ABRUPT TRANSITIONS FROM PROLONGED DROUGHT TO WET INTERVALS: CLIMATIC TRIGGER FOR HILLSLOPE EROSION EVENTS IN THE COLORADO PLATEAU, NORTHEASTERN ARIZONA, U.S.A

Dendrogeomorphological approaches have proven extremely useful in the study of hillslope dynamics in semi-arid regions. The vertical extent of exposure of tree roots provides a means of determining long-term erosion rates. In addition, identification of anomalous, negative departures in incremental growth of trees can be used to identify stressful periods, including those caused by root exposure in episodes of rapid erosion. We used both approaches to decipher a 400-year history of hillslope erosion. The clay-cemented sandstones of the study area (Jurassic Morrison Formation and Bluff Sandstone, San Rafael Group) weather rapidly and are highly sensitive to minor climate variation. Vertical erosional losses over the last 400 years averaged 18.8 cm/century. However, the tree-ring record indicates that erosion has been highly episodic. During the last 300 years, two of three major episodes of negative incremental growth anomalies in hillslope trees occurred after abrupt transitions from prolonged, multi-year drought to sustained, lengthy periods of above-average precipitation. The periods of anomalous, diminished growth are most plausibly explained as the consequence of rapid exposure of roots due to erosion that occurred at the onset of the wet climatic intervals. In contrast to hillslope trees in erosional hillslope environments, incremental growth in trees in nearby, geomorphically stable landscape positions closely tracked the regional precipitation signal. The most recent episode of negative growth anomalies exhibited by hillslope trees began within a few years after 1905 and was associated with the largest precipitation shift (drought to wet interval) in the last 400 years. A region-wide precipitation event in October 1907 that delivered heavy precipitation over several days is the most likely candidate for the storm that triggered the severe erosion. Significant declines in vegetation cover on slopes during extreme drought may make hillslope soils more prone to erosion if heavy precipitation events follow soon thereafter. The propensity of materials to be removed by erosion depends on sediment availability, which in turn depends on mechanisms controlling weathering and the retention of colluvium and soils, which in turn is dependent on lithology and vegetation responses.