Climatic variation and seed persistence: freeze–thaw cycles lower survival via the joint action of abiotic stress and fungal pathogens

Global climate change is altering thermal cycles in soils during late winter, a transition that may directly threaten seed survival via abiotic stress, facilitate infection by soil-borne pathogens, or both. Using field-collected soil and seeds of the perennial bunchgrass Elymus canadensis, we tested the hypothesis that soil freeze–thaw events limit survival within the soil through direct effects on seed persistence and amplification of soil pathogen attack using a factorial experiment that manipulated freeze–thaw cycles (constant freeze vs. freeze–thaw) and fungicide addition. Freeze–thaw treatment resulted in lower seedling emergence and delayed emergence time relative to constant-freeze controls. Fungicide-treated soils had greater emergence relative to untreated soils; the lowest seedling emergence was observed in no-fungicide, freeze–thaw-treated soils (<1 %). The strong effects of thermal variability and fungi on seeds were mitigated through interactions at the seed–soil interface, as subsequent experiments showed that fungicide and freeze–thaw treatments alone do not influence dormancy. Our work demonstrates that changes in freeze–thaw events directly limit seedling emergence, delay seedling phenology, and provide opportunities for fungal pathogens to limit seed persistence. As recruitment from seeds is a key determinant of plant population dynamics, these results suggest that climatic variation may generate unique consequences for populations under changing climate regimes.