How does flower-timing affect reproductive success in a woodland spring ephemeral?

Spring ephemeral flowers are an important food resource for early-season pollinators such as flies and native bees. Without ephemerals, these early foragers would go hungry because nothing else is in bloom. In return for their pollen and nectar, pollinators move pollen from flower to flower—an important step in plant reproduction that allows plants to produce seeds. But what happens when there is a mismatch between when the flowers open and when pollinators are active? Or what happens if a random frost comes through and destroys the flowers before pollinators are able to get to them? Luckily, many plants have evolved a bet-hedging strategy, spreading out when they flower across a growing season.

To test whether there are reproductive differences between spring ephemerals with earlier or later flowering times, my study used eastern spring beauty (Claytonia virginica) as a model species. Starting in late March, I went out to McDonald Woods in Glencoe, Illinois and did a daily survey of how many flowers were open, how many pollinators were visiting, and what the environmental conditions were like (i.e. temperature, sunlight, and soil moisture levels).

In late May, as the tree canopy begin to close, any Claytonia flowers that had been successfully pollinated turned into fruits which I collected and analyzed to determine the quantity and quality of seeds they produced. I measured reproductive success by analyzing seed viability, and I quantified it in two ways: the presence of formed plant embryos during seed x-ray analyses, as well as the seed’s ability to germinate under controlled greenhouse settings.

Interestingly, Claytonia that flower later in the season are able to produce as many seeds as earlier-flowering individuals, but these seeds tend to be less viable. This may be because, as the season progresses, Claytonia must compete for pollinator services with more and more flowering species that are coming into bloom. Another reason why later-flowering Claytonia produce less viable seeds might have to do with decreased levels of sunlight as the forest canopy develops. With less light available, plants may reallocate resources away from reproduction to vegetative growth. Finally, results from the germination experiment show a negative relationship between flowering time of the maternal plant and germination time of its offspring—that is, earlier flowering correlated to earlier germination. This suggests that the environment in which the plant embryo is conceived may have implications for the offspring. However, further research is needed to tease apart the mechanisms behind this relationship.

Understanding how natural variation in flowering time might affect reproductive success gives us a better framework for predicting how certain plant populations might respond to large-scale climate-induced shifts in flowering. For example, we know that climate change is signaling many plant species to flower earlier than they have historically been known to. For the earliest of bloomers, the lengthened growing season provides potentially high rewards with first access to important resources and pollinator services. However, these early-flowering individuals are at higher risk of experiencing more extreme weather events. Understanding how the timing of flowering relates to the natural biology of plant reproduction will be key to the conservation of many plant populations.

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