Spatial and temporal woodland patterns along the lower Turkwel River, Kenya

Spatial and temporal patterns of riverine woodlands in arid regions of Africa are poorly documented despite their considerable conservation value. We studied 1540 ha of riverine woodland in the lower Turkwel River floodplain, Kenya, between 1990 and 1998. Forty‐one woodland patches were mapped and their soil physical and chemical characteristics, tree species diversity, woody cover, tree density, wood volume and woodland regeneration were determined. The riverine woodland comprised nine vegetation types and a total of 14 woody species. Woodland patch mosaics were associated with microtopographical features and selected soil attributes. The most important woody species were Hyphaene compressa H. Wendl., Acacia tortilis (Forssk.) Hayne and Cadaba rotundifolia Forssk. The exotic Prosopis chilensis (Mol.) St. was invading parts of the riverine woodland. Overall, woody species diversity was low compared to similar riverine woodlands in East Africa. Tree density, wood volume and woody plant regeneration declined over the 8‐year study period, while woody cover was unchanged. Reduced tree density, wood volume and regeneration of woody species might be linked to changes in river flood patterns following the impoundment of the Turkwel Gorge Dam. It is suggested that spatially heterogeneous and temporally stochastic regeneration events, together with occasional tree mortality caused by channel abandonment, create the complex pattern of woodland patches in the lower Turkwel River floodplain. The mapped woodland patches may serve as monitoring units, which in future could reveal the interplay between changes in flooding patterns as a result of dam impoundment, anthropogenic disturbance and the well‐being of the riverine woodlands. Résumé Les schémas spatial et temporel des forêts riveraines des régions arides sont mal documentés malgré leur énorme valeur en terme de conservation. Nous avons étudié 1540 ha de forêt riveraine dans la basse plaine inondable de la rivière Turkwell, au Kenya, entre 1990 et 1998. On a répertorié quarante et une parcelles de forêt et on en a déterminé les caractéristiques physiques et chimiques des sols, la diversité des espèces d'arbres, le couvert forestier, la densité d'arbres, le volume de bois et la régénération forestière. La forêt riveraine comprend neuf types de végétation pour un total de 14 espèces ligneuses. Les mosaïques des parcelles de forêt furent liées aux caractères micro‐topographiques et à des caractéristiques précises du s