Collapse of riparian poplar forests downstream from dams in western prairies: Probable causes and prospects for mitigation |
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Authors: | Stewart B Rood John M Mahoney |
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Institution: | (1) Department of Biological Sciences, University of Lethbridge, T1K 3M4 Lethbridge, Alberta, Canada |
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Abstract: | Although historically abundant, the riparian poplar forests of the western prairies are now endangered as a result of the
damming and diversion of rivers in this region. Recent reports have described substantial declines of riparian poplar forests
downstream from dams in Alberta, Canada; Montana, North Dakota, Wyoming, Colorado, and Arizona, USA. The present report analyzes
the forest and hydrological conditions reported previously in order to clarify the causes of the downstream forest decline.
Dams were found to contribute to forest failure by (1) reducing downstream flows and/or (2) altering flow patterns to attenuate
spring flooding and/or stabilize summer flows. Reduced flows are reported to induce drought stress, which is particularly
lethal to seedlings and very old poplars. The artificial moderation of spring flooding may inhibit the formation of seedbeds
essential for seedling replenishment. Increased river valley development involving cattle grazing, agricultural clearing,
and direct harvesting of trees also contributes to forest failure.
Potential methods for mitigating the impacts of dams on downstream forests include downstream flow schedules that (1) retain
occasional spring flooding, (2) taper off rather than abruptly drop downstream flow, and (3) provide adequate flows throughout
the summer. Poplar forest stabilization and recovery can be further promoted by fencing to protect trees from livestock grazing
and trampling, or artificial site preparation such as cultivation or scarification to encourage poplar regeneration. |
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Keywords: | Poplars River damming Riparian vegetation Environmental mitigation |
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