The Canopy Photosynthesis Characteristics Of Different Continuous Cropping Generations â… -107 Poplar Plantation

Along with the intensification of production, the phenomenon of continuous cropping becoming more common. poplar as one of the most common species of plantation construction, the result of less productive which caused by continuous cropping is also becoming serious. Photosynthesis is the direct source of forest productivity,but the research of photosynthesis of different continuous generation of Poplar Plantation is less, the mechanism of continuous cropping effect on photosynthesis of poplar plantation has being unclear. In this study, the first generation plantation and the second generation plantation of Poplar as the research object which planted in the Gaoqiao forest farm, ningyang town, Shandong province. And the photosynthetic characteristics, canopy structure were measured in August and September, the determination standard include diurnal variation of photosynthesis in simple leaf, the response curve of simple leaf and photosynthesis, the canopy structure parameters such as leaf inclination、extinction coefficient 、leaf area index etc., the diurnal variation of light distribute in the canopy. The results showed that:Firstly, under the natural state, in August, the first and the second generation of middle and upper appeared the phenomenon of "midday depression",and lower did notdid not appear lower leaf net photosynthetic rate was higher than that in the middle and upper. At 8 a.m. of leaves of second generation show lower > middle > upper; noon, the first generation of middle and upper without significant difference, lower minimum; the second generation of middle and lower without significant difference, upper maximum. Except for above, all the time are shown as upper > middle > lower layer. In September, the first net photosynthetic rate of upper was higher than middle and lower, except at 6 p.m. Pn of lower layer is higher than the middle until 12 a.m, after 12 a.m middle higher than lower layer, Pn of upper of second generation are higher than middle,except at 10 a.m and 6 p.m. Pn of middle of second generation higher than lower layer, but lower layer and middle layer without significant difference at 10 a.m and 4 p.m.Secondly, The net leaf photosynthetic accumulation in August and September were higher than the two generation of forest forest. Net photosynthetic accumulation of the first and two generation in September was higher than in August.Thirdly, in the same photosynthetic active radiation, Pn of first generation show upper > middle > lower in August; Pn of second generation lower was higher than that of middle and lower, Pn of first and second generation showed upper > middle > layer in September, Pn second generation was higher than first generation.Fourthly, leaf area index and extinction coefficient of first generation is greater than second generation. Leaf inclination of second generation is greater than first generation. Crown size and thickness of the first generation was greater than second generation. Leaves leaf area of first generation were greater than second generation.The first generation lower total leaf area wasgreater than the second generation.First generation middle and upper total leaf area was less than the second. the total leaf area of the first generation was greater than second generation.Finally, the first generation and the two generation of the photosynthetically active radiation showed upper > middle > lower.In conclusion, the main reason of the decrease of photosynthetic productivity of the continuous cropping poplar forest is the decrease of the leaf area, especially the decrease of the leaf area of lower layer.