Micro environmental influences of Acacia Etbuica and Acacia Tortilis on herbaceous layer production Inimukogodo Rangeland, Laikipia District

Abstract

The influence of the canopy cover of mature Acacia etbaica and A. tortilis, on micro-environment was studied by measuring the ground layer Above ground Net Primary Productivity (ANPP), cover, species composition, soil water and fertility status in Mukogodo, Laikipia district. The study was done in the period of October 1992 up to July 1993 in an episodic stream catchment. Four sites were chosen in 3 different range sites, based on slope, initial ground cover, woody species density and depth of the soil. These areas were designated as Riverine (along the river) Glade, (a former cattle bonia), Bare open, (characterised by bare open patches in between the woody species) and Bare enclosed (similar to the Bare open, but enclosed at the beginning of the study). Three zones - canopy (up to the edge of the tree crown), sub canopy (extending to 1 and 1/2 radius of the canopy) and open (extending up to 3 radii of the crown) - were marked out around the above species, and replicated three times in each site. Herbaceous layer production was assessed by clipping and a temporally cage method, above ground plant cover was monitored using a point frame method. Soil fertility status was measured using standard methods. Soil water was monitored up to 90 em depth using a neutron probe. The canopy zone in the Riverine site had significantly less (P < 0.05) ANPP (3.37 tons ha-1) compared to the sub canopy and open zones (4.5~ and ~.32 tons ha-1 respectively). The Glade plot had at least 50 % (0.95 tons ha-l) less ANPP in the canopy and sub canopy zones compared to the open zone (1.78 tons ha-I) (significant at P < 0.05). In contrast, the canopy zones in the Bare open and Bare enclosed plots had at least 2 times more production in the canopy zone (2.9~ and 3.32 tons ha-I respectively) than in the open (1.37 - 1.49 tons ha-l) (significant at 5 Xll %). There was consistently more cover under the canopy zones of all the plots than in the open areas. The species composition under the canopy zone was more in abundance relative to the open zones. Mineral nutrients were also higher under the canopy zones than in the open zones in all the plots. Available soil water was consistently more in the open zone of the Riverine plot than in the under canopy zone. However, the opposite was the case in the rest of the plots. Available soil water explained at least 60 % of the variations in ground layer ANPP among the zones and the sites, while soil nutrients, especially, Nitrogen, Phosphorous and Potassium accounted for at least 30 % of the variations. Most of the soil water and soil nutrients under the canopy zone may have resulted from runoff [TOm the adjacent open areas, litter decomposition, and other exogenous sources like dung deposition from mammals. Conservation and management efforts of the range should give cognition to the differential response in ANPP to the presence of the woody species in various range sites.