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Friday, 15 March 2013

The benefits of flowering early


Bristol was a swirl of snowflakes and blossoms earlier this week. Monday on my walk the cutting wind was relentless. Yet, despite my frozen nose and numb fingertips, I stopped to admire the many splashes of colour along my route – a street lined with blossom-laden plum trees, front gardens lined with daffodils, heather and crocuses, splashes of primulas and even some snow drops in the local woods. As my teeth chattered despite my thick down coat, I did marvel at these early spring bloomers that have clearly found it to their advantage to flower despite cold temperatures, relatively short days, and a paucity of pollinators. So, what exactly are the advantages of being the first blossoms of spring?

A robin and crocuses, both soaking up some sunshine
on Thursday at the Botanic Garden.

Early woodland blossoms have access to more light

The first and perhaps most obvious advantage is that these early blossoms appear before the deciduous trees come into leaf, which gives them more access to light. Many of these early blossoms are naturally woodland flowers and so as soon as conditions become tolerable, these flowers put all of their energy into producing foliage and flowers before the forest canopy has formed. If successfully pollinated, the plant will produce a seed for dispersal. Then, as the forest floor becomes shaded by the trees above, the flower and foliage die back and any unused nutrients are returned into the roots or bulb. There will be no sign of these plants above ground for the rest of the year.

Though this may sound very much like a ‘get-up-and-go’ approach to flowering, the timing of when each species, and indeed each plant, flowers is incredibly complex and scientists have yet to figure out all the intricacies. It is affected by physical factors, such as soil nutrients, water, sunlight, day length and temperature, but it is also affected by biological factors, such as abundance of pollinators, herbivorous predators, seed dispersers and competition from other plants.  All of these factors may ultimately affect the reproductive success of the plant; flower too early and there may not be sufficient to set seed, but flower too late and the bird species that normally disperse the seed may have already migrated to over-wintering grounds.

There is less competition for pollinators

Though there are fewer pollinators about in early spring, there are also fewer blossoms to compete for their attentions. Insects that emerge in early spring or that forage throughout the winter, such as some bumblebee species, do not have a plethora of blossoms to choose from, so this increases the likelihood that the flowers that are out will get a visit.

There is more time for seed maturation

For some early season blossoms, such as fruit trees, there is an enormous investment in seed production, which takes time. The benefit in the end, of course, is a rather extravagant and often delicious means of dispersing seed great distances.

Early blossoms favour out-crossing  (Munguía-Rosas et al., 2011)

Fewer blossoms in early spring also mean that pollinators will travel greater distances between flowers. As a result, a flower may receive pollen from a more distant flower, which may be less similar genetically. It’s the floral equivalent of “bringing in new blood”, also known as out-crossing.  Perhaps the genetic material carried in that pollen encodes some increased resistance to frost or disease...or perhaps not. Most importantly, it is adding diversity to the population, which is the foundation for adaptation.

The crocuses in my front garden are in full bloom.

The evolution of early bloomers

There are not only differences in the time that plants flower between species, but also between populations of the same species and between plants of the same population. For example, the crocuses at the front of my garden, which have been exposed to more direct sunlight, are much further along than those in my back garden, which are shaded by a cedar hedge. This is a clear example of differences in the resources available in these two different growing environments.

However, consider a woodland covered with bluebells, what drives those first few bluebells to burst out before the others? It might be slight differences in their growing environments, but it is also in part a result of their genetic makeup. A sort of bluebell “aptitude” if you will that predisposes them to go to flower quickly - two separate bluebells, under the same growing conditions, may still flower at different times. Of course, it is therefore inevitable that those bluebells that are first to bloom will be pollinated by others that are in blossom, giving rise to new generations of early bloomers.

It might also be that environmental conditions are such that early bloomers are for some reason more successful in reproducing, perhaps because pollinators favour early bloomers (Munguia-Rosas et al., 2011). This will eventually drive the flowering time of entire populations earlier each season and over time this will become fixed within the genetic makeup of that population, in as few as three generations for some species (Galloway and Burgess, 2012).

Temperate plants tend to be more flexible with their flowering times

In temperate climates, there are much bigger differences in variables such as frost, temperature and day length across the landscape, between the seasons and between years. As a result, flowering plants in these regions exhibit tremendous variability in their flowering time – it is an adaptive flexibility that enables them to take advantage of the best growing conditions possible regardless of when they might happen (within reason of course).

If you’re going to be a risk-taker, be sure and have a plan B

Of course, many of these early spring blossoms have what could be considered a back-up plan. Snowdrops, crocuses, and daffodils are all capable of both sexual and asexual reproduction. So, if there is a prolonged heavy frost after these flowers have emerged or they are not successfully pollinated for any other reason during the season, then the bulbs will still form new small bulbs that are genetically identical to the parental bulbs. However, it is sexual production that brings genetic diversity to a population and this is what will allow a population to adapt to changing environmental conditions and resist disease.

There is a lovely clip here from the BBC’s Private Life of Plants on their website showing the progression of early blossoms as a British woodland bursts to life in the spring.

Well, I hope you are getting the opportunity to enjoy these early spring blossoms! Also, be sure to come out to the Botanic Garden over the Easter weekend to enjoy the Easter Sculpture Exhibition – amazing art, refreshments, and garden tours sounds like an ideal way to spend a weekend to me, I’m definitely going to be there!

Here are the references used above:
Galloway LF and Burgess KS. 2012. Artificial selection on flowering time: influence on reproductive phenology across natural light environments. Journal of Ecology 100: 852-861. http://www.cfbiodiv.org/userfiles/1111.pdf

Munguía-Rosas MA, Ollerton J, Parra-Tabla V, De-Nova JA. 2011. Meta-analysis of phenotypic selection on flowering phenology suggests that early flowering plants are favoured. Ecology Letters 14 (5):511-521.

1 comment:

  1. I have not posting about the benefits of being early blossom on my benefits blog

    would like to know, would you allow me if i posting something similar on my benefits blog?

    ReplyDelete