Ecological Research (2004) 19: 13–20 Blackwell Science, LtdOxford, UKEREEcological Research0912-38142004 Ecological Society of Japan1911320Original ArticleSynchronization by pollen couplingY. Iwasa and A. Satake *Author to whom correspondence should be addressed. Email: yiwasscb@mbox.nc.kyushu-u.ac.jp Received 28 December 2002, Accepted 16 June 2003. Mechanisms inducing spatially extended synchrony in mast seeding: The role of pollen coupling and environmental fluctuation Yoh IWASA 1 * and Akiko SATAKE 2 1 Department of Biology, Faculty of Sciences, Kyushu University, Fukuoka 812–8581, Japan and 2 Department of Entomology and Biology, 505 ASI Buildings, Penn State University, University Park, Pennsylvania 16802, USA. Trees in mature forests often show intermittent reproduction. Intensive flowering and seed production occur only once in several years (mast seeding), often synchronized over a long distance. Recently, coupled map models for the dynamics of individual energy reserves have been adopted to explain the phenomena. Even in a constant environment, the trees show a large between-year fluctuation in seed crops and the reproduction can be synchronized over the whole forest if the fruit production is limited by the availability of outcross pollen (pollen coupling). The model with local coupling in which trees are coupled by pollen exchange only with the neighbors shows that a strong synchronization of tree reproduction can develop over the whole forest that may be orders of magnitude larger than the distance of direct pollen exchange between trees. However, their fluctuation is close to the period-two oscillation, and is unable to explain observed intermittent reproduction of a longer interval between mast years. Finally the effect of common environmental fluctuation experienced by different individuals is studied, when the annual productivity and the reproductive threshold of trees fluctuate between years. In the absence of pollen limitation, environmental fluctuation correlated strongly between individuals (Moran effect) failed to produce a high positive correlation in seed production between individuals. If both pollen limitation and correlated environmental fluctuation are at work, a significantly large correlation was maintained. Hence, both pollen coupling and common environmental fluctuation are needed to explain synchronized reproduction with intervals longer than 2 years. Key words: coupled map models; forest trees; Moran effect; pollen coupling; synchronized reproduction. Introduction The variable and synchronous production of seeds by plant populations is called masting, or mast seeding (Kelly 1994). Many flowers and fruits are produced one year (called a mast year) but little reproductive activity occurs during the several subsequent years until the next mast year. Several explanations have been pro- posed to identify the evolutionary advantages of mast- ing. Masting might enhance the survival of seedlings by satiating seed predators (the ‘predator satiation hypoth- esis’; Janzen 1971; Silvertown 1980; Nilsson & Wästl- jung 1987). Alternative or additional adaptive functions of masting include improved pollination efficiency in mast years for normally outcrossing tree species (Nils- son & Wästljung 1987; Norton & Kelly 1988; Smith et al. 1990; Koenig et al. 1994; Shibata et al. 1998). However, these hypotheses on the evolutionary advantages of masting explain neither the mechanism causing masting nor how the adjustment of timing of reproduction is achieved. Isagi et al. (1997) proposed a very simple mechanis- tic model that considers the resource budget of each tree. Assumptions are: a tree gains a constant energy income every year from its photosynthetic activity, and that the tree may not reproduce while the energy reserve level is below a threshold. Once the energy reserve exceeds the threshold, the tree flowers and may have ovules fertilized by outcrossed pollen. The invest- ment to seeds and fruits produced from these ovules subsequently depletes the energy reserve of the tree. Depending on the degree of energy depletion, a single tree can show either constant reproduction over years, or fluctuating reproduction with a chaotic time series. If the seed and fruit production of a tree is limited by the availability of outcrossed pollen supplied by other trees in the same forest, different trees are coupled by