Flower Power: The Great North Wood in Bloom

Flowers are the sexual organs of the plant kingdom; they range from inconspicuous and green to big, flashy and vividly coloured and employ a range of strategies to transfer pollen from the male anther to the female stigma. Bees and other pollinators rely on a host of species with different flowering times to keep them supplied with nectar and pollen throughout the months that they are active. Wind pollinated trees bloom early, before their leaf buds burst, so that pollen can drift, unimpeded, through the leafless branches. In fact, flowers of some sort can be found throughout the year, even in bleakest mid-winter. Each year intrepid volunteer botanists wake up on New Year’s Day to take part in the New Year Plant Hunt, organised by Botanical Society of Britain & Ireland, and find hundreds of wildflowers in bloom, many of which are autumn flowering species taking advantage of our increasingly mild winters to flower later and for longer.

Woodland habitats support a multi-layered abundance of flowering plants, from the wildflowers and grasses of the woodland floor and the shrubs of the understory, to the trees themselves and the climbers that link the layers together. Across the remnant sites of the Great North Wood we find a host of woodland specialist species, combined with more recent introductions and the sun-loving plants of woodland edge and grassland that thrive where the woodland opens up into glades and meadows.

The flowers of many tree species use the wind to spread their pollen and are often overlooked because they lack the eye-catching petals of insect pollinated flowers. Oak (Quercus robur, Quercus petraea) for example is not known for its flowers but produces copious bunches of golden-green catkins in April and May. These are the male pollen-producing flowers; the female flowers that receive the pollen and ripen into acorns are small reddish and borne on short stalks called peduncles.

A plant that has separate male and female flowers within the same individual, such as oak, is called monecious. By comparison the yew (Taxus baccata), another wind pollinated tree, is dioecious; there are female yew trees with female flowers and male yew trees with male flowers. The latter produce visible clouds of yellow pollen in spring. Yew gender isn’t quite as simple as that though. The Fortingall Yew in Scotland is believed to be between 3,000 and 5,000 years old, making it one of the oldest living organisms in Europe. For most of its long life it has been recorded as male but it’s recently started producing fruits on one of its limbs and has become at least somewhat female.

The gender of ash (Fraxinus excelsior) is even more complex. The species produces dense clusters of purple and greenish-yellow flowers that can be male, female or hermaphrodite (containing both male and female parts within one flower). Individual trees may have flowers of one of these three genders but often have some combination of all three. Trees that produce both male and female flowers avoid self-fertilisation by staggering their ripening so that when their male flowers are producing pollen their female flowers are not yet ready to receive it. The female flowers become receptive after the male flowers have stopped producing pollen so they can receive pollen from other trees. This allows the trees to benefit from the advantages of outbreeding, producing offspring with high genetic variation and the ability to adapt to changing environments.

Not all trees are wind pollinated; the blousy petals of wild cherry (Prunus avium) and the sweetly scented blossom of lime (Tilia sp.) attract hordes of bees, hoverflies and wasps to spread their pollen far and wide. Ivy (Hedera helix) is also an important plant for insect pollinators, providing an abundant source of pollen well into November when many other plants have shut-up shop. The ivy mining bee (Colletes hederae), a recent arrival to our shores from mainland Europe, emerges in August and September, later than any other solitary bee, to take advantage of the crop of late season ivy pollen.

Another woodland climber, honeysuckle (Lonicera periclymenum), has evolved to attract a different insect pollinator. Its pale flowers release a heady scent and are pollinated largely by night-flying hawk moths, which are equipped with an extra long feeding tube (or proboscis) necessary to reach the nectar at the bottom of the tubular flowers. Bees cannot reach the nectar the conventional way but may instead pierce the flower tube to get at the sweet syrup inside.

Some flowers have gone beyond the usual adaptations of scent and colour to attract their insect pollinators. The broad-leaved helleborine (Epipactis helleborine) is an orchid that can be found at London Wildilfe Trust’s reserve New Cross Gate Cutting. It has greenish purple-tinged flowers and attracts bees and wasps that appear to become intoxicated after imbibing its nectar. It’s thought that the nectar is fermented with help from a fungus to produce alcohol. The bees and wasps can’t seem to get enough!

Another common plant of the Great North Wood, Lords-and-Ladies (Arum maculatum), has evolved a crafty way to ensure it gets the most out of its pollinators. Its unusual form, said to resemble male and female genitalia, has given rise to many imaginative folk-names such as Adam and Eve, soldier’s diddies, priest’s pintle, cuckoo’s pint, jack in the pulpit and sonsie-give-us-your-hand. The sheath-like structure is called a spadix and partially encloses a finger-like inflorescence (a structure that bears several flowers) called a spathe. The female flowers are at the base of spathe and the male flowers are little higher up. The plant produces a urine-like scent and raises its temperature by several degrees relative to its surroundings by metabolising starch. This attracts small flies that crawl into the spadix down to the base of the spathe where they deposit pollen on to the stigma of a female flower. The flies are trapped inside the spadix by a ring of hairs that separate the male and female flowers. The following day the hairs wither away, allowing the flies to crawl up the spathe, picking up pollen from the male flowers that have since ripened, and leaving the spadix to find another inflorescence.  

Many flowers of the woodland floor get the business of flowering done early to make the most of the light that reaches the ground before the leaves return to the trees and plunge the woodland into relative darkness. The plants then shrivel away completely drawing sugars and other useful molecules into underground tubers to wait it out until the following spring. These include lesser celandine (Ficaria verna), wild garlic (Allium ursinam), wood anemone (Anemone nemorosa) and bluebell (Hyacinthoides non-scripta), all known as ancient woodland indicators because of their association with very old woodland and all found in several sites across the Great North Wood. These plants are strongly suppressed by evergreen trees and shrubs such as cherry laurel, spotted laurel and holly, which deny them a much-needed burst of spring sunshine. These evergreen species have become dominant in several areas of the Great North Wood and during our volunteer workdays we’ve been thinning them to allow ancient woodland specialists like those above to endure.

Dog’s mercury (Mercurialis perrenis) is another ancient woodland plant that is present in small isolated populations in the area (I know of a patch in Sydenham Hill Wood and a patch in Long Lane Wood). It adopts a different strategy from the tuber-producing plants, maintaining its shade tolerant leaves all year round and eking out an existence with what little light reaches it though the canopy.

A third strategy is adopted by woodland plants like foxglove (Digitalis purpurea) that produce long-lived seeds that can lie dormant in the soil for many years until they sense light and are triggered to germinate. Where we have removed dense stands of cherry laurel we have seen several foxglove plants emerge, such as in Low Cross Wood, Spa Wood and Biggin Wood. These plants came from seeds that may have lain dormant for decades. Foxgloves are biannual which means that this year’s rosettes won’t flower until next summer after which they will die leaving their seeds to take advantage of the next window of light.

The flowers described above exist in a complex and fragile network of ecological interactions, mediated by availability of light, competition with other plant species and presence of preferred pollinators amongst many other factors. These plant communities are highly vulnerable to trampling and compaction and much of the work we do during our volunteer workdays is to limit these impacts. We hope that with some careful management the diverse and delicate woodland ecosystems present within the Great North Wood can continue to exist far into the future.

London Wildlife Trust launched the Great North Wood project in 2017 and now works with volunteers, community groups, landowners, and councils, to revive and reimagine this ancient landscape as a home for nature and people. The project is funded by Heritage Lottery Fund, the Mayor of London, Veolia Environmental Trust, Dulwich Estate, and Dulwich Society.

Thanks to Jeremy Bartlett for his description of lords-and-ladies and its pollinators:

https://www.jeremybartlett.co.uk/2018/03/16/cuckoo-pint-arum-maculatum/