Effect of Disturbance on Population Structure and Regeneration of Abies fargesii var. faxoniana (Rehder & E.H.Wilson): Mild Disturbance Promotes Growth, Severe Disturbance Leads to Decline.

Plant population structure and regeneration are effected by species traits, environmental conditions, and anthropogenic disturbance. Abies fargesii var. faxoniana (Rehder & E.H.Wilson), As a major constructive tree species in extreme high-altitude habitats on the northeastern Qinghai-Tibetan Plateau, the population structure, regeneration status, and development trends under different disturbance intensities remain unclear. We established plots of A. faxoniana natural forests under varying disturbance levels (seven each for mild, moderate, and severe disturbance), investigated population age structure and regeneration characteristics, and analysed population viability and predicted development trends using life tables, survivorship curves, and quantitative methods. The results showed that under light disturbance, the density of individuals and regeneration, and the survival rate were highest; the proportion of young individuals was largest; and the population structure exhibited an inverted 'J' shape, a growing population. Under moderate disturbance, the proportion of young individuals decreased, but the population was still growing. Under severe disturbance, the proportion of young individuals decreased significantly, and the population structure became spindle-shaped, a declining population. An increased level of disturbance led to higher mortality and lower life expectancy. The survival curve of populations under light and moderate disturbance followed Deevey type II (subtype B3), while populations under severe disturbance followed subtype B1. Under light disturbance, the population was stable and exhibited the greatest growth potential, under severe disturbance, growth potential was lowest. Under moderate disturbance, the population structure was most susceptible to change and sensitive to external disturbance. Disturbance significantly effects on the population structure, regeneration, and stability of A. faxoniana forests. Under light disturbance, the population structure remains stable with good renewal and significant growth potential. and the population can grow steadily after passing through the bottleneck stage. Under moderate disturbance, growth potential declines. Under severe disturbance, growth potential is severely insufficient, showing a declining trend. Therefore, populations under moderate and especially severe disturbances must be protected more effectively, alongside reasonable disturbance to promote population recovery and stability. Our findings reveal the effects of different disturbance intensities on the population structure and regeneration of A. faxoniana, providing a basis for the restoration and disturbance regulation of forest ecosystems in high-altitude regions.