Interactions Between Indigenous Southern Afrotemperate Forest Trees
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Interactions between indigenous southern Afrotemperate forest trees and arthropod diversity by Rudi Crispin Swart Dissertation presented for the degree of Doctor of Philosophy (Conservation Ecology) in the Faculty of AgriSciences at Stellenbosch University Supervisors: Prof. F. Roets, Prof. M. J. Samways Advisor: Prof. J. S. Pryke Department of Conservation Ecology and Entomology Faculty of AgriSciences March 2020 1 Stellenbosch University https://scholar.sun.ac.za Declaration By submitting this dissertation electronically, I declare that the entirety of the work contained therein is my own, original work, that I am the sole author thereof (save to the extent explicitly otherwise stated), that reproduction and publication thereof by Stellenbosch University will not infringe any third party rights and that I have not previously in its entirety or in part submitted it for obtaining any qualification. March 2020 Copyright © 2020 Stellenbosch University All rights reserved 2 Stellenbosch University https://scholar.sun.ac.za General summary Although small compared to other temperate rainforests in the southern Hemisphere, the southern Cape Afrotemperate forest complex is the largest in South Africa. While it occurs at temperate latitudes, it has strong tropical elements resulting from its paleo-history. Of the numerous species occupying forest ecosystems, insects comprise a major part of the total biodiversity, most of which occur in tree canopies. Prior to this study, little work had been done on insects in southern Afrotemperate forests in general, and no work at all has been done on the diversity and distribution of their canopy-inhabiting arthropods. Therefore, the aim here is to determine the extent to which various environmental factors affect the interaction between indigenous tree species and associated arthropod diversity in South African Afrotemperate forests. I first determine whether the context and contrast in which an individual tree grows (i.e. where it grows and what surrounds it) will impact its physiology and associated canopy arthropod diversity. I found that the contrast of vegetation surrounding an individual tree can affect leaf morphology, and, in turn, its ability to host particular arthropods, with trees with low contrast (i.e. surrounded by denser vegetation) revealing larger leaves and increased arthropod diversity. Furthermore, plant physiological features fluctuated according to the context in which a tree grows (natural, semi-natural, or planted vegetation), which affected associated canopy arthropods. Therefore, to optimally conserve local arthropod diversity using indigenous tree plantings in transformed landscapes, it is imperative to mimic natural tree context and natural variations in contrast. Forest arthropods maintain ecosystem health by driving ecosystem processes such as litter decomposition. I designed an experiment to compare the litter decomposition performed by arthropods vs. fungi, and determined which local factors influence variations in decomposition rates. In addition, I tested the home-field advantage (HFA) hypothesis at the tree-level. The HFA states that leaf litter decomposes more rapidly beneath plant species from which the leaves originate (home environment), than under other plant species. I demonstrated that arthropods perform the bulk of the decomposition function in these forests, and that their ability to do so varies significantly between different tree species, 3 Stellenbosch University https://scholar.sun.ac.za although the role of bacteria may also be substantial. Contrary to expectations, and despite selective arthropod responses toward different source leaves, HFA is not prevalent in this mixed forest system. Given the responses of arthropods to tree identity, tree context- and-contrast, and accompanying changes in plant physiological features, it was reasonable to assume that these factors may influence arthropods associated with the canopies of southern Afrotemperate forests. I therefore established the relative effects of tree species identity, plot characteristics, and plant physiology on the diversity and distribution of canopy arthropods. Tree species identity and differences in plant physiological features explained differences in arthropod diversity between individual trees. Individual trees surrounded by denser vegetation also had less diverse arthropod assemblages compared to trees in more open areas. I argue that in diverse mixed forests, tree crown heterogeneity is of significant importance in conserving arthropod diversity, driven not only by architectural variation, but also by fluctuating levels of light exposure. Differences in plant physiological features at the tree species level was accompanied by many effects on canopy arthropods, which would make generalisations of forest arthropod responses to anthropogenic changes difficult. As this study represents a first attempt to describe the diversity of arthropods in the canopies of southern Afrotemperate forests, I conclude by providing a synthesis of this diversity, placing it in a global context. I provide evidence that arthropod diversity in these forests is more similar to those of temperate forests than to arthropods associated with tree canopies in tropical forests. However, these forest canopies are ten-fold richer in species than the forest floors in this region. Combined with the high numbers of species sampled, many of which are undescribed, special conservation efforts is justified to protect southern Afrotemperate forest canopies across a wide biogeographical gradient. 4 Stellenbosch University https://scholar.sun.ac.za Algemene opsomming Ten spyte daarvan dat die suidelike Afro-gematigde woudkompleks klein is in vergelyking met ander gematigde woude in die suidelike halfrond, vorm dit die grootse woudkompleks in Suid-Afrika. Vanweë die unieke paleo-geskiedenis van die area, het die woude egter ‘n tropiese affiniteit. Van al die verskeie spesies wat woud-ekosisteme bewoon, is insekte ‘n groot deel van die totale biodiversiteit, en die meeste hiervan kan in boomtoppe gevind word. Voor hierdie studie, is baie min fokus geplaas op insekte in suidelike Afro-gematigde woude in die algemeen, en geen studie het gepoog om die diversiteit en verspreiding van boomtop-insekte te beskryf nie. Die doel van hierdie navorsing, dus, is om te bepaal tot watter mate verskeie faktore die interaksies tussen inheemse boomspesies en geassosieërde arthropoda diversiteit affekteer in Suid-Afrikaanse Afro-gematigde woude. Eerstens het ek bepaal of die konteks en kontras waarin ‘n individuele boom groei, die fisiologie en geassosieërde arthropoda sal beïnvloed. Ek het bevind dat die kontras van die omringende plantegroei rondom ‘n boom die blaarmorfologie en gevolglik die arthropoda beïnvloed, met bome in laer kontraste (omring deur digter vegetasie) wat groter blare en verhoogde diversiteit getoon het. Plant fisiologie het gefluktueer op grond van die konteks waarin ‘n boom groei (natuurlik, semi-natuurlik, geplant), wat geassosieërde boomtop arthropoda geaffekteer het. Om plaaslike arthropoda optimaal te bewaar deur inheemse bome te plant, is dit daarom van kardinale belang om ‘n boom se natuurlike konteks en variasie in kontras in ag te neem. Woud arthropoda onderhou gesonde ekosisteme deur ekosisteem prosesse soos blaar dekomposisie te dryf. Ek het ‘n eksperiment ontwerp om die blaar dekomposisie wat uitgevoer word deur arthropoda teenoor fungi te vergelyk, en om te bepaal watter plaaslike faktore die variasies in dekomposisie tempo sal affekteer. Verder, het ek die tuisveld-voordeel (TVV) hipotese getoets, wat stel dat blare vinniger afbreek onder plante vanwaar die blare afkomstig is (tuis), teenoor ander plant spesies (weg). Hier demonstreer ek dat arthropoda die meeste dekomposisie funksie uitvoer, en dat hul vermoë om dit te doen beduidend varieër tussen verskillende boomspesies, alhoewel die rol van bakterieë substansieël 5 Stellenbosch University https://scholar.sun.ac.za mag wees. Anders as verwag, en ten spyte van selektiewe arthropoda reaksies tot verskillende boom- spesie blare, blyk TVV nie van belang te wees in hierdie gemengde woudsisteem nie. Gegewe die reaksies van woudvloer arthropoda tot boom identiteit, boom konteks- and kontras, en gepaardgaande veranderinge in plant fisiologie, is dit redelik om te verwag dat hierdie faktore die boomtop arthropoda in suidelike Afro-gematigde woude kan beïnvloed. Daarom het ek die relatiewe effekte van boomspesie-identiteit, plot eienskappe en plant fisiologie op die diversitiet en verspreiding van boomtop arthropoda bepaal. Boomspesie-identiteit en verskille in plant fisiologie het verskille in arthropoda diversiteit tussen individuele bome bepaal. Individuele bome omring deur digter plantegroei het minder diverse arthopoda samestellings gehad in vergelyking met meer oop areas. Ek argumenteer dat, in diverse, gemengde woude, heterogene boomtop lae van beduidende belang is om arthropoda diversiteit te bewaar, gedryf nie net deur variasie in argitektuur nie, maar ook deur fluktuasies in lig blootstelling. Verskille in plant fisiologie by die boomspesie-vlak het gepaardgegaan met ‘n diverse verskeidenheid effekte op boomtop arthropoda, wat algemene afleidings van hul reaksies tot mensgedrewe veranderinge bemoeilik. Aangesien hierdie studie ‘n eerste poging is om die diversiteit van arthropoda in boomtoppe van suidelike Afro-gematigde woude te beskryf, sluit ek dit af deur ‘n volledige sintese te verskaf van hierdie diversiteit, en om dit in ‘n globale