Colonization of Stripped Branch Surfaces by Epiphytes in a Lower Montane Cloud Forest, Monteverde, Costa Rica Author(s): Nalini M. Nadkarni Source: Biotropica, Vol. 32, No. 2 (Jun., 2000), pp. 358-363 Published by: The Association for Tropical Biology and Conservation Stable URL: http://www.jstor.org/stable/2663865 Accessed: 04-05-2015 04:25 UTC

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SHARP, R. E., AND J. S. BOYER. 1986. Photosynthesisat low waterpotentials in sunflower:lack of photoinhibitory effects. Physiol.82: 90-95. ToMA,T., P. MATIUS, Y. HASTANIAH, KIYONO, R. WATANABE, AND Y. OKIMORI. 2000. Dynamicsof burnedlowland dipterocarpforest stands in Bukit Soeharto,East Kalimantan.In E. Guhardja,M. Fatawi,M. Sutisna,T. Mori, and S. Ohta (Eds.). EcologicalStudies 140, Rainforestecosystems of East Kalimantan:El Ninio,drought, fireand human impacts,pp. 107-119. Springer-Verlag,Tokyo, Tokyo, Japan. VON CAEMMERER, S, AND G. D. FARQUHAR. 1981. Some relationshipsbetween the biochemistryof photosynthesisand the gas exchangeof leaves.Planta 153: 376-387. Vos, J., AND P. J. OYARZUN. 1987. Photosynthesisand stomatalconductance of potato leaves-effectsof leaf age, irradiance,and leafwater potential. Photosynth. Res. 11: 253-264. WONG, S.-C., I. R. COWAN, AND G. D. FARQUHAR. 1985. Leaf conductancein relationto rateof CO2 assimilation. III. Influencesof waterstress and photoinhibition.Plant Physiol.78: 830-834.

Atsushi Ishida Forest EnvironmentDivision Forestryand Forest Products Research Institute(FFPRI) P.O. Box 16, Tsukuba NorinDanchi, Ibaraki305-8687, Japan. Takeshi Toma Tropical Rain Forest Research Center MulawarmanUniversity P.O. Box 1165, Samarinda 75117, East Kalimantan,Indonesia. Shigeta Mori Tohoku Research Center Forestryand Forest Products Research Institute(FFPRI) Morioka,Iwate 020-0123, Japan and Marjenah Tropical Rain Forest Research Center MulawarmanUniversity P.O. Box 1165, Samarinda 75117, East Kalimantan,Indonesia

BIOTROPICA 32(2): 358-363 2000

Colonizationof StrippedBranch Surfaces by Epiphytesin a Lower MontaneCloud Forest,Monteverde, Costa Rica1

Keywords: cloudforest;colonization; Costa Rica; epiphyte;Monteverde; secondary succession.

EPIPHYTES GROW ON SUBSTRATES THAT ARE DIVERSE AND EPHEMERAL. Canopy-dwellingplant communities are subject to disturbancessuch as wind, animal movements,waterlogging and subsequentsloughing, branchbreakage, and treefalls.These perturbationsresult in theirultimate death and decompositionon the forestfloor (Matelson et al. 1993). The rate of loss and replacementfor epiphyte communities in forestcanopies is unknownbecause the lengthof most studieshas been shortrelative to the timescale needed to monitorepiphyte colonization and growth. Some forestmanagement practices (e.g., epiphytic-moss harvesting for the floraltrade in the Pacific Northwestof NorthAmerica) and researchactivities (e.g., climbinginto treesand knockingepiphytes off) cause apparentshort- and long-termchanges in epiphytecommunities. Although moss collectors and researchersoften try to minimizedamage to the communitiesthey are harvestingor studying,the potentialfor their activities to affectcanopy plant communitieshas not been investigatedquantitatively. Littlehas been documentedon the amountof time and conditionsrequired for epiphyte communities to regeneratefollowing disturbances. Understanding epiphyte community dynamics under natural con-

1 Received8 July1998; revisionaccepted 1 February1999.

This content downloaded from 155.97.178.73 on Mon, 04 May 2015 04:25:12 UTC All use subject to JSTOR Terms and Conditions Notes 359 ditionsmay help mitigatepotential negative effects of humanactivities and contributeto our understand- ing of community succession. Althoughthe communitycomposition and abundanceof epiphytecommunities at singlepoints in time have been documentedin numeroustropical and temperateforests (Oliver 1930, McCarthyet al. 1986, Peck et al. 1995), the dynamicsof epiphytecommunities has been studiedonly rarelyand indirectly.Successional patterns of epiphytecommunities in temperateforests have been inferredby usingchronosequences of foreststands (Dudgeon 1923), toposequencesof treebranches (Stone 1989), or by aging the underlyingbranches in temperateforests through counting annual rings(Pike 1978). The generalpattern of epiphytesuccession on temperatetrees is thatearly colonizers are poikilohydric mosses and crustoseor fruticoselichens. These die in place and a layerof dead organicmatter accu- mulates,which serves as substratefor the fewvascular epiphytes that occur in temperateforests (Benz- ing 1995). The dynamicsof epiphytecommunities in tropicalforest ecosystems, however, is largelyunknown due to thehigh diversity of epiphytesand theinability to determinereliably the age ofepiphyte substrates. Studieshave been based eitheron communitiesexisting at a singlepoint in time (Yeaton& Gladstone 1982) or anecdotaldata. Observationsfrom tropical dry forests, for example, indicate that lichen colo- nizationof verticaltree trunks is extremelyslow, with areasscrubbed clean remainingvirtually bare 15 yearslater (D. Janzen,pers. comm.). In vegetationsuch as tropicalmontane cloud forests,in which epiphytesare highlydiverse and abundant,recovery from small-scale disturbance would be expectedto be veryrapid. With nearlyevery availablebranch and bole coveredwith diverseepiphytes, one would expectthat canopy communities on exposedbranch surfaces resulting from within-canopy disturbances in theseforests would be recolo- nized withina veryshort period. In this note, I reporton a decade of monitoringthe amount,com- position,and locationof epiphytecover following experimental stripping of largebranches on mature treesin Monteverde,Costa Rica. This is partof a landscape-levelstudy on theecological roles of epiphytes in tropicalcloud forests(Nadkarni et al. 1995, 2000). Fieldworkwas conductedfrom May 1987 to March 1997 in the MonteverdeCloud ForestReserve, a lowermontane moist forest in Costa Rica (10?18'N, 84?48'W). Annual grossprecipitation is ca 2200 mm, but actual wet depositionis much higherbecause of frequentwind-driven mist and fog (Clark 1994). The studyarea is in tropicallower montane wet forest(1550 m elev.) in LeewardCove Forest (Lawton & Dryer 1980), with a brokencanopy 12-25 m in heightand a densityof ca 150 trees/ha (>10 cm DBH; Nadkarniet al. 1995). The Monteverdeepiphyte flora is extremelydiverse and abundant (Nadkarni 1984, Ingram & Nadkarni 1993). Branch surfacesin the crown interiorof nearlyall maturetrees support epiphytes (bryophytes,herbs, woody shrubs,and hemi-epiphytes)in interwovenroot-humus mats up to 25 cm thick.The greatestaccumulations of humus are foundon junctionsof largebranches. Outer branches and branchtips are partiallyto completelycovered with bryophytesand small xerophyticherbaceous ,with verylittle or no accumulatedhumus (Nadkarniet al. 2000). Relativeto the forestfloor, the upper treecanopy experiencesmore wind (Lawton 1982), more frequentmist deposition (Clark, Nadkarni,Schaeffer, and Ghulz 1998), higherair temperaturemaxima, and more frequentwetting/ dryingcycles (Bohlman et al. 1995). At thissite, epiphytes fall from the canopythroughout the yearat a mean rateof 50-g drywt/m2/ yr in a varietyof forms,from individual plant partsto largemats thatcover whole branches(Nadkarni & Matelson 1992). When climbingin the canopy,one frequentlyencounters patches of exposedbark resultingfrom individual plants and whole mats sloughingoff branches. These "epislides"are apparent in all areasof the canopy.Exposed areason branchtips and branchlocations in the outercanopy appear to be relatedto bird foragingand movement;exposed areas on innerbranch locations appear to be relatedto large,mature epiphyte mats fallingoff due to theirown weight,or due to large arboreal mammalsunintentionally knocking them off (e.g., howler monkeys, tayras) or pullingthem off as they foragefor invertebrates in bromeliads(white-faced capuchins). In April 1987, my researchteam establisheda 4-ha intensiveresearch plot. We riggedca 35 trees fromthe most common familiesof treesin this forestwith mountain-climbingequipment, following Perry(1978). To sample the originalepiphyte community, we chose a randomsubset of all accessible branchesin fiveof the treeswe climbed (Table 1). All live and dead epiphyticmaterial was cut and

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TABLE 1. Identityof host tree and numberof cylindrats sampledfor which original and recolonizingepiphyte communities weremonitored from 1987 to 1997 in Monteverde,Costa Rica. NS = notsampled due to treeor branchloss duringthe 10 yearsof the monitoring period.

Numberof Numberof cylindratsfor cylindratsfor Tree original recolonizing Number Family Genus species community community 1 tuerckheimii 9 5 2 Moraceae Ficus velutina 12 3 3 Moraceae Ficus velutina 9 NS 4 Sapotaceae Pouteriaviride 12 5 5 Lauraceae Beilschmediapendula 12 NS

removedfrom cylindrats ca 75 cm in lengtharound innerbranches (within 5 m of the centraltrunk). Cylindratswere marked with small aluminumtags tied to the distaland proximalends of the plot. The samples were separatedin the laboratoryinto threecategories (vascular plants [leaves,stems, roots], bryophytes,and dead organicmatter [DOM]), the latterof whichconsisted of interceptedhost tree and epiphytelitter, crown humus, and invertebratefrass. There was virtuallyno lichencover or any exposed bare branch.All materialswere dried at 105?C for48 h and weighed. To quantifythe colonizingepiphyte communities, I visitedall of the cylindratsand noted and photographedthe colonizationof epiphyticplants at least one time per yearfor a decade followingthe stripping.In the courseof the monitoringperiod, some of the cylindratswere lost due to branch-and treefall.In June1997, I measuredcover of colonizingepiphytes by wrapping a clearacetate sheet around the branchand thenplacing another clear acetate sheet marked with a matrixof 100 dots (10 x 10 cm) at fiverandom locations around the branch segment. I countedthe number of dots that touched epiphytic plantsand characterizedeach "hit" into one of fivecategories: bryophyte, , crustose lichen, folioselichen, and bare bark.The measurementswere talliedand convertedto percentcover. Tree and brancheffects for the originaland colonizingplant communitieswere testedwith nested ANOVA. The plots were rankedby abundance (total biomassfor original communities, total percentcover for colo- nizingcommunities) and testedfor correlation. First,I expectedthe epiphytesto growback rapidly,since thisforest was repletewith abundant and diversecanopy plantson nearlyall treesurfaces. Also, "primarysuccession" of branchsubstrates (i.e., youngbranch tips in the canopyand stemsof saplingsin the understory)rapidly accumulated epiphytes withintwo or threeyears of growth.Second, I expectedthe incoming community composition to closely reflectthe originalcommunity (i.e., the communitywould rapidlyreplace itself).Third, I expected epiphytesto colonizeprimarily through encroachment from the sides of the clearedplots (i.e., growout fromthe edges of the exposedbranch segments). These expectationswere based on the literature(e.g., Rhoades 1995) and supportedby data on colonizingepiphytic bryophytes in a 50-yr-oldtemperate forest in the PacificNorthwest, (A. Cobb and G. Ramsey,pers. comm.; N. Nadkarni,pers obs.). Our obser- vations of earlycolonization following branch-stripping in that forestindicated that colonizationof branchesis relativelyrapid (up to 6 cm of elongationin the firstyear following removal of the original community),and that the compositionof the colonizingbryophyte community is verysimilar to the originalcommunity. The sourceof nearlyall temperateforest colonization is due to encroachmentfrom the sides of clearedcylindrats. The resultsfrom Monteverde were surprising. First, the rateof colonizationwas extremelyslow. For the firstfive years after stripping, no epiphyteshad colonizedthe exposedbranch sites; cylindrats were completelybare. Second, the returningcommunities differed strikingly from the originalcommunities. Dead organicmatter, bryophytes, and vascularplants, with no barebark or lichens,dominated the original communities.Crustose and folioselichens, with very little vascular plant colonization (Fig. 1), dominated the colonizingcommunities. Third, the patternof colonizationcontrasted with that of otherstudies. Instead of encroachment fromthe sides, epiphytesin the Monteverdecylindrats colonized the branchesfrom the bottomup.

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60 S50 Qor ~40 U eo ~30 0 20 I10

- Q e c C.) o0m (S

Epiphytetype

FIGURE 1. Abundance of components for the origi- nal (ori) community in percent total biomass and that of the colonizing (recol) community in percent total cover of epiphyte communities on inner branches of mature trees in Monteverde, Costa Rica. Crlich = crustose li- chens; follich = foliose lichens; bryo = bryophytes;vaspla = vascular plants; dom = dead organic matter,including intercepted litterfall,detritus, and crown humus.

During the sixthyear, I noted that some of the brancheshad algae and bryophytesgrowing on the undersideof the branch.I also noted thatfollowing precipitation events, the bottomhalf of the branch dried out more slowlythan the top half,due to the "shading"effect of the branchitself After eight years,on some of the branches,the bryophytecover from the undersideof the branchhad coalescedon the upperpart. By the tenthyear, small seedlings of vascularplants (tiny bromeliads and succulent-leafed Peperomiaand orchidspecies) were rooted in the accumulatedsubstrate. For the originalepiphyte mats, there was a strongtree effect on totalepiphyte biomass (P < 0.002). There was a brancheffect in the originalcommunities for only one of the fivetrees (tree no. 4, Table 1). There was no treeor brancheffect on the amount of total coverfor the colonizingepiphyte com- munities(P < 0.05). There was a weak positiverelationship (r = 0.22) betweenthe abundanceof the originalcommunity (total biomass) and the colonizingepiphyte community (total percent cover). These resultsindicate that the branchsubstrate of largemature branches may be inimicalto early secondarysuccession after stripping, even thoughthe generalenvironmental conditions and lifehistories of maturecanopy communities foster luxuriant epiphyte growth. Why mightcolonization of themature cloud forestepiphytes occur so slowly,and fromthe bottomof the branchup when epiphyteson young branchtips in the canopyand on saplingson the forestfloor colonize rapidly around the entirecircum- ferenceof the stem?Because theylack organicconnection to the pool of waterand nutrientson the forestfloor, mature epiphytes may require substantial and constantmoisture on theirimmediate substrate to survive;bare barkhas verylittle water and nutrientretention capacity. Tree barkin tropicalmontane forestsis generallysmooth with few "safespots" forpropagules or rootletsto hang onto, especiallyin heavyrainstorms. The earlycolonizers, which are surface-adheringand poikilohydriccryptogams, are capableof withstandingsubstantial periods of desiccation;however, they themselves take time and require some moistureto develop into substratesuseful for colonization by others,as suggestedby theirinitial presenceon the moisterunderside of the exposed branches.These colonizersprovide a substratefor mineralaccumulation from leaf leachate, water retention, and forphysical attachment of morecomplex formsof epiphyticplants that constitute later successional communities. Why does secondarysuccession of maturebranches after stripping proceed so slowlycompared to the rapid primarysuccession that takes place at canopy-levelbranch tips and on understorysaplings? Canopy-leveltwigs are verysmall in girth;they may be able to "capture"and retainwind-borne vegetative strandsof bryophytes,like spaghettion the tinesof a fork.Also, theirdirect exposure to blowingmist, which occurs most abundantlyat the outer edge of the canopy,provides a more constantsupply of moistureand atmospheric-bornenutrients than branchesin the crowninterior and on the forestfloor,

This content downloaded from 155.97.178.73 on Mon, 04 May 2015 04:25:12 UTC All use subject to JSTOR Terms and Conditions 362 Nadkarni wheremist coalesces into larger drops and is leachedof much of its nutrientcontent (especially nitrogen; Clark,Nadkarni, and Gholz 1998; Clark,Nadkarni, Schaefer, and Gholz 1998). In the understory,saplings present small-diameter branch substrates on which,as withcanopy branch tips,strands of fallingcryptogams can be snaggedand retained.The understoryenvironment is much moisterthan thatof the upper canopy,especially in the dryseason (Bohlman et al. 1995); thuswater availabilitymay not be as limitingas it is formature branches and branchtips in the canopy.This is supportedby the absence of lichenson saplings,whereas crustose, foliose, and fruticoselichens were commonon canopybranch tips. One scenariofor the developmentof undisturbedepiphyte communities on the innerbranches of maturetrees is a sequencethat may take the generation time of the tree.Rapid colonizationof branchlets by cryptogamsthat are snaggedand retainedoccurs when the treesare understorysaplings, and during subsequentprimary succession of canopy-levelbranch tips. As the treesage, thesecolonizers grow along with the increasein branchdiameter and the concomitantchanges in lightand precipitationregimes and the increasingdepth of accumulateddead organicmatter. The high biomassand diversityof the matureepiphyte communities may be due largelyto thepositive feedback that follows their establishment; once theyare establishedthrough the canopy tip and understorysampling sequence, they continue to accumulatewater and allochthonousnutrients (Nadkarni & Matelson 1991). This sequence of autogenicsuccession, in which the biotic communityfosters the perpetuationof suitablehabitat, would be brokenwhen the positivefeedback mechanisms are cut offby removalof the biotic community(as mimickedby theseexperimental manipulations). Following disturbance on large brancheswithin the canopy,a verydifferent habitat is created-bare bark,a broad,slippery surface that is unlikelyto snag threadsof bryophytes,and one subjectto rapiddrying. Thus, "recolonization"in this case is not reallyrecolonization, but rathercolonization of a verydifferent habitat into which propagules normallyfall and germinate.The highspatial variability of within-canopysubstrates requires large num- bers of samplesto characterizethese processes. Recovery may occur only aftera verylong time,if ever. Thus, matureepiphyte communities effectively may be as old and as potentiallynonrenewable as their host trees. Once the bryophyteshave grownup and aroundthe circumferenceof the branch,however, coloni- zation of the textured,water-retentive and nutrient-richersubstrate may be much more rapid than the initialstages observed in this study.My resultsindicate that thisphenomenon must be monitoredfor longer than a decade to documentthe next stages of successionand to ultimatelyunderstand how vulnerablethese mature epiphyte communities are to disturbance. The weak positiverelationship between the relativeabundance of the originalcommunity biomass and the amountof coverof the colonizingepiphyte community indicates a faint"ecological footprint" of the originalcommunity on the colonizingepiphytes (i.e., the abundanceof the originalcommunity helps determinethe abundanceof the colonizingcommunity). One mechanismfor this pattern could be thoseoriginal communities with the greatestabundance of epiphytesproviding more available prop- agules directlyadjacent to the site of colonization.Alternatively, the patterncould be attributedto the responseof both originaland colonizingcommunities to the conditionsat a particularsite; i.e., certain branchlocations may be good sitesfor both the original and colonizingcommunities due to theirphysical characteristics(e.g., exposureto mistand wind, branchdiameter, horizontal angle) or substratecharac- teristics(e.g., rugosebark). If eitherof theseexplanations is true,the ecologicalfootprint may become morepronounced when more timeelapses. The strongbetween-tree effects observed in mature,but not colonizing,epiphyte communities support this idea. Studiesof successionin arborealplant communities relate in interestingways to studiesof succession in terrestrialplants. For example,the colonizationof landslides,which constitutes primary succession, is analogous to colonizationof a new twig or the stems of an understorysapling. The post-landslide communityundergoes succession to matureforest with a concomitantchange in environmentalcondi- tions(e.g., attenuation of lightin the understory,accumulation of soil organicmatter). If the understory is strippedor damaged (e.g., the "socolado" treatmentof tropicallowland forests),it may take decades for the understoryto regrowin the verylow lightintensity, as comparedto the rapid developmentof vegetationon the newlycleared landslide. Long-term studies on the dynamicsof epiphyticplant com- munitiesin the canopy-on the orderof severaldecades-will provideinsights on ecologicalsuccession and restorationof arborealand terrestrialroot plant communities.

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I thankthe staffof MonteverdeCloud ForestReserve, the TropicalScience Center,and the Mon- teverdecommunity for use of the studyarea and logisticalsupport. Teri Matelson,Rodrigo Solano, Ken Clark, Doug Schaefer,and JackLongino helped with the ideas in this paper and with fieldworkin Monteverde.Discussions of resultswith members of theInternational Canopy Network,particularly Dan Janzenand RichardCellarius, were useful in gatheringanecdotal information and generatinghypotheses about the results.I thanktwo anonymousreviewers and Jon E. D. Fox, who gave helpfulsuggestions on the manuscript.This researchwas fundedby ResearchGrants and ResearchExperience for Under- graduatesGrant Supplements from the National Science Foundation(BSR 86-14935, BSR 89-18006, BSR 90-18006, BIR 96-30316, and DEB 96-15341), theWhitehall Foundation, the National Geograph- ic SocietyCommittee on Researchand Exploration,the Universityof CaliforniaSanta BarbaraAcademic Senate,and the EvergreenState College FacultySponsored Research Fund.

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Nalini M. Nadkarni EvergreenState College Olympia,Washington 98505, U.S.A.

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