种内竞争对树木的死亡，生长和更新的影响尚存争议，部分原因是我们对竞争强度随林龄增加的动态变化及其背后生态过程和机制缺乏了解。同种负密度制约（CNDD）会降低同种个体存活率和生长率进而降低种内竞争，由病原菌介导的CNDD倾向于在植物生长早期发生，而由资源竞争引起的CNDD在晚期更显著。我们前期研究发现在林龄小的天然油松林中根表病原菌相对丰度与种内竞争呈负相关，而在林龄大的油松林中呈正相关。据此，我们假设树木生长早期，由病原菌介导的CNDD降低种内竞争和生长率，同时影响由资源竞争引起的CNDD强度，进而影响晚期种内竞争和生长率，造成病原菌积累与早期种内竞争和生长率呈负相关，而与晚期种内竞争和生长率呈正相关关系。项目拟对不同林龄天然油松林中病原菌积累与种内竞争和生长率的关系进行广泛的实证评估，试图阐明病原菌影响天然油松林不同生长阶段种内竞争的内在机制，为我们准确预测森林动态变化提供科学依据。

No consensus exists about the influence of intraspecific competition on tree mortality, growth, and recruitment, in part because we lack understanding of the ecological processes and mechanisms involved in competition changes as trees increase in age. Conspecific negative density dependence (CNDD) could decrease survival rate and growth rate, and thus reduce intraspecific competition. CNDD mediated by pathogens prefer to occur in the early life stages of plants, whereas CNDD induced by resources competition become important in the late stages. Our previous study, conducted in three natural Pinus tabulaeformis forests, indicated that negative relationships exist in two younger forests between pathogens accumulation of roots and intraspecific competition, while positive relationship was found in the old forest. Base on this, we propose the hypothesis that CNDD, with pathogens as a major driver reduce the strength of intraspecific competition and growth rate in early life stages, and consequently influences the strength of CNDD induced by resources competition, which reduce the strength of intraspecific competition and growth rate in later period. According to this hypothesis, pathogens accumulation would negatively correlate with intraspecific competition and growth rate in early stages, whereas positively correlate in later period. The project intends to conduct a broad empirical assessment of whether these reverse relationships ubiquitously existed in natural Pinus tabulaeformis forest with different ages, to understand the mechanisms of how pathogens accumulation affects the strength of intraspecific competition in different life stages of plants, and hence provide scientific bases for accurately predicting changes in the forest dynamics.