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Large Trees, Fertile Islands, and Birds in Arid Savanna

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Large Trees, Fertile Islands, and Birds in Arid Savanna Journal of Arid Environments ()1999 41:61]78 Article No. jare.1998.0455 Available online at http:rrwww.idealibrary.com on Large trees, fertile islands, and birds in arid savanna W. R. J. Deanw, S. J. Miltonw & F. Jeltsch² *Percy Fitzpatrick Institute of African Ornithology, University of Cape Town, Rondebosch, 7700 South Africa ²Department of Ecological Modelling, UFZ-Centre for Environmental Research, Leipzig, D-04301 Germany ()Received 13 March 1998, accepted 24 August 1998 Large Acacia erioloba trees scattered through the sparse grassy vegetation of arid oligotrophic savanna are focal points for animal activity because they supply nest sites, shade and scarce food resources. Faeces, fallen nest material and carcass remains left below trees elevate levels of nutrients available to plants in the soil beneath large trees. Soil concentrations of N and K were two times greater, and P concentrations 2. 5 times greater under canopies of A. erioloba trees than in surrounding grassy shrubland. Plant species with fleshy fruits ()Boscia, Grewia, Lycium and Solanum spp. occurred in 8% of treeless plots and beneath 17% of A. erioloba saplings, but their frequency increased to 90% beneath large trees. Dead A. erioloba trees were replaced by matrix vegetation() 54% and large shrubs with fleshy fruits ()28% rather than by conspecifics () 17% . The distribution of fleshy-fruited plants in the Kalahari is thus dynamic and tied to the distribution of large trees such as Acacia erioloba. The shade beneath the canopies of large spreading trees was used by birds ()(particularly Kori Bustards Ardeotis kori and mammals mainly Springbok Antidorcas marsupialis, Gemsbok Oryx gazella, Blue Wildebeest Connochaetes taurinus and Bat-eared Fox Otocyon megalotis) as a resting place during the heat of the day. Large raptors ())1. 5 kg and vultures ())5 kg seldom perched on saplings, and more frugivorous bird species and individuals were seen on mature than sapling or dead trees. Nests of raptors, and the large communal nests of Sociable Weavers Philetarius socius were found mainly in large trees, and the nests of the Tree Rat Thallomys paedulcus were found in cavities in the stems of large trees. Acacia erioloba and the only other large tree, Acacia haematoxylon, apparently structure plant and animal communities and determine pattern and patch dynamics in this arid savanna. We suggest that their role in maintaining biodiversity in the Kalahari cannot be served by the homogeneous thickets of stunted acacias that develop where the vegetation is overgrazed. q 1999 Academic Press Keywords: Acacia erioloba; Kalahari; fleshy fruits; patch dynamics; seed dispersal; raptors 0140]1963r99r010061 q 18 $30.00r0 q 1999 Academic Press 62 W.R.J.DEANETAL. Figure1.GemsbokOryxgazellarestingintheshadeofalargeAcaciaeriolobatreeduringthe heatoftheday.Theincreaseddensityofvegetationlimitedtotheareaaroundthetreecanbe clearlyseen. Introduction Patchynutrientaccumulationsinoligotrophicsavannastendtobemaintainedand reinforcedbecausetheirenhancedproductivityattractsanimalswhichimportadditional nutrients()Scholes,1990.Largeisolatedtreesincreasesoilfertilitybeneaththeir canopiesandarethefociforanimalactivity(Belsky&Canham,1994 )(Figs1&2). AvifaunalspeciesrichnessiscorrelatedwithplantheightdiversityintheKalahari ()Pianka&Huey,1971,beingdependentonhorizontalandverticalstructuringofthe habitatbyscatteredtrees. InthearidsavannasoftheKalahariDesertofsouthernAfrica,whereradiationlevels arehighandthesandysoilsarepoorincations()Leistner,1967,shadedfertileislands beneathtreesarelikelytosupportdistinctiveplantassemblages(Belsky&Canham, 1994).Plantspecieswithahighprobabilityofreachingsubcanopysitesarebird-and mammal-dispersedspecieswithbarbedorediblefruits()Leistner,1967.Shadeand enhancedsoilnutrientlevelsimproveseedlingsurvivalofsomefleshy-fruitedtrees ()Manders&Richardson,1992.Moreover,distributionoffleshy-fruitedplantsis thoughttobecorrelatedwithsoilfertility,morespecificallywiththeavailabilityof potassium,andtobecorrelatedwithsoilfertility,ormorespecificallywiththe availabilityofpotassiumandnitrogen ()Milewski,1982;Hughesetal.,1993. Frugivore-dispersedplantsarethereforelikelycolonizersofsubcanopysites. Thedevelopmentofanutrient-richpatchbeneathatreeinopensavannahasbeen comparedwiththeformationofatree-fallgapinaforest.Belsky&Canham()1994 pointoutthatthedynamicsofthesepatchesdiffer:atree-fallgapinforestdevelops suddenlyandclosesgradually,whereasasubcanopypatchdevelopsslowlyandits durationislinkedtothelife-spanofthetree.Subcanopysoilchemistryandthe compositionofsubcanopyplantassemblagesshouldthereforevarywithtreeage. FERTILEISLANDSUNDERACACIAERIOLOBA 63 Figure2.White-backedVulturesGypsafricanusandaloneLappet-facedVultureTorgos tracheliotusrestingintheshadeofalargeAcaciaeriolobatree,andfeedingontheremainsofa Gemsbokkilledbylionsthepreviousnight. InaridKalaharisavanna()Fig.3wehypothesize dthat:1()subcanopysoilsbeneath theleguminoustreeAcaciaeriolobaarenutrientenriched;()2subcanopysitesare Figure3.AridKalaharisavannaalongtheNossobRiver,KalahariGemsbokNationalPark, SouthAfrica,showingscatteredlargeAcaciaeriolobatrees. 64 W.R.J.DEANETAL. Figure4.TheremainsofalargeAcaciaeriolobashowingcolonizationoftheareaaroundthe baseofthetreebygrasses,annualforbs(Cleomesp.) andasinglelargeindividualBoscia albitrunca(atreewithbird-dispersedfruits) . colonizedprimarilybyplantswithanimal-dispersedfruits(Fig.4);and(3)thatsoil fertility,andbirdandplantassemblagesvarywiththedevelopmentalstageofthetree. Studysiteandmethods Studysite ThestudywascarriedoutintheNossobRivervalley()approximately258209S,208309E intheKalahariGemsbokNationalPark()KGNPalongtheborderbetweenSouth AfricaandBotswana.Rainfallinthissemi-aridregion(200]250mmyear y1 ) occursin midtolatesummer()JanuarytoMarch.Meanmaximumandminimumtemperatures are37..48Cand1958CinJanuary ,and22.28Cand1.28CinJuly()vanRooyen,1984. TotalnitrogenandavailablephosphorusaregenerallylowinallsoilsoftheKGNPbut arehigherinalluvialthandunesoils()vanRooyen,1984. Thevegetationisveryopensavannawithgrasses(Schmidtiakalihariensis,Stipagrostis spp.)(andoccasionalsmalltreesAcaciaerioloba,A.haematoxylo n,A.mellifera)onthe dunes,andlargerA.eriolobatreesandfleshy-fruitedshrubs( Bosciaalbitrunca,Grewia spp.)(scatteredinamatrixofgrassandlowshrubs Rhigozumtrichotomumand Monechmaspp.)alongthemajorrivervalleys(Leistner,1967). Soilnutrientstatusandfertility InFebruary1993,onesoilcore()70mmdiameter=100mmdeepwastakenfromthe A-horizonbeneaththecanopiesofeachofthreeisolatedmatureA.eriolobatrees, withinthecanopyareaofthreedeadA.eriolobatrees,andatthreesitesinthe FERTILEISLANDSUNDERACACIAERIOLOBA 65 correspondingmatrixvegetation.Theseninesamplesitesweresubjectivelyselected. Sampleswereassayedfortotalnitrogen(N )(Kjeldah lmethod),organi ccarbon(C) ()WalkeyBlackmethodandforcitricacidsolubl ephosphorus(P),potassium(K)and sodium(Na)(Bra ynumber2method).One-wayANOVAsfollowedbyTukeymultiple rangetestswereusedtocomparethesoildata.However,highvariancesandsmall samplesizesnecessitatedincreasedconfidencelimitsforstatisticaltesting. Subcanopyandmatrixplantassemblages SurveysofperennialplantsweremadeinFebruary1993intheNossobRivervalley betweenKwang(c.20kmnorthofNossobrestcamp)andKaspersDraai(c.30km southofNossobrestcamp).Wedrove131-kmroadtransectsat3-kmintervals,and stoppedateveryindividualorgroupofA.eriolobatreesencounteredwithin20mof theroadoneachtransect.Ateachstop,vegetationbeneathA.eriolobacanopiesandin matrix()between-treevegetationwasassessedbyrecordingallperennialplantspecies visiblewithinonefieldofaspottingscopeatadistanceof20mfromtheroad(sample areaapproximately20m2 ).GroupsofAcaciaeriolobatreeswerenotmatchedinall casesbyamatrixsample.AllA.eriolobatreeswithinthesamplefieldwereclassified onthebasisofestimatedheightandbasaldiameteraseithersaplings(-5mhigh,erect canopy,-20cmbasaldiameter()b.d.,N s129),matur elivingtrees()20cmb.d., spreadingcanopy,Ns153)(ordeadtreesnofoliage,Ns 69).Vegetationcomposition inmatrixvegetation()Ns210wassampledatsitesthatwereaminimumof30m fromthenearesttreeorgroupoftrees.Forthepurposesofanalysisofestablishment sitesforAcaciaerioloba,sitescontainingsaplings()Ns129werealsoconsideredtobe matrixvegetation.Allplantsrecordedwereallocatedtooneoffourdispersalguildson thebasisoftheirfruitandseedmorphology:wind-dispersed(seedswithlargewingsor plumes)(,autochorousexplosivelydehiscen tcapsules),herbivore-disperse d(indehiscent, hardseedsenclosedingreenery,sensuJanzen,1984)(,andfrugivore-dispersedseeds encasedinfleshyfruits).Contingencytabletestswereusedtocompareobservedand expectedfrequenciesofdispersalguilds. Useoftreesbybirdsandmammals AsingleobserverrecordedallbirdsandlargenestsseenonA.eriolobatreesalonga 166kmroadtransectbetweenNossobandTweeRivierenon10]11March1994.Each treeoccupiedbyabirdornestwasclassifiedasasapling,maturetreeordeadtree usingthedefinitionsgivenabove.Birdspecieswereallocatedtofoodguilds:()1 insectivore,2()granivore ,()3insectivor e]granivore,4()frugivore ,5()carnivor eand()6 scavenger.MammalsandlargebirdsrestingintheshadeofA.eriolobatreesinthe KGNPwerenotedon3daysinFebruary1993andon2daysduringMarch1994. Contingencytableswereusedtocomparethedistributionofavifaunalfeedingguildsin eachofthetreestages. NomenclaturefollowsMaclean()1993for birds,Skinner&Smithers()1990for mammalsandArnold&deWet()1993forplants. Results Soilnutrientstatusandfertility IntheninesoilsamplesfromtheNossobvalley,N ()rs0..96,p-001andP 66 W.R.J.DEANETAL. concentrations()rs0..79,p-005werecorrelatedwithtotalorganiccarbon,andK wascorrelatedwithNa()rs0..93,p-001.ConcentrationsofN,PandKwere greaterinsamplesfrombeneathliveA.eriolobatreesthaninsoilsfrommatrix
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