Yellow willow catkins on a tiny branch.
©️ Margery Thomas

March on Hothfield Heathlands: A land of pollen!

It’s blowin’ in the wind. Millions of microscopic grains of it. Pollen.

At the moment mostly from trees on the reserve, as hay-fever sufferers may already be noticing. Hazel in sheltered spots was releasing pollen soon after Christmas, joined in February by alder. Through March to May, before growing leaves make it more difficult for pollen grains floating haphazardly about to land on the tiny female flowers, ash, beech, birch, sweet chestnut, oak, sycamore, willow species and giant redwoods among others will shed pollen into the wind. No need to attract pollinating insects so no large coloured petals or other enticements, energy goes into huge quantities of aerodynamic pollen, male flowers reduced to often showy catkins or cones, female flowers to unobtrusive stigma and ovary, positioned to optimise pollen landing on them.  Female flowers are not always on the same tree as the catkins; willows are dioecious, ie female and male catkins grow on separate trees. 

To enable tree pollen to travel and swirl about, survive bad weather and land successfully a grain is tiny, light, dehydrated with a sturdy outer casing protecting the cell containing the male DNA. Every species is unique, identifiable, about 15 to 100 microns small. Electron microscopes and macrophotography have revealed the myriad wonderful shapes and surface patterns of pollen, such as those etched on glass in the greenhouses at Oxford Botanic Garden. The oldest pollen found to date is in rocks over 120 million years old. The allergen in birch pollen was the first to be identified, over twenty-five years ago.

©️ Margery Thomas

Many herbaceous ie non-woody plants on the reserve are wind-pollinated, including the grasses that define the heathland areas, and the sheeps sorrel (Rumex acetosella, another dioecious plant) whose tiny female flowers, reduced to the essentials for fertilisation, cast a rusty haze over short grass. The heather (Calluna vulgaris) on the heaths is pollinated by insects and wind. A plant mainly of high rainfall areas, the anthers containing the pollen are protected from rain by closed petals, but wind can shake pollen out when insects aren’t flying. Studies have also shown that thrips that live in the tiny heather flowers move to other flowers thus transferring pollen. 

As the showier flowers start to open in order to produce the next generation, different techniques, attract the pollinator insects, bees, butterflies and moths. Energy goes into scent, colour on petals/sepals and markings to guide into the centre of flower where nectar may be produced, so that pollen, usually heavier and stickier than tree pollen, may be caught on fur or hairy legs, and taken to another plant. In the main bog, the insectivorous sundew Drosera rotundifolia has a challenge, needing insects to pollinate the small flowers that open one at a time, while needing insects to land on the leaves a few centimetres below to be consumed for nutrients. 

Some plants have developed a back-up to pollination by external forces, self-pollination, or cleistogamy, from the Greek kleistos for closed. On the reserve, in early autumn, two violets, Viola paulustris and reichenbachiana, both axiophytes or indicators of habitats of conservation importance, produce colourless flowers close to the rootstock almost underground, which produce some pollen which immediately fertilises the same hidden flower, resulting in the usual capsule of seeds at soil level which bursts virtually underneath the main plant. Likewise in the main bog, bog asphodel Narthecium ossifragum, another axiophyte, is known to resort to cleistogamy in very adverse conditions. 

Meanwhile the birds (and bees) are preparing to produce their next generations with nest-building and courtship. Please help us protect the nests at or close to ground level on the open heathland which are extremely vulnerable to disturbance. 

As the weather begins to warm and day length increases many of our bird species will start to think about nesting. On Hothfield low and ground nesting species like linnets, stone chats and yellow hammers are particularly sensitive to any disturbance. We are therefore asking for people to consider these species and walk on the paths and keep dogs on short leads whilst out in the open areas of the reserve. Together we can share these spaces with nature and encourage more rare species onto our amazing reserve like resident Dartford warblers and nightjars. The woodland periphery walk will still be a dogs under control zone but please bear in mind that livestock are also present on reserve. We will be asking for this until the end of the nesting season in September.