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Effects of warming on the early establishment of an African savanna tree

dc.contributor.advisorMidgley, Guy F.en_ZA
dc.contributor.advisorStevens, Nicola
dc.contributor.authorRandle, Matilda Rose
dc.contributor.otherStellenbosch University. Faculty of Science. Dept. of Botany and Zoology.en_ZA
dc.date.accessioned2019-11-15T15:25:34Z
dc.date.accessioned2019-12-11T06:50:59Z
dc.date.available2019-11-15T15:25:34Z
dc.date.available2019-12-11T06:50:59Z
dc.date.issued2019-12
dc.identifier.urihttp://hdl.handle.net/10019.1/107168
dc.descriptionThesis (MSc)--Stellenbosch University, 2019.en_ZA
dc.description.abstractENGLISH ABSTRACT: This study investigates the interactive effects of atmospheric warming and the presence of a C4 grass sward on the growth and mortality of establishing savanna tree seedlings (Senegalia nigrescens (Oliv.) P. J. H. Hurter). Atmospheric warming is one of the major drivers of global vegetation change, but has been little studied in tropical African systems. In cool temperature systems of the mid- to high –latitudes, where plant growth is predominantly controlled by climatic factors, warming has beenshown to drive an increase in plant growth and establishment. In systems of the mid – to low – latitudes, such as sub-tropical savannas, the potential role of warming is not well known, and the vegetation structure and functioning of these systems is controlled by the interacting impacts of fire, herbivory and climate. Wildfire and herbivory limit the seedling and sapling demographic stages of savanna trees. This stage is expected to be most vulnerable to warming because seedlings are sensitive to fluctuations in temperature and are highly dependent on water availability, a resource that is competed for strongly by C4 grasses. Furthermore, C4 grasses are unlikely to be as adversely affected by warming as C3 seedlings, due to differences in their photosynthetic pathways. Based on these arguments, this study tests the main hypothesis that there is an adverse interactive effect of warming and grass competition on tree seedling establishment in savannas. In order to test this hypothesis, I carried out a field experiment at Wits Rural Facility in Limpopo Province, South Africa during the 2017/18 growing season. Using passive open-topped, polycarbonate warming chambers, seedlings were warmed on average, by 1-2°C. Soil and plant water content were unaffected by warming but the presence of grass significantly reduced the relative water content of the leaves of establishing seedlings, suggesting competition for water between the different growth forms. Seedling growth rate was unaffected by warming when grown without C4 grasses, but a significant decline was shown by those grown with grasses above a daytime temperature threshold of 30°C. Likewise, seedlings grown with grasses suffered a 65% reduction in survivorship when warmed but those grown in the absence of grass suffered only a 15% reduction in survivorship. The results of this study therefore show that warming and the presence of grasses had an adverse additive effect on seedling survivorship, through which warming enhanced the dehydrating effect of competing grass on establishing seedlings, thus confirming the primary hypothesis. I propose that the cumulative stresses of carbon imbalance due to warming and grass competition for soil water drove this decline in tree seedling growth, resulting in higher seedling mortality with the implication of reduced successful establishment events under warmer conditions. This study makes an important contribution to understanding the impact of warming on African savanna species, in that it suggests a future decline in tree establishment under warmer conditions. To extrapolate these findings, a greater focus on understanding the impacts of warming on a range of savanna plant functional groups across the rainfall gradient, with other global change drivers, is required.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: Hierdie studie het die effek van verhoogde temperature in die atmosfeer en die teenwoordigheid van 'n C4-gras op die groei en mortaliteit van ‘n savanna-boomsaailing (Senegalia nigrescens (Oliv.) P. J. H. Hurter) ondersoek. Die temperatuur van die atmosfeer is een van die belangrikste drywers van plantegroei, maar die effek daarvan is selde in tropiese Afrika stelsels ondersoek. Binne klimaat stelses gevind in die middel- tot hoё breedtegrade, waar plantegroei hoofsaaklik deur die klimaat beheer word, is daar 'n direkte vehouding tussen verhoogte temperature en plantegroei sowel as die vestiging van nuwe saailinge. In klimaat stelsels gevind in die middel-tot lae breedtegrade, waar sub-tropiese savanna gevind word, is die effek van temperatuur nog nie duidelik nie. Dit word vermoed dat daar binne hierdie stelses 'n wisselwerking tussen vuur, oorbeweiding en klimaatsverandering die grootste drywer is vir plantegroei. Vorige studies het gevind dat veld brande en oorbeweiding die saadvorming en vestinging van savanna bome grootliks beinvloed. Hierdie stadium in die plant siklus is dus die mees kwesbaarste teen veranderinge in temperature. Saailinge is baie sensitief vir veranderinge in temperature en is baie afhanklik van die teenwoordigheid van water. Water is 'n natuurlike bron waar strek mededinging tussen saailinge en C4-grasse plaasvind. As gevolg van verskille in hulle fotosintetiese fisiologie, sal C4-grasse nie so negatief beïnvloed word deur herhoogde atmosferiese temperature soos C3-plante nie. Hierdie studie het dus die hipotese getoets dat daar 'n nadelige interaksie is tussen verhoogde atmosferiese temperature en kompetiese van gras op die vestiging van saailinge. Alle veldwerk was gedoen tydens die 2017 en 2018 groei seisoen by die Wits Rural Facility in die Limpopo Provinsie, Suid-Afrika. Saailinge was in die veld tussen 1-2° C passief verhit, deur polycarbonate strukture rondom plante in te stel. Die grond- en plant-water inhoud was nie deur die verhitting beïnvloed nie, alhoewel die teenwoordigheid van gras wel die relatiewe water inhoud van saailinge se blare aansienlik verminder het. Die vestiging van saailinge was darom aansienlik verlaag, wat 'n goeie indikasie is van die kompitiese tussen die verskillende groeivorme. Die groeitempo van saailinge was nie deur verhitting beïnvloed nie, indien die saailing in die afwesigheid van C4-grasse gevestig was. Daar was 'n groot afname in die groeitempo van saailinge wat moes kompeteer teen grasse bo daaglikse temperature van 30° C. Saailinge wat gevestig het in hoë temperature het 'n 60% afname in oorlewing getoon, terwyl saailinge in die afwesigheid van grasses slegs 'n 15% afname gewys het. Die resultate van die studie het dus aangedui dat verhitting en die teenwoordigheid van grasse 'n additiewe nadelige uitwerking op die oorlewing van saailinge gehad het. Dit kan dus afgelei word dat die druk van ‘n koolstof wanbalans as gevolg van verhitting, en die kompetisie tussen groeivorme vir groundwater, altesaam 'n afname in die groei van saailinge gehad het. Dit sal moontlik veroorsaak dat hoër saailing sterftes met voorspelde verhitting sal lui tot verlaagde saailing vestiging in warmer toestande. Hierdie studie help om die impak van voorspelde verhitting op Afrika-savanna spesies te verstaan. Om die breër gevolge van aardsverwarming te verstaan, moet toekomstige werk fokus op hoe verskeie savanna funksionele plant groepe deur die komponente van globale verandering, insluitend reёnval en temperatuur, beinvloed word.af_ZA
dc.format.extentxii, 59 pages : illustrations
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch University
dc.subjectGlobal warming -- Environmental aspectsen_ZA
dc.subjectSavanna plants -- Climatic factors -- Africaen_ZA
dc.subjectGrasslands -- Conservationen_ZA
dc.subjectTrees -- Seedling -- Growthen_ZA
dc.subjectUCTD
dc.titleEffects of warming on the early establishment of an African savanna treeen_ZA
dc.typeThesisen_ZA
dc.description.versionMastersen_ZA
dc.rights.holderStellenbosch University


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