On the basis of the NAPP determination of trees in a sample area (5mx5m or 5mx4m, 4-8 sample
areas/mu), and by surveying the amount of litter and herbaceous layer in the woodland, we have
obtained the NAPP of woodland.
2.2 Determination of soil moisture
Soil moisture was determined once every month with soil drill sampling method. Sampling depth
was 3-5m and sampling frequency was once every 20cm depth.
3. Results
3.1 Structure of community and NAPP of seabuckthorn woodland in Wuqi
3-year old seabuckthorn grows fast and its NAPP is 269g/m
2
. 3-4- year old plants begin root
shoot propagation. The 4-5-year old plants can cover land and become forest if the density is high
enough (more than 0.5 seedling/m
2
). Seabuckthorn with lesser density can form cluster shrubs. NAPP
of 8-year old seabuckthorn can reach its first peak of 600g/m
2
, and it has a tendency of stable growth.
NAPP of the 13-year old one is 645g/m
2
, and the total biomass production of weeds in the woodland
reached its peak value of 248g/m
2
.
3.2 Transpiration and water consumption of seabuckthorn woodland
In the semi-arid region of the Loess Plateau, water is one of main factors influencing the distribu-
tion and productivity of plants. Results show that the mean transpiration intensity of 4-6-year old
seabuckthorn in the air-seeding region of Wuqi is 0.377-0.628g/g.h during the growing season, and
that of 5-7-year old seabuckthorn in Guyuan is 0.394-0.911g/g.h. The transpiration intensity of
seabuckthorn has a rhythm of daily and seasonal variation (Tab. 1). Its rhythm is closely related with
solar radiation intensity, air temperature, atmospheric moisture, wind speed and weather condition.
The transpiration characteristics of seabuckthorn show a daily and monthly peak curve or double peak
curve. The transpiration intensity is usually stable, but has a large variation within a year. Therefore,
the results show that seabuckthorn has a defined adaptation to arid environment.
3.3 Water productivity of seabuckthorn woodlands
A shrub's water productivity is usually defined by the quantitative relation of its product and the
water used, which usually can be expressed by WUE. WUE of shrubbery is equal to the amount of dry
material produced by the shrub of a unit area consuming a unit water amount according to the
following formula (Li et al. 1990):
WUE = NAPP/Wc
Where:
WUE = water use efficiency (kg/hm
2
.mm), (t/hm
2
.mm) or (g/m
2
.mm);
NAPP = net aboveground primary production (kg/hm
2
, t/hm
2
, g/m
2
);
Wc = amount of total water consumption (mm).
The annual variation of total water consumption and transpiration water consumption of seabuck-
thorn woodlands in Wuqi, Ansai and Guyuan is shown in Table 1. During the period from 1981 to
1989, the annual mean total amount of water consumption, which includes water consumption for
transpiration, evaporation and runoff is 362mm, and the mean annual water consumption for transpi-
ration is 262mm, which accounts for 72.4% of the total. The total water consumption of vegetation in
wasteland is 366mm, of which water used for transpiration accounts for 13.9%. Total water consump-
tion for transpiration of seabuckthorn woodland is equal to 5.1 times that of vegetation in wasteland.
This shows that water consumption for seabuckthorn growth is much higher than that for vegetation in
wasteland. According to the co-efficient of transpiration and water consumption, the total water
consumption of seabuckthorn woodland producing 1g aboveground dry material in Wuqi, Guyuan and
Ansai is 755, 711 and 829g, respectively, and water consumption for transpiration is 551, 597 and
654g, respectively. Meanwhile, water consumption of vegetation in wasteland for transpiration are
644, 709 and 343g, respectively. The total amount of water consumption is 4501, 4250 and 2540g,
respectively. WUE of seabuckthorn woodlands in Wuqi, Guyuan and Ansai is 1.39, 1.53 and
