Potential for Use of Cottonwoods in Dendrogeomorphology and Paleohydrology

TitlePotential for Use of Cottonwoods in Dendrogeomorphology and Paleohydrology
Publication TypeThesis
Year of Publication1987
AuthorsClark, S
Academic DepartmentDepartment of Geosciences
DegreeMaster of Science
Number of Pages52
UniversityUniversity of Arizona
Keywordscottonwood, dendrochronology, dendrogeomorphology, escalante, fluvial system, fremont, harris wash, paleoflood, paleohydrology, paria, ponderosa pine, terrace sediment, tree ring, twenty five mile wash, utah

Fremont cottonwoods contain valuable environmental information that can be used to augment knowledge of fluvial systems. Cottonwoods have not been commonly used in dendrochronological studies because of difficulty in determining ring boundaries, uncertainty if growth rings are annual, as well as doubt whether riparian species cross-date. A new method of sample examination utilizing transmitted light permits clear view of ring boundaries, and resampling techniques suggest that the growth rings are annual. The cottonwoods studied are growing along Twentyfive Mile Wash and Harris Wash, both tributaries of the Escalante River in south-central Utah. Cross-dating was found among most of the cottonwood cores, except those from Harris Wash, which were approximately dated by ring counts. After application of rigorous dendrochronological methods, ring counts were deemed to be sufficient to estimate ages of cottonwoods, as the cores contain no missing rings and few false rings. Careful ring counts would accurately estimate the age of these trees to within 1 to 2%. The cottonwoods studied are partially buried by 2 to 4 m of terrace sediments. Dating of the trees provides a minimum age for the terraces of 130 to 227 years. Lack of cross-dating between the cottonwoods and nearby arid-site ponderosa pines indicates that these species respond to different environmental or climatic factors. The ponderosas are limited by lack of moisture, while correlation analysis suggests that the cottonwoods are limited by excess moisture. Soil saturation often causes a decrease in growth due to insufficient oxygen available to the roots. However, in years with very little precipitation, cottonwood growth appears to be limited by lack of moisture, and in these particular years a small ring occurs in the cottonwood series as well as in the ponderosa series. Growth suppressions in the cottonwoods correlate either with known floods on the Escalante or Paria Rivers, or with droughts. If the suppression is due to drought, a corresponding small ring occurs in the ponderosas. Timing of paleofloods can be interfered from suppressions in the early portion of the cottonwood chronology. Rates of alleviation were estimated at 0.9 to 3.0 cm/yr by dividing the amount of sediment above the basal root flare of the trees by the age of the trees. All of these methods would be especially useful in dendrogeomorphological studies on ungaged watersheds, before periods of record, or in watersheds where cottonwoods are the only tree species available.