Current issue: 55(2)
Under compilation: 55(3)
The aim of this study was to determine the effect of leaching of heavy metals (Cr, As, Cd, Cu, Ni, Pb, Zn, Co, Mo) and earth-alkaline metal, barium (Ba), on the percolation and ditch water quality from the forest roads that contained ash in the road structures. Water quality was studied in the immediate vicinity below the ash layers as well as deeper in the road structure. Water quality was also determined in the drainage water in ditches that crossed the forest roads. A mixture of wood and peat based fly ash was used in the road structures. The treatments were: 1) no ash, 2) a 15 cm layer of ash/gravel mixture, 3) a 20 cm layer of ash/gravel mixture, 4) a 25 cm layer of ash, and 5) a 50 cm layer of ash. Large variation in the concentrations of Cr, As, Cu, Ni, Pb, Mo and Ba in the percolation water, even within the same treatment, caused difficulties to generalize the results. The concentrations of Cr, As, Ni, Pb, Mo and Ba in water samples were high in some treatment plot lysimeters containing ash compared to the control (no ash). On the other hand, many lysimeters had low and similar concentrations in water samples in the treatment plots containing ash compared to concentrations in the control plots. The ash in the roads did not affect the concentrations in the ditches. The leaching is uneven and seems to take place only from some parts of the ash layer. Risk for leaching is minimal if such parts are not widely spread.
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.