1. Biology Division, University of Kansas, Lawrence, Kansas 66045-7534, USA
2. Department of Biology, Ludwig Maximilians University, 80638 Munich, Germany
3. Center for Research on Sustainable Agriculture CIPAV, A. A. 20591, Cali, Colombia
4. Fundacion Puerto Rastrojo, A. A. 241438, Bogotá, Colombia
5. Botany Department, Charles Darwin Research Station, Galapagos, via AP 17-01-3891, Quito, Ecuador
6. Conservatoire et Jardin botaniques de la Ville de Genève, Case Postale 60, CH-1292 Chambésy/GE, Switzerland

Correspondence to: Rolf Borchert¹ Correspondence and requests for materials should be addressed to R.B. (Email: borchert@ku.edu).

In tropical rainforests, 30–65% of tree species grow at densities of less than one individual per hectare¹. At these low population densities, successful cross-pollination relies on synchronous flowering. In rainforests with low climatic seasonality, photoperiodic control is the only reliable mechanism for inducing synchronous flowering^{2, 3}. This poses a problem because there is no variation in day length at the Equator. Here we propose a new mechanism of photoperiodic timekeeping based on the perception of variation in sunrise or sunset time, which explains and predicts the annually repeated, staggered, synchronous and bimodal flowering of many tree species in Amazonian rainforests near the Equator.

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