Variation in rates of seedling recruitment, growth, and survival can strongly influence the rate and course of forest regeneration following disturbance. Using a combination of field sampling and shadehouse experiments, we investigated the influence of propagule size and predispersal insect damage on the establishment and early growth of the three common mangrove species on the Caribbean coast of Panama: Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle. In our field samples, all three species exhibited considerable intraspecific variation in mature propagule size, and suffered moderate to high levels of predispersal attack by larval insects. Rates of insect attack were largely independent of propagule size both within and among trees. Our experimental studies using undamaged mature propagules showed that, for all three species, seedlings established at high rates regardless of propagule size. However, propagule size did have a marked effect on early seedling growth: seedlings that developed from larger propagules grew more rapidly. Predispersal insect infestations that had destroyed or removed a substantial amount of tissue, particularly if that tissue was meristematic or conductive, reduced the establishment of propagules of all three species. The effect of sublethal tissue damage or loss on the subsequent growth of established seedlings varied among the three mangrove species. For Avicennia, the growth response was graded: for a propagule of a given size, the more tissue lost, the slower the growth of the seedling. For Laguncularia, the response to insect attack appeared to be all-or-none. If the boring insect penetrated the outer spongy seed coat and reached the developing embryo, it usually caused sufficient damage to prevent a seedling from developing. On the other hand, if the insect damaged but did not penetrate the seed coat, a completely healthy seedling developed and its growth rate was indistinguishable from a seedling developing from an undamaged propagule of the same size. Similar to Avicennia, if an infestation did not completely girdle a Rhizophora seedling, it survived, but grew at a reduced rate. In summary, our experiments demonstrated that natural levels of variation in propagule size and predispersal damage by insects translate into significant differences in seedling performance in terms of establishment and/or early growth. Such differences are sufficiently large that they could influence the intensity and outcome of competitive interactions during forest regeneration.
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Acknowledgements We are very grateful for the field and laboratory assistance provided by R. Chisenhall, D. Hahn, A. Hammond, S. Lemos, D. Matias, and K. MacNeale, and for the logistical assistance provided by S. Lemos, F. Sanchez, R. Thompson, and the Visitors Services Office of the Smithsonian Tropical Research Institute (STRI). J. Abraham, N. Hausmann, J. McGraw, M. Metz, J. Skene, and two anonymous reviewers provided helpful comments on the manuscript. Identifications of insects were kindly provided by S. Scheffer (Agromyzidae) and S. Lingafelter (Curculionidae) of the Systematic Entomology Laboratory, Agriculture Research Service, US Department of Agriculture, S. Wood (Scolytidae) of the Monte L. Bean Life Science Museum, Brigham Young University, and J. Powell (Lepidoptera) of the Department of Environmental Science, Policy, and Manage- ment and the Essig Museum of Entomology, University of California, Berkeley. We thank STRI for allowing us to use the Galeta Marine Laboratory, and the MPF at UC Berkeley for conceptual inspiration. This study was supported by National Science Foundation grants DEB-9221074, DEB-9615887, and DEB-0108146 to WPS, and by the Committee for Research, U.C. Berkeley. It was conducted under research permits from the Panamanian National Institute for Renewable Natural Resources. We especially thank the Republic of Panama for preserving their forests and making them available for study.
- Forest regeneration
- Insect herbivory
- Propagule predation
- Seedling recruitment