Interactions of insect herbivory and multiple abiotic stress agents on two wetland tree species in southeast Louisiana swamps
Forested wetlands in Louisiana are hydrologically isolated from the Mississippi River, impounded by manmade structures, impacted by saltwater intrusion, and are sinking, resulting in more frequent flooding for longer periods. Additionally, defoliation of the two dominant trees, baldcypress (Taxodium distichum L. Rich) and water tupelo (Nyssa aquatica L.), occur frequently during spring. In Louisiana, the baldcypress leafroller, BCLR, (Archips goyerana Kruse) and the forest tent caterpillar, FTC, (Malacosoma disstria Hubner) defoliate up to 120,000ha of baldcypress-tupelo swamps. Restoration plans call for reintroducing Mississippi River water (diversions) to increase sediment elevation, promote natural regeneration, and enhance primary and secondary productivity. In this study, insect-tree-health relationships were evaluated in field and in multifactor, greenhouse and larval rearing experiments incorporating these environmental stressors and nutrient enhancements under simulated diversion conditions. It was ascertained from three field sites of varying tree density/health in Lake Maurepas swamps that nutrient augmentation (similar to N and P loading rates expected from 8000cfs diversion) increased stem growth of both tree species (more for cypress than tupelo) at moderately degraded sites. Nutrient augmentation also increased nitrogen in foliage, insect frass (insect feces), spring-clipped leaves, and abscised litterfall at all sites for both tree species. Nitrogen content of canopy foliage and litterfall was positively correlated with site health. These findings support the hypotheses that the swamps in Lake Maurepas are nutrient limited, and the existing trees can utilize, benefit, and act as nutrient sinks for nutrient-laden river water accompanying diversions. Positive insect responses to nitrogen found in lab rearing assays and field studies that defoliators are receiving nitrogen augmentation, and insect field populations might increase, particularly the FTC. The greenhouse study demonstrated that the combination of nutrient deprivation and flooding lead to the “poor” sapling growth, which was similar to the growth in the salt-stressed (3ppt) saplings. Encouragingly, when fresh water and nutrients were supplied to saplings after prolonged stresses of defoliation, flooding, and salt stress, growth was restored. Taken collectively, these studies demonstrated that river diversions are viable management tools for improving health and productivity of declining forested wetlands.
Archips, diversions, forest defoliation, Malacosoma, forest tent caterpillar, baldcypress leafroller
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