Northern Prairie Wildlife Research Center
Vegetation should be selected on a site-specific basis (Chapman et al. 1982). Knowledge of particular combinations of substrate, microclimate, nutrient and water level regime, and the dynamics of riparian plant communities in both time and space, will greatly aid in riparian ecosystem creation or restoration.
Because of their high edge-to-area ratio, riparian ecosystems have large energy, nutrient, and biotic interchanges between aquatic and terrestrial systems (Fredrickson and Reid 1986). Plant composition, habitat structure, and productivity are largely determined by the timing, duration, and extent of flooding.
The physiological impact of changes in water level on trees depends on the tolerance of the tree to maintain its present root system, the soil conditions, and water level changes. Short- and long-term impacts of water level changes on reproduction, roots, hormones, photosynthesis, and growth are discussed by Tesky and Hinckley (1977). The authors also present an overview of the physical and metabolic mechanisms of tolerance and soil- and water-level factors that affect plant response. The oxygen content, chemistry, and nutrient availability of the soil can influence how well plants respond to flooding. For example, high concentrations of sodium, manganese, aluminum, iron, nitrites, and sulfides during flooding are often responsible for plant toxicities.
Significance of a particular hydrological event for a plant species' distribution must be judged in a temporal perspective that takes into consideration the plant's length of life, its growing season, and particular times in its life cycle that may be more sensitive to submergence (Wakefield 1966). For example, growth of annuals and perennials of riparian zones on an alluvial fan of the Snake River near Clarkston, Washington, was related to somewhat different hydrologic phenomena. The lower distribution of spring annuals was related to winter flood peaks and time period during the growing season that a given level of the fan was exposed. The lower distribution limits of perennials appeared to be affected by exposure period during the growing season, duration they were submerged, and effects of flooding during critical periods of development (e.g., germination, seedling).
Assuming that stream flow is a major vehicle of seed dispersal to riparian zones, it is logical to consider controlled flooding as a method of vegetation establishment (Platte et al. 1987b). Timing of flooding is critical because seed viability of some species is short (e.g., less than 2 weeks for willows). Flooding should be avoided during periods when the stream is transporting noxious weeds.
Selection of plants for revegetation may involve not only consideration of native wildlife species, but also of plants that provide necessary resistance to erosive stream flows in heavily eroded areas. Revegetation specifications should be developed based on an inventory of stream hydraulics and other site conditions and a design that considers adapted plants and their erosion control characteristics, hydraulic limitations of revegetation, desired fish and wildlife habitat, and suitable methods of installation and maintenance (Carlson 1979). Carlson (1979) discusses these characteristics and methods for streamside revegetation efforts in the Pacific Northwest.
Usually, flood control maintenance in California is more involved with clearing riparian vegetation than planting it; however, efforts are being made to plant vegetation on highly erodible sites. On the Murphy Slough of the Sacramento River near Chico, willows and cottonwoods are being planted to aid in eventually reclosing the mouth of the Slough (Chaimson 1984). Benefits other than providing wildlife habitat include sediment deposition, velocity reduction, and redirection of flows. In Santa Clara County, California, revegetation along flood control channels was more successful when predominantly natives associated with riparian communities of the county were planted, weeds were removed from the site, and an irrigation system was used to promote initial growth (Goldner 1984).
Sediment texture also can influence establishment of riparian seedlings. On gravel bars of lower Dry Creek, Sonoma County, California, willow establishment was higher on bars where surface sediment size was less than 0.2 cm (McBride and Strahan 1984). Cottonwood (Populus fremontii) established more densely on areas of intermediate and large-sized sediments (0.2-1.0 cm), and mule fat (Baccharis viminea) dominated on larger sediments. Changes in gravel bar landforms can result in significant losses of established trees as well as young seedlings and saplings. Areas protected from swiftest currents are best suited to withstand high winter flows that can occur in this area.
A number of limiting factors may affect the success of bottomland hardwood plantings in the Southeast (Haynes and Moore 1988). Native regeneration relative to achieving a diversity of tree species was an important consideration on National Wildlife Refuges in this region. Other factors were: (1) drought during the growing season or a late freeze following plantings; (2) standing water and high temperature on sites with young seedlings; (3) flooding on sites where the species planted are not adapted for the duration or depth of flooding; (4) damage or destruction of seeds or seedlings by rodents, rabbits, or deer; and (5) poor seed viability or poor quality of nursery stock.
Field and experimental studies have demonstrated the influence of various environmental conditions on the species composition of bottomland hardwoods. Hosner and Boyce's (1962) study on the tolerance of various bottomland hardwoods to water-saturated soil indicated that occurrence of continuously saturated soil conditions for long, but varying, periods in bottomlands results in a competitive advantage for certain species (e.g., green ash [Fraxinus pennsylvanica], willows) and subsequently affects species composition of bottomland stands. Amount of exposure to direct sunlight and amount of litter and ground cover also can affect species composition, with cottonwood (Populus deltoides) and willow seedlings preferring direct sunlight and lack of litter (Hosner and Minckler 1960).
Selection of plant species for revegetation can be complicated by the fact that riparian communities are not always a distinct climax biotic community. On Sycamore Creek in central Arizona, Campbell and Green (1968) sampled a 21 mile sector of mountainous terrain and determined that due to large-scale changes in habitats caused by recurring floods, erosion, and deposition, development of immature stages of species is common, and the riparian association never reaches a climax stage because of these physiographic factors. Thus, in this semiarid location, the stream-channel vegetation is undergoing perpetual succession, which must be considered in efforts to restore this habitat.
A list of grass, broadleaf, and woody riparian species recommended for planting in various disturbed riparian zones is presented in Platts et al. (1987a). Monsen (1983) discusses planting conditions in riparian zones of the Intermountain Region and presents a list of grasses and broadleaf herbs recommended for riparian plantings within major plant communities of this region.
Wildlife values should be considered in both the selection of plant species and the structural arrangement of the plantings to achieve the highest functional use as wildlife habitat. For example, avian populations were rapidly enhanced by revegetating riparian zones with native riparian species of vegetation along the lower Colorado River (Anderson and Ohmart 1984). Under appropriate planting conditions, native trees grew 2-3 meters annually and shrubs matured and fruited the first year. Careful planning ensured almost immediate use of the area by a large and diverse avian population during most seasons. Clearing of the restoration site (if required) should be done selectively so that all native trees and all dead trees or trees with large dead snags are left intact to attract bird species that use snags as perches and cavities for nesting.
Seeding sites is less expensive than transplanting cuttings or seedlings (Ambrose et al. 1983; Haynes and Moore 1988). Direct seeding eliminates costs associated with growing seedlings in a nursery and is less time-consuming than transplanting seedlings. However, seeding of shrubs and trees is generally less successful than transplanting cuttings or seedlings (Ambrose et al. 1983). One exception to this is direct seeding of bottomland hardwoods of the Southeast (Haynes and Moore 1988). Direct seeding appears to result in some survival advantages with regard to climatic and soil conditions at the time of planting. For example, an acorn planted under adverse conditions would likely remain in a dormant state until germination conditions are satisfactory. On the other hand, a seedling planted under adverse conditions would be stressed and possibly killed. A disadvantage in direct seeding of acorns is that rodents can cause these plantings to fail by digging up and eating the seeds. But this has generally not been a problem except if acorns are planted under an existing canopy. Transplanting of seedlings appears to be a better method for lightseeded hardwood species in the Southeast (e.g., cypress [Taxodium spp.] and tupelo [Nyssa spp.]).
Sandrik and Crabill (1983) found that red maples (Acer rubrum), wax myrtles (Myrica cerifera), and bay species naturally reseeded disturbed sites from adjacent floodplain forests at the Amax Big Four Mine in west-central Florida. Red maples reached 15 feet in height 6 years after the experiment started. Survival of potted or bare-root trees varied with species planted.
Covering seeds is essential to most germination and seedling establishment. Various methods can be used to enhance success rate of the simple hand broadcast method of seeding, including seed drilling, hydroseeding, or cyclone seeders (Ambrose et al. 1983; Platts et al. 1987a).
Erosion control matting/blankets of dead plant materials or organic material provide temporary cover for exposed soils and moderate the effects of rainfall impact, runoff velocity, and blowing winds, and are particularly important when seeding slopes to provide protective cover for seedbeds, reduce evaporative losses, and stabilize seed location until germination (Abbey 1988). Matting made of straw, wood or coconut fibers, or synthetic materials costs more than simple layers of straw, but is more efficient.
Transplanting cuttings or seedlings is normally required to assure revegetation of trees and shrubs. Cuttings taken from local native stock are recommended (Anderson and Ohmart 1979; Anderson et al. 1984). Cuttings started in a nursery survive and grow better than direct plantings to the field (Anderson and Ohmart 1979, 1985). Fertilization and irrigation often are used to enhance initial seedling establishment. Fencing may be necessary to protect seedlings from wildlife (e.g., rabbits, deer) or cattle grazing.
Irrigation generally is required for successful riparian revegetation efforts in the arid Southwest (Disano et al. 1984). Irrigation for the first 150 days may be necessary for successful establishment of cottonwoods planted in these regions. In desert riparian areas, which are subject to prolonged and extreme desiccation, it is imperative to ensure that roots of the new vegetation gain access to the water table (Anderson and Ohmart 1979). Time of planting is important. Winter is the best time for planting desert riparian areas due to lower evaporation rates and thus greater saturation of soil from surface to water table. Trees or shrubs planted in winter will have a developed root system and suffer few side effects should the irrigation system fail.
During a severe 2-year drought in California, planting of various trees and shrubs along flood control channels in Santa Clara County without the use of irrigation systems resulted in the loss of about 75% of the plants (Goldner 1984). Later projects that incorporated a drip irrigation system with an emitter head placed under the mulch of each watering basin resulted in loss of only 10%-15% of planted trees and shrubs. Vandalism of the irrigation system and predation by rabbits and squirrels were responsible for most losses.
In river bottoms, water table depth usually determines moisture supplies to tree roots. In some cases, the water table may be so near the soil surface that it limits aeration, in others it may be below the reach of young roots. Juvenile cottonwoods benefit by water tables in the lower portion of the normal root zone; however, they will likely die in high water tables where soil is saturated for extended periods (Broadfoot 1973).
Large cuttings (13 to 20 feet long) of cottonwood (Populus fremontii and P. angustifolia) and willow (Salix nigra) were successfully established in areas of deep water tables (7 to 12 feet) on the Rio Grande floodplain south of Albuquerque, New Mexico (Swenson and Mullins 1985). Using dormant cuttings and planting cuttings at anticipated growing season water table depth (rather than above it) were recommended for best survival. Dormant poles of cottonwood and willow planted on plots with naturally fluctuating water levels had lower survival rates than those with constant water levels (Swenson 1988). Poles set above the growing season water table had lower survival, as did poles cut after breaking dormancy. Certain precautions are necessary when using this method, including fencing the area from livestock, avoiding flooding for periods longer than 3 weeks, and controlling beaver (Castor canadensis) activity.
In San Diego, the Escondido Creek Project design involves use of wastewater from a planned water reclamation facility to irrigate 2,000 native tree plantings over 4 ha of floodplain (LaRosa 1984). Wastewater must meet State and local regulatory agencies' criteria for levels of constituents (chemical, physical, bacterial, and other biological properties). Reclaimed water in California presently is used to irrigate fodder crops, greenbelts, golf courses, orchards, and vineyards. Problems of using treated wastewater include the higher content of salts and nutrients. Although many species proposed for planting are salt-tolerant, water management plans for irrigation must consider salt build-up in root zones, changes in groundwater quality, and other impacts of wastewater reuse. Enough water must pass through the soil profile to carry away dissolved minerals.