The decline of a riparian gallery forest in Devils Tower, Wyoming : causation and management techniques for restoration

Abstract

Devils Tower National Monument, Wyoming, (DT) has experienced a decline in cottonwood recruitment along the Belle Fourche River, leaving the remaining riverine forest in danger of disappearing. The National Park Service has requested information about the mechanisms behind this decline and possible management methods that could be used to restore forests to the Belle Fourche riparian corridor. Previous research has indicated that cottonwood seedlings require flood-deposited sediments and high initial groundwater for survival. A dam located 12 miles upstream of Devils Tower could be impacting the recruitment success of cottonwoods because of a decrease in seasonal flooding. If this decline is due to factors that managers have little control over, other methods should be explored to increase the success of future restoration efforts. This case study examined the physical attributes of the Devils Tower riparian area in comparison to a healthy, functioning, riparian gallery forest located on the nearby Powder River (PR) to learn the complex mechanisms that help support and sustain these healthy riparian ecosystems. Soil field tests, cottonwood surveys, and measurements of local groundwater patterns were used to compare the two sites. To determine the restoration potential of cottonwood gallery forests at Devils Tower, planting trials were conducted to provide a methodology for greater seedling survival, testing effects of residual herbicide, species planted, preliminary soil preparation including disking, herbaceous understory control, and irrigation. We observed higher groundwater and greater cottonwood age class diversity at the Powder River site, in contrast to the DT site. At DT, tree health was significantly higher (p=.0003) with shallower groundwater. Tree mortality was highest among boxelder and bur oak in sites with deepest groundwater levels. At the same time, irrigation at economically feasible levels had no significant effect on tree survival. This research suggests that site potential for cottonwood re-establishment is poor along the Belle Fourche riparian corridor due to deep groundwater tables (1.7-2.2m) and regulated flow patterns with limited sediment delivery to the floodplain environment. A successional shift resulting from regulated streamflow conditions now favors green ash over any other species.