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.