Beneath the flames: season of burning effects on growth in native grassland plants with underground storage organs

Underground storage organs (USOs) allow many perennial herbaceous plants to survive disturbance and seasonal stress by storing resources that support resprouting. However, the cost of disturbance may depend strongly on when it occurs relative to plant phenology and resource allocation cycles. In temperate grasslands, prescribed fire has historically been applied during summer or autumn when many herbaceous species are dormant. Increasing use of spring burns means fire can coincide with active growth and reproduction; yet, the implications for USO-dependent species remains poorly understood. We tested how burn season influences growth and belowground resource dynamics in four perennial species common to temperate native grasslands. Plants were subjected to simulated fire treatments in either spring (during active growth) or summer (after the main growth season). USO dry mass was measured as a proxy for stored resources, and post-fire phenological responses were monitored to assess the effects of subsequent growth and reproduction. Responses varied among species. Eryngium ovinum (Apiaceae) and Arthropodium strictum (Asparagaceae) rapidly resprouted after spring burning, and in some cases, reproduced within the same growing season. Summer burning coincided more closely with their vegetative dormancy. Both species generally had lower USO biomass following spring burns than summer burns. In contrast, Burchardia umbellata (Colchicaceae) and Bulbine bulbosa (Asphodelaceae) showed little within-season resprouting after spring fire and no clear differences in USO biomass between treatments. These findings indicate that the timing of fire can interact with species-specific phenological strategies to influence the costs of resprouting from underground storage organs.