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The Louisiana Office of State Parks recently undertook a project to remove all of the trees from the earthen mounds at Poverty Point State Historic Site in northeast Louisiana and replace them with grass. This was done in order to help preserve the mounds from the threat of uprooting trees and to make the site more authentic visually (there were no trees on the mounds when they were built). The tree removal project provided archaeologists and dendrochronologists (scientists who study tree rings) a unique opportunity to study the stability of the earthworks under these different vegetation covers. This research sought to compare the historical rate of soil erosion under the woodland cover with the rates of erosion as grass was established on the treeless mounds. What the future impact might have been, if the trees had been left on the mounds, was also modeled.

The study began with thorough documentation of the tree stumps and different attributes that might reflect past soil erosion (e.g., the location and length of exposed roots, and the presence of colluvial damming [the piling up of soil on the uphill side of trees]). Samples of some exposed tree roots were taken to investigate, using the tree rings in the roots, when they were first exposed to air and how much soil had been lost since that happened. The tree ring analysis did not provide conclusive results, but analysis of the other attributes indicated that significant soil erosion had occurred in the past. The trees acted like dams to trap the sediment on their uphill sides, but erosion occurred on their downhill sides.

Raw ring-width measurements are plotted for oak root sample.

Raw ring-width measurements are plotted for oak root sample.

The study next looked at whether there was erosion occurring after the trees were cut down and while grass was being established on the mounds. Using the tree stumps as benchmarks from which to measure soil loss, it showed that the process of erosion was continuing. It is not clear yet how the current rate compares to historic rates.

Finally, characteristics of trees previously uprooted on the site were used to predict how much the earthworks might have been disturbed by windthrown trees, if the trees had been left on the mounds. There is a strong relationship between the size of a tree’s stem and the amount of dirt disturbed when it is uprooted. A conservative estimate of nearly 1,000 m3 (35,315 ft3) suggests that removing the trees prevented significant additional mound erosion.

This research was made possible through Grant MT-2210-11-NC-09 from the National Center for Preservation Technology and Training (NCPTT).

Download “Tree-Ring Dating Investigation Report (2013-08a)” Stahle-Report.pdf – Downloaded 148 times – 1 MB

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