Dating hedges from subjective evidence

Certain “indicator” plants can provide clues to the origin of a hedge, e.g. Bluebell (Hyacinthoides nonscriptus) , dogs mercury (Mecurialis perennis) and primrose (Primula vulgaris) are all species occurring in or at the edge of woodland. There existence in hedges strongly suggests a woodland origin that possibly dates back to the assarts of the 12th – 14th century.

On the other hand we need to not overlook the possibility of secondary colonisation at a later date, especially in the case of bluebells that are sometimes escapees from gardens and waste piles.

Hawthorn as an indicator species

Evidence has shown that, providing clay is present as a soil type, the older a hedge is then the greater the presence and proportion of Midland thorn (Crataegus leavigata). During the last enclosure period of the 19th century there evidentially was a tendency to use

hawthorn (Crataegus monogyna) varieties. The reason for this change in hedge species planting may well have been due to the direct correlation between the increasing dissection of the leaves and the number of thorns on a plant, thereby providing a greater stock proofing barrier ability to the hedge.

Brambles as a possible indicator species

The use of predominant shrubs as an indicators species could well be expanded dramatically once more is understood about the nature of these variants. With 387 subspecies then, in theory, the older the site, the greater the opportunity for different species to colonise it. Identification, however, is difficult for the inexperienced as some features, such as stem colour will vary according to the amount of light or shade.

While bramble dating may help in the future, in our present state of knowledge, it still lies in the experimental stage and as such can not be more than subjective.

Trees as indicators of hedgerow age

Mature trees commonly found in hedgerows can provide a good judge of hedgerow age, by finding out the age of the trees themselves. This can be done easily with dead tree stumps, such as those provided by the pathogen Ophiostoma ulmi (dutch elm disease), especially in the southern areas, as well as hurricane damage, by counting the growth rings of the stumps.

If there are however, no stumps available then the age can be estimated by measuring the girth of the trunk at 1 metre from the ground. An increase in girth of about 1” per year is a relatively good average approximation for larger trees such as oak Quercus robur), ash (Fraxinus excelsior), beech (Fagus sylvatica), elm (Ulmus glabra)etc. This is providing that the tree is able to grow a full crown and is in the open, which is the usual case with hedgerow trees. Smaller trees such as apple (Malus sylvestris), cherry (Prunus avium), holly (Ilex aquifolium), hawthorn (Crataegus monogyna) , rowan (Sorbus acuparia) and white beam then the rate is nearer half an inch per year. Woodland or copse trees, where the competition is higher tend to increase at a rate of approximately half the full canopy rate.

This measuring method is known as the Forestry Commission rule of thumb and is the product of some 20,000 records showing that most of the larger tree species have a closely linked pattern of growth. Young trees tend to grow faster than the older ones but the 1 inch per year average tends to hold good.

Counting the rings of a tree stump will give a very accurate record of its age, or measuring the girth will give a close approximation. If this is greater than the hedge then the area may well have been woodland that was felled to make the fields. Perhaps more likely if several trees of the same age appear in the same hedge. If the trees appear to be younger than the hedge (worked out in the usual method) then they are probably later colonists or planted at a later age.

The ages of the trees compared to the hedge itself can provide complimentary confirmation as to the age of the hedge or may shout out loud if the age of the hedge and the age of the trees does not correspond at all. Thus pointing to a possible error.

Possible Age of Hedges by Colonisation of Tree and Shrub Species

Hawthorn is by far the commonest hedgerow shrub, and the two species, (Crataegus monogyna) and (C. laevigata), and occupy different locations and ages of hedge with C. laevigata hedges tending to be of a more ancient origin.

Elder (Sambucus nigra) is a small tree of about 10 metres in height if left alone, however when it is cut back, it grows at an outstanding rate and can kill off sections of other hedge plants due to shading. Coupled with its very fast growth rate and its favour of disturbed ground, spread by birds or small mammals eating the berries and then depositing the seeds with ready made fertilizer. Its relatively short life and fast growth rate tends to indicate that the hedge is relatively young or on impoverished soil. A caveat to this however, I have seen a very old hedge suddenly swamped with elder within 5 years after a rabbit fence was added along its length. So disturbance of the ground around a hedge can cause misleading interpretations.

Dog rose (Rosa canina), blackthorn (Prunus spinosa) and ash (Fraxinus excelsior) tend to occupy a middle position in colonisation and occurs in both species rich and species poor hedges.

Contrastingly hazel (Corylus avellana), field maple (Acer campestre) and dogwood (Cornus sanguinea) are more commonly found in a hedge with a high species diversity and as such have probably been used widely in the past for planting mixed hedgerows as they are today, in fact they are a staple addition in hedges that have been planted within the last 10 years.

Dating hedges from quantitive evidence

Other than the influence of man, the most powerful ecological factors influencing the rate of change in a hedge are colonisers by outside species and their competition with the already present. The greater the age of a hedgerow, then the more varied the flora, and consequently fauna should become.

This theory was tested by selecting 227 hedges in different parts of the country whose dates of origin could be determined from documentary evidence with some degree of certainty, then by estimating the number of species of shrubs occurring in lengths of 30 yards.

A correlation coefficient of +0.85 was obtained indicating a close positive relationship between the two variables. A figure of +1 would have meant a perfect linear correlation existed and 0 meant that there was no correlation, while a minus would indicate an inverse correlation.

An equation for estimating the age of a hedge from the number of shrub species in a 30 yard stretch = x+100y+30 where x = the age of the hedge in years

y = the number of shrub species in a 30 yd length.

The procedure is an approximate, so in practice a sampling distance of 30 yards is accurate enough. Given variation in the sequence and rate of change in hedgerows, making it not surprising that estimates from the equation are subject to appreciable error. On statistical grounds it is estimated that 95% of confidence limits lie within the range of + or – 200 years.

This procedure is applicable in all areas where traditionally single species hedges were planted.

Practical problems in hedge dating

This method of dating assumes that the rate of accumulation of woody species of plants is correlated positively with hedgerow age. Two questions must be asked if we are to evaluated more closely.

What evidence is there that changes do take place in the composition of hedges ?

To what extent do estimated ages of different hedgerows agree with evidence from documentary records, both over wide areas and in more localised situations.

Evidence for Change

Very little is known of the nature of changes in sequence in hedgerows as opposed to woodland or heathland due mainly to the neglect of the subject and possibly is the difficulty in ascertaining the originally planted species present. Management methods also change constitution of species presence e.g. elder (Sambucus nigra) the biggest problem is usually removed when hedges are laid but exists in greater numbers when the hedge is managed than when not, suggesting that management encourages regeneration from plant remains or colonisation by propagules predominantly from fruits and seed dispersal.

Other useful indicators that can help indicate hedge age.

Field maple (Acer campestre), found in many hedges on alkaline and neutral soils. It is however rare to find it with only one or two other species or in young hedges. It tends to start appearing when four or more woody species are present, ie circa 400 years plus.

Spindle, Evonymus europaeus, behaves in a similar manner to the field maple but coming in at a later date of circa 600 years plus. Its preference for calcareous soils prevents it from being a general age representative shrub due to its restricted distribution.

Species with a wide soil tolerance include hazel (Corylus avellana) and holly (Ilex aquifolium) while the wayfaring tree, Vibarnum lantana, and dogwood (Cornus sanguinea), are conspicuous colonists of calcareous soils and often establish themselves relatively early in a hedgerow.

Error in Age Prediction

The original sample of hedges from which the predictive formula was derived consisted of 227 hedges and covered a sizable part of the country which different soil conditions and management regimes as well as age and origin. With this in mind it is perhaps surprising and encouraging that this method of aging has stood the test of time so well and across many areas. Subsequent studies have supported and co-oberated the idea of an approximate linear relationship between a hedge and the number of shrub species that it comprised of.

However, anomalies do sometimes occur as seen in the table below, which summarizes data obtained in 3 villages in Shropshire. The 50 samples were each 50 yards (27.4m) long.

Documentary and estimated ages of 3 kinds of hedge in Shropshire.

All figures are to the newest 100 years.

While estimates for the ASSART hedges and those of the 15th – 17th century are predominantly in close agreement with the documentary evidence, the Commons Enclosure hedges differ considerably. This difference in years is explained by the agricultural records of the time, where the farmers and the land owners were planting any woody species found near to a hedge in order to save in the cost of buying the hedgerow plants.

This difference reinforces the need to cross check with local records and reliable documents because the correlations between species and age is evidentially fictional in this situation.

Another problem with the species/age equation are hedges that have been planted within the last 20 years and especially within the last 10, where it is now common place for the hedge to be planted with a diverse species type such as hawthorn (Crataegus monogyna) , hazel (Corylus avellana), field maple (Acer campestre), oak (Quercus robur), dog rose (Rosa canina) and/or field rose (Rosa arvensis), dogwood (Cornus sanguinea), spindle (Euonymus europaeus) , birch (Betula pendula) and holly (Ilex aquifolium). There then are occasionally a few other species thrown in depending on location and owners choice.