人工林生产力年龄效应及衰退机理研究进展

研究人工林生长规律具有重要的经济和生态意义。同龄林林分郁闭后，地上部净初级生产量随着林龄增加而降低的现象近几十年引起了林业工作者的兴趣和注意并成为研究热点。多数研究试图通过光合生理、林分营养、生物量分配和林分结构等随林龄的变化规律来解释林分生产力衰退机理。研究认为，林分郁闭后水分传输阻力的增加减少了树木的光合能力；林地养分的减少使得根系生物量分配增加，导致林分叶面积减少，树木光合能力下降；对资源的竞争使得树木优势度发生变化，资源利用率降低。光合能力、林分叶面积和资源利用率的降低以及根系生物量分配的增加是林分生产力衰退的关键，而林分呼吸和林木衰老的作用不大。今后深入研究树体水分运输及其补偿机制、逆境下根系的生长过程及适应机制，并跟踪研究林分生长规律，更有助于揭示人工林生产力衰退的实质。

Advances inresearch on the mechanisms of age-related productivity decline of planted forests

In forestry, it is of both economic and ecological significance to study the growth dynamics of plantations. In an even-aged plantation, the above-ground net primary productivity usually decreases with age following the canopy closure. The mechanisms and potential causes conducive to the decline in above-ground net primary productivity have attracted considerable attentions from forest ecologists and resource managers during the recent decades, and has become a hot topic of research in recent years. A number of hypotheses have been proposed, attempting to explain the net primary productivity decline through the change in foliage photosynthetic capacity, tree nutrition, allocation of above- and below-ground biomass, and stand structure, etc accompanying stand age. The decrease in leaf area following canopy closure is believed to be one of the significant contributing factors of reduced above-ground net primary productivity. The hydraulic limitation hypothesis ascribes the decrease in above-ground net primary productivity to reduced foliage photosynthetic capacity resulting from the increase in xylem hydraulic resistance as trees grow taller. But it fails to explain the rapid decrease in net primary productivity following canopy closure, which may be related to the compensation mechanisms adopted by trees of varied sizes. As a compensation for the decrease in hydraulic conductance as a tree grows taller, it usually increases its sapwood area in relation to foliage area, and accordingly, increases respiration of woody tissues. However, there is lack of experimental data to support the hypothesis that the increase in woody tissue respiration contributes the decrease in stand productivity. The nutrient limitation hypothesis postulates that gradual depletion of soil nutrient reserves may cause the decrease in net primary productivity of plantations. Because soil nutrient depletion may directly increase below-ground root biomass and relatively decrease leaf area and foliage photosynthetic capacity. This hypothesis may explain the decrease in net primary productivity observed in some old stands. There are other evidences that intense intra-specific competition for limited resources and resultant tree discrimination among individuals may, to certain extent, result in the decrease in resource utilization efficiency and net primary productivity. Although senescence-associated genetic variation as a tree is aging plays an important role in the decrease in tree growth, it appears tree size rather than age is more closely related to tree growth. In summary, the decrease in foliage photosynthetic capacity, foliage area and resource utilization efficiency as well as the increased allocation of below-ground root biomass are the major contributing factors of the decrease in net primary productivity of plantations, whereas tree respiration and senescence only play a marginal role in the decline of stand growth. In the future, it will be beneficial to understanding of the mechanisms of the decline in net primary productivity and plantation growth to study hydraulic conductivity and compensation mechanisms, root growth and adjustment under stressed environment, and monitor the dynamics of plantation growth.