English Bluebells

One springtime event I await with eager anticipation each year is the emergence of the bluebells. From late March a few nodding heads emerge sporadically across the woodland floor, and by late April they are out in full splendour, their blue hue carpeting the ground. 

A display of bluebells in a broadleaved woodland, Hampshire

 Compared to many wildflower species, the English or common bluebell (Hyacinthoides non-scripta) blooms early. This is an adaptation to its woodland habitat, to allow photosynthesis and accumulation of resources before the forest canopy becomes too dense and reduces light intensity at ground level. Bluebells can be considered ancient woodland indicators due to their slow rate of spread - populations spread up to 6cm per year.  In wetter areas, such as the west coast of the British Isles, H. non-scripta can also grow in more open habitats, where summer cover is provided by bracken (Pteridium aquilinum). Bluebells have been shown to flower later when growing in open areas than in the shade of woodland. 

Bluebells are geophytes - perennial plants with underground storage organs, in this case, bulbs. This means that when the leaves die back the plant isn't visible above ground. Splitting of the bulbs allows asexual vegetative reproduction, on top of sexual reproduction with seeds. The primary pollinators of bluebells include long-tongued bumble bees and hoverflies, which benefit from the early emergence of the flowers. Some bumble bees take advantage of the nectar source without pollinating the plant by biting through the base of the flowers. This may seem to breach the assumed mutualism of plant and pollinator, however, in some other plant species it has been shown not to reduce reproductive success, partly because bees frequently switch between nectar robbing and pollen collection.

The 5 annual phenological stages of bluebells

  

Left: Exposed bulbs of H. non-scripta in the early photosynthetic stage (early February)

Right: flowering bluebells (reproduction stage) in mid-March

English bluebells are widespread across the British Isles, with 25-50% of the global population found here. Despite their least concern conservation status, H. non-scripta faces threats at a local scale. Trampling of bluebells damages the above ground vegetation, reducing their capacity to photosynthesise, and therefore limiting growth rates and storage of nutrients. Bluebells take a long time to recover from the effects of trampling, partly because the new leaves grow from tissues which differentiated the previous year, so severe damage also prevents growth the year after trampling. 

Another potential threat to H. non-scripta is the illegal trade of wild-collected bulbs for use in horticulture. To prevent this, native bluebells are protected by legislation - the Wildlife and Countryside Act 1981 makes it illegal to dig up the plants or bulbs, and a later schedule of this act makes it illegal to trade wild bluebell bulbs and seeds. 

The most discussed threat to H. non-scripta is the introduced Spanish bluebell H. hispanica and hybrid species H. x massartiana. Both introduced species pose a potential threat through replacing native populations and reducing the genetic integrity of H. non-scripta. The level of risk of hybridisation is debated. Full interfertility between the native species and H. x massartiana is evident, and natural crossing is possible due to flowering synchrony and pollen transfer. The introduced species are widespread across the UK, and a survey by Plantlife found Spanish or hybrid bluebells in 1 in 6 woodlands. However, the non-natives appear to be concentrated around more urban areas, as they are grown as garden plants. If the rate of spread is as slow as the native species, the large existing populations of H. non-scripta will reduce the risk of genetic assimilation. On top of this, the introduced species of hybrid origin has significantly lower reproductive success than the native bluebell in mixed populations - regardless of the maternal plant, H. non-scripta is three times more likely to be the paternal plant than H. x massartiana. The asexual vegetative reproduction of bluebells may also limit genetic introgression.

Though English and Spanish bluebells share many characteristics, they can be distinguished when examined in more detail. A comparison of distinctive characteristics can be seen in the table below. Hybrid species have intermediate characteristics, though may often resemble the Spanish bluebell more strongly. 

An example of a non-native bluebell - the paler colour and lack of drooping are easily identified.

Properly disposing of garden waste containing introduced bluebells can help to stop the spread of invasive varieties, and non-natives can be removed by pulling up the bulbs, though care should be taken to correctly identify the species first. 

Some relevant websites and papers:

https://www.plantlife.org.uk/uk/discover-wild-plants-nature/plant-fungi-species/bluebell
https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/plants/wild-flowers/bluebell/
https://www.wildlifetrusts.org/wildlife-explorer/wildflowers/bluebell

Are native bluebells (Hyacinthoides non-scripta) at risk from alien congenerics? Evidence from distributions and co-occurrence in Scotland
Paternity analysis reveals constraints on hybridization potential between native and introduced bluebells (Hyacinthoides)
Plate 481. Hyacinthoides non‐scripta Hyacinthaceae