Saltcedar
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Introduction and Spread
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Restoration in the Southwest
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Restoration In The Southwest
Reasons to restore/vegetate
Where saltcedar infestations are extensive and broad control measures are practiced to eliminate the plant, ecological stability can be disrupted. Without special planning and care, saltcedar can rapidly reestablish or the area can be subject to invasion by other exotic species. In such instances, long-term, sustainable control is best accomplished through the establishment of native plant species with high competitive exclusion abilities following disturbance (Grime 1973). Native riparian woody species including cottonwood (Populus deltoides), Goodings willow (Salix gooddingii), and coyote willow (Salix exigua) have a rapid growth potential under low environmental stress and are able to competitively exclude saltcedar when seedlings are concurrently established after flooding (Sher et al. 2002). This ability is preempted however, under harsher environmental conditions such as soils with high salinity and drought (Grime 1979, Sher et al. 2002). Although their competitive abilities have not been tested, seepwillow (Baccharis glutinosa) and saltgrass (Distichlis stricta) may likely prove competitive on harsher sites following flooding.
Saltcedar is a facultative phreatophyte and adapted to a variety of habitat types. It thrives in upland situations and in areas where river flooding no longer occurs. Such environments can be difficult to revegetate as surface water may not be available and soil salinity is often high (Taylor and McDaniel 2003). Once saltcedar is removed, aggressive revegetation is required to limit invasions by other exotics including perennial pepperweed (Lepidium latifolium), Russian knapweed (Acroptilon repens), and Russian olive (Elaeagnus angustifolia).
Plants selected for revegetation must be competitive for essential resources such as light, nutrients, and moisture. Outcomes are often based on resource supply and consumption rates. The better competitor may be the species which is more tolerant to a reduction in resource availability (Tilman 1980). Although several perennial grasses have proven competitive abilities in combination with herbicide treatments at higher latitudes in the western United States (Benz et al. 1999, Young et al. 2002), species with similar traits adapted to the arid southwest have not been investigated. Anderson et al. (2003) contends that sites with excessive salinity cannot be revegetated effectively without costly soil amelioration. Currently, standard seed mixtures of 4-wing saltbush (Atiplex canescens) and alkali sacaton (Sporobolus airoides) are used in areas of higher salinity, but knowledge of their competitive abilities is lacking (Taylor and McDaniel 2003). Research is ongoing to evaluate the adaptability of additional species in harsh riparian environments which may also highlight their competitive abilities in the face of exotic invasions (Lair 2001).
Integrating saltcedar control strategies with revegetation
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Saltcedar control techniques and costs vary from individual plant control to large scale control programs involving a combination of techniques (Duncan 2003, McDaniel and Taylor 2003a, 2003b). The extent of the infestation and its location within the watershed will determine if individual plant or large scale control programs are warranted. Managers should also be cognizant of subsequent restoration processes and/or revegetation requirements when selecting a control strategy (Figure). For example, the mechanism for the establishment of native riparian vegetation is periodic flooding which provides scoured soil substrates for the germination of native, primarily aerially dispersed seed (Fenner et al. 1984). Although flooding still occurs on many sites, flood intensity is low requiring supplemental soil disturbance. This can be accomplished if mechanical saltcedar control is utilized (Taylor et al. 1999).
Natural, primarily herbaceous, recovery following saltcedar removal is also possible where flooding no longer occurs. Sites where high water tables persist within 1.5 meters of the surface support meadow or grassland communities dominated by saltgrass and alkali sacaton (Lindsey, 1948, Groeneveld and Or 1994). Natural recovery of these species has been noted on the Rio Pecos following saltcedar control where high connectivity between surface and groundwater predominates (Dudley et al. 2000).
Where hydrologic connectivity is low, artificial plantings will be required to restore sites (Taylor and McDaniel 1998, Dreesen et al. 2002, Anderson et al. 2003). Saltcedar control strategies should therefore avoid the use of mechanical means that promote annual and perennial herbaceous weed establishment (McDaniel and Taylor 2003a). These weeds potentially compete with planted materials for light, nutrients, and moisture resources and can dramatically influence the success of revegetation efforts (Anderson 2003).
Saltcedar control and revegetation in headwater areas
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Headwater areas are usually the more pristine areas of a watershed. Flood pulses are generally unimpeded by dams, levees or diversions for agricultural or municipal uses. Saltcedar infestations are generally limited to scattered stands or individual plants within robust native riparian forests and meadows. Individual saltcedar plant treatments such as the cut-stump or foliar methods over a period of 3-5 years can effectively eradicate infestations (Hughes 1965 Duncan 2003). Individual plant grubbing using tractors or front-end loaders equipped with stinger blades can also be employed, however equipment access is often limited. Revegetation is seldom required in headwater areas as natural recruitment processes for native species are unimpeded following periodic flooding.
Saltcedar control and revegetation in transitional areas
Transitional areas often represent a gradation from scattered infestations occurring in upper reaches to more extensive stands occurring in lower depositional basins of the watershed. Area geomorphology is represented by steep intervening canyons where the floodplain is narrow and the extent of native vegetation and exotic infestations can vary depending on floodplain width. Although desired, eradication is often not a practical management goal. Although hydrologic integrity may be compromised in transitional areas, periodic flooding may still occur and its severity may be high due to floodplain confinement. The potential for native species regeneration following saltcedar removal may therefore be high. Conversely, upstream dams may restrict sediment deposition in narrow floodplain areas resulting in channel incision (citation). Overbank flooding is therefore impaired and artificial planting will be required to revegetate areas. The contrasting hydro-geomorphologic conditions found in transitional areas will influence revegetation, and in turn, saltcedar control approaches. Where dense pockets of saltcedar exist within active portions of the floodplain and equipment access is possible, control programs which include a mechanical component may be desirable, whereas soil disturbance may not be appropriate where artificial planting are required in the absence of flooding to avoid weed establishment. Mechanically shredding standing saltcedar vegetation previously treated with herbicide (herbicide-shred treatments) can allow the removal of saltcedar vegetation aboveground avoiding soil disturbance. Either program may be restricted in very narrow transitional areas, where aircraft and/or ground equipment access are limited. Individual plant treatments would therefore be one of the few option available for control.
Saltcedar control and revegetation in depositional areas
Generally, the most extensive saltcedar infestations occur in depositional watershed areas. Hydrologic modification is most pronounced in these areas impacting the potential for natural regeneration following saltcedar control (Auble et al. 1994). In areas where natural fluvial processes persist, hydrologic connectivity is high, and native seed sources are available, naturally regenerated native communities provide high competitive exclusion abilities following saltcedar removal (Taylor et al. 1999, Stromberg 2001, Sher et al. 2002). Saltcedar control strategies should therefore have a mechanical element to prepare a seedbed for natural regeneration (Sprenger et al. 2002).
More often however, hydrologic integrity is poor in depositional areas resulting in low potential for the regeneration of native species and sites must be artificially revegetated. Many of these sites represent extremely harsh environments for artificial planting which limit revegetation success (Anderson et al. 2003). Considering these factors and the high costs for saltcedar control and revegetation (Table), sites should be prioritized based on revegetation potential in order to assure sustainable long-term saltcedar control. The determination of key variables including depth to water table and salinity will aid in the selection of saltcedar control areas with high revegetation potential (Sheets et al. 1994, Taylor and McDaniel 1998). As in transitional areas where artificial plantings are used, soil disturbance should be avoided to discourage weed growth and to limit the spread of herbaceous exotics which can occur when using heavy equipment.
Saltcedar often occurs within mature riparian forests restricting options for mechanical control or broadcast herbicide applications to preserve native vegetation. Saltcedar control is also complicated due to the need to remove saltcedar aboveground trunks and stems that perpetuate wildfire. The cut-stump treatment combined with chipping or shredding trunks and stems can be an effective control measure although costs are extremely high (Duncan 2003).
Determining revegetation potential
Extensive saltcedar infestations that require broad control measures can significantly disrupt site stability. Without special planning and care, saltcedar can rapidly reestablish or areas can be invaded by other exotic species such as perennial pepperweed (Lepidium latifolium), Russian knapweed (Acroptilon repens), and Russian olive (Elaeagnus angustifolia). In such instances, long-term, sustainable control is best accomplished through the establishment of native plant species with high competitive exclusion abilities following disturbance (Grime 1973). Native riparian woody species including cottonwood (Populus deltoides), Goodings willow (Salix gooddingii), and coyote willow (Salix exigua) have rapid growth potential under low environmental stress and are able to competitively exclude saltcedar when seedlings are concurrently established after flooding (Sher et al. 2002). This ability is preempted however, under harsher environmental conditions such as soils with high salinity and drought (Grime 1979; Sher et al. 2002). Although their competitive abilities have not been tested, native seepwillow (Baccharis glutinosa) and saltgrass (Distichlis stricta) may likely prove competitive on harsher sites following flooding.
Sites in all watershed areas can be generally prioritized based on revegetation potential prior to initiating saltcedar control (Figure). Assessing revegetation potential must include the determination of key variables including flooding potential and frequency and appropriate groundwater hydrology (Stromberg 2001). A soil survey is requisite for knowledge of soil texture, depth, and related salinity levels that ultimately influence replacement vegetation community types (Sheets et al. 1994, Taylor and McDaniel 1998, Anderson et al. 2004). Where the control of dense saltcedar within areas of poor hydrologic integrity and elevated salinity is anticipated, managers must realize that some sites may have limited revegetation potential due to these harsh environmental conditions (Anderson et al. 2004). Considering the high costs for saltcedar control and revegetation (Table), it is incumbent on resource managers to carefully select areas with high revegetation potential to provide long-term sustainable saltcedar control.
Restoration using controlled flooding
Controlled flooding coinciding with the natural seed rain of native species closely emulates natural regeneration processes. This technique is particularly effective when used in combination with mechanical saltcedar control which removes competing vegetation and provides light, soil minerals and nutrients for developing seedlings (Taylor et al. 1999). Native seed germination and plant growth is stimulated by soil disturbance and is influenced by key soil characteristics and hydrologic conditions inherent to the site. Flooding is also the natural process whereby soil salinity is reduced through leaching (Shafroth et al. 1995). Wetter sites with high groundwater and more frequent surface flow naturally develop vegetation suited for these conditions such as willow. Soil salinity is generally well leached at such locations. Drier sites with higher groundwater and limited surface flow are more characteristic of saltgrass meadows or mesquite brushlands (Bosque del Apache NWR, unpublished data).
On the Rio Grande and Rio Pecos, peak flood periods historically occurred in late May and early June as snow melted at higher elevations (Auble et al. 1994). Flood peaks were followed by precipitous drops in river flow as runoff moved through the river system. Native vegetation evolved to cope with these drying river conditions by quickly developing root systems to maintain contact with declining water tables. Although both native plants and saltcedar are established using controlled flooding, native plants are better able to survive dry conditions following flooding. For example, cottonwood mortality can be 70% after one year while saltcedar mortality can be over 90% (Dellorusso 1999, Taylor et al. 1999, Sprenger et al. 2002). These mortality rates result in a balanced mixture of native vs. exotic plants by the second year of growth. Cottonwoods are better able to compete for available soil nutrients and water and growth rates can be twice those of saltcedar (Dellorusso 1999). The resulting plant community is characterized by robust native species growth and saltcedar suppression in the understory.
Often, saltcedar can be easily controlled while preserving native seedlings through light discing in September following spring establishment (Smith et al. 2002). Native seedlings, particularly cottonwood, have deeper roots and heavier root structure than saltcedar seedlings allowing for high native seedling survival following light discing to control saltcedar. On more saline sites such as developing saltgrass meadows, light discing in July following spring seedling establishment can also control saltcedar while enhancing saltgrass growth by cutting and spreading remnant saltgrass rhizomes in moister soil substrates (Bosque del Apache NWR, unpublished data).
River flooding still occurs on some major southwestern river systems although less frequently than what historically occurred. Overbank flooding events are now managed by river regulatory entities through a network of flood control dams and levees to protect agricultural irrigation infrastructure and urban communities. During cyclical periods of abundant snowfall in mountain watersheds water may be available in excess of agricultural and urban needs. Riparian restoration can occur concurrent to water delivery from upper storage reservoirs to those further downstream by matching historic river flow patterns within existing levee systems. In a more controlled setting, flooding for riparian purposes is possible on areas such as state and federal wildlife refuges or tribal lands outside river levees utilizing appropriated irrigation water.
Restoration using artificial plantings and seeding
Many sites along the Rio Grande and Rio Pecos have no natural or controlled flooding potential and must be revegetated artificially. Weed competition can often limit the survival and growth of planted materials (Anderson 1988). Managers should therefore consider treating saltcedar monocultures using herbicide-burn control practices which result in limited growth of competing weeds. Factors influencing revegetation potential have been documented for more common plant species. For example, some species can survive through contact with the water table, therefore it is important to know water table depth and its annual fluctuation (Swenson and Mullins 1985). Plant species also have thresholds for survival and growth based on soil and salinity parameters (Taylor and McDaniel 1998a). Many of these sites have not benefited from the leaching effects of flooding and salinity levels remain quite high, preventing revegetation in some cases.
Considering the high costs associated with saltcedar control and revegetation, some preliminary information on these parameters should be gained prior to the selection of areas for saltcedar control. Preliminary site reconnaissance should evaluate water table depths through the establishment and monitoring of water table wells at key locations. Basic knowledge of soil salinity can be obtained using the electromagnetic induction method that measures apparent electrical conductivity across the soil profile non-invasively (Sheets et al. 1994). Although accuracy is limited beyond very low electrical conductivity readings, the instrument can help determine site suitability for some plant species that require low salinity levels for establishment and growth.
Following saltcedar control, detailed information is required to develop accurate planting prescriptions in the field to assure restoration success. The most practical method involves development of a grid system across the area with each grid cell consisting of one-half acre (Taylor and McDaniel 1998b). Soil samples are taken at the center of each grid, 15 inches below the soil surface and 18 inches above the water table and sent to a soil laboratory specializing in riparian revegetation for analysis to determine soil texture and electrical conductivity. From this information, a series of contour maps are generated outlining water table depth, soil texture and salinity. Field planting crews are provided with grid sheets outlining plantings based on species survival and growth tolerances to water table depth, soil texture, and soil salinity.
Cottonwood and willow trees and shrubs can be planted using dormant poles augered to the water table to establish forested areas (Swenson and Mullins 1985). This technique requires cutting saplings of sufficient length and small butt diameter during winter months. All lateral branches are trimmed leaving only 2-3 apical branches prior to soaking butt ends in water for a 10 day period prior to planting. On average, a 3-person crew is able to auger and plant 150-180 poles per day using a large production auger drilling machine (Taylor and McDaniel 1998b). Understory plantings can be made using 30 inch container nursery stock augered into water tables of generally 5 ft or less and at locations where electroconductivity reading are less than 8.0 (Fenchel et al. 1996). This technique relies on root development to the water table. Plantings are usually made in August and require supplemental water for a 1-2 month period. Planting density is about 100 trees or shrubs/acre, a density shown to benefit wildlife (Anderson and Ohmart 1982). Plant survival for both techniques is about 90% after 4 years with about a 24% annual growth rate (Taylor and McDaniel 1998).
Where water tables are deep and/or electroconductivity readings are high, other establishment techniques must be used. Rainfall harvest is one such method for establishing seedling shrubs where electrical conductivity levels are below 8.0 (Oaks et al. 1993). A road grader is used to construct a long shallow V-shaped water catchment. Seedlings are planted at 5 foot intervals at the bottom of the catchment and the banks are lined with plastic. Seedlings obtain supplemental water from surface rainfall or undersurface condensation funneled from the plastic to seedling root zones. Survival and growth rates are comparable to poles and containerized stock.
In areas of higher electroconductivity (8-14), seeding mixtures of 4-wing saltbush and salt tolerant grasses such as alkali sacaton are prepared and seeded following the onset of summer rains (Bosque del Apache NWR, unpublished report). Sites where electroconductivity levels are above 14.0 cannot be revegetated successfully (Anderson and Ohmart 1982). Seed can often take up to 4 years to germinate and growth can be slow depending on the timing and amount of late summer and winter precipitation.
Overall revegetation costs can range between $1,100 and $1,500 per acre (Taylor and McDaniel 1998b). These costs include site suitability potential analysis, plant materials, and labor. Over 80% of costs associated with revegetation are plant materials. Costs for plant materials can often be reduced for cottonwood and willows through harvest at natural nursery sites along rivers and ditches. Only 5% of total costs are associated with site suitability determination, quite low when the importance of this activity is considered.
Additional links and information:
BLM
Vegetation Treatments using Herbicides PEIS
(www.blm.gov/wo/st/en/prog/more/veg_eis.html)
Society
for Ecological Restoration
(www.ser.org/reading_resources.asp)
Guidelines for Planning Riparian Restoration in the Southwest
