| Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is a most important timber tree in southern China. In the last decades, because of the traditional extensive management and some inappropriate cultivated measures, such as monoculture of a tree species, control burning, over-all site preparation, clear cutting and continuous cropping at the same site, as well as the biological characteristics of Chinese fir itself, the decline of its forest productivity and soil fertility exists in common, which directly affects the sustained-yield management of Chinese fir plantation on site. It is a critical problem nowadays to reveal the' mechanism of productivity decline of Chinese fir plantations after continuous cropping and to explore techniques for long-term maintenance of the forest productivity.In northern Fujian province, the characteristics of tree growth and soil fertility of the Chinese fir plantations with different rounds of continuous cropping, different ages and different site index have been studied through fixed-size plot observations and extensive investigations. Practicable techniques, such as developing undergrowth and fertilization have also been explored for long-term maintenance of the productivity in Chinese fir plantations. The results obtained are as follows:1. Compared with the first planting, D.B.H, height and volume of the tree in the second and the third plantings in the continuous cropping system of Chinese fir decreased. For example, for the site index 16 stands, compared with those of the first planting, the mean tree height, D.B.H and volume of the second planting decreased by 9.27%, 12.56% and 22.85%, and those of the third planting decreased by 26.90%, 15.96% and 56.15%, respectively. The mean individual biomass of site index 16 stands from the second and the third planting of Chinese fir decreased by 18.45% and 42.94% respectively, compared with those of the first planting. The composition of biomass also changed, the ratio of trunk and bark decreased, but the ratio of branches, leaves and roots increased.2. The physical properties of soil were greatly influenced by the continuous cropping of Chinese fir. Especially in the mid-age stands, the soil volume weight increased, capillary, non-capillary and total porosity of soil under the second and the third plantings decreased, compared with the first one. Soil permeability also became worse. For example, for the site index 16 stand, capillary, non-capillary and total porosity of surface soil (0-20cm) under the second stand decreased by 23.33%, 22.03% and 23.06%, capillary and total porosity of surface soil under the third stand decreased by 43.91% and 27.8%, respectively. For the young plantation, due to the influences of site preparation and tending, the effects of continuous cropping on soil physical properties were not as clear as those in the mid-age stands.3. The greatest effect of continuous cropping on soil nutrient status occurred upon available P. Continuous cropping resulted in decreasing of available P to all of the stands. For example, for the site-index 14 mid-age stand, the concentration of available P in the surface soil of the second and the third plantings decreased by 35.82% and 7.68% respectively. For the site index 16 mid-age stand, the organic matter, total N and hydrolyzable N in soil under the second also plantation decreased by 43.98%, 15.11% and 28.86% respectively. But for the young Chinese fir plantation, except for the available P, most of the other nutrient concentration was not decreased, some were even increased.4. Total contents of Mn and Zn in this region were all lower than their average contents of the whole country, and those of Fe and Cu were relatively high. The contents of available Mn, Cu and Zn were relatively low, and most of soil in this region lacked the available contents of these microelements Mn, Cu, Zn, Along with the increasing continuous cropping of Chinese fir, these shortage could become more serious.5. Along with the increasing continuous cropping of Chinese fir, the acidity of the rhizophere soil was raised, and for the first and the second plantings, O. M. and total N of the rhizophere soil was relatively enriched. But for the third planting, they were relatively lacking. The available P in all of the rhizophere soil decreased.6. For the mid-age and mature continuous cropping plantation, the quantity of soil microbe decreased obviously. The organic carbon may increase in the soils of some continuous plantations, whereas the humic acid and humification coefficient decreased, i.e. continuous cropping resulted in lessening the complication and worsening the quality of soil humus.7. The long-term maintenance of Chinese fir plantation productivity was greatly influenced by ground clearance. Control burning and site preparation led to the decreasing of soil volume weights and the increasing of capillary porosity and total porosity, as well as to the water retaining capacity and soil permeability. Control burning and site preparation could also increase the quantity of microbes, and the contents of available N, P, K. Control burning might promote the survival rate andenhance growth of young Chinese fir trees in a short period. However, owing to the great losses of soil, water and nutrients resulted by control burning, it has been proved that control burning for continuous cropping on the same site is one of the major factors leading to the productivity decline of Chinese fir plantations.8. The type of under growth had clear effects on soil fertility of Chinese fir plantation. Compared with the dicranopteis type undergrowth, the nutrient contents of soil under Chinese fir stand with the pteridophyte type increased. For the same type undergrowth, the more abundant the undergrowth, the higher of nutrient contents in the soil. The contents of available B, Mo, Cu, Zn, in surface soil (0~6cm) under young Chinese fir stand with pteridophyte type undergrowth were higher than those with dicranopteis type undergrowth, but those in subsurface soil (6~30cm) of the former decreased.9. In the northern part of Fujian province, fertilization had some positive effects on the young Chinese fir plantation, but, in general, the effects were not remarkable. P fertilization could increase the biomass of different parts of the tree, In particular, the application of ammonium phosphate in a ditch could increase the amount of laves as well as the total biomass of the tree remarkably. P fertilization could also increase the P concentration and storage in one -year-old leaves and roots. Whether there is fertilization or not, leaf was the major P pool of Chinese fir stands.10. In pot experiment of Chinese fir, P fertilization increased its Fe and N absorption, and N and P associated fertilization increased its Mo absorption, P shortage induced Fe and N shortage, and N shortage induced Mo shortage.11. P utilization percentage by Chinese fir stands in Fujian was much higher than that of the pot experiment in Jiangsu. The highest P utilization percentage occurred with ammonium phosphate, which were 4.06% and 10.04% in pot and field experiment respectively. In the northern part of Fujian province, ammonium phosphate might be the best fertilizer for Chinese fir, and fertilization in a ditch was the best fertilization method.
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