Leaf-to-Twig Transfer Conserves Nitrogen and Phosphorus in Nutrient Poor and Enriched Cypress Swamps
The external and internal N and P cycling of cypress trees in natural and sewage-enriched cypress swamps in north central Florida was investigated. Based on monthly changes in foliar and twig nutrient pools sampled for a year, concentrations of N and P in leaves decreased over the growing season. Nitrogen and phosphorus concentrations in the twigs increased immediately prior to leaf abscission. In both swamps, nutrients were available for early-season leaf growth the next spring. There were significant differences between the natural and sewage-enriched swamps in N concentrations in twigs, N and P concentrations in foliage, and changes in P concentrations in foliage; seasonal changes in N and P concentrations in twigs and N in foliage were not different between swamps. Nutrient increases in leaves were very rapid during early spring, but by May there was a slow decrease of N and P, probably from being retranslocated back to the twigs. Concentrations of total organic N in throughfall were significantly enriched in the natural swamp relative to precipitation, while ammonium concentrations in throughfall were signficantly lower than in precipitation. Leaf fall plus foliar leaching, which were 1.25 kg P ha-1 and 20.3 kg N ha-1 in the natural swamp and 6.1 kg P ha-1 and 51.6 kg N ha-1 in the sewage amended swamp, returned 87 and 66% of the P and N, respectively, taken up by the trees each year in both swamps, indicating that more nitrogen than phosphorus on a percentage basis was permanently added to the above-water cypress each year. Adding sewage increased the importance of root uptake: 7.2 kg P ha-1 and 78.2 kg N ha-1 accounted for 34 and 48% of the uptake required for net primary production in the swamp receiving sewage effluent while in the natural swamp only 17% of the P (1.4 kg P ha-1) and 31% of the N (30.8 kg N ha-1) required for net production was supplied by root uptake. The internal biochemical cycle of leaf-to-twig transfer of nutrients (7.0 kg P ha-1 and 68.1 kg N ha-1 in the natural dome; 13.8 kg P ha-1 and 83.6 kg N ha-1 in the sewage-enriched swamp) was more important in conserving nutrients than leaf fall, leaf leaching, and root uptake, suggesting that the major nutrient conservation strategy in cypress swamps is redistribution of nutrients from abscissing leaves to more permanent tissue before leaf senescence. Forest Sci 32:900-913.
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