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First published online August 13, 2009; doi:10.3732/ajb.0800366 American Journal of Botany 96: 1603-1611 (2009) © 2009 Botanical Society of America, Inc.

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Limited flooding tolerance of juveniles restricts the distribution of adults in an understory shrub (Itea virginica; Iteaceae)¹

Ecology and Evolutionary Biology, Corson Hall, Cornell University, Ithaca, New York 14853 USA

ABSTRACT

Juvenile plants often have tight microhabitat associations because of specific requirements for seed germination and subsequent establishment. Due to their larger size, adults may be more adept at coping with stress. However, few studies consider the role of ontogeny in structuring plant populations, even though phenotypic expression can change through life history. In cypress–tupelo swamps of the United States, understory species often grow on microsites above floodwaters. In a field survey of one such species, Itea virginica, we found that more than 98% of seedlings occurred on elevated microsites, which were relatively infrequent. However, this strict association relaxed through ontogeny, with nearly 8% of subadults and adults rooted directly on the forest floor. We hypothesized that flooding inhibits juvenile establishment on the forest floor. In greenhouse experiments, we investigated the effects of flooding and substrate on Itea performance. Seeds had similar germination rates on drained swamp soil and cypress knee wood. Seedling growth was high on unflooded soil, but declined precipitously when submerged. Finally, performance of seedlings, but not older plants, decreased with flood severity. Our results highlight the importance of assessing stress tolerance over multiple life history stages because limitations of juveniles can constrain the distribution patterns of future ontogenetic stages.

Key Words: cypress– • tupelo swamp • Itea virginica • Iteaceae • life history • microhabitat association • nurse log • ontogenetic niche shift • regeneration niche • seedling establishment

Received for publication 28 October 2008. Accepted for publication 11 May 2009.

FOOTNOTES

¹ The authors thank N. Brunswig, M. Dawson, and the staff at Beidler forest for permission to conduct this research, logistical support, and encouragement. Funding was provided by the Andrew Mellon foundation, the Morley student research fund, and the Howard Hughes program (to A.L.). The authors thank C. Landi for help weighing biomass, N. Hairston and laboratory for various types of support, and M. Geber, T. Pendergast, J. Bellemare, J. Sparks, A. Flecker, S. Cook, the associate editor, and two anonymous reviewers for constructive comments on previous drafts.

² Author for correspondence (e-mail: jill.anderson{at}duke.edu); present address: Department of Biology, P.O. Box 90338, Duke University, Durham, NC 27708 USA

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