Fine root dynamics of Kandelia obovata, Rhizophora stylosa and Bruguiera gymnorrhiza in a mangrove environment in Okinawa, Japan

Abstract

The present study investigates the fine root dynamics including comparison of fine root production, mortality and decomposition of Kandelia obovata (S., L.) Yong, Rhizophora stylosa Griff. and Bruguiera gymnorrhiza (L.) Lamk. in the Manko Wetland of Okinawa, Japan considering the importance of fine root to the ecosystem carbon dynamics. Our objective was to investigate seasonal dynamics of fine root of the three species and explore whether there is variation among species using sequential core and ingrowth core methods. The root systems were separated into living and dead mass fractions and these were subsequently separated as very fine roots (<0.5 mm) and fine roots (0.5—2 mm). Total fine root production of K. obovata, R. stylosa and B. gymnorrhiza ranged from 4.7 to 7.1 Mg ha −1 y−1, 1.7 to 3.6 Mg ha −1 y−1 and 2.9 to 4.9 Mg ha −1 y−1, respectively whereas mortality ranged 2.9 to 5.9 Mg ha −1 y−1, 0.6 to 2.3 Mg ha −1 y−1 and 0.8 to 3 Mg ha −1 y−1. The mean fine root turnover of K. obovata, R. stylosa and B. gymnorrhiza were estimated as 3.42, 2.78 and 3.37 y−1 having a longevity of 3.46, 4.32 and 3.56 months, respectively. The amount of decomposition through sequential core and ingrowth core methods for K. obovata, was estimated at 47 and 28 g m−2y−1, respectively, whereas for R. stylosa and B. gymnorrhiza these were 6 and 11 g m−2y−1, and 15 and 19 g m−2y−1, respectively. Significant variation of soil salinity, soil N, soil pH among three species plots was observed although the result was insignificant in different seasons. Using ingrowth core method, we observed significant correlations of fine root production with soil salinity and soil pH, but the results were insignificant with sequential core data. Finally, the significant effect of soil pH was found in the regression model as the predictor of fine root production. Although the seasonal pattern was absent in fine root production, the seasonal change in the biomass and necromass was obvious. The high amount of fine root necromass available can be a great source of organic matter in the forest for carbon and nutrient cycling.