Old World climbing fern (OWCF) spores had lower germination rates (P = 0.0072) after being frozen for ≥ 3 h compared to < 3 h, and were highly susceptible to freezing periods ≥ 6 h with a 5.8- to 13.3-fold reduction in spore germination compared to controls. Freezing temperature did not affect germination of Japanese climbing fern (JCF) spores compared to controls (P = 0.32). OWCF gametophytes had reduced survival at all exposures to freezing temperatures compared to controls (P < 0.0001), and had < 0.5% survival for exposure times ≥ 3 h. The gametophytes of JCF had reduced survival at exposures to freezing temperatures ≥ 1 h (P < 0.0001) compared to controls. JCF gametophyte survival was 52.5% at 3-h exposure time, but was reduced to ≤ 0.1% at exposure times ≥ 6 h. All OWCF sporophytes exhibited 100% necrosis for all exposure times 24 h postexposure, but new growth from resprouts was observed for exposure time ≤ 6 h at 6 mo postfreezing. OWCF sporophytes' dry weight biomass was greatly reduced for all exposure times compared to controls (P < 0.0001). Results with nonlinear regression (P < 0.0001, R2 = 0.92) indicated that a single freeze for 2.4 h at −2.2 C reduced OWCF dry weight biomass to 0.01 g, a 10-fold reduction, at 6 mo postfreezing. These results indicate that JCF spores and gametophytes are more tolerant of longer exposure periods to freezing temperatures than OWCF, but OWCF exhibited the potential to spread further into northern Florida.
Nomenclature: Old World climbing fern, Lygodium microphyllum (Cav.) R. Br., Japanese climbing fern, L. japonicum (Thunb.) Sw.
Management Implications: Old World climbing fern (Lygodium microphyllum; OWCF) is likely to invade regions of USDA Plant Hardiness Zone 9A in northern Florida. Based on this study, OWCF will remain a minor component of the flora in northern Florida due to a single or multiple freezes that will affect all life stages of the fern. Spores and gametophytes of OWCF were susceptible to a freeze of −2.2 C at 3 and 0.25 h, respectively. Japanese climbing fern spores and gametophytes were more tolerant of a freeze at −2.2 C, with spore and gametophyte tolerance levels of 12 and 3 h, respectively. Nonlinear regression indicted that a single freeze of 2.4 h reduced OWCF dry weight biomass by a 10-fold reduction at 6 mo postfreezing compared to controls. The susceptibility of OWCF to a single freeze or multiple freezes will limit it spore production, gametophyte survival, and sporophyte growth. The higher tolerance of JCF to a single freeze could explain why JCF occurs throughout the southeastern United States, whereas OWCF is currently restricted to Florida. Land managers in natural areas of northern Florida should become familiar with the identification and habitat preferences of OWCF and report any occurrence to Florida Fish and Wildlife Conservation Commission's Invasive Plant Management Section.