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Janzen-Connell patterns are not the result of Janzen-Connell process: Oak recruitment in temperate forests

Abstract : According to the Janzen-Connell hypothesis, seedling mortality is greater close to conspecific (or closely related) adult trees because of higher enemy pressure, ultimately increasing local tree-species diversity. However, this pattern (i.e. a decline of seedling performance close to conspecific or closely related adults) could also result from other processes: (1) heterospecific adults might positively affect seedlings; (2) conspecific (or closely related) adults might negatively affect seedlings by causing a deterioration of the microenvironment. We tested these hypotheses, accounting also for sizes of adults. We planted oak-seedlings in a temperate forest, characterized their adult neighbourhoods, measured 26 microenvironmental conditions, seedling mortality during one year, budburst and leaf herbivory. We detected Janzen-Connell-like patterns (frequent lack of budburst close to conspecific adults; high seedling mortality close to closely related adults) that were consistent with the Janzen-Connell process. However, these patterns were either counteracted by non-Janzen-Connell processes such as a favourable microenvironment or were weak with little explained variance. We detected Janzen-Connell-like patterns that were not consistent with the Janzen-Connell process: proximity to heterospecific adults per se decreased leaf herbivory partly due to microenvironmental effects, such that a lower leaf herbivory decreased seedling mortality. Overall, the spatial pattern of tree recruitment may resemble that predicted by Janzen-Connell but result from different processes: notably heterospecific adults creating refuges from enemies, facilitating the establishment of oaks below non-oaks without hindering their establishment below oaks.
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Submitted on : Tuesday, April 11, 2017 - 10:04:52 AM
Last modification on : Wednesday, January 15, 2020 - 3:56:08 PM
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Maud Deniau, Vincent Jung, Cécile Le Lann, Thibault Morra, Phil J. Murray, et al.. Janzen-Connell patterns are not the result of Janzen-Connell process: Oak recruitment in temperate forests. Perspectives in Plant Ecology, Evolution and Systematics, Elsevier, 2017, 24, pp.72-79. ⟨10.1016/j.ppees.2016.12.001⟩. ⟨hal-01483989⟩

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