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 seed 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 conifer s€edlinss.ovth by physicailyand chemically excludingmore conpetitive vegetation.In plantalionsorer fiye yearsold on good siteswith deepsoils, grasses can be beneficial by excludingdeeper rooted shrubs.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 plants 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 seedling 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? seedlings.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 conifers, especially in cold soils. for Food and Forests,"held at OregonState Capability to begin root growth early and to Universityin 1983,speakers from the United rapidly expandabsorbing surfaces into large soil States,New Zealand,Australia, 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,shoot 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 forest 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 animals,and scalpshad fully occupiedthe clearedareas, in- increasedfire potential. cludingthe ground directlyunder the planted pines.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, shrub 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 pine seedlingsand Cochran, Pacific Northwest 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 plant 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-fir (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 germination 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 shoots 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 bacteria and blue-green seedlingsis bestshown in a studyof ponderosa algae,thereby rendering soil nitrogen levels too pine in Arizona(Rietveld