Influence of Overstory Composition on Understory Colonization by Native Species In

8/12/2019 Influence of Overstory Composition on Understory Colonization by Native Species In

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International Association for Vegetation Science

Influence of Overstory Composition on Understory Colonization by Native Species inPlantations on a Degraded Tropical SiteAuthor(s): John A. ParrottaSource: Journal of Vegetation Science, Vol. 6, No. 5 (Oct., 1995), pp. 627-636

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Journalof VegetationScience 6: 627-636, 1995? IAVS;OpulusPress Uppsala.Printedin Sweden 627

Influence of overstory composition on understory colonization bynative species in plantations on a degraded tropical siteParrotta, John A.

International Institute of Tropical Forestry, USDA Forest Service; P.O. Box 25,000,Rio Piedras, 00928-5000 PR, USA; Tel. +1 809 766 5335; Fax: +1 809 766 6302

Abstract. Patternsof understory olonizationby native andnaturalized rees and shrubswere evaluatedin 4.5-year-oldplantations f threeexotic treespecies,Casuarina quisetifolia,Eucalyptusrobusta, and Leucaena leucocephala, on a de-gradedcoastalgrassland itewith reference o overstorycom-positionandunderstory nvironmental onditions. 19second-ary orest pecieswereestablishedntheplantation nderstories(with a total area of 0.52 ha ), while no naturalregenerationoccurred n unplanted, hough protected,controlareas. Themajorityof these species(90 %)and the totalseedling popula-tion (97 %) were zoochorous, indicatingthe importanceoffrugivorousbats andparticularlybirds as facilitatorsof sec-ondaryforest species colonization.Understoryspecies rich-ness and seedling densities were affected significantly byoverstory omposition, he mostabundant egeneration ccur-ringbeneathLeucaenaand eastunderCasuarina.Understorycolonizationrates withinmixed-speciesstandswere interme-diatebetweenthose of single-speciesstandsof the trees com-prising heiroverstories.Significantnegativecorrelationswerefoundbetweenunderstory pecies richness andseedlingden-sity, and orest floordepthanddrymass,especiallyforsmall-seededornithochorouspecies.Highercolonizationratesnearthe peripheriesof plantationplots relative to plot interiorswere due in partto roosting site preferencesby frugivores,particularly ats.Thestudyresults ndicate hatoverstoryspecies selectioncan exert a significant influence on subsequentpatternsofcolonizationby secondary orest species and is an importantconsiderationin the design of plantationsfor 'catalyzing'successionon deforested,degraded ites.

Keywords: Biodiversity;Casuarinaequisetifolia;Eucalyp-tus robusta;Leucaena leucocephala;Naturalregeneration;PuertoRico;Restoration cology; Succession.

Nomenclature: Liogier(1982).

IntroductionIn tropical and subtropical regions where extensive

degradation of forests, rangelands, and agriculturallandsoften has severe social and economic implications, re-forestation programs designed and managed to satisfy

local needs for a diverse forest product mix are likely toassume an increasingly important role in the future(Brown & Lugo 1994). To date, tropical forestry re-search has made significant contributions towards therehabilitation of degraded tropical landscapes throughthe identification of stress-tolerant native and exotic treespecies and design of management systems for maxi-mizing plantation productivity under a wide range ofdegraded site conditions. Research on the effects ofvarious plantation species and management options onsoil improvement and nutrient cycling processes, con-ducted during the past 20 years, is likely to contributesignificantly to the development of sustainable silvi-cultural systems for degraded tropical lands.Given the high degree of dependence on a widerange of wood and non-wood forest products in manytropical countries, the traditional focus of 'wasteland'reforestation programmes on maximizing productivityof the planted crop (for fuelwood and/or timber) mayneed to be broadened if 'rehabilitation forestry' is togain public support and thus yield significant environ-mental and socio-economic benefits. Recent experi-ences in India and Southeast Asia have demonstratedthat community-based reforestation projects aimed atthe rehabilitation of species-rich secondary forests ondegraded lands can yield excellent results when tradi-tional forestry practices are adapted to facilitate regen-eration of locally valued native species (Anon. 1993;Poffenberger and McGean 1994).Recent studies in Puerto Rico (Lugo 1988, 1992;Parrotta 1992, 1993a; Lugo et al. 1993) and China(Brown & Lugo 1994) have shown that forest planta-tions established on degraded sites long devoid of nativeforest cover can act as 'successional catalysts', facilitat-ing recolonization of native flora through their influenceon understory microclimate and soil fertility, suppres-sion of dominant grasses, and provision of habitat forseed-dispersing wildlife. This 'catalytic' effect appearsto be a widespread phenomenon, supported by observa-tions and understory floristic data from plantations es-tablished under a variety of degraded site conditions,including mined lands, in India (Mathur et al. 1980;

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Parrotta, .A.Mathur& Soni 1983;Prasad&Pandey1985;Soniet al.1989; Bajaj 1990), Malaysia(Mitra& Sheldon 1993),PapuaNew Guinea D.Lamb1994,pers.comm.),Congo(H. de Foresta1994,pers.comm.),and Brazil(Goodetal. 1993;Parrotta1994pers.obs.).A numberof factorsaffectingtherate of plantationunderstorycolonizationby native (or naturalized) reeand shrub species are suggested by these and othersuccessionalstudies nplantations, bandoned astures,and minedlands in bothtropicalandtemperate egions.These factorsrelate to initial site conditions as well asplantation esignandmanagement ractices.Theformerinclude:the degreeof site degradation,particularlyheavailabilityof rootstocksand seed banks (Mathur&Soni 1983;Chaubeyet al. 1988a, b; Prasad& Pandey1989); landscape loristicsandproximity o native for-est stands, i.e., seed sources (Peterken& Game 1984;McClanahan 1986; Hardt & Forman 1989; vanRuremonde & Kalkhoven 1991; Dzwonko & Loster1992;Parrotta1993a;Dzwonko 1993);andpresenceofseed-dispersingwildlifepopulationsGuevarat al.1986,1992; McClanahan& Wolfe 1993; Mitra & Shelton1993). The latter include: tree spacing (A. Chiarucci1994,pers.comm.);plantation ge(Bhaskar&Dasappa1986;Chaubeyet al. 1988b;Lugo et al. 1993;Parrotta1993a),understorymanagementntensityanddegreeofprotectionfrom fire and other site disturbances Bajaj1990; Brown & Lugo 1994); and plantationspeciesselection, or overstorycomposition (Srivastava1986;Pande et al. 1988;Lugo 1988, 1992).The choice of species mayaffectunderstoryoloni-zation in severalways as treespecieswill differ n their'attractiveness'as roostinghabitatfor seed-dispersingbirds and bats (Debussche et al. 1982; Manders &Richardson1992;Mitra & Sheldon 1993);canopyar-chitectureand influence on understoryight, tempera-tureandhumidity egimes Richardsont al.1989;Parrot-ta 1993a);ratesof leaf litterproduction,decomposition,and litterchemistry(Suresh& VinayaRai 1988); andinfluence on soil biologicalactivityandotheraspectsofsoil fertility.At present,our understanding f how these factorsinfluence succession within plantationecosystems isvery limited. Systematicstudies arerequired o under-stand their relative importanceand implications forplantationdesign andmanagement o as to realize thepotentialof these systemsas successionalcatalysts.In this paper, the influence of overstory speciescompositionon patternsof woody tree and shrubspe-cies colonization is evaluated in young, single- andmixed-species plantations of Casuarina equisetifolia,Leucaena leucocephala and Eucalyptus robusta on aseverely degradedcoastal site in PuertoRico. Theseexotic species were chosen for studybecause of their

contrasting rowth ormsandecologicalcharacteristics,and theirextensive use in rehabilitation ndshortrota-tionplantationprogramsn the tropics.

Material and MethodsStudy area

Thestudyarea s located at theUniversityof PuertoRico's Toa Baja experimentalstation located on thenorthernAtlantic)coast of PuertoRico (18?27'N, 66?10'W). Annual precipitationaverages 160 cm and ismoderately easonal n distribution.Average dailytem-peratures angefrom 23.8 ?C in January o 29.4 ?CinAugust.The soils arewell-drained,alkaline,calcareoussands.Formerly upporting oastaldune forestvegetation,

the site has been subjectto frequentand often severedisturbancesduringthe past century,includingforestclearing, cattle grazing, cultivation, topsoil removal,andperiodicfires.Presently he surroundingandscapeis dominatedby residentialhousingor commercialandindustrialfacilities, with secondaryforest vegetationrestricted o roadsidesanda 10-haabandonedpasture,adjacentto the southernedge of the study site, sup-portingca. 20 nativeandnaturalizedreespecies.At the time of experimentalplantationestablish-ment,the studysite was dominatedby grasses,princi-pally Panicum maximum and Tricholaena repens, withca.30 speciesof herbsandvines,principallyAsteraceae,Papilionaceae and Euphorbiaceae; woody plants wereabsent(Parrotta1993b).

Experimentalplantationswere established n 1989using a randomizedblock design with threereplicate16mx 16 m plots of each of the following treatments:(1) Casuarina equisetifolia monoculture (CM);(2) Eucalyptus robusta monoculture (EM);(3) Leucaena leucocephala monoculture (LM);(4) Eucalyptus + Casuarina (EC);(5) Casuarina + Leucaena (CL);(6) Eucalyptus + Leucaena (EL); and(7) unplanted ontrol.

Theseplantationswere establishedaspartof alargerstudy oevaluatebiomassproductivity,nutrient ycling,andbiological nitrogenfixation in single- and mixed-species stands(Parrotta t al. 1994a,b). The initialtreespacingin all forest plots was 1 m x 1m; in the two-species reatments1:1mixtures), eedlingswereplantedin a checkerboardpattern.Plantationplots were sepa-rated from one anotherby buffer strips (unplantedal-leys) 5m in width,andtheunplanted ontrolplotswerelocatedoutside of the mainblock of 18 forestplots.The plantationswere neitherirrigatednor did they

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- Influenceof overstorycompositionon understory olonization-Table 1. Characteristics of the plantations'. Values following means indicate standard errors; n = 3 replicates per treatment.

Treatment2LM CM EM EL EC CL

Averagetreedensity(stems/ha)Averagestem diameter cm)Averagetreeheight(m)Above-groundbiomass(kg/m2)Fine root mass (kg/m2)Forestfloor mass (kg/m2)Forest loordepth(cm)Leafarea ndex (2.5-4.5 yr)Soil organicmatter %)

95007.08.4

7.2 1.80.25 + 0.120.73 + 0.07

1.3? 0.22.2 ?0.26.0+ 1.4

71007.79.7

10.5 + 0.40.19+0.101.05+ 0.102.5 + 0.23.1+ 0.15.5 + 1.0

480010.09.4

6.3 + 0.20.32 + 0.080.90 ? 0.03

1.5+ 0.22.6 +0.14.6 + 0.5

63507.79.8

10.0? 2.30.17 + 0.080.88 ? 0.08

1.5+0.22.1 ?0.17.3 + 1.4

440010.211.6

9.7 + 1.40.19 + 0.020.99 + 0.03

2.1 +0.52.5 + 0.18.3 + 1.2

83007.19.6

12.4+ 1.20.20 ? 0.111.08+ 0.052.4 +0.12.7 +0.15.6 + 0.6

1Plantation ge = 4 yr for treedensity,stemdiameter, reeheight, above-groundandfine rootbiomass;4.5 yrfor forestfloor mass anddepth;3 yrfor soil organicmatter.2 Treatments:LM = Leucaena leucocephala, CM = Casuarinaequisetifolia,EM = Eucalyptusrobusta,EL = Eucalyptus+ Leucaena, EC =Eucalyptus+ Casuarina,CL= Casuarina+ Leucaena.

receivefertilizeramendments.During hefirst 6 monthsafterplanting,the forest plots were manuallyhoed tocontrol weed competition.Thereafterall plantedandcontrol plots were protectedfrom understorydistur-bances,particularly razingandfire.Data collection and analysis

The floristic compositionof plantationunderstoryandcontrolplotswasmonitored1.0, 2.5, and4.5 yrafterplantation stablishment.For thesesurveyseachplanta-tionplotwas divided ntotwo zones: (1) a central11 mx 11m centralsubplot,and(2) a 168m2buffer,oredgezone, that ncludedthe threeperipheralreerows. Spe-cies richnessandpercentage over(6-pointcoverscale)were recordedfor grasses, forbs, vines, and woodyspecies in all plots. For tree and shrub species, allseedlings and saplingswere countedand theirheightsrecorded.Tree andshrub peciesencounteredn thesesurveyswereclassifiedwithrespect o theirdispersalcharacter-istics as autochorous,anemochorous,or zoochorous.Zoochorousspecies were furthercharacterizedas pri-marilybirdorbat-dispersed asedoninspectionof theirpropagulesandavailable iterature.Understory peciesrichnessandseedlingdensityfor trees and shrubswerecomparedamongtreatmentsusing a one-way analysisof variance(ANOVA), and between centraland edgezones of the plantationplots using pairedt-tests.Plantationunderstory loristic data were evaluatedthroughcorrelationandregressionanalyses,in relationto overstory composition and several structuralandenvironmental ariablesmeasured or eachplot duringthe periodfrom 2.5 to 4.5 yr afterplanting(Table 1).

These variablesncluded:otalabove-groundoverstory)biomass;fine root(


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Parrotta, J.A.Results and Discussion

Understory floristic developmentDuring the first two years, ground cover was gener-

ally very sparsein the plantationsrelative to the unplantedcontrol plots, but included most of the grass, herb, andvine species present at the site prior to plantation estab-lishment. Understory colonization by woody speciesdid not occur in any of the plantation or control treat-ments, with the exception of Leucaena leucocephala,seedlings of which were found in several of the plan-tation plots from the second year onward, the result ofnaturalregeneration by the planted Leucaena. Through-out this period the plantations were regularly visited bya number of frugivorous bird species which use theplanted trees as roosting sites, and the discarded seeds ofbat-dispersed tree species (Calophyllum brasiliense andTerminalia catappa) were first noted in the plantationalleys after ca. 2 yr.In spite of apparently regular seed inputs by roostingfrugivores, the virtual elimination of understory compe-tition by grasses, and the creation of light, temperature,and humidity conditions favorable for secondary forestspecies regeneration in the plantation understories fromthe second year onwards, it was not untiljust prior to the2.5-yr survey that such regeneration was first noted inscattered locations within the plantation area. Theseearly colonists included the two bat-dispersed speciesmentioned above, and Citharexylumfruticosum, a bird-

dispersed species that is very common in local second-ary forests. Thereafter, a rapid increase in both seedlingdensity and species richness occurred in the plantationplots. A very similar temporal pattern was earlier ob-served in Albizia lebbek plantations at this site (Parrotta1993a).

The apparent delay between the initiation of treeseed inputs and germination does not appear to be dueprimarily to physical environmental constraints (light,temperature, humidity) but rather to biotic factors re-sponsible for breaking seed dormancy and facilitatinggermination. Similar observations and analyses of soilfauna and microflora succession made in plantationsestablished on an abandoned pasture site in Costa Ricasuggest that delays in natural regeneration by nativeforest species, despite abundant seed rain, may be linkedto the rate of development of fungal and bacterialpopulations required for breaking seed dormancy (R.Fisher 1994, pers. comm.).By 4.5 yr, the plantation plots collectively supporteda fairly rich understory flora that included 19 species ofnative and naturalized trees and shrubs, representing 14families, within the 0.52-ha plot area surveyed (centralsubplots + edge zones). Most of the seedlings found inthe plantations were less than 50 cm in height, andusually less than 30cm; only a few individuals of Citha-rexylumfruticosum were between 1.5 and 2.0 m tall. Incontrast, no woody seedlings were found either in thecontrol plots (total area:0.08 ha) or elsewhere in the 0.5-ha unplanted, protected grassland surrounding the block

Table 2. Tree andshrubspeciescolonizing study plantationunderstories.Species Family Fruit ype Disperal Densityagent (No./m2)CitharexvlumruticosumCordiapolycephalaSchinusterebinthifiliusCalophyllumbrasilienseTerminaliacatappaBourreriasucculentaSpathodeacampanulataBurserasimarubaEugeniapseudopsidiumAndira inermisRoystoneaborinquenaMyrciasplendensIxoraferreaOcoteacoriaceaeRauvolfianitidaPsidiumguajavaCaseariaguianensisCedrelaodorataUnidentified axa

VerbenaceaeBoraginaceaeAnacardiaceaeHvpericaceaeCombretaceaeBoraginaceaeBignoniaceaeBurseraceaeMyrtaceaePapilionaceaeArecaceaeMyrtaceaeRubiaceaeLauraceaeApocynaceaeMyrtaceaeFlacourtiaceaeMeliaceae

FleshydrupeFleshydrupeFleshy drupeLargedrupeLargedrupeFleshydrupeDehiscentcapsuleDrupelikecapsuleBerryLarge drupeSlightly fleshy berryFleshyberryFleshyberryFleshyberryFleshy drupeFleshy berryDehiscentcapsuleDehiscentcapsule

Total 20.61

BirdsBirdsBirdsBatsBatsBirdsWindBirdsBirdsBatsBirdsBirdsBirdsBirdsBirdsBirdsBirdsWind

7.054.163.233.030.750.750.530.180.170.130.100.090.080.080.080.070.030.030.07

630

Total 20.61



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- Influence of overstory composition on understory colonization -

of plantation plots.Four secondary forest species: Calophyllum brasili-ense, Citharexylumfruticosum, Cordiapolycephala andSchinus terebinthifolius, comprised 85 % of the totalnumberof woody seedlings in the plantation understories(Table 2). A survey of the local tree flora indicated thatmost of the species encountered as colonists in theplantations could have been derived from parent treeslocated between 50 and 300 m from the plantation.The majority (68 %) of the tree and shrub speciesregenerating within the plantation area are bird-dis-persed, their fruits being fleshy drupes or berries con-taining seeds that are passed through the digestive tract.Three additional species - Andira inermis, Calophyllumbrasiliense, and Terminalia catappa, produce large-seeded drupes dispersed by bats, which carry the fruitsto their roosting sites, consume the fleshy outer layers,and drop the uningested seeds. At this site, seeds andseedlings of these chiropterochorous species display atypically clustered distribution, indicating a preferencefor certain trees as bat roosting sites, usually near theplot edges.In all, 84 % of the species and 97 % of the totalnumber of seedlings colonizing the plantation under-stories are zoochorous. As in other moist tropical forestenvironments, zoochory is the primary diaspore disper-sal mechanism for the vast majority of tree and shrubspecies occurring naturally in the vicinity of this site.The roosting sites provided by the plantation clearlyincreased the seed rain to the plantation understories

relative to the surrounding unplanted grassland areas.The importance of roosting sites for seed inputs intoopen habitats such as grasslands and mined lands hasbeen demonstrated in several recent studies (Guevaraet al. 1986, 1992; McClanahan & Wolfe 1987, 1993).

Only two anemochorous species were found in theplantations: Cedrela odorata and Spathodea campanu-lata. Their presence in the plantation understory, andabsence in the control plots, into which seeds of thesespecies presumably are also dispersed, suggest that fac-tors in addition to seed rain account for the lack ofregeneration of woody species in the grassland sur-rounding the plantation. At this site, and elsewhereunder similar conditions, these regeneration-limitingfactors might include competition with grasses and un-suitable microclimatic conditions (Nepstad et al. 1991;Guevara et al. 1992; Parrotta 1992, 1993a; McClanahan& Wolfe 1993).Plantation overstory and environmental effects

Plantation overstory composition appears to haveexerted a considerable influence over both understoryspecies richness and colonization rates by secondaryforest species, i.e., seedling density. Significant treat-ment effects were found with respect to the total numberof woody plant species in the plantation understories (P< 0.05, ANOVA), with mean species richness rangingfrom 3.0 + 0 to 7.7 ? 1.5 species/plot (289 m2) in theCasuarina monoculture (CM) and the Eucalyptus +

Table 3. Understory species richness and seedling density for tree and shrub species in plantation understories and control plots 4.5yr after plantation establishment. Values following means indicate standard error; n = 3 plots per treatment.1

PrimarydiasporedispersalagentTreatment2 Birds Bats Wind All speciesSpecies richness (No. species/plot3)LMCMEMELECCLControlSeedling density (No./m2)LMCMEMELECCLControl

5.3 + 1.3bc2.0 ? 0.6b4.3 + 0.3bc6.0 + 1.5c2.7 0.7bc2.7 + 0.7bC

Oa0.271 + 0.045C0.024 0.019ab0.036 + 0.012ab0.103 ? 0.035b0.024 + 0.005a0.031 0.007a0a

1.0 0.7abc0.7 ? 0.7abc1.3 0.7abc1.7 + 0.7bc0.7 + 0.3b2.3 + 0.3c0a

0.023 + 0.021ab0.006 + 0.006a0.024 ? 0.018ab0.029 + 0.01 ab0.004 + 0.002a0.043 + 0.008b0a

0.3+ 0.3a0.3 + 0.3a1.0 0bOa1.3 + 0.9abOa0a

0.003 + 0.003ab0.001 + 0.001a0.012 + 0.005b0a0.006 + 0.004ab0a0a

6.7 ? 2.2bc3.0 ? 0 b6.7 + 0.9bc7.7 + 1.5c4.7 + 1.7bc5.0 ? 0.6bc0a

0.314 + 0.026d0.031+ 0.016ab0.072 + 0.029abc0.128 + 0.046c0.034 + 0.009ab0.074 + 0.005bc0a1Similar uperscriptswithina column ndicatemeansthatwere similarwithin thesame seeddispersalmodeclass (P < 0.05, ANOVA). 2 Treatments:LM = Leucaena leucocephala,CM = Casuarinaequisetifolia,EM = Eucalyptusrobusta,EL = Eucalyptus+ Leucaena, EC = Eucalyptus+Casuarina,CL = Casuarina+ Leucaena.3Plot area= 289 m2.

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Parrotta, .A.

* Species with bird-dispersed eedsSpecies withbat-dispersed eeds

1 Species withwind-dispersed eeds

..... *.... Ii _gfl

LM CM EM EL EC CL ControlTreatment

* Species with bird-dispersedeeds1 Species withbat-dispersedeeds* Species withwind-dispersed eeds

LM CMLM CM EM EL EC CL ControlTreatment

Fig. 1. Understory colonization by native andnaturalized secondary forest tree and shrub spe-cies 4.5 yr after plantation establishment; treat-ment averages. a. Species richness. b. Seedlingdensity. Treatments: LM = Leucaena leuco-cephala,CM = Casuarinaequisetifolia,EM =Eucalyptus obusta,EL= Eucalyptus Leucaena,EC = Eucalyptus+ Casuarina,CL= Casuarina+ Leucaena.

Leucaena(EL) treatments, espectively(Table3).Similarbut somewhatmorepronouncedrendswerefoundwith respectto understory eedling density (P,

't 4.0

Z 2.0

0.0

0.30E 0.O25c,

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t)?o 0.05

0.00

632

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- Influenceof overstorycompositionon understory olonization-

Ewa,am.sv,ucliCLmC/)>10vl

0.400.350.300.250.200.150.100.05

* .U.U 0.0 0.50 1.0 1.5 2.0Forest loorDepthcm)

2.5 3.0

Fig. 2. Relationbetweenunderstorypeciesrichness or colo-nizing reesand hrubs nd orest loordepth t 4.5yr.y = 10.1- 2.4x (r2= 0.34;F = 8.17;P < 0.05).E6

CI

lucerLCl

while those with higherlitter accumulationrates weretheEC, CL, and CM treatments Table 1).Thus the differences in understorycolonizationamong reatments ppearobe stronglyrelated o differ-ences in forest floor depth and dry mass. Within therangesof litterdepth(0.8 - 2.8 cm) anddrymass (600 -1250g/m2)recordedat this site, the data suggest thatincreased litter accumulationacts as a progressivelysevere barrier o regenerationby small-seeded (bird-dispersed) reeand shrubspecies.This does not appearto be the case for large-seeded(bat-dispersed) pecies.Understory olonizationby thesespecieswas notinflu-enced by overstory compositionas noted earlier,norwere thereany significant rendsbetweenseedlingden-sities and litterdepthordrymass for thesetaxa.Appar-ently, the food reservespresentin the seeds of thesespeciesaresufficiently argeto supportdevelopmentofrootsystemsthatcanpenetratehelitter ayerand reachthe mineralsoil before they areexhausted.There wereno indicationsthat leaf litter chemistry,which variesgreatlyamongthe threeoverstoryspecies in this study(andstrongly nfluenceslitterdecompositionrates;au-thor, unpubl.),is influencingunderstoryregenerationthroughallelopathic nterference, shasbeensuggestedby some studiesfor LeucaenaleucocephalaandEuca-lyptusspp.(cf. Suresh& VinayaRai 1988).Amongthe otherplantation haracteristics ndenvi-ronmental actorsanalyzed,none weresignificantlycor-relatedwith eitherunderstory pecies richnessor seed-ling density(P< 0.05, ANOVA). A suggestive,thoughnot significant(P = 0.055, ANOVA), positive correla-tion was found betweenspeciesrichnessandoverstoryleaf area ndexof theplantationplots averagedover 12measurement atesbetween2.5 and4.5 yr.This trend s

0.0 0.50 1.0 1.5 2.0LitterDepth cm)

2.5 3.0

0.400.350.300.25:0.200.150.100.05 * 5

,, , .*, 1,600 700 800 900 1000 1100 1200 1300

LitterMass (g/m2)Fig. 3. Relation betweenunderstory eedling densityfor sec-ondaryforest trees and shrubs and measuresof forest flooraccretionat4.5 yr.a. Litterdepth:y = 0.19 -0.34 logIox(r2= 0.21; F= 4.36; P < 0.05).b.Litterdrymass:y= 2.28-0.73 logox (r2=0.28;F=5.52; P,

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Parrotta, .A.0.15

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interior edge interior edge interior edge interior edgeBirds Bats Wind AllSpeciesSeed Dispersal Mode

clearlymore attractiveas roostingsites for frugivorousbats thattransport arge-seededdrupes nto the experi-mental area. This is supportedby observationsof largenumbersof discarded eedsalongtheplot edgesrelativeto the centralsubplots.A similarpreferenceby frugi-vorous birds for trees, i.e., roosting sites, along plotborderrows is also apparent, lthoughdataon seed rainfor bird-dispersed pecies are not available to supportthishypothesis.Treesgrowing along most of theedgesof theplantationplots are,in general,larger hanthosewithin the centralsubplotsand may thereforeprovidemore accessible perches and/or greatervisibility (orecholocationpotential, n the case of bats)for roostingfrugivores.Implications for restoration forestry

The resultsof thisstudyhaveimportantmplicationsfor the design of plantations that can be used for'catalyzing'natural orest successionalprocesses,par-ticularlyon sites wheresoil seed banksandrootstocksofnative forestspeciesare ackingdue to theirdisturbancehistories. The choice of species that will initiallyformtheoverstoryappears ohave,atleastintheearly stagesof development,a significanteffect on the numbersofpropagulesanddiversityof zoochorouswoody speciescolonizingtheplantationunderstories.Tree species that form a thin canopy andproducefine, readilydecomposed eaf litter similar o Leucaenaappearvery well suitedfor this purpose.Such speciescan create a light- to moderately-shadedunderstoryenvironment hateffectively suppressesthe growthofgrasseswhile favoringthe germinationof manyearly-

Fig. 4. Comparison of understoryseedling densities (colonizing treesandshrubs)n plantationnteriorandedge zones at 4.5 yr. Statistical om-parisons of differences betweenmeans(paired -tests)based on com-bined data from all plantationplots:*** = P < 0.001; ns = notsignificant.

andmid-successional ropical orest treespecies.Theirrapid eaf turnoverand decompositionof nutrient-richlittercan result in significant mprovements n soil fer-tility as well as facilitate tree seed germinationandseedling development.Due to its high reproductive o-tential andcompetitiveadvantageover nativespecies,however, Leucaena leucocephala is not recommendedfor forestrestoration urposeson most sites.Rather,ess'weedy' species with similar architectural raits andlittercharacteristics hatregenerate ndcompetepoorlybeneath their own overstory or in surrounding,dis-turbed,habitatswould bepreferred.Therearenumeroussuchspecies,especially amongthe mimosoidlegumes,such as Albizia lebbek. In contrast, species such asCasuarina equisetifolia that tend to form dense crownscastingdeep shadein the understory ndproduce itterthat decomposes relatively slowly, and thus create aphysical barrier o regenerationby small-seeded sec-ondary orestspecies, are less likely to yield favorableresults.The importanceof frugivorousbirds and bats n thecolonizationof theplantation nderstories y secondaryforest tree and shrub species is also clear from thepresentstudy.The behavioralecology of these criticalseed dispersalagents in forest plantation ecosystemsmerits furtherstudy to better explain the understorycolonizationpatterns bservedat thissite and oprovideinsightsuseful for the design of rehabilitationplanta-tions that will attractseed-dispersingwildlife so as toaccelerate he rateof introduction f zoochorouspropa-gules and increaseunderstory pecies biodiversity.

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Acknowledgements. This work was conducted in coopera-tion with the Universityof PuertoRico (UPR)andsupportedin partby a grant o the UPRAgriculturalExperimentStationfrom the USDA under the Cooperative ForestryResearch(McIntire-Stennis)Act (projectPR-MS-8). Leaf area dataused in this studywereprovidedby Y. Castroand N. Fetcherundergrantsfrom the NationalAtmosphericand Space Ad-ministrationgrantNAG13-18)and he NationalScience Foun-dation(grantRII-8903827)to theUniversityof PuertoRico.

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Received 30 September1994;Revision received 20 February1995;Accepted9 March1995.

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