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Abstract: Dendrogeomorphologic approaches were used to study hillslope erosion and valley floor dynamics in a small drainage basin in the Colorado Plateau of northeastern Arizona, U.S.A. Root exposure in pinyon pines indicated hillslope erosion averaged 1.9mm/yr over the last 400yr, but erosion has been highly episodic. Negative increment growth anomalies in hillslope trees are interpreted as the consequence of rapid aerial exposure of roots by erosion. During the last 300yr, two of three major episodes of these growth anomalies occurred after abrupt transitions from prolonged, multi-year droughts to sustained, lengthy periods of above-average precipitation. The most recent episode of these growth anomalies began within a few years after 1905 and was associated with the largest precipitation shift (drought to wet interval) in the last 400yr. In contrast to trees on eroding hillslopes, increment growth of trees in more geomorphically stable landscape positions closely tracked the regional precipitation signal. Two major alluvial fills on the adjacent valley floor are also linked to the abrupt changes in precipitation regimes and the associated increases in delivery of runoff and sediments from slopes. The clay-cemented sandstones weather rapidly; rapid weathering and sediment production make slopes highly responsive to decadal precipitation changes. Significant vegetation declines on slopes during extreme drought make hillslope soils more prone to erosion if heavy precipitation follows soon thereafter. [Copyright &y& Elsevier]

Pinyon–juniper woodlands of the western United States frequently exist within topographically complex landscapes where varied slope aspect yields substantial, local microclimate variation. Vegetation composition and cover typically change markedly along the gradient of relatively mesic northern aspects to more xeric southern aspects. Ecohydrological processes including precipitation runoff, soil moisture storage, and erosion are strongly influenced by vegetation. In certain cases, reduction of plant cover may set self-enhancing feedbacks in motion that lead to further declines of both vegetation and soils, and in some cases, replacement of woodlands with more xerophytic vegetation. The first place such change is likely to occur is in the ecotone between the drier southern aspects and moister north aspects. We studied vegetation, soils, and soil erosion in two small (1–2 ha) drainage basins in northeastern Arizona where pinyon–juniper woodlands occupy northern aspects, grading to shrub-dominated vegetation on more xeric southern aspects. Mapping of soil thickness, use of tree-root exposure to measure long-term soil erosion rates, and data on tree mortality and establishment indicate that the ecotone between woodland and more xerophytic vegetation has apparently been shifting for centuries, with a reduction in woodland vegetation. Erosion rates on xeric aspects ranged from 14 to 23 cm per century in one basin and as much as 60 cm per century in the other basin. In contrast, mesic aspects showed either no net soil losses over the last several centuries or rates significantly less than on the xeric aspects. Exposure of small roots (< 5 mm diameter) of cliff rose ( Purshia stansburiana ) directly overlying bare bedrock surfaces indicates that the process of denudation is ongoing and probably expanding in ecotonal areas. Mesic and xeric aspects exemplify “conserving” vs. “non-conserving” ecohydrologic systems in terms of their capacities to retain water and soils. The contrasting sets of self-enhancing feedback dynamics on xeric vs. mesic aspects not only produce different states in vegetation and soils, they also set in motion the production of pronounced geomorphic contrasts that probably require centuries to millennia to develop. The ongoing ecohydrological transitions in the more xeric aspects are in the process of transforming those hillslopes from smooth, curvilinear, soil-mantled, sediment transport-limited slopes to detachment-limited slopes characterized by an expanding area of bare bedrock steps and cliffs, and this transition is probably irreversible. Predicted temperature increases over the next century for the region are comparable to the present-day soil temperature differential on xeric vs. mesic aspects at the site. Soil temperature is the principal driver of soil water evapotranspirative losses, and because of the interdependent linkages between soil temperature, soil moisture, weathering, production and retention of soil, vegetation, and hydrological response, relatively small temperature increases will likely accelerate the ongoing environmental changes in this and similar areas. [ABSTRACT FROM AUTHOR]