Effects of Nutrient Enrichment on Within-stand Cycling in a Mangrove Forest

Within-stand nutrient cycling is dependent on many factors, including primary productivity, nutrient-use efficiency, nutrient resorption, sclerophylly, decomposition, nutritional quality of plant tissue, and allocation to defense. The efficiency of these plantmediated processes depends on nutrient availability in the environment and inherent functional properties of plants. However, little is known about how nutrient availability will affect these processes in forested wetlands in the tropics. In a factorial experiment we fertilized 48 dwarfed Rhizophora mangle (red mangrove) trees along tidal-elevation and water-depth gradients at Twin Cays, a range of intertidal, peat-based offshore mangrove islands in Belize, Central America. Initial results indicated that phosphorus (P) deficiency is a major factor limiting primary productivity. Phosphorus-fertilized trees had a significant decrease in P-use efficiency and P-resorption efficiency, but a significant increase in nitrogen (N)-use efficiency and N-resorption efficiency in their leaves compared with controls and N-fertilized trees. Sclerophylly decreased dramatically in P-fertilized trees, while the nutritional quality of the plant tissue increased. Phosphorus fertilization did not affect P leaching from green leaves. We found no fertilizer effect on the decomposition rates of leaf tissue, possibly due to higher phenolic concentrations in the P-fertilized trees compared with controls and N-fertilized trees. However, belowground decomposition of cotton strips increased in the substrate associated with P-fertilized trees. Environmental conditions related to position along a tidal gradient may be as important as nutrients in controlling belowground decomposition.