Riparian Tree Species Distribution and Succession along the Lower Escalante River, Utah

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The Southwestern Naturalist





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The Escalante River is one of the few Southwestern rivers that has escaped impoundment. Livestock grazing and wood cutting have also been minimal because of the rugged canyon topography. It was thus possible to relate distribution of trees and successional status of the vegetation to geologic strata, river bank morphology, and river flow. Populus fremontii was found only on broad flood terraces in wide sections of the upper part of the study area. The oldest, largest trees of this species were located beyond the zone of usual flooding, probably rooting to the shallow water table in these sections of the river canyon. The major population of Tamarix pentandra was found in the section of canyon where large boulders provided protection from the full force of flooding water. Tamarix also inhabits medium and high flood terraces where river bank angle is steep and susceptible to only occasional flooding. Salix exigua occurred throughout the study area, but was least dense in the narrower, steeper stretches of canyon where Tamarix occurred. The high flood terraces supported the majority of each species population, especially the older age classes. Recolonization of lower terraces must occur from these higher terraces after each major flooding event. Tamarix will likely greatly increase in density if flood frequency and severity and average river flow is diminished by impoundments.

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