SHELTERBELTS FOR FARMLAND-(2)

3 Windbreak effects and structure features of forestbelts

Windbreak effect and structure features of forestbelts have close relationship. The main structure features, which influence windbreak effect of forestbelts, include density of penetrating sunshine spaces, ventilating wind coefficient, width of forestbelt, height of forestbelt, cross-section of forestbelt shapes, and conditions for having leaves or no leaves, etc.

3.1 Density of penetrating sunshine spaces
3.1.1 Concept and calculation ways
It is a ratio of projection area of penetrating sunshine spaces on vertical section of a forestbelt “s’” to total projection area on vertical section of a forestbelt “s”, (express by fraction or percentage), b stands for density of penetrating sunshine spaces.
b = s’/s×100%
In order to be accurate, b can also be counted by forest layers weighted average, such as b1, b2, b3, stand for the density of penetrating sunshine spaces of upper, middle and lower forest layers, individually, A, B, C stand for the thickness of forest layers. H stands for an average height of a forestbelt. The result can be attained with following formula.
b =(b1*A + b2*B +b3*C)/ H

3.1.2 Measure methods
We can use eye measurement, rectangular spectacle, taking Photograph, Probability analysis methods to measure the density of penetrating sunshine spaces of a forestbelt (the detail of measurement is omitted).

3.1.3 Density of penetrating sunshine spaces and the structures of forestbelts
The density of penetrating sunshine spaces is an important feature in the forestbelt structure., Hukumun.P. and KRHLGA.C. A in former Soviet Union have counted and defined the scale for fundamental structure types about density of penetrating sunshine spaces (table 1).

Table 1:The relationship between forestbelt structure type and density of penetrating sunshine spaces
structure types
density of penetrating sunshine spaces(%)
among trucks
among crowns
Dense
Sparse
ventilating
0-10
15-35
>60
0-10
15-35
0-10

This kind of division has a lot of advantages which indexes are clear, and easily to distinguish, certainly, we think that the best method is first to judge a fundamental structure by eyes, then, to distinguish it with density of penetrating sunshine spaces. For instance, a sparse forest belt has 0.3 of density of penetrating sunshine spaces. A sparse forest has 0.4 of density of penetrating sunshine spaces. A ventilating forest belt has 0.5 of density of penetrating sunshine spaces. A ventilating forestbelt has 0.7 of density of penetrating sunshine spaces. And so on.

3.1.4 The relation between density of penetrating sunshine spaces and sheltered effect
The density of penetrating sunshine spaces has close relationship with windbreak effect. Many experiments of scholars had shown, at first, effective protective distance of forest belt increases with increasing the density of penetrating sunshine spaces. When density of penetrating sunshine spaces reaches a certain point, meanwhile, the protective distance reaches the maximum; then, with the density of penetrating sunshine spaces increases continuously, however, the effective protective distance decreases. In north China, practices in production show that the most suitable structure of shelterbelts for farmland is narrow forestbelts, which have 0.25-0.3 of density of penetrating sunshine spaces. This kind of forestbelt not only has the most protective result, but also occupies smaller farmland.

SHELTERBELTS FOR FARMLAND-(1)

Man. Doqing
Ganzhu desert control Research Institute,Wuwei,China

1 -The concept of shelterbelts for farmland and their purpose
1.1 Definition
An artificial forest comprising of belts to protect farmland is called shelterbelt for farmland.
In addition, all shelterbelts, which are planted along the roads in farmland, riverbanks, ditches and the fringes of deserts close to the farmland, are also included in the same category.
Each shelterbelt for farmland is called a protective forestbelt, or a forest belt. The forest belts, which crisscross each other, are called networks of shelterbelts for farmland or networks of forests.
1.2 The purpose of the shelterbelts for farmland
The main purpose of the shelterbelts for farmland is to protect farming crops from natural disasters, particularly from the meteorological disasters, and to improve the farming eco-environmental conditions.
Along the desert fringes or moving dunes in oasis. The establishment of shelterbelts for farmland not only declines blownsand encroachment, such as, sand beating, cutting and piling in the farmland, but also forms a oasis shelterbelt system to improve the quality of the whole environment and to increase the productivity in the oases.
In recent years, people in China have been developing ecological agriculture and farming systems. On the whole, it is a kind of integrated utilization of plants in different space and time. It is on the basis of making fully to utilize sunshine, energy, farmland, labor and living beings to link with planting, raising, woods-fruit and manufacturing industry altogether. This can increase the agricultural benefit several times than the single planting trade. In the ecological agriculture, shelterbelts not only have a protecting function, but also provide a great economic benefit. So it is a protector, and in the same time is also a producer. In the future, with the development of technology of shelterbelts for farmland, the process which people set up shelterbelts for farmland is, in other words, the process that people establish integrated and sustainable agro-eco- systems is.

2. The structure of shelterbelts

2.1 Concept
The structure of forestbelts is size, amount and distribution of wind penetrating gaps in the forestbelt, that is, the distribution and the density of the branches and leaves of the trees.
The different structural forestbelts because of their differences in tree species, their penetrating sunshine space positions and distribution of each part of trees in the air, form some special external shapes. From vertical section of a forestbelt, through penetrating sunshine spaces and their distributions, evenness and layers of a forestbelt can be observed. From cross section of a forestbelt, the various geometric can be seen. These special features determine the function of ventilating condition and windbreak effect of a forestbelt.
The density of plantation (spacing in the rows and rowspacings), the width of a forestbelt (the number of rows) and the disposing patterns of tree species are the main factors of the forestbelt structure.

2.2 Fundamental types of forestbelt structure
According to the big or small ones and distribution of penetrating sunshine space, and windbreak features, the forestbelts are usually divided into three types; they are dense structure, sparse structure and ventilating structure.
2.2.1 The dense structure
The branches and leaves of the trees in the tree belt are crisscross each other during the growing period, the forestbelt is just like a wall and hardly has penetrating sunshine spaces. When the medium wind blows to the belt, it can hardly pass through. The most part of the air current (or airflow) passes from the top of the forestbelt and forms a wind free area or a weak wind area in the leeward of forestbelt fringe. After the air current gets over the forestbelt, the wind speed recovers to the initial speed soon. So the belt with the dense structure has a short protected distance.
Generally, the wide forestbelt which has many rows and three layer crowns to be formed by trees, sub-trees and shrubs in the same forestbelt is the dense structural forestbelt. Some narrow dense forestbelts, branchy trees, or fruticous community in which trees or bushes had been cut to form branchy densely form, also belong to the dense structural forestbelts.

2.2.2 Sparse structure
The penetrating sunshine spaces are evenly distributed on the vertical section. The wind, which blows to it, is divided into two parts, one part penetrates it, just like through a sifter, and forms many small vortex flows at the leeward of the forestbelt, another part goes over on the top of the forestbelt. So a weak wind area is formed at the leeward of forestbelt fringe. The farther the distance to the forestbelt is, the stronger the wind speed recovers. This kind of structure has a longer protective distance.
Usually, the forestbelt, which has two layers of forest-crowns, formed by trees and shrubs can form the sparse structure, or the forest, which has no shrubs, but the branchy trees can also form the sparse structure, too.

2.2.3 Ventilating structure
According to the distribution of penetrating sunshine spaces, this kind of structure has obvious two layers. One layer of tree crowns has smaller and evenly penetrating sun shine spaces, or has hardly penetrating sunshine spaces; another layer (underneath) of trunks has bigger paling-like penetrating sunshine spaces. When the wind blows to the forestbelt, one part goes over the top of the crown layer; another part passes through the underneath, because of Venturi effect, its speed in forestbelt is sometimes higher than that of the wildness. The wind begins to spread at the forest fringe of the leeward and becomes weak gradually; a weak wind area will form at a longer distance from the forestbelt. This distance is called mixed length. After this, the wind speed will be recovered gradually. So protected distance is longer.
Usually, the narrow forestbelt which are only comprised by trees, without shrubs or just with low shrubs in it, and which have evident clear height under the branches and leaves can form the ventilating structure.
Besides above three basic types, practically, there are many transitional types of these forestbelts, such as, upper layer sparse-lower layer dense structure, or upper layer dense-lower layer sparse structure, and etc. ....contineu.....