Litter Production and Turnover in Basin Mangrove Forests in Southwest Florida

Abstract

Litterfall, litter standing crop, and leaf decomposition of basin mangrove sites with mixed associations of Avicennia germinans, Rhizophora mangle, and Laguncularia racemosa adjacent to bay waters were compared to monospecific stands A. germinans located inland of the mixed associations. The objective was to determine whether basin mangroves with low tidal amplitude are characterized by lower litter productivity, longer litter residence times, and increased nitrogen recycling. Litterfall rates for the two mixed forests averaged 8.10 Mg°ha—1°yr—1 compared to 4.44 Mg°ha—1°yr—1 in the three monospecific sites. Peak litterfall occurred from August to October after increases in soil water salinity during June and July. Seasonal changes in surface litter mass occurred only in the monospecific forests and corresponded with higher leaf decomposition rates when tidal inundation frequency increased. As a result, surface leaf litter in a monospecific site > 200 m from the bay had a longer residence time than in two monospecific sites < 100 m inland. The rates of dry mass loss and organic carbon leaching of R. mangle leaves were lower than those for A. germinans, resulting in higher residence times of leaf litter in mixed (0.5 yr) than monospecific (0.2 yr) forests located <100 m from the bay. Leaf nitrogen decreased during senescence, and absolute nitrogen increased in leaf litter during decomposition on the forest floor. As a result, turnover rates of nitrogen in surface litter were lower than for dry mass. A comparison of litter dynamics and nitrogen recycling among riverine, fringe, and basin mangroves suggests that the relative influence of export and decomposition on litter dynamics depends on the magnitude of tidal inundation. Inland basin mangroves have low nitrogen recycling efficiency in litterfall, but higher internal recycling of litter on the forest floor, due in part to a longer litter residence time, nitrogen retranslocation prior to leaf senescence, and nitrogen immobilization during litter decomposition and peat deposition.