Responses of an African dwarf shrub, Indigofera spinosa, to competition, water stress and defoliation.

Abstract

spinosa, a dwarf shrub that is widely distributed in arid areas of northern Kenya, was subjected to water stress, defoliation and competition with grasses under a relatively controlled (garden) situation and in a natural setting. In the garden experiment, defoliation had no effect on total aboveground biomass produced or total N yield, but watering at 15-d intervals reduced both biomass and N yield. Allocation of both biomass and N to leaves was increased by defoliation, while allocation to stems was reduced. Root biomass was reduced by heavy defoliation at the 5-d watering frequency, but not at 10- or 15-d watering intervals. Allocation of biomass and N to grazers, as a proportion of the total yield, was not affected by watering frequency. A field experiment in a mature stand of I. spinosa was conducted to evaluate the effects of defoliation and competition with grasses on aboveground production, N yield and xylem water potential. Competition had a strong influence on all the measured variables, while defoliation had less of an impact. Plants under reduced competition had greater yields of biomass and N (by >200%) than plants under full competition. Defoliation reduced allocation to residual biomass and increased allocation of biomass to grazers. Seasonal trends in max. (pre-dawn) and min. (midday) xylem water potentials of I. spinosa showed few systematic differences among plants that received different treatments when there was adequate soil moisture (late Apr. to mid-June). During periods of severe drought, I. spinosa plants under reduced competition with grasses had significantly higher xylem water potentials than plants that experienced full competition. Defoliation of I. spinosa had little effect on water potential.