Man-made forest has grown rapidly global wise from 1990 to 2015. The percentage increase was from 4.06 to 6.95% of the total forest area between 1990 and 2015 . Man-made forest is playing an increasingly crucial role in timber production, environmental improvement, landscape rehabilitation and climate change mitigation . Natural regeneration is a vital for sustainable forest management. Naturally regenerated forests have the advantages of better plant establishment , self-regenerated material and high seedling densities [4, 5]. However, unreasonable managements make the planted forests unable to complete the natural regeneration [2, 6, 7]. Stand structure , litter density and grass cover , seedling adaptation [10, 11, 12], stand management [13, 14] and year-to-year stand conditions  are considered to be vital factors affecting natural forest regeneration. Natural regeneration of plantation is a long-term and complex process, which is affected by many factors. At present, the regeneration mechanisms of plantation are still poorly understood.
Sufficient seedling bank is the foundation to the success of natural forest regeneration [16, 17, 18, 19]. The structure and composition of natural tree regeneration is closely related to stand density , canopy opening [15, 20], seedling adaptability of tree species [16, 18, 19, 21] and forest management . Long term monitoring on tree seedlings revealed that the fraction of seedlings eventually reach to the sapling class is very small due to high seedling mortality [15, 22, 23]. The transformation from seedlings to saplings requires higher light intensity, which is hard to achieve under the forest. Usually the demand for light is closely related to the shade tolerance for specific trees . With the increase of seedling size, the proportion of non-photosynthetic tissues to total sapling biomass increased and so as respiration costs [25, 26, 27, 28]. Minimum light-demanding tree species rose with increasing seedling size, which led to the decrease of shade tolerance [29, 30]. Kunstler et al. considered that strong morphological adaptation was an important reason for long-term survival of Fagus sylvatica seedlings under forest. However, Sinz et al.  suggested the establishment and persistence of Fraxinus pennsylvanica seedlings beneath closed canopies showed no significant relationships with their morphological adaptation. Soto et al.  thought light and nitrogen interact to influence regeneration in old-growth Nothofagus-dominated forests in south-central Chile. Therefore, the current understanding of the dynamics of seedling banks under forest is still very limited.
Nonstructural carbohydrates (NSC) storage is a fundamental process that allows organisms to meet variable demand for resources during their development and buffer environmental fluctuations in resource supply. Phenology is a key factor influencing seasonal variability of NSC concentrations in trees [25, 33]. Reduced NSC reserve in the root system of Populus tremuloides and P. balsamifera seedlings during severe drought contributed to the root death of seedlings during the dormant season by compromising the frost tolerance of the root system. In shaded forest understories, NSC in tree seedlings would act like a buffer during the periods of negative net carbon balance against herbivores and diseases , defoliated  and suddenly shade increase , etc. Under severe shading, NSC in seedlings will be greatly consumed [38, 39], or even with very little remaining , which determines the survival time under the forest. However, Piper et al.  suggested that carbohydrate storage was not related to low-light survival in Nothofagus species, but supported the view that understory survival is primarily a function of net carbon gain. Imaji and Seiwa  found Quercus preferentially invested more carbon in defense than in storage. In addition, some studies have shown that the effects of NSC on seedling survival are related to water stress , seedling size , defoliation , and forest type . Therefore, the relationship between NSC and survival of seedlings has not yet formed a unified understanding.
China has the largest plantation area in the world. For a long time, plantation management pays too much attention to short-term productivity and economic benefits, but ignores its natural regeneration . Quercus acutissima, a deciduous tree belonging to Quercus in Fagaceae Family, is the main component tree of forest vegetation in warm temperate and subtropical regions of China, a pioneer tree in barren mountains and barren land and an excellent tree species for soil and water conservation, with high ecological, economic and landscape values. At present, there are few studies on natural regeneration of Q. acutissima plantation. Q. acutissima has certain shade tolerance, and its seedlings rarely die under more than 12% full light . Xue et al.  found there are a large number of seedlings in the secondary forest of Q. acutissima, but there are few saplings. In order to further explain the restriction mechanism of natural regeneration of Q. acutissima plantation, this paper takes the artificial pure forest of Q. acutissima, the main forest type of Mount Tai as the research object, and two questions were addressed: (1) the structural change of Q. acutissima seedlings under the forest in a long-term period; (2) how seedlings adapt with the individual growth.