Final answer:
The data provided mainly discusses the impacts of dams and dam removal on ecosystems. While it doesn't specifically provide evidence for reproductive failure of Rio Grande Cottonwoods before a dam's construction, changes in water levels or alterations in the ecosystem resulting from dam existence could hypothetically affect tree reproduction.
Step-by-step explanation:
The available evidence, as indicated by Howe and Knopf 1991, seems to suggest that the majority of the current mature Rio Grande Cottonwoods reproduced prior to the construction of the last major dam on the river. However, the question asked about any evidence of reproductive failure before the dam was built. As per the provided references, the main focus is on the impact of dam construction and deconstruction on the ecosystem, mainly on water quality, water levels, and aquatic organisms such as fish.
For instance, increased fish diversity and improved water quality were reported after dam removal. In context of Rio Grande Cottonwoods, if the existence of dams changed the river's water levels drastically or in some way altered the Cottonwoods' ecosystem, the trees might have experienced difficulties in reproduction. Without specific studies on Cottonwoods before dam construction, it would be challenging to definitively claim there was a reproductive failure before the dam was built.
The ecological value of dams and free-flowing rivers is a subject of ongoing research and debate. This has led to dam removal projects in many parts of the United States since the mid-1980s. These restoration experiments will provide valuable data for other dam-related projects.
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