Current issue: 56(2)
Microcatchment water harvesting (MCWH) improved the survival and growth of planted trees on heavy soils in eastern Kenya five to six years after planting. In the best method, the cross-tied furrow microcatchment, the mean annual increment (MAI; based on the average biomass of living trees multiplied by tree density and survival) of the total and usable biomass of Prosopis juliflora (Sw.) DC. were 2,787 and 1,610 kg ha-1 a-1 respectively, when the initial tree density was 500 to 1,667 trees per hectare. Based on survival, the indigenous Acacia horrida Span., A. mellifera (Vahl) Benth. and A. zanzibarica (S. Moore) Taub. were the most suitable species for planting using MCWH. When both survival and the yield were considered, a local seed source of P. Juliflora was superior to all other species. The MAI in MCWH was at best distinctly higher than that in the natural vegetation (163–307 and 66–111 kg ha-1 a-1 for total and usable biomass respectively); this cannot satisfy the fuelwood demand of concentrated populations, such as towns or irrigation schemes.
The density of seeds of woody species in the topsoil was 40.1 seeds/m2 in the Acacia-Commiphora bushland and 12.6 seeds/m2 in the zone between the bushland and the Tana riverine forest. Rehabilitation of woody vegetation using the soil seed bank alone proved difficult due to the lack of seeds of desirable species.
The regeneration and dynamics of woody vegetation were also studied both in cleared and undisturbed bushland. A sub-type of Acacia-Commiphora bushland was identified as Acacia reficiens bushland, in which the dominant Commiphora species is C. campestris. Most of the woody species did not have even-aged population but cohort structures that were skewed towards young individuals. The woody vegetation and the status of soil nutrients were estimated to recover in 15–20 years on Vertic Natrargid soils after total removal of above-ground vegetation.