The ancient agroecology of Perry Mesa: integrating runoff, nutrients, and climate

Understanding agricultural land use requires the integration of natural factors, such as climate and nutrients, as well as human factors, such as agricultural intensification. Employing an agroecological framework, I use the Perry Mesa landscape, located in central Arizona, as a case study to explore the intersection of these factors to investigate prehistoric agriculture from A.D. 1275-1450. Ancient Perry Mesa farmers used a runoff agricultural strategy and constructed extensive alignments, or terraces, on gentle hillslopes to slow and capture nutrient rich surface runoff generated from intense rainfall. I investigate how the construction of agricultural terraces altered key parameters (water and nutrients) necessary for successful agriculture in this arid region. Building upon past work focused on agricultural terraces in general, I gathered empirical data pertaining to nutrient renewal and water retention from one ancient runoff field. I developed a long-term model of maize growth and soil nutrient dynamics parameterized using nutrient analyses of runoff collected from the sample prehistoric field. This model resulted in an estimate of ideal field use and fallow periods for maintaining long-term soil fertility under different climatic regimes. The results of the model were integrated with estimates of prehistoric population distribution and geographical characterizations of the arable lands to evaluate the places and periods when sufficient arable land was available for the type of cropping and fallowing systems suggested by the model (given the known climatic trends and land use requirements). Results indicate that not only do dry climatic periods put stress on crops due to reduced precipitation but that a reduction in expected runoff events results in a reduction in the amount of nutrient renewal due to fewer runoff events. This reduction lengthens estimated fallow cycles, and probably would have increased the amount of land necessary to maintain sustainable agricultural production. While the overall Perry Mesa area was not limited in terms of arable land, this analysis demonstrates the likely presence of arable land pressures in the immediate vicinity of some communities. Anthropological understandings of agricultural land use combined with ecological tools for investigating nutrient dynamics provides a comprehensive understanding of ancient land use in arid regions.