Influence of vegetation on pollutant pathways

Reducing erosion by wind and water

Vegetation is very effective at reducing erosion by wind and water. Under trees, soil is retained and protected by the presence of roots and foliage whilst the input of organic matter through leaf senescence increases the binding capacity and therefore the stability of soil. The risk of particulate migration during site cultivation can be minimised by phased planting, establishing a nursery crop such as mixed grasses, and/or paying careful consideration to the timing of cultivation operations.

Trapping and absorbing airborne pollutant particles

Trees are also effective at trapping and absorbing many airborne pollutant particles with significant heavy metal content, so limiting the spread of contamination. For example, a 150 to 200 m wide strip of broadleaved woodland was shown to effectively immobilise over 450 kg per annum of airborne dust per hectare of woodland.

• Conifers are more effective scavengers of metal particulates than broadleaves.
• Broadleaves are often used in preference to conifers in areas of high pollution because of the annual replacement of foliage which decreases their accumulated load of toxic particles.

The effectiveness of deciduous woodland at capturing airborne particulates is reduced after leaf fall, but the efficiency of the filtering system can be increased by using a mixture of evergreen and deciduous species.

Retaining sediment material

Woodland has another important pollutant retention function, as it is effective at retaining sediment material (including sorbed organic, heavy metal and nutrient contaminants). Several studies have demonstrated that woodland is more effective at retaining pollutants bound to sediments than grassland – although the presence of the latter as either an understorey or adjacent strip can enhance further removal.

This research was funded by the Forestry Commission and Forest Research, in collaboration with the University of Reading.