Trees in urban areas

Trees in urban areas are very important factors of microclimate creation. They have a profound effects on surrounding environment because they affect air moisture level, temperature, solar radiation and air circulation – all above mentioned factors have a very important influence on quality of human life in urban areas.

In 21st century almost half of human population lives in urban areas. Big cities are not self-sufficient because they need high input of food and energy and they not give a good conditions for living. Almost always green areas are planned and managed when big cities are in the expansion phase. Human being does not bear isolation from nature therefore green areas in cities play a very important role in human wellness. Esthetic values of green areas and their role in erosion protection are not less important. Under conditions of intensive urbanization, when surface of green areas is getting lower because of high demand for new road system because of increasing traffic the role of proper management of green area is growing. Planning new green areas including new tree planting is very important because trees can protect cities against flood and ground sliding. It is worth to mention that investment in municipal infrastructure almost always leads to degradation of naturally occurred trees.

Environemntal conditions vs planting and replanting trees in urban areas

Urban areas have organized structure of buildings to deliver of service to human population. It is quite common that green areas have a marginal value during cities expansion when geotechnical works on a large scale have been done and natural green cover is totally destroyed and new plants are introduced after completion of investment.

Natural habitats for all trees are forest areas which were created by mutual influence of climate, soil and plants. Therefore typical forest habitats have been created in local and regional scales. In such habitats specific associations of soil microorganism have been developed.

Their function is to maintain dynamic equilibrium between organic and inorganic components of the soil environment. Where there is a high biodiversity of soil microorganisms higher plants have a very good conditions to grow and develop.

Particular role plays microorganism living in rhizosphere creating consortia which are able to maintain good living conditions or plant and themselves. In these consortia mycorrhizal fungi play a central role. Mycorrhizas live in very close symbiosis with host root system in many cases determining survival of many higher plant species.

Habitats which are found by urban planners for trees in urban areas are very frequently just hostile for new plants. Investing activities in urban areas made that natural conditions are irreversibly modified what create a very harsh conditions for planted tree seedlings.

Benefits from tree planting – planning

City councils are obliged by law regulations to create and maintaining municipal city areas playing recreational and esthetic functions. There are several institutions which were developed to plan and fulfill all activities connected with plan and manage green areas.

Anyway it happens that management practices lead to die-out of trees because they were performed not to help but to get rid of unwanted tree without time consuming and costly law procedures what unfortunately is typical during road investment.

Selection of trees species

The general rule for selecting of trees and shrubs for planting at urban areas is the best possible adaption them to specific habitats and to existing green areas to achieve the harmony which is visible as a self-sustainable system where maintenance works are minimized.

Also introduction foreign species should be performed as rare as possible always home species are better adapted to local environment.

When planting trees species which produce a lot of essential oils bearing in mind that conifers oil have effects similar to tranquillizers whereas broad leaved trees produces oils acting as stimulants. Also many specialists recommend planting trees which produce negative ionisation of air which has beneficial influence on humans for example – Scot pines, birches and lime trees.

Special attention should be given to avoid plating at recreation areas of trees and shrubs which whole plants or fruit are poisonous. There are most common poisonous plants which are sometimes planted in urban areas: yew, alder buckthorn, common snowberry, golden chain, black elder and mezereon.

Re-planting technology natural and technical conditions

We will try to give some pieces of advice how to solve this sometimes difficult problem. If we consider the fact that upper layer where almost all biological activity of the soil is concentrated is totally destroyed we will know the scale of problems which plant is faced. Also it is known that destroying of upper layer of the soil inevitably leads to reduction of the amount of available nutrients. This effect is multiplying when leaves and cut grass are removed because growing media for microbes is removed.
Problems connected to unfavourable environmental conditions is usually solved by application of mineral fertilizers and organic manures, appropriate tillage practices and selection of stress tolerant trees and shrubs species. In a longer time horizons those metods are imperfect. The best solution is to give to the plant – within rhizosphere – proper microbial communities which in long time can positively modify soil conditions.
First of all focus should be concentrated on mycorrhizal fungi which are able to create close symbiosis with host i.e. higher plant (tree) roots systems and on associated helper bacteria consortia which enhance plant growth and activity of mycorrhizas themselves. Such a complicated and complex consortia are able to increase of plant health status sometimes even in extreme harsh environment.
Why mycorrhizas and other microflora living in rhizospere are so important for tree and particularly for re-planted trees? Answer to this key question is rather long becaue those microbes play a different roles:
· improve water supply (because of network created by the system of mycelium absorption surface can increase even several hundred times);
· enable better access to nitrogen pool in the soil – also from organic compounds which are otherwise unavailable for plants;
· improve supply of roots in phosphorus, iron and other micronutrients when they occur even in forms unavailable for plants;
· are able to deliver passive protection (competition for habitats, induction of resistance) and active (antagonism) against pathogenic for root system fungi;
· because of higher photosynthesis rate they stimulate above-ground plant growth;
· because of phytohormonal activity increase root system growth;
· increase winter and frost resistance of plants;
· improve plant general appearance which is not caused by over-fertilization;
· improve growing conditions irrespectively to soil acidity;
· increase resistance to salinity stress;
· participate in creation of good soil texture;
· play a lot of functions which mechanisms are not fully understand – detoxication and immoblisation of heavy metals and permanent organic pollutants including PAHs et.
Even if we can observe only some of mentioned benefits of mycorrrhizas’ activity we can conclude that this important in nature phenomenon as mycorrhiza should be utilized in any operations connected with tree planting and replanting especially out of forest soils. Particularly difficult a task is re-planting of old trees under urban conditions. Nowadays it is quite possible and many precious trees in urban areas can be saved.
Performing this operations important factor is fulfilling some basic rules which can increase certainty that re-planted tree will continue growth under new environmental conditions.
· Time of planting
Re-planting has to be performed from the beginning of period of absolute dormancy to the end of relative dormancy. In deciduous trees it is a period between start of foliage color change in the autumn to bud breaking phase in the spring for conifers from the end of August (only Pinaceae) after the phase total lignification of annual growth to the phase of bud development in the spring (end of April);
Size of tree
· Currently because of new technologies applied virtually all trees can be re-planted. The biggest tree re-planted by Polish company has trunk of diameter of 300 cm.
· Methods and biological aspects of re-planting
· Trees which are planned to re-planting (particularly big ones) should be prepared to this operation at least two years earlier. During this period root system in a of soil which will be replanted with roots should be as dense as it is possible. To achieve such an effects roots should be gradually cut. At this stage specific mycorrhizal inoculation can be also performed.
· Other remarks
All details of re-planting technology have been published in many original papers but some remarks can be given here. After re-planting in a new place no fertilizers should be applied because of high sensibility of root system. The best time is to excavate tree when soil is frozen.
Trees which are planted in harsh conditions (at urban areas or close to highways) suffer from shock which is caused by several physiological stress because of rapid change of soil and other environmental conditions. The older re-planted tree is the longer stress lasts. It can be recommended that home species should be used instead of tree of foreign origin. Stress can be released by biological methods if to rhizosphere of re-planted trees some beneficial soil microbes will be introduced. It should be reminded that majority of trees planted in urban areas originates from nurseries where mycorrhization is not performed. Also it means that replanted trees usually are not inoculated by mycorrhizas and what is typical in trees for re-planting that fine roots which frequently live in symbiosis with mycorrhizas are lost that during preparing trees for operation of re-planting. Therefore artificial mycorrhization has a very important role to play.

Summing up: perspectives of mycorrhization and suitability of new technologies

Nowadays more frequent is an opinion based on results of extensive studies that trees which do not have active and living mycorrhizas are subjected to bigger stress after re-planting than trees with mycorrhizas. It has to be mentioned here when is the best time of applying artificial mycorrhizas:
· trees from nurseries should have mycorrhizas introduced during production, the best time for inoculation is start of seedlings growth;
· if trees from nursery were not mycorrhized inoculation can be done during planting on permanent place;
· trees planted to new place can posses mycorhizas but usually they do not. It is caused by the fact that before re-planting 80% of root system is lost.
Summing up this problem it can be underlined that mycorrhization is very important for trees re-planting. This is connected with different functions of mycorrhizas. The sum of this functions is better adaption of tree to harsh environmental conditions and passive and active protection of root system by mycorrhizas. To release the re-planting stress biological support in the form of mycorrhizas should be supplied by the grower.
The final effect is the higher survival rate for tree planting under the harsh conditions as urban areas, shelter of high traffic roads or other habitats not favored plants’ growth.
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2. Pokorski J., Siwiec A.: [Management of green areas] Kształtowanie terenów zieleni. Warszawa 1998.
3. Niemirski W.: [Management of green areas] Kształtowanie terenów zieleni. Wyd. Arkady, Warszawa 1973.
4. Siewniak M.: [Trees in the city centers. Gardens and lawns] Drzewa w centrum miasta. Ogrody, ogródki, zieleńce – nr 1,1999; s. 76-83.
5. Szczepanowska B.H.: [Trees in the city] Drzewa w mieście. Wyd. Hortpress, Warszawa 2001.