The definition of afforestation is included in the Nature Conservation Act as: “individual trees, shrubs or clusters of trees or shrubs which are not a forest (…) or a plantation, together with the land on which they occur and the other components of the vegetation cover of that land”. They may take the form of smaller or larger groups of trees, belts, avenues or individual trees. Mid-field trees and shrubs are trees and shrubs growing outside the forest, tall vegetation among fields, meadows, buildings, along roads, baulks, coastlines of reservoirs and watercourses.
Afforestation is a very important element in the protection of the agricultural environment. Numerous studies have shown that by appropriately afforesting the landscape, it is possible, among other things, to increase the water retention of the environment, improve water quality, reduce water and wind erosion of the soil, strengthen environmental resistance against crop pests and increase the tourist attractiveness of the area.
The role of trees in improving the water retention of the environment is through:
- weakening of wind power and reduction of evaporation of water from adjacent farmland;
- increasing the amount of water in the soil by slowing down surface runoff;
- forest litter has a high rainwater retention capacity;
- longer retention of snow cover. Tree cover slows the rate of spring snowmelt by about 5%, reducing water erosion and reducing the risk of snowmelt flooding.
The presence of wooded areas, especially around water bodies and watercourses, has a significant impact on their purity and water quality. Strips of trees and shrubs are able to intercept and neutralise much of the nitrates and phosphates that enter with groundwater and surface runoff into water bodies and watercourses.
The most important role, however, is played by mid-field woodlots and shrubbery in inhibiting wind power and limiting wind erosion. Alongside water erosion, wind erosion is one of the main types of erosion (the others being of lesser importance: snow erosion, tillage erosion and so-called mass movements). It is the movement of soil material over the soil surface under the influence of the wind. The detrimental effects of wind erosion are the blowing away of the topsoil, the mechanical destruction of plants and the exposure of their root system, and the pollution of water and air by soil particles. As a result of erosion, the soil loses not only the finest mineral particles, but also what is most valuable in soil – humus. The soil particles that are blown away carry with them not only mineral and organic components, but also residues of plant protection products, which can contaminate the air, land or water on which they fall.
The effects of erosion – such as a reduction in the rooting zone, loss of organic matter and nutrients and a deterioration in soil structure – can lead to a decline in agricultural production, as depriving the soil of a 1cm thick humus layer means a reduction in yield of 2 to 4%.
The soils most susceptible to wind erosion are sandy soils, dry organic soils and organic-mineral soils. The topsoil is most vulnerable to wind erosion, down to a depth of 5cm. It is estimated that about 28% of Poland’s area is at risk of wind erosion, which, with a significant proportion of poor soils most susceptible to this type of erosion, poses a significant risk of arable land degradation.
Depending on the height and width of the tree and shrub belt and the tree and shrub species present, wind speeds can be reduced by up to 30 to 50 per cent in open, flat areas. This prevents, or significantly reduces, the blowing away of soil material, but also crop lodging, fruit and leaf drop, and provides shelter from the wind for livestock. It is also not insignificant that the numerous trees and shrubs provide a benefit for many pollinator species and a habitat for other organisms, such as insectivorous birds, that are beneficial to agricultural crops.