Choosing the right time to flower

The fall crocus (Colchicum autumnale) is visible all over Denmark now, making a strange sight – a plant with large flowers and no leaves, blooming just a few days before the first frost is likely to occur. The timing of flowering in plants is very important to their evolutionary fitness – the wrong choice can lead to no seeds being produced and then no offspring! It should be no surprise that different species have evolved a range of different flowering strategies and timing to exploit their local environment. This crocus is likely taking advantage of the limited number of other species competing for bee pollinators at this late date. Of course this strategy is risky – frost can destroy the flower, and bees may no longer be active when the flower is available.

Here’s another alternative strategy, taken by Senecio soldanella – bloom as quickly as possible, even when environmental conditions are harsh (here, a snowfield at approximately 13,000′ elevation). Early flowering ensures that in a given growing season, reproduction is guaranteed to occur, or at least be attempted. The danger here is similar – harsh environmental conditions, and potential lack of pollinators.

A more typical approach is to wait for the peak of the growing season when conditions are good and many pollinators are available because of the high abundance of other species. This is the strategy taken by Aquilegia coerulea (columbine). However this too has its risks as climates begin to change rapidly. The cues that induce flowering in plants may not be the same as those that induce the arrival of pollinators like bees and hummingbirds. A rapid climate change could desynchronize these systems, leading to disastrous consequences for both. A recent study from the Rocky Mountain Biological Laboratory published in Ecology shows some evidence for this in the United States.

In the tropics, the seasonality of climate is highly reduced, so there can be fewer environmental causes of flowering. In many cases species flower year-round or idiosyncratically when resources are available. Here is a small mosquito-pollinated orchid from Costa Rican tropical moist forest that takes such a strategy.

And of course, not all flowers are what they seem – some are not even flowers at all! Above you are seeing an Arabis mustard plant covered in a Puccinia monoica rust fungus. The yellow coloration looks attractive to pollinators, which come by in search of pollen or nectar but only succeed in further spreading the fungus to other plants! Of course, this strategy only works when the fungus has evolved to take the same strategies as would be taken by a real flowering plant. This is a beautiful example of mimicry and parasitism first discovered at the Rocky Mountain Biological Laboratory in the 1990s and described in Nature.

Edit – a comment from an expert, Mel Harte: Pollinators that come DO get some nectar, because the rust does force the formation of extra floral nectaries, apparently (and that makes sense, since the spores will really get stuck onto the pollinator as it forages). So they fly away with a sweet treat, and lots of spores to disseminate!

So look closer at the next flower you see – it is telling you a story shaped by many long years of evolution!

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