What are the ecological functions of garden tree species?

In the application of unique ecological functions of garden tree species, the ecological and environmental benefits such as purifying air, reducing pollution and regulating climate are the most basic and important to maintain ecological balance and improve human living conditions. Therefore, as a symbol of human conquest of nature and high civilization, a city must pay attention to the improvement of its ecosystem from the early stage of construction and try to avoid the destruction of natural ecology by urban development.

With its unique way of existence, the city has greatly changed the natural environment and ecosystem of itself and its surrounding areas. The construction, reconstruction or expansion of a city will change the original natural landscape without exception. Whether it is plains, hills, lakes or beaches; Whether it is hillside, valley, mang forest or plateau; Will lose the tranquility and harmony of the past, replaced by noise and struggle. Rows of buildings, criss-crossing streets, dotted squares and communities have replaced towering forests and vast vegetation; Arrogant smoke and dust storms disturbed the blue sky. The disappearance of natural vegetation, the imbalance of natural ecology and the deterioration of natural environment are all negative effects brought about by improper development of modern cities. The primary problem of modern urban development planning is to ensure the existence of enough green plant communities, because the regulation of natural vegetation on the ecosystem is irreplaceable by human control. Giving full play to the ecological functions of urban "heart" and "ecological air conditioning" with garden tree species as the main body is helpful to maintain the ecological balance in the process of urban operation, improve the environmental conditions for urban residents to work, study and live, and build a modern development trend of garden-style cities.

(A) fresh air, adjust and improve the space environment

The normal content of oxygen in the atmosphere is 2 1%, and that of carbon dioxide is 0.03%. However, due to the concentration of population in modern cities, people breathe in oxygen and exhale carbon dioxide, and a lot of oxygen and carbon dioxide are consumed when various production and living fuels are burned. So at present, many cities in the world have broken the natural balance of the atmosphere. The content of oxygen is less than 20%, and the content of carbon dioxide can reach 0.5% ~ 0.07%, which has threatened human health. In particular, although carbon dioxide is a non-toxic gas, when the concentration in the air reaches 0.05%, people's breathing is uncomfortable. In addition, the global "greenhouse effect" caused by the increase of atmospheric carbon dioxide content has also brought increasingly serious disasters to human living environment.

The special function of green plants to absorb carbon dioxide and release oxygen through photosynthesis is several times to dozens of times that of lawn construction in the selection and application of garden tree species. Usually 1 ha of broad-leaved forest can absorb 1000 kg of carbon dioxide and release 750 kg of oxygen every day during the growing season. If an adult needs to breathe in 0.75 kg of oxygen and exhale 0.9 kg of carbon dioxide every day, it needs per capita 10 ~ 15 square meters of forest area or 25 ~ 30 square meters of grassland area. If the oxygen consumption and carbon dioxide emissions of various fuels during urban operation are added, the per capita green space area will be increased. For example, in Canberra, the capital of Australia, the green area accounts for 58% of the total urban area, and the per capita green area is 70 square meters. The average urban green area proposed by the American government is 40 square meters. As early as 1969, the United Nations published a report on urban green space planning, proposing that the per capita green space in cities should reach 60 square meters and the per capita green space quota in residential areas should be 28 square meters.

The main photosynthetic organ of garden tree species is leaves, so tree species with lush foliage and large leaf surface area can obviously adjust the balance of oxygen and carbon dioxide in the atmosphere, especially the selection and application of evergreen broad-leaved trees. Not only should trees and shrubs be widely planted on both sides of streets, residential quarters, factories, mines, institutions and other densely populated areas to increase the area of photosynthesis, but also we should vigorously develop suburban parks, develop suburban scenic spots, build urban forest parks, create green spaces to the maximum extent, increase the oxygen content in the air, and improve the air quality on which human beings depend. For example, Japan stipulates that a city with a population of 5 million must be equipped with a suburban scenic spot with an area of 1 000 hectares, which can accommodate 654.38+million tourists every day. According to the quantitative index of absorbing carbon dioxide per square meter, the ability of trees to purify air is divided into three categories. The first index value is higher than 2000g: persimmon, Robinia pseudoacacia, Albizia Albizia, Paulownia, Luan tree, Prunus mume, Prunus armeniaca, Xifu Begonia, Wei Zi, Fenghua Rose, Peach Blossom, Bauhinia and Lingxiao. The second index value is 1000 ~ 2000g: mulberry, Ailanthus altissima, Sophora japonica, Torch Tree, Cotinus coggygria, Fraxinus mandshurica, Populus tomentosa, Acer truncatum, Walnut, Crataegus pinnatifida, Pinus bungeana, Hibiscus, Ligustrum lucidum, Lilac, Rosa roxburghii, Lonicera japonica, Forsythia suspensa, precious wood, etc. The third category, the index value is lower than 1000g: Platanus acerifolia, Ginkgo biloba, Magnolia grandiflora, Liriodendron chinense, cherry blossom, brocade flower, rose, Tang Di, Chimonanthus praecox and chicken hemp.

In recent years, the "lawn fever" popular in China has been misunderstood in many areas, regardless of the level of urban construction, especially whether there are forest vegetation conditions in the suburbs. Blindly cutting down trees and planting grass will make the atmospheric environment that can't make ends meet more unbalanced, and its negative effect will far outweigh the vanity brought by superficial beauty. At present, from the perspective of landscape ecology, although adaptability has been considered in plant selection and configuration, the unreasonable phenomenon of community and community structure is still obvious, the most obvious example is grassland scale. Objectively speaking, from the perspective of spatial combination (aesthetics) and citizen demand (society), grassland and sparse grassland are indispensable. However, from the ecological point of view, grassland is a fragile ecosystem of a single species, and its maintenance needs corresponding management and timely renewal. This kind of investment is necessary, but it should be moderate. It is believed that only grassland is beautiful, only grassland has a sense of modernity, and the more grassland has a stronger sense of the times, the prejudice of blindly pursuing fashion regardless of actual needs exists in some planners, environmental designers and even a few landscape designers to varying degrees. The prejudice of individual decision makers naturally contributes to the spread of this phenomenon, which is based on foreign countries and feels great. The construction of coastal green space in a city, not to mention cutting down a large number of windbreaks, endangers the ratio of production to investment, biodiversity, unit leaf area coefficient, green visibility and so on from the ecological point of view. As far as "seeing the sea near the sea" and "using the sea near the sea" are concerned, "seeing the sea" is of course the pursuit of visual effects. As oceans and beaches, citizens including foreign tourists are the most direct users, that is, sightseeing, leisure, swimming and other activities. How to properly solve the problem of seeing and using still embodies the unity of ecology and landscape, and the simple beauty of landscape without ecological foundation is difficult to last. At present, the measures of returning farmland to grassland are positive measures after serious reflection. However, the "citizen square" which is heating up in China is mainly composed of architectural sketches and road pavement, which is more beautiful and less green, which fundamentally violates the purpose of building an urban ecological green space system and is worse than the above misunderstanding, and must be seriously corrected.

Garden tree species have the functions of absorbing heat, cooling and evaporating water, and have good functions of regulating and improving air temperature and humidity. During the growth of trees, 99.8% of the water absorbed from the roots will be evaporated, leaving only 0.2% for photosynthesis, so trees can effectively improve the relative humidity over the woodland. In spring, trees begin to grow, absorbing a lot of water from the soil and then releasing it into the air. At the same time, the forest land reduces the wind speed, the water vapor is not easy to spread, and the relative humidity in the forest land can be increased by 20% ~ 30%. In summer, the huge roots of trees, like pumps, constantly absorb water from the soil and then transpiration from the branches and leaves to the air. 1 hectare broad-leaved forest can transpiration 2500 tons of water in summer, which is equivalent to the evaporation of reservoirs in the same area and 20 times higher than that of land in the same area. According to the measurement, the daily transpiration of Pinus tabulaeformis per hectare is 43.6 ~ 50.2 tons, and that of Populus canadensis is 57.2 tons. Because of the strong transpiration of trees, increased water vapor and moist air, the air humidity of green land is 25% ~ 35% higher than that of non-green land, which is an important reason for the fresh air of woodland, thus creating a cool and comfortable living climate environment for people. Before defoliation in autumn, the growth of trees gradually stopped, but transpiration was still going on. Although the air humidity in green space is not as high as that in spring and summer, it is still higher than that in non-green space. In winter, the wind speed in woodland is low, the turbulent exchange in the air is weak, and the evaporation of soil and trees is not easy to spread, so the absolute humidity in woodland is generally higher, and the relative humidity is also higher than that in non-green land (10% ~ 20%).

The environmental temperature in the tree planting area is often lower than that in the building area, because shading can reduce direct sunlight and consume a lot of heat to transpiration the water absorbed from the roots. Especially in summer, the temperature of woodland is 3 ~ 5℃ lower than that of non-woodland and even about 10℃ lower than that of building area. The cooling effect is more obvious in forest parks or under densely shaded street trees. In hot summer, the surface temperature of bare land without trees far exceeds the atmospheric temperature at that time. When the maximum temperature of the open square at the height of 1.5 m is 3 1.2℃, the maximum surface temperature can reach 43℃, while the surface temperature in the green space is much lower than that in the open square. According to the temperature measured at the same time, the asphalt pavement is 36 ~ 45℃, the farmland soil ground is 28 ~ 32℃, the city center temperature is 27.5℃, and the forest temperature is 24.5℃. Because trees need to absorb 16.7 kilojoules of solar heat to produce 1 g of carbohydrates, which is equivalent to 2500 liters of carbon dioxide contained in the atmosphere, it is cooler where there are trees. Because of the large proportion of cold air, it falls to the surface, which leads to the increase of temperature difference between large forest land and other areas, thus promoting the circulation of air, so green space can create a good heatstroke prevention and cooling environment for people.

Due to the dense population and industrial concentration, the phenomenon that the temperature in the central area of the city is higher than that in the surrounding areas is called the heat island effect. For example, the average temperature in July in Beijing is 65438 0.6℃ higher in Tiananmen Square in the city center than in the suburbs. There are about 60,000 square meters of "heat island" in Shanghai, which is higher than that in the suburbs1℃; The annual average temperature in downtown Los Angeles is 65438 0.5℃ higher than that in suburban rural areas. Different from the loose and wet underlying surface in rural areas and covered by plants, the underlying surface in cities is mostly paved with bricks, cement and asphalt. , large heat capacity; The building wall with alternating density and staggered height increases the component of radiant heat, and its density reduces the diffusion of reflected heat. As a result, the average urban temperature rises and the temperature difference between day and night decreases, resulting in the heat island effect.

Around 65438+1July 6, 1995 15, the downtown area of Guangzhou was suddenly overcast with heavy winds, followed by downpours. However, there is basically no rain in the suburbs and it is calm. Meteorologists believe that this is caused by the "heat island effect". According to experts from Guangzhou Central Meteorological Observatory, the convective cloud originally formed in the area from the south of Sanshui City to the west of Nanhai City moved eastward at a speed of 30 km/h around 14 the day before yesterday, and developed rapidly after entering Guangzhou at around 15: 30, resulting in a strong thunderstorm of 1.5 m/s and a gust of magnitude 6, which lasted for 2 hours and the rainfall was 46.66. When the rain belt moves to the northeast, it weakens and disappears at an extremely fast speed.

1 July, 996/noon1day 1 1, a cloud and rain mass in southwest Shanghai increased rapidly after it entered the western suburbs. In the urban area, it just turned into a rainstorm, and the maximum precipitation in the urban area in 24 hours exceeded 160 mm, but at the same time, some suburban counties such as Fengxian and Jinshan only dropped by more than ten millimeters. Yao Zhizhan, director of Shanghai Central Meteorological Observatory, pointed out that atmospheric circulation is the first factor to be considered. But the location of the rain belt does not tell everything, and the environment of the city itself does have a great influence on rainfall. As we all know, the so-called "heat island effect" makes the temperature in the urban area significantly higher than that in the surrounding suburbs because of less greening and more reinforced concrete buildings. "Turbid island effect" refers to the concentration of industrial and mining enterprises in urban areas, which causes serious dust accumulation in the air. Dust and other pollutants are the most needed "condensation nuclei" for water vapor in clouds to become rainfall. In addition, due to the concentration of buildings in urban areas, the ground conditions are much rougher than those in suburban counties, the wind speed in urban areas will be greatly reduced, and weather systems such as strong rain belts will stay over urban areas for a long time, thus increasing the total precipitation.

(2) Purify the environment, slow down and monitor air pollution.

With the expansion of urban construction scale, the development of industrial production and the increase of population density, all kinds of energy consumption is overloaded and the discharge of three wastes exceeds the standard. When it goes beyond the self-purification and autonomy of the city, it will cause environmental pollution problems that endanger the normal operation of the system, human survival will be threatened by its own development, and urban development will be destroyed by its own construction.

According to the report released by the United Nations 1995, at present, only 20% of the urban residents in the world breathe the air that meets the acceptable standards, while about/KLOC-0.8 billion urban residents breathe the air that contains excessive sulfur dioxide and soot. The cities with the highest sulfur dioxide content in the air are Milan, Tehran, Seoul, Brazil, Rio de Janeiro and Sao Paulo, Paris, Madrid, China, Beijing, Shenyang and Xi 'an. From 65438 to 0999, the World Health Organization measured the concentrations of total suspended particulate matter, sulfur dioxide and nitrogen dioxide in the atmosphere of 272 cities in 53 countries around the world, and re-launched the top ten polluted cities in the world, including Beijing and Lanzhou in China. Shenyang, as a polluted old city that has been on the list, withdrew from this list.

Toxic gases emitted in industrial production are the main source of air pollution. For example, the main harmful gas produced by smelting enterprises is sulfur dioxide, which is large in quantity, widely distributed and harmful. Hydrogen fluoride is another highly toxic gas produced by kilns, phosphate fertilizer plants and glass factories, which is 20 times more harmful to human body than sulfur dioxide. According to the environmental statistics of China in' 98, the air pollution in China is still dominated by soot, which is at the level of severe pollution. The average annual concentration of urban sulfur dioxide is between 3 and 248 μ g/m3, and the national average is 66 μ g/m3. The average annual concentration of nitrogen oxides is between 4 ~ 140 μ g/m3, and the national average is 45 μ g/m3. The average annual concentration of total suspended particles is between 32 ~ 74 1 μ g/m3, the national average is 29 1 μ g/m3, and the national average annual dust fall is15.3t/(km2/month). During the period of 1997, the total emission of sulfur dioxide in China was 22.66 million tons, including industrial emission17.72 million tons, and the rest was domestic emission.

The high content of sulfur dioxide in the air is caused by the special underlying surface formed by the high-rise buildings, dense public facilities and criss-crossing streets in the city, and the special climatic conditions formed by the interaction of a large number of pollutants such as heat, waste gas and smoke in people's daily life. When the concentration of sulfur dioxide in the air reaches 0.00 1%, people will find it difficult to breathe and cannot work for a long time. When it reaches 0.04%, people's glottis will spasm and suffocate, and they will soon die. However, the sulfur dioxide released into the atmosphere by combustion combines with the water vapor in the atmosphere and falls with the rain to form acid rain (rain with pH < 5.6). According to the monitoring of Zhejiang Environmental Protection Bureau, the coverage area of 1997 acid rain has reached more than 80%, and the acid rain rate has reached 63.3%, that is, two out of every three rains are acid rain on average. The pH value of vinegar (organic acid) in China is usually 3, and acid rain (inorganic acid) with pH value of 3.32 has been detected in a certain place in this province, and its acidity is close to that of vinegar.

Many garden trees can absorb harmful gases. 1 ha Cryptomeria fortunei can absorb 60 kilograms of sulfur dioxide per month, and citrus leaves absorb more sulfur dioxide than Cryptomeria fortunei. Through the determination of sulfur absorption capacity of some common garden trees, it is found that Ailanthus altissima and oleander not only have strong resistance to sulfur dioxide, but also have strong absorption capacity of sulfur dioxide. Under the condition of sulfur dioxide pollution, the sulfur content of Ailanthus altissima leaves can reach 29.8 times of normal value and 8 times of oleander. Others such as coral tree, Lagerstroemia indica, pomegranate, cinnamon, Magnolia grandiflora, palm, Elaeagnus Elaeagnus, Ginkgo biloba, Sabina vulgaris, Cephalotaxus grandiflora and so on. , but also has a strong ability to resist sulfur dioxide. Robinia pseudoacacia, Ligustrum lucidum, Paulownia, Platanus acerifolia, Buxus macrophylla, etc. Both have strong fluoride resistance and absorption capacity. In addition, hibiscus, acacia, phellodendron amurense, poplar, bauhinia, wisteria, Amorpha fruticosa and so on have strong resistance to chlorine and hydrogen chloride. Lagerstroemia indica can absorb low concentrations of mercury. Most tree species can absorb ozone, among which Ginkgo biloba, Cryptomeria fortunei, Cinnamomum camphora, Pittosporum fortunei, Cyclobalanopsis glauca, Ligustrum lucidum, oleander, Robinia pseudoacacia, Platanus acerifolia and Forsythia suspensa have great effects on ozone purification. Some trees can also absorb ammonia, lead and other harmful gases. Therefore, in areas that may cause harmful gas pollution such as sulfur dioxide, choosing garden tree species with strong resistance according to specific occasions can play a good "harmful gas purification field" (table).

Table of resistance of garden tree species to harmful gases

Smoke and dust in the air and dust emitted by factories are also harmful substances that pollute the environment. Although the weight of these dust particles is very small, their total amount in the atmosphere is amazing. According to estimates, the total amount of smoke and dust discharged from 1997 in China reached15.73 million tons, including industrial smoke and dust12.65 million tons. The total industrial dust emission in China reached15.05 million tons. In many industrial cities, the average annual dust fall per square kilometer is about 500 tons, and in some cities with very concentrated industries, it is even as high as 1000 tons. Every time a city burns 1 ton of coal, it will emit 1 1 kg of smoke and dust. The dust produced by crushing industrial raw materials also contains particles such as carbon and lead. Coal dust, smoke dust and toxic gas particles have great influence on the fine dust floating over the city. Because of their different sizes and weights, their stay time, floating distance and settling speed in the air are also different. When the dust reaches a certain density and distribution height, it will form a fog barrier. Fog barriers reduce the visibility of the atmosphere over the city, and the intensity of solar radiation (especially ultraviolet radiation) received by the ground is generally only 3/5 of the original solar radiation energy, even lower in industrialized cities. The sunshine hours in cities have also decreased accordingly. For example, London, once known as the "foggy city", had only 82% sunshine hours in the city center. Especially in winter, due to the low distribution of fog barriers, the urban sky with coal as the main energy material is gray-black; The formation of inversion layer is not conducive to the diffusion of harmful gases, and it is easy to cause abnormal meteorological conditions (such as increasing rainy days, warming in winter, abnormal rainfall and so on). ), in severe cases, even lead to a large number of poisoning and suffocation of organisms.

Proper selection and application of garden tree species can absorb and filter dust in the air. On the one hand, because of the dense forest canopy, it has a powerful effect of reducing wind speed and promoting the decline of large particle dust carried in the airflow; On the other hand, because the surface of leaves is uneven, hairy and secretes viscous oil or juice, it can absorb a lot of floating dust in the air. After being washed away by rain, dusty trees can restore their dust retention function.

The total leaf area of trees is dozens of times the area occupied by trees, and the forest with 1 square meter has a filter leaf area of 20 ~ 75 square meters, so trees have great ability to absorb smoke and dust. According to the preliminary determination of general industrial areas in China, the dust content in the air in green areas is lower than that in non-green areas by 10% ~ 15%. When the leaves are dense, the dust content in the air under the forest is about 20% ~ 28% less than that in the open square; Even in winter when the leaves fall off, the tree crown can reduce the dust content in the air by more than 5%. Therefore, on the street with garden trees, the dust content in the air at the distance of 1.5 m (human breathing zone) is lower than that in the area without greening by more than 30%.

Spruce, pine, elm, Pu Shu, juniper, ginkgo, beech, poplar, willow, sycamore, phoenix tree, locust tree, melia azedarach, camphor tree, magnolia grandiflora, holly, privet, pittosporum, heather, oleander, coral tree, Chinese holly, boxwood. Therefore, it is very helpful to plant suitable garden trees on both sides of dusty roads and between industrial and mining areas and residential areas to give full play to the function of their natural air filters. Tree species with strong smoke resistance are: torreya grandis, torreya grandis, nanmu, osmanthus fragrans, gardenia, hibiscus, Chongyang wood, triangle maple, pentagonal maple, tallow, Gleditsia sinensis, cherry blossoms, Chimonanthus praecox, hydrangea, Quercus acutissima and so on.

In addition, the natural growth of trees is closely related to the surrounding environmental conditions. In the case of environmental pollution, the toxicity of pollutants to trees takes many forms, and people can analyze and identify the environmental pollution according to the "signals" sent by trees. This kind of tree species sensitive to environmental pollution is called "environmental pollution indicator tree species" or "monitoring tree species".

Different sensitivities of garden tree species to harmful substances of environmental pollution play an important role in monitoring environmental pollution. For example, cedar is very sensitive to harmful gases, especially during the growth period of new buds in spring. When it encounters the harm of sulfur dioxide or hydrogen fluoride, the needles will turn yellow and dry. Therefore, it is often possible to find the source of pollution that emits waste gas around it. In addition, Chinese pine, larch, masson pine, rose and apple are sensitive to sulfur dioxide. Grapes and apricots are sensitive to hydrogen fluoride. Platanus acerifolia is sensitive to ammonia (see table).

List of Monitoring Sensitivity of Garden Tree Species to Harmful Gases

When trees are affected by air pollutants, they usually leave scars on their leaves. Different pollutants and pollution levels have different symptoms. Sometimes the harm of pollution to trees is not to show the symptoms of leaves, but to affect their internal physiological and metabolic activities, leading to the decline of growth, dwarfing of plants, smaller leaf area, early defoliation and falling flowers and fruits. In the case of pollution, the pollutants absorbed by trees will also change some of their own components. These visible symptoms, physiological metabolism produced by trees or the difference of internal components become the preliminary basis for judging the type, concentration and range distribution of harmful gases in air pollution, which can be used to analyze and judge the situation of environmental pollution for further detailed judgment and treatment. Using the sensitivity of garden tree species to harmful substances to monitor environmental pollution is not only economical and convenient, but also simple, which can play a positive role in purifying the atmosphere and protecting the environment as a "greenguard".

(3) Wind prevention, soil consolidation, noise elimination and shock absorption, flame retardancy and disaster avoidance.

The wind-proof effect of garden tree species is remarkable. In winter, woodland can not only reduce the wind speed by 20%, but also keep the wind quiet for a long time, which can reduce the cold wind in cold winter. The greater the wind speed, the more significant the windproof effect of trees; The more trees, the better the windproof effect. It is windy in spring. When the airflow passes through the woodland, it consumes the energy of the airflow through the interception, friction and shielding of trees, thus reducing the wind speed. The windproof function of trees is to protect the windward side of forest belt, and the protection range is 3 ~ 5 times of the height of forest belt; The leeward side can reach 20 ~ 25 times the height of the forest belt. Tree species selection of shelter forest requires stable root system, tough branches and strong wind resistance; The tree is tall, the branches and leaves are lush, and the windproof effect is good; This kind of tree has long life, barren resistance and easy management. Such as Italian poplar, Canadian poplar, pecan, Taxodium ascendens, Taxodium ascendens, Metasequoia glyptostroboides, Castanopsis kawakamii, Ginkgo biloba, Liriodendron chinense, Pinus elliottii, Pinus thunbergii, Pinus tabulaeformis and Pinus sylvestris var. mongolica. Quercus mongolica, Quercus cyclobalanopsis, Casuarina equisetifolia, etc.

The function of soil and water conservation of garden tree species is manifested in the place where there are trees, and the soil is not easy to be eroded by rain. When it rains, the canopy can intercept 10% ~ 20% of rainwater, thus reducing the splash of rainwater on the soil. The litter layer in forest land can improve the water absorption and permeability of the surface and block the surface runoff. 1 hectare forest land stores 300 cubic meters more water than no forest land. According to the test, it is estimated that the time required for the topsoil 20 cm deep on land to be washed away by rain varies greatly with different vegetation coverage: woodland is more than 570,000 years, grassland is more than 80,000 years, cultivated land is 46 years, and bare land only takes 18 years (that is, contemporary people can suffer). The process of soil formation is very slow. The differentiation and formation of 1 cm topsoil requires natural changes of 100 years. Once lost, it's hard to get it back. Therefore, in urban construction, it is undoubtedly an emergency remedial measure to strengthen the function of soil and water conservation, especially for the proper selection and application of garden tree species on the steep slopes on both sides of roads or around ponds.

Urban population is concentrated, traffic is frequent, engineering buildings are springing up, and the roar of various machines and motors is loud, which not only makes people fidgety, easy to fatigue, and reduces labor productivity, but also leads to adverse symptoms such as hearing loss and neurasthenia, which seriously affects their health. According to estimates, when the noise exceeds 70 decibels, human health is damaged; When the noise reaches 90 decibels, people can't work long hours. Noise pollution in urban areas of China is still very serious, and most cities live in moderate pollution level. Among them, the influence of life noise is great and tends to expand, and traffic noise has the strongest influence on the environment. The equivalent sound level of road traffic noise in China ranges from 67.3 to 77.8 dB, and the national average is 7 1.0 dB (length-weighted). Among the 49 urban roads monitored, the sound level exceeding 70 decibels accounts for 54.9% of the total monitoring length. The equivalent sound level of urban environmental noise is 53.5-65.8 dB, and the national average is 56.5 dB (area-weighted). In addition, the noise of each functional area generally exceeds the standard.

Dense trees can absorb and block noise. According to the measurement, the forest belt with a height of 14m and a width of 20 ~ 30m can basically eliminate the noise caused by high-speed vehicles. Another example is that the street building is 5 ~ 7 meters away from the red line, and planting trees and greening can reduce the noise by about 15 decibels. The absorption and blocking of noise by trees is due to the scattering of sound waves by trees. When the sound wave passes, the branches and leaves swing, which weakens the sound wave and gradually disappears. At the same time, the pores and coarse hairs on the blade surface have the function of absorbing sound waves. Practical research has proved that trees and shrubs with low branches and short crowns have stronger noise resistance than trees with high crowns. The noise prevention effect of shelterbelts with the same number of trees, evacuation trees or multiple spaced narrow forest belts is better than that of complete wide forest belts.

The scattering of thermonuclear weapons and the diffusion of radioactive materials have a great relationship with topography and features. Wood is a big barrier, which can block the radioactive diffusion of radioactive substances and play the role of filtration and absorption. Oak forest can absorb all neutron-γ mixed radiation with the dose of 15 Gy, without affecting the growth. Reasonable selection and application of garden tree species can also reduce the vibration caused by explosion and reduce losses.

Many garden tree species also have the function of fire prevention, which can prevent the fire from spreading. Tree species with fire protection function usually have the following characteristics: the tree contains less resin, and the branches and leaves contain more water, so it is not easy to produce flame when it catches fire; Trees have strong germination and regeneration ability and root tillering ability, and can be regenerated quickly after being burned. Evergreen trees with good fire resistance include coral tree, Cinnamomum camphora, camellia, camellia oleifera, Podocarpus, mosquito mother tree, star anise, oleander, pittosporum, Ligustrum lucidum, Cyclobalanopsis glauca, Buxus macrophylla, Ilex cornuta, palm and so on. Deciduous trees include Ginkgo biloba, Quercus acutissima, Ailanthus altissima, Robinia pseudoacacia, Populus alba, willow, paulownia, Platanus acerifolia and Liquidambar formosana. Among them, the coral tree has the most remarkable fire prevention effect, even if all its leaves are burnt, there will be no flame. The fire resistance of ginkgo biloba is also outstanding. Even if all its leaves are burned in summer, it can still germinate and regenerate. In winter, even if most of the trunk is burned, it can continue to survive.

Parks, street green belts and other areas with dense planting of garden trees are also excellent places for earthquake refuge. 1July, 976, Beijing was affected by the Tangshan earthquake. Park green space 15, with a total area of more than 400 hectares and more than 200,000 evacuated residents. At the same time, the earthquake is not easy to cause trees to lodging, and trees are safe places for shock absorption; After the earthquake, we can make full use of trees to build sheds and solve the urgent need of temporary outdoor life. Therefore, in the construction of modern cities, the selection and application of garden tree species in earthquake prevention and disaster reduction can not be ignored.

(4) Benefiting the body and strengthening the body, and enhancing and displaying social functions.

There are 37 kinds of bacilli, 26 kinds of cocci, 20 kinds of filamentous bacteria and 7 kinds of budding bacteria in urban air. According to the investigation in Changshu City, Jiangsu Province, the content of bacteria per cubic meter of air is 3.35% in forest area, 14. 1 1% in forest edge and 309.94% in urban area.

The selection and application of garden tree species can reduce the number of bacteria in the air. On the one hand, due to the reduction of dust in the air in the tree planting area, the carrying amount of bacteria is reduced; On the other hand, trees can secrete a large number of fungicides, which can kill pathogenic bacteria or pathogenic protozoa (such as amebic dysentery and trichomonas vaginalis). The leaves of birch and Populus alba can kill all protozoa in 20 minutes, while eucalyptus citriodora takes only 2 minutes, Platanus acerifolia takes 3 minutes, Sabina vulgaris takes 5 minutes and Pinus bungeana takes 8 minutes to kill protozoa. The bactericide released from eucalyptus citriodora leaves can kill pneumococcus, Shigella, tuberculosis, various inflammatory cocci and influenza virus. 1 ha juniper Berlin can secrete 30 kilograms of bactericide every day, which can kill diphtheria, tuberculosis, typhoid fever, dysentery and other germs. The water extract of Sanguisorba officinalis root can kill pathogenic bacteria and dysentery bacilli of typhoid fever, paratyphoid fever A and paratyphoid fever B within 65438 0 minutes. 0. 1 g rough buds on the ground can even kill flies in 1 s. There are also some volatile oils released by tree species, such as eugenol, cinnamon oil, cinnamon oil, lemon oil, etc., which also have bactericidal effects, especially pine forest, cypress forest and camphor forest.

When people come to quiet, natural, comfortable and leisure forest green space from noisy workplaces and intense work, breathe fresh air and enjoy pleasant scenery, they will feel relaxed and refreshed. People exchange feelings in a fresh and beautiful natural environment and cultivate their own self-cultivation, which is beneficial to health. According to the measurement, people are in a green environment.