last update April 9, 1998
The fragility of arid soils combined with heavy use by both wheeled andtrackedvehicles in the Mojave and other deserts in the southwest has lead toincreased levels ofairborne dust. This airbome dust with particulate materials of less than2.5 microns indiameter is now recognized as a serious threat to human health. Control ofthis dust israpidly becoming an important aspect of land management. The Goldstonetank trailpresents a difficult management challenge. It is an important access routefor tanks andother vehicles to the westem half of the central corridor, but is locatedupwind of a posthousing development. This positioning exposes military personnel and theirdependents todangerous pm 10 laden dust.
To reduce airbome dust generated by traffic on the trail, the IntegratedTrainingArea Management (ITAM) office at Fort Iiwin contracted the installation ofa 3/4 mile longnative plant windbreak. Exotic plant species, traditionally used forwindbreaks, havehigher maintenance and water requirements and may adversely affect nativeecosystems.These factors make the use of native species an attractive altemative.Harsh desertconditions often make establishing nursery grown plants uncertain andlabor intensive.The need for altemative construction techniques that facilitateirrigation, improve plantsurvival and reduce labor requirements is critical if windbreaks are to beinstalled on a largescale at Fort Irwin.
The purpose of this demonstration project was to test the efficiency ofaltemativeirrigation techniques and native plant species in windbreak construction.A combination offour native desert species were used to gauge suitability. Along withusing native species,this windbreak also featured several novel irrigation design conceptsincluding: buriedperforated drainage pipe; delivering large volumes of water directly intothe rooting zone;and a water catchment area that during winter months will passivelyharvest rainfall anddirect it toward the windbreak base.
Windbreak Site
The windbreak extends approximately 314 of a rnile along the Goldstone tank trail between GPS coordinates 527900E, 3903672N and 526911E, 3903077N. The windbreak is located north east of the housing development area running parallel to and situated between Outer Loop Road and the Goldstone tank trail (Figure 1). The windbreak is bisected twice by seasonal stream courses. Where these cross, plantings and earthworks were omitted to prevent blockage of the water course. Topography on site consists of rolling alluvial hills typical of Mojave Desert bajada areas. The tank trail itself has been graded and is generally level with less than ten percent changes in slope. Soil on the tank trail is rocky, highly compacted and contains large amounts of fine dust, the result of mechanical grinding by heavy vehicle traffic. Consistent with heavily disturbed desert ecosystems, soil organic matter on site is extremely low averaging less than 1 percent. Typical of desert soils, nitrogen levels are low with ammonium averaging 0.51 ug/gm and nitrate averaging 7.79 ug/g.
Materials and Methods
The windbreak project was implemented in three phases. These phasesconsisted ofpreparatory site grading, irrigation system installation. and planting.Critical to the gradingphase was the incorporation of two irrigation methods; excavation of atrench for theirrigation pipe and grading a catchment area to passively accumulate anddirect surfacerunoff to the windbreak base.
Site Preparation. Site preparation was carried out using personneland equipment arrangedby ITAM. Severe compaction and the extremely rocky nature of the soilrequired that thesite be ripped to facilitate grading. Approximately seven meters of theGoldstone tank trailwas ripped along the eastern edge, the side closest to the housingdevelopment.
The objective of grading was twofold. The first objective was theexcavation of a trench toinstall perforated drainage pipe, the primary means for irrigation. Thesecond was toconstruct a water catchment area. The most efficient way to complete bothobjectives wasto blade a slope for the catchment area (approximately 10 %) and continueuntil a trenchwas created for the perforated pipe. The excavated soil material waspushed upinto a berm, later used to bury the pipe. A motor grader was used to bladethe ripped areato a depth of 30 to 45 centimeters and form the catchment area, trench forirrigation pipeand planting berm.
lrrigation Pipe Installation. Upon completion of grading, 50 to 100ftlong sections ofthree inch diameter flexible ABS perforated drainage pipe was installed.Sectionlengths were varied in response to slight variations in topography and tominimize airentrapment. The pipe ends in each section were bent upwards and staked inplace withrebar to create watering standpipes to be filled via water truck, Thestand pipes werecapped in two different ways to prevent animal entrapment; with hardwarecloth bent overthe tops and the use of manufactured caps. Once laid out and staked inplace,the pipe was buried by first pushing soil from the catchment area thenfrom the top of theberm using a motor grader.
Planting. Windbreak planting was completed on four separateoccasions April 12, 14, 26and 28. The windbreak was planted with four different native desert plantspecies,mesquite (Prosopis glandulosa), bladderpod (lsomerisarborea), desert willow (Chilopsislinearis) and allscale (Atriplex polycarpa). The numbersplanted are shown below inTable 1. The mesquite used was outplanted from 2x2x8 inch plant bandsgrown by theL.A. Moran Reforestation Lab in Davis California. The bladderpodoriginated from twodifferent sources; half was grown in 2x2xlO inch plant bands by the JoshuaTree NationalPark nursery in nearby Twenty-nine Palms, California and the other halfgrown in halfhigh (2 gallon) containers at the SERG greenhouse in San Diego,California. The desertwillow was outplanted from one gallon containers grown by Tree of LifeNursery in SanJuan Capistrano, California and the allscale grown in 3x3x14 inch plantbands by BorregoValley Growers in Borrego Valley, California.
| Scientific Name | Common Name | Number Planted | Percentage of Windbreak |
| Prosopis glandulosa | mesquite | 391 | 47% |
| Isomeris arborea | bladderpod | 128 | 16% |
| Chilopsis linearis | desert willow | 265 | 32% |
| Atriplex polycarpa | allscale | 40 | 5% |
Planting holes were dug approximately 1.5 meters apart in two staggered rows on both sides of the perforated drainage pipe, filled with approximately three liters of water and fertilized with a approximately 15 grams of preplant fertilizer prior to planting. Watering basins were shaped by hand around each plant to facilitate initial hand watering and each plant protected with a 2 foot high Supertube~ treeshelter secured with rebar. All plants received additional water in the basin following planting. Several sections of the windbreak planted on April 12 and 14 suffered losses and these were replanted on April 26 and 28. The windbreak upon completion of planting is shown in Figure 2.
Results and Discussion
Survivorship was recorded almost two months after the last planting dateon June 18. Bythis time many of the plants had begun to show noticeable growth. Manybladderpod,mesquite, and desert willow, had grown out of the two feet talltreeshelters. Some of thebladderpod were in flower and had already set fruit. Many of the desertwillowthat had suffered from transplant shock and died back were resproutingfrom the base.Survivorship results recorded after two months may give an indication asto which speciesmay be more resistant to harsh summer conditions. However, due to theshort time intervalbetween planting and sampling these numbers may be slightly distorted byreplanting thatwas completed approximately two weeks after the final installation.Mortality at the time ofreplanting can give an indication of which species are more susceptable totransplant shock,require more care during planting, or may be best outplanted from largercontainers.Because only two weeks had elapsed after planting and the small number ofplants involvedno attempt was made to distinguish replanted individuals from thosealready present and nosurvivorship comparisons between replanted and original plantings can bemade.Approximately 11% of the mesquite, 35% of the bladderpod, 12% of thedesert willow,and 20% of the allscale were replanted. This represents 127 plants (43,44, 31, and 8individuals respectively) or 15 % of the total number of plants installedin the windbreak.These numbers represent plants that succumbed to transplant shock or weredamagedduring planting rather than perished due to harsh desert conditions.
Based mortality at the time of replanting, it appears that bladdelpod isthe most sensitive totransplant shock and least resistant to rough handling during planting.Bladderpodoutplanted from 2x2xlO inch plant bands were the most sensitive. The rootsof theseplants were poorly developed and often the root ball fell apart duringplanting. Bladderpodout planted from 2 gallon half highs were easily removed from thecontainers, sufferedvery few losses and were flowering in June. Both the 2x2xlO inch and halfhighbladderpods were the same age. Rather than consider above ground size asthe only criteriafor judging the quality of container plants, it is important to considerhow consolidated theroot ball is and how difficult it is to remove the seedlings from theircontainer.
Allscale also had a high mortality transplant rate, approximately 20%.This percentage maybe inflated because so few allscale plants were planted and represents 10out of 50 plants.The large size, 3x3x14 inch, of the allscale plant bands made it moredifficult to excavatesuitable holes and awkward to remove the plant band. As with thebladderpod, the rootball may have been damaged during planting and may have contributed tohigh transplantmortality. These large plant bands are designed to biodegrade under moremesicconditions, however, fear that the paper would not degrade under desertconditions led toremoval of the plant bands prior to planting. Leaving plant bands in placeduring plantingwould facilitate planting and leave the root ball intact.
Survivorship of recorded two months after planting should give an betterindication of species performance in windbreak constructions. After twomonths theallscale appears to have the highest survival, at 98% with mesquite thenext highest at91%. Survivorship for bladderpod averaged 85 %, close to that for desertwillow at 84 %.Survival rates appear slightly lower in the southern most section of thewindbreak fordesert willow and allscale. This may reflect the rockier soil substrate inthis section or thefact that this was the first section planted. While it may be premature tojudge which specieswill have the highest survival and be the most successful, based oncurrent survivorshipdata, it appears that mesquite may prove to be the most successful speciesfor futurewindbreak installations.
Overall survivorship as of June 1997 is high with 729 of the 824 (88%)plants remainingalive. This coupled with observed growth suggests that rooting into themoist soil near theirrigation pipe has taken place. If irrigation into the perforated pipe iscontinued soilmoisture will remain high and drought stress eliminated. This should allowsurvivorship toremain high. Plant spacing on the windbreak was intended to absorb amortality rate ofapproximately 30% without seriously affecting performance.
Recommendations
During construction of future windbreaks, several slight designmodifications couldimprove future installations. Flrst, as the irrigation pipe was buriedseveral passes werenecessary to bury the pipe. Pinning the irrigation pipe with u shapedpencil rebar toprevent movement as the pipe was covered should facilitate burial. Inorder to encouragedeep rooting the perforated pipe should be buried well within the rootzone. As thewindbreak was planted, it became apparent that in several sections therewas not enoughmaterial in the planting berm to deeply bury the perforated pipe. Careshould to be taken infuture installations to either dig the trench for the irrigation pipedeeper, or make an extrapass with the grader and blade additional soil on top of the pipe.
As plants were installed, watering basins were constructed around eachplant to facilitatesurface watering and maintain the plantings until roots reached deepersoil moisturesupplied by the perforated pipe. This proved inefficient, labor intensiveand problematicbecause the plantings were made into the side and top of the windbreakberm. In manycases construction of these watering basins called for reshaping the bermby hand andnumerous repairs after basins failed to hold water. Surface watering inthis manner is theleast efficient method of delivering irrigation water. Temporary 2 inchdiameter x 20 inchtall PVC deep pipes buried 5 inches deep, in contact with the root zone,would deliver 1liter of water directly to where it is needed without the evaporativelosses associated withsurface watering. This would be a more efficient method of deliveringwater. With over800 plants installed on the windbreak, the cost of temporary deep pipeswould be offset byimprovements in survival, reductions in transplant shock and savings inlabor andreplacement plants.
At the time of planting the desert willow were not acclimated to FortIrwin conditions.Undoubtedly this contributed to transplant shock and many desert willowshed their leavesin response to outplanting. Replanting was probably premature and some ofthe desertwillow replaced were not dead. Where ever possible it is important toallow plants toremain on site two weeks prior to transplanting for acclimatization.
The mesquite was the most resistant to transplant shock. This is probablyowing to therobust genetic nature of mesquite and the shorter size of the plant bands.It was easier toremove and keep intact the root ball of 8 inch plant bands that themesquite were planted in.One problem encountered with the smaller size was damage to the rootsystem as theprotective treeshelters were installed. In some cases the crown of theseedling was severedfrom the root system by the edge of the treeshelter. Great care must betaken when bothinstalling and removing the treeshelter so as not to disturb the fragileroot system ofoutplanted shrubs. Leaving plant bands in place during planting wouldfacilitate plantingand leave the root ball intact. The question of plant bandbiodegradability and the possiblebenefits and drawbacks to leaving plant bands in place at Fort Irwin needsinvestigation.
Windbreaks constructed in this manner are most appropriate for sites with: (l) gentle slopes of 10 degrees of less, (2) sites with a minimum of rocks to interfere with grading and planting (3) sites with firm compactable soils, sites with highly erosive sandy soils should be avoided to prevent loss of the berm and catchment areas during intense rainfall (4) sites with water truck access (5) positioned upwind of areas to be protected.
