Tree Shelters
Reduced
Growth
and
Survival of Underplanted Red Oak Seedlingsin Southern Iowa Robert E. Bardon, North Carolina State University,Raleigh,NC 27695, and David W. Countryman and Richard B. Hall, Departmentof Forestry, Iowa State University,Ames,Iowa 50011.
ABSTRACT: A majorconcernin themanagement of northernredoak(QuercusrubraL.) is thedifficultyin regenerating standsthathavedenseunderstories ofshadetolerantspecies. A replicatedstudyin southernIowa indicatedthat overa 5 yr period, tree sheltershavea major impacton establishment of underplanted,1-0, northernredoakbarerootstock.Shelteredseedlings hadincreased heightgrowthduringthefirst 3 growing seasons. Butduringthenext2 growingseasons, annualheightgrowthofshelteredseedlings declinedtoa level of annualheightgrowthsimilarto thatof nonsheltered seedlings. After5 growingseasons, shelteredseedlings wereapproximately 30 cmtaller thannonsheltered seedlings. Shelteredandnonsheltered seedlings wereonethirdto two-thirdstheheightof theshelters.At theendof thefifih growingseason,totalmortalityfor sheltered seedlings was40 and55%, whereas for nonsheltered seedlings, mortalitywas26 and 28% at theMcNayand Stephenssites,respectively.Tree sheltersmay be a viable alternativein open areas (e.g., clearcutsor plantations),but in this study, tree sheltersreducedboth growth and survival when used to protect underplanted, 1-0, red oakseedlings. North.J. Appl. For. 16(2):103-107.
A continuing concern inthe management ofnorthern red oak(QuercusrubraL.) is thedifficultyin regenerating new standsto replace those harvested(Holt and Fisher 1979, Johnsonet al. 1984, Crow 1988, Lorimer 1989). The difficultyin regenerating redoakstandsis thatmanyoakforests in partsof EasternandCentralUnitedStateshavedeveloped denseunderstoriesof shade-tolerantspecies(Schlesinger 1976,Ehrenfeld1980,Coder1985).Thus,asred oak stands areharvested,theygraduallyconvertto moreshade-tolerant species(e.g., sugarmaple,Acer saccharumMarsh.;Americanbeech,FagusgrandifoliaEhrh.)becausethereis little or no advancedred oakreproduction(Sander1977). Thereis muchresearchonwaysto successfully establish red oakregeneration. Researchers havefocusedon planting improvednurserystock,herbicidecontrolof competition, removalof understory andoverstoryvegetation,andcombinationsof thesemethods(Johnson 1976,Loftis1983,Johnson etal. 1989,SchultzandThompson 1991,TeclawandIsebrands 1991,Minter et al. 1992). No•: D.W.Co•untryman isthecorresponding author, andhecanbecontactedat (515) 294-7703;Fax: (515) 294-2995;E-mail:
[email protected].
R.E. Bardonwasformerlywith the Department of Forestry,Iowa State University.JournalPaperNo. J-17077of theIowa Agriculture andHome Economics Experiment Station,Ames,Iowa.ProjectNo.2172,supported by Hatch Act and State of Iowa Funds.
Currentresearchhasalsocenteredonusingtreesheltersto establish redoakregeneration. Lantagneetal.(1990),Zastrow andMany (1991), andMinter et al. (1992) havestudiedthe effect of tree shelterson red oak seedlingsplantedin harvestedforestopenings.Lantagneet al. (1990) concluded that tree sheltersusedin a Michigan clearcutpromotedheight growthby improvingmicro-environments of red oak seedlingsandre-allocatinggrowthfrombranchesandstemdiameterto shootelongationfor shelteredseedlings. Zastrowand Marty (1991) concludedthat tree shelterspromotedheight growthand survivalby improvingmicro-environments of red oak seedlings.Minter et al. (1992) concludedthat tree shelterspromoteincreased heightgrowthby increasing relative humidity,reducingplant transpiration losses,and increasinglevelsof CO2.This heightgrowthmaycomeat the expenseof basaldiameterandroot growth. Researchhasdocumented thebenefitsof usingtreeshelters.Thesebenefitsare:(a)promoting heightgrowth(Lantagne et al. 1990),(b) reducingbrowsedamage(Tuley 1985,Potter 1991), (c) producingan environmentwith a higherrelative humidityandcarbondioxideconcentrations thantheambient environment(Minter et al. 1992), (d) sustaininga longer
durationof initial growthflush(Kittredgeet al. 1992),(e) improvingsurvivalrates(Tuley 1985, Potter1991), and(f) increasing earlygrowthrates(Tuley 1985,Potter1991).The NJAF16(2)1999 103
benefits of tree shelters based on short-term studies of less
than 5 yr havebeendocumentedextensively,whereasless hasbeenpublishedon the impactof tree sheltersusedfor longerperiodsof time (i.e., 5 or moreyears).The objective of thisstudywastodetermine theimpacttreeshelters haveon establishing1-0, red oak seedlingsunderplanted in upland
mixedhardwood standsovera 5 yr period.
Study Site This studywas conductedwithin two upland,mixed, hardwoodstandsof 2.3 ha each,locatedin LucasCounty in southcentralIowa. The standsare locatedon the McNay Research Farm at 40ø57 ' N and 93ø26 ' W and the Lucas
Unit of the StephensStateForestat 40ø57' N and93ø30' W. The McNay site had a history of grazinguntil approximately 30 yr ago. The Stephenssite was harvestedapproximately50 yr ago.The siteindexfor uplandoak (base age= 50) (Carmeanet al. 1989) is 20 m for boththe McNay and Stephenssites. Both the McNay and Stephensstandswere thinnedof all nonoak trees greater than 2.54 cm diameter breast height (dbh) duringthe winter of 1990-1991, just prior to the study.Cut treeswereharvestedby chainsaw,chipped, andhauledfrom the sites.Stumpsweretreatedwith picloram (Totdon RTU), accordingto the label, to prevent resprouting.After thinning,theMcNay standhad95 trees/ ha with an averagedbhof 43.4 cm. The averagestocking
level(Gingrich 1971)was48%stocked or14.1m2/ha. The Stephensstand, after thinning, had 282 trees/hawith an average dbh of 26.7 cm. The average stocking level
(Gingrich 1971)was60%stocked or 15.9m2/ha. Mean annualprecipitationfor April throughSeptember for southcentralIowa is approximately62.81 cm. During 1992 and 1993, precipitationlevels were above average; 83.72 cm and 107.44 cm respectively.Justthe opposite occurredin 1994, with an averageof 40.95 cm of precipitation.
Materials
and Methods
Nine hundred 1-0, red oak bareroot seedlings were
lifted in April 1991 from the Iowa Departmentof Natural ResourcesState Nurseryin Ames, Iowa, and the initial height (21.4 cm, Stand.Dev. = 11.2) and basaldiameter (0.5 cm, Stand.Dev. = 0.12) were measured.Initial height is thedistancefrom therootcollar to thetip of theterminal bud. Basal diameter was measured at the root collar.
The study design was a randomizedcomplete block design.Blockswere 46 m on eachside,with sevenblocks at the McNay site andeightblocksat the Stephenssite. Sixty seedlings were randomly selected and underplantedin each block of both sites, using a twoperson,gaspoweredauger.The augerbit was 20.5 cm in diameter.The seedlingswereplantedin six rows,with ten seedlingsperrow at a spacingof 1.2 m betweenseedlings and 3 m between rows.
White tree sheltersthat were 120 cm tall and approximately 8-10 cm in diameterwererandomlyassignedto the 104
NJAF16(2)1999
first five seedlingsor the secondfive seedlingsin each row. The shelterswereplacedon the seedlingsat the time of planting and were placedaccordingto the directions providedwith the shelters.Tubex© (Treessentials,St. Paul, MN) brandof tree shelterwaschosenon the basisof its availability,but doesnot imply endorsement of this brandby the authors. Data werecollectedeachfall. In orderto measureheight andbasaldiameterat ground-line,theshelterswereraisedup off theseedlings. If theseedlingwastallerthantheshelter,the shelterwas not raised to measureheight, but was raised slightlyto measurebasaldiameter.After the seedlingwas measured,the shelterwasplacedbackon the seedling. Damageto seedlings by browsewasalsomonitoredand wasnot a significantfactorin the study,with anybrowsed seedling beingremovedfromheightdataanalyses. No more than1% of the seedlings werebrowsedduringthe study. Size, growth,and mortality data were analyzedusing theGeneralLinearModel Analysisof VarianceProcedure (SAS InstituteInc. 1988). The GLM procedurewasused sincetreatmentshave unequalcell size. Results
Height
Annual height growth for nonshelteredseedlingsremained fairly constantover the 5 growing seasonsat the
McNayandStephens sites(Figure1A-B). Sheltered seedlingshadincreased heightgrowthduringthefirst 3 growing seasons, reachingtheirpeakin annualheightgrowth by the end of the third growingseason(Figure 1A-B). Shelteredseedlingsgrew 11 cm morein heightduringthe thirdgrowingseason, onaverage,thannonsheltered seedlingsat theMcNay site(P >f= 0.01) and16.4cmmorein height,on average,than nonsheltered seedlingsat the Stephenssite (P > f = 0.01). Annualheightgrowthof shelteredseedlingsdeclinedduring the last 2 growing seasons, to anannualheightgrowthsimilarto nonsheltered seedlings(Figure 1A-B). Because of theincreased growthin thefirst3 yr, sheltered seedlings weresignificantlytaller thannonsheltered seedlingsby theendof thesecond growingseason ontheMcNay site (Figure2A) with a similarpatternoccurringon the Stephens site (Figure2B). Shelteredseedlings remained significantly tallerthannonsheltered seedlings throughthe fifth growingseason, but the heightdifferencedecreased throughthisperiod.At theendof thefifth growingseason, shelteredandnonsheltered seedlingswereone-thirdto twothirdsthe heightof the 120 cm shelter(Figure2A-B). Basal Diameter
Differencesin basaldiametergrowthbetweensheltered andnonsheltered seedlings werenotstatistically significant overthe5 growingseasons for theMcNay andStephens sites (FigurelC-D). By the end of the fifth growingseason, nonsheltered andshelteredseedlingsgrew0.1 cm and0.07 cmin basaldiameter,respectively, at theMcNaysite.At the Stephens site,basaldiameterof nonsheltered andsheltered seedlings increased by 0.08 cm and0.07 cm respectively.
4O
McNay
A
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9
20
•o
60
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40
McNay
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0
4o
Stephens
Stephens
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:
80
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•
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._•
E
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._•
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-20
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•E•0.2McNay • 0.1 .-
McNay
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2nd
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• 0.2 Stephens
o
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non-shelter 0 5th
initial
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1991
1992
1993
1994
1995
planting growing season
grow ingseason
Figure 1. Mean annual height growth and basal diameter growth for underplanted, sheltered and nonsheltered red oak seedlings at the McNay and Stephens sites. Growth is based on seedlings alive at the end of the fifth growing season.
Mortality
Within thefirst 2 growingseasons, red oakseedlingshad significantdifferencesin total mortalitybetweensheltered andnonsheltered seedlings, withnonsheltered seedlings having greatertotal mortalitythan shelteredseedlingsat the McNay (P >f= 0.01) (Figure2C) andStephens (P >f= 0.01) (Figure2D) sites.By the end of the fifth growingseason, shelteredseedlings hadsignificantlygreatertotal mortality thanthenonsheltered seedlingsat theMcNay (P >f= 0.01) (Figure2C) andStephens(P >f= 0.01) (Figure2D) sites. Total mortalityof shelteredseedlings at theendof thefifth growingseason was40 and55%,whereas thatofnonsheltered seedlingswas26 and28% at theMcNay andStephenssites, respectively. Discussion
In thisstudy,treeshelters reducedbothheightgrowthand survivalwhen usedto protectunderplanted,1-0, red oak seedlings. Thereductionin growthandsurvivalof sheltered seedlingsoccurredafter an initial periodof rapid growth duringthe first 3 growingseasons. The rapid growthis
Figure 2.
Mean height (cm) and mean mortality (%) for
underplanted,sheltered,and nonshelteredred oak seedlingsat the McNayandStephenssites. Heightisbasedon seedlingselive at the end of the fifth growing season.
probablyrelatedto theimprovedmicroenvironment with in the shelter(Lantagneet al. 1990, ZastrowandMarty 1991, and Minter et al. 1992).
The reductionin growthandincreasedmortalityfor shelteredseedlingsin this experimentcouldbe the resultsof unbalancedshoot-rootratio, competitionfor nutrientsand otherresources, netlossof dailycarbongain,or a combination of these factors.
Zaczek (1994) and Burgeret al. (1996) showsheltered seedlings haverapidheightgrowthinitially,whichcreatesan unbalanced shoot-root ratio.Thisinitialrapidheightgrowth for shelteredseedlingscomesat the expenseof the root system, whichprobably resultsin lessheightgrowthin later years,asexperienced bythesheltered seedlings afterthethird growingseasonin thisstudy. By theendof thethirdgrowingseason, competition levels wouldbehigherin thestandmakingit difficultfor seedlings to competefor necessary nutrientsandlight. This stress, whichis typicalin thefield,cancauseredoakseedlings to stoptop growthand allocatephotosynthate to root growth andstorage(Dickson1991).If sheltered seedlings haveless developed rootsystems thannonsheltered seedlings by the NJAF16(2)1999 105
endof thethirdgrowingseason, sheltered seedlings couldbe experiencing greaterstressin the field, thereforeallocating morephotosynthate to root growthandstoragewhencomparedto nonsheltered seedlings. This patternof photosynthateallocationcouldexplainthe declinein heightgrowth followingthethirdgrowingseasonfor sheltered seedlings. Hansonet al. (1987) foundthat if light levels were not sufficientfor the red oak seedlingsto fix enoughcarbon dioxideto offsetmaintenance respiration,mortalitywouldbe inevitable. Combining thelackof lightcausedbyunderstory and overstoryvegetationwith the crown of the sheltered seedlingbeingconstrictedby the small diametershelter, allowinglessleafareato beexposed forcapturing photosyntheticradiation,as well as increasingself-shading,could createconditions in whichtheseedlings cannotfix enough carbondioxideto offsetmaintenance respiration. Thiscould lead to an increasein mortalityfor shelteredseedlings.A nonsheltered seedling,eventhoughundersimilarfield conditions, does not have a similar microenvironment as that
createdby a shelter.Thisdifferencein themicroenvironment
couldaccountfor the nonsheltered seedlingfixing enough carbondioxideto maintainheightgrowthandhavegreater survivalthana shelteredseedling. For short-termstudiesin which protectedseedlingsare underplanted, theseedlings probably arerelyingontheirstored carbohydrates to support theirgrowthandmaintaintheirexistence.Theotherpossibility isthatthesheltered seedlings inthese short-term studies havenotyetbecome enough outof balance in the shoot-rootratio to becomefatal. Thus, sheltersmay be beneficial underspecific conditions in whichseedlings receive enough lightandotherresources needed forseedlings to grow outof thetopof theshelterwithin3 yr. The useof sheltersby themselves is in no way a solution to establishing red oak regeneration in underplanted situations.Soundmanagement andtheuseof largeplantingstock (Johnsonet al. 1986, Schultz and Thompson1991, and Teclaw andIsebrands1991) are all part of successfully reestablishing red oakregeneration. Somepossibilities when workingwith sheltersthat werenot exploredin thisproject would be to remove the shelters at the end of the second
growingseason for thoseseedlings thathavenotreachedthe topof theshelter,orto uselargerdiametershelters. Removingtheshelterswouldallowpreviouslyshelteredseedlings to fully exposetheircrownsto incomingphotosynthetic active radiation.Placingshelterson seedlingsfor only 2 yr might boostinitialseedlingsurvivalandheight.If deerbrowseisthe limitingfactorin establishing the seedling(Walters1993), thenthismaynotworksinceseedlings in thisstudyafter5 growingseasons wereonly40-60 cm tall. The secondpossibility,usinglargerdiameter shelters (e.g.,20 cm,Teclawand Isebrands1991),wouldallowseedlings to developcrowns thatwouldhavemoreleafareaexposed andlessself-shading. Conclusion
In this study,tree sheltersreducedheight growthand survivalof underplanted, 1-0, red oak seedlingsafter an initial period of rapid growthduring the first 3 growing 106 NJAF16(2)1999
seasons. By theendof thefifth growingseason,shelteredand nonsheltered seedlings wereone-thirdtotwo-thirds theheight of the 120 cm shelters,with total mortalitysignificantly greaterfor shelteredseedlings. Thereductionin growthand survivalof sheltered seedlings in thisexperiment couldbethe resultsof unbalanced shoot-root ratio,netlossof dailycarbon gain, competitionfor nutrientsand otherresources,or a combinationof thesefactors.Tree sheltersmay still be a viable alternativein clearcutsor plantations,but further researchis neededif treesheltersareto be usedin protecting andestablishing underplanted, 1-0, red oakseedlings. Literature
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