last update April 8, 1998
As a result of ground maneuvers of both live fire exercises and force onforce military training activities much of the soil's cohesivelichen/gravel crust has been disturbed and shrub cover has beensignificantly diminished or, in many areas, eliminated. Evaluation ofLandsat Thematic Mapper images of NTC, by the California Gap Analysis(1994), reveals large expanses of mixed barren lands throughout theSouthern, Central, and a portion of the Northern Corridor. These barrensurfaces contribute to an increase in eroded lands from the forces of windand water. The results are elevated dust levels, gully formation, and theloss of realistic environmental conditions for desert training. Restoringvegetative cover is the best long term solution for erosion controlbecause it can reduce precipitation sheet flow, retain soil particles, andincrease soil porosity (Ricklefs 1982). For example, the extensivesurface root systems associated with many common desert plants (e.g.Larrea tridentata) tend to trap fine sand and soil particles,holding the soil surface in place.
To assist in rehabilitating disturbed lands at NTC, the Soil Ecology andRestoration Group (SERG), in association with the Integrated Training AreaManagement office (ITAM), has developed a five year plan to identifydamaged areas, produce a prioritized list of sites in need of erosioncontrol and provide a revegetation plan and management recommendations.This report examines sites in the Southern, Central, and NorthernCorridors of NTC.
METHODS
RESULTS OF SOIL ANALYSES
Soil nitrogen, organic matter, and pH varied little between thedisturbance sites. However, all of these sites appear to have deficientsoil nitrogen and organic matter levels compared to the local'undisturbed' soils (primarily collected from creosotebush scrubcommunities) [table 1].
| Disturbance | TKN (mg/gm) | Organic matter | pH |
| Heavy | 0.076 | 0.75% | 8.12 |
| Medium | 0.152 | 0.66% | 7.94 |
| Low | 0.123 | 0.82% | 7.36 |
| Undisturbed | 0.353 | 1.53% | 7.66 |
Soil collection and analysis was not intensive and sufficiently random to provide statistically valid comparisons between sites (this was beyond the scope of this project). However, the data provides a good overview of the soil problems associated with the disturbance areas at NTC. Desert soils are typically alkaline, with low organic matter, and nitrogen content. The soils at NTC, however, have extremely low nitrogen and organic matter content.
The large denuded sites are unlikely to be naturally recolonized by shrubsdue to thelack of nitrogen available to support the large energy budget in seedlingdevelopment. Thisalso indicates that direct seeding alone is less likely to succeed.Densely planting containergrown seedlings (with its greenhouse potting mix) can help restore thesoil creating windeddies that catch air borne, nutrient rich, silt and clay particles, droplitter, and create smallislands of fertility where annual and perennial volunteers may establishand continue thecycle. Low organic matter is an indicator of low nitrogen content, and italso indicates areas more likely to undergo erosion. Organic debris bindsto the soilparticles, increase soil porosity and holds moisture. These properties arebeneficial for soilfertility as well as erosion control.
MANAGEMENT APPROACHES
Desert areas may take centuries to recover from disturbance by humanactivity withoutactive intervention and restoration work. This is not surprisingas establishment and succession of any kind in this severe environment isnaturally slowand disturbance makes these conditions much worse. The uncertainty of theclimate andthe extreme conditions make revegetation difficult. Listed below aregeneral landmanagement and rehabilitation practices recommended for use at theNational TrainingCenter. The letters associated with each technique indicate whether itprimarily addressesdust abatement (D), water erosion (E), or replacement of vegetative cover(V).
A. Protect sites from further disturbance when possible D,E,V
The adverse effects of disturbance on critical sites should be minimizedby limitingaccess to damaged areas periodically or reducing the level of impact. Forexample, landmay recover from 80% destruction much faster than from 90% destruction.Rolling restperiods may help badly damaged plants recover. Limiting use in areas whenplants aregrowing and may be able to set seed may also help (late February throughMay). Specificlocations for activity reduction should be determined from LCTA plotsurveys and SiteRehabilitation Plans.
B. Seed collection V
Seed collection should be given priority, as seeding can help reestablishplants by directseeding or container production. If possible, seeds from localstands, with similar slope and aspect as the proposed revegetation site,should be collectedas they are more likely to be adapted to local conditions than seeds fromdistant sources.Seeds should be thoroughly dried to prevent mold during storage. Ingeneral, the seedsshould then be stored in a very dry warm (85 -100 °F) place for a fewweeks followed by aone to two day quarantine in a sealed container with moth larvicide tokill seed eatinginsects and eggs. The seeds can then be stored for several years (formedium to long lived[hard covered] seeds) in a cool dry room (protected from granivores) or ina refrigerator forseveral more years. The seed lots need to beidentified with detailed label including the species and variety (ifknown), the location ofthe collection (including seed zone if known), the elevation, soil type,date of collection,and the collector.
C. Decompaction E,V
Areas with heavily compacted soil recover very slowly. Tanks and otherheavyequipment can cause compaction to considerable depths, and individual tanktracks fromWorld War II training maneuvers can still be seen. Compacted smoothsurfaces increase sheet flow across the area and provide no seedgermination sites. Deepripping is desirable using a large tractor and 36-48" ripper bars. Deepripping facilitateswater infiltration, seed germination, rapid root growth and improvesoverall plant survivaland growth.
D. Surface shaping D,E,V
Roughening the surface can reduce wind speed across the ground and reducedustproduction. Improving moisture retention is also critical and can beaided by the following methods of surface shaping.
1. Pitting
Slopes can be pitted with equipment or hand tools to create pits that willreduce erosion andincrease soil moisture. The goal is to create pits that will endure andtrap blowing silt, seedand soil symbiont inocula.
2. Imprinting
Imprinting desert soils with a shaped roller has been very effective in increasing infiltration. The imprinter produces a pattern of pits and catchment areas that concentrate water and also trap blowing silt and seed.
3. Furrowing and swaling
Soils can also be left rough by plowing to leave furrows or plowed to makeridges orswales. If these are aligned with contours they will capture rainwater andreduce erosion aswell.
4. Microcatchments
Shaping the ground to concentrate available rainfall has been very effective for vegetation establishment in deserts. The design of microcatchments has been studied in detail in the Negev Desert of Israel and guidelines are available to choose appropriate designs for selected species in given soils and climates. A typical microcatchments might concentrate water from 30 m2 . Microcatchments can reduce salt concentrations at the planting spot and have proven more effective than one hand watering for the establishment of saltbush in the Negev Desert. Larger catchments have been used to create mini-parks.
E. Soil amendments D,E,V
Soil amendments such as bark chips, large wood chunks, local plant litter, and straw are desirable to reduce dust production. These mulches can also improve plant survival and establishment. They provide a number of benefits including wind protection, reduced evaporation, increased infiltration, rainwater retention, reduced erosion, and improved plant microclimate. Straw can be used if it is crimped, spaded or furrowed in. Rice straw is preferred as it is very durable and less likely to contain weeds. Bundles of rice straw and broom corn set vertically into the soil have worked well in restoration efforts, probably by limiting wind erosion and increasing infiltration. Rock mulches are economical (if stones are present), durable, and aesthetically pleasing in non-traffic areas. They can also facilitate plant reestablishment.
F. Gully control E
Gully control is difficult in any environment and particularly hard in thedesert. Highintensity rains, rapid runoff from denuded areas, and steep slopes canmake gully controlcomplicated. Stabilization methods include check darns (rice straw bale,brush, Christmastrees, or rock), riprap, or sand bags. Pitting slopes and reestablishingshrubs on the slopeswill also help reduce erosion. Furrows running out of gullies can spreadwater and reduceerosion. Using rolling dips on roads instead of culverts is recommended toprevent pointconcentration of water. Straw flakes in contour trenches are alsoeffective.
G. Direct seeding D,E,V
Direct seeding is relatively inexpensive and growth from seeds in a favorable year will often surpass transplants, but direct seeding is extremely vulnerable to drought and seed harvesters (ants and rodents) and frequently fails completely. Direct seeding trials generally only succeed one year in ten. Seeds can be planted by broadcasting or drilling by hand or machine. Ideally, direct seeding should be done after heavy rains or flood events. Planting seeds in pits or imprinted areas can improve germination and survival. The addition of large bark chunks to the soil may further improve survival by providing improved microsites. Good germination and considerable survival was achieved at Anza Borrego Desert State Park by direct seeding saltbush into pits with large chunks of bark. Generally, annual seeds and grass seeds tend to germinate and grow with a much higher success rate than perennials (although germination may be zero for any given species depending on environmental conditions). Collecting native grass (e.g. Achnatherum hymenoides, Pleuraphis rigida) and annual, biennial, and sub-shrub (e.g. Escholzia minutaflora, Baileya multiradiata, Adenophyllum cooperi) seeds and growing it in containers commercially may provide a good, constantly available source of seeds.
H. Container plants D,E,V
Multi-stemmed shrubs and trees such as creosote bush and mesquite are goodcandidates for revegetation and restoration efforts, but costs ofcontainer planting limit theuse of containers over large areas. They can be used in bands orvegetation islands toreduce dust generation. These bands and islands should be deep ripped andfurrowedalong the contour. Once established, container plants will improve siteconditions for otherplants by trapping fine soil, organic matter, and symbiont propagules,increasinginfiltration and water storage, and providing protection from the sun andwind.
I. Protecting natural volunteers D,E,V
Protecting naturally established plants can be one of the most inexpensiveoptions forimproving vegetation recovery. Protective screening and supplemental waterfor volunteerplants (and, in many cases, root crowns of recently defoliated shrubs) maybe one of themost cost effective options available for restoration workers.
J. Vegetation islands D,E,V
Concentrating resources to create resource islands may provide greaterbenefits thanless intensive treatments over a larger area. These islands can provideseed and inoculumfor surrounding areas. Resource islands apparently play a major role inthe development ofdesert ecosystems. Transplanting clumps of shrubs into the center ofbarren areas is a lowcost method of promoting resource island formation.
K. Artificial soil stabilizers D,E
Many of the disturbance sites at NTC are so large, with so much traffic,thatrevegetation may not always be practical. Soil stabilizers are oftenapplied to roads toimprove road stability and suppress dust. Generally, the chemicals bindthe fine soilparticles together to form a hard, protected surface. There are manycommercially availabledust control agents, such as Soil Sement, soybean oilsoapstock, and ligninsulfonate toname a few. Each have characteristics which are best for specific soiltypes, climaticconditions, and road uses. Each also may have different drawbacks such asthe ability topenetrate into the groundwater, soil contamination (i.e. creating asterile soil in whichplants cannot grow after a road has been abandoned), state compliance,suppressantduration, and cost. All of these conditions must be considered beforeinvesting in a dustcontrol treatment. Any artificial stabilizer chosen should meet thefollowing essentialcriteria: all (100%) of the damaged sitemust be covered; the stabilizer must not inhibit volunteer plantgermination, growth orsurvival; erosion must initially be prevented and the threat reduced withtime; the stabilizermust be easy to apply and reapply without special (difficult to acquire oruse) equipment;and the cost must not be prohibitive.
Relatively few controlled studies of soil stabilizers have been conductedin the MojaveDesert comparing ease of use, durability, effects on plant growth andcost. We suggestthat these be performed before recommending any one treatment. Inaddition, theenvironmental side effects should also be investigated by chemists andother environmentalprofessionals.
L. Monitoring Sites D,E,V
Monitoring specific sites will help direct future management decisions. Several sites should be used as experimental areas, where several treatments are applied and compared. To properly assess results, the experiments must be laid out with a statistically valid design (all aspects controlled for, full factorial design, etc.) and monitored with valid techniques (which can be designed and implemented by SERG personnel or an ITAM staff member familiar with experimental design). The most meaningful cost effective and simplest variable to measure would probably be number of leaves (not to be confused with leaflets), for most species (such as Ambrosia dumosa, Prosopis glandulosa, Isomeris arborea). However, height may be more meaningful for species which do not branch much or have small or slender leaves (such as Hymenoclea salsola, Oputia sp., and others). The sites should then be monitored seasonally, and the data analyzed using the appropriate statistical tests. M. Selecting Appropriate Species for Revegetation V
When a disturbance area is completely surrounded by a relatively undisturbed community, it may be appropriate to conduct vegetation sampling to determine the appropriate species and numbers to revegetate the site. There are many techniques available. Each test, however, is not appropriate for all ecosystems or even all community types within a system. For NTC we recommend the sampling protocol as accepted by the California Native Plant Society~ This method is relatively easy to learn, quick to perform, and it is the accepted technique of many organizations. With the CNPS method frequency, cover (using the transect), and total species (by surveying the quadrate) can be determined. For areas which do not occur near intact native vegetation, GIS analysis looking at elevation, slope, aspect, precipitation, and soil type may be used to identify model communities elsewhere.