The persistence and germination of fern spores in fire-prone, semi-arid environments

Abstract

In addition to population regeneration, the spore phase provides ferns with a capacity for dispersal through space and time (if spores are able to survive for long periods), and buffers their populations against environmental extremes. Inhabiting dry and fire-prone environments provides specific challenges to ferns, and the traits of fern spores in relation to these challenges are not well understood. Their shallow, dense and fibrous root networks mean that the loss or establishment of fern populations in disturbed ecosystems may influence soil stability, and indicate a keystone role in ecosystem function and ecological restoration. Here, we examine the requirements for, and limits of, spore persistence and germination of three Cheilanthes Syn.Fil. (Pteridaceae) species, namely, optimal conditions for spore germination and prothallial growth, sensitivity to temperature extremes and spore longevity. Viability of fresh spores was assessed following exposure to temperature extremes (-20 to 250°C) or after incubation at a range of temperatures (10-35°C) and in light or dark conditions. Viability of spores from herbarium voucher specimens was also assessed, covering 3-65 years of storage. Maximal germination occurred among spores incubated between 20°C and 30°C in light. Further temperature variation within this range resulted in a ×10 difference in prothallial size. Germination was unaffected by 10min of exposure to temperatures up to 100°C, but was reduced after exposure to temperatures of -20°C and 110°C. Germination of herbarium-stored spores demonstrated longevity of up to 15 years. This longevity, combined with high spore density in field soils (including at depth to at least 10cm), very high germination under ideal conditions, and microscopic size, describes a disturbance-mediated propagule persistence strategy. Spores are non-persisting and will germinate if exposed to light and water, but do persist if buried. Germinable spore can be found 5cm below the soil surface, and can persist for at least 15 years, but the light requirement for germination means that persistent spores can achieve germination only if exhumed. Elucidating these traits reveals a unique pathway for plant persistence, and contributes to the development of the restoration capacity of arid-environment ferns.