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Population Sex Ratios and Spatial Distribution of Ocotea Tenera (Lauraceae) Trees in a Tropical Forest

Nathaniel T. Wheelwright and Anne Bruneau
Journal of Ecology
Vol. 80, No. 3 (Sep., 1992), pp. 425-432
DOI: 10.2307/2260688
Stable URL: http://www.jstor.org/stable/2260688
Page Count: 8
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Population Sex Ratios and Spatial Distribution of Ocotea Tenera (Lauraceae) Trees in a Tropical Forest
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Abstract

1. Ocotea tenera (Lauraceae) is a dioecious understorey tree which occurs in the lower montane forests of Monteverde, Costa Rica. In a natural population of O. tenera trees whose ages were estimated to range from 14 to 30 years old, staminate trees slightly outnumbered pistillate trees but the sex ratio was not significantly different from 1:1. Staminate and pistillate trees were non-randomly distributed with respect to each other. The probability that a tree's nearest neighbour was of the opposite sex was much higher than expected by chance. 2. Males were also more common in two experimental plots, one established in 1981 and the other in 1984, although only one of the plots showed a significantly male-biased sex ratio. Spatial distributions of staminate and pistillate trees in both experimental plots were indistinguishable from random. 3. Individual trees in both the natural population and experimental plots monitored over a 10-year period occasionally switched in different years from producing almost exclusively staminate to almost exclusively pistillate flowers, and vice versa. Sex ratios within plots changed over time because of variable sexual expression between years rather than differential mortality. Once trees were established, mortality was low. 4. It is suggested that non-random spatial distributions in older O. tenera trees in the natural population may be caused by labile sexual expression modified by the presence of neighbouring trees.

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