Compensatory growth of the African dwarf shrub Indigofera spinosa following simulated herbivory

Use of long-term herbivory studies in understanding the effects of Livestoc k grazing on dwarf shrubs of arid zones of Africa is uncommon. Moreover, re search has seldom focused on monitoring a 4-5 yr effect of herbivory at the level of individual plants. This study provided information on field-based experiments and simplified statistical modeling to test compensatory growt h responses of individuals of the African dwarf shrub Indigofera spinosa in northwestern Kenya. From August 1986 to January 1990, we simulated livesto ck grazing during dry and wet seasons and over the full year, whereby plant s were defoliated during both wet and dry seasons. Individuals of I. spinos a (n = 480 plants) were subjected to one of five clipping intensities: uncl ipped control (0%), light (30%), moderate (50%), severe (70%), and very sev ere (90%) clipping; and defoliated of regrowth (i.e., new biomass). Our fin dings showed that rainfall, clipping regimes, and seasons of treatment infl uenced the compensatory growth response. Rainfall more than residual biomas s influenced regrowth, while plants with greater residual biomass produced more regrowth than those with less. Optimum residual biomass was achieved u nder the 30% clipping level, while least was maintained under the 90% level . We separated compensatory growth response into under- and overcompensatio n. We showed that overcompensation occurred under some conditions but not i n others. The shrub displayed relative overcompensation with a compensatory ratio (CR) > 1.0 for three continuous years with light clipping regime dur ing the wet season defoliations (WSD). In two of five years there was overc ompensation with the dry season defoliations (DSD), but undercompensation ( CR < 1.0) with full-year defoliations (FYD). On average, under the moderate , severe, and very severe clipping regimes the plants had undercompensation . Patterns' of change of cumulative regrowth and its derivative, relative g rowth rates (RGR), provided different compensatory responses. RGR was more positive at lower cumulative regrowth but gradually declined and became neg ative when cumulative regrowth was maximum. The exception was in WSD where RGR remained positive for three years. DSD by far achieved greater cumulati ve regrowth than WSD and FYD. However, compared to the controls, FYD (excep t under light regime) exactly overcompensated for total "biomass budget" wh ile WSD overcompensated under all clipping regimes except very severe. DSD under the light and moderate regimes overcompensated by +343.6% and by +202 .7%, respectively. The study showed that I. spinosa combines tolerance with compensatory growth response to cope with a wide array of herbivory and se asons of use. The shrub may be grazed under light regime during the wet and dry seasons as opposed to the full-year grazing which is unsustainable.