Bulb bankruptcy: energy dynamics in Eucomis autumnalis, a South African mesic grassland geophyte

Abstract

Intense grazing and trampling by livestock reduce forb populations in South African mesic grasslands by limiting photosynthesis, thereby depleting carbohydrate reserves in underground storage organs (USOs) required for persistence. The effects of simulated herbivory on above- and below-ground growth, bulb starch reserves, and water content in the bulbous geophyte, Eucomis autumnalis, were assessed over a 468-day pot experiment, with seasonal dynamics described using Generalised Additive Models. Four intense summer and autumn defoliations reduced seasonally integrated above-ground dry matter (-83.7%), bulb mass (-51.7%), starch concentration (-32.3%), total starch pool (-68.9%), and bulb water pool (-35.1%), while increasing bulb water content (+29.4%) relative to controls. Bulb growth and starch pools (bulb mass x starch concentration) were most strongly suppressed during late summer and autumn, when control plants increased bulb mass and replenished starch reserves. Repeated defoliation forced plants to divert assimilates to regrowth, disrupting seasonal source-sink dynamics and driving progressive depletion of reserves; bulbs were consequently pushed towards energetic bankruptcy, potentially unable to sustain future growth and persistence under chronic grazing. These findings highlight the need to avoid high stocking densities, protect plants from intense autumn grazing, and provide periodic full-year recovery rests to conserve geophytic forbs in mesic grasslands.