Facultative Annual Life Cycles in Seagrasses

Plant species usually have either annual or perennial life cycles, but facultative annual species have annual or perennial populations depending on their environment. In terrestrial angiosperms, facultative annual species are rare, with wild rice being one of the few examples. Our review shows that in marine angiosperms (seagrasses) facultative annual species are more common: six (of 63) seagrass species are facultative annual. It concerns Zostera marina, Z. japonica, Halophila decipiens, H. beccarii, Ruppia maritima, and R. spiralis. The annual populations generally produce five times more seeds than their conspecific perennial populations. Facultative annual seagrass species occur worldwide. Populations of seagrasses are commonly perennial, but the facultative annual species had annual populations when exposed to desiccation, anoxia-related factors, shading, or heat stress. A system-wide 'experiment' (closure of two out of three connected estuaries for large-scale coastal protection works) showed that the initial annual Z. marina population could shift to a perennial life cycle within 5 years, depending on environmental circumstances. We discuss potential mechanisms and implications for plant culture. Further exploration of flexible life histories in plant species, and seagrasses in particular, may aid in answering questions about trade-offs between vegetative and sexual reproduction, and preprogrammed senescence.

Automated summary

Most plant species have either annual or perennial life cycles, but there are facultative annual species that can be either annual or perennial depending on their environment. While facultative annual species are rare in terrestrial plants, they are more common in marine angiosperms, particularly seagrasses. Six of the 63 seagrass species are facultative annual, producing significantly more seeds than their perennial counterparts. These facultative annual species can transition to an annual life cycle when exposed to specific environmental factors such as desiccation, anoxia, shading, or heat stress. A large-scale coastal protection experiment demonstrated that the facultative annual seagrass population could shift to a perennial life cycle within 5 years depending on the environmental conditions.