Growth and survival of Prosopis africana provenances tested in Niger and related to rainfall gradients in the West African Sahel John C. Weber a, *, Mahamane Larwanou b , Tougiani A. Abasse c , Antoine Kalinganire a a World Agroforestry Centre (ICRAF), BP 320, Bamako, Mali b Universite ´ Abdou Moumouni, Niamey, Niger c Institut National de la Recherche Agronomique du Niger, Maradi, Niger 1. Introduction Tree improvement programs must understand patterns of intraspecific variation in tree species in order to effectively manage and conserve their genetic resources. In regions with pronounced environmental gradients in temperature and rainfall, tree popula- tions may exhibit continuous variation in growth, phenology, survival and other adaptive traits in relation to these gradients (Langlet, 1971). This clinal variation is generally assumed to reflect the result of natural selection along the environmental gradient. Clines have been reported in many conifer and hardwood species from the temperate zone (Morgenstern, 1996), but there have been very few published reports of clinal variation in tropical hard- woods. The West African Sahel is a semi-arid transitional zone between the relatively humid savannah woodlands to the south and the Sahara Desert to the north. In general, mean annual rainfall decreases from south to north and from west to east in this zone (Sivakumar and Gnoumou, 1987; Sivakumar et al., 1993). One would expect, therefore, the existence of clinal variation in growth and survival of native tree species distributed throughout this region. Prosopis africana (Guill., Perrott. and Rich.) Taub. (Leguminosae, sub-family Mimosoideae) is the only native Prosopis in Africa. It has a natural distribution from Senegal to Ethiopia in the north, from Guinea to Cameroon in the south, and from Uganda to Egypt in the east; but it has disappeared from extensive parts of its range due to over-exploitation, such as excessive cutting of stems and branches resulting in limited natural regeneration (Pasiecznik et al., 2004). In West Africa, it extends throughout the Sudanian and Guinean ecozones in the southern part of its range and into the Sahelian ecozone in the northern part of its range. It does not tolerate habitually dry sites, preferring 600–1500 mm annual rainfall. Trees produce a deep taproot, grow slowly but can attain a height of 20 m in natural stands, and can be coppiced for successive harvests. Trees in natural stands generally have an erect form, but they may be multi-stemmed due to forking in the lower trunk. The breeding Forest Ecology and Management xxx (2008) xxx–xxx ARTICLE INFO Article history: Received 10 January 2008 Received in revised form 30 April 2008 Accepted 6 May 2008 Keywords: Adaptation Cline Drought Conservation Reforestation ABSTRACT Prosopis africana (Guill., Perrott. and Rich.) Taub. is important for farming and pastoralist communities in the West African Sahel. It provides a range of essential products (wood, fuel, food, fodder and medicines) and services (soil fertility improvement), and is one of many native tree species maintained in the traditional agroforesty system, known as the parkland. However, this and other valuable species are disappearing in many parts of the region due to over-exploitation and, perhaps, due to increasingly drier conditions in the region. A provenance/progeny test (275 families from 28 provenances in Burkina Faso and Niger) was established at one location in Niger to investigate geographic variation in the species. Mean annual rainfall decreases from south to north and from west to east in the seed collection area. Given these gradients, one would expect clinal variation in growth and survival of P. africana and other tree species that occur naturally in the region. Tree growth (height, number of primary branches, stem diameter at 20 cm and 1.3 m) and survival were analyzed at 11 years. Variation due to provenances was significant for growth variables and almost significant for survival. Variation due to families within provenances was not significant for any variable. Provenance means for growth and survival increased from the more humid to the drier parts of the sample region. The clines in growth and survival with regional rainfall patterns suggest that tree improvement and conservation programs should collect seeds from populations in the drier zones for planting and conservation in the West African Sahel. This may be particularly important when one considers the trend in global warming. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +223 2223375; fax: +223 2228683. E-mail address: johncrweber@aol.com (J.C. Weber). G Model FORECO-11100; No of Pages 8 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco 0378-1127/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.05.004 Please cite this article in press as: Weber, J.C. et al., Growth and survival of Prosopis africana provenances tested in Niger and related to rainfall gradients in the West African Sahel, Forest Ecol. Manage. (2008), doi:10.1016/j.foreco.2008.05.004