Growth and Survivorship of Red Mangrove Seedlings Under a Mangrove Canopy and in a Saltmarsh Community in Northeastern Florida

Abstract

• In northeastern Florida, the red mangrove has established in saltmarsh communities. • Seedling growth was compared between saltmarsh and mangrove microhabitats. • Temperatures at the sediment surface were similar between microhabitats. • Inundation was greatest during storm events and high tides. • Greater growth in the Spartina microhabitat was likely due to significantly greater light availability. • Shaded conditions in under the mangrove canopy likely limited growth. In the United States, Rhizophora mangle L. populations have established in saltmarsh communities at the northern extent of its range in Guana Tolomata Matanzas National Estuarine Research Reserve, near St. Augustine, FL. This two-year study monitored the growth and survivorship of propagules planted in the intertidal band dominated by Spartina alterniflora Loisel and mangroves. Propagules in the S. alterniflora microhabitat grew more than twice as fast, had a greater number of leaves per plant, and produced more sets of branches and flower buds than those growing under the mangrove canopy. Differences in growth were likely due to the 2.7 times greater mean monthly insolation in the Spartina microhabitat during the spring-summer growing season. Individuals persisted beneath the mangrove canopy and may take advantage of increased light availability from canopy gaps created by extreme storm events. Despite the 67% mortality from herbivory, wind gusts, storm surge, and extreme high tides, surviving R. mangle seedlings reached maturity in the unshaded and warmer microclimate of the low-lying saltmarsh vegetation. Spartina alterniflora may serve as a facilitator during the cotyledon phase of R. mangle and did not inhibit the success of the seedling establishment, providing a mechanism by which dispersed R. mangle propagules may spread northward into saltmarsh communities.