Compensatory Growth of the African Dwarf Shrub Indigofera spinosa Following 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 modeling 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 I. spinosa (n ϭ 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 biomass influenced regrowth, while plants with greater residual biomass produced more regrowth than those with less. Optimum residual biomass was achieved under the 30% clipping level, while least was maintained under the 90% level. We separated compensatory growth response 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) Ͼ 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 Ͻ 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 clipping regimes except very severe. DSD under the light and moderate regimes overcompensated by ϩ343.6% and by ϩ202.7%, respectively. The study showed that I. spinosa combines 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.

Effects of simulated shoot and leaf herbivory on vegetative growth and plant defense in Acacia drepanolobium

“…Compensatory growth has also been found in herbs (Whitham et al 1991, Paige 1992, Alados et al 1997, Cebrian et al 1998, Meyer 1998, Lennartsson et al 1998, Paige 1999, shrubs (Oba 1994, Tolvanen and Laine 1997, Oba et al 2000, and trees , Dangerfield and Modukanele 1996, Kudo 1996, Bergströ m et al 2000) (see also review in Strauss and Agrawal 1999). Responses to herbivory may depend on the timing of herbivory, the type and extent of herbivory, the availability of resources in the environment to support regrowth (McNaughton 1983, Rosenthal andKotanen 1994), and the grazing/browsing history of the plant (McNaughton 1983, Paige 1992, Lennartsson et al 1997.…”

Effects of domestic livestock on the vegetation of the Knersvlakte, South Africa

“…Therefore, the plant overcompensates for the experienced losses by producing even more flowers (MCNAUGHTON 1983). This mechanism has been demonstrated for other African species, for example the dwarf shrub Indigofera spinosa (OBA et al 2000) and Acacia drepanolobium (GADD et al 2001) and could also apply for D. schoenlandianum which seems to be a palatable species, as its seeds were very frequently found in dung (compare 3.7). However, with higher grazing pressure, the increased production of flowers might be insufficient for overcompensating the more severe losses.…”

Response ofArundo donaxL. (giant reed) to leaf damage and partial defoliation

“…A number of studies summarized in Nowak and Coldwell (1984) indicated that partial defoliation resulted in increased photosynthetic rates for leaves that remained. Increased plant growth rates have been reported for the African dwarf shrub Indigofera spinosa exposed to different levels of clipping (Oba et al 2000).…”

“…Some plants respond to loss of leaf tissue by increasing growth rates (Oba et al 2000;Verges et al 2008), altering resource allocation patterns (Sun et al 2009), or increasing the photosynthetic rate (Nowak and Coldwell 1984;Meyer 1998). The effects of defoliation and leaf damage on growth characteristics of clonal species have not been studied as extensively as in other species (Esmaeili et al 2009).…”