Environmental requirements of trees
In cities, we find different species and botanical varieties of trees, from different backgrounds, differing in climatic factors, soil and biotic. The ecological requirements of these plants and their properties are therefore significantly different. In addition, hybrids and cultivars are planted, which in turn may also differ in requirements and properties from the starting species.
Considering the suitability of a tree for given ecological conditions, its requirements and properties should be comprehensively considered. Sometimes only one property can make a species unsuitable. For example, elms, due to the low light requirements, warm, air and soil humidity, soil fertility, their ability to grow roots of compacted soil and resistance to air pollution could be the basic component of the greenery of our cities (as it used to be), however, the widespread occurrence of the so-called. Dutch disease, which completely destroys the elms, it made, that the usefulness of these trees had fallen to almost nil.
Individual environmental factors can also replace and complement each other. In the table 2 the requirements and biological properties of the more important tree species are given. These information are necessary to perform basic beauty treatments. Due to the bad ecological conditions in the city, the table contains the minimum requirements. Complementary comments on the factors and properties listed in the table are provided below.
Light. Among woody plants we distinguish species of the so-called. shade bearing, with low light requirements, and light-requiring species. Shadow trees have crowns with dense foliage and are useful for covering shady areas, etc.. Light-hungry trees, for example, larches, poplar, pine trees, birch trees, olchy, they are distinguished by crowns with transparent foliage, the leaves are mainly distributed on the outer parts of the crown. Even only the side shading of such crowns distorts them and accelerates the dying off of the branches.
Temperature. The thermal requirements are largely related to the light requirements. However, the table mainly considers the low temperature resistance. The assessment mainly concerns the behavior in the so-called frost hollows and resistance to spring frosts, especially dangerous for species that begin their vegetation early.
Air humidity. Insufficient air humidity in the urban environment is one of the important factors limiting the cultivation of woody plants in cities. Trees with low requirements in terms of air humidity, like for example. Acer and gundo, some species of the genus Populus, Robinia, they withstand urban conditions much better. Ocean climate trees, e.g.. fir or beech, set higher demands than trees of a continental climate (pine, birch). Trees showing strong cuticle transpiration, e.g.. Abies alba, A. grandis, Pseudotsuga taxifolia, they are unable to restrict their transpiration by closing their stomata. Therefore, these species have high requirements as to air humidity. For example, low air humidity is a limiting factor for the presence of pine trees in the city and in the lowlands, despite the fact that this species is relatively insensitive to air pollutants. High air and soil humidity at the upper edge of the forest - in natural limb forests - compensates for some unfavorable ecological conditions, e.g.. heat shortage.
Air purity. There is a wide variation in the sensitivity to air pollution among trees. In urban conditions, it is the most common air pollutants, the so-called background, we include sulfur oxides, nitrogen oxides, hydrocarbons and lead compounds (the last two come mainly from internal combustion engines). also, depending on the type of industrial plants in a given city or district, there are also numerous other phytotoxic pollutants. The sensitivity of trees also depends on the overall health of the tree. It is very possible, that in industrial areas, the greater viability of black pine than that of Scots pine is due to this, that the former is less often infected by diseases and pests. In addition, the action of individual pollutants is enhanced, when it is combined with the simultaneous presence of other harmful factors. The sensitivity of plants to air pollution is a relative phenomenon - it depends on the type of pollution, on the concentration of the active substances, the development period of the plant and other environmental conditions. It is essential, that worse environmental conditions increase the sensitivity of plants, and good - they reduce.
Type of soil. Here we mainly take into account the requirements as to the structure and mechanical composition of the soil. The soil in the city is often a substrate that differs in physical and chemical properties from natural soils. Instead of soil, you can also often find rubble and rubble in cities. These are the so-called soil destructs. In downtown Warsaw, most woody plants grow on such anthropogenic formations. Their reaction is strongly alkaline (pH 8), and the optimum pH for most trees ranges from 5.5 to 7. The mechanical composition of the soil also influences other tree properties, e.g.. the ability to root, and the development of mycorrhiza necessary for some species.
Soil moisture. Insufficient urban soil moisture is a common phenomenon. This is mainly due to the surface drainage of rainwater. In urban conditions, trees often require water to be replenished by watering. Many trees are not suitable for cities because of too high humidity requirements (e.g.. most species of spruce, limb). In airy soil, due to the condensation of water vapor from the atmospheric air, a certain increase in humidity occurs. Amount of water, which soil gets in this way, can be significant, as it comes to approx 50 mm per year. This water is obtained in the summer, so during the period of the greatest water shortage in the soil. Unfortunately, this process in urban conditions is strongly limited.
The content of minerals. The knowledge of the requirements of trees in terms of soil abundance is particularly important for employees of green areas due to the necessity of constant supplementation in parks and green areas of minerals removed with the mulch. It must be said, that most of these ingredients are found in the leaves.
Air content in soil. This factor is usually underestimated, however important, and in urban conditions it is very unfavorable. As a result of strengthening the pavement (plates, asphalt) trampling, location, vanity, the possibilities of soil aeration are limited. Gases accumulate in concentrations that are harmful to the roots. For example, CO2 accumulates, which already in concentration 10% is harmful, and in a concentration of 30-50% it is highly toxic to the roots. Under certain conditions, methane can also accumulate, for example, due to leaks in the gas mains or anaerobic putrid processes (with the access of oxygen, methane is decomposed by bacteria into CO2 and H2O by the activity of bacteria). Hydrogen sulfide and ammonia are also produced during anaerobic putrefactive processes. Ammonia can also be released as a result of nitrogen fertilization in the form of urea and ammonium, just under the conditions existing under the impermeable strengthened surfaces. Plant roots emit mainly carbonic acid, and in the case of oxygen deficiency - other toxic acids like: formic, oxalic, acetic. Therefore, the absorption of many nutrients decreases in the case of oxygen deficiency, for example calcium, manganese, iron, which is directly caused by a change in the soil pH. For normal development, tree roots need a higher oxygen content in the soil air; it should be approx 10%.
Table. Habitat requirements and biological properties of some tree species.