Research on the Net Productivity and Nutrient Accumulation of the Natural Forest of Populus cathayana ×P. simonii

The relationship between the tree age and the amount of biomass of different organs of average tree could be expressed by a Logistics function.The biomass and net productivity of various organs and the whole forest increased with the increase of age and was represented by a parabola,so was the composition ratio of the main stem,while that of branch,leaf and bark decreased.The average maximum increment of biomass of sample tree and net productivity of the forest occurred at the age of 31～33,and the annual maximum increment occurred at the age of 20～27.The leaf assimilation rate of stem got the maximum at the age of 24,and decreased rapidly after 33-year-old.As for the content of nutrient elements of different organs,the amount of Ca was the largest,and that of N and K the second,while that of P and Mg the leas The nutrient amount in leaf was the largest,and that in stem the least.More than 80% of nutrients were concentrated in the branch,leave,bark and root.It is suggested that the clear cutting age should be not more than 33-year-old,and the cutting remains should be left in the field to maintain the soil fertility instead of moving away.