Tamarisk prevalence in the Rio Grande/Río Bravo del Norte

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Rodríguez, Rockie Marie

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This thesis describes tamarisk prevalence along the Rio Grande/Rio Bravo del Norte in the west Texas and north Chihuahua. Known as the Forgotten River, the borderland riparian area of study is arid and dam regulated. The region has also experienced extensive tamarisk establishment that has coincided with reduced stream flow and a diminishing native cottonwood presence. This thesis investigates whether tamarisk is a symptom of ecosystem change and evaluates the hypothesis that attributes riparian degradation to tamarisk encroachment. The first chapter of the thesis presents both sides of the tamarisk controversy. Two different assessments of tamarisk occurrence exist in popular environmental and scientific literature. One popular hypothesis suggests that tamarisk is responsible for the degradation of the Rio Grande/Rio Bravo del Norte. The information, easily accessible to the general public through influential publications suggests eradication of the shrub as a means to restore stream flow regime. The scientific assessment of tamarisk is associated with a different hypothesis, that tamarisk is not the cause of stream flow degradation and that eradication of tamarisk will not restore stream flow of the Rio Grande/Rio Bravo del Norte. The second chapter of the thesis describes tamarisk’s introduction and floral attributes. The private market and government agencies promoted the introduction of tamarisk on the east coast. By 1926, tamarisk had reached the borderlands of the U.S. and Mexico. Information on floral attributes of tamarisk, its high stress tolerance, expansive root system, and reproductive strategies help in understanding why the shrub flourishes in hydrologically variable arid conditions such as those of the study region. The third chapter expands on the reproductive requirements of exotic tamarisk and native cottonwood vegetation. As native flora’s germination is dependent on natural flood pulses, they are vulnerable to stream flow disturbances. Tamarisk responds more flexibly to flood event variances and can adapt and establish better in human managed ecosystems. The difference in germination needs between tamarisk and native riparian vegetation reveals how stream flow may have influenced tamarisk establishment. This chapter also examines whether tamarisk affects stream flow through consumptive use of water. Evidence from studies of tamarisk evapotranspiration rates does not support the hypothesis that tamarisk is primarily responsible for diminishing stream flows. This chapter concludes that tamarisk does not cause riparian degradation through either vegetative displacement or excessive water consumption. The fourth chapter considers the geographical and historical environmental context of the study region. The naturally arid setting coupled with increased demand for water resources led to the construction of Elephant Butte Dam. Analysis of stream flow data reveals disruptions in natural flood pulse dynamics that favored tamarisk propagation. The case of tamarisk encroachment along the Rio Grande/Rio Bravo del Norte serves as an example of how alterations in an ecosystem can manifest throughout the system in complex and unpredictable ways. Drawing informed conclusions on tamarisk’s occurrence on the Rio Grande/Rio Bravo del Norte, this thesis establishes that tamarisk is a symptom of riparian ecosystem change, not a principal cause



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