The age of hedges - Botanist Max Hooper correlates number of species in English hedgerows with centuries in age

Whole Earth Review, Summer, 1995 by Charles Elliott

FEW YEARS AGO AN ENGLISH BOTANIST NAMED Max Hooper had a brilliant idea. It had to do with Hooper as studying the effects of insecticides on plants when he noticed that English hedges vary a lot. Some were composed of no more than a single species, usually hawthorn, and these tended to run in straightlines. Other hedges, especially the wandering kind beloved of landscape artists and the English generally, often had an impressive range of shrubs and trees, from hazel to oak to several kinds of roses. Some, in fact, contained upwards of a dozen different species in a short stretch. Obviously the variation meant something. But what?

Hedges are ubiquitous in Britain. Estimates of just how many there are differ widely, but the best guess is half a million miles or more. You see them everywhere, especially in the southwest, and also along the Scotch and Welsh borders wherever the land is not too high or rough. Not every farmer loves a hedge -- starting in the Napoleonic era, when high grain prices encouraged bigger fields, a gradual process of grubbing out hedges has been going on, particularly in East Anglia and the East Midlands. Even so, plenty remain. Some regions have so many roadside hedges that it is hard to see the scenery, and around our house near Monmouth, in Wales, the hills are absolutely patchworked with them. (They are substantial hedges, too, made to hold Welsh mountain sheep, and they do. If you want to get through one, take an ax and figure on spending an afternoon.

To a naturalist, a hedgerow offers many delights. It is a shelter for birds and such little mammals as rabbits, shrews, and (obviously) hedgehogs, and a habitat -- sometimes the only really comfortable habitat left -- for dozens of plant species from cowslips to cow parsley. Hooper, however, viewed his hedges with different eyes. Soil types failed to explain the variation in the number of tree and shrub species in particular hedges. So, in most cases, did climate. Could it be, he wondered, that the variation reflected the age of the hedge? Was it possible that we had here a sort of gigantic botanical clock?

To find out, he tracked down a selection of 227 hedges that could be dated with some accuracy from documentary evidence: old deeds, charters (some going back to Saxon times), the Domesday Book of 108 6, monastic records, old maps. The hedges were scattered across England from Devon in the south through Gloucestershire to Cambridgeshire, Huntingdonshire, and Lincolnshire in the Midlands. Then, with some help, Hooper started counting species in randomly chosen thirty-yard sections of each hedge. Arbitrarily excluded were brambles such as blackberries (find a hedge without blackberries! I and woody climbers such as ivy (ditto), but those counted included about fifty other common shrubs and trees ranging from hedgerow hawthorn (Crataegus monogyna) to euonymus (known around here as spindlel). The results were fascinating.

Within a surprisingly narrow range, Hooper found that the number of species in thirty yards of a given hedge correlated with the age of the hedge in centuries. A hedge known to have existed a thousand years ago had at least ten species in it, an 800-year-old hedge eight species, a 100-year-old hedge just one. The margin of error could be as much as one or two centuries, but more often than not the numbers jibed. There was something strangely satisfying and at the same time slightly unearthly about the idea that hedges aged this way. Nicest of all, of course, was the fact that anybody with a shrub and tree guide could go out, pace off thirty yards, count species, and tell how old a particular hedge was.

Plenty of people did this once Hooper's Rule received some publicity, among them several thousand schoolchildren enlisted by their science teachers to survey hedges all over the country. As the data came in, one fact about British hedges emerged with splendid clarity: There is an amazing number of very old hedges, hedges older by far than the oldest stagheaded oak squatting in a deep park, hedges to shame the antiquity of Tolkien's most elderly ent.

Previously, most historians of the countryside assumed that the great majority of British hedges dated only from the time of parliamentary enclosures, when powerful landlords managed to get bills passed permitting them to enclose -- and take over -- open fields traditionally farmed and grazed in common. The addition of hedges created pastures where cows and sheep could be left without herdsmen, thus further enriching the wealthy (and incidentally driving a considerable number of impoverished farmers off to urban slums or to America). Enclosures got started in the 1600s, reached a peak late in that century, and continued until the middle of the nineteenth century when there wasn't much left to enclose. So if in fact most hedges were enclosure hedges, then, at the outside, most hedges ought to be 200 to 300 years old.

We now know, thanks to Hooper's Rule, that less than half of Britain's hedges are the result of parliamentary enclosures, and most of these are in the East Midlands. Elsewhere, there are miles and miles of old hedges still leafing out every spring. A quarter of the hedges in Devon are over 800 years old, another quarter more than 700. In certain parts of Devon and Suffolk, researchers found that eighty percent of the hedges contained between six and ten species, suggesting that they were between 600 and 1,000 years old. (Some apparently went back even further than that, though nobody seems about to pinpoint a Roman hedge using Hooper's Rule.