Browsing by Author "Williamson, David"

Now showing 1 - 8 of 8
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    Can Climate-Driven Change Influence Silicon Assimilation by Cereals and Hence the Distribution of Lepidopteran Stem Borers in East Africa?
    (Elsevier, 2016) Calatayud, P. A.; Njuguna, E.; Mwalusepo, Sizah; Gathara, Mary; Okuku, G.; Kibe, A.; Musyoka, B.; Williamson, David; Ong’amo, Georges; Gerald, Juma; Johansson, Tino; Subramanian, Sevgan; Gatebe, E.; Le Ru, Bruno P.
    In East Africa, lepidopteran stemborers such as Chilo partellus and Busseola fusca are major constraints to production of maize, which is the main staple food crop in the region. Cereals depend on silicon (Si)-based defences to fight off herbivores. Using altitudinal ranges in the East African highlands as ecological surrogates for inferring climate change, it was shown that Si concentrations in soil and maize decreased with altitude. This was attributed, in part, to low temperatures at high altitudes, which negatively affected Si assimilation by maize. Experiments showed that B. fusca was more susceptible to Si than C. partellus. Hence the predominance of B. fusca in the highlands and of C. partellus in the lowlands could be partly explained by altitudinal differences in Si concentrations in maize plants. Therefore, a rise in temperature due to climate change should enhance the plants’ Si assimilation and as a result C. partellus might move into the higher altitudes and increasingly displace B. fusca.
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    Characteristics of Soils in Selected Maize Growing Sites along Altitudinal Gradients in East African Highlands
    (2015) Ong’amo, George; Njuguna, Elijah; Gathara, Mary; Nadir, Stanley; Mwalusepo, Sizah; Juma, Gerald; Kimani, Jackson; Landmann, Tobias; Williamson, David
    Maize is the main staple crop in the East African Mountains. Understanding how the edaphic characteristics change along altitudinal gradients is important for maximizing maize production in East African Highlands, which are the key maize production areas in the region. This study evaluated and compared the levels of some macro and micro-elements (Al, Ca, Fe, K, Mg, Mn, Na and P) and other soil parameters (pH, organic carbon content, soil texture [i.e. % Sand, % Clay and % Silt], cation exchange capacity [CEC], electric conductivity [EC], and water holding capacity [HC]). Soil samples were taken from maize plots along three altitudinal gradients in East African highlands (namely Machakos Hills, Taita Hills and Mount Kilimanjaro) characterized by graded changes in climatic conditions. For all transects, pH, Ca, K and Mg decreased with the increase in altitude. In contrast, % Silt, organic carbon content, Al and water holding capacity (HC) increased with increasing altitude. The research provides information on the status of the physical-chemical characteristics of soils along three altitudinal ranges of East African Highlands and includes data available for further research.
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    Holocene Explosive Eruptions in the Rungwe Volcanic Province, Tanzania
    (2010-07) Fontijn, Karen; Ernst, Gerald G. J.; Elburg, Marlina A.; Williamson, David; Abdallah, Edista; Kwelwa, Shimba; Mbede, Evelyne I.; Jacobs, Patric
    The Holocene explosive eruptive record of Rungwe and Ngozi volcanoes of the Rungwe Volcanic Province in Tanzania was reconstructed based on detailed stratigraphic field evidence combined with whole-rock major and trace element analyses of tephra samples. This reconstruction is supported by 25 new radiocarbon dates on palaeosols that provide additional constraints on the Holocene tephro-chronostratigraphy. We show evidence of two catastrophic Ngozi eruptions and five Rungwe pumice fallout deposits, and also identify several more intercalated poorly preserved pumice and ash deposits. The Ngozi eruptions probably played a role in shaping the present-day caldera. The Rungwe record includes a ca. 2.2 km3 deposit of a Plinian-style eruption dated at ca. 4 ka, a sub-Plinian one at ca. 2 ka and at least three additional smaller-scale fallout deposits. The Rungwe explosive eruptive record shows that the volcano has been frequently active in its late Holocene past. This study highlights the need for volcanic monitoring in the region and presents herewith the first basis of future volcanic hazard assessment.
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    Macrofossil evidence of Late Holocene presence of Aldrovanda vesiculosa L. in Central-Eastern Europe (Poland) and East Africa (Tanzania)
    (Elsevier, 2015) Gałka, Mariusz; Bergonzini, Laurent; Williamson, David; Majule, Amos; Masao, Catherine A.; Huguet, Arnaud
    To date, the seeds of waterwheel plant (Aldrovanda vesiculosa) were only found in Europe, in sediments of the Eemian and Mazovian Interglacials. The absence of A. vesiculosa seeds in European Holocene deposits is probably due to unfavorable climatic conditions, i.e. lower temperature, during this period: in temperate zones, A. vesiculosa reproduces mainly in a vegetative way; it rarely blooms and only occasionally produces seeds. In this paper, subfossil seeds of A. vesiculosa were identified in two peat bogs: one (Sucha Kobyła) in SE Poland and another one (Kyambangunguru) in SW Tanzania. Single seeds of this plant were found in a peat layer from the Polish site formed after 1600 AD as well as in sediments accumulated around 440 AD in the Tanzanian one. To our knowledge, this is the first time that seeds of A. vesiculosa are found in Holocene deposits in the world. Our findings provide new data on the distribution of A. vesiculosa over the Late Holocene.
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    Potential Feedback between landuse and climate change in the Rungwe tropical highland stress a critical environmental research challenge
    (Elsevier, 2014) Williamson, David; Majule, Amos; Delalande, Manuella; Mwakisunga, Benard; Mathe, Pierre-Etienne; Gwambene, Brown; Bergonzini, Laurent
    Characteristic trends in landuse and climate highlight critical challenges in future resource management along the tropical topoclimosequence (TCS) of the Rungwe Volcanic Province (RVP), a major headwater biodiversity and food reservoir of Tanzania. TheRVPorography supports a coherent distribution of agroecosystems and livelihoods, from hot irrigated lowlands to endangered afromontane forests above 1500 m.a.s.l. Recent increases in deforestation, land fragmentation and soil denudation/compaction in the densely populated TCS were combinedwith a strengthenedneedandconsumption ofwaterin the lower, warmer and drier end-member. Consistent with a regional decline of the long rains, a considerable (up to 30%) decrease in annual rainfall and a pervasive decline of the Lake Masoko (LM) aquifer testifies to a strong aridification trend. We suggest here that current landuse and demographic trends likely amplified the hydrological response of the TCS to regional and global warming. Testing such a hypothesis, however, requires improved local monitoring, to allow scaling and quantification of local hydrological budgets associated with landuse impacts, and evaluationofthecontributionoftreesandagroforestrysystemsto mitigating the aridification trend.
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    The Rungwe Volcanic Province, Tanzania – A Volcanological Review
    (2011-11) Fontijn, Karen; Williamson, David; Mbede, Evelyne I.; Ernst, Gerald G. J.
    The Rungwe Volcanic Province in SW Tanzania is a densely populated area that is considered volcanically active. As part of the East African Rift System, a significant control of tectonic activity seems to exist on the location and also potential destabilization of volcanic edifices. Three large volcanoes, Ngozi, Rungwe, and Kyejo, dominate the landscape and all show contrasting eruptive behaviour in the recent geological past. Kyejo volcano is a flow-dominated volcano that had a historic lava flow eruption. Lake sediment cores, drilled in Lakes Malawi, Masoko, Rukwa, and Tanganyika, provide a record of frequent explosive eruptions in the last few tens of thousands of years. In combination with on-land stratigraphic observations, they constrain the minimum eruptive frequency of especially Rungwe and Ngozi volcanoes. Both volcanoes had Plinian-style eruptions in the Holocene. The most striking documented Rungwe eruption, the ca. 4 ka Rungwe Pumice, is a rare case of a Plinian eruption in near-wind-free conditions. Furthermore, the Rungwe Pumice, just like any other Rungwe tephra deposit, does not show any evidence of pyroclastic density current deposits. Apart from explosive eruptions at a range of scales happening every few hundred years at Rungwe, the volcano also experienced at least two sector collapse events generating debris avalanches. All existing evidence shows that the Rungwe Volcanic Province is prone to future significant explosive eruptions. To further assess, quantify and mitigate volcanic hazard risks, extensive and systematic multidisciplinary geological research, and both volcanic and tectonic monitoring are needed.
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    Solar and anthropogenic imprints on Lake Masoko (southern Tanzania) during the last 500 years
    (Kluwer Academic Publishers, 2006-10-21) Garcin, Yannick; Williamson, David; Bergonzini, Laurent; Radakovitch, Olivier; Vincens, Annie; Buchet, Guillaume; Guiota, Joël; Brewer, Simon; Mathe, Pierre-Etienne; Majule, Amos
    The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years. Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this remote basin during colonial and post-colonial times.
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    Solar and anthropogenic imprints on the Lake Masoko (southern Tanzania) during the last 500 years
    (Springer Science+Business Media B.V. 2006, 2006) Garcin, Yannick; Williamson, David; Bergonzini, Laurent; Radakovitch, Olivier; Vincens, Annie; Buchet, Guillaume; Guiota, Joël; Brewer, Simon; Mathe, Pierre-Etienne; Majule, Amos
    The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years. Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550– 1850), confirming that the LIA in eastern Africaresulted in marked and synchronous hydrological changes. However, the direction of response varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this remote basin during colonial and post-colonial times.