Baldcypress and water tupelo responses to insect defoliation and nutrient augmentation in Maurepas Swamp, Louisiana, USA

Abstract Forested wetlands in Louisiana are hydrologically isolated from the Mississippi River, impacted by saltwater intrusion, and are sinking, resulting in more frequent flooding for longer periods. These problems are exacerbated in some areas by impoundment from man-made structures. Additionally, defoliation of the two dominant trees, baldcypress ( Taxodium distichum L. Rich) and water tupelo ( Nyssa aquatica L.), frequently occur during spring. In Louisiana, the baldcypress leafroller, BCLR ( Archips goyerana Kruse) and the forest tent caterpillar, FTC ( Malacosoma disstria Hubner) annually defoliate an average of 120,000 ha of baldcypress–tupelo swamps. In combination, these factors have led to severely degraded forested wetlands with reduced basal area, dieback and tree mortality over extensive, formerly productive swamplands. Restoration plans call for reintroducing Mississippi River water by diversions, to increase sediment elevation, promote natural regeneration, and enhance primary and secondary productivity. Nutrient augmentation of baldcypress and water tupelo trees in Maurepas Swamps (similar to NO 3 − loading rates expected from 229 m 3 /s diversion) increased radial growth (cm 2 ) of both species (especially baldcypress) in degraded forest stands. Nutrient augmentation also increased nitrogen in foliage (insect frass (feces), spring clipped leaves, and abscised leaf litter) for both tree species. Nitrogen content of canopy foliage and leaf litter was positively correlated with site health. These findings support hypotheses that swamps in southeastern Louisiana are nutrient limited, and existing trees can utilize, benefit, and act as nutrient sinks for nutrient-laden river water accompanying diversions. Defoliation of these swamp species, although not uniform among years, did not result in losses in radial growth, but increased overall litter deposition for baldcypress. An increase in nitrogen content in insect frass in response to nutrient augmentation indicates defoliator populations might increase as a result of nutrient influxes.