Evaluation of Prosopis Juliflora Productivity for Carbon Stocks and Animal Feeds in Selected Dry land Sites, Magadi Sub-county, Kenya

Abstract

Factors affecting Prosopis juliflora productivity and its spread patterns on the hillslopes and floodplain landscapes were evaluated in the drylands of Magadi subcounty, Kenya. The above-ground biomass and carbon stocks were estimated using allometric equations based on several tree dimensions. The productivity of Prosopis pods was assessed over a period of ten months from a random sample of 320 Prosopis trees on the floodplains and hillslopes landscapes. Time series trends for prosopis biomass, monthly rainfall, temperatures; cattle, sheep and goats populations were analyzed from 2000 to 2014. MODIS NDVI and NPP (250m) data was used to determine P. juliflora biomass and its temporal spread dynamics. Four categories of eight plots measuring 900m2 each were randomly selected, and fenced off into dense, dense managed, moderate and sparse densities respectively. The managed dense plots had the Prosopis trees pruned and spaced at five meters apart. Of the 320 Prosopis trees, 128 were randomly selected for destructive sampling for fresh weights determination for the development of Prosopis biomass estimation models. The mean monthly temperatures increased from 330C in 2000 to 370C in 2014; rainfall decreased from 600mm in 2000 to 250mm in 2014 and the divergence from the long term mean rainfall (450mm) decreased from 585mm to 403mm. At the same time cattle population decreased while sheep and goats populations remained stable. P. juliflora spread correlated positively (r=0.2; P<0.05) with mean monthly temperature and negatively (r=-0.4; P<0.05) with rainfall and other vegetation cover in floodplains but not in the hillslopes parts of the study area. It also correlated negatively with cattle populations (r=-0.4; P<0.05). In 2008, herbaceous cover, shrublands, and open trees together with bare areas constituted 50%, 30% and 22% respectively, out of which 70% had been replaced by Prosopis by 2014. The Prosopis NDVI in the floodplain had a lower mean (0.49) while on the hillslopes it was 0.78. The annual 2 rate of Prosopis spread was 0.13 km2, 4.76 km2 and 13.09 km2 respectively in the floodplains, while in the hillslopes, the rate of spread was less (5.36 km2, 5.37 km2 and 5.29 km2) in 2000, 2006 and 2013. The basal diameter was the most important dimension among among diameter at breast height, crown width and tree heights in estimating. The dense managed category in the hillslopes held the highest Prosopis biomass (44.13 tons/ha) followed by dense unmanaged category (43.68 tons/ha). The dense unmanaged category of the floodplains had lower estimates (34.15tons/ha) followed by dense managed category (28.01 tons/ha). The moderately and sparsely dense categories in both landscapes recorded the lowest Prosopis biomass estimates (18.75 and 3.47tons/ha in hillslopes and 12.72 and 5.09tons/ha in floodplains). Prosopis biomass increased in the dense Prosopis clusters in hillslopes landscape, but there was no biomass change in the moderate and sparse density clusters. Management increased pod production in the hillslopes landscape. Prosopis pods were produced throughout the year, but peaked in the middle of the rain season. There are viable Prosopis biomass and pod quantities in the dense managed hillslopes landscape to sustain Prosopis based trade in carbon stocks and animal feeds production. Keywords: Carbon, climate, feeds, livestock, P. juliflora, pods, productivity