History of Roadside Verges | birkin-wildlife

History of Verges

History of Hedgerow and Roadside Verges

By the end of the 18th century, the roads in England where in a very bad state , pot holes and thick mud abounded. Then with the introduction of new road construction methods by Telford and Macadam, it became no longer necessary to dodge pot holes and as consequence roads could become narrower. This over hall of the national road structure started in the early 19th century and gradually spread though the whole country. The excess space between the new road and the boundary, usually a hedge, that was no longer needed for pot hole dodging manoeuvres, became disused and colonised by plants, thus creating the verges that we know today.

In current times the verges have become an important part of the road network, ie;

providing added visability to drivers at bends and junctions,
as places to park in an emergency,
as areas where road repair equipment and materials can be stored temporally,
as places for drains and soak aways to remove surface water,
as a means of providing structural support to the road surface,
providing a visual link between the road and its surroundings,
separation of pedestrians, cyclists and horse riders from the carriageways where larger vehicles travel,
acting as significant reservoirs of plants and animals - particularly in areas where hedges have been removed,
as wildlife corridors and a means of plants spreading from one area to another.

The portion of the verge nearest the road edge is subject to constant disturbance ie; Throwing up of mud and water in wet weather, Pollution from salting and exhaust fumes. Further from the road the conditions are more stable and have a rich community of annual and perennial plants.

The road network was only metalled from the early 19th Century onwards, the verges are comparatively younger than the hedges that standby them. In circa 1972 it was estimated that there were up to 0.415 ha of maintained land/km of road (1.65 acres/mile). Using the Department of Transport figures (1987) of 225,450km (140,100 miles) which would work out at approximately 93,560ha (231,190 acres) of managed land. To which we could then add a further 3645ha (9,000 acres) of banks, burns, tree plantations. This total of 97,200ha (240,000 acres) provides a vital and very large habitat area for plant and animal species. The average verge width of areas of country is very difficult to work out due to the way that they vary from area to area and county to county.

Width of verges by class.

Class 1 roads a A category;

Class 2 roads are B;

Class 3 roads are Minor roads with 4.3m+ of metalling;

Unclassified roads have < 4.3m metalling or untarred.

Many counties are some what typical in that they possess large numbers of low category roads with very narrow verges, however when you look at the average width of the trunk roads and A roads you will notice a massive difference. This difference may be partially explained by the fact that some of the roads tend to pass through cuttings and valleys or are associated with industrial sites or residential areas.

On verges that adjoin a hedgerow, than both these areas share a common number of herbaceous plants. Many of these plants provide food for a variety of birds, animals and insects, with the hedge providing shelter. However when the adjoining hedge is removed all that remains of this once virile source for the wildlife is the verge.

Motorway Verges

Most modern roads and their verges tend to be about 100 years or more in age, with the coming of tarmaccadam etc, but quite a few are much older still. However, the first section of motorway was only completed in 1959, so even the oldest is still under 60 years old. The design of the motorway network has altered over the years and varies from one area of the country to another. The ministry of Transport, in 1968, laid out general specifications and guidelines for two or three lanes of traffic with a lane width of 12 feet (3.7m) and a central reserve of 13 feet (4m). On each side was a hard shoulder of 10 feet 6 inches (3.2m) that was bordered by a verge of at least 5 feet (1.5m). However the verge size can vary tremendously in size shape deviation and gradient as well as geology. These guidelines were published in "Layout of roads in rural areas" (London: HMSO).

The soil that is excavated from the bed of the motorway is generally piled up on the sides forming banks. This is the cheapest method of getting rid of the spoil, but also acts as a sound deflector as well as creating a barrier for any vehicles that leave the carriageways.

The geology of a given area dictates powerfully the size, shape and gradient of the bordering verge and is bound by the hardness, durability, stability and rate of erosion of the local rock structure. The standard verge gradient is 1:2 (45%) but this varies tremendously. Rocks such as chalk, limestone or granite are very stable and relatively hard, so the gradient can be increased. Clays and sandstones form lighter soils with higher erosion rates, so the gradient will be lower and needs the roots of the verge plants to hold to structure together and create a stable environment. Shale and metamorphic slate, on the other hand are famously unstable and highly prone to slides and cascades so often require strong metal barriers or heavy duty metal netting to hold them in place.

Where the superficial geological layer is quite thick and the excavation is relatively shallow then the soil of the verge is likely to be of the same composition as the surrounding land, thus making it hardly surprising to find local plant colonisers from the surrounding area. However the soil

will have been through some changes before being colonised, such as drying, erosion, changed aeration and compaction from heavy plant machinery. The drainage system and properties will also be altered.

Sometimes there is insufficient soil to create a sufficient barrier, so soil needs to be brought in from other sites. This extra soil is brought in from local sites or from other parts of the motorway construction that have a surplus but occasionally quarry or colliery waste is used. For economic reasons the areas sourced should be as close to the site as possible. Given the geological structure of the uk then many different types of substrata and soil are utilised thus creating a patchwork of soil types which can favour different colonisers. Even without this action there would still be variation of the soil types, due to the fact that the geological stratum changes on average every 20 - 25 miles (40km), with some areas, such as Cheshire, changing every mile of two.

Origins of Motorway plants

During the construction of a motorway, the soil verges are appropriately prepared to make a good seedbed prior to seeding. Which is then done as soon as possible to avoid soil erosion. The seed rate will vary according to the gradient, the greater the gradient then the higher rate of seed per hectare.

Where the gradient is to steep for machinery to safely travel then a form of seeding known as Hydro seeding is used. This is basically a mixture of organic mulch, seed and fertilizer which has a gel like consistency and provides everything that the seed needs for germination and growing. This mixture is then sprayed onto the banks by a tanker where its gel like qualities enable it to stick to the banks

The seed mixtures that are used again will vary but are basically of two types, grass seed mix and flower meadow mix. The flower seed mix will have native wild flower seed with very little or no grass seeds in. The grass seed mix will have 4 - 6 species of grass seed and usually some white clover as a nitrogen fixer.

As well as herbaceous plants, trees and shrubs are also planted. The forestry commission and the department of the environment choose suitable areas for planting with preferences for sites that hide unsightly objects, soften hard outlines of structures such as bridges and help to deaden noise levels in built up areas.

The most common tree species planted are field maple (Acer campestre), sycamore (Acer pseudoplatanus), silver birch (Betula pendula), scots pine (Pinus sylvestris), oak (Quercus robur), beech (Fagus sylvatica) and ash (Fraxinus excelsior).

Common shrub species planted are hazel (Corylus avellana), goat willow (Salix caprea), hawthorn (Crataegus monogyna), blackthorn (Prunus spinosa), and elder (Sambucus nigra).

Although the majority of flora found on a verge stems from the initial planting and seeding, this state is unlikely to last for long, before being penetrated by other plant species. The rate and extent of this invasion is dependant on the nature of the terrain, the number of dormant seeds present in the topsoil, the introduction of seeds from elsewhere (wind blown seeds from surrounding areas or seeds carried on passing traffic and / or birds) and management methods.

Verge Plant Diversity

The species diversity of verges by comparison with hedgerow diversity, is brought about with considerable differences. Whereas hedgerows age and species composition have a strong correlation, other factors such as maintenance, geology (e.g. chalk or peat subsoils) and climate have very little bearing.

With verges, however, this is very different, the most dramatic in terms of effect are obviously mowing and herbicides, both of which cause drastic changes of the flora in question. Other factors concerning the diversity of the plants and animals consist mainly of:

Aspect; South facing verges tend to have a greater diversity than North facing ones due to the better illumination levels as well as a longer growing season; ie. Quicker to start in spring and still growing after the north aspect verge is in shade during the autumn.
Pollution; such as run off and splashing or over spreading of grit and salt during the winter months. Although some plants are actually adapted to this environment and flourish very happily with reduced competition from other species. Another pollutant is fuel and oil spillage as well as litter.
Disturbance; of passing traffic aiding seed dispersal, crushing by vehicle tyres parking on verges and trampling by pedestrians walking along the verge.
Acid/alkaline balance; when the soils are alkaline (ie. On chalkstone areas from leaching from the chalkstone road base.) a particularly efficient coloniser is the greater plantain (Plantago major).

Estimating Plant Diversity in Verges

About the simplest method of estimating the diversity of the verge flora is to create a transect across the verge at right angles to the road, 1 metre apart, laid parallel. This can be done with lengths of timber or pieces of string which are then bisected at regular intervals along the transact forming quadrats usually 1m² , and the numbers of each species noted.

A slightly more comprehensive method is to estimate the relationship between the total number of plants and the number of individuals per species that fill a quadrat.

Various correlation equations are available but probably the simplest is this one that follows that will discover the diversity index. One of the biggest advantages with this system is that when compared to the previous one, the individual species do not need to be identified. Merely being able to distinguish one from another is sufficient. So thus said the survey can be carried out by less skilled botanists which can be a time saver once the original species listing has been completed. These can then be identified by an index of letters or numbers.

For example:
I = greater plantain 10 plants
II = buttercup 9 plants

III = dandelion 8 plants
IV = stinging nettle 7 plants
V = red clover 6 plants
VI = daisy 5 plants

Total = 45

D= N (N-1) D= Diversity index

å n(n-1) N= total # of individual plants

n= # of individuals/species

å = summation

So using the example in the equation would be worked out thus;

D=45 x (45-1) = 1980

å = 10x(10-1)+9x(9-1)+8x(8-1)+7x(7-1)+6x(6-1)+5x(5-1) = 310

1980 divided by 310 = 6.387 or 6.4

This diversity index can provide us with a means of monitoring management strategies or pollution problems. If we found that the index had dropped from 6.4 to 3.2 for instance, then it would be possible to flag a problem area in need of attention.

It is likely, however to be a great deal of help in measuring the rate of colonisation due to the drastic management methods of verges and from traffic and salt pollution.

Pollution and its effects

Pollution of verges is as a direct consequence of the traffic passing. The effect of this pollution are increased directly by the increase in traffic volume and not surprisingly the volume of traffic found on motorways lays down higher amounts of pollutants than any other roadway types in Britain.

The main pollutants and their effects are as follows;

Litter
The worst and most obvious pollutant of verges is litter thrown from passing vehicles. Bottles can smash and cause injury to pedestrians and larger mammals or even in very hot dry spells be the cause of fires, acting like a magnifying glass. If they do not break then they can become a death trap to small mammals and several flightless invertebrates who may find a way into the bottle but cannot then climb up the slippery sides and consequently starve or cook in hot weather.

Plastics are not quite a lethal as glass but because of their polymer qualities take very long times to degrade and are very unsightly. In very large amounts they shade the ground and prevent plants growing, acting a plastic weed guard used by gardeners.

Food waste will break down quickly and may even be taken as food by mammals, invertebrates and if it rots even by the plants themselves, although very large amounts in local areas may cause over fertile soils with its own problems on the flora growing there.

Carbon, nitrogen and sulphur oxides
Emitted by vehicle exhausts along with tiny fractions of unburnt fuel. Oxides of nitrogen are atmospheric pollutants and usually consist of a mixture of NO (nitric oxide) and NO2 (nitrogen dioxide) in varying amounts.

The presence of nitrogen oxides in the air is beneficial to the biochemistry of some the plants by way of increasing the synthesis of amino acids, and thus increasing the growth of the plant. However, there are some species to which this effect is toxic.

Roadside pollution by oxides of nitrogen are usually accompanied by SO2 (sulphur dioxide) which contributes a large constituent of acid rain but studies are on going as to its effect on plants.

Salt

Sodium is known to play an important part in flocculation of clay (the action of the small clay particles clumping together). Therefore improving the soil texture and crumb structure as well as aeration and drainage. However an excess of sodium causes the flocculation process to break down and once again the clay particles form a dense compacted and impermeable layer

During the course of a winter an estimated 1.5 million tonnes of salt are applied to Britain’s’ roads in order to stop icing. The rate of application varies according to the number of frosts as well as the severity. An average frost requires about 5.4 tonnes per kilometre lane, were as in a hard frost it amount may double.

One of the biggest problems with salt is the residual effect that can remain in the soil for a considerable time. It tends to reach highest concentrations by march, gradually reducing over the summer and then rising again through the winter, often starting again in November onwards.

Another variable in the concentration levels of salt is hot dry periods when the evaporation effects are greater then the rainfall leaching rate.

The verges tend to be covered up to 2 metres from the road edge, either directly from the gritter or flicked up from car tyres and obviously central reservations that are hit from both sides will tend to have much larger concentrations then the road verges. These high sodium areas are where the plants are most effected and the damage causes bare patches which look unsightly but are usually filled again during the summer by re-colonisation of other plants. Trees and shrubs, however, which are growing in this zone tend not to fare as well and the damage to them is more obvious. Experiments were done on 11 common species found at road sides and each plant was a, sprayed with a saline solution on the stems and leaves and b, salt was incorporated into the soil. the tolerance for sprayed leaves and stem ranged from 91% to 100% concentrations. The soil tests range was much greater with sea buckthorn (Hippophae rhamnoides) and grey willow (Salix cinerea) both having a 100% tolerance, down to goat willow (Salix caprea) having 62% tolerance and hawthorn (Crataegus monogyna) as low as 61% saturation tolerance.

As seen above the saline concentrations can have a negative effect on some of the flora species found on a verge. It can however, be beneficial to others that have evolved in a coastal environment and have a high salt tolerance. These plants are known as Halophytes and include plants such as sea plantain (Plantago maritima), sea spurrey (Spergularia marina), sand spurrey (S. rubra), sea aster (Aster tripolium) and relfexed meadow grass (Puccinellia distans) plus several others.

The reflexed meadow grass has become one of the most widespread and successful halophyte colonisers which favours disturbed, compacted and poorly drained soils and produces large numbers of small light seeds that are well adapted for dispersal in the slipstream of passing vehicles and attaching to the bare patches of soil created by the deaths of other non-halophytes.

Lead compounds

Lead compounds are no longer such a threat as they once were due to the vast majority of cars now running on lead-free petrol. Not so long ago, however, most petrol engines had lead added to the fuel to help lubricate and prevent engine ware. The lead that was emitted by car exhausts is a complex mixture of compounds, but the principle once that is found in soils and vegetation bordering roads is PbSO4 (lead sulphate).

By 1990 some 9,000 tonnes of lead was added to fuel as tetra-alkyl lead fluid. Of this about 60-70% is emitted from vehicle exhausts into the atmosphere. A large amount of this is deposited in an area of about 50 metres on either side of the road way with about 10-20% extending further out to approximately 100 metres. Rural road concentrations can be quite low (<10ug/g dry weight) but at 30 metres on a busy motorway it can rise to 10 times this amount. This statistic has a variability in that about 4 times as much lead compound is deposited on grass than bare soil and rough hairy surfaces of plants will accumulate lead at 10 times greater rate than smooth leaved plants. So a verge vegetation of hairy plants will absorb a higher amount of lead compound and reduce its spread pattern. Trees also feature in this in that the soil under a tree has approximately 2 times the amount of grass in the open.

Lead pollution and its effects on plants

Extensive research has been done and shows us that the effects of lead on plants, unsurprisingly, is generally a negative one. High concentrations in the soil tend to produce a more adverse reaction than the same deposits on the leaves, showing that the intake of lead is predominantly through the root system rather than through the leaves. Further more when plants were tested with high pollution rates they showed a high concentration of lead in the roots but very little transfer to the shoots of the plant. other than inhibit growth of roots and shoots, additional effects may be an imbalance of mineral nutrition and a reduction in the rates of photosynthesis and respiration.

Like the halophytes certain species appear to be far more resistant to lead than others. One such of these species is ribwort plantain (P. lanceolata).