COMPENSATORY GROWTH OF THE AFRICAN DWARF SHRUBINDIGOFERA SPINOSAFOLLOWING SIMULATED HERBIVORY

Abstract

Use of long-term herbivory studies in understanding the effects of livestock grazing on dwarf shrubs of arid zones of Africa is uncommon. Moreover, research 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 mod- eling to test compensatory growth responses of individuals of the African dwarf shrub Indigofera spinosa in northwestern Kenya. From August 1986 to January 1990, we simulated livestock grazing during dry and wet seasons and over the full year, whereby plants were defoliated during both wet and dry seasons. Individuals of . spinosa (n = 480 plants) were subjected to one of five clipping intensities: unclipped control (0%), light (30%), moderate (50%), severe (70%), and very severe (90%) clipping; and defoliated of regrowth (i.e., new biomass). Our findings showed that rainfall, clipping regimes, and seasons of treatment influenced the compensatory growth response. Rainfall more than residual influ- enced regrowth, while plants with greater residual produced more regrowth than those with less. Optimum residual was achieved under the 30% clipping level, while least was maintained under the 90% level. We separated compensatory growth re- sponse into under- and overcompensation. We showed that overcompensation occurred under some conditions but not in others. The shrub displayed relative overcompensation with a compensatory ratio (CR) > 1.0 for three continuous years with light clipping regime during the wet season defoliations (WSD). In two of five years there was overcompensation 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 growth rates (RGR), provided different compensatory responses. RGR was more positive at lower cumulative regrowth but gradually declined and became negative when cumulative regrowth was maximum. The exception was in WSD where RGR remained positive for three years. DSD by far achieved greater cumulative regrowth than WSD and FYD. However, compared to the controls, FYD (except under light regime) exactly overcompensated for total biomass budget while WSD overcompensated under all clip- ping regimes except very severe. DSD under the light and moderate regimes overcompen- sated by +343.6% and by +202.7%, respectively. The study showed that L. spinosa com- bines tolerance with compensatory growth response to cope with a wide array of herbivory and seasons 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.