Grass response to clipping in an African savanna: Testing the grazing optimization hypothesis

It has been suggested that grazing could stimulate the aboveground dry mass and/or nitrogen yields available to grazers (i.e., the grazing optimization hypothesis, GOH), but the actual importance of this effect is still controversial. The GOH has only been tested for a few grasslands and one savanna ecosystem, but not for the moistest and warmest grasslands and savannas of the world. The objectives of this study were to test the GOH in the humid savanna of Lamto (West Africa) by analyzing the growth of grasses in response to a field trial with three levels of clipping X two levels of fertilization. We quantified the effects of clipping and fertilization on the dry matter and nitrogen yields to grazers (i.e., mass or nitrogen amount in clipped-off tissues during the experiment) and on the remaining yield (i.e., mass or nitrogen amount in residual phytomass at the end of the experiment) over a three-month period. Total phytomass yield, the sum of yield to grazers, and the remaining yield was maintained under moderate clipping frequency and fertilization as compared to control conditions. Both clipping frequencies decreased the remaining phytomass yield as compared to control plots. Clipping frequency significantly increased nitrogen concentrations in the total yield, in the remaining yield, and in the yield to grazers. Total nitrogen yield and nitrogen yield to grazers were 65% and 91% higher on the plots experiencing moderate clipping frequency with fertilization as compared to control plots. The study shows that grazers in this humid savanna system could potentially modify ecosystem processes in such a way as to partly alleviate nutritional deficiencies, but, only in the presence of increased nitrogen availability.