“Environmental veto synchronizes mast seeding in four contrasting tree species”
Michał Bogdziewicz, Magdalena Żywiec, Josep M. Espelta, Marcos Fernández Martinez, Rafael Calama, Mateusz Ledwoń, Eliot McIntire, and Elizabeth E. Crone (Aug 2019)
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Adverse weather events can synchronize trees reproduction
Can adverse weather events help plant to reproduce?
Variable, synchronized seed production, called masting, is a widespread reproductive strategy in plants. How plants are able to synchronize reproduction is a longstanding question. One leading idea is that plants need to accumulate sufficient resources after depletion following seeding, which creates annual variability of reproduction. Synchrony is then induced by the efficiency of mass flowering for outcross pollination. Plants that reproduce asynchronously do not spent resources on seeds due to pollination failure, so they flower again and again until other plants do it as well, when flowers can get pollinated. Similar resource carry-over can be induced by external factors preventing reproduction – environmental veto – like frost or other adverse weather events. In this case, preventing plants from developing seeds in one year helps to put all plants on the same reproductive schedule.
The authors of this study used four species differing in their masting strategies: Two species (pine and rowan) are “flowering masting” species, i.e. plants where mast seeding is driven by variable production of flowers. Two other species were oaks, and these are “fruiting masting” species, i.e. species where annual variation in seed production is a consequence of variable ripening with relatively constant flower production. Based on that difference in life history, they predicted that synchronization in the “flowering masting” species would be driven by density-dependent pollination success, while in the second group by correlated environmental veto. Indeed, positive density-dependence of pollination was much stronger in rowan and pine, while veto was much more frequent in oaks. However, in all four species, the veto was strong enough to drive synchrony without the need to invoke any other mechanism. Susceptibility to adverse weather conditions may benefit masting species, as it allows them to synchronize seed production.
Synchronized and variable reproduction by perennial plants, called mast seeding, is a major reproductive strategy of trees. The need to accumulate sufficient resources after depletion following fruiting (resource budget), the efficiency of mass flowering for outcross pollination (pollen coupling), or the external factors preventing reproduction (environmental veto) could all synchronize masting. We used seed production data for four species (Quercus ilex, Q. humilis, Sorbus aucuparia, Pinus albicaulis) to parametrize resource budget models of masting. Based on species life history characteristics, we hypothesized that pollen coupling should synchronize reproduction in S. aucuparia and P. albicaulis, while in Q. ilex and Q. humilis environmental veto should be a major factor. Pollen coupling was stronger in S. aucuparia and P. albicaulis than in oaks, while veto was more frequent in the latter. Yet, in all species, costs of reproduction were too small to impose a replenishment period. A synchronous environmental veto, in the presence of environmental stochasticity, was sufficient to produce observed variability and synchrony in reproduction. In the past, vetoes like frost events that prevent reproduction have been perceived as negative for plants. In fact, they could be selectively favored as a way to create mast seeding.