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Ecology and Restoration of Riparian Vegetation in the Arid and Semiarid West: Lessons
Julie Stromberg, Arizona State University, Tempe, AZ, and Duncan Patten, Montana State University, Bozeman, MT
The hydrology, geomorphology and biology of riparian ecosystems in the arid and semiarid
West have been altered by many types of land and water uses, and many efforts are
underway to restore desired ecological conditions to these systems. This presentation
discusses lessons learned from rivers and springs in Arizona, California, Utah, Nevada
and Montana subjected to restoration activities as well as those that serve as reference
ecosystems. Through case studies we demonstrate the following:
1. Passive vs. Active Restoration. One lesson learned is that many riparian ecosystems
have high resilience. Passive restoration, or removal of stressors, thus can be an
effective approach to pursue prior to taking active restoration measures. If implementing
active restoration (e.g. modifying channel morphology or restoring flows), patience
is required to allow for natural recruitment of riparian plants.
2. Stream Flow Restoration. Another lesson learned is that stream flow regimes exert key influence on riparian vegetation structure. Development of environmental flows should be river-specific and based on stakeholders’ desired ecological outcomes. Adaptive management should be an integral part of the use of environmental flows allowing for adjustments when appropriate.
3. River/floodplain Connectivity. A third lesson is that maintaining river/floodplain connectivity is essential. By removing structures that reduce river/floodplain connectivity and/or designing environmental flows that create periodic overbank flooding, riparian floodplain processes are re-established. Re-establishment of river/floodplain connectivity may require appropriate contouring of riverine surfaces including meanders and point-bar gradients.
4. Riparian Plantings. Although many plant species will recolonize on their own, plantings can be useful to hasten vegetation development. Prior to planting, soil traits, stream hydrology, and other physical conditions need to be analyzed to ensure that plantings will survive.
5. Range of Variability. Another lesson learned is that the range of restoration targets of riparian plant communities varies greatly. The climatic and hydrologic variability that characterize dryland regions produce a range of plant communities through time. In particular, past extreme disturbance events create legacies that produce high temporal change in attributes such as vegetation abundance.
6. Irreversible Outcomes. Yet another lesson learned is that some environmental changes are irreversible. Withdrawal of groundwater, for example, may cause long-term changes in riparian and wetland plant communities as a consequence of long-term changes in aquifer storage.
7. Potential Irreversible Conditions. Finally, legal and political mandates or decisions
may “trump” ecological management preventing maintenance or restoration of riparian
and wetland plant communities.