Effects of environmental factors on tree seedling regeneration in a pine-oak mixed forest in the Qinling Mountains, China

Natural seedling regeneration and tree establishment are affected by various environmental factors. In this study, we established eight, eight, six, and four independent forest stands (each stand was further divided into five subplots) respectively based on the altitudinal gradient, stand density, slope location, and slope aspect to investigate the effects of environmental factors on tree seedling regeneration in a pine-oak mixed forest. The results indicated that the seedling density was significantly higher at altitudes of 1,283 m to 1,665 m, whereas the sapling density did not differ with altitudes. The seedling and sapling density decreased significantly at 1,835 m. The seedling densities on the upper slopes were much higher than those on the middle and lower slopes, whereas the sapling density had no difference. The sapling density decreased gradually from the southwest (20A degrees-75A degrees) whereas it increased on the shady slopes to the northeast (40A degrees). The seedling density increased from southwest (20A degrees) to northeast (40A degrees). The seedling and sapling densities increased with the stand density (850 trees ha(-1) to 1,525 trees ha(-1)) whereas the sapling density was significantly lower in stands (1,900 trees ha(-1)). Principal components analysis showed that the slope aspect and stand density had more important roles in tree regeneration in this study region compared with the other two factors. Therefore, our findings suggest that it will be beneficial to keep stands at a moderate density on shady slopes. Appropriate thinning of higher density stands is also expected to promote the natural regeneration of pine-oak mixed forest.