Does spring defoliation affect energy reserves in a South African mesic grassland geophyte?

Abstract

Overgrazing reduces the abundance of mesic grassland forbs, including geophytes. The potential negative effect of spring grazing, aimed at exploiting high-quality grass forage, on energy reserves in underground storage organs (USOs) of a mesic grassland geophyte, Merwilla plumbea (Lindl.) Speta, was investigated. In a pot trial, plants were subject to three defoliation treatments: spring, summer, and a double cut in both seasons, alongside an undefoliated control. Seasonal changes in herbage productivity, non-structural carbohydrates (NSCs), and starch levels in bulbs were measured. There was a pronounced significant seasonal pattern in NSC and starch depletion in spring with replenishment during summer through to winter. Although spring-defoliated plants experienced a 47% delayed recovery in starch levels mid-summer compared to controls, they ultimately achieved comparable aboveground biomass, bulb mass, and NSC and starch levels by the end of the growing season. While starch played a crucial role, NSCs were less affected by defoliation. Spring defoliation did not result in lasting impacts on carbohydrate reserves or macronutrients, suggesting that M. plumbea may be resilient to infrequent, intense defoliation, including in spring. Research is needed on defoliation effects in other seasons, especially during the late-season USO replenishment period, and the role of other stored energy and macronutrient reserves.