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Dendrochronological evaluation of the El Niño effect in East-Africa. Case study: Isoberlinia tomentosa (Harms) Craib et Stapf

Valérie TROUET student laureate

°1974 Belgium
Bio-engineer in land and forest management, Universiteit Gent, Belgium, 1999

Dendrochronologische evaluatie van het El Niño-effect in Oost-Afrika. Gevalstudie : Isoberlinia tomentosa (Harms) Craib et Stapf

Uncommon dry periods possibly linked with enormous forest fires, and even unusual rains in the tropics are frequently reported topics in press articles. All these climate events are directly related to immeasurable human suffering. The uncommon climatic phenomena are considered to connect with the El Niño effect. In equatorial Africa this effect encompasses a rising of temperature and rainfall (the warm phase) followed by a cold phase with a decrease of both.
To be able to anticipate the calamitous consequences of these climatic fluctuations, it is important to understand thoroughly this phenomenon. Therefore scientific data are needed that can be arranged in a sufficiently long time series (at the least some decades). Because long environmental series of data are scarce in Africa, useful data should be reconstructed. Climate reconstruction is possible from growth rings of trees. In areas where seasons are well marked, the growth in thickness of the trees occurs mainly by formation of annual discontinuous wood layers, clearly visible as concentric rings on cross sections of the trunks. The rings are thicker the years when tree growth is good, for example thanks to high rainfall; they are thinner the unfavourable years. For Africa, this method was not possible up till now.
The graduate thesis of Valérie Trouet aims at studying the effect of El Niño on tree growth.
Because the dendrochronological approach is new for East-Africa, three preliminary questions should be answered:
(1) do East-African trees show any growth rings ? (a wood anatomical approach gave an answer to this question)
(2) do the patterns of growth rings cross date among trees ? (stem discs from Tanzania have been collected for this purpose)
(3) is it possible to extract information on El Niño from the growth ring profile of a population of trees ? (this has been answered by a statistical approach).
Significant relations between the El Niño and the growth ring width have been found. This suggests that a climate reconstruction from tree ring data is possible.
Valérie Trouet succeeded in producing a pioneering work in the study of the impact of El Niño on the African ecosystems. There is no doubt that the sustainable development of the latter necessitates a good knowledge of the effects of El Niño, which can be favourable, but sometimes also disastrous for the productivity of the forests, of crops and of fisheries.