Climatic Signals in Tree Rings of Burkea Africana and Pterocarpus Angolensis from Semiarid Forests in Namibia

Trees (2004) 18:442–451 DOI 10.1007/s00468-004-0324-0 ORIGINAL ARTICLE Esther Fichtler · Valerie Trouet · Hans Beeckman · Pol Coppin · Martin Worbes Climatic signals in tree rings of Burkea africana and Pterocarpus angolensis from semiarid forests in Namibia Received: 17 September 2004 / Accepted: 28 January 2004 / Published online: 30 March 2004 Springer-Verlag 2004 Abstract Tree-ring studies contribute worldwide to the both sites. The response at the westernmost site to two understanding of climate and its relation to tree growth. ENSO indices indicates a tree growth decrease during El Long tree-ring chronologies serve as climate proxies for Nio years, which are generally dry in southern Africa. the reconstruction of past, pre-instrument climate and its recent change. In tropical regions, the availability of ex- Keywords Tree rings · Semiarid Africa · El Nio actly dated tree-ring chronologies is limited. The den- Southern Oscillation · Burkea africana · Pterocarpus droclimatic potential of two dominant species from dry angolensis forests in northern Namibia was examined in the study presented in this paper. Both species (Burkea africana Hook and Pterocarpus angolensis DC) were sampled at Introduction two sites (ca. 900 km apart), and the response to several climatic variables, including ENSO indices, is studied. To understand natural climate variability and the magni- All specimens showed distinct growth rings and cross- tude of possible human impact on it, high resolution, long- dating between radii was successful for all trees. Species- term climate data are needed (Bradley et al. 1996). The specific mean curves were built for both sites. The mean availability of such data becomes more urgent in critical curves of different species of the same site synchronised regions, where the environment is closely linked with its significantly, allowing the construction of a site-specific climate (Yadav and Singh 2002). Because the major part chronology. Synchronisation between sites was not pos- of southern Africa suffers from poor infrastructure and sible, but spectral analysis of the chronologies implied low socio-economic development, the consequences of that both show similar long-term (6.7 year) oscillation extreme weather or climate anomalies are often devastat- patterns. B. africana is more sensitive to rainfall variation ing to both people and property (Fauchereau et al. 2003). than P. angolensis at both sites. Growth response to High vulnerability to weather and climate hazards, asso- rainfall was positive, but a time-lag in the reaction oc- ciated with the growing population, make southern Africa curred between the sites, corresponding to the time-lag one of the regions where potential changes in the hydro- of the beginning of the rainy season. Air temperature logical cycle due to global warming could lead to extreme showed a negative correlation with stem increment at negative impacts on societies (Shulze et al. 2001). Climatic time-series, as provided by instrument-ac- E. Fichtler ()) · M. Worbes quired records, are very limited in Africa, in time as well Institute of Agronomy in the Tropics, as in space (Anyamba and Eastman 1996). Proxy data University of Gttingen, offer a potential solution to this problem. Tree-ring data in Grisebachstrasse 6, 37077 Gttingen, Germany e-mail: [email protected] particular often have a fixed annual resolution and can Tel.: +49-551-393753 produce absolute time-series (Cook 1992). If trees show Fax: +49-551-393759 annual rings, dendrochronology is doubtless the safest and most reproducible way of age dating (Worbes 2002). V. Trouet · H. Beeckman Long-term ring chronologies are a potentially powerful Laboratory for Wood Biology and Xylarium, tool for analysing demographic trends and ecological Royal Museum for Central Africa, Leuvensesteenweg 13, 3080 Tervuren, Belgium factors influencing growth in tropical trees (Vetter and Botosso 1989). Previous tree-ring studies have shown the P. Coppin dendroclimatic potential of several tree species in south- Laboratory for Forest, Nature and Landscape Research, ern Africa (February 2000; Gourlay 1995; Stahle et al. Katholieke Universiteit Leuven, 1996, 1999; Trouet et al. 2001, 2004). Basic knowledge of Vital Decosterstraat 102, 3000 Leuven, Belgium 443 the relationship between environmental variables and tree Tree species growth is crucial for the prediction of future growth re- Both tree species are common in southern African woodlands. sponses to climatic variation (Pumijumnong 1999) and to Burkea africana (Red Syringa) is a medium sized tree usually 8– such long-term phenomena as El Nio Southern Oscilla- 10 m tall, with a maximum height of 20 m (Coates Palgrave 1997; tion (ENSO) (Cook 1992). Storrs 1995). It occurs in various types of woodlands over a wide Recent climatic change may bring about rapid envi- range of altitudes and habitats, but is most characteristic for hot, low-lying areas (Coates Palgrave 1997). Leaf fall takes place from ronmental changes that may have extreme impacts on tree May to September and new leaves flush from August to December. growth (Jalilvand et al. 2001), as tropical tree growth is Flowers appear from August to November whereas fruits ripen sensitive to the timing and variation in local climatic re- from February to October but can remain on the tree for a very long time (Coates Palgrave 1997; Storrs 1995). The wood is hard, heavy gimes (Enquist and Leffler 2001). Global warming could 3 also partly be related to the enhanced influence of ENSO (865 kg/m at 12% moisture content, Goldsmith and Carter 1981) and tough. Because of its small tree stature the wood is limited in on southern African rainfall (Fauchereau et al. 2003). The use. reconstruction of the regional influence of ENSO on cli- Pterocarpus angolensis (African Blood wood) is a medium mate, can only be derived from information acquired in sized to large tree up to 16 m high, but reaching 28 m under ideal the centre of the relevant region and therefore proxy data conditions (Coates Palgrave 1997; Storrs 1995). It occurs in woodland and wooded grasslands. Leaves fall from May to June need to be acquired locally (D’Arrigo and Jacoby 1992). and new ones emerge from September to October. Flowering takes This study examines the potential of tree rings of two place in the months August–December while fruiting occurs from indigenous species in Namibia for dendrochronological January to June (Coates Palgrave 1997; Storrs 1995). P. angolensis studies and the connection between tree growth and cli- is very resistant to fire. It is one of the best-known, most generally used and most valuable timber crops in southern tropical Africa mate for these species. Up to now no chronologies are (Coates Palgrave 1997) with a density of 640 kg/m3 at 12% available for Burkea africana Hook or Pterocarpus an- moisture content (Goldsmith and Carter 1981). golensis DC from Namibian stands, although for the latter a short chronology from Zimbabwe exists (Stahle et al. 1999). Special emphasis is placed on the connection be- Tree-ring analysis tween tree growth and the ENSO phenomenon. All samples consisted of full stem discs. The discs were dried in open air and polished progressively with sanding paper bands up to grit 600. Then the samples were cleaned from dust with compressed air. Material and methods Tree-ring structure was analysed under a stereo-microscope with low magnification. Ring boundaries were marked starting Study sites from the wane edge to the centre, with special attention paid to the determination and registration of wedging rings. At two sites in northern Namibia, Ondangwa (56170S, 59150E) 0 0 Ring widths were measured to the nearest 0.01 mm using and Katima Mulilo (3017 S, 1724 E), 32 individual trees of two LINTAB equipment (Rinn and Jkel 1997). To avoid any inaccu- species (Burkea africana Hook, Caesalpiniaceae; Pterocarpus an- racy the discs were readjusted during the procedure to measure golensis DC, Papilionaceae, both Leguminosae) were sampled perpendicularly to the rings. during the dry season of 1998 (Katima Mulilo) and 2000 (Ondang- wa). Total annual rainfall is lower in Ondangwa (454 mm) as compared to Katima Mulilo (672 mm). The climate of both sites is Cross-dating and indexation characterised by an annual dry season of 5–7 months with less than 50 mm precipitation (Fig. 1), which lasts in general from May to Growth curves are compared visually and statistically to bring ring- October. width series in a synchronous position (Pilcher 1990; Worbes 1995). Successful cross-dating indicates the influence of an external growth factor on tree growth in a region (Eckstein et al. 1981). To gain reliable results the overlap of cross-dated time-series should reach a minimum of 40 years (Pilcher 1990). The Student’s t-value expresses the degree of affinity of two time-series (Baillie and Pilcher 1973): pffiffiffiffiffiffiffiffiffiffiffi n À 2 Student0s T ¼ r pffiffiffiffiffiffiffiffiffiffiffi 1 À r where n is the number of values and r is the correlation coefficient. In addition to this the “Gleichlufigkeitskoeffizient” (GLK) is used in dendrochronology to investigate the success of cross-dating. The Gleichlufigkeit is a measure of the year-to-year agreement between the interval trends of two chronologies based upon the sign of agreement, expressed by the following equations (Schweingru- ber 1988): 1 XnÀ1 G ¼ ½G þ G ðx;yÞ n ix iy À 1 i¼1 where G, Gleichlufigkeit; n, number of values; and x, y, series. The indexation and calculation of the GLK as well as the Stu- Fig. 1 Annual distribution of rainfall (bars) and temperature (lines) dent’s t-value were performed with time-series analysis and pre- at Ondangwa (white) and Katima Mulilo (black) 444 sentation (TSAP) software (Rinn and Jkel 1997). In addition mean NOAA Climate Prediction Centre (http://www.cpc.ncep.noaa.gov/ sensitivity values were calculated for all chronologies to provide a data/indices/index.html).

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