30 Days of Tennessee Archaeology 2015, Day 25
University of North Texas
The Mississippian Period has long been seen as a time of extreme and rapid change, from intensive agriculture to the growth of large population centers and then ultimately to the collapse of the Mississippian polities in the Southeast. The collapse around AD 1500 to AD 1700 represents a complex synergy of causal processes, such as environmental degradation, conflict, drought induced water and food resource stress, and of course European contact. People react to environmental change in many different ways. These reactions do not happen on a global scale, but rather on a level where people are living their daily lives.
To examine these decisions requires a closer look at the changes in agriculture over time, especially as related to drought. One way to do this is to determine the amount of moisture that was available to crops, then determine whether crops may have failed within a given year, or at least to what degree they may have failed. Examining locations of agricultural lands near villages and the amount of potentially productive land can provide valuable information on the local environment and to the Mississippian people’s subsistence. Studying soil moisture and crop failure can complement the field of paleoethnobotany, which we saw on Day 11. My ongoing research focuses on several major sites around Nashville in two watershed subbasins – the Harpeth and the Lower-Cumberland. However, for today, I want to focus on the Mississipppian site of Mound Bottom, located just to the west of Nashville and by the Harpeth River.
The amount of crops that fail within a year can a have tremendous impact on a village’s decisions of whether to stay or migrate elsewhere, or in further developing and refining food storage/preservation techniques or trade decisions. Some scholars have examined changes in climate and crop production during the Mississippian time period (see Anderson 1996, 2001). The changes in crop production seem to coincide with large societal shifts, such as with shifts to larger aggregated and fortified villages.
During the Mississippian period, there was significant annual and intra-annual variability in precipitation, which may have had a major impact on agricultural productivity (Aharon et al. 2012; Anderson 1996, 2001; Anderson et al. 1995; Stahle and Cleveland 1992). If precipitation is significantly high, soil can become saturated; if significantly low, crops will reach wilting point. Both scenarios result in crop failure. Using hydrological modeling, or examining the science behind water available to crops, we examine changes in the spatial and temporal distribution of soil moisture around sites to evaluate the impact of fluctuating precipitation amounts on agricultural potential. To assess the impacts of climate on food resources, I am evaluating water conditions across the landscape to identify areas that were not suitable for crops during drought conditions, and therefore would have impacted food production. Soil moisture is the amount of water present within a defined space, like a soil column. It is essentially the inputs, like precipitation, minus the outputs, like evaporation, or water running off the surface.
To determine moisture conditions that may have been available to crops, we take into account four different variables including soils, topography, vegetation, and climate (annual precipitation and temperature). To determine annual precipitation and temperature, we use data from tree-rings that was collected and analyzed by dendrochronologists, or someone who studies tree-rings. By focusing on crop failure, we will be able to tell whether declines in agricultural productivity were due to a loss of plants or a decrease in the overall production of individual maize stalks. This allows us to determine whether certain areas within a watershed, surrounding major human population centers, were more or less susceptible to crop failure and how that relates to population movements. Let’s look at the area surrounding Mound Bottom.
Soils can affect these inputs and outputs because differing size sediments affect how long water remains in a particular place. For example, in a sandy soil, water will quickly drain, thus it is not available very long for crops. In addition, we must also consider slope and topography. The angle of a slope can also highly affect outputs, since a steeper slope would cause water to move more quickly downhill, thus, a flat area would be better for soil moisture. Aspect – or the direction that a slope faces – can also affect evaporation and transpiration, thus moisture retention. Solar radiation is more intense with a southern facing slope, thus less moisture is retained. In the figure to the left, you can see the places that will have the most evaporation in red, thus less moisture. If you have a really dry year or several dry years, then the red areas are not going to be very good for having water available to crops. However, if you have a really wet year or several wet years, then the blue areas may end up having too much water for crops.
One drought period recorded by Meeks and Anderson (2013) occurred around AD 1288 to AD 1308. Before this period, the Middle Cumberland area had a period of large mound building – including Mound Bottom – in addition to many scattered smaller villages. However, around the AD 1250s, settlements shifted from smaller spread out villages, to larger aggregated villages with larger and more intensive fortifications. People began abandoning the lower and central portions of the Harpeth subbasin, and moved to the south to places such as Fewkes Site in Williamson County.
This project is in its early stages but it will be able to reveal new insights into the impacts of drought on the prehistoric peoples’ farmlands, and whether it was a major contributing factor to societal shifts in the Mississippian society of Middle Tennessee.
Aharon, Paul, David Aldridge, and John Hellstrom. 2012. Rainfall Variability and the Rise and Collapse of the Mississippian Chiefdoms: Evidence from a DeSoto Caverns Stalagmite. Climates, Landscapes, and Civilizations 198:35-42.
Anderson, David G. 1996. Chiefly Cycling and Large-Scale Abandonments as Viewed from the Savannah River Basin. In Political Structure and Change in the Prehistoric Southeastern United States, edited by John F. Scarry, pp. 150-191. University of Florida Press, Florida.
Anderson, David G. 2001. Climate and Culture Change in Prehistoric and Early Historic Eastern North America. Archaeology of Eastern North America 29:143-186.
Anderson, David G., David W. Stahle, and Malcolm K. Cleaveland. 1995. Paleoclimate and the Potential Food Reserves of Mississippian Societies: A Case Study from the Savannah River Valley. American Antiquity 60(2):258-286.
Meeks, Scott C., and David G. Anderson. 2013. Drought, Subsistence Stress, and Population Dynamics. In Soils, Climate and Society: Archaeological Investigations in Ancient America, eds. John D. Wingard, and Sue E. Hayes, pp. 61-83. University Press of Colorado, Boulder, Colorado.
David W. Stahle and Malcolm K. Cleaveland. 1992. Reconstruction and Analysis of Spring Rainfall over the Southeastern U.S. for the Past 1000 Years. Bulletin of the American Meteorological Society, 73(12), 1947-1961. doi: 10.1175/1520-0477(1992)073<1947:RAAOSR>2.0.CO;2. Available at: https://www.ncdc.noaa.gov/cdo/f?p=519:1:0::::P1_STUDY_ID:16458. Accessed on August 31, 2015.