Revegetation Methods for Urban Streams: The Use of Salix laevigata and Populus fremontii Nalini Rao Environmental Sciences Abstract Riparian zones require a healthy plant base to provide ecosystem functions such as erosion prevention and sequestration of chemical pollutants. In this project, the methods of riparian restoration using two common riparian trees, Salix laevigata and Populus fremontii, were explored along a small stream in an urbanized setting of northern California. Revegetation success was studied by comparing the survival of one group of rooted cuttings versus another group planted directly into the soil. Establishment and growth were assessed at two elevations along the creek bank. The goal of the project was to determine both the optimal rooting method and location along the bank for maximum cutting survival and growth, and to compare the success of the two plant species. During the first spring season following planting, all methods resulted in pole survival with no appreciable differences resulting from rooting method. The average number of leaves and stem length was 19.1 and 7.4, respectively. Bank location was a significant determinant of growth for S. laevigata and not significant for P. fremontii. The S. laevigata cuttings grew more than the P. fremontii perhaps because of different optimal growing seasons, as willows tend to initiate growth earlier in the spring. Despite an initial lead in leaf formation by pre-rooted plants, the directly planted cuttings grew more than the rooted cuttings, perhaps because handling of the cuttings twice resulted in double transplantation shock. The use of both species represents a viable strategy for urban stream restoration, and consideration should be given to both rooting method and bank height.Introduction Riparian zones are important transition areas between aquatic and terrestrial ecosystems. The vegetation is also useful as the basis for in-stream food webs, but has been degraded worldwide as a result of human activities in urban, agricultural and wildland settings. Relatively recent human activities, such as the increase in agricultural runoff, streamflow diversions, and deforestation, have degraded riparian zones (Auble 1994). Increased sedimentation, lowered water tables and other impacts have caused the decline of the cottonwoods (Populus spp.) in many parts of North America (Rood et al. 1995). Increased air pollution and industrial waste have caused the continual decline of willows (Salix spp.) in northwest Russia (Kozlov et al. 1999). Periodic soil desiccation has contributed to the failure of riparian vegetation in Mississippi (Pezeshki et al. 1998). Decreases in land quality causes decreases in the resiliency of riparian vegetation, and thus in the ecosystem as a whole. Riparian zones provide many ecosystem functions, including intercepting excess chemicals and nutrients that flow from the land into the streams, armoring stream banks to decrease erosion, and supporting a diversity of invertebrate and vertebrate species (Gregory et al. 1991). Riparian zones are often identified as corridors for the movement of animals and the dispersal of plants (Gregory et al. 1991, Harris 1999). Healthy riparian plants are essential to providing an effective buffer zone (Wade 1995). It is often desirable to manipulate the vegetation in order to establish diverse communities of animals and plants on either side of the buffer zone, which is the area that collects the inflows of the other ecosystems (Wade 1995). While some riparian zones are isolated from one another, migrating birds use riparian corridors, as well as connected riparian vegetation patches, as refueling resources (Skagen et al. 1998). Some isolated riparian zones include urban streams that occur along the migration path. When riparian zones are degraded, the riparian vegetation is harmed, and is unable to support birds as well as invertebrates. In urban streams and streams near agricultural lands there are increasing efforts to revegetate the degraded stream banks (Harris 1999). These are preliminary steps in restoring the ecosystem functions of the riparian zone, and will provide recreational and economic amenities in the urban environment (Wade 1995). In California, revegetation efforts often include the planting of willows because they are native riparian species which establish rapidly and provide useful services in riparian environments (Friedman 1995). These plantsare dominant native riparian plants, and require ample soil moisture for growth (Pezeshki et al. 1998). Cottonwood (esp. Populus fremontii) is also native to California’s riparian zones, and can be used in revegetation efforts (Friedman 1995). Cottonwoods are especially sensitive to the absence of natural disturbance, particularly flooding. They require conditions of channel movement of bare soil caused by flooding to establish seedlings because the seeds germinate on open gravel beds and other clear substrates. Regulated urban streams inhibit such movement (Auble et al. 1997). Another problem is that high creek banks result from highly incised urban streams, and may cause loss of contact with the water table. The steepness of the bank and lowered water tables cause the plants to be far from the water table. This distance can affect the water availability and consequently the growth of the plants. Some willows may survive at the water’s edge better than at higher heights above the creek bank in wetland environments, and some willow species tolerate dryness better than others (May 1995). Species that are found at higher elevations survive less successfully close to the water’s edge, and researchers recommend that considerations should be given to water table elevation when planning a restoration strategy (Pezeshki et al. 1998). The objective of this study was to test the effectiveness of two factors for promotion of riparian vegetation. The first was a test of differential establishment of plants depending on bank height along a small creek within an urban setting. My project compares the growth of red willow, Salix laevigata, and cottonwood, Populus fremontii, at two different heights along a creek bank. S. laevigata was chosen because studies have shown that it has an effective root system that will tolerate moderate water table lowering (Williams and Matthews 1990). Plants from of the genus Salix are often used in restoration projects, but Populus
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