Plant species composition and diversity depending on piospheres and seasonality in the southern rangelands of Kenya

Introduction: Scarcity of water in the rangelands of Kenya has led to the introduction of piospheres. Previous research has however produced contrasting results on the effects of piospheres and seasonality on Shannon-Wiener's diversity index, and therefore, this information is still deficient. In this study, the impact of these piospheres on plant species composition, diversity, and richness was assessed. Methods: Vegetation sampling was done during both the long rains (April) and the dry season (August). Three piosphere types (dam, trough, and a seasonal river) were studied using 0.25 m(2) quadrats to sample vegetation at intervals of 20 m along 100-m transects. Four 100-m transects were used per piosphere (north, east, south, and west directions). Shannon-Wiener's diversity index was used to determine species richness and composition. Two-way ANOVA was used to determine if piospheric distance had an effect on species diversity, richness, and evenness using GenStat 15th edition. Results: A total of 22 grasses and 29 forbs were recorded in the study area. The most abundant grasses near the piospheres were Eragrostis tenuifolia (12.9%) and Cynodon dactylon (10.6%). E. tenuifolia is an increaser species and was therefore most abundant in areas of severe grazing while C. dactylon has been known to be tolerant to grazing. The most abundant forbs were Crotalaria brevidens (37.5%). The Shannon-Wiener diversity index significantly increased (F = 25.07, P = 0.001) with distance from the three piospheres owing to the high grazing intensity near the piospheres and was significantly different between piospheres, being higher (F = 10.05, P = 0.001) at 20 m from the river (1.2 +/- 0.1) compared to a similar distance from the dam (0.9 +/- 0.1) and the trough (0.8 +/- 0.2). This was probably because the trough was smaller in size compared to the other piospheres, thereby concentrating more grazing animals per unit area and causing a greater impact on plant species. Conclusions: The study demonstrated that species diversity, richness, and evenness were low near the piospheres due to heavy grazing. Similarly, the lowest species diversity, richness, and evenness were recorded near the trough compared to the dam and the river. This study therefore recommends that range reseeding and rest period be done to rehabilitate degraded areas and facilitate plant regeneration. Larger piospheres should also be used to minimize animal impact per unit area.