Effects of Substrate Disturbance on Secondary Plant Succession; , California Author(s): D. V. Prose, Susan K. Metzger and H. G. Wilshire Source: Journal of Applied Ecology, Vol. 24, No. 1 (Apr., 1987), pp. 305-313 Published by: British Ecological Society Stable URL: http://www.jstor.org/stable/2403806 Accessed: 28-11-2015 02:51 UTC

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This content downloaded from 128.226.37.5 on Sat, 28 Nov 2015 02:51:47 UTC All use subject to JSTOR Terms and Conditions Journalof Applied Ecology (1987) 24, 305-313

EFFECTS OF SUBSTRATE DISTURBANCE ON SECONDARY PLANT SUCCESSION; MOJAVE DESERT, CALIFORNIA

BY D. V. PROSE, SUSAN K. METZGER AND H. G. WILSHIRE

U.S. GeologicalSurvey, 345 MiddlefieldRd. Menlo Park, Ca. 94025 U.S.A.

SUMMARY

(1) The effectsof substratedisturbance on perennialplant successionin the Mojave Desert wereassessed at threemilitary camps abandoned for40 years. (2) Soil compaction,removal of the top layerof soil, and alterationof drainage channel densitycaused significantchanges in perennialplant cover,density, and relativespecies composition. (3) Long-livedspecies, predominantly Larrea tridentata,were dominant in all control areas but percentagecover and densitywere greatlyreduced in areas wheresubstrate alterationswere significant. (4) Pioneerspecies such as Ambrosiadumosa and Hymenocleasalsola had percentage covervalues similarto or greaterthan controls in mostareas wheresubstrate alterations weresignificant, and thesespecies were dominant in themajority of disturbedareas. (5) Where substratealterations were insignificantin disturbedareas at one camp, Larrea was thedominant species as in thecontrol.

INTRODUCTION Human activitiesthat disturbvegetation commonly also modifythe soil substrate. Substratedisturbances reported in studiesof secondaryplant successionin the Mojave Desert includesoil trampling,trench backfilling, and road gradingduring power and pipe-lineconstruction (Vasek 1975a, b; Lathrop & Archbold 1980a, b); excavationof borrowpits for road-building materials (Vasek 1979/80);soil tramplingin townsites of abandonedmining communities (Webb & Wilshire1980; Webb, Wilshire & Henry1983; Webb, Steiger& Turner1987); tillage of agriculturalfields (Carpenter, Barbour & Bahre 1986; Romney,Wallace & Hunter1979); and soil tramplingand loadingby soldiersand vehiclesin militarymanoeuvre areas and base-camps(Lathrop 1983).These disturbances commonlyproduced substrate alterations that persisted for decades afterdisturbance, suchas soil compaction,destruction of thesoil profile,and/or soil loss bydirect removal or byerosion. Since these alterations are detrimentalto vegetationgrowth (reviews of this topic include Rosenberg 1964), most studies listed above conclude that substrate alterationsslow therate of plant succession. However, few of these studies quantitatively assesssubstrate alterations in disturbedareas. Webb & Wilshire(1980) and Webb,Steiger & Turner(1987) measuredsoil compactionin miningtowns abandoned for 50-75 years and foundthat minimally-compacted areas supportedvegetation assemblages similar to those in undisturbedareas, whereasplant cover and speciescomposition were signifi- cantlychanged in highlycompacted areas. The purpose of this studywas to assess the effectsof differentdegrees of substrate disturbanceon perennialplant successionin Larrea tridentata(Sesse & Moc. ex DC)- Ambrosiadumosa (Gray) communitiesin the easternMojave Desert. This area was 305

This content downloaded from 128.226.37.5 on Sat, 28 Nov 2015 02:51:47 UTC All use subject to JSTOR Terms and Conditions 306 Substratedisturbance and succession designatedthe in 1942and was used formilitary training exercises for2 years(Meller 1946).Three abandoned base-camps offered a numberof studyareas thatwere disturbed in varying degrees. Natural substrate and vegetationproperties varied betweencamps, allowingcomparison of successionaltrends between slightly different micro-environmentsin the Mojave Desert.

STUDY AREA

The threeabandoned military camps-Ibis, Clipper,and Iron Mountain-were located in the easternMojave Desert of California,in a sparsely-populatedarea betweenthe townsof Ludlow,Calif., Needles, Calif., and Parker, (see Prose (1986) forexact locations).The elevationof Camp Ibis was 525 m,Clipper 575 m,and Iron Mountain350 m. Each camp encompassed1000-2600 ha, and consistedof a networkof graded dirt roads and clearedareas used fortent sites, vehicle parking lots, and otherpurposes. The regionalclimate is arid,with annual rainfall during the period 1940-84 averaging 11 2 cm at Needles (located within61 km of camps Ibis and Clipper), and 7 9 cm at Iron Mountain.Average January and Julytemperatures at Needles duringthis period were l1 10C and 35 50C, respectively(National Climatic Center,written communication, 1985). The camps wereestablished on gentlysloping, coalescing alluvial fans (bajadas). The fan surfacesat Ibis and Iron Mountain were incisedby numerousshallow drainage channels. Clipper was astridea more stable surfacewith large, widely-spacedwash channels.Average soil texturesin the0-30 cm depthrange were gravelly sand at Ibis and Iron Mountain and loamy sand at Clipper. The plant communityat all camps was creosote bush scrub (Munz 1974). Larrea tridentatawas the dominant species in undisturbedareas withAmbrosia dumosa subdominant. There was some variationin communitycharacteristics between camps (see Table 1).

MATERIALS AND METHODS

Exact locationsof the camps and theirlayouts were obtainedfrom National Archives recordsin Washington,D.C. and fromaerial and ground-levelphotographs taken when the camps were active. Soil and vegetationmeasurements were made in minimally- disturbedcontrol areas and adjacenttent sites, parking lots, and roads.The controlswere traversedby somevehicles during training exercises, but many vehicle tracks remain and thussignificantly-disturbed areas could be avoidedwhen sampling. Records indicate that thetent-sites were cleared of vegetation by soldiers using hand-tools and subjectedto foot trampling.Vehicle parkinglots were also manuallycleared of vegetationbut were subjectedto intensiveuse by a wide varietyof armouredvehicles and lighterjeeps and trucks.Roads wereconstructed by removingthe upper 10-20 cm of soil by gradingand wereheavily used by bothvehicles and personnel.Camp Clipperwas activefor a shorter timethan the other two camps; operationswere transferred from an adjacentcamp that was subjectto flooding.The campswere abandoned with no apparentattempt to reclaim them,and no signsof recentdisturbance were present in sampledareas because of the remotenessof thearea; theroads are theonly areas likelyto have beenrecently disturbed by vehicles.

This content downloaded from 128.226.37.5 on Sat, 28 Nov 2015 02:51:47 UTC All use subject to JSTOR Terms and Conditions TABLE 1. Density(plants ha-1) (D) and percentagecover (C) of perennialvegetation at threeabandoned military base-camps; Mojave Desert. *Significantlydifferent (P< 005) fromcontrol.

Camp Ibis Tent Parking Tent Parking Tent Parking Control site lot Road Control site lot Road Control site lot Road g

Long-lived D C D C D C D C D C D C D C D C D C D C D C D C Krameriaparvifolia 790 2-0 - - Larrea tridentata 584 7-4 127 1-7 84 0-5 43 0-4 726 11-3 299 5-4 175 3-0 100 1-3 200 5-5 167 2-2 84 0-9 Opuntiaechinocarpa ? - ? - ? ?75 0-1_ - - _ _ -______Opuntiaramosissima 83 0-1 - - - - Yucca schidigera 124 0 1 - - - - Total long-lived 1581 9-6 127 1-7 84 0 5 43 0-4 801 11-4 299 5-4 175 3-0 100 1-3 200 5-5 167 2-2 84 09 - - Long-lived opportunistic Ambrosiadumosa 2959 7-4 3043 8-6 3449 9-3 791 3-3 1274 1-9 1501 4-6 650 20 700 4-3 233 1-0 2150 3-9 750 1-8 100 0 3 :> Short-lived Bebbiajuncea - - - - _ _ ?------17 0-1 ------Dyssodiaporophylloides 83 0-1 Enceliafarinosa - - - - 52 0 2 ------?17 0-1 18 0-1 67 0-5 0 Enceliafrutescens 331 0-4 457 0-4 1501 2-7 832 1-8 ------Hymenocleasalsola - - 249 0-9 42 033100 - - - 0-4 1267 6-4 - - Lepidiumfremontii ????? 33 0-1 50 0-1 - - Porophyllumgracile - - 167 0-1 42 011------Stephanomeriapauciflora 207 0-2 41 0-1 84 0-1 43 0-1 ------32 0-2 132 0-5 91 0-3 - - Total short-lived 621 0-6 914 1-5 1721 3-4 875 19 - - - - ?49- - - 0-3 282 1I1 1426 6-9 67 0-5 w Totals 5161 17-6 4084 11-8 5254 13-2 1709* 5.6* 2075 13-3 1800 10-0 825* 5.0* 800* 5.6* 467 6 8 2599* 7 2 2260* 9.6* 167* 0 8*

to

This content downloaded from 128.226.37.5 on Sat, 28 Nov 2015 02:51:47 UTC All use subject to JSTOR Terms and Conditions 308 Substratedisturbance and succession Soils and surfacehydrology studies Soil compactionwas assessed by the bulk densitymethod (Blake 1965). Recording penetrometermeasurements were not used because severelydisturbed areas were compactedbeyond the workingrange of the instrument(Prose & Metzger1985). Bulk densitywas calculatedfrom oven-dried soil samplesthat were collected with a piston- drivencylinder (volume: 224 cm3). Thirtysamples were collectedfrom soils between shrubsand drainagechannels at depthsof 0-10 cm,10-20 cm, and 20-30 cmin each study area. Particle-sizedistribution was determinedon additionalsamples by thehydrometer method(Day 1965). Bulk densityvalues wereaveraged for the 0-30 cm depthrange in each control, and disturbed area and differencesbetween means were analysed statisticallyat a confidencelevel of 95% withan appropriatet-test. Drainage channeldensity was analysed at Camps Ibis and Iron Mountain because vegetationwas concentratedalong wash channelsand alterationof surfacedrainage characteristicscan significantlyaffect vegetation (Schlesinger & Jones1984). The number of drainagechannels was recordedon traversesperpendicular to thedominant drainage directionin each studyarea. Densityof channels in each area was calculatedas numberof channels per 100 m, and control and disturbedpairs were analysed with a t-test. Measurementswere not made at Camp Clipperbecause dissectionof the surfacewas minimalin the studyareas.

Vegetationstudies Perennialshrubs were sampled in 20 m2rectangular plots at Camp Ibis and in 50 or 100 m long x 2 m wide belt transectsat Camps Clipperand Iron Mountain.The plots and transectswere alignedperpendicular to the dominantdrainage direction to minimize errorscaused by concentrationof vegetationalong wash channels.A stratified,random samplingtechnique was used (Greig-Smith1983) and the numberof plots or transects requiredfor a representativesample was determinedfrom spacing and speciesarea curves (Brower& Zar 1977) of the dominantspecies, Larrea and Ambrosia,in the controls. Sampleareas in the controls and disturbedareas were 240 m2(n = 12) at Camp Ibis,400 m2 (n = 20) Camp Clipper,and 600 m2 (n = 30) at Camp Iron Mountain. Larrea plants occurringin clumpswere considered individuals if the main stemsof visually-distinctshrubs were separated by a distanceof at least 1 m. Density(number of individualsha- 1) and percentagecover of each shrubspecies were determined for each studyarea. Distributionsof cover values were normalizedby angulartransformation (Campbell 1974)and differencesin totalpercentage cover between controls and disturbed areas weretested for significance (P<0 05) witha t-test.Total densityvalues foreach studyarea werealso analysedstatistically. To analyse the adaptabilityof species with particularlife-histories to substrate alterations,species were categorizedas eitherlong-lived, long-lived opportunistic, or short-lived(Vasek 1983; R. H. Webb, personalcommunication 1986).

RESULTS

Soils and surfacehydrology Bulk densitymeasurements were highlyvariable but indicatedthat disturbedareas weresignificantly compacted between 0 and 30 cm depth,except at Camp Clipper,where

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1.9

.8- E

c 1-6/

1-5 I Control Tent Parking Road lot

FIG. 1. Soilbulk density at threeabandoned military camps, Mojave Desert. (0) CampIbis; (C) Clipper;(A) IronMountain. Vertical bars represent standard deviation. *Significantly different fromcontrol (P<0 05).

onlythe road had significantlyhigher density values thanthe control (Fig. 1). The deep, fine-grainedsoils at Clipperhad thelowest overall bulk densityof all camps. Drainage channeldensities at Camp Ibis werenot statisticallydifferent in thecontrol and disturbedareas (Table 2). At Iron Mountain,channel densities were similar in the controland tentsite, but were significantly lower in theparking lot and road thanin the control.Surfaces between channels in theparking lot contained fluvial deposits of silt and sand.

Vegetation Total plant densityand cover in all roads and the parkinglot at Clipper were significantlylower than in the controls(Table 1). At Iron Mountain,total densitywas significantlyhigher in the tent site than in the controlbecause of an abundance of Ambrosiaseedlings. The parkinglot at thiscamp had a significantlyhigher total density and coverthan the control because of an abundanceof Hymenocleasalsola (T & G). All otherdisturbed areas had total densityand cover values similarto those in respective controls. Long-livedspecies were greatlyreduced in disturbedareas compared to controls. Larrea tridentatawas negativelycorrelated with disturbance at all camps,and was very

TABLE2. Drainage channeldensity at threeabandoned militarycamps; Mojave Desert. Channeldensity= numberof streamsper 100 m. n =number of traverses. *Significantlydifferent from control (P< 005) Ibis IronMountain Standard Standard n Mean deviationn Mean deviation Control 3 8 5 1 21 3 116 091 Tentsite 3 12-0 0.99 2 108 0 35 Parkinglot 3 120 1 51 2 5 7* 1 06 Road 2 10 1 049 2 6.4* 1 13

This content downloaded from 128.226.37.5 on Sat, 28 Nov 2015 02:51:47 UTC All use subject to JSTOR Terms and Conditions 310 Substratedisturbance and succession sparseor notpresent in theparking lots and roads. The dominantspecies in thecontrols, Larrea was subdominantin all disturbedareas exceptthe tentsite and parkinglot at Camp Clipper.Other long-lived species occurred in thecontrols at Ibis and Clipperbut werenot presentin disturbedareas. Ambrosiadumosa, the sole long-livedopportunistic species, had highercover values in disturbedareas than in controlsexcept in the roads at Ibis and Iron Mountain. This specieswas dominantin overhalf of the disturbedareas. Short-livedspecies were present in undisturbedareas at all camps, but were encounteredin measuredareas only at Ibis and Iron Mountain.Disturbance generally caused an increasein thesespecies, particularly of Enceliafrutescens (Gray) at Camp Ibis and Hymenocleasalsola at Iron Mountain.

Relationof soil bulkdensity and perennial shrub cover Percentagecover of perennialswas chosen forcomparison with changes in soil bulk densityrather than densitybecause cover is consideredto be of greaterecological significance(Daubenmire 1968). Total coverwas significantlyreduced in theextremely- compactedroads comparedto controlsat all camps (Fig. 2). Cover of long-livedspecies, predominantlyLarrea, decreased with increased soil density.Ambrosia dumosa, the long- livedopportunistic species, and short-livedspecies, had highercover values in compacted areas thanin controlsexcept in theextremely compacted roads at Camps Ibis and Iron Mountain.Thus, relativespecies composition shifts from Larrea-dominant assemblages in non-compactedareas to assemblages dominated by short-livedspecies and/or Ambrosiain significantlycompacted areas.

DISCUSSION Soil compactionwas significantin mostdisturbed areas 40 yearsafter the military camps wereabandoned. Webb & Wilshire(1980), Prose (1985), and Webb, Steiger& Wilshire (1986) reportedthat various desertsoils remainedcompacted as a resultof human disturbancesfor similar or longerperiods of time,probably because animalburrowing and plantroot growthare theprimary agents of soil-looseningin low desertvalleys but animalsand plantshave difficulty displacing highly-compacted soils. Soil compactionhas been shownto inhibitplant growth in manylaboratory studies (for example, Rosenberg & Willits1962), and compactioncaused significantdecreases in coverof desert vegetation in disturbedareas (Rowlands 1980; Webb & Wilshire 1980). In this study,Larrea tridentatawas foundto be highlysensitive to soil compaction.These observationsof recentlydisturbed areas indicate that establishedindividuals can survivemoderate disturbances,but Larrea has a verylow rate of germinationand seedlingsurvivorship evenunder ideal laboratoryconditions (Barbour 1968). Because the tent sites and parking lotsat thecamps were evidently only cleared of above-ground vegetation before use, some Larrea root-crownsmay have survivedand producednew foliageafter the camps were abandoned.The virtualabsence of Larrea in thecamp roads maybe due to extremesoil compaction,combined with the effectsof road constructionwhen gradingprobably destroyedmost root-crowns. In addition,nutrients essential for plant growth may have been strippedby gradingwhich removed 10-20 cm of soil. Charley& Cowling(1968) foundthat removal of thesurface 5-10 cm of soil significantlydepleted nutrient reserves and caused a significantreduction of perennialcover after13 yearsof successionin a desertsaltbush community in Australia.

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- 1818 (a) (b) lb~~~~~~~~~~~~~~~~~b

PL TS 12 -

\ETS

6 \ ER F L-;R I' ~~PLEQ .

3-3 L..L. I _- C__ ; {

1-50 1 60 1 70 1 80 190 1-50 1-60 1-70 1-80 1-90

18 Bulkdensity (g cm-3)

15 -

12 -

PL 9 U S

6 -j 0\)K 3-

AlI 1'50 160 1I70 /180 1-90 Bulkdensity (g cm-3)

FIG. 2. Relationof soil bulk density and vegetation cover at threeabandoned military camps,Mojave Desert:(a) Camp Ibis; (b) Clipper;(c) IronMountain. (U) Total percentagecover; (0) long-livedperennials; (El) long-livedopportunistic; (A) short- lived.C, control;TS, tentsite; PL, parkinglot; R, road.

Coverof Ambrosia dumosa in mostdisturbed areas was similarto or higherthan that in thecontrols. This speciesis a commonpioneer in disturbedareas in the Mojave Desert (Vasek 1979/1980;Lathrop & Rowlands 1983) apparentlybecause of its abilityto reproduceboth asexuallyand by efficientdispersal of abundant seeds. Reductionsof Larrea in disturbedareas mayhave furtherenabled Ambrosia to re-establishquickly by reducinginterspecies interference. Such interferencewas observedin a westernMojave Desert Larrea-Ambrosiacommunity by Fonteyn& Mahall (1981). Short-livedspecies such as Encelia frutescensand Hymenocleasalsola were more abundantin most disturbedareas than in controlsat Camps Ibis and Iron Mountain. These specieswere early colonizers of disturbedareas elsewhere-in the Mojave Desert (Vasek 1979/1980;Webb, Wilshire & Henry1983), partly because theyinhabit naturally 'disturbed'areas such as drainagechannels. Hymenoclea was greatlyincreased in the

This content downloaded from 128.226.37.5 on Sat, 28 Nov 2015 02:51:47 UTC All use subject to JSTOR Terms and Conditions 312 Substratedisturbance and succession parkinglot at Iron Mountain.In thisarea, alterationof surfacedrainage characteristics may have created favourablegrowing conditions for this species. Runoffcapacity frequentlyexceeds that of the reduced number of drainage channels as indicatedby fluvial depositson inter-channelsurfaces. A similar,frequently-disturbed environment exists in largedrainage channels where Hymenoclea occurs naturally. At Camp Clipper,Larrea was the dominantspecies in the tentsite and parkinglot. These were the only disturbedareas examinedwhere substratealterations were not significant,and wherepioneer species did not dominatevegetation assemblages. Thus, it appearsthat disturbed Larrea-Ambrosia communities can re-establishclimax conditions quicklyin termsof relativespecies compositionif substratealteration is minimal.If disturbancecauses significantsubstrate alteration, pioneer species assume dominance because theyare adaptive to a wide range of disturbancewhile substratealterations inhibitre-establishment of Larrea and otherlong-lived species. Substantial amelioration of substrateconditions must take place in highly-disturbedareas beforeLarrea re- assumesdominance in accordancewith classical successiontheory.

ACKNOWLEDGMENTS We thankDenise Polk forher assistancein the field.Frank Vasek and RobertWebb providedvery helpful reviews of themanuscript. This projectwas supportedby a grant fromthe U.S. ArmyResearch Office; contract No. 104-84.

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Prose,D. V. (1986). Map ofareas showingvisible land disturbances caused by twomilitary training exercises in the MojaveDesert, California. U.S. GeologicalSurvey Miscellaneous Field Studies Map MF-1855,Reston. Romney,E. M., Wallace, A. & Hunter,R. B. (1979). Revegetationstudies pertinent to theBarstow JO MEwR solar thermalpower system and topotential power plants elsewhere in theMojave Desert: UCLA 12/1224UC 11, Laboratoryof Nuclear Medicine and RadiationBiology, University of California,Los Angeles. Rosenberg,N. J.(1964). Response of plants to the physical effects of soil compaction. Advances in Agronomy, 16, 181-196. Rosenberg,N. J. & Willits,N. A. (1962).Yield and physiologicalresponse of barleyand beansgrown in artificiallycompacted soils. Soil Science SocietyProceedings, 26, 78-82. Rowlands,P. G. (1980).Recovery, succession, and revegetation inthe Mojave Desert. The Effects of Disturbance on DesertSoils, Vegetationand CommunityProcesses with Emphasis on OffRoad Vehicles.A CriticalReview (Ed. byP. G. Rowlands),pp. 76-119,U.S. Bureauof Land Management,Riverside. Schlesinger,W. H. & Jones,C. S. (1984).The comparativeimportance of overland runoff and meanannual rainfallto shrubcommunities ofthe Mojave Desert. Botanical Gazette, 145, 116-124. Vasek,F. C., Johnson,H. B. & Eslinger,D. H. (1975a).Effects of pipeline construction on creosotebush scrub vegetationof the Mojave Desert. Madrono, 23, 1-13. Vasek,F. C., Johnson,H. B. & Brum,G. D. (1975b).Effects of power transmission lines on vegetationof the MojaveDesert. Madrono, 23, 114-130. Vasek,F. C. (1979/80).Early successional stages in Mojave Desert scrub vegetation. Israel Journal of Botany, 28, 133-148. Vasek,F. C. (1983).Plant succession in theMojave Desert. Crossosoma, 9, 1-23. Webb,R. H. & Wilshire,H. G. (1980).Recovery of soils and vegetation ina MojaveDesert ghost town, Nevada, U.S.A. Journalof AridEnvironments, 3, 1-i5. Webb,R. H., Wilshire,H. G. & Henry,M. A. H. (1983).Natural recovery of soils and vegetationfollowing humandisturbance. Environmental Effects of Off-Road Vehicles. (Ed. by R. H. Webb & H. G. Wilshire),pp. 279-302.Springer-Verlag, New York. Webb,R. H., Steiger,J. W. & Wilshire,H. G. (1986).Recovery of compacted soils in MojaveDesert ghost towns. Soil Science Societyof AmericaJournal, 50, 1341-1344. Webb,R. H., Steiger,J. W. & Turner,R. M. (1987).Dynamics of MojaveDesert shrub vegetation in the PanamintMountains. Ecology, 50 (in press).

(Received11 November1985; revisionreceived 15 October1986)

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