The tree species Xylocarpus granatum is commonly described as occurring in the upper intertidal zone of mangrove forests, but mature trees are occasionally found at lower elevations. In the Utwe River basin, on the Pacific island of Kosrae, we investigated the relative importance of several biotic and abiotic factors that may control the intertidal distribution of X. granatum. Factors we evaluated included differential seed predation across the lower, mid, and upper intertidal zones and seedling responses to salinity, tidal flooding, and shade. Seed predation was 22.4% over the first 34 days and varied little among zones or in gaps versus under the forest canopy. By day 161, there were still no differences in seed mortality, but a significant difference was found in seedling establishment, with much greater establishment in the upper intertidal plots. X. granatum seedlings in a greenhouse experiment exhibited greater growth in freshwater than seedlings in 23 ppt salinity, which is typical of salinity levels found in the mid intertidal zone in our field study sites in Micronesia, where mature X. granatum trees are generally absent. Seedlings grown in 23 ppt salinity, however, exhibited few visible signs of stress associated with patterns in growth. Seedlings grown in a simulated tidal flooding treatment (with 23 ppt salinity) also showed few signs of stress. Growth declined dramatically under 80% shade cloths, but there were few interactions of shading with either 23 ppt salinity or simulated tidal flooding. Differential seed predation is not likely to be the primary factor responsible for the intertidal distribution of X. granatum on Kosrae. However, seedling tolerance of flooding or salinity may be more important, especially relative to a potential contribution to secondary stress mortality. Other factors may ultimately prove to be more critical, such as physiological effects of salinity on seed germination, effects of tides on seed dispersal and rooting, or differential herbivory on seedlings.

Disturbance is an integral component in mangrove forest dynamics, influencing forest structure, composition, and function. The impacts of human disturbance, however, threaten mangrove forests throughout the world. Small-scale wood harvesting on the small Pacific island of Kosrae, Federated States of Micronesia, provided an instructive scenario for exploring the dynamics of human disturbance. Natural disturbances on the island are rare, but the growing island population harvests mangrove trees for firewood and construction materials, placing pressure on the forest. In order to determine recent harvest rates, we estimated gap ages by developing a time scale for mangrove wood decomposition and by quantifying growth rates for Rhizophora apiculata and Bruguiera gymnorhiza seedlings. Stump and log decomposition patterns were useful in aging gaps, although some patterns were more reliable than others. Seedlings of both species added approximately 5 nodes/year depending on light conditions. The island-wide harvest rate was 10% over the last 10 years, but the rates varied widely among different parts of the island. Rhizophora apiculata has been harvested preferentially, and a dearth of young trees where harvesting has been heaviest portends a decline of this highly desired species in the forest. Socio-economic data substantiated some but not all of the trends we observed. Even on a small island, local differences in both natural and anthropogenic factors are important to understanding forest dynamics.

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Background Development and application of DNA-based methods to distinguish highly virulent isolates of Fusarium oxysporum f. sp. koae [Fo koae; cause of koa wilt disease on Acacia koa (koa)] will help disease management through early detection, enhanced monitoring, and improved disease resistance-breeding programs. Results This study presents whole genome analyses of one highly virulent Fo koae isolate and one non-pathogenic F. oxysporum (Fo) isolate. These analyses allowed for the identification of putative lineage-specific DNA and predicted genes necessary for disease development on koa. Using putative chromosomes and predicted gene comparisons, Fo koae-exclusive, virulence genes were identified. The putative lineage-specific DNA included identified genes encoding products secreted in xylem (e. g., SIX1 and SIX6) that may be necessary for disease development on koa. Unique genes from Fo koae were used to develop pathogen-specific PCR primers. These diagnostic primers allowed target amplification in the characterized highly virulent Fo koae isolates but did not allow product amplification in low-virulence or non-pathogenic isolates of Fo. Thus, primers developed in this study will be useful for early detection and monitoring of highly virulent strains of Fo koae. Isolate verification is also important for disease resistance-breeding programs that require a diverse set of highly virulent Fo koae isolates for their disease-screening assays to develop disease-resistant koa. Conclusions These results provide the framework for understanding the pathogen genes necessary for koa wilt disease and the genetic variation of Fo koae populations across the Hawaiian Islands.