Effects of environmental variables on seedling-sapling distribution of Qinghai spruce (Picea crassifolia) along altitudinal gradients

•Seedling frequency has normal distribution along altitude gradient.•Moss has inhibition and facilitation effects on the seedling establishment.•An optimal depth of moss for germination of seedlings has been found. Understanding the effects of environmental variables on seedling-sapling distributions can help gain insight into changes in population recruitment in the context of global warming. Six transects were used to survey the distribution of seedling-saplings and the variability in environmental variables along altitudinal gradients. In addition, laboratory experiments measuring the effects of moss on Picea crassifolia seedling emergence were conducted to identify seedling development characteristics of Picea crassifolia. The results show that the seedling-sapling distribution of Picea crassifolia is determined by altitudinal gradients over the large scale and is influenced by moss at the small scale. We found the largest number of seedling-saplings occurring at middle altitudes (3050–3250m a.s.l.). The crucial abiotic factors in the seedling-sapling distribution for this species are precipitation at lower altitudes and temperature at higher altitudes. Moss mats can promote the recruitment of Picea crassifolia when the moss depth is less than 5cm and moss cover is less than 50%. Conversely, with increasing moss depth and cover, the effects are shifted towards a negative relationship. Moss can promote moist conditions, implying that recruitment may be facilitated by moss in lower altitude areas where drought stress may occur. However, both air temperature in moss layers and soil temperature below moss are reduced, suggesting that the presence of moss may result in a shortened growing season for Picea crassifolia. More research into the seedling-sapling distribution of Picea crassifolia at both the large and small scales is necessary for artificial forestation and forest management, especially for those alpine plant species that are sensitive to climate change.