Abstract: Vitex shrubland is a typical secondary shrubland in mountainous area of Huairou County and also in the low-mountainous area of north China. Over 80% of the biomass of the shrubland are contributed by its dominant species-Vitex negundo var. heterophylla. Due to frequent cutting, the aboveground biomass of Vitex shrubland was continually removed, and that of its underground accounts for 55% of the total biomass. In 1986, the primary net production of Vitex was 6363.6 kg·ha-1·a-1, in which 72.4% came from its roots, only 12.4% was contributed by the stems. The highest growth rate of the roots of Vitex appeared from May to July and the average productivity in this period was 49.7 kg·ha-1·d-1. The highest growth rate of stems with the average of 8.69 kg·ha-1·d-1 was found from July to October. Obviously, the photosynthetic products had firstly transported to the roots and were accumulated, and then transported to the stems. The allocation of photosynthetic products in the different parts of shrub organ showed that photosynthetic products were accumulated in ball root, then transport to other parts of roots. The top of stems was the first part where photosynthetic products were accumulated, then other parts of stems. A comparison with the biomass of other Vitex shrublands in the similar enviroment in Beijing area shows that biomass of the shrublands in north slope is larger than that in south slope. Comparing the biomass of Vitex shrubland with the biomasses of artificial forests in the same area with similar habitat we found that trees in the site occupied by the shrubs which we studied, can offer 5 times biomass than that of the shrubs.

Key words: Vitex shrubland, Biomass, Seasonal dynamics