Frequent defoliation rapidly and progressively reduces the vigour of a perennial mesic grassland forb, Thunbergia atriplicifolia

Abstract

Forbs in South African mesic grasslands are ancient, numerous, and diverse, thriving with fires and light herbivory but declining under chronic, severe grazing and trampling. This study tested the hypothesis that repeated herbivory progressively weakens forbs by reducing their underground growth and storage capacity, ultimately threatening their persistence in grazed grasslands. Over two years, Thunbergia atriplicifolia, a forb with tuberous storage roots, was subjected to frequent, severe defoliation during the growing season in a replicated pot experiment. Recurrent defoliation significantly reduced stem length, stem number, and tuberous and fibrous root biomass in the subsequent spring of each year. Notably, underground growth declined more sharply in the second year (75.6%) than in the first (60.4%), despite fewer defoliations and higher rainfall. While defoliated plants initially compensated with increased above-ground growth, this strategy proved unsustainable, leading to a severe reduction in below-ground reserves. These findings indicate that persistent defoliation reduces the resilience of forbs, diminishing their competitiveness against grasses and threatening their long-term survival. To conserve the high forb diversity of mesic grasslands, precautionary management with reduced defoliation frequency and intensity and periodic whole-year rest periods are recommended, alongside adaptive management that adjusts grazing intensity and timing based on regular monitoring.