Tree-Ring Research in Semi-Arid West Africa: Need and Potential Item Type Article Authors Tarhule, Andover; Hughes, Malcolm K. Citation Tarhule, A., Hughes, M.K. 2002. Tree-ring research in semi- arid west Africa: Need and potential. Tree-Ring Research 58(1/2):31-46. Publisher Tree-Ring Society Journal Tree-Ring Research Rights Copyright © Tree-Ring Society. All rights reserved. Download date 26/09/2021 14:50:07 Link to Item http://hdl.handle.net/10150/262542 TREE -RING RESEARCH, Vol. 58(1/2), 2002, pp. 31-46 TREE -RING RESEARCH IN SEMI -ARID WEST AFRICA: NEED AND POTENTIAL ANDOVER TARHULE Department of Geography University of Oklahoma Norman, OK 73019, USA and MALCOLM K. HUGHES Laboratory of Tree -Ring Research University of Arizona Tucson, AZ 85721, USA ABSTRACT High - resolution paleoclimatic data for West Africa are needed to provide context for contemporary climatic and ecological dynamics. Six hundred trees (22 botanical families, 43 genera and over 70 species) from semi -arid West Africa were evaluated for their suitability for dendrochronological research; specifi- cally ring development. The samples were classified as `potentially useful', `problematic', or `poor' based on the presence and distinctiveness of annual rings, ability to achieve crossdating between radii using skeleton plots on at least some samples, circuit uniformity, ring wedging, and variability of ring widths. Samples were classified as potentially useful if (a) they exhibited distinctive annual rings that could be identified and counted with little uncertainty and be independently verified by a second person with little or no error, (b) crossdating between radii could be successfully achieved, at least on some samples, (c) the rings were generally consistent throughout the stem cross section, (d) ring wedging was minimal (in the relative sense) or absent, and (e) the ring widths were variable, indicating the possibility of climatic sen- sitivity. Seven species, including five from the Caesalpiniaceae family (Cassia sieberiana, Cordyla pinnata, Daniella oliveri, Isoberlinia doka, Tamarindus indica), and one each from Mimosaceae (Acacia seyal) and Verbenaceae (Gmelina arborea) families, that most closely satisfied these criteria were classified as 'po- tentially useful'. The `problematic' category includes those samples that satisfied some of the criteria but for which greater diligence is required to detect rings. Eight species from three families were classified in this category. Finally those samples on which ring detection appears futile given current methods and techniques were classified as `poor'. Most of the samples classified as `potentially useful' belong to three botanical families, Caesalpiniaceae, Mimosacae, and Verbenaceae. These results are consistent with the findings of other studies, and therefore support further investigation of the potential of West African trees for tree -ring analysis focusing on these families. Furthermore, inability to crossdate between trees and to explain several ring anatomical features underscores the pressing need for comprehensive field studies of cambial activity during the growing season, and for the identification of dormant seasons. This requirement, and other difficulties discussed suggest a need for increasing the local dendrochronological expertise in West Africa. Keywords: Soudano -Sahel savanna, tree - rings, Africa west, climatic variability, drought. INTRODUCTION logical change (Charney1975;Hulme1992; El- tahir and Gong1996;Nicholson2000).These The Soudano -Sahel savanna region of West Af-changes manifest most vividly in pronounced and rica is experiencing significant climatic and eco-persistent decrease in annual rainfall and stream- Copyright © 2002 by the Tree -Ring Society 31 32 TARHULE and HUGHES a 1.5 series are therefore too short to establish whether 1.0 the persistence and magnitude of the present 0.5 - drought are unprecedented in the recent climatic 0.0 history (i.e. the last several centuries) of the re- -0.5 - gion. Knowledge concerning the magnitude of -1.0 - contemporary climatic variability relative to pre- -1.5 - C vious events is important for the planning and -2.0 management of several activities in the region, in- 30 cluding water resources, agriculture and ecological 25 - systems. For example, if it were demonstrated that a20 - persistent, multi -decadal droughts are part of a low frequency pattern that is integral to the climate, r15 then the design and planning for long -lived pro- ÿ10 - jects such as water supply systems would need to 5 incorporate the risk for such events during the use- 1940 1950 1960 1970 1980 1990 2000ful life span of the project. On the other hand, establishing that the events are unprecedented Figure 1.(a) Pronounced rainfall decline in West Africa, up- could also be useful for seeking the most likely dated from the index of rainfall variability developed by Lamb (1985). The index is obtained by (i) expressing each station causes and explanations for the changes, as well rainfall as a standardized departure (i.e. departure from the as for developing new planning criteria that ac- mean over the entire period of record, divided by the standard commodate the new climatic regime. These obser- deviation); and (ii) calculating the regional average from the vations point to a need for extending the available standardized station departures for all the stations used, in this case 41. (b) Discharge of River Niger at Koulikoro (Mali). hydro -climatic time series back in time, using proxy sources. Unfortunately, the Soudan savanna has few flow discharge (Figure 1) that began in the latesources of proxy climatic information. Qualitative 1960s throughout the region; progressive declinehistorical information on landscape, river dis- of the levels of Lake Chad (currently measured atcharge, droughts and famine exist in various forms a mere one -fifth of its size in 1963; Coe and Foley including travelers' accounts and diaries, folklore, 2001) and a southward shift in the location of rain- oral history, colonial records and palace chronicles fall isohyets (Todorov 1985; Breman and Kesslerfrom the great empires and kingdoms (Nicholson 1995; Le Barbé and Lebel 1997; Tarhule and Tar -1978, 1979, 1996; Apeldoorn 1981; Tarhule and hule -Lips 2001). Since the onset of these changes,Woo 1997). However the authenticity -and there- concerted research efforts have revealed the broadfore scientific utility -of this information has yet processes involved but the specific causes -i.e. theto be established. Nicholson (1996) succinctly dynamic interrelationships among the many pos-elaborates the major limitations inherent with the sible causal factors - remain poorly understood.archival sources, including the lack of scientific The uncertainty concerning the causes and signif-rigor, the fallibility of memory and oral tradition, icance of on -going changes derives at least in partthe fact that the information is frequently second, from the short length of scientifically measuredor third hand, observer bias and the tendency to hydro -climatic data against which to evaluate thegeneralize a single observation at a point over changes. Indeed, only for limited areas of Westlarge areas. Tarhule and Woo (1997) attempted to Africa does one find systematically measured andquantify the magnitude of rainfall deficits during written records for key climatic variables, such ashistorical famines and droughts by implementing rainfall and temperature, that date to the first de- a simple optimization procedure for the period cade of the 20th Century. Only during the 1930'swhen both sets of data were available (1895- did synoptic stations become generally established 1954). The authors calculated the magnitude of cu- throughout the region. The available hydroclimatic mulative rainfall deficit corresponding to various Dendrochronology Prospects in West Africa 33 folklore droughts during the instrumented period,the mid -latitudes and have limited time and re- but provided no objective means for validating the sources for devoting effort to evaluating tropical estimates for events predating the beginning oftrees for dendrochronological research. measured data. Nevertheless, spurred by rapid environmental To date, paleolimnological investigations of thechange and the successful development of precise- Kajemarum oasis in the Manga Grassland ofly dated chronologies in South America and South northeastern Nigeria have furnished much of the Asia, more concerted effort was brought to bear in information on Holocene climate for the Sahel.an attempt to establish the potential of African Street -Perrot et al. (2000) found evidence of multi -trees for tree -ring analysis during the 1990s. Stable decadal to centennial -scale droughts in the 5500 -et al.(1995) developed tree -ring chronologies year paleolimnological sequence of dust deposi-based on Vitex keniensis and Premna maxima tion in the oasis, prompting them to declare thatgrown in plantations in Southeastern Mt. Kenya. the post -1968 Sahel drought is not unique. John - Stahle et al. (1997) also developed a 200 -year dat- athan et al. (1998) provided insight into variationsed chronology for Pterocarpus angolensis at in salinity and solute composition in the oasis, Hwange National Park, Zimbabwe. Maingi (1998) both of which reflect hydroclimatic changes in theevaluated 19 species sampled from the Tana riv- vicinity of the lake. Salzmann (1996) and Wallererine forest of Kenya. He identified four species and Salzmann (1999) determined that the modern(Acacia