Effect of environmental variables on germination of Japanese climbing fern spores

Background/Question/Methods

Lygodium japonicum (Japanese climbing fern) is an exotic, invasive vine that has become a problem in forests of the southeastern United States. Typically starting as a few scattered individuals, invasions of Japanese climbing fern can escalate into dense, tangled masses that overtop trees, smother understory vegetation, and alter fire behavior. Invasions are especially problematic in plantations managed for pine straw as bales may serve as vectors for L. japonicum dispersal. Most research has focused on the control of mature plants, though improved understanding of the environmental conditions under which the fern germinates may better inform managers to detect or prevent early establishment. In this study, we evaluated how temperature and the presence of pine litter affected L. japonicum spore germination timing and rates. Pots of native, sterilized loamy clay sand were inoculated with L. japonicum spores and placed in growth chambers set to the following temperatures: 30, 20, 18, 16, 15, and 10 degrees Celsius. Additionally, half of the pots in each chamber were covered with a thin layer of pine litter while the other half were left exposed. Pots were monitored weekly for presence/absence and percent cover of gametophytes over five weeks.

Results/Conclusions

Fern spore germination was influenced by both temperature and presence of pine straw. In general, higher temperatures spurred germination more quickly and over a greater percentage of pots than lower temperatures. At 30 degrees Celsius, nearly all pots germinated by the second week while only 44% of pots exposed to the 20-degree treatment germinated by the third week. A marked drop in germination can be noted with each decrease in temperature until a critical threshold is reached between 10 and 15 degrees, at which point no germination occurred over the five-week period. Additionally, pots under litter germinated about one week earlier and displayed higher final percent cover averages than exposed pots: 24% higher at 30 degrees, 27% at 20 degrees, 21% at 18 degrees, and 2% at 16 and 15 degrees. This difference was especially significant in pots exposed to colder temperatures; for instance, uncovered pots exposed to the 15-degree treatment did not germinate at all while 67% of pots under litter eventually did. It is possible that litter insulates L. japonicum spores and thereby enables germination in temperatures that would otherwise be inhospitable, which may actually make pine straw plantations even more susceptible to Japanese climbing fern establishment.