Belowground productivity of mangrove forests in southwest Florida

Abstract

Studies in belowground dynamics are limited mainly due to the difficulty of studying roots despite wide recognition of its importance. This dissertation focused on methods for analyzing mangrove roots, root responses to phosphorus and flooding, and variation in root production between forest types. Techniques to separate live and dead roots such as colorimetric, fluorescence, buoyancy, and visual assessment were compared. The traditional method of visual assessment combined with root buoyancy was accurate, fast, and applicable to larger samples. Additionally, techniques such as rhizotrons, root ingrowth cores, and root image analysis were useful to study mangrove roots. Root and litter production, and hydro-edaphic conditions were determined seasonally for one year at eight sites (fringe, basin, and scrub forest types) in Southwest Florida. Root production was equal or greater than litter production showing spatial variation, especially in biomass allocation. Correlations with soil variables indicated that above and belowground processes respond differently to environmental conditions. The combined root+litter production was a good predictor of flooding and nutrient stress. Mangrove root production and morphology responded to nutrient enrichment (additions of nitrogen or phosphorus) depending upon forest type and stress factors interacting with resource acquisition. Root production increased with low nutrient availability and high flooding, and decreased with high salinity and nutrient availability. Nutrient enrichment increased the specific root length and surface area at the basin-monospecific site, and decreased the specific surface area at the scrub forest. Greenhouse experiments were conducted to study root dynamics of Rhizophora mangle and Avicennia germinans seedlings in response to phosphorus availability and flooding regimes. Mangrove species differed in their tolerance of flooding, and their plasticity to nutrients availability. The more flood tolerant species, R. mangle, was slower growing. The faster-growing species, A. germinans, exhibited limitations to flooding and changes in root morphology that altered the surface area for absorption of nutrients. These results indicate a trade-off between root strategies to tolerate flooding and to acquire nutrients. This study contributes to a better understanding of how mangrove ecosystems function. Additional work in other geographic areas and sedimentary settings is needed to provide a broader perspective on belowground processes in mangrove systems.