50 Common Trees in Hong Kong Volume 2 Trees and Nature
Trees and Nature
Hong Kong is a modern and densely populated city. It is quite unbelievable that
more than 40% of the land is used to build country parks. Perhaps someone
may ask why we need to reserve such a large area for trees and nature. In fact,
trees, human beings and nature are closely related to each other. Below are
some functions of trees in nature.
1)
Photosynthesis
One of the great talents of trees is it can carry out photosynthesis to
absorb carbon dioxide and release oxygen. Such a gas exchange is vital
for the Earth and all organisms, yet it is often neglected.
We always believe that the supply of oxygen from aerosphere is right and
proper. In fact this is due to green plants. The leaf of plants contains
chlorophyll and water which are necessary for carrying out photosynthesis.
Plants adsorb carbon dioxide at night. When the sun comes out, plants
will use chlorophyll and water to carry out photosynthesis and produce
glucose and oxygen. Oxygen is the most important element for
metabolism in all organisms. No organism can survive without oxygen. It
is estimated that about one-third of the Earth’s oxygen comes from the
Amazon rainforest.
Besides, the fact that trees adsorb carbon dioxide benefits human beings.
Excess carbon dioxide will lead to greenhouse effect and therefore rise in
temperature. The amount of trees governs the level of carbon dioxide in
aerosphere, which should be kept around 0.03%. Many developed
countries try to minimize the release of excess carbon dioxide from cars
and factories. They also limit all those activities which damage trees, such
as lumbering. All these measures are to ensure an optimum level of
carbon dioxide in aerosphere to prevent greenhouse effect.
2)
Green reservoirs and natural retaining wall
When you go hiking, have you ever discovered that some clear water
coming out from cracks of rocks? Have you thought about where this
water comes from? This is an effective way to regulate water flow in
nature, which is a very interesting and useful ecological phenomenon.
Trees need soil to grow. The roots are used to adsorb nutrients in soil as
well as support the huge tree trunk. The roots are grasping the soil so
tightly that the soil is protected very well by the roots. Soil contains a lot of
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50 Common Trees in Hong Kong Volume 2 Trees and Nature
gaps so that roots can easily get into the soil. When it rains, water will
seep through the soil and retain in the gaps. This reduces water directly
flowing into the reservoirs and prevents them from overflowing. The water
in the gaps will flow into underground slowly according to permeability and
geographical conditions of the slopes. After some time, the water will
gather in the brook and then the reservoir. This results in better water flow
control. When we hike, we see some water flowing out from the slopes.
This indicates that the soil has stored a lot of rainwater. In this respect,
trees reduce the harm of flooding.
From another point of view, trees protect the slopes and prevent landslide
to occur. There is evidence that slopes without trees are more easily
damaged by landslide than those with trees to protect.
3)
Climate moderator
a) Decreasing temperature in forest
People like to walk under tree canopy to enjoy the cool air if it is a hot
day. Tree canopy blocks most of the sunlight. Besides, sunlight is
used in evaporation and photosynthesis. Therefore the temperature is
lower under tree canopy.
b) Increasing humidity in forest
In autumn and winter, trees release water to the air by evaporation for
growth. This increases humidity in dry seasons and makes people feel
more comfortable.
c) Air exchanger
In similar conditions, the temperature of forests is lower than that of
cities. The reason is that trees adsorb most of the sunlight. This
enables air exchange between forests and cities and thus lowers the
temperature of cities.
4)
City protector
a) Reducing wind speed and blocking sandstorm
Dense canopy reduces wind speed and makes the sands in the air fall
off to the ground. The hair and sticky material on the leaf surface
screen most of the dust and sand in the air. We may have heard of the
fact that trees reduce the destructive effects of sandstorm in
international news.
b) Noise proof
Trees reduce and diverge noise. Sound wave is weakened by the
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50 Common Trees in Hong Kong Volume 2 Trees and Nature
movement of tree branches.
c) Air purifier
Trees reduce bacteria in air. Poplar, Birch and Lemon-scented gum
release disinfectant material to kill pneumonia, dysentery and
diphtheria bacteria.
d) Fireproofing
The woods of some trees possess low fat and water which are ideal
for fireproofing purpose. They would not be ignited easily. Trees such
as Japanese Ternstroemia, Wild Camelia and Buddhist Pine prevent
hill fire to spread about. The crowns of tall trees block most of the
sunlight. This hinders the growth of shrubs which is the main cause of
hill fires.
5)
Beautifying the environment
Nowadays many international cities try to create an image of “green city”.
This is because planting trees make cities more beautiful.
In order to have good town planning, apart from good architectural design
and transportation network, architects would add “green” elements to
high-rise buildings to increase “green area” in cities as well as make cities
more vivid. This could be done because tree changes its image and mood
according to different seasons. In spring, flowers blossom; in summer,
there is fragrance everywhere; in autumn, fruits ripe; in winter, it gives us a
cool image.
To conclude, trees are good for rest, ornamental purposes and promoting
cultural activities. People could relax themselves and release pressure of
city life.
6)
Food for animals
Living things rely on each other in nature. Trees have to think of ways to
disperse pollens and seeds. Some plants use their flowers, leaves and
fruits as birds’ food. This helps pollination and propagation.
Generally speaking, most trees use fruits to attract birds. Examples are
trees of the family Moraceae such as Small-leaved Banyan, Opposite Leaf
Fig, Peepul Tree, White Mulberry and Paper Mulberry. Many birds such as
Chinese Bulbul, Bulbul and Spooted Dove like eating the fruits of
Moraceae trees. Moraceae trees usually grow in cracks or caves. It is
because most birds excrete the seeds there after eating the fruits. Apart
from the fruits of Moraceae trees, those of Azedarach, Bishopwood, Snow
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50 Common Trees in Hong Kong Volume 2 Trees and Nature
Berry and Ivy Tree are also very attractive to birds. Local native species
helps species diversity. For instance, the fruits of Acronychia, Chinese
Alangium, Cassia, Chinese Hackberry, Morris's Persimmon, Lingnan
Garcinia, Turn-in-the-wind and Wild Strawberry are the major food of birds
and animals in forests. Some of them are leiothrix, Formosan gem-faced
Civet, Macaca Cyclopis and boar. They all help dispersing seeds.
Some plants use insects to help propagation. This could be done by
providing leaves for larva to eat. Examples of such leaves are those from
Kassod tree and Sunshine trees. They are the favourite food of larva of
pale yellow species butterflies. The leaves of the family Lauraceae attract
broad-tailed swallowtail butterflies while those of Banana Shrub and
Magnolia attract Common Jays. Other trees such as Lantana, Chinese
Hibiscus and Cuban bast provide honey. Trees such as Castanopsis,
Camphor tree, Gordonia, Sweet Gum, Nanmu Tree, Pond Spice and Silk
Oak attract butterflies, bees, wasps and bumblebees. They help
spreading pollens.
7)
Other uses of forest
People have been using trees for long. Although super alloy and genetic
altered food are very common in modern society, we still rely trees to
provide food and wood.
a) Fruits
The fruits we usually eat are mainly from trees. Examples are apples,
oranges, longans, lychees and carambola.
b) Drinks
Chinese people like drinking tea. This gradually becomes one of the
Chinese cultures. Tea tree then becomes an important research topic.
Other drink: coffee tree (for making coffee), cacao (for making
chocolate drinks), coconut trees (for making coconut drinks).
c) Building materials
People have been using wood in building for a long time. For example,
Camphor tree is used as insect repellent, beech and walnut as floor
material and nanmu as pillars. There are many more examples than
we could list. First, you can count the number of furniture items in your
home that are made with wood!
d) Medical purposes
Long time ago, people have been using plants for curing different
diseases. With modern technology, trees and other herbal plants
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50 Common Trees in Hong Kong Volume 2 Trees and Nature
produce different compounds when they grow, some of which are
used to make medicine for killing bacteria. For instance,
i.
Saponin: Cough medicine, cardiant and water pill.
ii.
Alkaloid: Bitter, toxic, yet useful in curing many diseases.
Examples are alkaloid of cinchona, alkaloid of periwinkle and
Ephedrine.
iii.
Fragrant volatile oil: Stimulant, insect repellent, painkiller.
On the other hand, many herbal medicines used by herbalists are
extracted from different parts of plants. Bark, leaf and fruit of many
trees are used for medical purposes.
In Hong Kong, common trees that are used for such purposes are
Chinese Holly, Microcos, Cape Jasmine, Silk Oak and Screw Pine.
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