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.