Freeze-thaw effects on the biology of seeds and seedlings from exotic and native plants

Abstract

During the cold-moist conditions of winter, stratification occurs that alleviates diaspore dormancy in many species enabling spring germination. Winters in southeastern United States frequently have warm spells, lasting a few days, interrupted by cold (freezing) periods. In this study, the effects of a freeze during stratification and following a warm spell on diaspores and seedlings of locally common exotic and native plants were determined. In the laboratory, diaspores were exposed to freezing temperatures, following stratification and stratification + warm spell, that simulated a cold front passage and then they were incubated at warm temperatures. Freezing at both -10C and overnight reduced viability and germination in some of the species. Only seeds of two exotic Lonicera species germinated during the simulated warm spell, and these seedlings survived down to -5 but none survived overnight at -10C. In a common garden experiment, Celtis, L. japonica and L. maackii had cotyledons before a freeze; diaspores of other species only had roots. Seedlings of L. japonica and L. maackii had moderate survivorship, whereas those of Celtis had low survivorship. In the field, diaspores were sown inside and outside (control) of open top chambers (OTC). Viability of four species did not differ between the OTCs and control, probably due to the failure of the OTCs to warm above ambient conditions. However, viability of the native, but not exotic, species was greatly reduced with a warm spell interruption during winter. Thus, freezing may differentially affect seed viability and germination and seedling survivorship of exotic vs. native species and potentially influence competitive interactions between them.