Riparian-zone restoration
Riparian zones are significant in ecology, environmental management, and civil engineering because of their role in soil conservation, their habitat biodiversity, and the influence they have on fauna and aquatic ecosystems, including grassland, woodland, wetland or sub-surface features such as water tables.
The perceived need for riparian-zone restoration has come about because riparian zones have been altered and/or degraded throughout much of the world[1] by the activities of mankind affecting natural geologic forces.
[1][2] Restoration efforts are typically guided by an ecological understanding of riparian-zone processes and knowledge of the causes of degradation.
Riparian-zone disturbance falls into two main categories: hydrologic modifications that indirectly impact riparian communities through changes in stream morphology and hydrologic processes, and habitat alterations that result in direct modification of riparian communities through land clearing or disturbance.
Dams are built on rivers primarily to store water for human use, generate hydroelectric power, and/or control flooding.
Dams can also cause substantial changes in downstream riparian communities by altering the magnitude, frequency, and timing of flood events and reducing the amount of sediment and nutrients delivered from upstream.
However, the lack of flooding caused by altered hydrology creates more favorable conditions for the germination of saltcedar over cottonwoods.
Particularly in arid regions, shallow groundwater, seeps, and springs provide a more constant source of water to riparian vegetation than occasional flooding.
[4][7] For example, Fremont cottonwood (Populus fremontii), and San Joaquin willow (Salix gooddingii), common riparian species in Arizona, were found to have more dead branches and experienced greater mortality with decreasing groundwater levels.
[11] Channel straightening and levee construction eliminate these areas of deposition, creating unfavorable conditions for riparian vegetation recruitment.
[2] A lack of flooding has been shown to decrease the amount of habitat heterogeneity in riparian ecosystems as wetland depressions in the floodplain no longer fill and hold water.
[9] Because habitat heterogeneity is correlated with species diversity, levees can cause reductions in the overall biodiversity of riparian ecosystems.
For example, in an arid South African ecosystem, grazing was found to cause a reduction of grasses, sedges, and tree species and an increase in non-succulent shrubs.
[15][16] Fencing prevent stock from depositing feces directly into waterways and trampling the banks; planting reduces surface runoff.
[1] Riparian zones may be particularly vulnerable to invasion due to frequent habitat disturbance (both natural and anthropogenic) and the efficiency of rivers and streams in dispersing propagules.
For example, feral burros along the Santa Maria river strip bark and cambium off native cottonwoods, causing tree mortality.
[2] The complete removal of dams and flow-altering structures may be required to fully restore historic conditions, but this is not always realistic or feasible.
[6] However, simply restoring a more natural flow regime also has logistical constraints, as legally appropriated water rights may not include the maintenance of such ecologically important factors.
However, this type of restoration can be logistically difficult: in many cases, the initial straightening or modification of the channel has resulted in humans encroaching into the former floodplain through development, agriculture, etc.
[19] Since the completion of the first phase of restoration, a number of improvements in vegetation and wildlife communities have been documented as the conversion from uplands back to wetlands has begun to take place.
[1] Ensuring the establishment of native species is considered vital in preventing future colonizations of exotic plants.
[13][31] By simply restoring hydrologic processes such as periodic flooding that favor riparian vegetation, native communities may regenerate on their own (e.g., the Cosumnes River floodplain).
[4][25] One potential hindrance to passive vegetation restoration is that exotic species may preferentially colonize the riparian zone.
For example, restoration of soil biota, including symbiotic myccorhizae, invertebrates, and microorganisms may improve nutrient cycling dynamics.
[22] Ultimately, a combination of approaches taking into account causes for degradation and targeting both hydrology and the reestablishment of vegetation and other life forms may be most effective in riparian zone restoration.
