFNM), and an archaeological site north of the Phoenix basin along Cave Creek (CC). During spring, summer, and fall of 2008, soil properties were compared across three landscape features: 1) agricultural rock alignments that were near the archaeological site 2) geologically formed rock alignments that were located 0.5-1 km away from settlements; and 3) areas both near and far from settlements where rock alignments were absent. Annual herbaceous plant biomass was also collected in each location. To explore the effect of alignment and surface soil geomorphology on soil and plant properties, the physical properties of alignments and surface soils were measured. At AFNM, presence of rock alignments, distance from archaeological settlement, and time of year were significantly associated with soil physical properties and nutrient concentration. Patterns of potential nitrogen mineralization rates (pNmin) and herbaceous plant growth varied spatially and temporally. In contrast, at CC, time of year is the only factor associated with soil physical properties, while patterns of pNmin are associated with distance from archaeological features and time of year, and biomass was associated with the presence of alignments.In two areas of central Arizona, topographic modification and seasonality affect current ecological processes and soil properties in distinctly different ways. At AFNM, relatively well-built rock alignments have altered soil properties and processes while less intact alignments at CC have left few legacies. By exploring the effects of season and landscape modification on soil properties and processes, the effects of prehistoric agriculture on current arid and semi-arid ecosystems can be better understood.