Ecological effects of Festuca costata on the Drakensberg grasslands.

Abstract

Fire and herbivory are important in determining the vegetation structure in savannas and grasslands. In addition, anthropogenic activities have significantly contributed to alterations in the relative abundance and distribution of plant species in many grasslands through increased nitrogen deposition and elevated concentration of carbon dioxide in the atmosphere. Using a field experiment, the study explored the effect of neighbouring plants, defoliation, and fire on the ecological performance of a major C3 grass, Festuca costata, whose range is expanding in Afro-temperate grasslands of the Drakensberg Mountains in South Africa. The experiment had three factors: fire at two levels (burned, unburned), defoliation by clipping 50mm above-ground surface, at four levels (no defoliation, defoliation of focal plant, defoliation of neighbouring plant within a 300 mm radius, and defoliation of both the focal and the nearest neighbouring grass within a 300 mm radius from the focal plant). The third factor was plant neighbourhood types at three levels (the focal plant growing alone, focal plant growing with other F. costata plants, focal plant growing with any other grass species) for a total of 24 treatment combinations with five replicates. Using a pot experiment under garden conditions, the study evaluated the influence of soil nutrient amendment with 140 KgHaˉ¹ limestone ammonium nitrate in the performance of F. costata. The variables of interest in both experiments were; above-ground biomass production, tuft circumference, number of tillers and specific leaf area of the focal plant. Generalised Linear Models in SPSS were used to analyse the data from both experiments. The results showed that burning had a significant negative effect on above-ground biomass production, number of tillers and tuft circumference of the focal plant (P < 0.001), with the unburned grassland showing greater mean values for all parameters than the burned grassland. Defoliation had significant negative effects on above-ground biomass production, number of tillers, tuft circumference while it showed strong positive influence on specific leaf area of the focal plant. Neighbourhood types were significantly negative for above-ground biomass production, number of tillers and tuft size of the focal plants (P< 0.001 in all cases). Nutrient addition reduced the number of tillers of F. costata and tuft size and showed no significant effects on above-ground biomass production and specific leaf area. Defoliation significantly increased SLA, above-ground biomass production, number of tillers and tuft circumference were not affected.. Plant neighbourhood types showed significant negative influences on tuft circumference. Defoliation accounted for the most variations in reduced tuft size, above-ground biomass and number of tillers, however, it resulted in increased specific leaf area. These results suggest that defoliation by herbivores and use of fire may be complementary tools for managing high altitude grasslands under climate change. The findings of both the field and pot experiment suggest that F. costata may not be tolerant to selective herbivory. In addition, the study shows that the increase in nitrogen availability due to anthropogenic activity may have a negative influence on the performance of F. costata.