Preventing negative influences from impermeable pavements
Trees growing in housing estates and cities use less and less the normal soil habitat. The soil is very often covered with different materials or so strongly compacted, that rainwater and air access becomes difficult, and often impossible. The worst soil conditions exist in the streets and squares, completely paved, where only small holes are left for trees. The size and quality of the soil surface is crucial for a growing tree to grow and develop. The soil surface surrounding the tree trunk should be loose, porous structure, allowing easy soaking of water and free access of air to the roots.
Drawing. The influence of the type of surface on the tree: a - for normal growth, trees require loose and permeable soil on an area of at least the size of the crown projection; b - the root system of the tree surrounded by an impermeable surface is deprived of rainwater and free air access, with the result that most of the roots are kept close to the hole; c - semi-permeable or permeable surface can ensure the use of rainwater and very good soil aeration.
The size of such an area should be at least the projection area of the crown (Lynx. a). Such large areas can only be provided for trees growing in parks and green areas. Trees growing in the streets, usually on pavements covered with concrete slabs, they most often use the surface of open soil in the form of the so-called. bowls around the trunk. The area of this bowl usually does not exceed 1-2 m². The roots of a tree planted in such a place remain close to the open surface for many years (Lynx. b), where the water enters and where the air permeates. However, these places are exposed to trampling, or siltation with impurities flowing from the pavement surface and they become impermeable after some time. Some improvement in keeping this small area in proper condition is achieved by frequent loosening, adding loosening materials etc.. However, digging and loosening the soil in the bowls is labor-intensive and cannot be carried out frequently. Therefore, such surfaces are usually protected with various types of gratings, based on the reinforced edges of the opening or concrete slabs with a special slotted structure enabling filling with fine aggregate or loose soil and sowing with grass. A good protection ensuring high permeability of the surface are cobblestones or small cobblestones laid on a 15-20 cm gravel bed. The gaps between the stones should be covered with coarse sand. The devices securing the soil next to the tree should not hinder the movement of passers-by. So they cannot, for example,. show unevenness. This is especially important on sidewalks with heavy pedestrian traffic. The bowls around the trees are usually built this way, that their edges are slightly lower than the surrounding pavement. It allows, provided that the slopes are shaped appropriately, for rainwater to drain into the bowl. However, such solutions are not recommended there, where salt is used to dissolve snow, because the salty water that runs off the pavements gets into the soil and can destroy the tree.
An unfavorable factor, and often the parking lots built there are a threat to trees growing on large lawn areas. The construction of the car park includes:. in. by creating a surface adapted to the load of vehicles. Most often it is done this way, that a heavy road curb is set up in the vicinity of the tree. Usually, encountered roots hinder this, so they get cut. The soil is removed to a depth of 30-40 cm, and in its place a heavy one is laid, impermeable surface, usually clinker on a concrete foundation. Due to the lack of oxygen, the roots beneath it die. Leaving a narrow strip of the lawn in such cases does not protect the water supply and oxygen access. Maintaining a lawn in such conditions is extremely difficult (Lynx. a).
Drawing. Rules for the construction of parking lots: a - placing heavy curbs in close proximity to trees, building impermeable surfaces and leaving narrow strips of the lawn, are the biggest mistakes in the construction of parking lots; b - making the surface in the vicinity of the existing tree of openwork plates on a gravel base is beneficial for the tree root system, because it facilitates the access of water and air.
A much better solution for trees and not worse for cars is to make a permeable surface in the vicinity of trees made of openwork plates. Paving made of sorted stone or cubes, although providing sufficient permeability, cannot be recommended, due to the high laboriousness of execution. Openwork slabs are increasingly used in the construction of parking surfaces. Laying them does not require the removal of a thick layer of soil, often enlarged to give the area the required slope. Such plates, laid on a coarse sand or gravel base, make up the surface, which can absorb large amounts of rainwater and does not require slope shaping. The high permeability of this type of surface results in a permanent improvement of the required properties of the soil substrate. Access to the tree trunk can be limited without building a curb, only by using very practical special triangular asphalt concrete elements, which simply lay on the surface.
When arranging all surfaces in the vicinity of trees, avoid placing various rigid materials close to the trunk, like for example. heavy curbs, concrete surfaces, etc.. Such elements not only expose tree roots to damage during construction, but they themselves may be damaged by growing and thickening roots in the years to come.
