The growth response of selected Zambian hardwood species to changing climatic conditions
Thesis (PhDFor)--Stellenbosch University, 2019.
ENGLISH ABSTRACT: Factors such as precipitation and temperature are key to the growth of trees. The expected change in growing conditions, i.e. rise in temperature and reduction in precipitation will influence tree growth, wood structure and on wood quality. Due to lack of instrumental data, little is known about how Miombo trees growing in Zambian Miombo woodlands have responded to climate change. The objective of this study was to examine the extent to which climate variables related to water availability and temperature during cambial periods shape wood anatomical properties of three hardwood species growing in Miombo woodlands in Zambia, namely Brachystegia spiciformis, Burkea africana, and Isoberlinia angolensis. The species were selected based on value, distribution across the climate zones, dendrochronological potential, and relative ease of coring. To understand how Miombo trees growing in Zambia have responded to climate change, the ring structure and wood anatomical properties were related to known extreme climatic events. Sample materials, in the form of increment cores, were collected from areas differing in water availability. From the three climate zones, sites with climate data were selected and their aridity determined based on mean annual precipitation and mean annual temperature using De Martonne’s Index. At each site, 15-20 living trees were selected for sampling. For each tree, diameter at breast height (1.3 m), total height, bole height, and crown diameter were measured, after which two increment cores were then obtained from breast height at 90° and 180° to the wind direction. After collection, the cores were labelled to indicate site name, species, tree number, and core number for ease of identification in the lab and then placed in a core holder. In the lab, the cores were prepared for ring measurement and analysis using standard dendrochronological procedures after drying. The cores were then placed on a wooden mount and their ring structure studied under a microscope attached to a computer with ring measurement and analysis software. For wood anatomical studies, a Nano-CT scanner was used to obtain images from prepared cross sections representing wood formed during dry and very wet years. To compare the growth response of each species to different sites, ring structure was studied. Ring analysis revealed that mean sensitivity negatively correlated very well with mean annual precipitation. For all the species, sensitivity was significantly different between dry and wet sites. Sensitivity was high on the drier sites while complacent growth occurred on the wetter sites. In all the species, about 20% of the ring width variance could be explained by precipitation, but growth was unresponsive to temperature. The second part of the study looked at how the trees responded to extreme climate events. Each of the species was looked at separately. In all the species, data analysis revealed statistically significant (p<0.05) differences in fibre and vessel characteristics between those formed during wet and dry years, and those from dry and wet sites. Studies on effect of ring width on density revealed that, typical of diffuse-porous woods, density was independent of ring width. Fit functions developed from models based on projected values of precipitation under climate change scenario RCP8.5 revealed that cell wall thickness, which will increase by an average of +10.6 µm in all the species, will have the biggest influence on wood density. Wood quality will therefore change because of climate change. The study proved that in the absence of long-term data measured across the climate zones on the same trees, tree-ring studies can provide answers to questions on how particular trees react to adverse effects of climate change.
AFRIKAANSE OPSOMMING: Faktore soos reënval en temperatuur is die sleutel tot die groei van bome. Die verwagte verandering in groeitoestande, as gevolg van klimaatsverandering, d.w.s. styging in temperatuur en vermindering in reënval, sal boomgroei, houtstruktuur en houtkwaliteit beïnvloed. Dit is onseker hoe Miombo-bome wat in die Zambiese Miombo-woude groei, sal reageer op hierdie klimaatsverandering. Die doel van hierdie studie was om te ondersoek in watter mate die klimaatsveranderlikes die houtanatomiese eienskappe van drie loofhoutsoorte wat in Miombo-boslande in Zambië groei, naamlik Brachystegia spiciformis, Burkea africana en Isoberlinia angolensis, beïnvloed. Die spesies is gekies op grond van waarde, verspreiding oor die klimaatsones, dendrochronologiese potensiaal en relatiewe gemak van inkrement-boring. Steekproefmateriaal, in die vorm van inkrementboorsels is versamel van gebiede wat verskil in die beskikbaarheid van water. Van die drie klimaatsones is terreine met klimaatdata gekies en hulle droogheid is bepaal op grond van gemiddelde jaarlikse reënval en gemiddelde jaarlikse temperatuur met behulp van De Martonne se Indeks. Van elke terrein is 15-20 bome gekies vir steekproefneming. Ringstruktuur is bestudeer om die groeireaksie van elke spesie op verskillende terreine te vergelyk. Ringanalise het getoon dat gemiddelde sensitiwiteit negatief korreleer met gemiddelde jaarlikse neerslag. Vir al die spesies was sensitiwiteit aansienlik verskillend tussen droë en nat plekke. Sensitiwiteit was hoog op die droër terreine, terwyl selfbewuste groei op die natter terreine plaasgevind het. Vir al die spesies kon ongeveer 20% van die ringwydte-variasie verklaar word deur reënval - en groei het nie op temperatuur reageer nie. Tipies vir diffeus-poreuse hout, was digtheid onafhanklik van ringwydte. In die tweede deel van die studie is die reaksie op uiterste klimaatgebeure geanaliseer. Vir elke spesie en terrein is een uiters nat en droë jaar gekies vir houtanatomiese studies. Vir alle spesies was verskille in vesel- en seleienskappe tussen dié wat gedurende nat en droë jare gevorm is, en dié van droë en nat plekke, statisties beduidend. Fit funksies ontwikkel uit modelle gebaseer op geprojekteerde waardes van neerslag onder klimaatsverandering scenario RCP8.5 het getoon dat die selwand dikte, wat met 'n gemiddeld van +10.6 μm in al die spesies sal styg, met die grootste invloed op houtdigtheid het. Houtkwaliteit sal dus verander as gevolg van klimaatsverandering.