Placed on the Internet October 22, 2001
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Kenneth D. Lair, Ph.D.
Research Botanist, Restoration Ecologist
Ecological Research and Investigations Group
Bureau of Reclamation
Abstract
EXPERIMENTAL REVEGETATION STRATEGIES AND TECHNOLOGIES FOR
RESTORATION OF NATIVE SHRUB/GRASS PLANT COMMUNITIES
ON SALT CEDAR INFESTATION SITES
Vegetative restoration of sites impacted by salt cedar (SC) invasion (and subsequent control) presents technical and conceptual challenges, particularly within the context of biological or herbicidal approaches. Research is often driven by evaluation of control effectiveness, with secondary emphasis on ability of sites to sufficiently recover vegetatively for site stabilization and habitat value enhancement. On drier, saline sites not amenable to seasonal flooding or irrigation, recovery of desirable vegetation may be the most limiting factor for site enhancement. Increased emphasis is needed on development and evaluation of revegetation techniques for sites where potential for natural vegetation recovery following SC control is limited. This is especially true where natural or artificial recovery of willow (Salix spp.) and/or cottonwood (Populus spp.) is limited because of unavailability of supplemental water (via over-bank flows, shallow water table, or irrigation).
Amount and density of standing biomass (live and dead) remaining after control, seedbed preparation, and time frame to achieve levels of control sufficient to favor vegetation establishment and site protection/stabilization, are problematic in mature SC stands. Presence of dense standing dead material poses limitations in relation to seeding techniques, seed interception in aerial applications, and shading impacts. Undisturbed soil surfaces impacted by SC leaf litter accumulation, salinity, hummocky micro-relief, nitrogen limitations, and possible livestock trampling compaction also restrict potential for successful revegetation. Absence of vesicular-arbuscular mycorrhizae (VAM) specifically symbiotic to native revegetation species (especially grasses and shrubs), because of the long duration of salt cedar occupation in mature stands, may also be a significant constraint.
While salt cedar sites may recover naturally after control measures, especially in less dense stands, this needs to be evaluated in light of the definition of "recovery" and an acceptable time frame for it to occur. What density, diversity, cover, canopy structure, and habitat value of the understory are being achieved? Over what time period are desirable habitat values achieved, especially in the interior of large, dense, monotypic infestations? If this "recovery" takes 10 years or more, with the first 1-5 years dominated by ruderal weedy species, recovery of habitat values and improved water salvage may be significantly delayed. A prime objective should be to shorten the time frame for recovery to 3-5 years by eliminating or reducing an extended weedy phase, and where feasible, shorten the early- to mid-seral perennial phase. Some sites may need initial establishment of earlier seral species in order to adapt to harsh environmental conditions until the site stabilizes (from the standpoints of organic matter recovery, energy flow and nutrient cycling, if not for erosion control). Other sites may facilitate later seral species and accelerated successional strategies, particularly if amenable to seedbed preparation techniques.
What are the critical knowledge gaps and limiting factors in salt cedar revegetation for which limited research or field experience exists? Within the context of vegetative recovery in more aridic, dense, monotypic salt cedar stands with no (desirable) understory, major information needs include:
· Efficacy of, and techniques for seedbed preparation and seeding application in dense standing dead material. Revegetation is difficult on drier sites with dense, mature, monotypic infestations in the absence of soil surface disturbance (i.e. some form of seedbed preparation). Different methods of achieving desirable growth medium conditions need testing through varied means of seedbed preparation to enhance soil-seed contact, salinity reduction in surface soil layers, VAM inoculation, etc.
· Impact of the rate of salt cedar reduction on seeded species selection, seedbed preparation and seeding techniques, and ability of seeded vegetation to minimize salt accumulation at the soil surface. Design of seeding mixtures that accomplish dual goals of adapted grass/forb/shrub community establishment and achievement of acceptable fringe or transition/interface habitat for affected wildlife species (e.g., the federally endangered Southwestern willow flycatcher (Empidonax trailii extimus) will assume higher priority.
· Techniques to reduce the time frame for establishment of adequate levels of cover, diversity, and production for Habitat restoration, and prevention of (resistance to) secondary adverse environmental consequences. Seeding/planting technology in artificial revegetation of salt cedar control sites should be considered with respect to other aggressive weeds that may be present and potentially increase in the absence of salt cedar [e.g., arundo (Arundo donax), perennial pepperweed (Lepidium latifolium), Russian knapweed (Acroptilon repens), leafy spurge (Euphorbia esula), yellow starthistle (Centaurea solstitialis)]. Minimizing potential for capillary rise and salt accumulation at the soil surface during salt cedar reduction is also a prime concern.
· Consideration of non-traditional seeding practices that minimize competition between seedlings. In order to re-establish shrub-grassland complexes in semi-arid and arid plant communities, reduction of inter-specific, intra-mixture competition during the establishment phase is critical. Seeding or transplanting desired species of forbs and/or shrubs first (often with a nurse or cover crop, as needed), with interseeding of desired grasses following the first growing season, may be advantageous. This sequence facilitates establishment of forb/shrub taproot systems into deeper soil layers without competition from aggressive, fibrous root systems of grasses in upper soil profile layers.
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Placed on the Internet October 22, 2001
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