Photos from the Field: Recruitment Distance and Tree Size

Anna Sugiyama. (2015). Recruitment Distance from the Nearest Reproductive Conspecific Increases with Tree Size in Tropical Premontane Wet Forests. Biotropica, 47: 526–535. doi: 10.1111/btp.12241

Studies have shown that median distances of plants from the nearest reproductive conspecific (recruitment distance) shifts outward with increasing age or size class, and that plant spatial distribution changes over time in a predictable manner. However, observations and empirical evidence for such predictable changes are limited, and underlying mechanisms explaining such patterns for a wide range of individual sizes are not fully explored. In Costa Rican premontane wet forests, I empirically tested whether recruitment distance changes in a predictable manner with increasing size for five animal-dispersed tree species by considering all post-germination sizes. Specifically, I tested the Janzen–Connell hypothesis and the colonization hypothesis by considering distance, density, size, herbivory, biotic infection, and light availability simultaneously. Recruitment distance increased with increasing size (16–22 m) for the four non-pioneer species, suggesting eventual regeneration success for seeds dispersed away from reproductive conspecifics. During the 2 years of this study, I found positive distance-dependent survivorship and light availability were important for post-seedling survivorship, in agreement with the Janzen–Connell hypothesis and the colonization hypothesis, respectively, but only for seedlings. However, seedlings did not escape aboveground herbivory or biotic infection better when the seeds were dispersed greater distances. Results highlight the importance of seed dispersal for successful regeneration, and suggest that changes in spatial distribution over time may be predictable in the vicinity of maternal trees for some non-pioneer tree species.

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View at study site, Las Cruces Biological Station.

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Patches of forest exist as small islands in Coto Brus county, southern Costa Rica.

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Conspecific individuals of all sizes were recorded in transects radiating from fruiting trees.

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Light environment was assessed by taking hemispherical photos.

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Germination tests were conducted prior to this study to identify the smallest individuals.

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Seedlings of Drypetes brownii. About 0.8% of all seedlings were albino, all of which died within a year.

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Seedlings of Ficus tonduzii.

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A seedling of Lacistema aggregatum.

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A seedling of Quararibea aurantiocalyx.

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Seedlings of Tapirira mexicana.

(All photos: Sugiyama)