David J. Cooper
Department of Earth Resources
Colorado State University
Fort Collins, CO 80523

Abstract
TIMING OF TAMARISK INVASION IN THE UPPER COLORADO RIVER BASIN,
AND ITS EFFECTS ON FLOODPLAIN FORMATION AND RIPARIAN VEGETATION

Abstract:

The timing of tamarisk establishment on floodplains has been determined primarily using three approaches; air and oblique photo analysis, historical records analysis and dendrochronology. Photo analysis requires careful analysis of historical photos to attempt to identify plant species and the first appearance of tamarisk. This approach is widely used providing researchers with a tool for analyzing vegetation change. However the difficulties of identifying tamarisk on poor quality older photos, particularly where other plants are present make this approach difficult to apply and errors can result. A written historical record by explorers and botanists provides a more exact but spotty record of the timing of tamarisk establishment in many areas. Dendrochronology, the science of tree ring analysis, can provide very accurate dates for the oldest plants in a region. However the difficulties of analyzing shrubs and woody plants on floodplains where they are buried by fluvial sediments make this the least used approach.

Channel narrowing has been has been documented on many streams in the West. Approaches for narrowing include repeat photo analysis, topographic resurvey of historical stream gauge sites, stratigraphic analyses, and dendrochronology. The strongest approaches result from the application of all of these approaches at a site. Annual peak stream flows have declined during the later part of the 20th century on some rivers reportedly due to climate change have occurred. Thus, channel narrowing likely would have occurred, and the role of tamarisk in this narrowing is unclear. In other areas, tamarisk's role in channel narrowing and sediment accumulation on floodplains is unequivocal. Where tamarisk has caused channel narrowing the most important question to address is the restorability of river channels.

Tamarisk with its readily dispersed seed, long period of seed dispersal, rapid germination and fast growth allows it to readily colonize bare fluvial sediments. Its broad tolerance for substrate type, low water requirements and flood tolerance makes it well adapted to life on arid region floodplains. It has formed dense thickets along many streams and lakes. Many people blame tamarisk invasion for the decline of native cottonwood and willow. However, there is relatively poor evidence for this. Herbaceous plant communities that would have dominated canyon riparian zones likely have declined. Tamarisk however is a typical early successional species. It is shade intolerant and cannot reproduce within its own stands.