U.S. DEPARTMENT OF THE INTERIORBUREAU OF LAND MANAGEMENT
|Barstow Field Office|
Afton Canyon Riparian Restoration Project Fourth Year Status Report
"Can you live without the willow tree? Well, no, you can't. The willow tree is you." John Steinbeck (1902-1968) Abstract: In 1996, the Bureau of Land Management completed its fourth year of work on the multiyear Afton Canyon Riparian Restoration Project. The objectives of this ambitious restoration project are to control exotic plants, particularly the phreatophyte saltcedar (primarily Tamarix ramosissima, T. parviflora), restore critical native plant community structural elements and minimize adverse effects to the riparian zone within the Afton Canyon Area of Critical Environmental Concern. The goals of this riparian, or streamside, project are to improve the proper functioning condition status of a small southwestern stream segment and to restore a specific site's key Potential Natural Community elements. Treatments have resulted in a fairly high degree of saltcedar control and native revegetation is progressing slowly. Project work appears to have improved this stream segment's proper functioning condition from a "non-functioning" to a "functioning at risk" status. Project activities and accomplishments to date are summarized and current trends/status are discussed in relation to planned objectives and goals.
The exotic plant group dealt with herein is commonly referred to as saltcedar (Tamarix ramosissima, T. parviflora, T. chinensis), or tamarisk, and are a group of phreatophytic plant species introduced to North America from Eurasia in the 1800s for ornamental, shade and soil stabilization purposes. As many as 54 species of this group have been formally recognized, though not all are considered invasive. Collectively, those species considered invasive have come to occupy over a million acres of sensitive riparian and wetland habitats, ranging from Mexico to Canada. This exotic plant has established itself in nearly all southwestern riverine systems and many wetland or marsh areas critical for native plants and animals. Saltcedar's propensity for adversely affecting an area's hydrology and geomorphology, displacement of native plants and degradation of both wildlife access to water and plant/ground structure necessary to meet most native bird and mammal species' requirements for foraging, breeding and nesting, has been well documented (Blackburn et al, 1982; Cohan et al, 1978; Kerpez and Smith 1987). These identified impacts have elevated saltcedar control to the forefront of riparian restoration work in the American Southwest.
Characteristic of opportunistic invaders (Bossard 1992), saltcedar often becomes established in "edge" or border areas of riparian or wetland habitats, and frequently forms extensive growth stands when local, conducive circumstances present themselves. In the case of saltcedar in North America, such circumstances commonly present themselves in terms of native plant community or water system disturbance related to flooding or lack thereof, fire, herbivory or drought, with a few exceptions. Saltcedar infestations have been documented in a few isolated spring systems lacking any observable form of native plant community or water system disturbance, though these stands of growth are usually small in nature and rarely completely displace the native plant community.
In contrast, those riparian and wetland areas where the water table is between 1.5 and 6 m from the ground surface (Campbell and Dick-Peddie 1964) and are, or have been, subject to disturbance via altered water flow or fire regimes, woodcutting, livestock grazing or vehicle use impact, frequently form dense, monotypic stands of saltcedar (Egan et al, 1993; Horton 1977). The native plant community, in such instances, is often severely fragmented, degraded in vigor and/or completely displaced by the invading saltcedar. However, saltcedar control has been shown to be feasible at some sites, at least for the short term, through intensive restoration efforts (Barrows 1993, Bureau of Reclamation 1992, Lovich et al, 1994).
Exotic villain or adaptive victor, temporarily vanquished invader or thriving monoculture, saltcedar is a formidable survivor and is here to stay. The only identified, ecologically freindly recourse to widescale displacement of our native riparian communities appears to be control of this invasive plant in some fashion, providing protection for our remaining pockets of native riparian vegetation and restoring those sites most capable of supporting a competitive, healthy native plant community. The Afton Canyon Restoration Project is an example of a recent attempt to do just that-remove and control saltcedar at a manageable level at an appropriate site, concurrent with restoration of some of Afton Canyon's potential native plant community attributes.
In addition to significant resource extraction impacts, Afton Canyon has been subject to profound human influences, both within and upstream of the canyon, in the last century. Primary among these were river staightening for railroad construction, flood control, water diversion, groundwater pumping, off-highway vehicle (OHV) free-play activities, the introduction of several exotic plant species such as saltcedar, and broadscale aerial herbicide application (1959-63) to control camel thorn (Alhagi pseudalhagi), a plant species toxic to cattle. The functioning condition of the canyon's riparian zone (discussed further under project components and accomplishments section), though likely degraded from pre-railroad conditions, is thought to have been in "properly functioning" condition until the late 1960s. Sometime following upstream dam construction(1968) and the flood of 68-69, this functioning condition rating of the Afton Canyon river segment is considered to have slipped to a "functioning at risk" condition, likely due in part to altered river flows, continued perennial cattle occupation of the canyon floor, increased OHV travel within the central riverine channel and upstream demands upon regional groundwater. Saltcedar is suspected to have exploded in density during the early 1970s, and recent USGS saltcedar core samples preliminarily indicate that many oldgrowth saltcedar trees at Afton became established in the mid to late 1960s. Afton Canyon's ecological condition is considered to have continued a downward trend to a condition of "non-functioning," by the late 1980s.
In a relatively short timeframe, saltcedar displaced an estimated 75 percent of the native vegetation within the canyon, with drastic ramifications upon the diversity and abundance of native plants, aquatic animals, mammals and particularly avian species. Few open surface water areas existed prior to initiation of project work and wetland emergent "green strip", width as well as length, were thought far less than the site's potential. Forb, grass and shrub community diversity/abundance were considered depauperate and only one cottonwood tree remained in the canyon as of 1992. Surviving mesquite and willows were similarly stressed in vigor or dying and few age classes were present. The effective floodplain had been reduced severely and large old growth saltcedar dominated terraces lined a primarily dry, saltgrass (Distichlis spicata)/young saltcedar dominated, meanderless river segment. Saltcedar's common impact upon both plant and animal species at this site was determined to be this invasive plant's tendency to alter habitat structure, whether vegetative, water or soil related.
The far western, upstream reach (approximately 2 miles length) of the Mojave River at Afton Canyon, supporting in the neighborhood of 300 acres of riparian habitat, was selected as the initial study area or pilot, for the long term restoration project at Afton. Following completion of this western stream segment, project work is planned to continue as funds/personnel permit, in a downstream direction for approximately five additional miles, supporting roughly 400 additional acres of riparian habitat.
PROJECT COMPONENTS AND ACCOMPLISHMENTS
1) protection of existing native vegetation to ensure future native plant seed stock;
Project monitoring has relied heavily on the use of a qualitative evaluation process referred to as Proper Functioning Condition Assessment (PFC). This assessment is conducted by an interdisciplinary team of specialists and is designed to assess riparian area/wetland functioning condition by analyzing the interaction among geology, soil, water and vegetation. The central premise of this assessment process is that "riparian areas are functioning properly when adequate vegetation, landform, or large woody debris are present to:
A) dissipate stream energy associated with high waterflows, thereby reducing erosion and improving water quality;
Riparian and wetland areas are considered to be functioning properly when there is adequate structure present to provide the listed benefits applicable to a particular area (BLM 1993). It is essential that PFC assessment be based on an riparian area's capability and potential. Although Ecological Site Inventory (ESI) has not been conducted at Afton Canyon for the purposes of defining site capability/potential, evaluation of the study area was completed using historical references, current onsite data and Mojave River relict site information, alluding to Afton Canyon's site potential.
Project accomplishments following the first four years of work, including ACEC management actions completed immediately prior to project initiation, have included:
1) area route designation emphasizing riparian area protection and rerouting of Mojave Trail vehicle use out of central riverine channel;
Study area trend analysis is encouraging in that the 1990-96 trend for average bare ground cover, following a rapid increase between 1990-93 concurrent with initial saltcedar treatment and a major flood event (1993-94), is gradually decreasing while the 1990-96 trend for average non- persistent litter, persistent litter and live vegetation, following an initial decrease between 1990- 93, are gradually increasing (Figure 5). Average saltcedar frequency and number of perennial plant species trends for the period 1990-96, depicted in Figure 6, partially explain the attribute changes over time shown in Figure 5. Average bare ground, initially rather high in 1990, became even higher with project saltcedar burning/cutting in 1993, particularly with the addition of a high sediment depositional flood following treatment, and then began gradually decreasing as native plants began to reestablish themselves; average non-persistent litter, persistent litter and live vegetation on the other hand, decreased following saltcedar treatment work in 1993 and have all gradually increased with continued project work.
Individual stream reach transect trend analysis (T1-wet, T3-moist and T6-dry) depicted in the Saltcedar Frequency 1990-96 Trend of Figure 6 is also interesting in that it confirms previously ground-identified retreatment needs in the far western edge of the study area or "dry stream reach," an area receiving the least amount of retreatment to date. The amount of retreatment in this upstream reach area may also have had a bearing on the static to slight decline change depicted in the dry stream reach (T6) of Number of Perennial Plant Species 1990-96 Trend of Figure 6, though it is more likely that this apparent trend is a function of both water availability and lower levels of saltcedar retreatment, compared to the other, wetter stream reaches.
Both photoplot and trend analyses indicate that saltcedar has become a less dominant feature and that native plants have become more of a dominant feature, within the study area in the time span 1990-96. Wetland emergent "green strip" width and length have increased markedly and native plant diversity/productivity within and downstream of the study area have increased exponentially. In terms of overstory restoration, young cottonwood, willow and mesquite trees now number in the hundreds, though over 7000+ trees have been planted to date (suspected survival rate of 15-20%). As far as understory plants, a patchwork of forbs, shrubs and grasses have replaced a monoculture of saltgrass and bare ground. However, several sites within intensely burned areas remain unvegetated five growth seasons post project burns, possibly due to boron or salt accumulations.
A small amount of cottonwood/willow recruitment and reestablishment has occurred since project work was initiated, but most willow/cottonwood reestablishment has resulted from project pole plantings. Since project initiation, screwbean mesquite, desert willow (Chilopsis linearis), smoke tree (Dalea spinosa), baccharis (Baccharis emoryi, B. glutinosa), quailbush (Atriplex lentiformis), common reed (Phragmites communis), and grasses such as alkali sacaton (Sporobolus airoides), species largely absent from the study area prior to project work, have also reestablished themselves within the study area.
Wildlife species diversity, abundance and degree of use appear to have increased both within and downstream from the study area with the plant community changes noted above. Animals not regularly observed within the study area, particularly pisciverous, insectivorous, and accipitrine bird species, are now regularly encountered in both winter and breeding seasons. Aquatic invertebrates, amphibians and small mammals all appear to have increased in diversity and abundance, though no extensive surveys have been conducted to verify these apparent increases. A few areas where humans experienced difficulty crawling through old growth saltcedar stands prior to treatment, are now being used as desert bighorn sheep (Ovis canadensis nelsonii) foraging/water access areas.
During the first four years of project work at Afton Canyon, the effective floodplain has also increased in size, as well as in the number of braided channels that remain vegetated throughout the year with native wetland emergents (plant species necessary for maintenance of riparian soil moisture). Low to average winter water flows appear to be conveyed over a broader area in 1997, when compared to pretreatment profiles. Point bars are revegetating and the stream appears to be in greater balance with the water and sediment being supplied by the watershed in 1997 when compared to that occurring in 1990. The riparian zone appears to be widening and sinuosity, width/depth ratio and gradient appear to be in greater balance with the landscape setting, following project work conducted to date. Based on the 1990-96 changes noted in this document, pertaining to all major sections (hydrologic, vegetative and soils-erosion deposition) of the Proper Functioning Condition Standard Checklist, the proper functioning condition rating for the study area has increased from a "non-functioning" condition (Figure 2) to a "functioning at risk" condition, with an upward trend (Figure 3).