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Philip M. McDonald ForestService, U.S. Departmenlof Agriculture,Redding Calilorn a 96001

Grassesin YoungConifer Plantations- Hindranceand Help

Abstract

Grassesofien hind€r the establishnentofconifers in plantationsby preemptingresources, chemically excluding naiural lings (allelopathy),attracting insectsand aninals, and increasingthe fire potential. Grassessenerally are not desirablein con- ifer plantationsless rhan five yearsold, but alter five years,they can aid s€edlinss.ovth by physicailyand chemically excludingmore conpetitive vegetation.In plantalionsorer fiye yearsold on good siteswith deepsoils, grasses can be beneficial by excludingdeeper rooted .0n poor sites with shallo* soils, grass€sand sbrubsofi€n conpete rhroughout the profile and no bpnefira..ruFs ro.onifFr seedlingsby convertingto grassas.

Introduction catalogued-1500species in the United States alone(Logan 1982).Not only are grassesexten- Crassesin young conifer plantationsoften lead siveabove ground, but they occupymuch area to conflicting points of view. Most forestersad- below ground as well. Nearly 85 percent of the vocatelittle or no grassin conifer plantations. total standing crop of live in North Mostrange managers favor at leastsome grasses, American grasslandsis below ground (Trappe providedthat theyare palatable. Do grassesdeter 19Bl).The combinedroots and root hairs of a or enhanceconifer survival and growth? single4-month-old cereal rye plantgrown in the Should plantations contain grassesthat lower laboratoryhad a total root surfacearea ol237 seedliuggrowth, but facilitategrazing? Or should n1 (2554 lfl and a total length of 623 km (387 plantations contain no grasses(or other com- mi) (Robbinsand Weier 1950,125-126). These petingvegetation) and thereforeno grazing,but valuesare many times greater than for conifer promotehigher seedling Thesequestions $owth? .Although rye grass plants develop reflect the needfor inforrnationon conifer plan- much fasterthan mostperennial grass seedlings, tations when grassesare presentor likely to be the magnitudeof root and root hair development so.Needed even more is the understandingthat belorvground demonstrates the strongcompeti- grassescan both hinder and help. The key is to tive nature of grassesrelative to young knowwhen and where. conifer seedlings.And somegrasses begin root growth At an InternationalSymposium on "Foothills long before , especially in cold soils. for Food and ,"held at OregonState Capability to begin root growth early and to Universityin 1983,speakers from the United rapidly expandabsorbing surfaces into large soil States,, , and the United volumes allows the grassesto capture scarce Kingdompresented papers on how bestto utilize moistureand nutrient resources,Fueled by these the forage presentin developingconifer planta- resources, growth also is rapid.Together, tions (Hannaway1983). In the United States, grassroots and foliage preempt site resources to studieson growingtrees and livestocktogether the point thar conifer seedlingseither cannot are underway in several areas including the becomeestablished or, onceestablished, cannot PacificNorthwest, California, and the Southeast. grow at the potentialof the site. That opportunities exist for the production of foragewhile a coniferplantation is developing Most plantationsconsist of plantedor seeded into a not only is a fact, but alsois of great coniferson ground denudedby someform of site interest.Increasing production frorn the same preparation.In this disturbedenvironment, the landis attractiveto a worldever in needof more oppodunityfor grassesto becorneestablished is food and fiber. great,especially if seedalready is presentor a Ecologically,grasses are markedlysuccessful seedsource is nearby. worldwide. 0ver 5000 species have been

NorthwestScience. Vol. 60. No. 4. 1986 271 GrassesAs A Hindrance createdaround seedlings in rhespring. In June, survival was 93 percent and height growth ex- Grasseshinder seedling establishment in young cellent.By August,few seedlingswere alive. coniferplantations by preemptionof resources, Massesof grassroots from plantsbordering the allelopathy,attraction of insectsand ,and scalpshad fully occupiedthe clearedareas, in- increasedfire potential. cludingthe ground directlyunder the planted .Young grassplants Proomnf ^n nt Rocnr rrnoc also becameestab- lishedin the scalpedareas and addedto the com- In northeasternCalifornia, where soil rnoisture petition.Together, the grassplants robbed the wasthe majorlimiting factor,Roy (1953) found pinesof essentialsoil moisture.Significantly, the thatsurvival of plantedpines varied with ground tallest grassplants in rhe plantation were those coverof shrubsand grasses.After lwo years,sur- borderingthe scalps(J. L. Jenkinson,Pacific vivalranged from bestto worston bareground SouthwestForest and RangeExperiment Station, with no stones,slash, open stony ground, pers,commun.). grass cover,and cover.Another study in north' Not only areconifer seedlings negatively af- easternCalifornia sholred that 80 percentof pon- fectedby grasses,but so are conifersaplings and derosapine (Pinuspond,erosa Dougl. ex Laws. smallpoles. Growth of pole-sizedpines on rela- ,tar.ponderosa) planted seedlingsdied when in tively shallowsoils in easternOregon was hin- a so$n mixture of one-year-oldgrasses (Baron dered by bitterbrush (Purshiatridentata), After 1962), 30 percent when grasses Only died no were removingthe bitterbrush,perennial Idaho fescue present. (Festuca idahoensis) invaded and decreased In a laboratoryand field studyon root com- diametergrowth of the pineseven more (P. H. petitionbetween ponderosa seedlingsand Cochran, Forest and Range grassesin Arizona,grass roots grerv faster than ExperimentStation, pers. commun.). In north- pine roots. Main roots of mountain muhly easternCalifornia, where relarively shallow soils (Muhlenbergia montana) ar'd Arizona fescue predominate,different combinationsof shrubs (Festucaarizonica\ elongated50 percent faster and grasses{ere createdbeneath a standof pine and consequentlyoccupied a givensoil volume poles.After five years,basal area growth of pines soonerthan pine roots.Root weights of grasses increased28 percent over the control when alsowere greater than pine seedlingroots after grasseswere removed and 6 percentwhen broad- two years(Larson and Schubert1969). In a re- leaved shrubs were eliminated (Gordon 1962). lated experimentwith the samegrass species, Wright (1978)noted that similarly-sizedstands most ponderosapine seedlings,planted three in Arizona were partially thinned by grasses. months after the grasses,were suppressedor In theseexamples, reductions in conifersur- dead (Rietveld 1975).Furthermore, the two vival and growth can be attributed to preemp- "an a\^csompdcmonslralion sludiesconslilutcd tion of scarcFr.sour"es by the moreaggressive power of the competitive of fescueand muhly" grasses, (Rietveld1975). Scalping,or the removalof grassesand forbs Allelopathy frorn smallareas around conifer seedlings, was Allelopathyis the process*hereby a chemical evaluatedon east-facingslopes in Mendocino substancereleased from one inhibitsthe County,California. Grasses and forbs werere- presenceor growth of anotherplant in the same movedin 1.8-mand 0.9-m (6-ft and 3'ft) squares or nearbyhabitat (Tukey 1969, Rietveld l9?5). aroundDouglas- (Ps eud,otsuga menziesil seed- Allelopathicinteraction between grasses and Iings. The 0.9-m(3-ft) square was "not sufficient other plant speciesmost often occurswhen the clearingof grassesand forbs to providegood sur- olherplanlc are smalJ. Inhibition of vival" (Adams1975, p. 15-17).A similarstudy and radicle or shoot elongation are typical. of young ponderosaand Jeffrey pir'e (Pinusjef- y'el) seedlingsin the foothills of the central Althoughextracts of manyplants are toxrc Sierra Nevada showedsimilar results.Deep to other plantsin the laboratory,few demonstrate scalpsl.l-m by 1.2-m(3.5-ft by 4-ft)wide r,rere toxicity in the field. Toxic metabolitesin the

272 McDonald natutal environmentare neutralized by several live and decayingbroomsedge signifi- factors, including adsorption on soil colloids, cantly reduced pine stem, root, and needle microbial activity, or possibleinactivity because lengths.Significant reductions in ovendryweight of xeric conditions(Tukey 1969).Grasses that for bothstem and root portions also were noted. showtoxicity to conifer seedlingsin the field are Toxicity can affectplant communitycomposi- importantto forestersand ecologists. tion. Communitymakeup often resultsfrom toxin productionin Fewspecies of grasses,relative to the total sone speciesand toxic tolerance (Muller numberin the United States,have been screened in orhers and Muller 1956).In Oklahoma, for grass (lnslido for toxicity in the laboratory or field. Sixteen example,the annual threeawn grasses(Table l) havedemonstrated toxicity in oligantha\ effectively invades the early seral (Helianthus bioassays(Rietveld 1975, Heisey and Delwiche sunflower annuus) commu nily il 1983).Most also are toxic in rhe field, or sus- becauselolerates the allelopathic environment pectedof being so, createdby the sunflower.In turn, threeawndom- inates for severalyears by allelopathicallysup- Allelopathyin grassand its effect on conifer pressingnitrogen-fixing and blue-green seedlingsis bestshown in a studyof ponderosa algae,thereby rendering soil levels too pine in Arizona(Rietveld 1975). A statistically lowto supportspecies in subsequentseral stages significantreduction in pineseed germination (Wilson and Rice 1968).Such a processcould af- rate,total seedgermination, and initial radicle fect plant successionand alsoconifer seedling developmenttook place from chemicalspredom- growth. inanrly in live foliage, and to a lesserextent in deadresidues of Arizonafescue and mountain Attractionof insectsand animals muhly. Site preparationand subsequentvegetation con- trol measuresoften causeshifts in vegetationand TABLE l. Gras speciesknown or suspectedto be toiic in the field. populationdynamics of associatedorganisms. 0n the west-facingslopes of Mount Shasta,Califor- Common Name Scientilic Name nia, thousandsof acresof brushfieldwere cleared to establishconifer plantations. 0ne largecom- partmentwas treated with herbicideto createfuel Wesrern wheargrass Big squirreltail for broadcastburning. After a successfulburn, Squirrehail ponderosapine seedlingswere planted. Several Cheatgrass years later, shrub seedlingsand sprouts were Blue grama againtreated with .Conseguently, com- Tinothy peting vegetationshifted from woodyshrubs to Sofi chess Ripgut brome grasses(Eromzs spp. and Srrpa spp,),Wingless Medusahead Ellmus caput ned.usae grasshoppers (Brad,ynotesoDesa) became Broonsedge ,4n,l,ro p ogo n Diryi n icu s epidemic. A year later, pocket gophers Threeawn (Thomomys spp.) increasedsubstantially. The Iohnsongrass Sorghun halepense Bernudas.ass shift from shrubsto grassand correspondingin- Sudangrass Sorghun sud.anese creasein grasshoppersand pocket gophers Rabbitfootgrass Polwogon nonspeliensn causedthe loss of most of the pine seedlings (McDonaldand Tappeiner1985). Competitionfrom grassis commonplacein In the southeasternUnited Srates,loblolly pine plantationsthroughout the southrvestand pine (Pinus laeda)seedlings decreased in abun- is recognizedas being important in reducing danceand sizeif growingin fieldsof broomsedge ponderosapine survivaland growth(Pearson (Andropogondrginiczs). To ascertainthe cause 1942,Larson and Schubert1969). Larvae of the of this phenomenon,Priester and Pennington south$eslernpine tip moth(Riyacionia npomcx (1978)prepared live, dead,and decayingshoot icaaa)a\so can reduceheight growth of young extractsof broomsedgeand appliedthem to four- ponderosapines, sometimes by as much as 55 week-oldloblolly pine seedlings. The extractsof percentover a six.yearperiod. And deformation

Crassesin Young ConiferPlantations 273 from repeated attacks can render seedlings are associatedwith increasedfire frequency,at worthless as crop (Jenningsand Stevens least in ponderosapine forests. l9B2).Most atracksby this moth occur at heights Relativelo mostshrub or lreecommunities, lessthan 3 m (10 ft) aboveground. Consequently, the spreadrate of fire in grassesis often greater. competition from grassand repeatedinsect at- A comparisonof fire spreadin tall grassand tall tackscan keep pine seedlingsbelow this height chaparral,for example,showed that fire rnoyed for years, each year compounding the loss of 50 percent faster in grass(Albini 1976). seedlinggrowth. Fire frequencyand spreadrate are likely to On the RogueRiver National Forestin south- increasewhen grasses are present in youngcon- ern Oregon,a native needlegrass(Stipa califor- ifer plantations.And fire may do more goodthan nica)became the principal herbacpous species in harm to the grasses.Increases in grassproduc- a ponderosapine plantation.In spite of scalp- tion, numbers,, and seedsare likely within ing aroundeach seedling to control the grassand one year after burning, and recoveryof peren- baiting to controlthe pocketgophers, survival nial grassesis rapid. after the first year was25 percent.Crass invaded the scalpedareas and the pocket gopherpopula- GaassesAs A Help tion recovered after the first growing season. Grasseshelp conifer seedlingsin young planta- After elevenyears, less than one percentof the lionsby exclusionof r.ompetitireplant species pinesremained-over 1000seedlings had been and allelopathy. lost after the first year. Pocket gophers "were the main causeof mortality" (Hermann 1963), Exclusionot CompetitivePlant Species The vole(Microtas spp.) also or"cupies grassy As noted, grasses areas and posesa threat of damageto conifer competestrongly with conifer seedlings, seedlings.The Oregor.vole (Microtus oregon) is especiallywhen the seedlingsare srnall yet found in a wide range of habitats in the Pacific and not established,After establishment, which generally years, grasses Northwest,especially on sitesoccupied by grasses takes 5 may ac- growrh, and forbs common to logged or burned areas tually aid conifer seedling Although the grassescan negatively growth, (Borrecco e! al- 1979).Damage, in the form of affect conifer generally clippedterminal and lateral shootsand basal woody shrubs are even more debarking,can be seriousin young conifer plan- competitive. tations (Lawrenceet al. 1961,p, 16-17). A well documentedstudy in central Califor- nia portrayedthe potential that grasseshave for Increased Fire Potential controlling woody shrubs (Schultz et al 1955). The shrub stand consistedof wedgeleafcean- Convertingground.orer in a planlationto one othrts(Ceanothus cun eatus), chaparral whitethorn or more speciesof grass often increasesfire (C. leucodermis),deerbrush (C. integerrimus), and potential. Moreover, the greater availability of Mariposa manzanita (Arctostaphylo s marip osa). resourcesresulting from the fire can allow the It wasburned in the fall. The followingApril, new grassto gain in size and vigor. seedlingsnumbered over 839,500 per hectare On the east slopesof the Cascades,Blue (339,500/acre).Ryegtass (Lolium multiflorum) Mountains,and northern Rockies,ponderosa and hardinggrass (Phar,aristuberosa var. pine is part of the climax community.Where stenoptera),each irt a separatestudy, lyere seeded grassesand someshrubs cornprise the under- into the ashesjust after the fall rains.A uniform story, fire frequencyis estimatedto be every 6 stand of grass was established for each. In to 22 years.In the centralRockies, where the August, data showedthat grass cover greater forest understoryis predominantlygrasses, fire than 55 percent for ryegrassand 45 percentfor occursevery l2 to 25 years.In contrast,fire fie- hardinggrasseliminated all shrubseedlings, quencyin the widespreadponderosa pine-bitter- Excavationsin seededand unseededryegrass brush communityof west-centralNorth America plots related competitionbetween grass and is estimatedat 50 years(Wright l97B).These data shrub seedlingsto soil moisture.Where ryegrass indicatethat predominantlygrassy understories had been seeded,shrub seedlingswere 5

274 McDonald centimeters (2 inches) tall and roots had shrub seedlingshave become esrablished. (Dara penetrateda maximumof 28 centimeters(ll on file at the PacificSouthwest Forest and Range inches).The ryegrassroots had grown downward ErperimentStation. Redding. California). 137 centimeters(54 inches).The soil moisture The phenomenonof grassexcluding shrubs deficiencyzone, as determined by the wilting was also noted in a fuelbreak study in central point for sunflowers,extended dor.vn122 cen- California. Part of the open fuelbreak prepared timeters(48 inches).In the unseededplots, shrub by a tracto. was seededwith perennial grasses seedlingsaveraged 20 centimeters(8 inches)in and part wasleft bare(Schirnke er oL 1970).After heightand rootsextended to a depthof 109centi- five years natural regeneralion,including pon- meters(43 inches).The soil moisture deficiency derosa pine and whiteleaf mar'zanita (Arc- zone extended down only 15 centimeters (6 tostaphylosviscida), were abundant inches). in the open area. Plants of these specieswere fewer and Near Mount Shasta,California, dense, mixed- smaller when growing with grasses(Table 2). speciesbrushfields are beingconverted to con- In the coolerand more moist environmentof ifer plantations. Here, a long-term ecological the 0regon Coast Range,several clearcuttings studyhas shown (Strpa that a native needlegrass werebroadcast burned and seededin the fall with spp.) invaded areas where the shrubs were re- perennialgrasses and legumes.The grassmix- moved.0nce the grassbecame established,and tures wereevaluated for effect on westernthim- this happened5 to years 6 after the pineswere bleberry (Rubuspaniflorus) and red alder (Alnus planted,the plotsthat wereinitially free of shrubs r&6ra). Bentgrass (Argrostis lezzis) had the still werefree lB yearslater. Shrubsfrom greatestimpact on westernthimbleberry, with all producedon large plants in windrowsnearby or plots showing a significant reduction in both possiblyfrom dormant seedsin the soil, had not coverand frequencyduring the fifih year. Bent- successfullyinvaded (McDonald theseareas and grass and dwarf intermediate wheatgrass 0liver 1984).The mechanism for shrub exclusion (Agropyon intermediam)had the greatestimpact could be physical modification of the environ- on red alder, with significant reduction of cover ment, emission of toxic substancesfrom Slipo. and frequencyin the fifth year (Klingler 1982). or both.But whaleverthe process. the grass ex- cludedthe shrubsand resultedin statisticallvbet- The impactof gass in reducingwoody shrubs ler heightand diamcter growlh of thepines (30 was also demonstratedin central and southern to 375 percentbetter than control)(McDonald California. After , a standardpractice is and Oliver 1984). to solr annual ryegrass(Lolium muhiflorum) as a ground coverfor reducing runoff and erosion, Two comparativelynew studiesin northern In the relatively dry habitar of a large burn, California also demonstrate the capabilitv of Lolium cover reduced jimbush ceanothus grassto excludeshrubs, Both studies invoired (Ceanothussored,iatus) seedling density tenfold testinga numberof manualand chemicaltreat- and slowedshrub height growth (Griffin 1982). mentsthat wereaimed at reducingshrub popula- tionsand enhancing conifer seedling growth. In Allelopathy one study,a speciesof perennialbrome (Bromus spp.)had formed a densestand where deerbrush Just as some species of seedlings are had beeneliminated. In the other,Pacific needle- hinderedby toxic substancesfrorn grasses,so do grass(Stipa caffirnico) and rescuegrass(-Bromzs other speciesof seedlingsbenefit. Toxic sub- catharticus\badbecome abundant where green- stances from grassescan reduce competitive manzanila(A rc!ost ap h ylo s patula) aw) Sierra shrubs,undesirable trees, and evenother grasses. chinkapin(Cosrozopsis sempe ruirens) werc killed. In one instance,toxic chemicalsfrom brome- In both studies,the treatedplots weie aboutO.O4 grassin old fields markedlyreduced brornegrass hectares(0.'l acre) in size and surrounded by seedlings(Meyers and Anderson1942). But such abundant seed producing shrubs. In addition, documentedexamples are few,and moreiesearch numerousdormant shrub seeds in the soil were is needed.As noted, only a fraction of grass an almostcertainty. Yet, annualsampling has speciesin the United Stateshave been tested for shownthat oncethe grassbecame abundant, no emissionof toxic chemicals,and an evensmaller

Grassesin Young Conifer Plantations 275 TABLE 2. Characteristicsof ponderosapine and {hiteleaf nanzanita seedlingsafter 5 yearsin a grassaDd grass-freeeDviron- nenr in cental California.

Density Height Cover per .{XXX hectarer (percentl Perennial Manzanita Manzanita Manzanita Annuals g"asse€

]\"arursl rs.4 19.8 24.1 25.2 7.8 15.6 32.1 0.6 Graa6es 1.9 2.2 t3.2 t3.5 0.2 0.3 t9.2 64.0

proportionof shruband tree specieshave been For five-year-old conifer plantations on screenedfor susceptibilityto suchchemicals. But medium-to-goodsites, many grassplants per the potential of grassesto help young conifer 1.8-m(6-ft) radius probably haveless effect than plantationsalso exists.Foresters and resource manyplants of woodyshrubs. Medium and bet- managersshould be awareof this phenomenon ter sitesusually are associatedwith soilsover and watchfor eramplesof it. 0.9-m(3-ft) deep, and conifersaplings are lessim- pactedby grassesthan by deeper-rootedshrubs. lmplicationsFor Management For five-year-old,"onifer planlations on Survival and grovth of young conifer seedlings poorer sites with soils generallyless than 0.9-m dependson the availabilityof resources,espe- deep,the relativerooting depth of shrubsand cially soil moisture. In California and southwest grassesis not as important.In general,roots of Oregon,where long, hot, dry summersare typical, conifers,shrubs, and grassesgrolv through the soil moisture usually is the limiting factor on soil to a rocky layer. Here, root extensionbe- nearly all sites,regardless of quality. Competing comeslateral and plantscompete for resources vegetation further reduces rhe already limited on rop of the rocky layer. No classof plants has soil moistureresources. And the better site qual- an advanlage. ity, the faster and more completelythe site becomesoccupied. Degreeof risk is probably fairly high within Cirenthat grasses often are slrong compeli. a 0.9-m(3-ft) radius of bare ground around each tors,the question"How muchgrass is too much seedling.Less risk is associatedwith a 1.5-m(5-ft) for conifer survivaland grovth?" often is asked. radius of clearedarea. Early indicationsare that The answerdepends largely on ageof the planta- radii of 1.5m (5 ft) or largerare required to con- tion, soil depth, and degreeof risk. 0nly general trol shrubs (Fiddler and McDonald 1984).The guidelinesare presented,and exceptionsare largerradii maybe necessaryto controlgrasses probable. as well. For newlyplanted conifer seedlings, one grass Increasingevidence suggests that early con- plant in a I.8-m (6-ft)square (about 0.9-mor 3-ft ifer seedlinggrowth is closelyrelated to amount radius)around the seedlingearly in the season of resources,especially to availablesoil moisture. is probably too much (McDonald l9B3).The Any moisture used by competing vegetationis singleplant likely vill becomea largeplant with that much lessavailable for conifer seedlingsur- a root systemlarger than that of the coniferseed- vival and growth. And given a limited amount ling. And after the productionof seed,more grass of moisture,the advantagelies with thoseplants plantssoon will be present.Moreover, if one grass or speciesthat are presentfirst. Their roots are plant is present,the oddsare that other seeds unirnpededby competitors'roots or chemicals and plants of grasswill be presentand they too or mycorrhizae,and they are free to exploit an will matureand produce seed. For thesereasons increasedsoil volume.In turn, shootgrowth also and becauseof extensiveroot developmentand benefits.A coniferseedling can capitalizeon this capture of soil resources,grasses should be processif completeo. near-completecontrol of eliminatedin young conifer plantationsuntil the competingvegetation is accomplishedduring the seedlingsare established. first few years. After five years, gra6s can

276 McDonald increasewith little risk of a major reduction in chemicalsemitted from these grassescould conifergrowth because the seedlingsare nor- becomeincreasingly important in the nearfuture. mally well established. Certainly,more information on the allelopathic Resourcemanagers need a varietyof tech- potential of different grassspecies in different niquesto counteractdeleterious vegetation in environmentsis needed. coniferplantations. Manual techniques, chemi- cals,grazing animals, fire, and biologicalcontrols The effectivenessof grazing animalsto con- (grassesreplacing shrubs, for example)figure im- trol woodyshrubs in youngconifer plantations portantlyin controlling or eliminating unrvanted curently is being evaluated.In the past,the vegetation.These techniques, used singly or to- generalfeeling was that grazing animalsand gether,are necessary to accomplishthe rangeof youngconifer seedlings did not mix. In addition biologicand economicobjectives of resource to the paperspresented at the InternationalSym- "Foothills managerstoday. posium on for Food and Forests" Biologicalcontrols probably will not be as im- noted earlier,a recentstudy in northern Califor- (Thomas portant as the other techniques,overall, but nia 1984)indicated that damagefrom shouldalways be consideredand usedwhere pos- sheepto two-to three,year-oldponderosa pine sible.Long-term costs may be lower,especially and Douglas-firseedlings can be minimal,pro- if additionalrelease treatments are not needed. vided rhat timing and durationof grazingare In sensitiveareas, biological control may be the closelycontrolled. Furtherrnore, controlled graz- only techniqueavailable. ing seemsto favor the establishmentof grassand to prohibit new shrubs. AIIelopathy,both as an advantageand a dis- CIosecooperation be- tweenrange managers advantage,should receive increasing considera- and foresterscould mean young tion in ieforestation. It is significant that suc- that coniferplantations with a combined cessfulmanual, chernical, or releasetreat- yield will be more productivebecause the grasses mentsoften lead to grassunderstories in young excludenew shrubsand the animalskeep the old plantations.The effect on conifer growth by shrubsfrom expandingor producingseeds.

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ReceioedI July 1985 Accepted,for publication27 December1985

278 McDonald