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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 pollutions that 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.
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).
