Winter 1992

Is Restoration of Riparian Systems a Viable Mitigation Tool?

Stephen A. Laymon
Kern River Research Center, Weldon

Mitigation for riparian habitat lost during development projects is an issue of increasing importance. In the last 10 years, four major conferences have been held regarding structure and function of riparian systems in California and the last two highlighted restoration. Despite this interest, surprisingly little is known about the factors that determine success of an individual restoration site or even how success might be defined. The first step in defining the success of a restoration-mitigation site lies in developing a firm understanding of the goals of the project. Examples of project goals are:

(1) creation of habitat for a certain endangered species (e.g. yellow-billed cuckoo or Bell's vireo); or
(2) creation of forest of a certain type (e.g. open-canopy mature valley oak woodland or closed-canopy mature cottonwood-willow riparian forest).

In the first example, an understanding of the habitat requirements of the target species is needed. For most animals in riparian systems, habitat requirements are poorly known. In tests of California Department of Fish and Game's Wildlife-Habitat Relationship Models in riparian habitat along the South Fork Kern and Sacramento rivers, I have found a poor fit between the model's predictions and the actual breeding birds. Therefore, detailed research on habitat requirements of target species will likely be needed prior to the start of the project. In both the first and second example, a site must be chosen that can support the target plant community. For example, low terrace sites will not support oaks due to frequent inundation and a high terrace site will not support willows because the depth to ground water is too great. Sites with high salinity or extreme pH values may not sustain the necessary plant species. On the Kern River, yellow-billed cuckoos are nesting in five year old natural regeneration sites, but have not yet nested on restoration sites of the same age. Natural, low terrace willow-cottonwood riparian forest regenerates after flooding and densities of young trees are much greater than on restoration sites. Much thinning takes place on natural reforestation sites as the forest matures. In the short term there are great differences between natural and planted sites, but we know now we will still not understand the critical factors affecting the outcome of restoration sites. Resources to conduct restoration are scarce, the mission is critical, and we cannot afford to repeat our failures.

To avoid repeated failures, we must have the integrity to report failures, as well as our successes. This is difficult for all of us, but may be most difficult for consultants when the next contract depends on the success of the last. Reporting of failures is vital for two reasons: (1) often much more can be learned from them since we are more likely to ascertain the factors that caused sites to fail than to succeed; and (2) we don't want others to make the same mistakes. A free flow of information is vital to learn what we need to know to succeed. There is no room for proprietary information in this field. It is too soon to know if mitigation through restoration is a viable tool. It has been pointed out that restoration projects for their own sake often succeed, but when they are done for mitigation they often fail. This is an interesting observation and a study of the reasons behind this would take us a long way towards where we need to be in our understanding of restoration. Steve can be reached at (619) 378-3345.