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

Abstract

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.

Environmental gradients in the Turkwel riverine forest, Kenya: Hypotheses on dam-induced vegetation change

“…The river is, however, strongly meandering for most of its course, and the dynamics of the lower reaches have contributed to some notable exceptions from the simple cross-sectional profile in terms of abandoned channels and steps in floodplain elevation (Stave et al, 2003). Generally, the tree canopy attains a height of 20 m along the riverbanks and the forest extends 1-3 km on either side of the river channel (Oba, 1991).…”

“…The current distribution of indigenous vegetation along the Turkwel River is also being disrupted by an invasion of the exotic Prosopis chilensis (Mol.) St. (Stave et al, 2003).…”

“…The Turkwel Gorge Dam was completed in 1990 and has long raised concern about possible forest degradation (Adams, 1989;Oba, 1991;Stave et al, 2003). However, research has not yet been conducted on the relationships between altered river discharge and forest composition.…”

Flood variability and spatial variation in plant community composition and structure on a large arid floodplain

“…These few have tended to focus predominantly on tree distributions (e.g. Hughes, 1988;Stave et al, 2003) despite herbaceous plant life often being more widespread in these environments (Higgins et al, 1997). Consequently, the role of flooding in determining spatial patterns in plant community composition and structure of hydrologically variable arid floodplains is poorly understood.…”

Invasive Plant Species and Biomass Production in Savannas

“…Rested crop fields have vegetation and soil disturbance and represent early stages of plant succession. Enhancement of regeneration in native woody species and the invasive Prosopis juliflora in rested and abandoned farms in floodplains in savannas has been reported however the invader eventually becoming dominant (Muturi et al, 2009;Stave et al, 2003;Oba et al, 2002). Fire is a key ecological disturbance in savannas which can play a role either in suppressing potential invasive plant species that are not tolerant or promoting those that are tolerant.…”