Active or Passive Riparian Restoration
By Irv Schiffman
Active and passive restoration are two approaches to the restoring of riparian areas whose ecosystems have been changed by newly introduced uses, frequently agriculture or grazing. Active restoration is where the affected land is managed by planting specific vegetation and the use of other methods in order to achieve a particular structure. Passive restoration is where minimal activities are undertaken and the riparian area is allowed to reestablish on its own.
At River Partners we find that active restoration is the best way to restore lands that have been seriously altered. Letting nature take its course once the land has been cleared of agriculture is, of course, cheaper, but it is also very short of vegetative diversity and does not begin to provide the major benefits derived from designed intervention.
There are a number of reasons why passive restoration will not restore the historic riparian ecosystem. For one thing, the original river system along which the native vegetation grew often no longer exists. River systems are by nature highly disruptive with uneven water flows, flooding episodes, sediment transport and frequent course changes. In their natural habitat, riparian vegetation plants evolve and thrive under these conditions. Cottonwood trees growing along the river, for instance, produce masses of seeds that take root on the wet and muddy gravel bar where they germinate while continually irrigated by the receding floodwaters. Their growth along with other indigenous species contribute to the establishment of a rich riparian forest ecosystem.
Enter man and the changes made in the natural order. Dams are constructed, river flows and sediment transport are affected, meanders are disrupted, and exotic plants are introduced into the environment. Cottonwoods seeking to re-establish themselves under such conditions no longer have the help of a beneficial river system and, further, must now compete with such exotics as star thistle, Arundo and Johnsongrass that have become part of the changed environment. As they attempt to grow, the cottonwood seeds are shaded out by the exotic interlopers. In sum, the natural succession pattern has been altered and it is difficult, if not impossible, to recreate the previous riparian ecosystem.
Beyond the intrinsic constraints of passive restoration, however, the advantages of active restoration are obvious. The flora that we plant in the riparian zone not only thrive under the altered conditions, but generally succeeds in attracting targeted species such as the Valley Elderberry Longhorn Beetle (elderberry bushes) and the Least Bell’s Vireo (mugwort understory and shrub-sized willows). The density and arrangement of the riparian plants are structured so that flood convergence on the site is controlled, and by the end of our three-year weed control program, the new plantings have gained a strong advantage over the exotics.
The benefits of active as opposed to passive restoration are illustrated below:
The project shown here was planted in fall 2006 at the Vierra Unit of the San Joaquin River NWR. The field in the center of the photo was left to passive restoration (it is surrounded by a small levee that allows floodwater to enter and exit the field as the river moves up and down). As can be seen, the plant cover is sparse despite being immediately adjacent to excellent sources of seed material, although there is some cover. What can’t be seen is that the recruitment has been only black willow and cottonwood—very low diversity. In contrast, fields actively restored right next to this one have a nice establishment of native herbs (creeping wildrye, mugwort, and gumplant), shrubs (blackberry, rose, buttonbush, sandbar willow, coyote brush), and six different species of trees (Oregon ash, box elder, black willow, arroyo willow, valley oak, cottonwood).