Nature 2016 October

Hedgerows – not being ant-sized we miss so much

A unique feature which characterises the British landscape is its hedgerows. Returning home from a holiday abroad as the plane descends through the  clouds, it is the patchwork of fields demarcated by hedgerows that provides the first confirmation that you are indeed landing in the UK and not France, Belgium or elsewhere in Continental Europe

Hedgerow origins are both ancient and modern. Neolithic farmers arrived around 5000 years ago clearing (assarting) forest trees and modifying the woodland edges to create field boundaries. In the Chilterns we need to wait until the Bronze Age settlers and the later Anglo-Saxon Charters for hard evidence of hedge creation and management. Mind you Grim’s Ditch, which is in places a ditch-bank-hedge, is thought to have been an early form of sheep barrier. Meanwhile, drovers routes, old parish boundary lines and holloways, which are all still discernible crossing this area, include remnants of hedgework, coppicing etc. Enclosure surveyors in the 19th century marked out straight edged field boundaries that were planted up with hawthorn and blackthorn.

As varied as their purpose, it should not be forgotten that except in rare situations hedges are principally man-made structures and though their construction tends to follow certain ground rules their form varies greatly depending on how they are managed by man and the variety of component trees and non-woody plants that comprise them. Further transformation of the hedge occurs over time as trees die or new ones spring up and different flora is accidentally or purposefully introduced and agricultural methods change.

The hedgerows of the Chiltern Hills have their own distinctive signature. Three trees provide the foundation blocks: hazel, blackthorn and hawthorn, the latter being the most dominant and appearing as the most prolific tree in 97 percent of all hedges. Other trees which are often found include ash, beech, eld maple, sycamore, hornbeam, elder and holly. It is also possible to find an occasional oak, a stand of dogwood, spindle tree and honeysuckle. Until the 1980s we would include elm in this list, but today it is a rarity and examples are more often miserable stumpy examples. All hedgerow trees and woody plants had an important purpose, value and often an additional connection back to religious (pagan or Christian) beliefs: notable examples being gospel oaks, willow wands and ‘the holly and the ivy’. The crab apple provides an example of a tree purposely planted or nurtured as a valuable source of autumn fruit for human consumption. Not long ago the hedgerow’s purpose was important as a larder, a source of seasonal fuel, a pharmacy or for a supply of emergency timber.

The hedgerow is not only an aggregation of trees, shrubs and herbs. Wildlife from the largest mammal or most colourful bird to the insect that lives but a day all rely on a hedge to shelter, feed, reproduce and rear young, or seek its prey. A hedgerow is a complete ecosystem and for many animals a corridor or superhighway connecting one otherwise isolated wood or pasture from the next. A typical section of hedge a few 100 yards long can easily contain over 3000 species of plant, animal or fungi. I read an insightful comment recently which observed that we humans are far too big to see most hedgerow wildlife easily. Not being ant-sized we miss so much. Even then we would still overlook the immeasurable variety of microorganisms.

The following examples might suffice to provide an idea of the variety and complexity of the interactions of wildlife connected to hedges. Trees ensure the overall solidity and impenetrable barrier. Each tree also provides food or habitats for their own signature collections of insects. Oak surpasses all other trees in terms of the sheer variety of different species it hosts – an unknown number but well in excess of 400 different invertebrates, mainly insects.

On the other hand hawthorn provides a good example of the typical interactions between the tree and some insects which only live on this tree. First there are several species of midge, minute parasitic flies which lay their eggs on leaves, stems or in the flowers where fruit develop and this irritates the hawthorn and causes it to grow galls within which the midge larva feed. There are even secondary insect parasites that only infect hawthorn parasites.

In common with all hedge trees the hawthorn has several of its own leaf-eating beetles all with wing cases bearing distinctive speckling and striping  colourations. There are at least 25 moths dedicated to causing mischief only with the hawthorn including the eponymous hawthorn moth. The caterpillars carve out subcutaneous tunnels in a pattern unique to that species which afford them some vital extra protection and access to the more nutritious-rich tissues. The hawthorn shield-bug has a bright green mantle spangled with red splashes. Shape and camouflaging gives it a measure of free range as it nibbles the crimson ripe haw fruit.

I have left the most bizarre interrelationship to last. A soap opera played out amongst insects indigenous to hawthorn. Centre-stage is a jumping louse which uses its needle-like mouth to extract sap from the softer stem. Known as psyllids and like their garden relatives the aphids, they excrete a sweet liquid, known as honey-dew. This nutrient-rich waste product is much sought after by ants which milk the lice. In return the ants are very protective of their milk-herd and will attack any predator that invades the territory. That is all but one parasitic wasp which resembles the ants and is able to mimic their behaviour. In so doing they have free range to roam around the colony of lice, injecting them with eggs.

Benjamin Franklyn is recorded as saying “Love thy neighbour, yet don’t pull down your hedge.” This was picked up by the American writer Robert Frost who included this proverb in his poem Mending Walls “Good fences make good neighbours”. So aside from the benefit to nature of hedges, they help to sustain a healthy relationship if not friendship between those living cheek by jowl.

So next time you pass along a path bounded by a hedge take a closer look. Identifying the trees and plants of a hedge can be a challenge to those unfamiliar with leaf shape, ower colours and bark, so pocket a handy tree and plant guide. I still use Keble Martin’s Concise Flora in Colour – out of print but still available second-hand. A good starting point for beginners is The Wildflower Key by Rose and O’Reilly. Mind you, armed with a 3G smartphone and some decent connectivity (!) there are several useful online guides.

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