Soil seedbanks are shaped by the timing of fires in a Mediterranean‐type ecosystem

Abstract

Many plants rely on soil seedbanks to persist in fire‐prone ecosystems. However, knowledge of plant responses to fire is largely based on above‐ground dynamics. Quantifying how fire influences the seedbank of a diverse range of species will improve fire management. Here, we aim to understand how the timing of fires influences species occurrence in the soil seedbank, and how this relates to species traits, in a Mediterranean‐type ecosystem. We sampled the soil seedbank across 57 sites that represent a range of fire frequencies (1–9 fires in 81 years) and time since fire (1–81 years). Through a 15‐month germination experiment, we identified 39,701 germinates from 245 plant species. Using nonlinear models, we quantified the responses of 75 species' soil seedbanks to fire history and compared these to above‐ground responses. Fire influenced species' soil seedbanks according to seed longevity and species' life‐history traits. We observed a response of 15 species' seedbanks to time since fire: All were species with long‐lived seed, including eight annuals, five resprouters and an obligate seeder. Similarly, we observed a response of 17 species' seedbanks to fire frequency, 16 of which had long‐lived seed. Extensive periods without fire (>40 years) increase seedbank availability for annuals that form a long‐lived seedbank, and frequent fire (every ≤15 years) depletes it. We did not detect a consistent influence of fire on species from several other functional types. Many (53%) species found in the soil seedbank were not found in the above‐ground vegetation, and seven of the 22 modelled displayed an above‐ground influence of fire. Synthesis and applications: Fire frequency influences the probability of species occurrence in the soil seedbank. Frequent fire depletes seed availability of species that depend on long‐lived soil seedbanks for persistence. Yet, other species, including perennials with short‐lived seed storage, appear to be resilient to the frequency of fires experienced to date. We suggest fire management should aim to generate variation in fire frequencies within the landscape, including areas of low fire frequency, tailored to maintain rich plant diversity within the soil seedbank.