The role of sexual reproduction in the maintenance of established Zostera marina meadows

Abstract 1.For clonal plants the role of sexual reproduction in the
maintenance of populations can vary widely. Some species are dependent on
repeated seedling recruitment. For other species, interactions between
adults and seedlings within existing populations can affect seedling
survival and limit sexual reproduction in existing populations. Genetic
studies of seagrass populations increasingly suggest sexual reproduction
is important for the resilience and stability of their populations, but as
of yet little observational data supports these findings. Because seagrass
populations provide important ecosystem services and are threatened with
increasing anthropogenic impacts, understanding their reliance on sexual
reproduction is evolutionarily and ecologically important. 2.The goals of
this study were to determine: 1. if seedlings of a marine angiosperm,
Zostera marina, establish and recruit within existing Z. marina meadows;
2. if interactions between seedlings and surrounding adult shoots
influence the survival of established seedlings. To meet these goals,
surveys estimated seedling establishment and tracked seedling survival
within multiple populations. Manipulative experiments then tested the
impact of neighboring adult shoots on seedling survival and the overall
trajectory of experimental plots with and without sexual reproduction. 3.
A three-year survey identified established seedlings within Z. marina
meadows each year. Additionally, concurrent seed addition experiments
indicated seed supply could influence seedling establishment rates. A
survey tracking the survival of tagged seedlings, as well as the height
and density of surrounding adult shoots, showed adult shoots may
negatively impact seedling survival. Experiments then demonstrated that
seedlings without neighboring shoots survived longer than those with
neighboring shoots. Lastly, two transplant garden experiments comparing
the survival of plots with and without seeds highlighted that seedling
recruitment is likely most important to maintain bottom cover where
disturbances generate gaps in the adult population. 4. Synthesis. This
study demonstrates that seedlings do establish within existing seagrass
meadows, and that some survive to recruit into the adult population.
Competition with existing vegetation, however, can be a factor
compromising seedling survival. Sexual reproduction may thus most likely
occur in, and be most important for, clonal plant populations that
experience seasonal disturbance.