EFFECTS OF CLIPPING ON SIZE AND TILLERING OF NATIVE AND AFRICAN GRASSES OF THE BRAZILIAN SAVANNAS (THE CERRADO)

African grasses are becoming major invaders of crop fields, degraded rangelands, and natural reserves of the Blazilian savannas (the cerrado). One of the main hypotheses proposed to explain the successful spread of these species suggests that they are more grazing tolerant than native grass species. To test this premise I compared African and native grasses for tillering and size change responses after clipping. It was hypothesized that African species would tiller more but be shorter after clipping than native species. Four African and five native grasses were clipped in the field at three different periods: early wet season, late wet season, and dry season. Control plants were not clipped. Plant height and number of tillers were measured. Plant base circumference was also measured in the caespitose species to determine whether or not species were spreading laterally as a result of tillering. Comparisons between control and clipped plants were made one month after clipping. Control and early wet-season clipped plants were also followed for one year. Defoliation decreased plant height in all species; tillering increased only in plants of the African species clipped during the late wet season; and plant base circumference did not increase in any caespitose species. There is evidence that only in the African species Hyparrhenia rufa did the circumference of the base increase as a consequence of tillering. A time trend analysis revealed that plant height of both African and native species was affected by both clipping and time and that plant base circumference was affected by clipping only in H. rufa. The trend analysis also showed that tillering was not stimulated by clipping in any of the species and that tillering was more a function of time than treatment. Clipping significantly reduced tillering in the African species Melinis minutiflora. It is concluded that tillering is not necessarily a response by grasses to defoliation, and that the invasion of cerrado by African grasses is not the consequence of the higher grazing tolerance of the invading species.