Flowering synchrony is the amount of overlap between flowering periods of plants in their mating season compared to what would be expected to occur randomly under given environmental conditions. A population which is flowering synchronously has more plants flowering (producing pollen or receiving pollen) at the same time than would be expected to occur randomly. A population which is flowering asynchronously has fewer plants flowering at the same time than would be expected randomly. Flowering synchrony can describe synchrony of flowering periods within a year, across years, and across species in a community. There are fitness benefits and disadvantages to synchronized flowering, and it is a widespread phenomenon across pollination syndromes.
History Synchronous flowering has been observed in nature for centuries. Sources from the ninth and 10th centuries noted the interannual synchrony of bamboo species. Early scholarly work focused on interannual variation in the form of mast seeding in tree species such as pines and oaks. An early proposed explanation for masting was the resource management (or weather tracking) hypothesis. This suggested that trees produced large amounts of seeds in response to favorable resource availability and weather conditions. Subsequent research has shown that while weather and resource availability may act as proximate mechanisms for interannually synchronized flowering, the ultimate driver is adaptive evolution for increased mating opportunities. Early studies of synchronous flowering were biased towards trees species, which typically exhibit higher within-year synchrony than herbaceous species. The field has since expanded to include more herbaceous species. Researchers have also begun to investigate biotic drivers of synchrony, such as pollinating mutualists and herbivorous antagonists. Community and global patterns of flowering synchrony are emerging across species. Such broad patterns are prone to disturbance by anthropogenic change such as global warming and the introduction of invasive species. Little has been done to examine synchrony across plant functional groups (i.e. trees and herbaceous annuals and perennials), though differences in pollination syndromes complicate such analyses. More work is needed to understand how global shifts in flowering plant communities will reshape ecosystems.
Scales of synchrony Synchronous flowering can occur across a season (intrannual synchrony) or across multiple years (interannual synchrony) within a species, or across coflowering species in a community. Populations and communities can exhibit multiple types of synchrony simultaneously.
Within-year synchrony The synchrony of a population of flowering plants can be described within a season by how many plants are flowering at given points in time, and the distributions of individuals' flowering start and end dates. More synchronized populations have lower variances for the period of time during which individuals are flowering. The point in time at which the most plants in a population are flowering is commonly described as "peak flowering." When more plants are flowering simultaneously, there are more mates and mating opportunities available for individual plants. Self-incompatible plants, which constitute about half of all flowering plants, must outcross in order to reproduce. Within-season synchrony can increase the probability of successful outcrossing by donating pollen to, or receiving pollen from, a viable mate. In the case of wind-pollinated Juncus rushes, which exhibit multiple flowering pulses in a season, synchronized flowering allows plants to hedge their bets on the population experiencing appropriate environmental conditions for reproduction during at least one of the pulses.
Across-year synchrony In plant species which flower every year, complete across-year synchrony has been achieved. Plants which do not flower every year can achieve varying degrees of synchrony. Resource consumption can dictate how frequently a dioecious or gynoecious plant flowers, as producing seeds is a significant resource investment. When a plant flowers asynchronously in a year in which few other individuals are flowering, it has few mating opportunities. Plants which are not well-pollinated do not invest much in seed production, which can allow them to flower again in a short time. This can re-synchronize individuals, because when they are well-pollinated and invest energy into seed production, they have limited resources to invest in flowering the following year. Some species have highly canalized synchronous flowering cycles. Many bamboo species exhibit synchronous flowering return intervals, with some as long as 120 years. A proposed mechanism demonstrated that such extreme intervals can arise as a mutation and spread in a population when they align with the ancestral interval. For example, a plant with a mutation to flower every four years has many mating opportunities if it cycles with a population that flowers every two years, allowing the mutant to reproduce and pass on the four-year trait. Phylogenetic methods can reveal how across-year synchrony evolved in populations.
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