Agricultural slope management and soil erosion in Tlaxcala, Mexico.

摘要

Geoarchaeological methods are used to reconstruct a sequence of agricultural change and human impact on the environment in Tlaxcala, from the arrival of the first farmers at 900 B.C. to the present. The main focus is on the development of agricultural terracing and the degradation of farmland on slopes. The bulk of the field data consists of stratigraphic sections through terrace fills and other slope deposits at the multi-period settlement of La Laguna, and Holocene alluvial sequences in four different drainages.;There is no convincing evidence of agricultural terracing in the Formative or Classic periods. Instead, farmers relied on a form of swidden cultivation that involved the periodic burning of secondary scrub on unterraced slopes. It caused severe soil erosion, the development of barren wastelands, and accelerated the cut-and-fill cycles of streams. The favored response to population growth was not to intensify the use of sloping land, but to clear and degrade ever greater expanses of it.;Bench terraces appeared in the Middle or Late Postclassic period. They were used to reclaim previously degraded slopes by building up artificial cultivation surfaces behind risers faced with stone. Their construction was encouraged by the Aztec system of land tenure and a market for maguey products. Most were destroyed in the Early Colonial period, as maintenance ceased due to the catastrophic depopulation of the countryside. Renewed land degradation was encouraged by a boom in sheep ranching and the consequent overgrazing of slopes.;The large estates that took over much of the farmland in the Late Colonial and Modern periods employed a dual strategy. The more degraded and distant fields were managed as rough pasture and degraded further. Others were reclaimed and improved by means of sloping-field terraces with ditches, and earthen berms planted in maguey. Much of their yield was destined for distant urban markets, but the agricultural colonization of the plains diminished the overall importance of sloping land.;The history of Tlaxcalan agriculture shows little direct or inverse correlation between population density and land degradation. It brings out the importance of long-term local trajectories in shaping land use at any particular moment.