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.