Abstract:

Afforestation by water-harvesting improves the soil water environment in forest owing to increase of runoff volume collected in the planting strip, so that the productivity of forests is influenced. According to the model of “the double best” and the relationship (the water productivity-WP) between the present situation of water environment improved and the forest production in the different model of afforestation by water-harvesting (forest density), the rational density of afforestation by water-harvesting has been chosen. With the decrease of forest density, the water amount harvested in the planting strip is increased and the soil moisture in available zone of root system is improved, so that the function curve of tree's water productivity changes from linear to logarithmic upward. When the forest density are respectively 1,000 per hm² (in 5 years old ) and 840 per hm² (in 10 years old), the relation curve of tree's WP(WP_t) to transpiration is parabolic, which shows that at this moment water is not a limiting factor of production. Namely, the suitable density values of 5 years old and 10 years old black locust forest are respectively 1,000 per hm² and 840 per hm². Under the suitable density, the WP values of individual tree are respectively 6.7144 g·kg^-1 and 12.1632 g·kg^-1, which accord with the standards of “the designed individual pattern with high-yield” model, and the WP values of forest are respectively 8.4103 kg·hm^-2·mm^-1 and 21.6646 kg·hm^-2·mm^-1, which can be considered as “the optimum population structure” model. It can be applied in both theory and practice to choose the model of forest based on the relationship between water and forest production in the developing of water-saving forestry and in the improvement of water use efficiency.

Key words: Afforestation by water-harvesting, Water productivity(WP), Rational density of stands, The model of &ldquo, the double best&rdquo